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FINAL I REPORT
DEVELOPMENT OF \ A LASER
DIODE -P-RESSURE SENSOR
(DSS FILE NO. 04SB-FP941-3-2262)
SEPTEMBER 1985
!AKEM OCEArlOGRAPHYLTD SIDNEY, BRITISH COLUMBIA, CANADA
1.
2.
- ; -
TABLE OF CONTENTS
T ABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
INTRODUCTION
Review of Pressure Measurement Technologies
1.1 Principles of Pressure Measurement
1.2 Pressure Measuring Devices
1.2.1 Mechanical Elements
1.2.1.1 Elements Based on Gravitation
1.2.1.2 Elastic Elements
1.2.1.2.1 Bourdon Tubes
1.2.1.2.2 Diaphragm Elements
1.2.1.2.3 Bellow Elements
1.2.2 Electrical Pressure Transducers
1.2.2.1 Strain Gauges
1.2.2.2 Capacitive Pressure Transducer (BKS)
1.2.2.3 Piezoelectric Pressure Transducers
1.2.2.I.f Magnetic Pressure Transducer
1.2.2.5 Resistive Pressure Transducer
1.2.2.6 Other Electric Transducers (Paroscientific)
1.2.3 Other Transducers
1.3 References
filterferometers
2.1 General Properties of Light and Interferometers
2.1.1 Plane Waves, Photons and Light Intensity
2.1.2 Coherence of Light
2.1.3 General Properties of Interferometers
Page
;
iv vii 1
7 9
9
9
9
9
10
11
11 12
12 12
13 13
14
15 16
17
17 17
20 26
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TABLE OF CONTENTS (CONT'D)
Page
2.2 Two Beam Interferometers 35
2.3 Multi Beam Interferometers 38
2.11 References 43
3. Laser Diodes 44
3.1 Intrinsic Properties of Laser Diodes 44
3.1.1 Laser Diode Structure 44
3.1.2 Laser Emission 46
3.1.2.1 Intensity 46
3.1.2.2 Frequency and Wavelength 50
3.1.2.3 Polarisation 58
3.1.2.4 Angular Characteristic 60
3.1.2.5 Optical Power and Injection Current 60
3.1.2.6 Aging, Drift and Deterioration 63
3.1.2.7 Noise 63
3.2 Laser Diodes Under Optical Feedback 66
3.3 References 71
4. Experimental Bench Model Sensor 75 1.1
4.1 Design Considerations for the Bench Model Sensor 75
4.1.1 Operational Concept 75 i ! ,
lj..l.2 Key Parameters 77 t~--::
lj..2 Supporting Systems 78 r.' .. k.!
lj..2.1 Laboratory Characteristics 78 W
4.2.2 Visual Aids 78 I .
4.2.3 Pressure Simulation 79
4.2.lj. Current Source 81
4.2.5 Thermometry 81 I".:.:
G
lj..2.5.1 Thermocouple Approach 84
4.2.5.2 AD590 Electronic Thermometer 90
I ,
5.
6.
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TABLE OF CONTENTS (CONTD)
4.3 Measured Intrinsic Laser Diode Properties
4.3.1 Experimental Setup
Page
92
92
4.3.2 Voltage Drop, Optical Power and Noise vs. Drive Current 96
4.3.2 • .1 Mitsubishi ML 4102 96
4.3.2.2 Hitachi HL 780 I G 96
4.3.2.3 Ortel LDS3-H 98
4.3.3 Optical Power vs. Time: Drift and Noise 103
4.3.4 Optical Power and Temperature 103
4.3.5 Summary of Laser Diode Performance 106
4.4 Measured Properties of Laser Diode Bench Model Sensor 108
4.4.1 Optical Power vs. Displacement: L(d)r; Calibration Curve 110
4.4.2 Optical Power vs. Drive Current: L(I)d 117
4.4.3 Influence of Mirror Reflectivity and Reflector Geometry 124
4.5 Demonstration Sensor 125
4.6 Summary of Tests 127
4.7 References 129
Alternative Approaches
5.1 Alternative Operational Concepts and Devices
5.2 References
Conclusion
130
130
132
133
7. Appendices
A: Sensor Output Forms for Displacement, Between d=IO\l17) 135
and d= !.25mm
B: Sensor Output as Function of Drive Current. 147
C: Manufacturers Test Data of ML 4102, HL 780lG and LDS3-H. 156
D: Properties of CW - Ga Al As Lasers 161
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LIST OF FIGURES
INTRODUCTION
Fig. 0-1
Fig. 0-2
CHAPTER 2
Fig. 2.1-1
Fig. 2.1-2
Fig. 2.1-3
Fig. 2.1-4
Fig. 2.1-5
Fig. 2.1-6
Fig. 2.1-7
Fig. 2.2-1
Fig. 2.2-2
Fig. 2.3-1
Fig. 2.3-2
Fig. 2.3-3
Fig. 2.3-4
CHAPTER 3
Fig. 3.1-1
Fig. 3.1-2
Fig. 3.1-3
Fig. 3.1-4
Fig. 3.1-5
Fig. 3.1-6
Fig. 3.1-7
Schematic Diagram of Laser Diode Sensor
Schematic Diagram of Laser Diode Membrane Type Pressure
Sensor
Light Described as Wave of an Electric Field
Coherence of Light
Spatial Coherence of Light
Interference of Two Beams of Equal Amplitude
Interference of Two Beams of Equal Intensity - Intensity
Variation with Phase Angle.
The Fringe Problem
Calibration Curve l (L1 Cf ) Conventional Set Up of a Michelson Interferometer
Michelson Interferometer for High Mechancial Stability
Improvement
Reflection of a Plane Wave in a Plane Parallel Plate
Formation of Multiple-Beam Fringes
Multiple Beam Fringes in Transmission
Light Rays for Multiple Beam Interference in the Set Up of -
the Laser Diode Sensor
Typical DH - Laser Diode Structure
Definition of Threshold Current and External Quantum
Efficiency
Dependence of Thermal Conductivity on AI Mole Fraction
Frequency Bandwidths of Semiconductor Lasers
High Resolution CW Spectrum of a Ga As Laser
Energy Shift Among Laser Modes
Typical Spectra for Index Guided and Gain Guided Lasers
2
2
18 21
23 27
29
31
33 36 37
38 39
39
42
45 48
51
52 52 55
56
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LIST OF FIGURES (cont'd)
Page
Fig. 3.1-& Wavelength Hysteresis Effect 57 Fig. 3.1-9 Polarization of Laser Diode Output at Different Current 59
Levels
Fig. 3.1-10 Polarizaton of Three Adjacent Longitudinal Modes 59
Fig. 3.1-11 Far Field Laser Beam Divergence for Index Guided Laser 61
Fig. 3.1-12 Far Field Laser Beam Divergence for Gain Guided Laser 61 Fig. 3.1-13 Power /Injection Current Characteristics for Index and Gain 62
Guided Lasers
Fig. 3.1-11j. Power Loss of Laser Diodes Over Time 64
Fig. 3.1-15 Noise of Laser Diodes as Function of Current and Frequency 64
Fig. 3.2-1 Changes of Laser Characteristics Under Feedback: 69
Hysteresis, Wavelength Shift and Undulation
Fig. 3.2-2 Changes of Laser Characteristics Under Feedback: 70
Transient Response, Line Broadening and Required Output
Form for Laser Diode Sensor.
CHAPTER Ij.
Fig. 1j..2-1 Block Diagram of the Optical Feedback Laser Diode Bench 76 Model Sensor
Fig. 1j..2-2 Hysteresis and Displacement of Stacked PZT - Piezoelectric 80 Pusher ~Burleigh PZIj.O)
Fig. 1j..2-3 Circuit Diagram of Laser Diode Constant Current Source 82 Fig. 1j..2-1j. Block Diagram for Temperature Measurement on Laser 85
Diodes with Thermocouples and Temperature Control
Fig. 1j..2-5 Circuit Diagram for Thermocouple Amplifier 86 Fig. 1j..2-6 Circuit Diagram and Layout for Peltier Element Driver 87. 88 Fig. 1j..2-7 Circuit Diagram for AD 590 Electronic Thermometer 89 Fig. 1j..2-& Thermocouple - and AD 590 Thermometers 91 Fig. 1j..3-1 Circuit for Measurements with ML Ij.l02 Laser 93 Fig. 1j..3-2 Variable Bias Source for Noise and Temperature Drift 95
Measurements
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LIST OF FIGURES (cont'd)
Page
Fig. 4.3-3 Voltage-Current Characteristics, Optical Power and 97
Amplitude Noise of ML 4102 Laser Diode l.."_:
Fig. 4.3-4 Voltage-Current Characteristics,Optical Power and 99
Amplitude Noise of HLP 7801 G Laser Diode ! .
Fig. 4.3-5 Optical Power and Amplitude Noise of LDS3-H Laser Diode 100
Fig. 4.3-6 Amplitude Noise and Drift of ML 4102 Laser Diode 101 i
Fig. 4.3-7 Thermal Drift of ML 4102 Laser Diode 102 t Fig. 4.3-8 AD 590 Thermometer Head Bonded to Laser Diode Heat 104
Sink l" Fig. 4.3-9 Correlation of Temperature Changes and Optial Output 105
Power for HLP 780 I GLaser
Fig. 4.4-1 Experimental Set Up for Laser Diode Bench Model Sensor 109
Fig. 4.4-2 Experimental Bench Model: Set Up and Details of Laser 111
Diode Interferometer
Fig. 4.4-3 Bench Model Sensor Output (Fringes) 112 i
Fig. 4.4-4 Declining Fringe Size with Distance 114 I Fig. 4.4-5 Noise Limited Displacement Resolution of 2x10-12 m 114 I .
Fig. 4.4-6 Envelopes for Max and Min of Fringe Signals 115 I Fig. 4.4-7 Fringe Modulation vs. Large Mirror Displacement 116
1 1
Fig. 4.4-8 Fringe ~odulation vs. Mirror Displacement for Four Laser 118 I Currents (Tracking Range)
Fig. 4.4-9 Fringe Modulation vs. Mirror Displacement for Four Laser 119 [ to-:..-
Currents Outside Useful Tracking Range
Fig. 4.4-10 Power/Current Characteristic for Bench Model Sensor for 120 i I· 1
Fringe Maxima and Minima
Fig. 4.4-11 Largest Possible Output for Bench Model Sensor 121 i Constructed From Threshold Shift b
Fig. 4.4-12 Details of Fringe Top at I = 19.2 rnA 123
Fig. 4.5-1 Laser Diode Interferometric Module in Demonstration 126
Pressure Sensor
Fig. 4.6-1 Parameters Governing Sensor Calibration Curve 1I L (d) 128
t~ I'.' . ~ ," ..
Table 2.1-1
Table 4.2-1
Table 4.3-1
Appendix D
-vii-
LIST OF TABLES
Temporal Coherence of Radiation Sources
Comparison of Thermometer Properties
Data of Laser Diode Performance
Properties of CW - Ga Al As Lasers
25
83
107
161
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INTRODUCTION
Since the first recorded measurement of barometric pressure by Torricelli
1643 with a fixed cistern mercury instrument the need to measure pressure in a large
variety of applications has led to a highly specialized and refined field within
metrology and instrument technology. Partial pressure of 10-16 bar are measured by
commercial inverted magnetron masspectrometers and experimental situations for
the fusion of atoms require pressures counted by megabars. Within this range of over
22 orders of magnitude new applications demand for still higher resolution and larger
accuracy and thus require a fresh and innovative approach.
This report summarizes the development work for a novel interferometric
pressure sensor, proposed by SEAKEM OCEANOGRAPHY LTD (1) with applications
for physical oceanography in mind. Pressure measurements of high resolution and
accuracy are needed here in conjunction with temperature and salinity measurements
to derive reliable water density. Other typical applications include depth meters in
submersibles, tide gauges and wave height meters.
The basic sensor scheme utilizes the back refletion from an external mirror
into a laser diode (Fig. 0-0 to modulate its power output, which is detected by a
photodiode. This scheme has been reported as early as 1976 for sensor applications
and investigated as acoustic sensor in 1980 (2) and since suggested also for
measurment of other excitations (3) (4). Detailed information on these sensors is not
provided, they reached however a vibrational displacement resolution of"'9 X 10-14
m (corresponding to a phase shift of about 7 X 10-7 rad) observed at a favourable
frequency of > 1 KHz and possibly using lock-in techniques for noise reduction.
Published signal forms show a ripple of about 1 % and consist of non-monotonous,
asymetric fringes with sharp, unsteady changes at displacement intervals of half the
wavelength of light. For best signal modulation the laser was operated close to
threshold and therefore using only partly the full power of the laser.
The proposed pressure sensor is in its operational concept an electro-optical
transducer, where the pressure sensing element is formed by an elastic membrane
(Figure 0-2). The membrane position is read by remote sensing from a miniature
semiconductor laser using an interference technique: one of the two laser mirrors
("facets") is also used as a beam splitter, (BS, Fig, 0-0 which permits part of the
light, "T", to pass to reach the reflecting membrane and to return to the laser ("signal
wave" S). The other part of the light which is reflected back into the laser medium
acts as reference wave and interferes here with the signal wave. The phase
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LASER DIODE
~ ~ ~) EXTERNAL
PHOTO- R+S R+S R~'; REFLECTOR DETECTOR ~ ...:A- I;)
S ~ ~S
BS • • VARIABLE POSITION
Fig. 0-1 Schematic diagram of laser diode displacement sensor. BS: beam splitter; R: reference wave; S: signal wave.
I
I
EJ I.e.
M
Fig. 0-2 Schematic diagram of laser diode membrane type pressure sensor: M: membrane; R: reflector; LO: laser diode; PO: photodiode.
- 3 -
difference between reference and signal wave is proportional to the distance of the
external reflector from the laser facet and provides an amplitude modulation of the
laser beam, which can be calibrated against pressure. The construction of such a
calibration curve leads to problems for interferometers if the mirror travels
distances larger than one quarter of the wavelength of light. This problem,
sometimes referred to as fringe counting, requires ;l.n electronic memory for the
sensor to remember its absolute calibration, which is unacceptable for a precision
field instrument. A novel simple technique is proposed here and partly developed
which makes the electronic memory unnecessary and interferometric devices in
general more practical. This method is described in more detail in 2.1.3 "General
Properties of Interferometers" and in 4.4 "Measured Properties of Laser Diode Bench
Model Sensor". Following conventional interferometer designs (6) a sensor would
require several additional components such as beam splitters and 1/4 wave plates and
therefore measure several centimeters. The careful analysis of the basics of
interferometers however, led also here to practical solutions and opens for the first
time the doors for the development of a truly small size complete interferometer,
which can be as small as 3 X 3 mm 2 with an estimated resolution exceeding 10-12 m
and the possibility of covering with several dozen fringes a displacement range of
about 10-4 m.
With typical dimensions of fractions of one mm for the laser chip itself, low
power consumption of less than 100 mW under contino us operation and the need for
only two additional components, - an external mirror membrane and a miniature
photodiode - the sys!em offers itself for applications where dimensions, weight and
controlled operating power become determining factors, like for the application in
remote instruments, self-sufficient oceanographic stations or moored buoys.
While proven suitable for dynamic applications the concept of the optical
feedback laser diode pressure sensor -- short OFS - has to be tested and developed
here for a static pressure sensor system where the concern for long term stability and
drift becomes dominant over frequency response and short term stability. Such a
sensor has to meet and beat the performance of existing devices, as for example
manufactured by Paroscientific Ltd. (5)
The goal which the development of the OFS is aimed at can be characterized
by the following specifications:
resolution:
accuracy:
stability/drift:
recalibration:
time response:
storage temperature:
operational temperature
error due to temperature:
pressure ranges:
or:
or:
possibility of battery operation
extreme rigidness
compactness
- 4 -
10-lf to 10-6
10-lf
10-8/h (over 3 months)
after 10lfh (1 year)
1O-2s to 1 s
-lfO to +lfOoC
-2 to +300
10-3 (at -lfOoC) of full scale
1 atm + 30%
600 atm
0-1 atm
These specifications are somewhat flexible as a developmental goal. Resolu
tion, stability and accuracy give a realistic estimate for the expected performance of
the new sensor. Laser based interferometric laboratory devices exceeding these
specifications are used in standards for time and length measurement, indicating the
usefulness of the interferometric principle for high precision instruments.
While the reduction of parts required for the sensor head to only three
components (photodiode, laser, external mirror) permits many favourable features as
size and thermal and mechanical stability of the sensor, it leads to the usage of
components for more than one function in the sensor and the sensor components
become highly coupled.' The design of such a system requires therefore a much
deeper understanding of intrinsic components as well as the integrated sensor system
than required for the traditional design of systems composed of isolated components.
This and the specifications require the careful selection "of materials,
components and operation techniques in order to develop the sensor system. The first
part of this report reviews existing pressure measuring techniques, identifies success
ful materials and approaches, while the following part reviews those basic principles
; .
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of interferometric devices and lasers which allow the identification of operational
and design parameters and their useful range. An experimental bench model OFS is
designed and built as well as support systems providing electrical and mechanical
isolation from the environment, "noise" free electrical power and high resolution
temperature measurement. Measurements are carried out to determine resolution,
thermal impact and stability of the OFS, to optimize the output and determine the
range. Measurements show that short term resolution can exceed the specifications
and a range exceeding 4 orders of magnitude could be therefore achieved. Long term
stability measurements were unsuccessful due to experimental difficulties, as will be
discussed. A model sensor has been built to test assembling techniques and miniature
components on a scale, which represents dimensions of a field pressure sensor. The
final conclusion section summarizes the experience gained, possible applications of
the OFS, its strongholds and weaknesses as they present themselves to date.
This report has been written with the intent to provide enough details in both
theoretical background and experimental techniques to permit design and construc
tion of a highly integrated prototype sensor system.
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REFERENCES
1. "Development of a Laser Diode Pressure Sensor" -- An Unsolicited Proposal submitted to the Department of Supply and Services Canada, Hull-Quebec, by SEAKEM OCEANOGRAPHY LTD., 2045 Mills Road, Sidney B.C. V8L 3SI, April 1983.
2. A. Dandridge, R.O. Miles, T .G. Giallorenzi, Electr. Lett. 16, 25 (1980) 948.
3. "Diode Laser Senses Physical Changes" - "Patents" in "Lasers and Applications" 4, 5 (1985) p. 225-226.
4.
5.
A. Dandridge, R.O. Miles, A.B. Tveten, T .G. Giallorenzi, Proc. European Conf. Integrated Optics, London, 1981, p. 110.
R.B. Wearn, N.G. Larson. "The Paroscientific Pressure Transducer --Measurement of its Sensitivities and Drift", Applied Physics Laboratory -University of Washington, Report APL-UW 8011, Aug. 1980.
6. J.P. Legendre, "Displacement Measurement with a Michelson Type Interferometer", National Research Council of Canada, Div. Electrical Engineering, Report No. ERB 939, NRCC No. 19829, Nov. 1981.
I I ' ,--"
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1. REVIEW OF PRESSURE MEASUREMENT TECHNOLOGIES
Objective:
The following chapter reviews principles and technologies presently used for
pressure measurement, and identifies weaknesses and limitations as well as materials
and techniques useful to be considered in a new sensor.
Vacuum technology will be mentioned briefly, where principles may be
transferred to instruments working at higher pressures.
This chapter consists in parts of a review by G.C. Zoerb (1).
-1.1 Principles of Pressure Measurement
The definition of "pressure" as "force per area unit" (or equivalently the
change of momentum per time and area element) is applied to the solid, liquid and
gas phase. Pressure is measured as a difference to a reference point; with reference
to absolute zero "absolute" or "total presure" is measured. With reference to the
variable atmospheric (barometric) pressure "gauge pressure" indicates pressure above
and "vacuum" indicates pressure below atmospheric pressure. The atmospheric
pressure is not constant, but varies with elevation, latitude and also with time and
temperature. Another reference point, the "standard atmospheric pressure" defined
as 1 bar= 750 Torr is used frequently in technical applications.
Although legally pressure is measured in "pascal" in agreement with the
internationally accepted 51-system, for practical and historical reasons the units (bar)
and (Torr) or (mm Hg) are still frequently encountered.
1 pascal = I N/m-2
I bar = 105 pascal = 750.062 Torr = 750.062 mm Hg
Devices for measuring pressure range from very simple liquid filled U-tube
manometers to intricate electrical pressure measuring systems. In general, liquid
manometers, Bourdon tubes or other mechanical pressure measuring elements are
used when a static (or steady) pressure is determined. For dynamic pressure or when
remote reading is desired a pressure transducer with associated electronic equipment
is used. A transducer is defined as a device which converts one energy form
proportionally into an other energy form as for example mechanical energy being
converted into electrical energy. Transducers are convenient for data processing
- 8 -
and read out in pressure sensor systems. At various times transducers have been
called "pick ups" and "sensing elements".
To measure means "to compare an unknown quantity to a known reference
(2)" and an accurate measurement depends much on properties and quality of this
reference source. The implementation of the reference source forms the physical
principle of the device, while all subsequent functions serve data acquisition,
processing and display. It is through these latter functions that many devices become
transducers.
The majority of the mechanical pressure elements construct their reference
from two of the three (only) mechanical forces: il inertial force (or weight) is used in
liquid manometers and the free piston type instruments. The most common liquid is
pure mercury, which has a high density (g :0 13.5951 g/cm3). A column of 760
mm will subject to an acceleration due to gravity of 9.80665 m/s2 - exert a pressure
of 1 atm = 1.01325 bar (exact). iil elastic force (Hook's law) is used in membrane and
Bourdon tube type pressure gauges. iii) Pressure gauges using frictional force are not
in wide use, appear however being applied in vacuum technology (3).
In addition to the forementioned principles we find in vacuum technology
gauges depending on the transport of heat (and mass) or electrical charge, which can
be calibrated for specific gases into pressure readings. Although reliability and
accuracy of these devices is only marginal they do presently represent the best
approach for the vacuum regime.
While the potential limit of accuracy of the device depends on the properties
of the reference pressure the resolution is both a property of reference source and
read out system. For a mercury column 'of densitY!i = 13.5951 g/cm 3 and an
(assumed) acceleration of g = 9.80665 m/s2 accuracy and resolution implied by these
numbers is about 10-5. The practical limit will likely be less, due to impurities of the
mercury, thermal effects, knowledge of the local gravitational constant g and effects
of the readout and display system. Mercury columns are used as standards for
pressure up to about 4 bar with an accuracy of 10-4 and are used in the form of the
McLeod gauge for vacuum to 10-4 Torr with an accuracy of about 10-2 for the upper
and 10-1 for the lower pressure range.
I
I I
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1.2 Pressure Measuring Devices
1.2.1 Mechanical Elements
1.2.1.1 Elements Based on Gravitation
The long list of purely mechanical pressure gauges based on gravitation as
reference includes the liquid column types: fixed-cistern barometer, U-tubes for
absolute pressure or pressure differences, well type manometer, inclined tube
manometer, ring-balance manometers and bell gauges. The dead weight tester (or
piston gauge) utilizes known masses placed on a free moving piston of known area to
provide a pressure reference and is used as a standard for pressures up to and above
7000 bar. Typical accuracy is 10-3 to 1 O-~ of full span.
Various readout and display systems have been devised (1) to increase the
sensitivity of the sensing element and utilize the potential of accuracy and resolution
of the applied method discussed above. The instrument sensitivity defined as
instrument response to the excitation pressure is characterized by the ratio of output
signal and sighal pressure. Different systems use laser systems, balances, optical,
acoustical, electro-optical and electromagnetic principles to increase the readout
accuracy of columns to about 0.05 mm and use the devices as sensor heads for remote
data processing (transducers).
1.2.1.2 Elastic Elements
Pressure devices based on elastic elements may be classified into three
categories: Bourdon tubes, metallic diaphragms and bellows.
1.2.1.2.1 Bourdon Tubes
Bourdon tubes are among the most commonly used elements and were
patented 18~9 by Eugene Bourdon. The element consists of an oval or flattened shape
tube, which becomes circular when pressure is applied. Different geometrical
configurations of the tubes are used to provide sufficient pointer movement without
gear trains. Typical applications are pressure ranges between 0-1 bar and 0-3500 bar;
instruments for 0-7000 bar are available. Precision gauges reach accuracies of 10-3
- 10 -
to 5 X 10-3 of full scale readings.
Accuracy is classified by the American National Standards Institute (ANSI) by
the following grades:
AA for test gauges with an error less than 1/2% of the graduated scale
A
B
for high grade commercial gauges with an error smaller than 1% of the
scale range within the middle half of the scale and I 1/2% along the
rest of the scale
for commerical grade gauges, where the error must not exceed 2% of
the scale range within the middle half of the scale and 3% along the
rest of the scale
Precision Bourdon gauges are compensated for temperature effects. Most
frequent failures are due to wear of the linkage system which am plifies the tube
movement and thereby increases the overall sensitivity of the instrument.
1.2.1.2.2 Diaphragm Element
Metallic diaphragms are used for measuring relatively low pressures.
Diaphragms may be flat or corrugated disks. The corrugated diaphragms are used in
larger diameters than the flat disks and thus produce greater linear deflections at
lower stress. Larger size and lower stress, however, reduce dynamic response. When
two corrugated discs are joined together at the outer edge the units are referred to
as a capsule, which -.: evacuated -- are used widely in aneroid barometers.
Non-metallic (or: non-elastic) diaphragms are used for measurement of low
pressure or vacuum. The very flexible diaphragms are made from leather, neoprene,
polyethylene, teflon or silk and the reference pressure is generated by a spring.
Devices are available for ranges 0 - I X 10-5 bar to 0 - 0.3 bar gauge pressure.
Flat metal diaphragms are used among others in strain gauge transducers and
capacitance transducers. The ranges covered here in vacuum are (II) 10-3 bar full
scale to about 10 bar ~ full scale with the ability to measure absolute values
approaching 1.3 X 10-8 bar. Under these conditions a 1 Torr - full scale membrane
moves approximately 10-11 m for the resolution limit.
Error sources in connection with diaphragm pressure elements are hysteresis,
non-linearity, and temperature effects. The prestressed (or tensioned) metal
r·, ~.- _.
,
i
!
k
diaphragm shows little inherent hysteresis, is however dependent on the process of f'¥
- 11 -
stretching and tensioning. Hysteresis accounts for less than 10-2% of the reading
error of the instruments. Apart from temperature effects non-linearity accounts for
larger parts of the total error. (For a complete vacuum instrument including
electronics non-linearities may contribute 80% to the total error.)
Fundamental to the design of diaphragm type sensors is the selection of
materials which have an optimum of both thermal coefficients and media compati
bility and Inconel is frequently chosen 0). Temperature effects both the zero
calibration and the span (range) calibration for the instrument. Properly designed
sensors show an "uncontrolled" zero point drift coefficient in the order of 2 X 10-2%
of full scale per oC and a span coefficient of 6 X 10-2% of reading per oC. By
temperature control these coefficients can be reduced by a factor 10-100. Total
error for present technology is between 5 X 10-2 and 3% of reading ~ zero and span
coefficient (for MKS Baratron gauges). It is expec~ed that the measuring capabilities
of diaphragm (capacitative) instruments can be extended to 10-10 bar at the low end
and 70 bar at the high end.
1.2.1.2.3 Bellow Elements
Metallic bellows are made from a piece of seamless tube into a one-piece
expansible or collapsible member. The axial movement - frequently amplified by a
linkage system - is used to indicate pressure. Typical application spans are from 3
bar to about 5 bar. The axial bellow stroke is approximately 5 to 10% of the length.
Hysteresis and zero slrift problems are more prevalent with this element than with
Bourdon tube and flat diaphragm. Materials are brass, phosphor bronze, stainless
steel, Monel or beryllium copper.
1.2.2 Electrical Pressure Transducer
The potential for compactness, dynamic properties and convenience of data
processing have made the combination of elastic elements and remote electrical
readout systems very attractive. The mechanical signal is here transformed into a
convenient form by one of the following means:
i) strain gauges
ii) ca paci ti ve
iii) piezoelectric
- 12 -
iv) magnetic (inductance or reluctance)
v} resistive; moving contact
vi) others
1.2.2.1 Strain Gauges
Since its introduction in 191j.0 electric strain gauges have become the most
important transducers for the measurement of pressure and other variables. Within
the group of bonded strain gauges presently metal gauges and piezoresistive semi
conductor gauges are distinguished with different sensitivities: the gauge factor -
defined as unit change in resistance per unit change in strain - is for a wire or foil
gauge 2, but exceeds 120 for semi-conductor gauges. Bonded strain gauges find
applications including in flat metal diaphragm pressure transducers, piston type
pressure cells, Bourdon tube and bellow pressure transducers. Devices can be built
with high frequency response and for high pressures up to 3500 bar.
A large variety of unbonded strain gauges is on the market, including wire
type, wire with dc-differential amplifier module and semi-conductor strain gauges.
1.2.2. 2 Capacitive Pressure Transducer
Capacitive type pressure transducer normally consist of a metallic diaphragm
between two volumes. Stationary plates are placed at each side of the diaphragm. A
pressure difference will decrease the capacity to one plate and increase it to the
others. Change in capacitance is sensed by an 10KHz AC signal across the plates.
Small size, high dynamic response, high temperature resistance and good linearity and
resolution are major advantages. Disadvantages are thermal drift, sensitivity to
vibration and the necessity for relatively complex electronics. Differential pressure
as low as 10-8 bar can be sensed.
Capacitive pressure transducers resemble in mechanical design and functions
of the components very much the laser diode sensor discussed in this report.
1.2.2.3 Piezoelectric Pressure Transducer
In most transducers the piezoelectric material is in direct contact with a flat
metallic membrane, which deflects under pressure load. The piezo material is backed
I
I I·
,. i
i.
, .. i
- 13 -
by solid material, so that the reference pressure is determined by the elastic
properites of membrane and piezo element. Several natural materials exhibit an
piezo electric effect, including the crystals quartz and tourmaline. Rochelle salt and
ADP are grown artifically. Certain ceramics can be produced to exhibit piezoelectri
city, which they lose at temperatures above their "Curie point". For the most
common ceramics, barium titanate and lead zirconate-titanate (PZT) these tempera
tures are about 1500C and 4000C. Main advantage are good frequency response and
reproducability if temperature is controlled. Disadvantages are sensitivity to
temperature and "electric noise" environments due to high electric impedance. The
latter makes the transducer unfit for static applications.
1.2.2.4 Magnetic Pressure Transducer
Two classes can be distinguished: those, whose operation is based on a change
of inductance and those, that depend on the change of reluctance of a part of the
magnetic circuit. The electic circuits are low impedence circuits, capable for high
currents. Sensing elements are diaphragm, bellows, Bourdon tube and U-tube.
Inductance type transducers are frequently of the linear variable differential
transformer type which basically measures small displacements. The full range
deflection of the diaphragm may be as small as 100 /"'" • Small volume changes due
to membrane flexing and no frictional load on the membrane are major advantages,
as well as small size and possibility to operate off 60 Hz power supplies.
Reluctance type pressure transducers use frequently diaphragm or Bourdon
elements. Typical pressure ranges are 0 - 10-2 bar to 0 - 350 bar at medium
frequency response (50 - 5000 Hz).
1.2.2.5 Resistive Pressure Transducers
Transducers of this category include pressure sensitive wires, carbon pile
pressure transducers and moving contact transducers. The latter consists usually of a
mechanical sensing device and a lever system which moves a contact point over a
distributed resistor made from wire, carbon film or conductive plastic.
A pressure transducer based on the pressure (or force) proportional change of
material resistance has been described 1957 (5). The principle of operation reminds
on the early carbon microphones, where pressure waves change the contact between
carbon particles and thus change the bulk conductivity. In the forementioned
- 14 -
instrument a mixture of rare earths is processed to produce a pressure sensitive
material, which is made to form wafers with contact surfaces on each side. Pressure
variation can be measured directly with an ohm meter since the change in resistivity
with pressure is high.
1.2.2.6 Other Electric Transducers (Paroscientific)
Another unique method relies on the change of the natural vibration
frequency of an elastic resonator under stress. Wires under tension and a quartz
string under compression have been used. The devices are "inherently digital"
because they produce changes in digital patterns portional to pressure and a brief
review on technology is given in (6).
The Paroscientific pressure sensor employs a quartz crystal, which changes
its resonant frequency under compression and gives a digital readout. The vacuum in
the pressure capsule provides the reference state for absolute measurements.
Advantages of this device are digital output, accuracy comparable to primary
standards, insensitivity to environmental factors, minimum size, weight and power
consumption and good utility of the readout.
These transducers have achieved remarkable performance. Available ranges
are between 0 - I bar and 0 - 690 bar. The quartz crystal oscillates at ~O KHz
without stress and the frequency drops to 36 KHz under fuJI range condition. Key
point for the success of this sensor is the construction of a high-Q resonator providing
a small bandwidth in the oscillator spectrum. The Q claimed is in the order of ~OOOO.
The calibration equation used to relate pressure to frequency proved accurate within
70 to 100 X 10-6 of the sensors full scale pressure (7) at a single temperature within
the range -50C to + 300C; Resolution of 3 X 10-6 of full scale pressure as claimed by
the manufacturer may be conservative (7). Hysteresis varies but is under 70 X 10-6
of full scale pressure and temperature effects are smaller than 175 X 10-6% full
scale pressure. The sensor is widely used in oceanographic applications, and was
extensively tested (7).
I 1-. ~.: .:
I
[
L.:.
- 15 -
1.2.3 Other Transducers
Many other pressure transducers have been described in the literature and
only few will be highlighted here.
A device known as hypsometer is used for indirect pressure metering and
measures the changes of the boiling temperature of a liquid in equilibrium with its
vapour for changing pressure. Equilibrium temperature and pressure of the vapor
liquid system are directly related and accuracy and speed of response are limited only
by the temperature sensing means. As altimeter the hypsometer is proven superior to
aneroid instruments and sensitive enough to resolve 1/3 m altitude (3 X 10-5 bar).
Other optical transducers seem possible. A review of the literature (8)
produces however only devices with a purely "dynamic" response (acoustic sensors,
hydrophones) with excellent sensitivities. Another group of optical sensors with
related properties are accelerometers, which are based on the displacement of a mass
due to acceleration and an elastic force serves as reference source. At least four
different readout principles are used: Mach Zehnder interferometer (9), (10),
Michelson interferometer (11), (16) and simple intensity modulation by the relative
position of two apertures (14, (18) ("optical lever"). All these instruments are applied
to dynamic acceleration measurements.
A noteworthy intensity readout system using bulk components is described as
ear Iy as 1959 (17) and reached outstanding resolution with a conventional
incandescent light source: the change of 10-10 rad of a mirror orientation has been
resolved with a response time of 0.25 s. The device has been used to detect changes
in the refractive index of a gas of 10-9, measurements of velocity as low as 30
14"" {year and "it appears that a displacement of 10-14 could be detected."
The piezo-optical effect has been used to encode pressure - or force -
information on light (15), (16), (19). Similar to the piezo-electric effect also the
piezo-optical effect is relatively temperature sensitive. This indicates that the
thermal (lattice) vibrations within the crystal or molecular structure of the material
reach dimensional changes, which can be compared with the changes caused by
pressure or optical stimulation.
- 16 -
1.3 REFERENCES
1. G.C. Zoerb "Pressure and Vacuum" in "Instrumentation and Measurement for Environmental Sciences", 2nd Edition, Bailey W. Mitchel ed., ASAE, St. Joseph, Michigan 49085, 1983. Special Publication 13-82.
2. Private communication, Dr. Telle, Lab 1.22 "Physikalisch Technische Bundesanstalt", D-33 Braunschweig, West Germany.
3. Mentioned as "Spinning Rotor Friction Gauge" in catalog "Measurement &: Control", MKS Instruments, Inc, Barlington MA 01803.
4. J.J. Sullivan, "Modern Capacitance Manometers", Transducer Technology, July! Aug. (1979) 22.
5. O.B. Clark, Prod. Engineering 28, 10 (1957), 106-109.
6. J.M. Paros, "Digital Pressure Transducer", Measurement and Data 56: 10, 2 (1976)
7. R.B. Wearn, N.G. Larson "The Paroscientific Pressure Transducer -Measurement of its Sensitivities and Drift". Applied Physics Laboratory -University of Washington, APL-UW 8011, August 1980.
8. T .G. Giallorenzi et al. IEEE QE 18, 4(1982) 626.
9. A.B. Tveten et al, Electr. Lett 16, 22 (1980) 854.
10. A.D. Kersey, D.A. Jackson, M. Corke, Electr. Lett. 18, 13 (1982), 561.
11. A.D. Kersey, D.A. Jackson, M. Corke, Opt. Com. 45, 2(1983), 71.
12. A. Dandridge, A.B. Tveten, Journ. Lightwave Techn., Vol. L T -2, (1984) 73.
13. T. Shuidri, K. Kyuma, M. Nunoshito, Appl. Opt. 22,11(1983) 1771.
14. G.A. Rines, Appl. Opt. 20, 19(1981),3453.
15. W.B. Spillman, Appl. Opt. 21, 15(1982),2653.
16. D.A. Jackson, A.D. Kersey, M. Corke, Electr. Lett. 18,5(1982),227.
17. R.V. Jones, J.C.S. Richards, J. Sci. Instr. 36 (1959) 90.
2
18. R.O. Cook, C.W. Hamm, A. Akay, J. Sound and Vibration 76, 3(198Il, 443.
19. W.B. Spillman, D.H. McMahon, Appl. Opt. 21 (I9) (I982), 3511.
i,
i ..
I i L:-,
)'.: ~ .. -, I· l:::.:
i
,
I
- 17 -
2. INTERFEROMETERS
The review of pressure measuring instruments in the last chapter revealed
that some instruments under development use an optical transducer step between the
sensing element and the final electric readout unit. Some of these instruments use
interferometers which, because of their capability of high resolution and accuracy
are· also used for precise time and length standards. The following chapter will
review some properties of these instruments before their application in an extremely
compact variation - the laser diode sensor - is discussed for pressure measurement.
Objective:
A review of the principles of operation of dual beam and multiple beam interfero
meters identifies components and their functions as well as the control parameters
necessary for the design and operation of the compact laser diode interferometric
sensor.
2.1 General Properties of Light and Interferometers
2.1.1 Properties gf Plane Waves, Photons and Light Intensity
It is customary. in optics to describe the properties of electromagnetic
radiation simplified either as "photon" particles or as "waves". The physical
properties of the latter can be acurately described by sin or cos functions of the
electric field; for mathematical convenience however they are described by a vector
function with complex amplitude where the real (or imaginary) part is only considered
of physical significance. A plane wave is illustrated in Figure 2.1-1.
The mathematical representation for a plane wave travelling in positive x
direction is given by
-'"
£(x,t) - E .. (2.1-1)
- 18 -
-y:
~ -F
Fig. 2.1-1 Light described as wave of an electric field. Eo: constant maximum amplitude, A: wave length
a) polarization in y - direction b) polarization in y-~ plane
!'o.
, .
I. ''':'''.',
I
I
I l i I L._
- 19 -
-The scalar part of Eo describes a space and time independent, constant amplitude maximum of the electric field !lnd the vector gives the direction of polarization (here in the y-z plane). The exponential part describes the phase of the
wave. The first exponential term describes the propagation direction of the wave • ... The propagation vector k gives the direction relative to the coordinate system
represented by x. The length of the propagation vector j'k I = k is the wave number
k= 2Tr/~. The second exponential term describes the oscillation frequency W = 211:"
or propagation velocity c of the wave, related through c =i\." where c is the vacuum
speed of light. For media other than vacuum the speed is reduced by the index of
refraction n to v = c/n. The first two terms describe the time and space dependent
phase of the wave. The last term introduces a time and space independent phase 'f with respect to the origin as reference.
It is easy to see the various properties of a travelling wave described by this
relationship: "to describes the maximum amplitude and polarization of the wave, the
first exponential term the spatial variation of this maximum amplitude, the second
term the time variation of the amplitude for a fixed location and the final term an
initial phase or the phase relative to another wave.
The intensity "I" of light is defined as the time average of the amount of
energy crossing per unit time a unit area perpendicular to its flow. The average -< E2 > is used for el:ctromagnetic waves to measure this intensity (I), where only
the real parts of the wave functions are taken. For optical frequencies oscillations of
the electric field vector can not be resolved in contrast to radio frequencies and the
detector averages over times long compared to a period giving
(2.1-2)
where the latter applies to plane waves. It is customary to drop the factor 1/2 and
refer to
I
as intensity. Optical radiation detectors - as the eye, photodiodes or photographic
plates - are intensity detectors and their "quadratic" properties make interferometers
possible, as will be shown further down.
- 20 -
If the wave amplitudes are squared by themselves all phase related informa
tion is lost in this process: phase angle, frequency and propagation direction are not
obtained and require an additional "conditioning" of the wave before it enters the
detector in order to be obtained.
The vector properties of the light wave amplitudes permit application of all
methods of vector calculus, including addition and subtraction of the (electric field)
wave amplitudes and the decomposition of the polarization of the amplitude vector
into two orthogonal components.
Other light properties are not adequately described by the wave concept: the
light intensity calculated as I = Eo2 would permit infinite resolution and therefore
neglecting the quantum nature of light. The resolution limit is given by the photon
energy
E hI,) (2.1-3)
where h = 6.62517 X 10-34 joule and \) = cIA is the light frequency. The intensity in
the photon concept is then
I :: N hI) (2.1-4)
where N is the number of photons per unit time and area.
The photon concept is also applied, where radiation interacts with matter as
for example in absortion and emission processes in lasers.
Instruments, which encode their measurement signals by intensity modulation
("amplitude modulation" or "amplitude sensors") have a typical resolution of 10-3 or
10-4, although photon counters permit a higher resolution.
2.1. 2 Coherence of Light
An additional important property of light, the coherence, is not incorporated
in the wave model described above: The above model describes the very ideal
situation of a wave, which originated at times t = - at;;> and will continue to a
similar future point, and which has in addition no lateral boundaries in contradiction
to any real physical situation.
I:' r-'.
l.
! .'
- 21 -
Fig. 2.1-2 Temporal coherence of light. a) perfect wavetrain; b) limited coherence due to phase changes.
c) Spatial coherence of light.
- 22 -
In the real world "white light" is most common, consisting of a mixture of
short wave trains with different frequencies, directions of propagation and different
polarization. Even within the wave train the phase may be interrupted and the lateral
dimensions of the waves are small.
The concept of "coherence" is introduced as a general statistical description
of such radiation fields in terms of correlation functions (5). Interference is the
oldest and simplest example of a technique to establish the correlation between two
light beams or the correlation between light emerging from two different points on a
light source. In this now somtimes called "classical" concept of coherence (in order
to distinguish from efforts starting around 1960 for more detailed descriptions)
temporal coherence describes the time, over which a wave train does not exhibit a
phase disturbance and can thus be "correlated" or compared to another wave train of
equal polarization, frequency, and propagation direction (Fig. 2.1 - 2a). This
coherence time is equivalent to the damping time of a damped oscillator and
therefore related to the "bandwidth" or "line width" A ~ of the light frequency:
Ll t" I
(2.1-5) .-During the time Il tc light travels dlc = c·Ate. which is the length of the
coherentwave train. Light is termed quasi-monochromatic, if 11 v,,'" V.
Spatial coherence describes the lateral confinement of a (plane) wave train at -
the source, over which no phase disturbance occurs, see Figure 2.1-2. A mathema-
tical formulation can be derived from Figure 2.1-3 as follows: The source f is
viewed from some distance at different locations PI, and P2 separated by Ll u and
appears from each position under the solid angle .d.Jl2. .• The spatial coherence length
tau is then the maximum separation between PI, and P2, under which the same phase
of the wave fronts originating at.:f' are obtained and the relationship holds
(2.1-6)
(A u)2 is called area of coherence of for the source f . Combining both coherence effects defines a volume CA u)2 . a l c, the
coherence volume. The energy of one "photon" within this volume would have no
characteristic distinctions, as there are no "phase" bumps etc. within this volume and
I i I '.'
i
- 23 -
f
Fig. 2.1 - 3 Spatial coherence of light: ;fextended lightsource, ~u: separation of two pinholes,LJ..n.angular separation of pinholes
- 24 -
therefore it would be impossible to resolve the position of a photon more accurately
than this volume (2). This volume is therefore compared to the elementary phase cell
of size h3 in thermostatistics and termed energy mode in optics. The fundamental
difference however appears due to different statistics to be applied: while in the
thermodynamic elementary "mode" a maximum of one particle exists there is no limit
for the number of photons per elementary cell. Photons are "bosons" in their
statistical nature. This permits by means of many photons per mode a virtually
unlimited resolution of the mode volume. The number of photons per second
available in a mode (degeneracy''> is a figure of merit for the light source to resolve
or carry information:
(2.1-7)
where the factor 1/2 takes care of two polarization states and it,,), the "radiance,"
describes the intensity distribution over the line profile expressed in photons per unit
area, solid angle, unit frequency interval and unit time.
A narrow emission line light source has a bandwidth of A \I = 108 Hz or a
coherence time A tc .,.10-8 equivalent to £I e c ~3 m. The degeneracy is then about O%> 10-3. A laser running in TEMoo longitudinal mode can have a power of several
watts and bandwidth of 100 KHz are not rare. Thus Atc .. 3 km and rf--- 1014 (5).
Taking this last figure and comparing it with typical figures for laser diodes we
obtain A '" ~ 100 MHz and a power of several milliwatts which would provide a
degeneracy of about d'~ 109 and higher.
From the discussion above it is obvious that the extraordinary properties of
lasers are not so much its coherence length but its enormous photon numbers
available for a single mode. It appears that either multimode - few photon light
sources exist for the development of instruments such as incandescant light sources
or gas discharges or high photon number - few mode light sources. The intermediate
regime leaves room for future developments of light sources.
Table 2.1-1 summarizes the temporal coherence properties of some sources
of radiation.
I . t j
I··
- 25 -
Table 2.1-1 Temporal Coherence of Radiation Sources (after (3) )
Source
HF oscillator (lMHz)
Quartz oscillator (lMHz)
Visible spectrum (Light)
He - Isotope Lamp
He - Ne Laser
Semiconductor Laser Diode
Linewidth Bandwidth
s-1
102
10-3
2xl014
107
3 .... 104
108
Coherence Time tc
s
10-2
103
5xl0-15
10-7
0.3 ..• 10-4
10-8
Coherence No. of Coherent Length Ic Wavelengths
m
3xl06 104
3xl011 109
10-6 3
30 6xl07
3x I 04 ... 1 08 3xl0 10 •.. l0 14
3 6xl06
- 26 -
2.1.3 General Properties of Interferometers
In the last sections properties of light and the possibilities to encode k',
information in intensity {"amplitude modulation"} was discussed in the (plane) wave
and particle picture of light. Another possibility, the encoding by polarization, has
been used in instrumentation as mentioned in 1.2.3 but will not be further discussed.
These two approaches exhaust the possibilities to decode information contained with ~
the maximum amplitude of the electric field, Eo, in 2.1-1 of plane waves.
As mentioned earlier the "phase information" contained in the exponential
term ?f plane waves described by 2.1-1 is lost for detectors. Interferometric
techniques are used to reconstruct this information. In interferometers a signal
containing coherent wave train is correlated (or compared) to a coherent reference·
wave train. Changes in coherence time, spatial coherence, frequency or wavelength,
direction of propagation and phase angle, even in polarization and electric field
amplitude are detected by simply bringing the signal and reference wave trains
depicted in Figure 2.1-11 to an overlapp. The electric field vectors of reference field
Er and signal field ~ are added (as vectors) and form a new resulting radiation field -- . ..... -Et. A detector provides the mathematical operation of formin!1; the square of Et = Es -"
+ Er as intensity proportional output. For plane waves we obtain: ... e. i (k,X ~ t.Jr t + r,. a))
(2.1-8)
(2.1-9)
Where slightly different frequencies W =: 2 1T'Vr and Ws are permitted, as long as
the frequency difference L1 ~.~ {tJr ~l.:J~/is smaller than the linewi.dth f:. Vc; = . J1 We 271"
determind by the temporal coherence.
- 27 -
Eor
Eos
Fig. 2.1-4 Interference of two beams of equal intensity but different phase angle 'f.
371"
- 28 -
The total intensity is then -"
Er :> (2.1-10)
I 5 1 ,.. t Jsr (2.1-11)
where Jsr is the interference term. If WI"'" Co> 5 this is
and the last two terms describe a new travelling amplitude wave with frequency
(ill.,) 1= l4Jr - wsl. This heterodyne interferometry will not be further discussed here.
For CJr :c,)s (homodyne interferometry) we have
(2.1-13)
which shows the dependence of the interference term on the amplitude and the
phases of the waves.
The first term Ll k x describes the interference due to different propagation
directions which fo.rm an angle between the wave fronts. This may be caused by
shear or tilt of the radiation fields ("misalignment of mirror").
The second term describes the relative delay of the wavefronts t1"t
(2.1-14)
d p is the difference in "optical path" both wave fields have travelled caused by
differences in the index of refraction "n" or geometrical path length "I".
I
Ie.,
I .
i . ,
i:. I",:;:';
- 29 -
If the maximum amplitudes of reference and signal wave are equal,
IEori = I Eosl the interefrence signal can be written as (2.1-11,2.1-13):
It-(x) = 2 I [I + Cos (tl/{; + !JC{'(X)j (2.1-15)
If (x) ~ I COS 1. (1 t1 i<; + A rex) ) (2.1-16)
... and if L1k :: 0 both wave fields are propagating into the same direction ("co-
linear") we obtain
(2.1-17)
Interference maxima and minima appear for phase shifts corresponding to
half the wavelength of the lightwave, the maximum intensity is four times the
intensity of signal or reference field and in the minimum zero intensity is obtained,
see Figure 2.1-5.
r ~I
r---"'#'--f..--~'-----'t:.'f Fi g. 2.1-5 Inf..erferonco oC Lwo beama oC equal intoll8ity~ \'Rl"iat.ioll of int.t'.llsity Wit.Jl
phaoe diff .... nc. tJ. If [1]
- 30 -
It can be seen from 2.1-16 that an intensity change in time at the detector
indicates only a change of either the amplitude, polarization, wave vector or optical
path difference, not, however, which quantity has changed. This property as well as
its sensitivity makes interferometers delicate but at the same time very universal
instruments, as it can be adopted to measure either of those variables.
The intensity maxima (or minima) are termed "fringes." The "fringe" output
from interferometers causes two difficulties in the further processing of the data,
.;.'~; ,-,-
sometimes referred to as "fringe problem." ['.-'
Let us assume a sensor signal produces a monotone increasing phase
shift Al(/').., Figure 2.1-5 and eq. 2.1-17. This results in a i) periodic and ii) non
monotone intensity signal, which does not describe the phase angle r uniquely and is
therefore degenerated: major parts of the information on the "phase" are not
completely recovered due to the "squaring" properties of the detector.
The problem arises from the phase being a complex or vector quantity, which
requires definition of both the scaler length and angular direction to define any
arbi trary phase P( 'f i, see Figure 2.1-6a.
Alternatively the projections of the phase on the real axis "r" and imaginary
axis "i" characterize P uniquely, Figure 2.1-6b. With increasing phase angle t1 if the
phase P rotates on a circe I with radius 2fT. The projections on the r-axis correspond to
the known result 2.1-15 Of we neglect unessential details here\ given in Fig. 2.1-6a:
T (2.1-18)
However the dotted vector could produce the same signal, if the point pI, is moved in
"-" direction and only the additional information of the "direction" (sign of A r) will
identify P uniquely within a full period. This corresponds to measuring the i
component:
12.1-19\
and the phase P(I.1?,) is reconstructed as:
12.1-20)
I I,:
i.
l ' , !
- 31 -
III GJ ~
'" l- e
'" .... ""e :::IGJ -+ : o e .... 0 ..... : ... 9- / III e
<l GJ 0 "t:! U ~ .... a.. > I 0 s: "" .... ...
<l ...... Q.O - -...
"" ~o
" .:;; !. CO
'" ..... GJ~
'" .c .... 111 e . ........ GJ
0", >. e
..Q GJ· ...
"''''' I • e .... ... '" • .cGJ ue
I GJ 0 1II"t:! ",ee .c n::I'~ ... .c
I GJ .... I.f- c.'po-OO:J: ~
I ellle 0 0
9- .,..~.,..
.... "' .... <l '" '" N .... N
I .,.. 0'",
"" "" GJ .... GJ .... e ....
I U~U
'" '" V ""OJ"" "'''''''' I .c:::l.c UIIIU
'" .. ~!Q I e 0
GJ ~ :::I ~"'''' .Q c·,.. 00..0 5-'", E ....... '" .... e OJ "t:! :::I "'"t:! e < III .... GJ
"" "t:! N ----- ----- '+- I ....
GJ~i; .c<l"" I- ...
Ie I .... .
N . '" ....
..... Q. .....
0
- 32 -
within one full period. Counting of "periods" or "fringes" can solve the remaining
problem. A different solution will be pointed out further down.
The previous considerations are interpreted as follows: For complete phase
recovery an additional measuring channel has to be provided. This channel has to be
independent from the first (orthogonal basis vectors "i" and "r") and contain phase
information. Cos and sin functions are related by a constant phase difference of 1l' /2
correpsonding to II / I.f in wavelength and it is a common approach to create a
separate channel by choosing a polarization 900 to the first channel and delay the
wavefronts with 11 /4 waveplates ("retarders"). These components are however bulky
in comparison to laser diodes and cannot be applied to the highly integrated laser
diode interferometer, as will be discussed later (Chapter 5). Two other possible
approaches are "differentiation" of the signal and operation of the interferometer at
two wavelengths.
Due to the principle of energy conservation there exists in every interfero
meter a fringe system which is complementary to the first, thus explaining the total
intensity amplitude of 21 (eq. 2.1-15) for either of the fringe systems. "Complemen
tary" means the systems are out of phase by 1800 and if "System I" is observed in
"transmission" direction on a dielectric mirror "System 2" will be observed in
"reflection". Metallic mirrors with absorption are known to introduce additional
phase shift. It is however not possible to implement in this way the second, phase
shifted signal channel with the laser diode sensor. Other methods are described in
the literature (9), (J 0), (J I), (J 2).
The problem of periodicity led to counting of fringes to establish the exact
relationship for the phase. A different solution can be obtained, if an additional,
montone modulation of the intensity with the phase angle A. if is provided: in this
event the phase point p(d.r) rotates along a spiral and permits the unambiguous
recording of P over many periods by two intensity measurements of the "i" and "r"
components. This results in monotonically decreasing or increasing fringe sizes of
both "i" and "r" signals, see Figure 2.1-7. The length of the vector
p (2.1-21)
gives an unambiguous measure for the phase angle and the information encoded in it.
This is the calibration function for the instrument.
I I
I
~','.
210
r
"I" SIGNAL a) b)
Fig. 2.1-7 Calibration curve I (tlf)
a) Monotone declining intensity represented by length of vector up" solves periodicity problem of fringes.
b) Envelope provides calibration curve I (b.~).
t:.1{J
______ w
w
- 34 -
While the outlined approach is necessary if output of more than one fringe is
produced a simple measurement of the "i" or "r" signal is sufficient as calibration
curve p(Ar), if only output of one half fringe is produced.
Interferometers are used to measure five different quantities:
i) geometrical measurements, length 1
ii) measurement of refractive index, n
iii) measurement of the ratio between wavelength A from a standard source
and mechanical length 1
iv) comparison of two wavelengths
v) measurement of spectral line profiles
Each of the quantities can be used to encode pressure information, although only
method i) and iii) follow the traditional approach of mechanical gauges reviewed in
Chapter 1.
In summary the fundamentals of optical radiation and interferometers have
been reviewed, permitting an insight into design parameters for interferometric
sensors in general. The discussion should have revealed why interferometric sensors
are quite delicate, but sensitive instruments. Further the "fringe problem" has been
identified as loss of information from the complex phase during the transformation
(performed by the detector) to a real number, the intensity. Other than in
radiowaves no alternate detector is available. Introduction of a second measuring
channel "i" overcomes this problem. Additional intensity modulation provides the
unambiguous construction of a scalar function P(A'() which is the calibration for the
instrument.
The technological implementation of the interferometric principle will be
discussed only as much as is necessary to understand the very unique properties and
functions of components of the laser diode interferometer under 3.2.
i·--·
- 35 -
2.2 Two-Beam Interferometers
Interferometers are commonly classified by the method of creating reference
and signal wave into instruments where a wavefront is divided into two or more
sections ("wavefront division") or by splitting the wave amplitudes ("amplitude
division"). The latter class will be considered here only. A further classification
distinguishes between two-beam and multiple beam interference. The laser diode
interferometer shows characteristics of both types.
A typical example for a two-beam interferometer is the Michelson-instru
ment, Figure 2.2-1, where the wave amplitudes are split by beam splitter BS into
reference path R and signal path S. After returning from the mirrors M I and M2 the
beams (or radiation fields) are united again by BS before reaching the detector D.
Varying the mirror position by All causes a difference in the optical path length of
2 II ilo n between signal and reference arm and cause a phase difference
2.1r T (2.2- I)
Generation o~ the 900 shifted "i" signal can be achieved in various ways: A
polarizer inserted behind the light source followed by a}t. /4 retarder plate, which
generates two perpendicularly polarized and by;>' /4 delayed beams which pass
through the interferometer arms. A polarizing beam splitter in front of the detector
can separate the "i" or "r signal before the wavefronts are.detected by two detectors.
Another variation of a Michelson interferometer is given in Figure 2.2-2.
This set up (9) uses retroreflectors for greater mechanical stability. The two 1800
complementary fringe systems are used for signal improvement and 900 - delay
technique gives full reconstruction of the phase in conjunction with electronical
fringe counting.
'\;;[;g
MI
R II
• as
~ ~2
(2) /
LIGHT SOURCE / '''-- -v-~------
Fig. 2.2-1
~ 12
S
Wo
Conventional set-up of a Michelson Interferometer: r·11' M2: stationary and movable mirror; 0: detector; S: signal arm.
;:;-. ;.;
AI2
BS: beam splitter; R: reference arm;
-~,....----;-
w
'"
MIRROR
FIXED
CORNER
THIN fiLM POLARIZING
CUBE BEAM-SPLITTER
REFLECTOR )C ~O/~O
BEAM
SPLITTER
... DETECTORS 0, I
MOBilE CORNER REFLECTOR
Fig. 2.2-2 llichelson interferometer for high mechanical stability and signal improvement [9].
MIRROR·
/d.
<l="t> w .....
- 38 -
2.3 Multi-Beam Interferometers
The Fabry Perot inteferometer is the best known multi-beam interferometer
and forms the principle of nearly all laser resonators. Here several radiation fields or
beams are created by multiple reflection from two partly reflecting parallel surfaces
as given ins Figure 2.3-1.
As outlined in 2.1 for two beams the reflector and transmitted amplitudes of
the respective electric field vectors are superimposed and the respective intensities
R (reflection) and T (transmission) are obtained. If absorption can be neglected the
intensities are normalized such, that the incident intensity is equal to one:
R + T I (2.3- J)
The phase difference between successive beams is according to Figure 2.3-1:
d::. nl h cos Q' (2.3-2)
and the intensities observed in the arrangement of Figure 2.3-2 are for the ratio of
reflected (r) and incident (i) waves:
F Sin 2 ..sf T(r}
(2.3-3)
and for the transmitted and incident waves
I (t)
I (2.3-4) -I (iJ - + F S('n 2 J' I z.. where
F ~R
(1- Rt (2.3-5)
r ' i , .
i
i r . L .
I····· ,
i L'
!-
- 39 -
s c,
W
n n
n'
T n
n' h
1 •
£, £2 EJ Fig. 2.3-2 i g. 2.3-1 Reflection of a plane W8\"O in a plano parallel plnte! 1] Illustrat.ing ronnation of multiple 'beaJ\l fqngcs of equal incJinatiol:
with a plane parallel pIat.ol.1J
7 -
Fi g. 2.3-3 lll1ltiplu beam fri.n~ of l'qunl inclination in transmitted light: ratio lUI/lUI ofttRlumlitk'tJ. aud iliCidf;>ut intc:onsiti~ lLi a function of phoso diffcrt'nco d (m is an inwb"Cr). [1]
- 40 -
The latter ratio is shown for various reflectivities "R" in Figure 2.3-3 and it becomes
clear, that high reflectivities provide a transmission filter, which permits good
transmission for only very small phase shifts. For low reflectivities "R" also F is
small and the nominator of (2.3-4) can be expanded and retaining only first order
terms it becomes
t-f (I-ecscf') (2.3-6)
The intensity varies in the form characteristic for the two beam interferometer,
although the modulation depth is very shallow. In Figure 2.3-1 this corresponds to
interference of only two beams CI and C2 or EI and E2.
A small change in geometrical separation "h" of the reflecting surfaces, Fig.
t
2.3-1, produces - due to multiple reflections - a large path difference between the i._
first and say the two hundredth beam resulting in a more drastic change in output
than for the two beam configuration. For a sensitive instrument the operational
point would be chosen at half maximum intensity with high reflectivity. As will be
seen in Chapter 4.4 the laser diode sensor operates in this regime.
If absorption is present, as for example in metallic films, the intensity ratio
for the transmitted intensity is reduced by a factor: r(t}
2 J
) /fF.5tj,/f (2.3-7)
where now holds for the intensities:
/2+R + T I (2.3-8)
The phase is changed upon reflection an additional amount e. If one of the surfaces ,
is made from an absorbing material constant phase difference" between transmitted
and reflected intensities will be obtained.
- 41 -
In a laser diode the resonator forms a Fabry Perot cavity with plane parallel
surfaces similar to Figure 2.3-1, where the angle of incidence is 900. Fresnel
reflection under this condition is for n = 3.5 about R = «n-ll I (n+j))2 = 0.30 and the
intensity maxima for the resonator curve are fairly wide, as will be seen from tests,
Chapter 11.11.
As the principle idea behind a Fabry Perot interferometer is the splitting and
delaying of wavefronts before they are combined again we can think about a further
modification by adding an additional mirror on one side, say "D" in Figure 2.3-11. The
rays between D and B, etc. will now be produced by interference and therefore
appear to be produced by an "effective" reflectivity from the combined surfaces D
and F. Changing the distance d between the first (laser) cavity and external mirror -permits a control over the phase of the wavefronts DB and thus the intensities of the
"C" radiation field. If reflectivities at D and F are Iowa sinusoidal modulation will
be observed at "C". Neglecting the complications due to the presence of the active
laser material this is the scheme of operation for the laser diode sensor (13), which
will be reviewed in more detail in section 3.2 after ·the general properties of laser
diodes are reviewed.
n
n'
n
I ~'
e I
- 42 -
LASER
DIODE
d
1111//11 EXTERNAL MIRROR
Fig. 2.3-4 Light rays for multiple beam interference in the set-up of the laser diode sensor.
1
I,
- 43 -
2.~ References
1. M. Born, E. Wolf. "Principles of Optics", ~th edition, Pergamon Press, New York, 1970.
2. H. Weber, G. Herziger. "Laser-Grundlagen und Anwendungen", Physik-Verlag GmbH, Weinheim-W-Germany, 1972.
3. K. Tradowsky. "Laser-Kurz und Bundig", Vogel-Verlag, Wurzburg - West Germany, 1968.
~. A. Nussbaum, R.A. Phillips. "Contemporary Optics for Scientists and Engineers", Prentice Hall Inc., Englewood Cliffs, N.J., 1976.
5. W.H. Steel. "Interferometry", 2nd edition, Cambridge University Press, Cambridge - Great Britain, 1983.
6. A. Yariv. "Quantum Electronics", 2nd edition, John Wiley & Sons, New York, 1975.
7. M. Garbuny. "Optical Physics", Academic Press, New York, 1965.
8. S.G. Lipson, H. Lipson. "Optical Physics", 2nd edition, Cambridge University Press, Cambridge - Great Britain, 1981.
9. J.P. Legendre. "Displacement Measurement with a Michelson -Type Interferometer", National Research Council of Canada, NRCC No. 19829 -ERB 939, November 1981.
10. J.W. Edgerton, K.L. Andrew, Rev. Sci. Instruments 1>5(2), 1971>, 219.
11. W.H. Southwell, Appl. Opt. 19(16), 1980,2688.
12. C.O. Weiss, Forschungsbericht BMBW, K73-11>, (1973). Institut fuer Plasmaphysik, Universitaet Hannover, West Germany.
13. A. Dandridge, R.O. Miles, T .G. Giallorenzi, Electr. Lett. 16, 1980, 91>8.
j'
- 44 -
3. LASER DIODES
Objectives:
The following chapter reviews properties and operational parameters of laser
diodes of present Ga Al As technology for both intrinsic operation and operation
under optical feedback. This serves to identify favourable and unsuitable laser
structures, parameters and operational conditions.
3.1 Intrinsic Properties of Laser Diodes
The following review in. Chapters 3.1.1 to 3.1.3 is based in large parts on a
summary given in (I).
3.1.1 Laser Diode Structure
A semiconductor represents a pn-diode in the geometry of a suitable
resonator. Three different pn-structures are utilized for the production of laser
diodes: pn-homo-junctions, pn-singlehetero structures (SH) and ppn or nnp double
hetero structure (DH). Liquid phase epitaxy is used to manufacture the structures.
Figure 3.1-1 shows a typical DH-structure.
The active re~ion (n- or p-Gal_x Alx As, xl!: 0) is sandwiched between an-and
a p-Gal-y Aly As layer with x<y and x,y mole fractions. These layers have a larger
energy band gap and a smaller index of refraction than the active region. Conse
quently a potential well forms in X-axis for the injected charge carriers as well as a
dielectric waveguide for the propagation of the light waves.
For lateral confinement (y-axis) of the active region a number of different
technologies led to the development of various DH-structures with different proper
ties, including: oxide-strip laser, V-groove laser, buried-heterostructure (BH) laser,
channeled substrate planar (CSP) laser and transverse-junction-stripe (TJS) laser.
- 45 -
•..• _ .. .--.•• _ ..... ..,I.;::..""7?" ... O'::
----------~--~~~x y
IR - radiation z
metal contact
P 'Go Al As l-y y
P 'Go Al As acti ve 1 ayer l·x x
n -Go Al As . l-y Y
n- Go As (substrate)
meta 1 contact
Fig. 3.1 - 1 Typical DH- laser diode structure (from (1)).
DH-Iaser diodes are classified by their lateral wave confinement into two
groups: in "gain guided" (gg) lasers the lateral wavefront confinement is achieved by
the lateral distribution of the charge carriers, which provide the optical gain in the
laser (2). The second group includes "index guided" (ig) laser diodes, where the
specially designed index profile provides a waveguide effect (3). Laser mirrors are
provided by the (I 10} cleaved facets (yz-plane) of the Gal-x Alx As crystal. The high
refractive index of the crystal material of n = 3.6 results in a Fresnel-reflection at
900 of about 30%. Due to the high optical gain in the laser material this is
sufficiently high to obtain optical feedback and laser oscillation.
,
i
r·'.'-' :-: .. : I
I i
I .
I I i :
r-.-.-
! •
- 46 -
3.1.2 Laser Emission
3.1.2.1 Intensity
A bias voltage in forward direction on the laser structure reduces the
depletion region of the pn junction and electrons are injected as minority carriers
from the n-Gal_y Aly As layer to the p-Gal-x Aly As layer, which forms the active
region. We discuss the emission of light in terms of the band gap model of the pn
junction with valence - and conduction band. It is then a necessary condition for the
emission of light that the conduction band of the active layer contains more injected
charge carriers (electrons) than to be expected under thermal equilibrium conditions.
The disturbed semiconductor tries to return to equilibrium condition by recombina
tion of injected electrons in the conduction band with the holes (minority carriers) in
the valence band. The excess energy is radiated in form of a photon (spontaneous
emission). The energy of the photon is roughly equivalent to the gap between
conduction - and valence band of the active layers of about 1.4 eV, corresponding to'
about 800-900 nm wavelength in the near infrared region. The emitted wavelength
can be shifted by variations of doping type and - concentration in the active layer.
For cw operation of the laser diode a steady-state population inversion between
conduction - and valence band must be maintained within the active layer. This is
obtained by minority carrier injection. Additional to the first laser condition
(population inversion) the optical gain of the stimulated emission process must be
sufficiently large (second laser condition) to compensate for losses of the laser
resonator. Under these conditions a spontaneous photon with an energy smaller or
equal to the difference of the quasi-Fermi-niveans of p- and n- layer is no longer
absorbed. It stimulates the recombination process described above and thus creates a
"secondary" photon ("stimulated emission") with identical characteristics to the
primary photon. This leads to the extraordinary coherence and light properties
discussed in Section 2.1. The threshold of laser oscillation is characterized by
= R, R1. 0.1-1)
stating that the losses per unit length, ct, are compensated for by optical gain g (per
- 47 -
unit length) and the effects of the resonator formed by two reflectors with
reflectivity RI R2 at a separation I and a resonator "round trip" length L = 21. For
some lasers the gain "g" is very high so that no reflecting elements are required. For
semiconductor laser diodes a small reflectivity (of 3096) is sufficient to obtain laser
oscillations.
At small diode currents "I" spontaneous emission dominates ("LED-mode")
giving low spatial and temporal coherence, while at high current the stimulated
emission process prevails ("Laser-mode") leading to larger spatial and temporal
coherence, narrow linewidth of emitted light and high intensity per mode. The
transition from dominating spontaneous to stimulated emission and laser action is
characterized by a point, the threshold current !th· Due to the temperature dependance of gain - and loss factors of the active
layer the threshold current changes according to
(3.1-'21
exponentially with the temperature Tal o"f the active layer, where I*th is the "ideal"
threshold current of the active layer without self-heating effects. (1* th would be
obtained in pulsed operation) 1* th 0 is the "reduced" threshold - .current at 0 K and , To, a material constant with a typical value between 100-300 K. To depends on diode
geometry, doping concentraton in the recombination region as well as the energy gaps
at the heterostructures (I+l.
During cw- operation a considerable fraction of the electric power is
dissipated into heat by two processes: j) ohmic heating according 12r (r: electric serial
resistance of laser diode; typical 0.05 - 5 ohms) and jj) radiationless recombination of
electrons and holes: U I (U: pn-junction voltage,~ 2V). In order to limit the active
layer temperature Tal to acceptable values an efficient heat transport from the
active layer is required. This is usually done by connecting the laser diode efficiently
to a passive eu-heat sink. The combined heat losses lead to an incrase of the active
layer temperature over the heat sink temperture Ths of
I1T IV (u I +- I?'r) Rr 0.1-3)
where RT is the thermal resistance between laser diode and heat sink with typical
I l
f-,
1_.
- 4B -
values 20 - 1500C/W. According to eq. 3.1-2 the temperature increase of the active
layer increases the "ideal" threshold current 1* th:
(3.1-4)
A steady state cw- operation is reached for the laser diode according to eq. 3.1-1 to
3.1-4 only when the (four) independant parameters which govern the laser emission:
electrical serial resistance, "thermal resistance, threshold current and its thermal
dependance are balanced among each other.
Thermal cycling reduces the lifetime of the laser diode considerably (6) and
should be avoided in precision devices. The design of a good heat sink and effective
heat transport between laser active layer, heat sink and bonding of the laser chip is in
its difficulty sometimes compared to the design of specific laser properties (7).
Other parameters effecting laser emission include electric and magnetic
field, pressure applied to the laser crystal and air convection. The latter three
effects have been observed in this lab and are not documented in the open literature.
LEO mod.
Fig. 3.1 - 2 effi ci ency
lUI
current I
e ~~\ Tl.xt • hV 'FfJ
Definition of threshold current and external quantum
- 49 -
The optical power L of a laser diode is characterized as function of the
current above threshold by the external differential quantum efficiency "l. ext, Fig.
3.1-2, which describes by how many photons the laser output is increased for a
certain increase of injected electrons:
::: e hv (3.1-5)
where e,h,V are electronic charge, Planck's constant and frequency of light.
The quantity
AL AI (3.1-6)
(operating efficiency) is obtained as slope from power measurements. The slope may
vary slightly with aging. Typical values (5) are 0.30 ~ ""2 ext":::: 0.75 corresponding '1;\ it? R . ""'vv
for 11 = 830 nm, he = 0.67 ~ to 0.45 ~ 4. op = 1.12 MR·
Any shift in threshold current Ith either due to thermal effects or to change
in gain will be observed in dramatic changes in optical power.
The gain g is found to be related to current empirically (5) as
f3 I WI (3.1-7)
where (3 is a proportional factor and therefore
11:1-, :- (-, [d\+ -'. f3 L
(3.1-8)
where R I, R2 are the laser mirror reflectivities, which form a Fabry-Perot resonator.
In this approximation a change of the effective reflectivity R I (or R2) as discussed in
2.3 will shift the threshold current by Ll Ith and therefore modulate the laser output
by
ilL 0.1-9)
i ,
- 50 -
From these simple considerations a high operating efficiency is useful, however not
critical, as lowest and highest typical values differ only by a factor two. This simple
approximation points out also the importance of high current stability and thermal
effects for the laser diode performance: the required optical power resolution and
current stability (and repeatibility) are of the same order of magnitude giving for
Lo = ImW and a resolution of 10-~ a current stability of better than 100nA over the
longest time scale considered. Similar estimates for thermal effects lead to an
acceptable temperature fluctuation of the order of AT = I mK.
Doping AI-mol concentrations 0 = X = 0.~5 lead to direct band gap materials
and can be used for laser production, while higher concentrations provide indirect
band gap materials, which are used in passive devices such as wave guides or
modulators (20).
Thermal conductivity is a strong function of AI mole fraction, see Figure 3.1-
3 (from (20) ) and therefore thermal properties and emitted wavelength are not
independent. AI provides a shift to shorter wavelengths (7) and low AI - content laser
may prove more stable.
3.1.2.2 Frequency and Wavelength
The frequency properties (wavelengths) of the light emitted from a semi
conductor laser diode are determined by many parameters including the emission
band of the transition, the gain curve, the resonator modes and finally properties of
the laser mechanism which reduce the Iinewidth further, see Figure 3.1-~.
The wavelength range of the emission band is a material property; for Ga As
the centre is around 8~0 nm (9), (10) while the band is between 800 and 900 nm (10).
Variation of the doping concentration x shifts the emission wavelength between 730
and 900 nm for Gal-x Alx As laser (9).
Commercially available devices emit in the range 790 nm (optical recording)
to 850 nm (indicating that outside this range the mixed crystal is not very stable).
Wavelength around 790 nm are still (barely) visible for the human eye and therefore
favourable for alignment.
- 51 -
• j 02 t-
.- ~
00 ~.l.-J....l..J~.l~'~' -'--w_
i , t , .J
OO 02 04 06 08 '0
Fi g. 3.1 ~ 3 Dependence of Ihl!rm:.1 L"UlHlu..:tivity un :\1 mille fractiun.
[
a)
I b)
3
/ /
I
4
",
",
- 52 -
2 -....... 1
'-.. -<.' "
\
Fig. 3.1 - 4 Frequency bandwidths of semiconductor band 2: gain profile 3: resonator mode bandwidth a; at low gain b) at high gain
.t.m&O, ~.o,Aq,,-.
iAA.L8Al Cm.n.q+1l
(m.n.q)
(m.n,q +2) (m,n-+I,q+I' (m.,n+ I ,CI )
Cm.n+l.q + 2)
(m.n+2.q+~ Cm.n+2,q" Cm.n+2,q)
8376 8380 8382 ·8384
WAvELENGTH" (A,)
I'
"
lasers: 1 emission 4: laser linewidth
S-12.5fLm L. 37~~m T· n·x
I'1'I- 150rno
I ·170mo
Fi g. 3.1 - 5 High resolution OJ spectrum of a GaAs laser
- 53 -
The gain curve Figure 3.1-11 depends as discussed in the previous section on
current, temperature, mirror reflectivity (or photon concentration in the laser).
Typical gain curves have about A'/I = 5 nm (FWHM) and permit the development of
several longitudinal modes (27). At low gain few modes develop, while high gain
permits also the development of side modes.
The laser diode sensor requires true single mode operation. Laser modes will
be discussed in some further detail.
A semiconductor laser diode represents a plane parallel Fabry-Perot etalon.
The distribution of the inversion state density in the z-y plane, Figure 3.1-1, permits
excitation of the fundamental transverse mode TEoo as well as the higher orders
TEzy = TEOl, TElO, TEll etc. For DH-structures the active layer is ~ 0.8 "' ... in
z-axis, permitting only one mode in this direction. The active layer measures in
y-axis ~ 20 fi"" permitting the excitation of higher order modes in y-direction: only
the modes TEoo, TEOl, TE02, are permitted. The active layer width in y-direction,
refractive index gap perpendicular to the active region and the injection current
determine which of these modes get excited. Transversal fundamental mode
operation is obtained by reducing the active layer width. Sometimes lasers operating
in transverse fundamental mode are called misleadingly and unprecisely "single mode"
lasers, without regarding longitudinal mode properties.
Typical transversal mode separations are in the order of 0.01 nm (12) to 0.02
nm, corresponding ot 10-20 GHz. Laser line width are 1-300 MHz, with 50 MHz - 70
MHz as a typical value (53) and cavity modifications have been used to reduce the
linewidth down to 1 KHz (511). These data in connection with the discussion of
eq.3.l-l2 shows that for multi-transversal mode lasers a temperature control of
better than 0.5-1 K is required if transversal mode hopping noise has to be avoided.
For the excitation of the longitudinal modes TEoom, TEoom+ 1 the inter
ference condition holds:
2 Yl~ e (3.1-10)
where i\.., are the wavelengths of the modes in (integer number) and nm is the index
, -r
I I
L-_.'
i·-
- 54 -
of refraction at the active layer of i\ ... For typical laser diodes holds:
1=300 ,.."" ,(1m = 3.6, ~ = 830 nm and 1'1'1 = 2600 are the number of half wavelengths
r. per resonator length I. Differentiation of 3.1-10 gives
+ oU e (3.1-11l
which permits the calculation of the mode separation by setting dl = 0, dm = -I and
for i\ = 830, nm = 3.6, dn/d i\ = 1.1 X [0-3 nm- I (4) we obtain (depending on
resonator lengths 1) from
2Y1 ... e (J -~: j~ J (3.1-12)
longitudinal mode separations of II i\ m,m-I ~ 0.2 ....• 0.6 nm, see Fig. 3.1-5.
Thermal load will according to 3.1-11 change the cavity length and index of
refraction, which will result in two effects: i) for small temperature changes Ii T
only wavelength shift will occur at a rate
~ 0.0065 nm/K - 0.06 nm/K
and ii) for A T~ 2 K wavelength shift and mode-jumping result in an overall rate
0.25 nm/K
The latter is also obtained from the approximation of 3.1-12 (8)
Lll\ 2 Yleff l
0.3 VI"" I K
with neff = 4.4, 1\ = 830 - 870 nm, I = 250,.. ... indicating a considerably different
index of refraction of the active layer than generally adopted. - For communication
applications temperature is controlled to .:t. 0.5 K by a Peltier cooler using a 10K ohm
thermistor as reference. Current requirement for a 4 T = 40 K temperature
difference is about IA. Longer wavelength laser diodes emitting at 1.3 and 1.6 r .... are more temperature sensitive. Although the high resolution spectrum given in
- 55 -
Figure 3.1-5 looks very "clean" it is very deceiving, as pointed out by D.T. Cassidy
(15): such spectra are time averaged over long time scales and the difference
between long and short time averaging is apparent from Figure 3.1-6, taken from
Cassidy (15): at a detector time resolution of At~l ns the spectra are smeared out,
indicating rapid wavelength shifts, which will produce (phase) noise in the laser diode
and reduce the coherence length. I I . I
no feedback
A v J\. I I I
BII BIO B09 wavelength (nm)
a) T1me-averaged output sperua or the )aser Cor a bias current. or 24.5 mA and no intentional feedback.
00 ~ Q)
c: Q)
00 c:
'in o Q) ~ u .5
(
increasing wavelength b) Time-resolved output spectra oelhe laser. The dat.a for this
figure Were recorded simultaneously with the recording of From it is apparent that the energy shifLS among the modes
This information is not obvious from the dat.a of
Fig. 3.1 - 6 Energy shifts among laser modes
In gain guided lasers more than one mode contributes usually to the output
which results in a shorter coherence length. This is desirable for video-disc players
([ 9).
The line width of single mode Ga AI As laser diodes is significantly larger than
the Schawlow-Townes formula for laser line width predicts (52). Assuming a
Lorentzian profile the coherence length can be predicted from the linewidth by
= 7T AV
with v = velocity of light in the medium and .1 \? the linewidth. For Ga AI As the
product of optical power Lo and Av is 70-120 MHz mW with 80 MHz mW most
typical and the coherence length can be estimated as rule of thumb as lC<m) = 0.8 Lo
(mW).
For distributed feedback lasers the overall thermal wavelength shift ii) is
reduced from 0.25 nm/K to about 0.07 nm/K (which may be due to lack of mode
hopping) (20). Temperature increases also linewidth and therefore reduces coherence
length.
f'~ 1'.-··;
i I
VI OJ
+> .~
VI <::
.:':l <:: .~
OJ > .~
oj..>
'" ~
1 1
a)
" r 843
1l i 1h U i1h
__ ~'~I~IUJl1lul~I~I ______________ ___
I \ 1 12 i1h 1,3 i1h
1 ' I, "111""t!.uL~Wj~1u ... u II I I I I I 1
1,4i1h t6i1h
I I b)
84S nm 847 790 792 nm 794
Fig. 3.1 - 7 Typical spectra for a) index guided and b) gain guided laser diodes at three different drive currents (from (1)).
'" '"
- 57 -
78or------------------.
E c::
~.
~.
o· •• 0 ...... ..
P=3mW Rf=Q.32 R,=O,32 l=2SO}lTTl ~ .. ;.-: ..
0-i·::·~ ..
/ (a)
770~2~0~--~3~0----~40~---750~
E c::
:c I-
'" Z w
785
u:\ 780
~
TEMPERATURE ("C)
P=3mW Rt=0.76 R,=Q96 l=250)lm
20 30 40 TEMPERATURE
(b)
50 (·C)
a) l..asin.: v.:a\·elen~th·\·s·temP<'r8ture cha~8('Lcristics under the et.Jnslnnt3-mW output power.
Fig. 3.1 - 8 Wavelength hysteresis effect b) due to changing current (from (22)).
b)
_ 775 E c::
Rf=0.32 R,=0.32 I =250)lm
(a)
~ E
01-=> a.. l=> o l-
S :c §
E c::
772 0~--~20--~~40--~-6~cr~·
780
CURRENT (rnA)
Rf=0.76 R,=0.96
(b)
i= 779 ,.''': r:.r 0- r
'" z W ...J
~ 778 ~ L .. -··
777 0~~~2~0~~~4~0~~~60~
CURRENT (rnA)
~ E -
01-=> a.. l=> o l-
S :c
'" ::;
Li~hl·('urrcnl chnracteristics and w8\·f"len~th·('urrent charactl'ri~tiC'S of typicnl VSIS la::oers.
a) due to changing temperature
i i.-
I
[
- 58 -
.Monolithically Peltier cooled diodes are described in (20), and frequency
stabilization for heterodyne-type communication is described in (21).
Typical spectra from index guided and gain guided laser diodes are given in
Fig. 3.1-7.
Hysteresis effects in emitted wavelength as function of temperature or
current cycling is described frequently 00, (22), see Fig. 3.1-8.
3.1.2.3 Polarization
The active layer of a laser diode is built as a plane parallel wave guide and
according to the conventional electromagnetic model of the diode (3) two polariza
tion modes exist: for the !ransversal !:Iectrical polarization, TE, the electrical field
vector oscillates parallel to the plane of the active layer, while in the transversal
magnetic polarization mode, TM, the field vector oscillates perpendicular to the
plane. TE and TM fractions of the polarizaiton are governed by the different
reflexion coefficients RTE and RTM, which are functions of active layer thickness,
index of refraction and index jump along the active layer. For DH-Iasers it is
RTE» RTM (4), which explains the strong TE-polarization in laser diodes (4). Laser
emission from DH-Iasers is generally not linear polarized 0) as shown by measure
ments. Investigations for index guided and gain guided laser diodes indicate, that the
emitted radiation from comercially available laser diodes is at least 7096 linear
polarized and neighbouring modes show identical polarization. Figure 3.1-9 shows the
polarization properties for three different current levels above threshold and Figure
3.1-10 shows the polarization of three adjacent modes. In each case the intensity for
0= 00 (polarization parallel to active layer) is largest 0).
Ul Q) .~ ..., .~
Ul <= Q) ..., <= .~
Q) ,.. .~ ..., '" ~ Q)
'-
90· /
'"
O· e
90·
;,1.95 ;Ih
;,1.60 hh
i,1.11 ilh
Fig. 3.1 - 9 Polarization of laser diode output at different current levels (from (1)).
-- ;;;;m l;~: ';'~-:--- , l ___ _
.h793.34nm
.h 793.17nm
,1.,793.00nm
I I 1
90· O· 90· ( e ')
Fig. 3.1 - 10 Polarization of three adjacent longitudinal modes (from (1)).
,-,-,,-.-, . ~~:
CJ1
'"
- 60 -
3.1.2.1; Angular Characteristic
The small geometrical dimensions of the active layer (thickness 0.2 - 0.8,.", ,
width 1.5 - 20 l"'" , length 200 - 8001'''' ) provide a very compact radiation source
with a typical emitting area of 0.3 - 16 l"',.,;2 .•
Laser diode geometry causes diffraction and typical divergence angles are
30-500 perpendicular to the active layer and 5-200 parallel see Fig. 3.1-10 and
Fig. 3.1-12. A laser diode shows stable radial characteristic if for injection levels 1 =
21th no shift in the maxima of near -and far field characteristic are observed and no
redistribution of intensity distribution and beam divergence are obtained (1;).
Index and gain guided laser diodes show different angular stability. From 18
index guided laser diodes only two showed instable characteristic, while from 12 gain
guided laser diodes four were instable at higher currents. Instable laser diodes show
also the tendency to oscillate at higher transversal modes and/or show (longitudinal)
mode jumping, which can sometimes be observed as "kink" in the power current
characteristic.
3.1.2.5 Optical Power - Injection Current (Diode Characteristic)
Index guided and gain guided diodes show - apart from stability - different
features 0) for the onset of laser action:
ig: 15;:!! ith* 85 m A
gg: 75~ith~175mA
and spontaneous emission is higher for gg-Iaser, see also Figure 3.1-13.
Although present lab technology permits cw- laser operation of up to 50 and
200 m'W typical commerical powers are 1-15 mW. For I mW at I t< ... t the power
density is 108 W /cm 2 which would approach the highest permitted power densities for
pulsed operation 00 ns at 10 Hz repetition rate) of high quality of dielectric mirrors.
Good metal coating mirrors permit 100 W /cm 2 - 1000 W /cm2 and the absorption of Al
around 800 nm may prevent usage of AI-coated mirrors.
- 61 -.
II
~ .~
VI <=
r OJ +' j. t8S ilh <= .~
OJ j. tSO ilh >
.~
+'
'" ~ j. tlO ilh OJ S-
30° 15° 15° )
Fig. 3.1 - 11 Far field laser beam divergenz for index guided laser (from (1).
II 1..
J _______ j,l}O i1h _____
j, 1.30 ilh .~
VI <= OJ +' j, 1.05 ilh <= .~
OJ >
j, 0.90 ith ...., '" ~ OJ s- I
30" I I I i
I I I i 30" 15° 0° 15° 15° 0" 15° 30°
)
Fig. 3.1 - 12 Far field laser beam divergenz for gain guided laser (from (1)).
I I
~':j
! :: f:":
i -
I
I ,--,
I h~ 3D"
3r- t 6
2 LD ~ 4i / LD 1 +' +' :::J :::J
.B-2 0-+' :::J :::J 0 0
'- '-OJ OJ 25 50 Vl 50 100 150 Vl
'" '" ~ ~
r2~ 161 - / emission
L/.w:t LD 5 4j ~/ LD2
L /mW
2 , spontaneoUS
I --/'/
50 100 150 50 100 150 current i/mA ) current ilmA----?>
I 9 6
6 LD 6 l / LD3
3 2
a) I ::! I I b) 50 100 150 100 200 300
Fig. 3.1 - 13 Power/ injection current characteristic for three laser a) index guided b) gain guided
(from (1)).
en N
- 63 -
3.1.2.6 Aging, Drift, Deterioration
Aging is noticed by the shift of threshold current to higher values and shift of
the emitted spectrum to larger wavelengths at constant temperature of the heat sink
(1), while threshold current increases. Aging may be caused by various effects,
including migrating crystal defects ("dislocations") in the resonator volume ([6),
mirror degradation induced by photochemial erosion, harmful moisture or vapour (17)
and aging of the electrical contacts at the heat sink.
The optical power output at constant current and temperature of the heat
sink has been investigated for 18 ig-lasers (1), (18) of which 16 showed very little
deterioration. From 12 gg lasers 8 showed little change over time, see Figure 3.1-14.
Estimated deterioration rates are from this figure in the order of 9:: = 10-20 n W /h.
Barry, who also describes deterioration with an Arrhenius function, gives a value for
the threshold current shift due to aging of 1t = IfA/h, which would result in
.5 .. W /h if the operating efficiency is assumed as 0.5 W / A, see 3.1.2.1. -Lifetime
of laser diodes has dramatically improved over the last years (about factor 2 per
year) and only newest data should be trusted. Rates published 1980 (19) claim
changes of 10% of Ith for 103h for uncoated cleaved laser facets and 1 % for A1203
coated facets.
The data provided are only accurate enough for a rough instructive estimate.
Assuming for LD5 a deterioration rate of 12 nW /h at a power level of 12 mW gives an
estimate for the time of 106 h before the power has dropped to zero. This number is
in line with modern (i 985) laser diodes. For a resolution limit of an intensity sensor
of 10-4 this power change is reached within 102 hours.
Temperature increase reduces the lifetime considerably (Arrhenius-function):
T = + 30 K reduces the lifetime by factor 15 (26).
3.1.2.7 Noise
Intensity and frequency fluctuations are observed during the operation of
laser diodes and the noise maximum at threshold can be used to determine the onset
of laser action. Relative noise power is defined as 20 log (dL/L) where L is the laser
intensity, dL the rms fluctuation of intensity and a 10-5 intensity fluctuation
corresponds therefore to a -100 dB noise. Results are subsequently normalized to a
bandwidth of 1 Hz for comparison reasons. Figure 3.1-15a,b shows the noise increase
i I
I
i L.
i i •
I i i
!
L • 3: 4 E 0-::> 0- 3 0-::> 0 a:
2 w 3: 0 0-
00
L
i
- 64 -
15-
mW LO 5
~.-·--.-.--11-&.- .. -.-.-.-.-.-. 10-
LD 19 --0--0-0_ 0 __ 0_0 -0-0-
0_ 0 -0-
L07 -0-_0_0--0-5- 0_0_._. -.-LO 3 --. '--'-'-'--' -'-
i03
. --. --"
i04 h )
i.ll0 mA
i= 120 mA
i·115mA
i=250mA
Fig. 3.1 - 14 Power loss of laser diodes over time (from (1)).
'" " -0. UJ -100
'" (5 Z
0 -100
" 0 " UJ > ;::
w 0 -110 "' 0 0
z
< -120 ... UJ a:
0 -120
0
0
00 -130 ,. 2. 3. 40 o. 10 100 1000 10000
CURRENT mA FREOUENCY Hz Power output and absolute value of the intensity noise (arb.
a) units) as a function of laser driving current for I.he Dii laser. b) Frequency dependence of the intensity noise (1 Hz B/W) of the three lasers tested: ., T JS: 0, CSI·; 0, D II.
Fig. 3.1 - 15 Noise of laser diodes a) around threshold b) as function of frequency (from (23)).
- 65 -
around threshold and the frequency dependence. For frequencies S 1Hz no published
data are available; this is however the region of greatest interest for the pressure
sensor.
Noise from back and front facet are only partly correlated (23). It is believed
that noise is generated within the neighbourhood of the laser cavity due to local
current and temperature fluctuations which create scattering centers and destroy
momentarily the fixed phase relationship of a wave train. Lattice mismatch at the
heterojunction interface might be part of the cause, a smaller change in Al
concentration might then reduce the problem and quaternay structures (used for 1.3
and 1.61"'" band) may completly avoid the problem (23).
Current modulation (2 - 4mA at 50 - 200 MHz) has been used (24) to reduce
noise levels by as much as 20 dB. The higher frequencies give the better results. The
current modulation results in a frequency modulation of the laser spectrum, which
leads to an interaction between the modes (including modes from different cavities).
Similarly it is conceivable to modulate the external radiation field in case of back
reflection. Times for longitudinal mode switching is reported as 10 - 20 ns (25).
Diode manufacturers suggest similar current modulation frequencies for noise
reduction. Note that the suggested modulation frequencies fall within typical line
width of semiconductor lasers. There will exist an upper optimum modulation
frequency above which the noise can not be reduced. The coherence length will be
effected and influence on drift and low frequency noise should be investigated for the
sensor.
Noise may be caused by the interaction of several modes. Typical intensity -
ratios in "single mode" lasers between main mode and satellite modes are few
percent. Mode competition noise is largely suppressed by a laser described in (22),
where the satellite intensitities are suppressed to 1:1250.
i I , .
[
I !
, , L.
!: i:'-:: t:::".:
- 66 -
3.2 Laser Diodes Under Optical Feedback
The usage of laser diode properties modulated by an external reflector as
sensor has been suggsted as early as 1976 and this scheme has been since investigated
(15), (26) to (1j.9). A recently renewed interest comes from the communication
industry where the need for a reduced linewidth becomes more and more important,
and where back reflections from fibre ends disturb proper laser performance (1j.8).
Early theoretical models considered only the intensity of the light reflected
back into the cavity (29) by introducing an "effective reflectivity" for one laser facet
and investigating the effect of changed photon density (or intensity) on laser gain.
Cassidy (15) distinguished coherent and incoherent back reflection and finds small
amount of incoherent back reflected light im portant for the laser output. Acket et
al (1j.7) and Hammer (1j.9) have presented very recently models and approaches, which
regard the characteristics of light (intensity, phase) and include complex laser
structures (1j.9). Not included so far are geometrical aspects of the beam (beam
diffraction), and realistic photon lifetimes in both cavities. The theoretical analysis
of laser diodes operating under feedback has only. in the last few months approached a
state, where the models will provide useful information on any experimental situation
including laser diodes and backreflection.
The phenomena induced by optical feedback are numerous, and many of them
seem to contradict each other: wavelengths shift (27), induced mode jumping (50),
noise increase (37)(?0), noise reduction (37), linewidth reduction (to 30 KHz)
(32),(33),(3Ij.),(1j.5) and broadening (3Ij.)(36), oscillations or self pulsation of output
power (36), wavelength stabilization (3Ij.),(29),(30),(31),(1j.5), low frequency wavelength
instabilities (32),(31j.) (phase noise), hysteresis and bistability (27).
Some of the phenomena listed do not permit a simple interpretation (27). A
possible reason may be the use of lasers of immature technology which showed
transversal instability, oscillated in multilongitudinal modes and did in general not
have the highly stable performance of present technology lasers. In addition some
general laser features "invite" complex behavior of laser diodes under feedback:
- 67 -
i) a broad gain spectrum, typically about 5 nm, permits the develop
ment of several longitudinal modes; inter modal energy swapping
has been identified as major phase noise source.
ii) sensitive dependence of refractive index on temperature (51);
iii) strong dependence of active medium refractive index on the
excited carrier density.
Both iil and iii) explain high temperature sensitivity of the laser and
wavelength shift.
Theoretical investigations of the influence of optical feedback on laser
performance concentrate on a) power variation, b) wavelength shifts c) noise
generated in steady state and dynamic conditions. The latter indicates also the
stability of a steady state. The cause for the effects can be traced back to variations
in the refractive index in the active layer due to thermal effects (for low frequency
changes) or carrier densities (at high frequencies). Several authors introduce a
feedback parameter "e" which allows some classification of the observed effects and
some interpretation:
(
Where "f" is a fudge factor to describe inaccurate focusing of the backreflected light
on the laser facet «117) f=0.16), "b" is the ratio of changes in real and imaginary part
of the index of refraction of the active layer (-0.5 ... -11.6), caused by the current
changes J 'Le the photori round trip time for the external cavity, tot photon lifetime
(inaccurate, see (4-5)) or round trip time in the laser cavity, "R" laser facet
reflectivity (0.32) and "r" the power reflected back (range probably: <10-8 <r< 0.251.
The last bracket describes the intensity of the interacting light, (or more
accurate: the electric field amplitude of the light coupled back to the laser), the
first likely an additional phase shift and 'reo / 'Tel is a measure for the numbers of
modes involved, and can vary according to experimental conditions over more than 11
orders of magnitude.
[
r r-,.
i
! .
I
I
l.
I
l
I" "
- 68 -
The laser active medium acts as a light amplifier increasing the light best
suited for the gain profile. These light properties can be dominated by the laser
cavity or by the external cavity. Especially for 'reo > "t,more modes are provided by
the external cavity, which permit intermode coupling.
Three cases are distinguished:
C < I, encountered for small external cavities, even at high power levels of
backreflection (r< 0.25) (Operation of laser diode sensor).
With a phase variation between the interfering light fields from laser and
external cavity a sinusoidal wavelength increase is obtained which goes hand in hand
with a decrease in power and vice versa. Wavelength shifts are "small" (observed: 2-
8 nm for a phase difference between ?-tIl. o. .. J A/~ (lJ.lJ.).
C - I larger wavelengths shifts with jump; instabilities around "jump" phase angle,
noise spikes ( lJ.3 ); power fluctuations are to be expected.
~ best investigated, because ~ » 'lit permits usage of additonal components,
situations encountered in most sensors and transmission networks; with monotone
increases and decreases of the phase over more than 2-n it is observed: bi-stability
(lJ.7)(27) hysteresis (lJ.Z)(27), power- and wavelengths jumps (lJ.7), self-modulation (lJ.2)
and undulation of the L-I characteristic, (lJ.2), (27), correlation between noise and
undulation (27). Self-mode locking is predicted (lJ.lJ.). In this regime falls also an
extensive investigation on noise properties and noise reduction (by 30dB) (lJ.1). The
investigation indicates favourable conditions for shorter cavities.
Summarizing: an external cavity laser diode sensor should operate in the C < I
parameter field, favouring short external cavities and reduced laser facet reflecti
vities.
Noise reducing techniques operating around 50-70 MHz can be applied for low
frequency applications such as the laser diode sensor and can improve the signal to
noise ratio by up to 3 orders of magnitude.
- 69 -
o Co ~O'NEA SiJPPL Y
F'.'I,r lIoII'liQOR
PHOTO-CELL
I . . XV-RECORDER 1----f--·.---.. .J
MONOCHROMATOR
. COUPi..ING CIRCUIT
Block diag:ra-m of the experiment.
-9
C~~I7~O~-~IM~-~1~90~-~2~OO~ CURR[N~ (mA)
b)
a) OUIi'U1 \"l"r~u!, ,UfJcnt l"Uf'\l'!/. wi:h :Jnd witlhlUl l':\!crnal fl'l't.l·
b:u·\;. IIr!iih:rl'l<>i~ i!ii ~t'l'lI Ul Ih:.l1 wilh fCl'dh;u:k.
(from (27))
• ,
; . .D~ , .
• a
"0 no "" "'" CURRENT (mA)
Spectral changes and output undula.tion with current increase. LR is greater by SO jJ.m than the nearest intcgr:ll multiple of LD-
W,'" FEE[)8.&CK.
(from {27))
''''''' c) Sp.:&:tI.J1 t.:h.:.m!X'Ji and \Iuqlul undubtiun "ith current incfcOlsc
when I'N 1.'t1u;Jb,:.an inll.'~r:.ll multirlr.: uf /'0'
(from (27))
Fig. 3.2 -b) and c)
1 Change of laser characteristics under feedback spectral changes
a) hysteresi 5
I
I
I !
i i
Ic_ L--",.
k
- 70 -
'0' B\:'~ CU~g~"'1 It (m':';
a) 111(' tr:lpsi.:m outpUt rc:;ronsc to :l current pulse :Jt \'arious points on th~ um.lu!:.Jh:d /.-/ cuh'C'. Pub,~' hcil:hl W::IS 32 rnA. whit:h ",":IS sur~'riml'll~l'd lln th ..... ok bi:.s l'UTh.'nt. •
) r-.h'a~un'lll)htll"('tlrrl·lIt V!'i !:as,'r hi' Ii!',·, .Iistal,,"" : lilT a 111111 jl~JlL . Thl' Ilhllh ... ·urn·1I1 is I'Tnl"lrl illll:lllllllll' laS4'r n':" fan'I,".1 Pllt
,11I\\'I'r •
a. FREE RUNNING 1..s.r. 900 MHz
0.4 ! 0 .0.4 0.6 0.2 0.2 0.6
l'-"~' l.s.r. 66 GHz.
t I
201001020
c. 0.06", FEEDBACK f.s.r. 66 GHz.
2010 0 10 20
j 1'1 I I 'l ~.~ . • ' .~ J I-,".-.. ... y ....
d. 0.20:, FEEDBACK
1.s.r. n GHz.
200204060
.' e. 0.3'7, FEEDBACK
/ , 2010Ql020
GH.
1.s.T. n GHz.
b) Fa.bry Perot spectra of the CSP laser diode for varying amounts of optical feedback. The data were obtained using a variable frcc·spectral range.
Fig. 3.2 - 2 response b)
Change of laser line broadening
characteristics under feedback a) transient c) required output form for laser diode sensor
- 71 -
3.3 References
1. A. Abou-Zeid, "Spektrale Untersuchungen an Kommerziellen Ga Al As-Laserdioden". PhysikaJisch-Technische Bundesanstalt, Bericht Me-56, ISSN 0341-6720, Braunschweig - West Germany, 1984, in German.
2. F.R. Nash, J. Appl. Phys., 114(973), 11696.
3. R. Lang, Jap. J. Appl. Phys. 16 (J 977), 205.
4.
5.
H. Kressel, J. K. Butler, "Semiconductor Lasers and Heterojunction LED's," Academic Press (J 977).
Bell Northern Research intern training course on "Fibre Optics"; printed course material, 1981.
6. W. Schmid, B. Flade, K. Hoeing, R. Eggert, "A Precise, Programmable 850 mm Optical Signal Source", Hewlett Packard Journal, 36, J(J 985) 7.
7. Private communication, C. Look, Northern Telecom, Corkstown -Ottawa/Ontario. P.O. Box 351 Station C.
8. NEC Laser Diode - Application Manual, AN8280 I, California Eastern Laboratories, Santa Clara, CA 95050.
- 9. H. Weber, G. Herziger, "Laser - Grundlagen und Anwendungen".
10.
11.
12.
13.
Physik Verlag GmbH, Weinheim/Bergstrasse, West Germany, 1972, in German.
R. Daendliker, "Laser Kurzlehrgang", AT-Verlag, Aarau-Stuttgart, West Germany, 1981, in German.
J.D. Barry, I~EE QE 20, 5 (9811) 478.
Zachos, Ripper, IEEE QE 5, 29, (969).
M.J. Adams, M. -Cross, Solid State Electronics, III (971) 865.
Ill. T.L. Paoli, IEEE QE 16, 7 (975) 489.
! i( --;
- 72 -
15. D.T. Cassidy, Appl. Opt. 23, 13 (984) 2070.
16. H. Kressel, H.F. Lockwood, J. Phys. Suppl. 35, C3 (974) 223.
17. F .R. Nash, R.L. Hartmann, IEEE QE 16, 10 (1980) 1022.
18. A. Abou-Zeid, "Physikalisch Technische Bundesanstalt Jahresbericht 1981", p. 128, Braunschweig West-Germany 1981, in German.
19. L.J. van Ruyven, "A Semiconductor Laser for Optical Disc Systems", Solid State Laser Development E1coma, Philips Research Laboratories, AA5656 Eindhoven, The Netherlands, No. CQL-Il/80, 1980.
20. S. Hava, R.G. Hunsperger, H.B. Sequeira, J. Lightwave Techn. LT-2,2 (1984) 175.
21. T. Okoshi, K. Kikuchi, Electr. Lett. 16, (980) 179-181.
22. S. Matsui, H. Takiguchi, M. Taneya, O. Yamamoto, H. Hayashi, S. Yamamoto, S. Yano, T. Hijikata, Appl. Opt. 23, 22 (1984) 4001.
23. A. Dandridge, H.F. Taylor, IEEE QE 18, 10 (1982) 1738.
24. K.E. Stubkjaer, M.,B. Small, IEEE QE 20, 5 (1984) 472.
25. M. Ito, T. Kimura, IEEE QE 15 (1979) 542-544.
26. "CQL I 0 Semiconductor Laser for Information Readout". Electronic Components and Applications 3, I (1980), Amperex Electronic Corp., Slatersville Div., Slatersville R.I. 02876.
27. R. Long, K. Kobayashi; IEEE QE 16,3 (1980), 347.
28. A. Dandridge, R.O. Miles, T .G. Giallorenzi Electr. Lett. 16,,25 (1980) 948.
29. C. Voumard, R. Salathe, H. Weber, Appl. Phys. 12, (1977) 369.
30. C. Voumard, R. Salathe, H. Weber, Opt. Comm. 13,2 (1975) 130.
- 73 -
31. C. Voumard, R. Salathe, H. Weber, Appl. Phys. 7, (1975) 123.
32. L. Goldberg, H.F. Taylor, J.F. Weller, Electr. Lett. 18, 9 (982) 353.
33. L. Goldberg, A. Dandridge, R.O. Miles, T.G. Giallorenzi, J.F. Weller, Electr. Lett. 17, 19 (1981) 677.
34. L. Goldberg, H.F. Taylor, A. Dandridge, J.F. Weller, R.O. Miles, IEEE QE 18,4 (1982) 555.
35. L. Goldberg, H.F. Taylor, J.F. Weller, IEEE QE 20, 11 (1984) 1226.
36. R.O. Miles, A. Dandridge, A.B. Tveten, H.F. Taylor, T.G. Giallorenzi, Appl. Phys. Lett. 37, 11 (1980) 990.
37. R.O. Miles ,A. Dandridge, A.B. Tveten, T.G. Giallorenzi, H.F. Taylor, Appl. Phys. Lett. 38, 11 (1981) 848.
38. S. Saito, Y. Yamamoto, Electr. Lett. 17,9 (1981) 327.
39. A. Olsson, C.L. Tang, IEEE QE 17, 8 (1981) 1320.
40. K.E. Stubkjaer, M.B. Small, IEEE QE 20,5 (1984) 472.
41. T. Fujito, S. Ishizuka, K. Fujito, H. Serizawa, H. Sato, IEEE QE 20, 5 (1984) 492.
42. N. Ogasawara, R. Ito, T. Sasaski, T. Osada, Jap. J.Appl. Phys. 21, 10 (1982) 1465-1471.
43. S.J. Chua, T.C. Chong, IEEE QE 20, 11 (1984) 1243.
44. T. Kanada, K. Nawata, IEEE QE 15,7 (1979) 559.
45. F. Favre, D. Ie Guen, J.C. Simon, IEEE QE 18-10 (1982) 1712.
46. G. Wenke, Y. Zhu, Appl. Optics 22, 23 (1983) 3837.
47. G.A. Acket, D. Lenstra, A.J. den Boef, B.H. Verbeek, IEEE QE 20, 10 (1984) 1163.
[ i I
- 74 -
48. W. Bludau, R. Rossberg, Appl. Opt. 21, 11 (1982) 1933.
49. J.M. Hammer, IEEE QE 20,11 (1984) 1252.
50. J.W.M. Biesterbog, A.J. den Boef, W. Linders, G.A. Acket, IEEE QE 19 (1983) 986-990.
51. Landolt Boernstein, Numerical Data and Functional Relationships in Science and Technology, New Series Group III Vol. 18, Elastic, Piezoelectric, Pyroelectric, Piezooptic, Electrooptic Constants and Nonlinear Dielectric Susceptibilities of Crystals, Springer Verlag Berlin 1984.
52. R.C. Youngquist, Appl. Opt. 24, 10 (1985) 1400.
53. A. Mooradian, "Laser Linewidth" in "Physics Today" 5 (1985) 43.
54. C.O. Weiss, private communication, Lab. 1.22, Physikalisch Technische Bundesanstalt, 0-33 Braunschweig, West Germany.
l .
I I ..
i··-. , .
- 75 -
4. EXPERIMENTAL BENCH MODEL SENSOR
Objective:
i) The Bench Model Sensor (BMS) has to demonstrate the technical
in
feasibility of an optical feedback laser diode sensor applied to static rather
than dynamic situations, as having been demonstrated to date. This includes
the demonstration, that indeed the elements required for the construction of
a unique and monotone sensor calibration curve, as described in details in
Section 2.1, "General Properties of Interferometers" can be obtained in
measurements.
The BMS has to provide sufficient evidence, that the technical
specifications given in "Introduction" are feasible for short and long term
applications.
4.1 Design Considerations for the Bench Model Sensor
Objective:
This section summarizes the operational concept of the BMS system as well
as derives key parameters for the development of subsystems.
4.1.1 Operational Concept of the Bench Model Sensor
The BMS is d«:signed to allow both measurement of displacements less than a
fringe providing with one measuring channel a unique calibration curve, as well as
tests of displacements involving several dozen fringes. The basic operational concept
is given in the block diagram Fig. 4.2-1. The solid blocks have been used during the
tests described in Section 4.5 and 4.4 while dashed boxes represent subsystems which
did not become available during the project. Housing and membrane design were to
be tested at he next stage of the development, as indicated in the "Introduction".
The effect of pressure moving the external mirror is simulated by the combination of
a mechanical translation stage, see 5.1.3. The laser diode current is supplied by a
high quality constant current source which is specifically developed at SEAST AR
INSTRUMENTS LTD. for sensor applications. A temperature control unit did not
become available. A precision thermometric circuit developed for this application
provides temperature readings from the laser diode heat sink with a resolution of
about I mK.
r-------, I housing & 1
~~:.m~;~.:_J
pressure simulator. calibration
r - ---------, I noi se reduci ng : I RF-modul ator 1
I 1-100 11Hz 1 '-----r --u...J
i i
constant current source
power supply , (battery)
photodiode rprp;ver I-__ -I~I optical feed-, I
• back laser ._ diode module ~------'
temperature measuring (cont ro 1 )
A I I
1 r--------, : wave length ; 1 control 1 1 _______ ...J
DC offset
integrator. low pass fil ter 1--.---11 osci 11 oscope
• T ~ 50 ms
r------~ r------, ;low noise: I AID 1
L -I front end (- - -~ converter : - - -L~m~if~e~J L_l~ bi':'_J
~ chart recorder T ~ 50 ms
r -- -----"j 1 data 1
- .. processi ng. : : display I L _______ J
Figure 4.2-1 Block diagram of the optical feedback laser diode bench model sensor .
. )::~ IT;;: ,.
...., '"
- 77 -
The photodiode receiver provides the demodulation for the analog signal (see
2.1.3) which is - if necessary processed - displayed for time scales T =- 50 ms on an
oscilloscope with a maximum sensitivity of ImV/div or for T2: 50 ms on a chart
recorder with full scale sensitivity of I mV.
The analog DC measurements of low signals (mY) at a high bias level of
several volts proved cumbersome due to (thermal) drift. An alternate route
consisting of a signal conditioning front end analog amplifier, a 16 bit AID converter
and a data processing and display group was in a planning stage.
4.1.2 Key Parameters for Subsystems
The following considerations provided the data necessary for the development
of the low noise conltant current source and the thermometer: A theoretical model to
predict signal levels did not exist; the published output forms (J), (2), resembled the
output of a two beam interferometer and for simplicity a linear relationship between
displacement and change in output power was assumed for the calibration curve
resolving parts of a fringe. Typical output power changes of few mW have to be
resolved to better than 10-7W. A pin-photodiode (RCA silicon high speed, C30920E)
has a responsitivity (at" = 830 nm) of 0.6 A/w, providing a resolution of 6 X 10-8 A
(and smaller). Under experimental conditions a dark current of 1.5 X 10-8 provides
an offset while noise density calculations indicate a noise contribution of about
(6.2 X 10 -3 nA) and will not be significant.
Comparing the optical power changes to be resolved by the sensor with power
changes induced in the laser diode light source by a change in forward current I or
active layer temperature T, we obtain with literature data and the formula presented
in Section 3 the permissible limits AI = 10-7 A, AT = I mK. These numbers exceed
specifications for communications application, where 4T ~ 0.5 K is acceptable and
compare well with performances achieved during the development of precision
sensors (3) and - laser performance evaluation (4), (5).
The modulation for noise reduction should operate at frequencies in the range
10 MHz.c.t\ '4-00 MHz and permit a modulation of the undisturbed laser power Lo of
LI 1.0 ILo = 10 - 50%. This corresponds for an operating efficiency of 0.5 wI A to 5
-10 rnA current modulation.
- 78 -
4.2 Supporting Systems
Objective:
These systems control the environment of the sensor, provide: alignment aids
for the invisible IR radiation, pressure simulation and calibration, stabilized low noise
constant current for the laser diode and measure the temperature close to the laser
diode.
4.2.1 Laboratory Characteristics
All tests are carried out in the optics laboratory of SEAST AR INSTRUMENTS
LTD., which is for this purpose specifically equipped as follows: A Micro-G Vibration
isolation table (Technical Manufacturing Corporation, USA; top dimensions: 8 X 4
feet; 8 inches thick) rests isolated from the building in ground floor level on its own 5
ton conrete fundament. The low 2 Hz resonant frequency of the table damping
system reduces walking noise in tests with a calibration hydrophone by more than a
factor 30 and reduces the damping time to by more than a factor 4.
Shelves are suspended from the ceiling above table center, which keeps 50 Hz
humm from electrical instrumentation off the table top for the sensitive
measurements.
The lab is built as a one layered electrically shielded room with metal door,
screened air inlet/ outlet and a single electrical ground point.
Electrical power is provided through a noise rejecting isolating transformer.
The laboratory door is provided with a combination lock for safety
considerations.
4.2.2 Visual Aids
The IR radiation for the laser diodes used for the tests falls into the near
infrared region, 780 - 850 nm and is only at relatively high power levels visible for
the human eye. Laser operating personnel is instructed to wear protective eyeglasses
for safety reasons.
Phosphor screens, converting IR to visible light if previously exposed to UV
light, are used for beam alignment. A B&W TV camera/monitor system is installed
(Sony Electronic Viewfinder AVG 3200), which is sensitized to IR radiation and used
in connection with a stereo microscope (Wild, M7A) with camera adaptor to monitor
I
I:
I L,
j. I
- 79 -
alignment of laser, photodiode and mirror. The total magnification of microscope,
TV camera and monitor is 255 X and dimensions of about 51'm are resolved.
~.2.3 Pressure Simulation
The displacement of the pressure sensor membrane and the mirror attached
to it are simulated in the BMS by the movement of a translational stage (DaedaI)
which is magnetically coupled to a combined piezoelectric/mechanical micrometer.
This permits a jerk free movement of the translation stage over 2.5 cm with a
resolution of better than 25tt m by the mechanical micrometer. Fine adjustments in
the position are carried out with the piezohead (Burleigh, PZ ~O) providing a
maximum scan over 18 ~m at 1000 V input (Burleigh PZ 70 Power supply, HV Op
Amp). The translation stage supports a mounting platform with a second
piezoelectric pusher (PZ ~O, Burleigh) which contains the mirror element. The
mounting platform can be adjusted in 3 angular dimensions to align the mirror surface
plane parallel to the laser facet.
Hysteresis and drift of the PZT elements require careful considerations
during the measurements. The piezoelectric pushers are characterized in two sets of
experiments. Hysteresis is investigated incorporating the PZT elements in a
Michelson interferometer using a He-Ne laser (i\ = 632 nm) as light source, counting
the fringes as function of the applied voltage. Results given in Figure ~.2-2, show the
hysteresis and indicate an average displacement of l}'l'" per ~0.2 V applied to the
PZT element.
A second experiment is carried out with the BMS, increasing only the
distance and measuring the required voltage increase to the next maximum and
minimum. Averaged over ~~ fringes (corresponding to ~~ X 11/2 = 18.~ /'1m) a
displacement of ll"m requires 5~.5 ! 30% volts. This compares reasonable with
dynamic measurement giving 53 V / m.
The results presented show that stacked PZT elements can not be
incorporated into the design of a high precision pressure sensor without problems. It
is however, possible to correct electronically for non-linearities (IO), (II).
Noise of the commercial PZ 70 power supply of more than 80 mV-pp limit the
positioning accuracy of 1.5 nm at best.
FRINGt:S pm
30-1
9 28
26t8
24
22 ~7 20
6
18
16 J..5 14
I
4 12
10 t 3
8
61-2 4
2
o 50
-~):::~
I
I FRINGE f;{ 1 = 632·81 , nm 2 2
100 150 200 250 APPLIED VOLTAGE
F:- ,"";.,,~-;-- '~:'." ';,: r-'
Fig. 4.2-2 Hysteresis and displacement of stacked PZT piezoelectric pusher (Burleigh Pl 40).
300 350 400 450
::::;::::; ~~---- ----::~j
ex> o
- 81 -
4.2.4 Current Source
A constant current source has been developed to provide low noise, stabilized
currents up to 200 rnA for laser diodes. Power is provided by a 12 V gel cell and the
input power is thoroughly filtered to prevent surges to reach the sensitive and fast
reacting laser structure. The current is dialed up by a precision potentiometer which
is calibrated in a calibration curve. Presently o.ccasional noise bursts can reach 10
tJA. The circuit diagram is given in Figure 4.2-3.
A new constant current source is designed providing HF-modulation
capability at MHz frequencies for noise reduction, low frequency modulation for
temperature stabilization or optical power stabilization, a 100 nA noise floor and 412-
digit readout.
These data compare favourable with those achieved elsewhere (4) where a
digital nanovoltmeter (Schlumberger Solartron 7075) and a (Hewlett Packard) A/D
converter are incorporated in the current control scheme which provides a laser
current resolution of 100 nA, fluctuations over several hours ~ 150 nA and long term
fluctuations of -= 500 nA.
4.2.5 Thermometry
Threshold and emitted optical power of laser diodes are sensi ti ve functions of
the active layer temeerature, which has to be regarded for the application in sensors
of high sensitivity or applications involving heterodyne or homodyne techniques. The
temperature can only be deducted, either from j) changes of the laser performance
due to external stimuli such as pulsed operation or ii) from measuring temperature at
a spot close to the active layer. The latter technique effects laser performance only
little and is preferred.
In a steady state the temperature decreases from the "hot" active layer
through different layers of the laser structure to the "cold" heat sink, which by itself
may consist of a combination of a diamond or Si crystal heat sink mounted on a
passive Cu-heat sink and/or a thermoelectric (TE) cooler. Any point between the
"hot" active layer and the "cold" heat sink can be used to characterize the laser
+12
IK2
1/2 + +
560 122 2'5V"1 10/25 10K
Fig. 4.2-3 Circuit diagram of laser diode constant current source.
" ::::;i~~ , .. ·i:."· . ~.' -;-,,-----.
BNC
VN" IRF 4K7
IK 20n
~F--~
ex> '"
- 83 -
active layer temperature and therefore used as temperature reference point in a
feedback loop to stabilize the laser temperature. In this case the reference point
temperature is compared electronically to a chosen "set" temperature and the TE
heat sink regulated to obtain zero difference between set point and reference
temperature. For best performance of the feedback circuit the reference point and
TE cooler have to be kept as close as possible to the active layer. Stabilization of
the reference point temperature to D. T ....,1 ml( have been reported from various
laboratories (3) to (6). (6) reports difficulties to obtain good temperature stabilization
due to "bulky" components prohibiting the close contact between active layer and
reference point thermometer. Size, power dissipation, heat capacity and resolution
have to be considered in the choice of the thermometer. Electronic circuits AD 590
(3), (4), (6), thermistor (5) and thermocouple are possible choices:
Table 4.2-1 Comparison of Thermometer Properties
AD590 Thermistor Thermocouple
Size large medium small
Heat capacity large medium smaIl
Power large, I mW large/medium small
dissipation
Resolution good good good
Output I fA A/K, absolute current/voltage voltage
Form linear logarithmic linear
- 84 -
The thermocouple approach indicates major advantages over thermistor and
AD590 device; the implementation of the technique proves however difficult as will
be seen in the next paragraph.
1J.2.5.1 Thermocouple Approach
Due to the thermoelectric (Seeback) effect the junction between two metals
(or semiconductors) provides an EMF ("voltage" output) characteristic for the metals
chosen at a given temperature. If two junction pairs A and B are connected in series
and junction B is kept at a known reference temperature TB the voltage measured
across the open end is proportional to the temperature difference T A - TB = AT. If
the junctions are made from thin wires of d = 25/" m diameter an extremely small
thermometer sensor results which can even resolve temperature differences on teh
laser diode surface (300 X lfOO ,.. m). The following operational scheme has been
suggested; Figure If.2-lf: The temperature T A (reference point on the laser diode) is
transferred to a larger body, reference block B, in the following way: The junctions
A and B provide at temperatures T A :f: TB a non-zero signal which is detected and
amplified in the "zero" detection amplifier and provides the control signal for the TE
driver 1, which changes with the TE cooler 1 the temperature of the reference block
to T A = TB: the temperature is "transferred" from the inaccessible laser diode
surface to a large block, where it is mesured by the AD590 electronic thermometer.
This output can be compared to a "set" laser diode temperature and a derived signal
controls the TE driver 2 and TE cooler 2 to keep the laser diode at the "set"
temperature.
The thermocouple system is designed to meet the following specifications:
The thermocouple junctions were to be soldered from wires approximately ~ = 251"m
at a length of t = 100 f'm and provide an output of 30 t' V /K {chromel-alume1l with
good linearity over -2 to + 300C and a resolution of 1 mK. The time response of the
system is to be 10-1 to 10-2 s.
The thermocouple amplifier uses a chopper stabilized "zero" detection
scheme and has been developed in cooperation with Sea-Met, Ottawa (7). All
components of the thermocouple thermometer scheme including the AD590 have been
built and tested as components, see circuit diagrams Figure If.2-5 to Figure If.2-7.
I t .
1 i I>
I l .. :
I
l
I·
"SET"
TEMPERATURE
- 85 -
A B LASER - REFERENCE DIODE BLOCK
• I I
I I
I TE I TE COOLER 2 COOLER I
• I I I I
TE "ZERO" DETECTION TE DRIVER 2 AMPLIFIER DRIVER I
-.-
Fig. 4.2-4 Block diagram for temperature measurement on laser diodes with thenno couples "A", "B" (--) and temperature control (---).
. AD 590
; I
I I
I I I I
Jf"6V
r'., . &~~ J 7 ' "" -- ,~" ·10. ,-. • " A,
C • '<., ,~ " .., no ; -. ~:/o 'I~lc ~~;;,' --~:"rii' 8hJ"T,/" - (' ~ , c .• _ "c.':~I'. 1 l;filli "
_':\-0----- LJ,e.,-, ~Cl b/' ]1:_0_'. '~,1.,8 'r--J''-2 C.2~'" 2/
S
( , ->~-~,,-. "", '0 .1!'J(.~L~, ,--, !, "'iii' , . I ' " J '. , , S,~", ~y"'L --" ". <; '''. -AA' __ """ n. " __ ~, ", ..
8 _ .. ,n 'J --', 27./10 ~~~,~'~:o_ ~ SiR\'52~']l' "" ~7,K r", '17K L~1~ i c.t J
<. ~70 -., ~70 i I' --'" 1 ::rl
1.."'_>-----, :r~c..k". mC.II""'~I'S.
._~ ... , ~ ~
~ .6_ tC /. AIOuO::' po., '-;l;.,.,
S c I "1 10 ,<" ... R •
•
"
.~
,; loLL ~, 1 I eVo, IC'" ]<B
/,. ,,",I 1 I"" ,~ J" + '" , +' "cr-- IANT. 11
'°'"
~, -6 E ,Lt,i!>a ~ 13 ,~~ 4
I - » P.J3 , ~L'
i Bl - ~ 1--.1
4 I c.9 :.... <r C' C/5"J, .711~TI.o.I(l l- . (. , " 11 I""'T 1/16 • 17
5,'; . ,': I ~."C T -ll.0~ o 6 ... 0<" ')O,~_'-_L. -
A'. • 0 .... '~O('J' J; ..... vll:..
V3~ 1(..6 T~ ,-:..8
1(1 TO Ie.", ;"-"'-}-1'-1-1
I II II -b, I I L_L .. 1
f:.1;·,' ...... ~ /.:..l To I{.~ wlTtl .1/-:'0 ~ ,111,)",,1< -:. '~1,.. ...
C. ",-'-'Nil '''0,,·1'
! <.'2..71'\
l~ L·~-~h
t --- ------' "11 221< ~~ i VoCl
--21'f~ .0 £,e..", : H/~~~_~~~~
v"
(.7
SO~ Kit 6awRI°w.
= 1.o" ~701~
1 I '5 EA sri i<.. /"g7f.'""O.lT"; . \ls~
-
G.S....,Q,
.:rJ 130~"f'CI
flN<- c,.r.
C6 T .1/"
1:,<"''''<'00'0< A,"'P. 11'1 ..... 1183-71-
Fig. 4.2 - 5 Circuit diagram for thermocouple amplifier
;;:::" '::"':
co en
I i
I S"llL .... n u oN( INPuT
J,
•
TIl. "
RI OC'<>
L~Gl 1 I I..1
31'+-.J, -1(.\.... -
" .1
., lOt{ :
R. ~1<7
I:~t HU
Q> R. I"
'l'llvLJc.
-'-1 O~
TIP 12.0
R, .. I .J\..
s·
j .::J"'l.. PI .fj,.c"II'
-l1v
·_·-k ~ ;;;:;'~j" ..., R', 1'-<" , ". I ..n.. i
1----'< 5. i :---o. I
j;~"I;h I C3
1" loV~ti'~'" ~/--INO
j
," ---- ( r ) TIP 12.2
'-ol LUI Rt::: o I K SqPflll'\
"''''I
~K7 -+--,----4 ;: In S'w / ':qRlt
,
,* .:r,3. : ... , ......
7,,1.'" ;:",1' .. :,.,; ,;. .... .,), .....
.".~~~ I (J"4 " I C7 c~ . .1! ~ '.:-,: I '_01 100J" p/u T' T""r
." c,o ell 1 -=- B"2.
r~ ~rL'/O( ,."-~--Il .... c,...l---.L----·s
I
5e.A ....... 7ftol lN~,/l.\Jtr-E-..,"T-".J LT •. .... -..- 01/ 1.
rl leli.
.... ·Pe .. r.' ... EL,:....,~ ..... 7 t:.J;'I"t.1t.. '~~-7'1
Fig, 4.2 - 6a Circuit diagram for Peltier element driver
CXl .....
~ :
r
Fig. 4.2 - 6b
1 I, !
>, "II"?iL " I · -,--
.1
I:;", i ,.
!
>~" 110 I 0-1
Layout for Peltier element driver
[;,' ':: ~ .: : ... L .-~
:)fJ.O;;'T".... 1J..I'5:-",;J·'i\E.LJI~
··'·"'0" ';l.~ "," {rI
f'~(11F1( ft.,-,.-tt,vr !j';'I'I~h... ,-;"J - 7 4-P C [j. Y,uT - 8~·7<I·Ptl"l D-'
::::::i~( -.~.-- :::.
ex> ex>
+v
(jfJ"3 Bu nOM 02 VIEW 01 T.>52
. ~J ~ 1:1 ~~'k!<r ~
1 .,
J.,f\.
.)'::... R2:" R3 oil IN - . 3KO' 8K3 S;Z -li~A / OFFSET
AOJ.O ~ ~,-f-2-- I
IC2~~ 1 CI it·,· J" OFFSET.
~4
RIO. 3" K
2
MCI45S BUFFER 'I ·./To "P~T 0 - + _(f1 I
RI2 . 2JK <,
G
RI4
RI5 20K
~---+-------08UFFE" ~7
4K7 SPM~ I ':':'SKG
AOJ.O '. VV'v~--+v~'>
Ie K -v
lOUTPU"
S?A.N
IN 1 '0--- _ [C~ I COMtJotl . . -----"
GROUNOO I . __ i '- ,I
SENSORO---~---L---4 ,~
EXCITATION
€. ') oilV
O)--r--'--- . INO~. .114.7/161. .n I' 7AN1: -=-
9~ONVERTER ..L0UTPUT
021 IN 4\A8
R& 2KO
INO
T-'- J~ ,..t4
o ~ r TAtliT .. : -=-__ I-o'-.T
SEASTAR "INSTRUfAENTS "rfN 10" .-·OEC 22/64 -:nA B , ....
-v ~ CURR£N.I. .1:9~T~r,E AMP I
BUFFER (AO:5~0·~MPI·: I a.3:74.02pc
Fig. 4.2 - 7 Circuit diagram for AD 590 -electronic thermometer .
0>
'"
- 90 -
The reference block contains the TE cooler (Peltier element, Me1cor USA)
which is connected over alternating layers of good and bad heat conductors (silver,
alu foils) for even heat distribution to the thermocouple junction B and the AD590
thermometer. The assembling of the thermocouples A and B and the reference block
prohibited usage of the fine wire diameters of 25 r""'. Larger wire diameters did
not become available so that the complete system could not be tested.
Preliminary tests with different thermocouple materials (Cu and silver plated
Cu) and very thick wire diameter (.3 mm) provided a calibration curve showing a
major temperature offset.
Instead measurements were conducted with the AD590 thermometer, which is
described in the following section.
4.2.5.2 AD590 Electronic Thermometer
The AD590 temperature transducer is an integrated circuit. When a supply
voltage between four and thirty volts is connected, it provides a current output which
is proportional to the absolute temperature at a sensitivity of I,. A/K. The
electronics, see Figure 4.2-7, is temperatue compensated for a range (0 ~ T ~ 30 0C.
The output is connected to a digital voltmeter, and the display (in volts) has to be
multiplied by a factor 20 to obtain temperature in centigrades. Resolution is about 1
mK and accuracy (which exceeded available laboratory equipment) is believed to be
of similar order.
The thermometer has been successfully applied to laser diodes and optical
power fluctuations can be related to temperature changes of the order of few mK.
This is discussed further" in Section 4.3.4.
Thermocouple and AD590 thermometer are shown in Figure 4.2-8.
i
Ii
t
::-.
- 92 -
4.3 Measured Intrinsic Laser Diode Properties
Objective:
Tests of intrinsic laser diode properties are carried out to verify and
supplement manufacturers specifications and include the folio wig evaluations:
threshold current Ith; calibrated optical output power vs. forward current (LfI), noise
vs. current, shifts of threshhold current and noise pattern due to degradation, thermal
drift.
The following lasers were used in tests: HLP 7801 G (Hitachi), MM laser for
compact disc reading, ML Iil02, ML 3101 SM laser (Mitsubishi) with low noise. These
lasers are delivered in a hermetically sealed can. The LDS3-H SM (OrteI) Laser is
mounted on an open heat sink, which permits access to both laser facets. In addition
HL lliOO SM open heat sink lasers (Hitachi) were obtained, arrived however -after 10
months -too late for being incorporated into tests. Manufacturer test data sheets for
these lasers are in Appendix C.
1i.3.1 Experimental Set Up
In all tests described the laser diodes are connected to passive heat sinks
(min. size required Ii X Ii cm 2). The HL and M L laser diodes contain a photodiode
monitor at the back facet of the laser and allow for a compact set up. Here a metal
box (Hammond Box) serves as heat sink and package for resistors and necessary BNC
connectors. The LDH laser diodes are mounted on a Cu sheet permitting access to
the front facet to position the external mirror and to the back facet to position a
photodiode for the output signal. The Cu-heat sink with laser is mounted on a Alu L -
profile which extends the heat sink and contains connectors and resistors for laser -
and photodiode. Metal box and Al profile bar are provided with mounting rods, which
-together with a set of fixtures - permit the positioning of the laser on the vibration
free table.
The electrical circuit for ML 1i102 Laser is given in Figure, 1i.3-1. The laser
mount serves here as star - ground point for all instruments connected. Forward
current If is either measured across a 10 l'l. resistor or read from a calibraton curve
of the power supply, which relates dial settings to output current within 0.1 mAo -
-I
~
n n [] ~
[J , i
~:. , , ! ,
~] , ~> ,
::
" , , [] 0
}~-: , f , l' ~.
I
0 , - [
f
D ~-" , " " " n ;~"
~.
[]
I
~.: ~
"1 ' ' ... ~ ~.:
I' t.:·· ,,::
..
I !
uJ ':--.-: .... '
U
U [;1 :' ~:
C
- 93 -
12V SCOPE
1111h-,:r--r.r~ CHART REC.
20 K
PO
II 12 V
LO
SCOPE
r-- .., I I I I I I lOt( If
L __ -1
CONST. CURRENT
SCOPE
Fig. 4.3-1 Circuit for measurements with HL 4102 laser
- 94 -
Grounding of "+" or "_" connection of laser diodes and photodiodes is not
standardized in packaging and has to be considered for each laser diode type anew.
Noise signals are only few m V usually on a signal measuring volts and have to
be measured in DC mode for stability and drift measurements. A simple bias network
is inserted between photodiode and oscilloscope or chart recorder, which provides a
variable bias, see Figure 4.3-2.
The LDS3-H laser does not contain a photodiode and a RCA -C30920E is
removed from its can with a file and drill press and mounted to a x-y-z positioner
(DaedaD permitting a resolution of 5 ,.. m. The positioning of the diode, which looks
under an angle of 250 at the back facet of the laser diode, has to be optimized for
two conflicting objectives: j) the sensitive area of ,-1 mm2 should capture most of
the radiation field to provide a high signal. This requires a short distance to the
laser, ideally~ I mm. With the can removed from the photodiode the laser heat sink
permits a closest distance of 2 mm. ii) back reflection and scattering off the
photodiode affects the laser performance at levels < 10-8. Mounting under an angle
and a large distance are desirable here.
A Hamamatsu S 1337-1010 large area (98 mm2) photodiode (on request
manufactured without window) has been obtained for a working distance of ~ 10 mm,
arrived however too late to be tested.
Alignment of the photodiode is carried out as follows: A photodiode at a
distance of 15 cm from the laser front (under 250 angle) is used to monitor laser
output and indicates if the photodiode on the laser back produces significant output
changes. The back photodiode is postioned in x-y-z direction for optimum output.
A 60 MHz Iwatsu SS 5710 oscilloscope with two separate channels and
sensitivity range from I mV/div to 10 V/div is used for most measurements. Some
noise measurements are done with a Teletronix 468 digital storage scope. Chart
recorders used are one channel chart recorders, Omni Scribe, Houston Instruments
with a maximum sensitivity of I mV full scale.
Temperatures quoted are air temperatures in the lab, which usually differ
from the table top and laser heat sink temperature significantly.
il
, -] L· L,
no' U
n", I
i.
[J
r]
! 1
[I , .
i]' ,
: J
U
HOTODIODE
- 95 -
o SCOPE OR CHART RECORDER
Jt------------------~O
Fig. 4.3-2 Variable bias source for noise and temperature drift measurements.
- 96 -
4.3.2 Voltage Drop, Optical Power and Noise vs. Drive Current
Mitsubishi ML 4102 (No. 84-220549) SM
Voltage/Current Characteristics
The voltage/current characteristic of a laser diode, see Fig. 4.3-3a, shows the
typical features of a diode. Note that around threshhold no changes are visible.
Optical Power and Amplitude Noise
Optical power and noise show dramatic changes ar6ung threshold (here at
T = 17.50C, , = 29.2 rnA), see Figure 4.3-3b. Above threshold the power increases
linearly with drive current at a rate "top = 0.41 mW /mA for the ML 4102 (deducted
from manufacturers information). The absolute noise (which is taken as the
"eyeballed" peak to peak variations observed at a scope sweepspeed of 1-5 ms/div -
data obtained in this crude method agree well with more sophisticated methods) rises
sharply around threshold, reaches a characteristic peak at , = 1.06 'th before leveling
off to a constant absolute noise power. The arrow indicates mode jumping to the
next longitudinal mode due to increased active layer temperature: also single mode
lasers operate only in limited current regions truly in single mode. The jump is
generally not noticeable in the L/' characteristic.
As a power meter was not available calibration of the measured laser output
(in Volts) to absolute power (mW) is obained, by dividing the slope above threshold
A L(V)/ A 'p(mA) by the operating efficiency "l op(m W /mA).
Typical noise levels for the ML 4102 obtained in this manner are,.., 0.7
(shoulder) - 1 ,. W (peak) if mode hopping (3 - 7 f'< W) is disregarded.
The diode is rated at 3 mW for' = 39 rnA and emits at i\ .. 784 nm for
T = 250 C.
Hitachi HLP 7801 (No. 4C3243) MM (in can)
Voltage/Current Characteristics
" .. ]. ~ ,
[]
[.1. U
1]'.' , . i: <
u [j
The voltage/current characteristic shows similar features to the ML 4801 :]
characteristic, see Figure 4.3-4a.
q J
FORWARD CURRENT
If (rnA)
20
10
- 97 -
a) 0 J...,.====r:=:::.......,..--.....,.--......
b)
1·40 .
OPTICAL POWER
(V) L 4·0
15mW
3{)
1·0 2·0
1·0 0'5
1·501·60 VOLTAGE (V)
1·70
()JW)
1·80
NOISE
(mV)
3
1·0 2
O'
1~~ __ ~~==~==;=:;==;:~:L~~~I.f~ 0+ 14 16 18 20 22 24 26 28 30 32 (mA)
Fig. 4.3-3 a) Voltage-current characteristic b) optical power L and amplitude noise of ML 4102
laser diode
- 98 -
Optical Power and Amplitude Noise
Both optical output L and noise (Figure 4-.3-4-b) show the charactertistic
increase around threshold. The "kinks" in the L/I characteristic are rarely observed
and only typical for "instable" laser operation. Operating efficiency is derived from
data sheet for this particular laser as 1'Z op = 0.23 mW /mA and used for the absolute
calibration. The laser operates at 1\ = 793.6 nm (T = 250 C) in multi mode.
The noise curve peaks again around I = 1.06 Ith at noise levels of 1 r W. In
early measurements the shoulder dropped to 0.5 I" Wand degraded for unknown
reasons within 2 months to about 0.9 f" W. Mode hopping noise has been observed at
levels up to 7 - 10 !'! W. SIN ratios between 102 - 104- appear possible.
Shift of threshold current due to temperature is Observed at a rate of 0.6
mA/K.
The general impression of "instability" of this MM laser is confirmed under
feedback conditions.
Ortel LDS-3H (No. L2232) SM, open heat sink
Voltage/Current Characteristic
Has not been measured, (1.84- V at 3 mW power It. manufacturer).
Optical Power and Amplitude Noise
Both optical output power and noise agree in general features with the other
lasers.
The operating efficiency for this laser (No. L2232) is 41[ op = 0.387 mW /mA for
a threshlilold current Ith = 17.5 rnA. The lasing wavelength is 836.4- nm and maximum
rating 3 mW.
The noise shows the characteristic maximum around I = 1.06 Ith with
1.75 )A Wand drops only little to a shoulder value of 1.6 tt W. While the noise
shoulder was confirmed in a second measurement the noise around threshold changed,
see Figure 4-.3-5.
No mode jumping is observed and the laser gives the impression of being very
"stable" .
" .. U
'.1 J
n u U J
If (rnA)
60
50
40
30
20
10
a) O~ ~ ~ 2-0 1-4 1-6 1-8
VOLTAGE (V)
OPTICAL POWER L
(V) 4(mW)
8
711-5
6
5 rl-O
4 I
3-1
0-5 2 .
I I r I ,
x
NOISE
(pw,f (mV)
1-5
60 b) ol~J;- ,,"=;/l th=49mA I
48 52 56 '" I 0 64 44 (rnA)
Fig_ 4.3-4 a) Voltage-current characteristic; b)'optical power and amplitude noise of HlP7801 G laser diode.
- 100 -
OPTICAL NOISE POWER
()JW <mV) L
(V) . (mW) 2·0 2
12 12
, ....... I "-
10 , )t- 10 I
1·5 / 1·5 I .,)/-"1.
/ "/.
8 I 8 I I I
6 1·0 I 1·0 6 I I I
4 I J 4 ~
0·5 I 05 I
I 2 I 2
J
]</ ,c-)\
'th= 17·6 mA 0 0
12 14 16 18 20 22 24 (mA)
Fig. 4.3-5 Optical power and amplitude noise of LDS3-H laser diode.
J ~
N·· ~- -
E,.
tj.· "
]
!"m ~H-I+-II!-H~ 'Ii . ,
1111111111111111IIIII _ t 1 • I
H=!
.... C) ....
-102 -
I ; -l ! ! :':!,Q
--1
_. -,~
. 0:1' . en
• •
-'
!] . c·
'J .. . •
- 103 -
4.3.3 Optical Power vs. Time: Noise and Drift
For long term stability and noise mesurements the photodiode output is
biased as described under 4.3.1 and Figure 4.3-2. The biased output is displayed with
a chart recorder. A 40 min record of a ML 4102 laser diode is given in Figure 4.3-6.
If occasional "jumps" are neglected as well as long term drift a noise of :!: 0.4 m V is
measured. Under the operating conditions (I = 1.03 Ith) scope measurements indicate
between 1 -2 mV noise. The discrepancy can be explained by the slow time response
(~O.I ms) of the chart recorder, which acts as a low pass filter. In this case noise
level over several minutes are:!: 0.4 mV = 0.18rW and maximum fluctuations
observed over 40 min are:!: I mV = 0.45 t' W in better agreement with expectations.
"Jumps" can be related to sudden temperature changes, as will be shown in
4.3.4 and will be avoided with thermal control of the laser; The experimental set up
proves very temperature sensitive; a slight blow from 2 m distance provides major
output changes. Measurements are carried out without persens in the laboratory to
obtain reliable results.
A noise level of :!: 0.4 mV = .2 I"W over several minutes would indicate a SIN
ratio of about 5000: I if temperature is stablized.
Noise reduction schemes briefly discussed in 3.2 have the potential to reduce
noise over -20 to -30 dB or serveral orders of magnitude.
Under these circumstances as well as favourable aging rates (See 3.1) long
term stability of four to five orders of magnitude appear feasible.
As within time frame no temperature control became available the laser heat
sink was mounted to the largest heat sink available: the top plate of the vibration
free table. In addition heat sources (lights) and sinks (cooling through wall) were
carefully balanced. A test record over 14 h is given in Figure 4.3-7. The laser
temperature changed by AT = 2.0 K giving an average rate of 2.4 mK/min.
4.3.4 Optical Power and Temperature
The AD590 sensor is bonded with thermal conductive epoxy (EPOTEK) to the
heat sink block of a Hitachi HLP 7801 G diode where the cap is removed, (Figure
If.3-8). To reduce convective cooling by air turbulence a styrofoam/aluminum housing
is fitted around laser diode and temperature sensor, permitting a small window for
the laser radiation. The photodiode output is biased and fed to a chart recorder, as
described previously. A second chart recorder and bias circuit B is connected to the
-104-
Fig. 4.3 - 8 AD 590 thermometer head bonded to laser diode heat sink
r
I I
• s.. QI
'" '"
'" QI ell = '" E :::I .... e I .... I<o = o
en I
M . ..r . ell
.r-LL.
- 106 -
AD590 output and a simultaneous plot of both outputs is presented in Figure 4.3-9, ~ which confirms that thermal fluctuations of the laser diode are responsible for
medium time power variations of the laser output.
Estimates for the heat transfer of slowly moving air at I cm/s indicate that
temperature changes of the diode may be caused by air convection. In a sensor the
laser diode should therefore operate in vacuum.
4.3.5 Summary of measured laser diode performance
While the LDS3-H laser gives the best overall impression during operation (no
mode hopping observed either at rising current or at various operating temperatures)
the wide beam angle is a slight disadvantage as well as the noise power which is about 9 a factor two higher than for the other lasers investigated.
Long term noise tests are of a quantitative nature. If temperature
stabilization and noise reduction techniques are employed a long term stability of
I: I 04 appears feasible.
Laser data are summarized in Table 4.3-1.
']
lJ ;) U
1 .. ] , ::..
Table 4.3-1 Data of Laser Diode Performance
noise:
mode (\ Ith * Lmax -tt op peak shoulder
pattern (nm) mA mW mW/mA ~W fAW
HLP 7801 G MM 793.6 50 5 0.231 I
(No.4C3243)
ML-4102 SM 784 30 3 0.41
(No.84-220-549)
LDS SM 83.64 17.5 3 0.387 1.8 1.6
*Ith measured at 250 C
mode
hop.
t<W
beam
dl/dT angle
mA/K Deg.
0.5-0.9 7-10 0.6
0.7 3-7
none 40/30
I-' 0 ....,
- 108 -
4.4 Measured Properties of Laser Diode - Bench Model Sensor
Objective:
It is the objective of the tests descirbed here to identify design parameters
and operating parameters as well as their limits for the laser diode pressure sensor.
The following tests have therefore been carried out: measurement of optical
power vs. drive current at fixed external mirror position of L(I)d, optical power vs.
displacement of fixed threshhold current LCd)I, optical power vs. displacement at low
reflectivity of the external mirror. Some limited tests have been done with different
reflectors.
The experimental arrangement for the tests is given in Figure 4.4-1. For
pressure simulation a triangle ramp from a (low frequency) function generator is
amplified with a DC-offset in a HV-operational amplifier (Burleigh PZ 70) to 500 -
1000 V. The DC offset ensures unipolar operation of the piezoelectric pushers. The
HV ramp drives the mirror mounted on the piezoelectric element periodically and
modulates the mirror position by up to 18 t' m corresponding to about 44 fringes for
?I = 836 nm.
The laser diode used in most tests and showing best results is LDS3-H. Some
tests have been carried out with ML 4102 and HLP 7801 G, which exhibit mode
hopping and permit only a narrow range of operation in drive current.
Photodiodes are the built in monitors (HLP 7801 G, ML 4102) or the external
diode as described in the last section. For some tests the signal is biased before it is
displayed by scope or chart recorder.
Typical ramp frequencies are 50 Hz for a display on the scope with 2ms/div.
For display on the chart recorder ramp frequencies are in the order of 1/20 Hz and
lower.
r] l L
! 1
, I
lJ : .•• 1 J
'J {
L
exar function generator
HV - op amp Burleigh PZ70
translation stage with ~
mirror
pressure simulation
constant current source
power supply
photo diode
,L,
~ laser diode H photo diode .. [ bias
interferometer
Figure 4.4-1 Experimental set up for laser diode bench model sensor.
oscilloscope
chart recorder
data conditioning and display
...... o
'"
- 110 -
1t.It.l Optical Power vs. Displacement L(dl,
Ortel LDS3-H
Most tests are carried out with this laser and the experimental set up of the
bench model sensor is given in Figure 1t.1t-2a with details in 1t.1t-2b. This picture
clearly shows the feasibility of a sensor occupying few mm2.
Fringe Resolution and Calibration Curves
For close distances between external mirror and laser facet the condition
C< 1 (see 3.2 "Laser Diodes Under Feedback") holds and relatively clean sinusoidal
modulation is obtained, as shown in the record Fig./t./t-3 (1.= 1.05 Ith; 2V /full scale; T
= 17.20C; 30 cm/min; AI-first surface mirror). Distance between fringe maxima is
~ /2 = 1t18 nm and the record presents a total displacement of about 2 I'm. A sensor
resolution of 1O-1t requires then a signal resolution of 10-3 V for multifringe
operation, which appears feasible with temperature stabilization and noise reduction
techniques.
A calibration curve constructed from only 1/2 fringe requires a signal
resolution of (better than) 1O-1t V which is for the unstabilized laser about 102 times
below present noise levels.
As discussed under 2.1.3 "General Properties of Interferometers", a practical
displacement interferometer sensor requires two phase shifted (900) monotone
declining fringe signals "r" and "i" to construct an unambiguous calibration curve and
therefore overcome the need for electronic memories. Figure 1t.1t-1t shows such a
fringe sequence (ramp voltage lower trace: 1000V, closest mirror distance ~I0l"m;
AI-mirror, I = 18.6 rnA ~ 1.06 Ith; scope: 0.2 V/div; t = 2 ms/div.(uncal.». About Itlt
fringes are obtained with a declining amplitude for increased distance to the laser
facet. The modulation depth of the signal declines from about 1.6 V by 1t2% to 0.68
V. The first 20 fringes (8.3 t<m) are strictly declining so that the fringe size can be
used for identification of the fringe number. If the fringe size Figure 1t.1t-1t is plotted
versus sensor displacement a point-by-point display of the calibration curve is
obtained.
'] , l
u
u [J
Fig. 4.4 - 2a Experimental benchmodel sensor set -up; from left: blac k x-y-z positioner for RCA photodiode; under microscope with TV camera: alu mount for LDS3-H laser heat sink; right: translation stage with micrometer/ piezoelectric head (far right) and piezoelectric element with mirror directly under the microscope.
-111-
Fig. 4.4 - 2b Bench model sensor under mi croscope, t he complete i nterferometer From left: external mir ror; LDS3-H laser diode, go ld: heatsink; Cu- passive heat sink and photodiode removed from protective can. Width of heat sink is 2 mm. This set up demo nst rates that a sensor module ca n be made to fit 3 x 3 mm 2!
(f)
~ ...J o >
2.0
1.0
0.0
, ... 1""/ "1' I \. 1 • I I ; iii ;. , ' .:1 I: i: I . , I J :; • i 1 : ; • : ~ ::. • •• ': .':'. •
- , ____ C •. _ I-----W---.:- --,. />- -C -~.- .. ;"\ ----.- or' - .--.. - - -,. r-' --~ -t-'-~-----\+--· .. -/"'-' -, .--. --+--i-- L __ , ,I,' , " , " " ," "I I l \ : ~i i . I' . : ",; . j' . ! ! \: i ' ; ~ : \ \ i :; : i _ ~ J; : : ; I : .. t . -'. . 1-' '; -. ; .. ,. ·1 .L I" ;-- , ....... - .-., .... ",/:" I.---~.-.• -I --II ... " .1- .. -
: : ' ' ; ~ I \ I . I . : ,I \: ; r, ; 1 : . ; I • , : : ; ! ;
___ ' __ I ___ o ___ .I\_J __ [ __ i ___ , ....;._....:~.L_~ f-:-----:--- -'\~-r·- I: _:..._~---LJjl:.-~-.-i-.-'-_ - !-bt-!--" , , ' " ,,~ \' 'I I,·'" I " "I 'I I' , ... I I I 1 I I I : : I' , : , . : I : I, 1 i,l 1 -I ! 1 I . I: I I' I I' I • r-:.-- :-.j- II T--'I-!--;--I'--t--l--~\-~/ -;--:--'1 -;\--1 'i,-T ; --r-LILI-1T-I---"- -',--1" \---r-1 -II.JJ ' .. I· I I I , ~, " ,\ '! ,\~ I" --; : I' 1 I f~ II Iii o! :--: -!T-; I ,--r- i ! j' -I :~ I ' 1 r! r- I- -'I
I I I I! I I I ' , I I .
. .,' --r---'--'-'o-- ---- -,-t 1l+ I--l ' " . -., . I ," I' "', . I I .. : ~ .: -, . -- -::: - . :.- I, 1 I' I , I 1 " I'· i \ I .. -/- . 1 I' .. - I' 1 ., I _ _ • ._ 1 ___ •• __ • • • I , I. 'I -
:.---[} 307-~ ~,-,--t---I-T-l- J~ -j -l-- i- : ---1- -1--:--~rT:----f--!t1--~ --. ~- -'-·l--·f-~T-, . I:::: -:: - ,'.. r- \ , \ I - 1 i I 1-1: 1'1 . <> .. - I . . I .. :.- !.:~.. .... "I I, _ 1- 'I' 1\ I' -.. : -:.. I \ f-I-"-r-I-~I--I---r-----r---t--~ r-------L-I- rTo-1--~- --f--i-- _C- -1- -'-:-ir-1 . , --- .... -- - - . I I' I . I ' .. . - - - -.j'-:- :.:j I . ., I. .. I 1 ' .. : .-- .. ' .' -'.' - 0H ~ ••• : ::.:_ ~~~ :~ ............ ' • '. I.. :~ '... ............. : . I ; .. '. . _ ...... __ ~" .. _." ._.~ ...... ~~ .. . . --L,o .. '''- .. -~. . "I~ .11-- .. - ....... 'I-t I" .1
1_" .- ... - .... . .. "'-" .... " ...
~::V ::!i :':: ~:J--:~:-;-:c ~-~- :-~~:I':-~-- :1~:~8':,':; ~~.-,.' ·-:F~ .. ---:, '.' ... ~: ::: -~: .::~: ~:: .~~~ 'i~ .~:: :~_l .. T .... ,! , .... J ,,- . ..-. 1- \ '1 . 'I .... '. ···-r .\ ·1---·· -... ,_ .. ,c", "";" ,,' !I~' :~:::: '," "'<: >' :~l';;:TTr:'-: .. ::.:~-t-_l.·· .1. ,I i .. · ••.. \ .... ::, {;:: :.:;; .; :::; ~:- : .
.,'_, ....................... 1 ..... J ...... , .1 ·1···· ... , . . 'T~T:T: .'~ -:-:·l·I-.-~ ····f:::· :-:1:::- -:.-,.- .... '~.: ----i -'-!--"~-'r"--' .,·1---;---1-:- .......... : ... ~~ .. :-:-. :,. '" 3," -:-:-:r:-:-:r'--1 '1 1,,· .. - .... ,.' .... \ 1" "'I . . ·1· . I ' ' I I~' 11-" .... , -.. ... .. ·1 .. · .......... · ...... ·1 . . ...• i.' ...................... .. . .... .. 1 I··· . ' i' I 1···· .. . .. .,.,~... -'" !"" .• ~. -, ... " ..... . .. _ .:.. ___ . . .... '" ...... . .... -.:.. .' ~. :":,,,:,:,_ .. ~o-: . __ ~:: .. -1 ___ .:.~:: .1----1 ... "._._ L. __ 1_ ........•. , .... _. - 0. " ....... 1-: - .. :J .• -, •• ~ _:...~ .:....._j
Figure 4.4-3 Bench sensor output as a function of the displacement of the external mirror. Closest distance from laser facet was about 10 11m •. (T ~ 17.2 C, If = 17.5 rnA) !
!--' !--'
'"
- 113 -
The parameters controlling fringe size and therefore the calibration curve of
the sensor are further discussed in the rest of the report.
Fringe size and sequence are very reproducable for the first 15-20 fringes,
where details depend on angular mirror alignment.
Maximum Sensitivity Within Fringe
Largest sensitivity is obtained keeping the mirror in quadrature condition in the first
fringe. A noise signal of less than 0.3 m V on the ramp signal modulates the mirror
position by about 2 X 10-12 m, see Figure 11.11-5. Under these conditions even talking
at a distance of I m causes modulation: in a quiet laboratory index of refraction is
constant and only distance modulations change the optical path changes A (n d)
observed by the interferometer. It is d = .10-6 m, dd = 10-12m; LI dId = 10-6. If d = constant the bench model sensor measures /ln/n = 10-6 For air n = 1.002 and An =
10-6. In a first approximation p "".9 ...... (n-1) and tJ.p/pN/)'+-I~ Combined with
the result from above we obtain Ap/p = 10-3 which is possible for the upper end of the
acoustic pressure scale. Modulation of the sensor by variation of the refractive index
appears' feasible. In the laser diode pressure sensor the interferometer should operate
under vacuum.
Tracking Ranges
The tracking range is defined here as the displacement range over which the
interferometer output is sinusoidal and permits the construction of a calibration
curve. Several tests have been run extending the mirror displacement to several mm
and entering the C >1 region of operation showing instability. Maximum and minimum
of the sinusoidal modulation are measured and the envelope plotted, see Figure 11.11-6
(I = 1.1 Ith' T = 21.60C, closest distance d = 10 II m). This plot reveals that the
average laser power is drastically affected by the mirror position several mm away.
Largest modulation occurs in a narrow range close to the laser diode. This is
emphasized, if only the modulation depth (max - min) is plotted, Figure 11.11-7 (I = 1.06
ith, T = 200C). The modulation signal drops from about 3 V to 100 mV within
150/'C m. Tracking is possible to about 1150 t< m, where signal fluctuations and non
sinusoidal output forms indicate onset of the C > 1 regions, which are interrupted by
Fig. 4.4 - 4 Strictly declining fringes are obtained with increasing distance from the laser facet . Closest distance: 10 1!JT1; largest distance: 28 1!JT1 .
... I
90 ~ -,
• - .-
I' ·It
, I I
f - -
10 I
i -
t ~"H
-
. ''''''' .. ~ LI ., ..",.,
111'_11 ... - --
,'1, ','-' . '-"''1 il .~ .. ,
""'" ',.
I,,~, "'lfP.l .'
Fig. 4.4 - 5 Noise of 0.3 mV on the HV ramp (upper trace; 1 mV / div) modul ates the laser output (lower t race; 10 mV/div). Mirror position is within the first fringe from laser facet; in quadl~ture. The noise limit corresponds to a disp lacement of only 2 x 10- m!
I I I I
I
I I
5'0
4{)
-..
~ ..J
~ .......
g ~3·0
..J
2·0
-LMOO MAX
t + t
- LMOO MIN
1'0 2.0 3·0 4·0 mm
Fig. 4.4-6 Envelopes for max (lmod, max) and min (Lmod , max) of fringe signal. Arrows indicate maximum and minimum in fringe size.
+
d
I-' I-' c.n
- 117 -
regions with sinusoidal modulation. The latter are spaced here at about 1.37 mm and
correspond to the optical path length 1 n of the laser cavity (I = 0.3 mm, n = 4.4).
Both the good tracking region and the onset of "erratic behaviour" - likely
due to mode swapping -- are investigated in tests, where the drive current is varied.
Results are given in Figure 4.4-8 and 4.4-9. Onset of "erratic behaviour" is in all
cases beyond a distance of 350/-1 m. The "erratic pattern" is repeatable and shows
similar features increasing with drive current.
Optimum drive current is found around I = 1.06 Ith (I9 rnA) for the largest
percentage of modulation (60-70%). This condition seems to coincide with largest
noise for the isolated laser.
A documentation of output forms for d = lOr m to d = 1.25 mm can be found
in the Appendix A for the LDS3-H operated at 1.06 Ith (I9 rnA). A maximum
modulation of 80% was observed for d = 10 t" m.
The change from the sinusoidal to non-sinusoidal output is gradually
increasing with distance here and obvious long before noticeable in the modulation
depth, Figure 4.4-8 and 4.4-9.
Other Lasers
Limited experiments have been performed with ML 4102 and HLP 7801G •
The latter shows a strong tendency to mode hopping, while it is possible to find a
range of drive current, over which results from ML 4102 are comparable with the
results of the LDS3-H laser.
4.4.2 Optical Power vs. Drive Current L(I)d
The optical power/drive current characteristics has been rnesured for the
LDS3-H laser at d = 8 rm, 50 rm and 250 ,.m and is given for maxima (constructive
interference) in Figure 4.4-l0a and minima (destructive interference) in Figure 4.4-
lOb. Both maxima and minima curves indicate an increase in threshold current. This
is in contradiction to findings by others(2), (8) and is predicted by theory for d > > n 1,
where the external cavity is large compared with the laser cavity. Further discussion
exceeds the purpose of this report.
The forward currents for which the largest modulated output is obtained is
marked by an arrow.
Operating efficiency of maxima and minima curves are identical to those of
L (mV)
60
50
40
30
20
10
- 119 -
Illth = 1.1 lllth = 1.07
l/~th = 1.04 l/Ith = 1.01
T = 18.50 C
T = 20.20 C T = 20.20 C T = 20.00 C
1·10 I'h
1·07 I'h
O+-------~----~------~------~------_r------_r--~ o 0·2 0·4 0·6
d (mm)
0·8 1·0 1·2
Fig. 4.4-9 Fringe modulation vs. displacement for four drive currents outside useful tracking range. LDS3-H laser. AL-mirror; ~ non-sinusoidal modulation.
~Ith (rnA)
0·8
0·6
0·4
0·2
~L + (mW)
0·3
0·2
0·1
+-o I •
o 10 20 30 40 50 60 70 80 d (pm)
Fig. 4.4-11 Largest possible output signal ~L for bench model sensor as constructed from threshold shifts lHth "+" (Fig. 4.4-10a,b) and signals "X" (Fig. 4.4-8).
.... N ....
-123-
Fig. 4.4 - 12 Details of fringe top at I ; 19.2 mAo 5 mV /di v, t ; 2 us/div, bias: 2.993 V. Amplitude variations correspond to about ± 4 mV "no ise ".
- 124 -
Noise
Noise measurements for the sensor configuration require the control of the
mirror position as well as a thermally stabilized laser source; both were unavailable.
Instead it is possible to bias the output and display parts of fringes on the
oscilloscope. (This method drives the amplifier into saturation and it is hoped that
they regain stable operating conditions sufficiently fast.) Figure 11.11-12 shows the
position of a fringe top resolved at t = 2 /"s/div and 5 mV/div. (It = 19.8 mA, bias
2.993 V). Over the exposure time about 20 fringes are observed. Amplitude
variations are of the order of + II mV and agree very well with noise levels of the
isolated laser diode for the drive current.
The single fringe traces exhibit variations < I mV and demonstrate how much
lower noise levels are for short time operation.
11.11.3 Influence of Mirror Reflectivity and Reflector QUafltity
First surface aluminum mirrors have a (power) reflectivity of about 86% for
(I = 830 nm (and 111% absorption), while a glass surface will reflect approximately
11%. Tests with 0.2 mm thick glass reflector give about 10% of the modulation depth
obtained from the Al mirror, both at I = 19 mAo Largest signals with the glass mirror
are about 170 mV for d = 10 r m.
The region of "non sinusoidal" signals (for Al at d = 550 /'< m and modulated
signals of 5 mY) appears at the same modulation depth and therefore closer to the
laser, reducing the tracking range of the sensor.
Apart from sensor output form the signal to noise ratio will determine the
tracking range and the parameters controlling the slope of the calibration curve have
to be identified. The comparison of tests carried out with different reflectors
demonstrate that the calibration curve slope is controlled by the laser cavity
properties for short sensor displacement.
A test is carried out with a GRIN collimating lens and a plane Al reflector
with encouraging results: on first trial a 25% modulation is obtained.
In addition a concave Al reflector (of minor surface quality) has been tried.
These reflector geometries require a larger distance to the laser, which increases
stability problems and have therefore not been investigated further.
- 125 -
4.5 Demonstration Sensor
A single mode ML 3401 laser, removed from its housing, is bonded ("Crazy
Glue") into the package of a Paroscientific pressure sensor to demonstrate basic
assembling techniques, see Figure 4.5-1. A 1 mm2 piece of first surface Al mirror is
bonded to a small Cu-block, which in return is bonded to the original lever system.
Pressure supplied to the hose expands the elastic bellow, which provides here the
pressure reference and moves the mirror towards the laser so that "low pressure"
corresponds to the largest displacement.
Initial concerns that the bonding might lead to overheating and destruction of
the laser proved unnecessary. The laser, which has similar characteristics to the ML
4102, can be operated in a limited current range without Inducing mode hopping by
the external reflector. Modulated signals are of the order of 25% of the output.
Problems encountered are the cutting of the small mirror as well as the
deveopment of a mounting scheme, which provides a parallel alignment to the laser
facet.
Tweezers on mounting brackets fixed to x=y-z positioners allow for fine
positioning of the components before bonding.
- 127 -
4.6 Summary of Tests
The output of the sensor, operated well above laser threshold and starting at
small displacements consists for increasing displacements of sinusoidal "fringes" -
similar to a two beam interferometer. The fringes decrease in amplitude and become
for an interim "non-sinusoidal" before they increase and symmetry is restored. The
pattern of increasing and decreasing fringes continues at intervals controlled by the
laser round trip time and is similar to a Fabry-Perot output of low finess and losses.
Losses in the external cavity are determined by mirror reflectivity (glass/AI)
and shape (plane/concave/focussing) and determine slope 1 in Figure 4.6-1 as well as
the maximum output obtainable in the sensor, point 1.
The modulated output envelope is identical with the calibration curve,
however only parts for small displacement are smooth enough for sensor applications
between point (2) and (5). The "erratic behaviour" (3), fringe distortion (5) and signal
to noise ratio (4) set practical limits to a range within the part labeled "slope 2".
This slope is controlled by properties and losses of the laser diode (gain, wavelength,
quantum efficiency). The maximum of the modulation (point (1) for d = 0 m) is
reached at the lower straight part of the L/I characteristic for fringe minima, Figure
4.4-10b. Any further increase in forward current increases only the signal bias and
the absolute output of the laser. This however seems to destabilize the sensor (wave
form distortion) and move point (5) closer to the laser so that an optimum operational
point exists.
The tracking range of the sensor is about 100 r m, where the largest
obtainable modulated output is between 0.4 - 0.04 m W •
Noise power could not be clearly determined. A pessimistic approach
suggests to take the maximum "eyeballed" noise power observed from the scope (t = 2
ms/div) at 1-21" W. It has been demonstrated however that air turbulence effects the
open heat sink configuration seriously and contributes to noise and drift. Shielding
from turbulence as well as thermal stabilization is necessary.
The noise appears greatly reduced at high time resolution and reveals the
stronghold of the sensor to operate at high time resolution (pulse pressure, vibrations,
hydrophone, accelerometer, current transformer, etc.).
In quadrature operation a displacement as small as 2 X 10-12 m has been
resolved.
- 128 -
~L
~Lmax - -- - -
~Lmin ~------- -I I I 4 I I , I , , ,. ,
TRACKING RANGE
Fi g. 4.6-1 Parameters governing sensor cali bration curve t. L; for explanation see text 4.6.
d
1] L
W :', ,,~
g fu:,
[] . ~:
B 0 0
- B
n "
[] i.
J g ;
~ :-~
B ,.
fJ ~ .
!J i ,
U
U ".
'] L
-126-
press ure
1
Fig. 4. 5 - 1 Laser diode interferometric module in demonstrat i on pressure sensor .
[
r
I I I I
•
- 129 -
1j..7 References
1. W. Bludau, R. Rossberg, Appl. Opt. 21, 11 (1982) 1933.
2. A. Dandridge, R.O. Miles, T .G. Giallorenzi,
Electr. Lett. 16, 25 (J 980) 91J.8.
3. F. Schumann, K.H. Tietgen, Elektronik II (J 98IJ.) 59-62.
IJ.. A. Abou-Zeid "Spektrale Untesuchungen an Komerziellen Ga Al As Laser
Dioden", Physikalisch Technische Bundesanstalt, PTB-Bericht PTB-Me 56,
1981J., ISSN 031J.1-6720 Braunschweig-West Germany,- in German.
5. S. T. Ho "Reduction of Frequency Noise in Semiconductor Lasers",
MSc Thesis, Dept. Electrical Engineering and Computer Sciences,
Massachusetts Institute of Technology, 1981J..
6. R. Kist, S. Drope, H. Woelfelschneider, "Fiber-F~bry-Perot (FFP)
Thermometer for Medical Applications" in Proceedings of the 2nd
International Conference on Optical Fiber Sensors, Sept. 5-7, 1981J.,
Liederhalle Stuttgart, West Germany, VDE-Verlag GmbH, Berlin 1981J.. West
Germany, 165.
7. Sea - Met Sciences Ltd., Dr. T. M. Dauphinee,
36 Ave., Ottawa, Onto KIS ON9, Canada.
8. R. Lang, K. Kobayashi, IEEE QE 16, 3 (1980) 31J.7.
9. T. Kanada, K. Nawata, IEEE QE 15,7 (1979) 559.
10. G. Hernandez, Appl. Opt. 17, 19 (]978), 3088.
11. S.M . Lindsay, I. W. Shepherd, Rev. Sci. Instr. 1J.8, 9 (] 977), 1228.
R g
• 11., ~J
J ~
D H
-- n g []
",1 ~j
'.] .' , . c
, 1
d
- 130 -
5. ALTERNATIVE APPROACHES
5.1 Alternative Operational Concepts and Devices
Two independent signals are required for the unanimous reconstruction of the
light wave phase and displacement d, as is discussed in chapter 2.1.3 "General
Properties of Interferometers". The cos and sin function fulfill these requirements
and the latter is obtained in an interferometer output by introducing an additional
phase shift tl'f = A Iii- (retarder) in signal -- or reference beam. This is usually
implemented by operating two crosspolarized and ~ phase shifted interferometers
on one beam and providing the same phase modulation in them. This approach fails
for the laser diode sensor because the waveguide structure-of the active layer acts as
a polarizer/analyzer with a high extinction ratio and permits operation in only one
polarization mode, in the plane of the layer.
External beam manipulations - as for example the interference of a part of
the laser output with another part which is retarded by }\ I Ii- -- will contribute an
additional time independent phase to the electric wave vector. Both the extreme
phase variations at light frequencies as well as a constant time independent phase
provide in the detector output only a DC offset. External beam manipulation can
therefore not provide the required second output form.
Differentiation of the output signal can be obtained by changing the optical
path A (n.d) by a known amount and measuring the change in output power A. L. For
vacuum operation n = 1 and Ad = f\ Iii- a displacement modulation of about 250 nm is
required, which can be obtained from a good single disk piezoceramic (L1 dIll u ~ 500
nml 1000 V).
Two Wavelength operation of the laser can provide two independent signal
forms within a limited range of operation and appears very attractive. Assuming a
phase difference A If = A I Ii- to be introduced by the wavelength shift ~ 1 - (\2 = £l..\ for the mirror displacement d we obtain
--
or
2.0-1 A
(5.1-1)
(5.1-2)
- 131 -
For d = 25 i\ we obtain !J)../ A = 1/200 or LL\ = 4 nm. Such a wavelength shift is
within the capabilities of present technologies (T - shift of laser environment;
pulsed/cw operation; wavelength shift by current modulation in the C = I region,
injection locking) and is a promising operational concept, permitting absolute
displacement and index of refraction measurements.
Utilization of the low correlation between laser diode forward and backward
output was considered for the generation of two independent interferometer output
forms.
Tracking of the mirror position by keeping the interferometer in quadrature
with a piezoelement was considered. Piezoelements are unsuitable for frequencies
smaller than I Hz and an additional dither would have to be introduced.
An "optical displacement" sensor changing i1 n rather than d was considered.
IJ n = 10-6 can be resolved for unaggressive gases containing no vapours.
A polarimeter arrangement was considered. It requires a larger external
cavity. Sensitivities are generally higher than for interferometers. Temperature
stability has been reported as problem.
A laser diode was exerted to direct pressure. An output modulation of about
25% can be expected. Laser manufacturers warn for dislocation migration in the
laser structure.
A combined electronic/optical oscillator was discussed which changes its
peak frequency due to external excitation. The operation of such a sensor uses the
principle of the paroscientific pressure sensor and requires a high-Q resonator, which
is here formed by a combination of electronic (transistor, diode) and optical
components (laser diode, optical fibre).
In all approaches the module size will be kept small to minimize disturbance
by thermal effects and mechanical noise. Materials of suitable mechanical and/or
thermal properties are quartz and glassy materials like Zerodur. Thermal properties
and mechanical stabilities are compared in ref. (j) - (3).
CO! );-
U
lJ. ,
- 132 -
5.2 References
I. S.F. Jacobs, S.C. Johnston, D.E. Schwab. Appl. Optics 23, 10 (1984) 3500.
2. J.J. Schaffer, H.E. Bennett. Appl. Optics 23, 17 (1984) 2852.
3. S.F. Jacobs, S.C. Johnston, G.A. Hansen. Appl. Optics 23,17 (1984) 3014.
n o I
-- n [J
[]
- 133 -
6. CONCLUSION
The present report highlights components and their functions of an optical,
highly integrated sensor system for displacement and pressure measurements. The
usage of components in multiple functions leads to a sensor system which can in its
size only be compared to VLSI circuits and therefore makes a very attractive sensor
system. The high degree of integration decreases on the other side the freedom in
free design parameters and - if laser design to required specifications is not
available - the selection of the proper laser characteristics becomes the most
important single design step. This step requires therefore a much more thorough
understanding of available technologies, components and their function within the
sensor than what is required for a design of discrete" components. Practical
experience from a bench model sensor are supplemented where necessary by data and
theories from the literature. The summaries provided here give a state of art review
for Ga Al As laser diodes as well as for the technology of laser diode sensors.
Experiments with the bench model sensor show its large potential to be
operated at high frequencies. With laser rise times of subnanoseconds the device can
be used for high resolution (time and spatial) measurements of displacement,
acceleration, refractive index changes, vibration or as hydrophone (sensitive area
0.1 mm2). In such application thermal stabilization of the laser becomes less
important. Combining the sensor with lock-in amplifier technique displacement
smaller than 10-14 are expected to be measured for signal frequencies of lOa KHz.
This corresponds to a resolution of 10-9 for displacements or refractive index.
The sensor module operation for static signals requires for Ga Al As lasers a
high degree of thermal stabilization and likely noise reduction schemes (current
modulation or HF modulation of light reflected back into the laser cavity). The
operation over a displacement range larger than one fringe is possible - requires
however additional electronics for either differentiating the signal by modulation of
the mirror position or two wavelength operation.
Operation for displacements smaller than one half fringe appears possible,
requires however stringent temperature control and noise reduction of the laser. The
sensor performance could not be tested for these conditions.
The construction of a demonstration sensor demonstrates that bonding can be
used as technique to mount the laser without inhibiting the cooling excessivily.
Production techniques have to be further developed for the precise mounting of the
external mirror opposite to the laser facet.
- 134 -
The work reported here focuses on the well established technology of Ga Al
As laser diodes, which is presently still more readily available than lasers based on
quarterny alloys. Developments supported by the requirements of telecommunica
tions industry have led to dramatic improvements in thermal stability of these lasers
operating at 1.3 or 1.6 JA m and should be considered for the further development of
the laser diode pressure sensor module.
If the laser diode sensor is compared to piezoelectric or piezoresistive
modules they show all best operation at frequencies well above 1 Hz. The laser
module as a receiver is however superior due to its extreme potential for compact
ness as well as its capability for high spatial (;!O .1 mm2) and temporal (~ 1 ns)
resolution.
f1 ~J
