hp an1550 6_high speed lightwave component analysis
TRANSCRIPT
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High-spe ed lightwavecomponent analysis
Application Note 1550-6
Characterizing
sys tem components
Laser and LED transmit ters
Ph otodiode receivers
External modulators
Optical components
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As lightwa ve tra nsm ission systems become m ore advan ced, component designers an d
ma nu factu rers m ust ma ximize the per form an ce of th eir devices. For example, one
par am eter often used t o specify digita l system performan ce is bit err or rat e. However,
it is difficult to specify individual components in such terms. Rather, fundamentalmeasu rement s such as gain, bandwidth, frequency response an d retu rn loss can be
appr opriat e. The Lightwave Component Analyzer (LCA) is used t o measur e th e linear
tr an smission an d r eflection char acteristics of a component as a function of modulation
frequency. Measuremen ts ar e calibrated a nd can be performed a t modulat ion ra tes up
to 20 GHz.
Table of Conte nts
Introduct ion
Genera l measurement t echniques and considera t ions 3The Lightwave Component Analyzer (LCA) family 4
Lightwave transmitter measu remen ts (E/O)
Modu la tion ba ndwidth , frequ en cy r espon se, a nd con version efficien cy 5
The effect s of bias on laser per formance 7
Laser pulse measurements 7
Laser reflect ion sensit ivity 8
Modula t ion phase response 8
Laser input impedance 9
Electro-opt ic e xternal m odulator me asurem en ts (E/O)
Modula tor bandwidth and responsivity 11
Lighwave receive r measurem ents (O/E)
Modu la tion ba ndwidt h, fr equ en cy r espon se, a nd con ver sion efficien cy 12
Photodiode pulse measurements 13
Photodiode modula t ion phase measurements 15
Photodiode output impedance 15
Optical compo nen t meas ureme nts (O/O)
Transmiss ion measurements 17
Fiber length and propagat ion delay 17
Fiber modulation pha se sta bility 18
Reflect ion me asurements 18
Methods for measur ing lightwave reflect ions vs. distance 18
Achieving both high resolut ion an d long ra nge 20
Electrical compo ne nt measu remen ts (E/E) 21
Appen dix 1: Signal relat ionships in op to-electric de vices 21
Appen dix 2: Operat ion in the t ime dom ain
Basic considera t ions 22
Range and resolu t ion 23
Improving measurement accu racy through ga t ing 23
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Introduct ion
General Measurem ent
Techn i ques a ndConsiderations
The concept of lightwa ve
component a nalysis is str aight-
forward. Measurements ar e made
of th e small-signal linear tra ns-
mission and reflection character-
istics of a va riety of lightwa ve
componen ts. A pr ecise electrical
(signal genera tor) or optical (laser)
source is used to stimulate th e
component under test an d a very
accurate optical or electrical
receiver measures t he t ransmit-
ted (or r eflected) signal. Sin ce
characterization over a range
of modulation frequencies is
required, the frequency of mod-
ulat ion is norma lly swept over
the bandwidth of interest.
Measurem ents a re typically
comprised of the appr opriate ra tio
of microwave modulat ion cur ren t
(or power) and lightwave modu-
lation power (see Figure 2).
While Figure 1 demonstra tes
th e basic concepts of lightwa ve
component analysis, the specific
measur ement processes are illus-
tr ated later. An a na lysis of how
various signa ls are used in the
measu remen t pr ocess is found
in Append ix 1, "Signa l Relation-
ships in O pto-electric Devices."
LWSource
LWReceiver
Display
Amp
O/O
E/E
RF Source
E/O
O/E
O/O, E/O, O/E, or E/E
Deviceunder test
Modulated Lightwave
modOpticalModulationPower
modElectricalModulationCurrent
InputDevice
Under Test Output
P : I :
E/O measurement =
O/O measurement =
P Out
I In
P Out
P In
mod
mod
O/Emeasurement =
I Out
P In
mod
mod
mod
mod
Laser
Photodiode
Fiber
Figu re 1. LCABlock diagram
Figure 2.Measurementsignals
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O/ O Measurem ents
Characteristics of purely optical
devices can a lso be measur ed. Inthis case, both the stimulus and
response are m odulated light. The
rat io measur ement is simply one
of gain or loss versu s modula -
tion frequen cy.
Measurement Process
To simplify th e pr ocess of mak ing
measur ements , LCAs ha ve a
built in Guided setu p featu re.
This will lead th e user th rough
the basic measurement setup
an d calibration featu res.
Measurement Calibration
The key to making accurate E/O,
O/O, or O/E m easur ements is
calibrated instrument ation. Th e
instru ment lightwave source and
receiver a re individua lly cha ra c-
terized. The systematic responses
of the components ma king up t heLCA can th en be rem oved, yield-
ing the response of the device
under test (DUT). (See Appendix
1, Signal relationships in opto-
electric devices for m ore det ail.)
The LCA Family
There are several instrum ents
in the LCA family. Their charac-
teristics are sum mar ized below:
Please refer to the Hewlett-Packard
Lightwave Test an d Measur e-
ment Catalog for a complete
listing of Lightwave Component
Analyzers a s well as other light-wave test equipment.
An LCA measu res input m odulat-
ing curr ent and output modulation
power a nd displays the rat io of the
two in Wat ts/Amp, either linearly
or in decibels.
O/ E Measurements (Photodiodes)
The m easur ement process for O/E
devices is s imilar to E /O devices.
The measu rement consists of th e
ratio of output electrical modu-
lation cur rent to inpu t optical
modula tion power. Slope respon-
sivity for O/E devices describes
how a change in optical power
produces a cha nge in electrical
current . Graph ically this is
shown in Figure 4.
The LCA measures th e input
optical m odulation power a nd
output modulation current and
displays the rat io of th e two in
Amps/Watt.
E/ O Measurem ents
(Lasers, LED's)
The measurement of an E/O
tr an sducer is a combinat ion ofinput modulating current and
outpu t optical modulat ion power.
Slope r esponsivity is used to
describe how a chan ge in input
current produces a cha nge in
optical power. Graph ically th is
is shown in Figure 3.
Responsivity Rs (W/A)= Pout/ IinRs (dB)= 20 log10 (Rs(W/A))/(1(W/A))
PoutmW
Iin mA
Figure 3. E/O sloperesponsivi ty
Responsivity Rr (A/W)= Iout/ PinRr (dB)= 20 log10 (Rr(A/W))/(1(A/W))
IoutmA
Pin mW
Figu re 4. O/Eslope responsivi ty
Modulation
LCA (nm) Frequency Range
HP 8702 850, 1300 or 300 KHz3/6 GHz1550
HP 8703 1300 or 1550, 130 MHz20 GHzFP or DFB
HP 8510/834201 1300 or 1550, 45 MHz20 GHzor FP or DFB
HP 8720/83420
1 See HP Product Note 8510-15.
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Figu re 5. E/Omodulation band-width measurement
Lightwave TransmitterMeasure me nts (E/O)
The LCA is used t o cha ra cterizeth e tra nsmission and r eflection
para meters of laser and LED
sources with respect to modula-
tion frequency. The transmission
measu remen ts to be discussed
include:
modulation bandwidth and
frequency response
conver sion efficiency
th e effects of bias
pulse measurements
reflection sensitivity modulation ph ase response
laser input impedance
Other laser m easurements includ-
ing linewidth, chirp, and RIN
ar e discussed in H P Applicat ion
Note 1550-5 (or 371).
Modulat ion Bandw idth,Frequency Response , andConversion Efficiency
Modulat ion ban dwidth r efers
to how fast a laser can be inten -
sity modu lated, wh ile conversionefficiency (responsivity) refers
to how efficient ly an electr ical
signal driving a laser is converted
to modulated light . Alth ough
responsivity is often used to
describe a sta tic or DC par am e-
ter, the conversion efficiency of
a device for modulation signals
is a dynam ic chara cteristic and
can be r eferred t o as slope
responsivity.
It is not un usu al for slope
responsivity to vary according
to how fast the electr ical signal
is varied. As th e frequency of
modulation increases, eventua lly
th e conversion efficiency will
degrade or roll off. The fre-
quency wher e th e conversion
efficiency drops to one-half of
th e maximum is the 3 dB
point (when dat a is displayed
logarith mically) an d det ermines
a laser s modulat ion ba ndwidth .
Distortion of modulation signalswill occur if the frequency response
is not flat an d th ere a re fre-
quency componen ts wh ich exceed
a laser s ban dwidth.
The measu remen t of modulation
bandwidth consists of stimulat-
ing a laser with a n electr ical
(microwave or RF) signal a nd
measu ring its response (modu-
lated light) with a l ightwave
receiver. Normally th e frequency
of an electrical signal into alaser is swept t o allow cha racter-
ization of the laser over a wide
range of modulation frequencies.
Measurement Results
and Interpretation
Figure 5 shows the m easure-
men t of the conver sion efficiency
of th e laser as a function of mod-
ulation frequency. The display
uni ts a re Watt s per Amp (the
vertical axis). In t his case, the
display is in a logarithmic format
where 0 dB represents 1 wat t
per a mp. The h orizontal axis is
modulation frequency, indicating
that the measurement is being
made over a wide range of fre-
quencies, in th is case from
300 kHz to 3 GHz.
As stated, this measurement
indicates h ow fast th e laser can
be modulated. This part icular
laser has a modulation ban dwidthof about 1.5 GHz. Beyond th is
frequen cy, t he conver sion effi-
ciency is gradually degraded.
There are two significant compo-
nents tha t limit t he modulation
bandwidth. One is the actual
const ruction of a laser including
the physical dimensions and
fabrication process. The other is
how efficient ly an electr ical signa l
is delivered to th e laser. (See
Laser input impedance.)Measurement Procedu re
An accura te measur ement
requires a user calibration. A
user calibration will allow the
LCA to remove the response of
the t est system, including the
electrical cables, optical fiber,
and the instr ument i tself. Prior
to the a ctu al calibrat ion st ep,
th e LCA needs to be configured.
This in cludes:
start and st op frequencies sweep type (linear or logarithmic)
number of measurement points
measurement sweep t ime
sour ce power level
Note: LCAs have a Guided
Setup feature t hat leads th e user
thr ough al l the s teps that a re
described here. Guided setu p is
accessed by press ing SYSTEM
key an d th e [Guided setup] soft-
key. The following t ext d iscusses
th e processes that th e guidedsetup executes.
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Figu re 7. E/Omeasurement
To perform a simple frequency
response calibration, th e con-
nections sh own in F igur e 6 must
be made. The analyzer mea suresthe a ppropriate path s so the fre-
quency and phase response of the
un known path (s) is/are then
char acterized. The a nalyzer then
uses t his inform at ion in conjunc-
tion with t he int ernal calibrat ion
data to generate an error matrix.
(The lightwave source and receiver
characteristics a re pre-determined
dur ing a factory calibration an d
stored in m emory. The st orage
meth od depends on th e type of
LCA used). The end r esult is th edisplayed response of the laser
under test alone.
After the calibration is complete,
one might expect t o see a flat
response at 0 dB indicating th e
test system r esponse ha s been
removed. When using an HP 8702,
th e display seen u pon completion
of th e response calibra tion pr o-
cess will not n ecessarily be a flat
line. The laser used in th e cali-
bra tion is sti l l conn ected a ndha s become the DUT. Thus, its
response is displayed unt il it is
replaced with th e actual tes t
device. When the HP 8703 cali-
bration is completed, no response
(other th an noise) is displayed
until an E/O test device is con-
nected between the electrical and
optical measurement planes.
In a ddition to the simple response
calibration, there are also the
resp onse plus isolation andthe response plus match cali-
brat ions. Th e isolation calibra-
tion is used for high insert ion
loss (low conver sion efficiency)
devices wher e an y signal leak-
age within the instrument m ay
be significan t r elative to the
actual signals measured. The
ma tch calibrat ion is used to
remove the effects of reflections
between the instrument electri-
cal test port a nd th e laser undertest. If the laser being tested ha s
a poor electr ical input ma tch, the
response and m at ch calibration
can pr ovide a significan t improve-
ment in measurement accuracy.
(The response an d ma tch cali-
bra tion is only available with the
HP 8703 LCA.) An exam ple of
the r esponse plus ma tch calibra-
tion is found in t he section on O/E
receiver measurements.
Once the setup a nd calibrat ionha ve been completed, th e laser
under test is connected an d accur-
ate measur ements can be made.
Accuracy Considerations
There a re several items to con-
sider with respect to measur e-
ment accuracy. These include:
Keeping all electrical an doptical connectors clean and in
good cond ition
Operating the test device in
l inear regions (unsat ura ted
conditions)
Avoid overdriving th e instr u-
ment receiver
Minimizing cable movement
Allowing the instru ment t o
warm-up
Keeping both optical an d elec-
tr ical reflections at a m inimum
(for tra nsmission measur ements)
Figu re 6. E/Ocalibrationconfiguration
HP 8702 HP 8703
HP 8340XSource
HP 8341XReceiver
HP 8702 HP 8703
E/O DUT HP 8341XReceiverE/O DUT
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The Effects of Biason Laser Performance
The frequency response of
a laser is also dependent onbiasing conditions. As th e DC
bias of th e laser is increased,
the bandwidth will generally
increase. This is typically due
to th e rela xat ion oscillation
characteristics that vary with
bias. The relaxation oscillation
phenomenon creates a r esonan ce
in th e frequency response, noise,
an d distortion of th e laser.
Figur e 8 is a composite of a
bandwidth measurement made
at th ree different bias levels.
(The horizont al a xis is log fre-
quen cy.) As bias is increa sed,
both responsivity and band-
width increase. For th is laser,
as bias rea ches a certa in point,
the high-end r esponse begins to
degrade.
Note in th e two lower tr aces
that the response tends to peakbefore rolling off. This is t he
region of relaxat ion oscillation.
Care must be tak en when mod-
ulat ing a laser in th is region,
becau se this is where n oise and
distortion properties ar e often
at th eir worst. (See HP Appli-
cation N ote 1550-5 (or 371),
Measur ing Modulated Light.)
Laser Pulse Measurements
Fr equency doma in informa tion
(modulation bandwidth) is related
to time domain perform an ceusing th e an alyzer s time domain
featu re. An LCA uses the mea-
sur ed frequency domain (band-
width) data and math ematically
man ipulates it thr ough a form of
an inverse Fourier tra nsform to
pred ict t he effective step an d/or
impulse r esponse of a laser. (See
Appendix 2, "Operat ion in t he
time domain;" Basic considera-
tions.)
Measurement Resultsand Interpretation
Figure 9 shows th e predicted
impulse response of a high-speed
laser. The da ta is displayed in
a linear m agnitude format (as
opposed t o logar ithmically in dB).
Several items of informa tion ar e
available from this measurement.
One is basic impulse width, which
is a measure of device speed. Two
time values a re shown. The PW
value is the time between mark-
ers at the half-maximum points.However, part of the response is
due to th e finite bandwidth of
the inst ru ment itself. The Net
PW value is impu lse response
with th e instru ment s response
removed.
Figure 8. Compositeplot of bandwidth at3 bias levels
Mark er 1, at 10.337 ns, is the
effective delay or propa gat ion
time through the laser device
from t he electr ical inpu t t o theoptical outpu t. The device ha s a
long len gth of fiber pigtail wh ich
is the main cont ributor to the
tota l delay.
Note also th at t here is a
secondary impu lse. This typi-
cally indicates the presence of a
reflection and re-reflection.
Figure 10 shows the pr edicted
step r esponse of th e sam e laser.
From this measur ement we can
determine r isetime, ringing, andovershoot performance. In gen-
eral, these parameters a re directly
related to the frequency response
of th e device. (For a compa rison
of time-doma in measu remen ts
generat ed by an LCA versus a n
oscilloscope, see Figur e 24, page
14 under Ph otodiode Pu lse
Measurements).
Figu re 9. E/Oimpulse response
Figu re 10. E/Ostep response
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Figure 11.Reflectionsensi t ivi tyse tup
Measurement Procedu re
Pulse measurements ar e gener-
ated by manipulating measured
frequency response da ta. Conse-quently, the measurement pro-
cedure is almost identical to that
used for th e modulation ban d-
width. (Potent ial differences
exist du e to requirements of th e
math ematical t ransform. See
Appendix 2, "Operat ion in th e
time domain.")
Laser Reflection Se nsit ivity
The frequ ency response of a
laser ma y be modified if light is
reflected back int o th e laser scavity. The r eflection sen sitivity
of a laser can be mea sur ed as
shown in F igure 11.
Measurement Procedu re
The measurement setup is simi-
lar to the measu remen t of mod-
ulation bandwidth. In addition,
a directiona l coupler is inserted
in the optical path (prior to cali-
brat ion) in order t o monitor th e
tra nsmitted light and minimize
th e instru ment s response to the
reflected light. The controlled
reflection is conn ected to the other
arm of the coupler. For an accu-
rat e measurement , it is essential
th at all optical reflections, except-
ing th e cont rolled reflection, be
kept at a minimum.
Typically, a las er s frequ ency
response with back-reflected
light is compared t o the response
when n o reflections are pr esent.
The r esponse calibration for the
reflection sen sitivity measu re-ment (un der th e Guided setup
menu ) norma lizes the frequency
response to a flat line when no
reflections a re pr esent. As the
back-reflection is increased, and
the polarization of the reflectedlight is adjusted for worst case
results, th e modulation response
will deviate from th is norma lized
tra ce and sh ow the r eflection
sensitivity.
Measurement Interpretation
In t his case, the r esponses for
severa l levels of reflections ar e
shown in F igure 12, a composite
diagram (th rough offsett ing
subsequent measu rements by
changing th e display reference
level). The magnitude and polar-
ization of the reflected light are
adjusted wh ile th e laser s outpu t
is monitored by the LCA. Depend-
ing on how well the las er is iso-
lated, and its inherent sensitivity,
the frequen cy response of the
laser can be significan tly impacted
by reflected light. In the worst
case, (a reflection of approximately
4 dB retu rn loss) the modulation
response shows a 3 dB pea k-to-
peak variation.
When a laser is used in an actualsystem, th e amoun t of back-
reflected light m ay be u nkn own.
Thus, it is desirable to develop
a r obust laser wh ose chara cteris-
tics will be consist ent over a diver-
sity of opera ting en vironm ents.
Modulat ion Phase Response
Idea lly, a laser s m odulat ion
envelope will exhibit a linea r
phase response versus modula-
tion frequen cy. If th e relat ive
pha se relationships of the mod-ulation frequencies do not remain
const an t, a form of distortion will
occur. The pha se response of the
laser can be displayed in two
ways. One way is to display the
pha se r esponse directly. The
second is to display th e pha se
resp onse in a delay form at .
HP 8702 (or 8703)
E/O DUT HP 8157/8Attenuator
v
HP 81000BRReflector
HP 11894Polarization
Adjuster
HP 11890/1Directional
Coupler
HP 8341XReceiver
Figure 12. Reflectionsensi t ivi ty for severallevels of reflection
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Measurement Procedu re
and Interpretation
Ph ase dat a is displayed by
simply choosing the dat a formatto be ph ase as opposed to th e
defau lt log mag. If th e DUT
ha s any significan t length in
either th e optical or electrical
pat h, some compensat ion in
length (thr ough the electr ical
delay function un der the Scale
Ref key) will be r equir ed for
viewing the ph ase r esponse of
the laser. In th is measurement,
10.315 ns of electr ical dela y is
added, because t he fiber pigtail
is about 2 m long.
The p ha se res ponse often fol-
lows th e frequ ency response.
The frequency response of this
laser rolls off at the same fre-
quency range where the phase
begins to deviate from a linear
response.
Figu re 14. E/Odelay measurement
Figu re 13. E/Ophase response
Sometimes the phase response
is easier to interpret an d use
when viewed in the delay dataform at . The plot of delay is us ed
to indicate t he effective time it
ta kes for a m odulat ing signal at
the input of the E /O DUT t o exit
the device as m odulated light.
Ideally, th is tr an sition time will
be th e sam e for all modulationfrequencies of interest.
Figure 14 sh ows the delay for a
3 GHz laser. The average pr opa-
gation t ime over the 3 GH z band-
width is near 6.3 ns.
Laser Inpu t Imped ance
The convers ion efficiency of a
laser is dependen t n ot only on
the inh erent pr opert ies of the
laser, bu t a lso on h ow efficientlythe electr ical m odulation signa l
is delivered to th e laser. High-
speed modulation signals are
general ly t ran smit ted to the
laser over tr an smission lines
with a 50 or 75 ohm char acteris-
tic impedance. Maximum power
tra nsfer will occur if th e input
impedan ce of th e laser is th e
same as the t ran smission line.
Unfortun at ely, the input imped-
an ce of an active laser is much
lower tha n th e tran smission sys-
tem used to drive it. Two problemsoccur when such an impedan ce
misma tch exists. First, a signifi-
cant amoun t of energy will be
reflected at the transmission
line/laser inter face. This r eflected
energy may eventu ally be re-
reflected a nd distort the desired
data signal. The second problem
is tha t t he reflected ener gy is
wast ed since it is never effec-
tively used t o modulat e the las er.
Thu s, th e overall conversion
efficiency of the laser is degra ded.
Measurement Procedu re
Figure 15 shows th e retur n loss
of a laser with a simple resistive
matching circuit as measured
on the component an alyzer. The
measurement is made by send-
ing a swept RF signa l to the laser
under test and measuring the
energy tha t r eflects back. The
setup a nd calibrat ion pr ocedur e
will depend on the model of LCA
used. In all cases, a calibrat ionkit containing k nown electr ical
reflection stan dar ds is required
to improve the accuracy of the
reflection measur ement s.
Figure 15. E/O returnloss measurement
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Measurement In terpretation
The retu rn loss over a 6 GHz
ra nge varies from a best case of
near ly 34 dB to a worst case of17 dB. It is not unu sua l for th e
reflection level to get worse as
th e modulation frequency is
increased.
Retur n loss is th e rat io of
reflected to incident energy
(10 Log (P refl/ P inc)). The larger
the retu rn loss magnitude, the
smaller th e reflected signal an d
the better t he impedance match.
Figure 16 uses the same mea-
sured dat a as th e return loss plot,except in th is case th e data is
displayed in a Sm ith Cha rt for-
ma t. A Smith Ch art is a form of
an impeda nce map. The display
shows the laser input impedan ce
as a fun ction of frequen cy. For
th is laser, the impedan ce is close
to 50 ohms over th e 6 GHz ran ge,
as t he response does not deviate
much from t he center of the char t.
The Smith Char t data presenta-
tion is selected under the Format key menu.
Figure 16. Retu rnloss in Smith chartformat
The impedance data from the
Smith Cha rt can be used to model
the inpu t str uctur e of th e laser.
The laser s effective inpu t imped-
an ce can be improved with a
mat ching network. Simple meth -
ods ar e usua lly resistive, while
more efficient bu t complex meth-
ods use reactive elements.
Im plications of
Impedance Mism atch
on Measurement Accuracy
When th e inpu t impedan ce ofth e E/O device under t est is far
from 50 ohms, a significant por-
tion of the electrical energy sent
to th e device will be reflected.
This r eflected energy can degrade
measu remen t a ccur acy. This is
typically seen a s r ipple in fre-
quency r esponse measurements.
Two techn iques ar e available to
overcome t his pr oblem including
th e response/match calibra tion
(discussed in O/E measurements)
an d gating (discussed in Appen-dix 2, "Operat ion in th e time
domain").
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Electro-opticExternal ModulatorMeasurements
External intensity modulators
can be cha ra cterized in mu ch
th e same way as laser sources.
This is an other class of E/O
measurements where the stimu-
lus is a swept frequency electri-
cal signal an d th e response out
of th e modulator is inten sity
modulated light . In par ticular,
modulation bandwidth, phase,
an d electrical impedance mea-
surements are made with the
component analyzer in t he sam e
configur at ion th at is used for
laser measurements.
However, a significant difference
exists due to the modulator being
a thr ee-port device. While the
frequency response of a m odula-
tor is often independent of th e
input optical power, th e respon-
sivity is not. Th e conver sionefficiency of th e modu lat or is
not only a function of th e elec-
tr ical inpu t, but also the level of
th e optical inpu t.
The LCA measu rement compar es
the output modulation power to
the input modulat ion current .
A responsivity in Wat ts per Ampis then computed an d displayed.
If the inpu t optical power is
increased, th e output modulation
will typically also increase. Th us,
the a ppar ent r esponsivity will
increase. This means th at t he
modulator responsivity mea-
sur ement is valid only for the
specific optical inpu t power t ha t
existed when the measu rement
was performed. Th e frequency
resp onse is t ypically valid over
a wide ra nge of input powers.
Figure 18 is a measu rement of a
wide bandwidth external modu-
lator. The unu sua l response at
the low frequency ra nge is due
to the efficiency of the electrical
impedance ma tching circuitry.
Similar t o the process used for
laser measurements, the phase
response an d electrical input
impedan ce can a lso be cha ra c-terized. The frequency domain
informa tion can also be used to
predict the step and impulse
responses.
Lasers are typically described
by an input current versus output
power relationship. The preferred
description for a modulat or is
often a n inpu t voltage versus out -
put power relat ionsh ip. Becau se
LCA measur ements assum e a
50 ohm m easurement environ-ment , the LCA modulator mea-
sur ement in Watts per Amp can
be convert ed to Watts per volt by
scaling (dividing) the measure-
ment by 50. With th e HP 8703,
th is can be achieved by setting
th e nu mer at or K (gain ) term of
th e coefficient model t o 50, load-
ing the model into memory, and
dividing th e dat a by memory.
These functions are under the
Display key.
HP 8703
CW LWSource
DUT
RF in
LW out
Figu re 17. E/Omodulator mea-surement setup
Figure 18.Modulatorbandwidth
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electr ical r eceiver. Normally th e
frequency of the modulation is
swept to allow examination of
the photodiode over a wide ran geof modula tion frequen cies.
Measurement Results
and Interpretation
The instru ment display of Figure
19 shows the conversion efficiency
of th e ph otodiode as a fun ction
of modu lat ion frequ ency. The
vertical axis display un its ar e
Amps per Watt an d th e horizon-
tal axis is modulation frequency.
In t his case, the vertical axis is
in a logarith mic format where0 dB (the center line of the dis-
play) represent s 1 Amp per Wat t.
12
Lightwave ReceiverMeasurements(O/E)
The measurements that t he LCAmakes on lightwave receivers
are in m any ways similar to those
ma de on lightwa ve sources. In
th is case, th e stimulus will be
modulated light an d the response
will be demodulat ed electrical
signals. Measurements include:
photodiode responsivity an d
modulation bandwidth
step and impulse response
char acterization and improve-
ment of the electrical outpu timpedance
As with the laser source, band-
width m easurements a re relevant
to pulse rise and fall times, while
impedance measurements ar e
important to minimize signal
reflections and maximize elec-
tr ical power tr an sfer. Optical
power reflections are discussed
in Optical components: Reflec-
tion measurement s.
Photodiode ModulationBandwidth, FrequencyResponse , andConversion Efficiency
As discussed ea rlier, photodiode
conversion efficiency refers to
how a change in optical power is
convert ed to a change in output
electr ical curr ent. As th e fre-
quen cy of modula tion increases,
eventually the receiver conver-
sion efficiency will rolloff. Thus,
th e device ha s a limited modu-
lation bandwidth.
The m easurement of modulation
band width consists of stimulat -
ing the photodiode with a source
of modulated light a nd mea sur-
ing the outpu t r esponse (RF or
microwave) curren t with a n
Figu re 19. O/Ebandwidth andresponsivi tymeasurement
The photodiode under test h as a
modulation bandwidth of appr oxi-
ma tely 1.5 to 2 GHz. The fre-
quency response a lso shows some
distinct resonances that will
impact th e time-doma in (step
or impulse) performa nce, as
shown in Figures 22 and 23.Measurement Procedu re
The measu remen t process is
virtua lly identical t o the laser
measur ement . An accura te
measurement r equires a user
calibration. This will allow the
LCA to remove the response of
the test system including the
electrical cables, optical fiber,
and the instr ument i tself. Prior
to the actua l calibration step,th e LCA needs to be configured.
This in cludes:
start and st op frequencies
sweep type (linear or
logarithmic)
number of measurement
points
measurement sweep t ime
sour ce power level
Note: LCAs have a Guided
Setup featur e tha t leads the
user th rough all the steps tha tar e described here. This is the
recommended measurement pro-
cedur e. Guided setup is a ccessed
by pressing the SYSTEM key
an d th e Guided Set up softkey.
The following text discusses th e
processes that the guided setup
executes.
To perform a s imple frequency
response calibration, th e con-
nections in Figur e 20 must be
made. The analyzer then mea-sures the appr opriate path s. The
frequency an d pha se responses
of th e un kn own pat h(s) is/are
then characterized. The analyzer/
system u ses this inform ation in
conjunction with the internal
calibration data to generate an
error ma tr ix. (The light wave
source and receiver chara cteris-
tics are predetermined and stored
in m emory. The st orage meth od
depends on t he t ype of LCA used.)
The end resul t i s that the f re-quency and pha se responses of
the ent ire test system are removed
from the measurement so that
th e displayed response is only
that of the ph otodiode under t est.
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13
After complet ion of the calibra -
tion, one might expect to see a
flat response at 0 dB indicat ing
the test system response has
been removed. When u sing anHP 8702, th e display seen u pon
completion of the response cal-
ibra tion pr ocess will not neces-
sar ily be a flat l ine. The O/E
receiver used in th e calibrat ion,
which is sti l l in th e measu re-
ment path , has become the DUT.
Thu s its response is now dis-
played. When th e HP 8703 cali-
bra tion is completed, no response
other th an n oise is displayed
until an O/E test device is con-
nected between t he electr icaland optical measur ement planes.
In a ddition to the simple response
calibrat ion, ther e are also the
response plus isolation and t he
response plus ma tch calibrations.
The isolation calibrat ion is used
for high-inser tion loss (low con-
version efficiency) devices, where
an y signal leakage within t he
instr u-ment m ay be significant
relat ive to the actua l signa ls
measu red. The ma tch calibration
is used to rem ove th e effects of
reflections between the instrument
electrical test port a nd t he ph o-
todiode under test. (The response
an d ma tch calibrat ion is only
available with the HP 8703 LCA.)
Once th e setup an d calibrations
have been completed, the instr u-
ment is now ready to make accur-
ate m easur ements. The receiver
to be tested is placed in the m ea-surement path and its response
can be seen, as in F igure 19
O/E bandwidth and responsivity
measurement, previously
shown.
Response an d
Match Calibration
The response and ma tch calibra-
tion is used to impr ove measu re-
ment u ncertainty when t he O/E
test device ha s a poor outpu t
mat ch. Impedan ce mismat ch
leads to s tan ding waves tha t
degrade th e measurement of
device responsivity. Typically,
th is problem is m ore pronounced
at higher modulation frequencies.
The response and match calibra-
tion uses network an alysis error
correction techniques to minimize
th e effects of misma tch. The cali-
brat ion requires a 1-port electr ical
reflection calibration in addition
to the thru tra nsm ission calibra-
tion for t he optical and electr icalpaths.
Figure 21 is a composite m easur e-
ment of a high-speed photodiode.
The lower tra ce is a measur ement
with only th e normal responsecalibrat ion. The upper trace, which
ha s lower ripple, is ma de using
the response and match calibra-
tion. The tr aces are intent iona lly
offset for cla rit y.
HP 8702 HP 8703
HP 8340XSource
HP 8341XReceiver
Figu re 20. O/Ecal ibration con-figuration
Figure 21. Responseand match cal ibration
The r esponse m atch calibration
can be executed by following
the steps in the Guided Setup
procedure.
Photodiode PulseMeasurements
To see what implications th e
device bandwidth and frequency
response have on the t ime domain
performa nce, the time domain
tra nsform can be used. This trans-
form uses t he measured frequency
response data t o predict the small
signal st ep and impulse responses
of the photodiode. (See Appendix 2,
"Operat ion in th e time domain;"
Basic considerations. )
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14
Figu re 22. O/Estep response
gat ing removes t he effects of
reflections and is discussed in
deta il in Appendix 2, "Opera tion
in th e time domain ;"Improvingmeasurement accuracy through
gating.
It is interestin g to compare th e
predicted time-domain response
with a tr ue time domain mea-
sur ement . Figur e 24 shows a
composite of the step response
generat ed by an LCA in com-
parison with the step response
when measured using a sharp
optical pu lse and a h igh-speed
HP 54120 oscilloscope.
The two measur ements agree very
well. It is importan t t o remember
that the oscilloscope measurement
displays the combined response
of the optical pulse, the oscillo-
scope, an d th e photodiode. The
LCA measurement can calibrat e
out the response of the test system
in order t o isolat e th e response
of th e DUT. The tr ace magnitu de
differences are du e to unequa l
instr um ent vertical scales.
Figure 22 shows the predicted step
response of th e sam e photodiode
whose bandwidth was measured
in Figure 19. There are severalpoints of interest. Th e tr an sition
from off to on or risetime (on
the order of 180 ps) is dependent
upon th e device ban dwidth
(roughly 2 GHz). There is some
ringing in the step response.
The frequen cy of th e ringing
corr elates d irectly to th e fre-
quency response resona nce at
3.2 GHz. Anoth er inter esting
chara cteristic is the seconda ry
step th at occur s roughly 600 ps
after th e initial step. This is dueto reflections with in th e device,
and is easier to understand by
viewing the impulse response.
Figure 23 shows the pr edicted
impulse response of the photodi-
ode using t he low-pass impulse
data tran sform. This measure-
ment provides several pieces of
informa tion. First, we see the
impulse width. The t ime between
the m ark ers is 123 ps at th e full-width ha lf-maximum points.
(This is due not only to the ph o-
todiode bandwidth , but a lso th e
finite bandwidth of th e instru -
men t its elf. The net pulsewidt his th e effective pulsewidth of th e
photodiode alone after r emoving
th e effect of th e inst ru men ts
bandwidth.) Another importan t
data point is noted by mar ker 1
at the pea k of th e response. This
value is 621 ps a nd is t he effec-
tive delay of th e ph otodiode or
in other words, the a verage
propagation t ime experienced
by the modulation signal from
the optical input to the electrical
outpu t. A second impulse is notedby mark er 2. This response is
due to an intern al reflection and
re-reflection. Th e r e-reflected
signal tra vels a longer distance
than the primary impulse, and
ther efore shows up with a rela-
tive delay.
Figu re 23. O/Eimpulse response
Figure 24. Compos ite
t ime domain mea-surements
The reflection in the photo-
diode has an adverse affect on
the frequency response of the
device. If this reflection could beremoved, the r esponse would be
improved. A technique called
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16
The dat a can a lso be displayed
simply as Retu rn Loss, th e ratio
of reflected to incident power
(10 Log (P re fl/ P inc).
Figu re 28. O/Ereturn lossmagnitude
Figure 29. Timedomain display of electrical reflec-t ions
Figu re 27. O/Ereturn loss inSmith chartformat
Measurement Procedu re
and Interpretation
The setup and measur ement
of photodiode ret ur n loss ar eidentical to th e procedure u sed
in characterizing laser r eturn loss.
See Laser inpu t impedan ce
on page 9. Figure 27 sh ows the
ret ur n loss of an optical receiver
measured with the component
an alyzer, displayed on a Smith
Char t. A Smith Ch ar t is a form
of an impedance map . The dis-
play shows the outpu t impedan ce
as a fun ction of frequen cy. For
this receiver, an electrical amp-
lifier follows the photodiode, sothe measur ed impedan ce is essen-
tially that of the amplifier. Over
the 6 GHz measurement ran ge,
the impedance stays reasonably
close to 50 Ohms (th e center of
the Sm ith Cha rt). The ideal case
would be for the impeda nce to
be a consta nt 50 (or 75) Ohms.
The Smith Chart data presenta-
tion is selected u nder the Format
key menu.
Using the time domain feature
of th e LCA can help to determine
the locations of any discontinu-
ities in the electr ical pat h of theph otodiode as sembly. Figur e 29
is a t ime/distance represent ation
looking back int o a ph otodiode
assembly. (This is the same pho-
todiode measu red on pa ges 12
to 14, where it wa s shown in a
tra nsmission measurement th at
there were significant reflections.)
SMAConnector
Joint50 Ohm
Transmission Line
PhotodiodeAssembly
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17
Optical Componen ts(O/O): Tran sm iss ionand Re f lect ion
Measurements
Consistent, accurate measure-
ments of multi-mode fiber band-
width ar e difficult to achieve using
an LCA, principally due to th e
fibers inherent instability in m ode
stru ctur e and distribution. How-
ever, frequency response data can
be used in a time domain forma t
to yield pr ecision length an d
propagation delay tra nsm ission
measurements and high-resolu-
tion reflection measu remen ts.
TransmissionMeasurements
Fiber Leng th andPropagat ion Delay
In the following example we want
to determine the length of a sec-
tion of singlemode fiber. The mea-
surement will be made by using
a modulated optical signa l with
a swept modulation frequency.
The ran ge and resolut ion a redirectly dependent upon the m od-
ulation frequency bandwidth and
the nu mber of measur ement
point s. A useful tool built into the
HP 8702 and HP 8703 that assists
in making time domain measure-
ments is th e tra nsform param -
eter s function. See Appendix 2,
Operat ion in th e time domain;"
Transform Param eters.
Measurement Results
and InterpretationFigure 30 shows the result
of th e fiber tr an smission m ea-
surem ent displayed in th e time-
domain. The frequency-domain
data has been t ran sformed to
predict t he impu lse response of
th e fiber.
Figure 30. Impulseresponse of a lengthof fiber
Placing a marker a t the peak
of the pulse indicat es th e propa-gation time th rough th e fiber,
24.429 ns. If we know the index
of refraction, we can calculat e th e
physical length of th e fiber. Con-
versely, if we kn ow the p hysical
length , we can calculat e th e fibers
index of refraction. The impulse
width is due t o the finite ban d-
width of the LCA and not th e
fiber itself.
Measurement Procedu re
The measurement setup is
stra ightforwar d. The swept m od-
ula ted optical source is connected
th rough a short piece of fiber to
the inst ru ment s light wave
receiver. A mea sur ement cali-
bration is required to remove the
tra nsmission path length an d
frequency response er rors of theLCA sour ce and receiver.
Care must be tak en in setting
the instrum ent sweeptime and
IF ba ndwidth , part icular ly for
long devices. This is because
th e LCA tun ed receiver cont in-
ues to sweep while the stimu lus
signal is delayed through th e
fiber. The m inimum sweeptime
for a given device delay is det er-
mined by the combination of IF
bandwidth, number of measure-ment point s, and th e frequen cy
span . There are no simple rules
to follow in set tin g th e critical
parameters. The best procedure
is to set th e sweeptime to a large
value, such a s 10 seconds, with
th e DUT conn ected, prior to per-
form ing a calibra tion. (If th ere is
no response, the sweeptime may
need to be increased furt her ). The
sweeptime is sequent ially reduced
un til the response cha nges. The
sweeptime is then increased backto a level giving a st able mea-
surement .
Figu re 31. O/Ocal ibration s etup
HP 8702 HP 8703
HP 8340XSource
HP 8341XReceiver
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18
Once th e measur ement calibra-
tion h as been performed, th e test
fiber can be conn ected between
th e short fiber and th e test sys-tem. The initial measurement is
ma de in the frequency domain.
Actual length measur ements are
determined through t he t ime-
domain t ran sform. Measurement
accuracy is discussed in Appen-
dix 2, Operat ion in th e time-
domain.
Fiber ModulationPhase Stabil i ty
In cert ain fiber optic microwave
link applications, it is importan t
for t he m icrowave signal to ha ve
a very stable phase response
relative to other signals propaga-
ting on different fibers or through
different media. If the index of
refraction varies with temper a-
tur e, or some other environmen tal
parameter, the carrier (l ight)
velocity an d th us t he m odulat ion
envelope will experience a rela -
tive phase sh ift.
Because we ar e attempt ing to
measure a change in the fiber
char acter is t ics , the setup a nd
calibra tion procedur es ar e dif-
ferent t han for m ost measu re-
ment s. In th is case, we calibrate
the instru ment with the fiber
un der test conn ected to the
instrument .
HP 8703
Figure 32. Pha sestability calibration
With th e fiber under t est in
place during th e calibrat ion, we
effectively rem ove an y respons e
present in an ambient environ-ment. Care must be taken th at
effects other tha n th e param eter
of interest (for example t emper-
ature) do not impact the measure-
ment. For instance, any bending
of th e cable after calibrat ion can
cause a change in the pha se
response.
For this measurement, the
device un der test is a 10 km
spool of fiber. The mea sur emen t
is made with a CW modulat ionfrequen cy of 10 GHz. In stea d of
sweeping frequency, th e m easur e-
ment is ma de over a 16 minute
time span. It can be seen t hat
the modulation phase response
does vary significantly with time.
In this measurement , the rela-
tive phase response begins a t
roughly 60 degrees (some phase
change ha s already occurred
between the time the calibration
was completed and t he measu re-
ment began). The pha se contin-ues t o cha nge to 180 degrees,
where th e an alyzer rolls over
to +180 degrees. For the given
time span , the total variation is
appr oximately 150 degrees.
As shorter lengths of fiber ar e
examined, the phase response
variance versus time will become
sma ller. However, other pa ra m-eters such as temperatu re or
physical stress can cause ph ase
variat ion, even over short ru ns
of cab le.
Reflection Measurements
In a high-speed fiber optic
system, reflected light can cause
a variety of problems and come
from several different sources.
Both distributed feedback (DFB)
and Fabry-Perot lasers are sen-sitive to light reflecting back int o
their resonant structures. Both
noise an d modulation chara cter-
istics can be degraded. In a com-
munication system, re-reflected
light can arr ive at th e receiver
an d potentia lly cause bit errors.
To minim ize th ese effects, it is
importa nt to char acter ize the
am ount of light th at is reflected
off of optical componen ts an d
determ ine where t he r eflections
occur.
Methods for Measu ringLightwave Reflectionsvs. Distance
In component development it is
often n ecessary to determine t he
physical location of the r eflection.
If ther e ar e mu ltiple reflections,
we mu st determine which reflec-
tions contribute significantly to
the total a mount of reflected light.
There are a variety of methods for
measu ring reflected light versu s
distance or position. Among t hesemethods are optical time-domain
reflectomet ers (OTDR), optical
coher ence-domain reflectometer s
(such as the HP 8504A precision
reflectomet er), and optical fre-
quency domain reflectometers
(OFDR). The LCA uses the OFDR
Figu re 33. O/Ophase measure-ment vs. t ime
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19
HP 8702
Test Port
HP 8341XReceiver
HP 8340XSource
HP 8703
TestPort
HP 11890/1 Figure 34.OFDR setup
technique. Each technique has
advanta ges an d disadvant ages.
(When m easur ements of total
retur n loss are required, withoutspatial informa tion, a power m eter
solut ion su ch as the H P 8153A
is used.)
Determining both th e ma gnitude
an d locat ion of reflections in light-
wave components require t ech-
niques beyond th e capabilities
of a multimet er or conventional
OTDR.
The LCA is well suited for
ma king h igh resolution reflec-
tion measu remen ts of light wavecomponen ts. Th e LCA does not
use a pu lse techn ique and con-
sequently does not suffer from
deadzone problems typical of
OTDRs. Instead, a wide band-
width swept frequency technique
is used, which leads to precision
location a nd resolut ion of each
reflection.
The set up for a r eflection
measurement requires that
th e light wave source be rout edto th e input of a dir ectional cou-
pler. The DUT is conn ected to
the coupler output a rm. The
coupled ar m is connected t o the
LCA receiver.
The r esolution of th e LCA in
OFDR mode is dependen t u pon
the modulation frequency range.
The wider the bandwidth, thehigher is the two-event r esolution.
The closest t ha t t wo reflections
can be and st ill be resolved is refer-
red to as response resolution. (See
Appendix 2, Operation in the
time domain;Basic considera-
tions.) A 20 GHz instr umen t band-
width can pr ovide 5 mm of two-
event resolution wh ile a 3 GH z
bandwidth can provide 33 mm
(in fiber). If higher resolut ion is
required, th e HP 8504 precision
reflectometer offers better tha n25 micron 2-event resolution. Mea-
surement sensitivity is enhan ced
thr ough trace averaging and set-
ting the LCA IF bandwidth to a
low value, su ch as 30 H z. This
usually slows the measurement
rate, but will reduce the effects
of noise. Sm aller reflections can
then be seen.
Once the frequency range ha s
been set, a calibration must be
perform ed. The simplest cali-brat ion is achieved by using th e
open-ended test port a s a Fresn el
reflection stan dard. This assu mes
tha t t he port is polished, clean,
an d in good condit ion. With t his
calibrat ion sta nda rd in pla ce,
the a nalyzer measur es the light
reflected off the test port as th e
frequency of modulation is sweptover th e selected bandwidth.
Thu s, the frequency response
imper fections of th e LCA are
ma th emat ically removed from
the measurement .
Figure 35 shows the reflections
from a light wave cable consisting
of th ree pa tchcords with simple
PC conn ectors. The ma gnitude
of th e r eflection for each conn ec-
tor is easily seen. Settin g the
index of refra ction t o 1.46, an dusing the marker functions, the
length of each pa tchcord can be
determ ined, at 1.514m, 1.761m,
an d 1.756m r espectively.
Figure 35. Multipleref lect ion measu rement
Measur ement accur acy is dis-
cussed in Appendix 2, Opera tion
in the time domain.
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(The sweeptime considerations
discussed in O/O tr an smission
measurements are even more
critical here, since the signal istr aversing the length of the fiber
twice before being detected.)
98.513 s of electr ical dela y is
added, and while in t he t ime
mode, the spa n is reduced to
show two reflections very close
together. These are th e Fr esnel
reflections at the two coupler
outpu ts, which ar e measured to
ha ve a different ial path length
of 18 mm . The t wo-pass mea -
surement technique has pr o-
vided m illimeter resolut ion atth e end of a 10 km cable.
Note: This technique is sus-
ceptible to alias r esponses. The
reflection a t t he inst ru men ts
test port, or where t he coupler
is conn ected to the fiber spool, can
potentially show up in a nother
region du e to th e limitations of
th e tr an sform. To determine if a
response is real or an alias, the
num ber of measurement pointsshould be chan ged and th e mea-
surement repeated. True events
will ma intain their locat ion, wh ile
alias events will move.
20
There are l imitations in the
OFDR technique. The higher
th e two-event resolution, the
smaller th e overal l measur e-ment range. For instance, the
20 GHz configuration with 201
measu remen t point s offers th e
best two-event resolution (5 mm
in fiber), but t he one-way ran ge
is only 1 meter. The mea sure-
ment ran ge can be increased by
increasing the nu mber of mea-
sur ement p oint s, or decreasing
the instruments frequency range,
which will in tur n degrade t he
two-event resolution. (See
Appendix 2, Operat ing in t hetime domain.)
Achieving Both HighResolut ion an d Long Range
Some measurement scenarios
require both high r esolution and
long range. This can be achieved
using t he LCA in a 2-pass
measurement technique. The
ana lyzer is first set up in a nar -
row bandwidth mode that pro-
vides a long enough ra nge to
locate th e region of inter est. Thepropagation time to the ar ea of
interest is deter mined. The LCAs
frequency range is then widened
to provide the two-event resolu-
tion required t o isolat e th e indi-
vidua l reflections. The electrical
delay equal to th e propagation
time t o the r eflections is added
to the measurement (using the
electrical delay function under
th e Scale Referen ce key). This
effectively pu lls th e r eflections
of inter est into the instr um ent sreduced range.
A High-resolution Measurem ent
of Differential L ength
To demonst ra te th is procedure,
a long s pool of fiber with a 1X2coupler at t he end was m easured.
The different ial length of the two
arm s of the coupler is t he desired
measurement.
The first ta sk is to locate the
coupler a t t he end of the fiber. The
spool is estimated to be 10 km
in length. The measurement span
is configured t o provide 12 k m of
ran ge. This requires a frequency
spa n of only 2.5 MHz. After a
response calibrat ion similar totha t described above, th e spool
an d coupler a re th en conn ected
to the test port, and the measur e-
ment of Figure 36 is generat ed.
Two reflections a re seen . One
at time 0, corr esponding to th e
fiber connection to the instrument,
and another over 98.513 s (two-
way) or 10.114 km (one way), at
the cable end.
The t ask is now to zoom in on
the cable end an d examine the
reflections in high resolution
mode. The an alyzer ban dwidth
is increased to 20 GHz an d placed
in set freq low pass mode. The
ana lyzer is then recal ibrat ed
with t he DUT disconnected.
Figure 36. OFDRmeasurement inwide span
Figure 37. Zoomin gin on the cable end
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21
Electrical Compone ntMeasurements (E/E)
Lightwave component analyzershave t he capability to operate a s
RF an d microwave network ana -
lyzers. They can th en be used t o
chara cterize the electr ical compo-
nent s used in lightwa ve systems
including amplifiers, filters, coup-
lers etc. For tu torial informa tion
on RF an d microwave network
an alysis please refer to the HP
"Vector Semina r" booklet (HP lit-
erature number 5954-8355.)
Appen dix 1:Signal Relat ionships inOpto-electric Device s
Signal Relationsh ipsUsed in ComponentMeasurements
The LCA measurement technique
is built upon concepts u sed in
cha racterizing RF and m icrowave
devices. S-para met er or s cat-
tering matr ix techniques have
proven to be convenient ways to
chara cterize device per form an ce.
The following section will discuss
how similar techniques ar e usedin char acterizing devices in t he
lightwave domain. This is intended
to show the basis on which E/O
an d O/E responsivity measu re-
ments are defined.
Figure 38 is a general represen-
ta tion of a l ightwa ve system,
showing input and output signals
in term s of term inal voltages,
input and output currents, and
optical modulat ion power.
S-para meters are used todescribe the tran smitted and
reflected s ignal flow with in a
device or network . For the model,
the following S-par am eters a re
defined:
S11=b1 (a2= 0)a1
S22
=b2 (a
1
= 0)a2
where:
a1 =V1 incident on E/O deviceZ0
= I1 Z0
b1 =V1 reflected from E/O deviceZ0
a2 =V2 incident on O/E deviceZ0
b2 = V2 transmitted from O/E device
Z0
= I2 Z0
It is interesting to note tha t
delta volta ges and curren ts
ar e used a s opposed to RMS
values. Th is is done because we
deal with m odulation signals in
describing lightwa ve tra nsdu c-
ers, where a cha nge in optical
power is proportional to a change
in electrical current or voltage.The overall system forwar d gain
is defined as:
S21 =b2
(a2= 0)a1
S12 = 0 (no reverse transmission
is assumed)
5050
PI
50
PO P
2
I2
II
PO
E/O O/E
PO
f
Rr(A)
WR
s(W)
A
50
Figure 38. Signaldefini t ions
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Data Subject to ChangeCopyright 1992Hewlett-Packard CompanyPrinted in U.S.A. 4/975091 6478E
With the t ime-domain tra nsform
turned off, the gate function may
rema in a ctive. The frequency
response is now shown, butwith th e effect of th e reflection
removed. It is apparent th at t he
reflection has a significant effect
on the frequency response. Thus,
gating p rovides a useful tool to
simulat e the resu lts of actu ally
removing unwanted responses.
The t ime-doma in gat ing function
acts a s a time ban dpass filter
tha t passes the primary response
an d removes the responses dueto r eflections. Once t he reflec-
tions ha ve been gated out , th e
measurement can be returned
to the frequency doma in. The
frequency response displayed is
as if th e reflected signals wer e
no longer presen t.
Figure 39 shows a photodiode
response th at is degraded due
to int erna l reflections.
Analyzing the r esponse in t he
time domain, the secondary
impulse is determined t o be
du e to a reflection.
Using th e gating function (part
of the tra nsform m enu), the time
gate or filter is centered an dthe spa n a djusted to reject a ll
but th e primary response. The
gate center is noted by the T
an d width by the two flag ma rk -
ers. The gate is turned on, and
the reflection response is
removed.
Figure 39.Degraded fre-quency response
Figure 40. Timedomain response(with ref lect ions)
Figure 42.Frequency responsewi th and wi thoutgating act ive (gatedtrace is offset)
Figu re 41. "Gated "time domainresponse
For more information aboutHewlett-Packard test and measure-ment products, and for a cu rrentsales office l isting, visit our website, http:www.hp.com/go/tmdir.