The document discusses various detection techniques in coherent optical communications, focusing on modulation methods like OOK, PSK, and FSK. It highlights the performance comparison of different systems including homodyne and heterodyne detection, as well as their respective error rates and required sensitivities for optimal receiver performance. Additionally, it addresses the impacts of laser line width and the use of error control techniques to enhance receiver sensitivity.

1. 1
Detection Techniques in
Coherent Optical
Communications
MEC

2. 2
Contents
• Introduction
• Modulation.
• Direct Detection OOK.
• OOK Homodyne System.
• PSK Homodyne System.
• Heterodyne Detection.
• Performance Comparison.
• Receiver Sensitivity.

3. 3
Coherent Light Wave System

4. 4
Modulation Techniques
Modulation:
• Amplitude Shift Keying – On/Off Keying.
• Frequency Shift Keying
• Phase Shift Keying.
Detection:
• Homodyne/Heterodyne Detection
• Synchronous/Asynchronous Detection.
• Choice of Modulation/Demodulation
determines receiver sensitivity.

5. 5
Modulation Formats

6. 6
Direct Detection OOK
• Equal Probability 1/0 - OOK data stream is
ON only half of the time on an average.
• Required number of photons per bit of
information is half the number required per
pulse.
• If and O electron-hole pairs are created
during 1/0 pulses, average no. of photons
per bit for unit quantum efficiency,

7. 7
Direct Detection OOK
• Chance of making an error:
• For direct detection OOK, about 10
photons per bit are required to get a BER
of 10-9.
• Difficult to achieve!, amplifiers and
detectors add thermal noise and shot
noise.
• Required received power level b/w 13 and
20 dB.

8. 8
Probability of Error
• Probability of error in decoding of any bit
where the error function
• Pe depends on peak signal to rms noise
ratio V/σ where V – signal amplitude, σ –
rms noise.

9. 9
OOK Homodyne Systems
• When a 0 of pulse duration T is received,
average number of electron-hole pairs
created is the number generated by local
oscillator

10. 10
OOK Homodyne Systems
• If voltage seen by the decoder
during a 1
• Bit Error Rate

11. 11
OOK Homodyne Systems
• To achieve BER = 10-9, V/σ = 12.
• Expected number of signal photons
created per pulse, As
2T = 36.
• Average energy of each pulse must
produce 36 electron-hole pairs.
• For equal probability of 1’s and 0’s,
average number of received photons per
bit of information = 18.

12. 12
OOK Homodyne Systems
• BER for OOK homodyne detection,
• For x ≥ 5,

13. 13
Homodyne Receiver
PSK Homodyne Receiver
Best theoretical receiver sensitivity
Most difficult to implement.

14. 14
PSK Homodyne Receiver
• Incoming optical signal (AS) combined with
local oscillator signal (ALO) using a fiber
directional coupler / beam splitter.
• Incoming signal weaker than LO signal.
• Beam splitter made almost completely
transparent.
• Phase change to denote bit change : AS
and ALO out of phase for 0, in phase for 1.

15. 15
PSK Homodyne Receiver
• For a 0, resultant no. of electron-hole pairs
generated,
• For a 1,
• Voltage seen by the decoder in the
receiver,
• rms noise
• For BER = 10-9, V/σ = 12, ALO
2T = 9 (η =
1).

16. 16
PSK Homodyne Receiver
• An average of 9 photons required to
achieve a BER of 10-9.
• PSK optical signal is ON all the time.
• For PSK homodyne detection,

17. 17
PSK Heterodyne Receiver –
Synchronous Detection
Photodetector output at IF.
18 photons/bit for a BER = 10-9
Microwave PLL generates local
reference phase
Baseband

18. 18
PSK Heterodyne Receiver-
Asynchronous Detection
20 photons/bit for BER = 10-9
Differential PSK – mixer to produce
positive or negative output
depending on whether there is a
phase change from the previous bit.

19. 19

20. 20
photons/bit

21. 21
Comparison
• Synchronous heterodyne OOK detection is 3
dB less sensitive than homodyne OOK,
• 36 photons/bit for BER = 10-9.
• Asynchronous OOK detection,
• 40 photons/bit for BER = 10-9, 3dB less
sensitive than DPSK.

22. 22
Comparison
• PSK gives best sensitivity for narrow line
widths.
• When line width >0.2% of the bit rate, PSK
sensitivity degrades quickly.
• FSK and OOK can be detected using
optical power measurement not sensitive
to phase noise, good performance
sensitivities below 60 photons/bit for line
width to bit rate ratios ~ 1.

23. 23
Solutions
• Laser line width limits system performance
when using PLL for simple transmission
codes.
• Error control techniques to relax laser line
width requirements and improve receiver
sensitivities.
• Receiver sensitivity increase (1 – 10 dB)
depending on code constraint length.

24. 24
Calculated Quantum Limited Receiver Sensitivity

25. 25
BER Improvement for (2,1)
Convolutional Codes
30 photons/bit

26. 26
Thank You