20ghz dqpsk optical modulator electrical bias optimiser
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Engineering
Self-Adjusting Serrodyne Loop Optical Frequency ShifterBenjamin LarcombeAcademic Supervisor: Professor A. Lowery
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 2
What is an Optical Serrodyne Loop Frequency Shifter? (Recap)
An optical Single Sideband Spectrum generator which uses one single Mode laser source.
Generates a spectrum of optical carrier frequencies separated by a fixed but variable frequency space.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 3
Motivation• To be able to produce an optical spectrum of closely spaced
carriers (subcarriers of a WDM channel) and modulate a single stream of data onto these subcarriers.
• Why?
• To test whether high data rates are achievable when subcarriers are more closely spaced together
• Because Using multiple modulators to independently modulate data onto each subcarrier, for lab testing purposes is totally unnecessary and extremely inefficient.
Background Theory An IQ (In phase/quadrature) DQPSK type Modulator can
1. Be DC biased at its three DC Bias Terminals
2. And Have two RF Quadrature Signals (Sin and Cos) applied to its RF terminals
Such that an Optical Frequency Shift can be produced at the DQPSK’s modulator Output.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 4
Frequency Shift produced by Linear time variant Phase shift Around IQ
plane.
DQPSK Optical Modulator
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 5
RF A
180° Bias A
-RF B180°
Bias B
-RF A
RF B
90° Bias C
Phase Shifts at Bias A,B, and C are controlled by Bias Voltages.
Optical Laser Input
Optical Frequency
Shift Output
When Bias Inputs A and B are optimized Output
Power Is Minimized, Assuming RF is Inactive
or 0
When two low frequency quadrature Dither Signals are applied to Points A and B, two
low frequency optical components are seen at the
Output, the dither signal frequency and twice its
frequency.
When Bias Point C is optimized, the second harmonic produced by the dither Signal Is
Minimized,
Upper Arms
Lower Arms
Optical Couplers
DQPSK Modulator Non Idealities A real nested DQPSK Modulator or Nested Mach Zehnder
Interferometer
– Includes Insertion Losses through its structure due to• Non-Ideal Optical Coupling/Decoupling• Optical path Losses
– Has a non-infinite extinction ratio (power is not equally balanced between the upper and lower arms).
– Is highly dependent on temperature. Over time DC Drift Voltages can effectively move the bias voltages away from the Optimised Points.
Thus the aim of this project is to keep DQPSK modulator biased so that a clean Optical Frequency Shift can be produced over a large time span.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 6
What was done this year?
1. A number of different methods that achieve biasing of a DQPSK Modulator were researched. The chosen method was then simulated in Computer Simulation using VPI software for verification.
2. Design and Development of a proposed system which includes a DE2 board a soft-core NIOS II processor that can achieve the required biasing method.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 7
What was done this year?
3. To test the proposed optimisation method using the developed system with the required RF signalling and Optical Power and Spectral Monitoring Devices.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 8
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Biasing Methods Study
• As mentioned, a number of different biasing methods were researched. One low cost method was found that achieved biasing for a IQ DQPSK Modulator used to modulated QPSK Data.
24th October 2012 9
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Biasing Technique
• “Asymmetric dithering technique for bias condition
monitoring in optical QPSK modulator”[1].
24th October 2012 10
Biasing technique uses two asymmetric
orthogonal low frequency sin wave dither signals with DC offset Voltage Controls at the three DQPSK Bias Points.
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Biasing Technique
24th October 2012 11
Using the two dither signals, biasing of Points A and B could be achieved by monitoring the output power of the dither signals frequency. The third bias Point C could be biased by monitoring twice the dither signals frequency (2nd order harmonic.
“These two biases (A and B) are easily monitored by using a first-
order lock in amplifier, because the optical power of the QPSK signal
strongly depends on BIAS1 (A) and BIAS2(B).”[1]
“Accordingly, cos(φ3) (Bias Point C) can be detected by the second-order lock-in Amplifier and Optical Power
Monitor.”[1]
Simulation
1. VPI Design of the DQPSK Modulator
2. Simulation of the DQPSK Modulator Under different DC Biasing Conditions using a TCL Script.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 12
Simulation Results
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 13
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Peak-to-Peak Amplitudes from Simulation
24th October 2012 14
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.50
1
2
3
4
5
6x 10
-6
Bias Voltage Drift Amount from Optimised Value
Ave
rage
Opt
ical
Out
put
Pow
er (
W)
Bias Dithering Sweep Characterstics for Points A,B and C
Converting the previous slides Simulation Waveforms into Peak-to-Peak Amplitudes Results. Thus Finding the Minimum Output Power for each point A, B and C Results in finding the Optimised Bias.
Proposed System Design
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 15
NIOS II Soft-Core Processor.
-Floating Point ALU
-Digital Band-pass Filtering
-DAC Voltage Offset Control
-Bias Control AlgorithmDAC C
0-1.3V
WM8731 16-Bit ADC with 32 kHz Sampling
DAC A 0-1.3V
DAC B 0-1.3V
Verilog HDL
Quadrature Dither Signal
Generators
Verilog HDL 4096 by 16 bit
RAM, Sampling
Buffer
The Input into the ADC for the project was an Optical Multimeter. This Optical Multimeter has
an Optical Power Meter with a bandwidth of 4kHz and an Output voltage range of 0 to 2V.
24th October 2012Self-Adjusting Optical Serrodyne Loop Frequency Shifter 16
Complete System Overview
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Results Vs Simulation
24th October 2012 17
0 50 100 150 200 250 300 350 4000
100
200
300
400
500
600
700
DAC A Electrode 10-bit Value
Pea
k-T
o-P
eak
Ave
rage
Pow
er
Dither Sweeps of Voltage Point A,B and C with Optical Power Monitoring of the first Harmonic (Dither Signal Frequency) for points A and B and Optical Power Monitoring of the second Harmonic (twice the dither Frequency) for Point C.
0 50 100 150 200 250 300 350 4000
100
200
300
400
500
600
700
800
DAC B Electrode 10-bit Value
Pea
k-T
o-P
eak
Ave
rage
Pow
er
0 50 100 150 200 250 300 350 4000
100
200
300
400
500
600
700
800
900
1000
DAC C Electrode 10-bit Value
Pea
k-T
o-P
eak
Ave
rage
Pow
er
Bias A Bias B Bias C
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Self Biasing Algorithm
1. On Start-up Bias Voltage Optimiser for Points A, B, C by finding minimum peak-to-peak positions over large range of DAC values.
2. Bias Voltage Optimiser with smaller range around previously found optimised Values.
3. After Optimised Bias Values are found, dither signals are turned off and DC values Set.
4. If Optimisation is required later because of DC drift at the bias points only a push of a button is needed.
24th October 2012 18
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Manual Tuning Requirements
• While the DC bias Tuning works, self tuning the of quadrature RF delay line is required; however find the DC optimised Values reduced the amount of time to tune the DQPSK Modulator Significantly.
24th October 2012 19
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Results with NO RF
For bias Voltage Optimisation to be successful, if the RF is off, the output power of the DQPSK modulator should be minimised, as both the top and bottom DQPSK arms cancel themselves Out.
24th October 2012 20
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
RF off Before Bias Optimisation.
24th October 2012 21
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Output Spectrum before Biasing with RF OFF
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
RF Off After Bias Optimisation
24th October 2012 22
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Output Spectrum After Optimisation with RF OFF
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Results With RF ON
For Bias Voltage Optimisation to be successful with the RF turned On, a single side band optical frequency shift should be achieved while minimising the laser’s original frequency and also the other harmonics.
24th October 2012 23
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Results With RF ON before Optimisation
24th October 2012 24
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Output Spectrum without Biasing with RF ON
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Results with RF ON After Optimisation Before RF Delay Line is Tuned
24th October 2012 25
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Optimised DC biasing with RF before RF Delay Line is Optimised
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Results with RF ON After Optimisation After RF Delay Line is Tuned
24th October 2012 26
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Optimised DC biasing with RF after RF Delay Line is Optimised
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Further Results After 1 Hour of Bias Optimisation
As mentioned, due to DC drift Voltages which move the bias voltages away from the optimised position, Self-Adjustment or tuning is required Over time.
24th October 2012 27
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Further Results After 1 Hour before Bias Optimisation
24th October 2012 28
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Output Spectrum After 1 Hour before Optimisation
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Further Results After 1 Hour After Bias Optimisation
24th October 2012 29
193.085 193.09 193.095 193.1 193.105 193.11 193.115-70
-60
-50
-40
-30
-20
-10Output Spectrum After 1 Hour after Optimisation
Optical Frequency [THz]
DQ
PS
K O
utpu
t P
ower
Spe
ctru
m [
dBm
]
Laser Centre Frequency
Output Spectrum
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Conclusion
Results and Simulation both agree and the self-Adjustment DQPSK Bias system work at achieving a single Optical Frequency Shift.
However further testing is required when the Serrodyne Loop is active.
24th October 2012 30
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
Future Improvements
The designed System’s DAC’s only have 10-bit resolution. To achieve finer tuning DAC’s with higher resolution would be required.
The Optical Power Multimeter Device has a different resolutions and a small bandwidth. An Optical Photodiode Low Frequency Monitor may again achieve a finer tuning accuracy.
24th October 2012 31
Self-Adjusting Optical Serrodyne Loop Frequency Shifter
References [1] H. Kawakami, E. Yoshida, and Y. Miyamoto, “Asymmetric dithering
technique for bias condition monitoring in optical QPSK modulator,”Electron. Lett., vol. 46, no. 6, pp. 430–431, Mar. 2010.
24th October 2012 32
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