1. Introduction to Fiber Optic Communication
Lecture VII
Introduction to Fiber Optic Communication
Ver 2
COHERENT DETECTION
Moshe Nazarathy All Rights Reserved

2. Coherent detection SNR limits (analog)
Analog coherent
Homodyne transmission:
Instantaneous SNR eval
(t-dependence dropped)
I&D IDEAL
PHOTON COUNTER LO
SNR (sig. pwr / shot-noise var)
at the output of a W Hz
LPF passing the signal
So, what’s the Big Deal?
but…the coherent performance
is practically achievable, DD performance is not !
Coh. Det. overcomes receiver thermal noise <<shot-noise
just better
(a factor of 4
in SNR)
<<to add “analog” SNR
for OADD and heterodyne
SYN/ASYN>>

3. Coherent detection – some advantages
Some key advantages of coherent optical communications:
Direct access to the received electric field, linearly accessible by optically coherent downconversion of the received bandpass optical field.
Availability of the field enables electronic (digital) mitigation of channel impairments (CD, PMD, NL)
Improved sensitivity with the LO power acting as a gain, in effect boosting the signal prior to electronic detection (overcome thermal receiver noise).
Improved frequency selectivity, allowing to use electrical filters in the RF domain to remove the noise around the optical carrier and sharply suppress adjacent optical channels in a DWDM system.

4. Coherent detection – some disadvantages
Needs more coherent lasers – lower linewidth
More complex receiver, requiring to mitigate the phase wander of the optical source and the fluctuations of optical polarization
Disadvantages mitigated by modern DSP

5. The Coherent Receiver Front-End: A linear Opto-electronic Downconverter

6. Building block for coherent and differential detection: The Balanced Optical Mixer
Assume signal and LO have same freq. - homodyne
Initially address a single polarization
(scalar treatment)
-port
“mixing product”
(*)
coupler
Proof:
Proof: Substitute in (*)

7. A pair of BALANCED optical mixers in quadrature - called optical hybrid implements the complex MIXING PRODUCT
mixing product

8. Coherent Homodyne Receiver Front-End (e.g. for QPSK)
optical hybrid
Coherent Homodyne Receiver Front-End (e.g. for QPSK)
Local
Oscillator (LO)
Phase
Info
Let i.e. assume the LO is aligned with the signal phase reference
(real axis of the signal constellation)

9. Polarization Diversity Hybrid
Opto-Electronic DownConverter xI
Signal x
Single-Polarization Downconverter I y xQ yI x
Single-Polarization Downconverter II yQ LO y
PBS
Polarization
Beam Splitter + _
coupler
Single-Polarization Down-Converter (Optical Demodulator)

10. Putting it all together: Coherent Receiver block diagram (homodyne or intradyne)
Sig. & LO
have nearly
the same freq.
ADC
ADC
DSP
ADC
ADC

11. Coherent
Receiver
with
Integrated
Optical
Front-end

12. Homo/Hetero-dyne detection with balanced Optical Mixer
SIGNAL & LO at same frequency (homodyne)
-port
“mixing product”
coupler
Now let SIGNAL & LO be at different frequencies (heterodyne)

13. Balanced coherent receiver with electrical quadrature demodulation and electrical/optical PLL
“Optical Voltage-Tuned-Oscillator”
Tunable laser
FIXED
VTO
Actually
decision-directed
PLL
Optical PLL
Note: Single-lane scalar version
Assume that a polarization controller rotated the input polarization signal to be
parallel to that of the LO. Alternatively, this is one of the two polarization lanes
of a polarization diversity scheme

14. Putting it all together “Classic” coherent heterodyne receiver
Each polarization lane feeds an electrically coherent
receiver extracting the IQ components by electrical downconversion
with cos/sin subcarriers

15. Coherent Homodyne BPSK Receiver
In this case the 2nd quadrature
is not necessary
as the noiseless part of
does not contain an imaginary part.
Assume that was tuned to be
real-valued (i.e. in phase or in anti-phase
with the possible values of

16. Binary Differential Phase Shift Keying (BDPSK)
Extract PD
The optical mixer
becomes a key
building block
in optical DPSK
realization
Differentially Coherent Detection T
DELAY
INTERFEROMETER
(DI) FRONT-END

17. (a) * (b) * Previous symbol DPSK reference Current symbol
Differential
vs. Coherent
Detection
Previous symbol
DPSK reference
Current symbol
(a)
DPSK DETECTION LO
LIGHT
SOURCE
(b)
COHERENT DETECTION * *

18. T T I-port Q-port QDPSK receiver front-end
The bias effects a rotation of the constellation: Typically

19. QDPSK receiver front-end
I-port T T
Q-port

20. Homodyne/Intradyne Coherent Receiver Technology considerations
X-pol.
Y-pol.

21. Coherent Transmitter block diagram Technology considerations
Alternative
View

22. 100G Coherent Polarization-Muxed QPSK (PM-QPSK) is the next step
Two phase DOFs and two polarization DOFs: 28 Gbaud operation
Parallel transmission of 28Gb/sec on each quadrature of each polarization: 4 parallel lanes
112Gb/s  2 polarizations 56 Gb/s each, QPSK (2 bits/sym), 28Gsym/sec

23. A formulation of COHERENT DETECTION MODELING and error probability performance - suited for communication engineers

24. Equivalent electrical circuit for optically coherent detection
Consider a coherent detection receiver comprising a strong optical LO,
which either tracks the frequency and phase of the incoming signal
(homodyne (HOM))
or is locked with an offset to the incoming frequency (heterodyne (HET)).
Perfect polarization tracking is further assumed for the purpose
of determining the ideal quantum limit sensitivities.
Associated with such a coherent receiver
is a conceptual black box with input consisting of
the real-valued bandpass received electric field
and with output taken as the mixing (signal x LO)
signal current component of the photo-detector
The I/O transformation defined by this black box
(input field to output current), is linear –
a mixing photocurrent component is generated
with complex envelope proportional to the real part of
Self-read
Opt RX
Front-end
+filtering LO
the noiselessly received analytic signal component of the electric field,
downconverted to the IF frequency (or complex envelope U/C to IF):

25. Coherent detection model (HOM/HET)LO
I&D IDEAL
PHOTON COUNTER
Coherent Gain
(LO boosting) factor
HET:
Just set
in the HET result
Homodyne:
HOM:

26. Full optical demodulator - 90 deg balanced hybrid – heterodyne
Coupling matrix
Signal is atten.
thru the coupler
but sig. currents
add-up
in amplitude + _
coupler
Same factor of 2
as in the single-ended
Single-Polarization Single-Quadrature Down-Converter
(Optical Demodulator)
Noise from
the two PDs
adds up
incoherently
doubling
in noise
power
Relative to a single-ended detector,
the SNR at the balanced detector differential output is halved (assuming same # of signal photons at input)
as sig. gain did not change, while noise doubled
However, setting same # of photons at the PD in both cases,
the SNR is double (due to the coh. sig. add.)

27. Full optical demodulator - 90 deg balanced hybrid – homodyne
Half the single-ended case
(and the DD terms
cancel out)
means phase error –
received constellation tilt
We shall assume that the carrier-recovery system effected + _
coupler
Single-Polarization Down-Converter (Optical Demodulator)
Lost a
factor of 2
in ampl.
due to input
splitting
splitting
factor

28. Full optical demodulator - 90 deg balanced hybrid – serodyne (for heterodyne just use upper branch)
drop IF carrier
for homodyne
Equivalent
system: + _
coupler
Single-Polarization Down-Converter (Optical Demodulator)

29. Full optical demodulator - 90 deg balanced hybrid – intradyne(for heterodyne just use upper branch)
drop IF carrier
for homodyne
Noise power summation
in balanced PD pair
Noise pwr
3 dB lower
than single-ended + _
coupler
Single-Polarization Down-Converter (Optical Demodulator)
Pwr SNR
3 dB worse
than single-ended

30. Equivalent electrical circuit for optically coherent detection
Proof of
with
HET
The detected analytic
optical field:
The detected photocurrent:
Older
version
absorbed in IF
filtering LO

31. Equivalent electrical circuit for optically coherent detection
Proof of
with
HOM
The detected analytic
optical field:
The detected photocurrent:
Older
version IF
filtering
Nulled out by OPLL LO

32. LO SHOT-NOISE limited ANALYSIS

33. Symbol SNR evaluation (single-ended det. , counting sig
Symbol SNR evaluation (single-ended det. , counting sig. photons right at PD)
The total photocurrent in each quadrature branch is then expressed as
HET:
HOM twice as large !
No squared-cos
averaging
HOM:
Assume real-valued1-D HOM
constellation: specifically BPSK
SYMBOL SNR EVALUATION
# of PHOTO-ELECTONS

34. Equivalent electrical circuit for optically coherent detection
Redo the derivation for HOM
for completeness
Self-study
Assume
SYMBOL SNR EVALUATION
# of PHOTO-ELECTRONS upon direct detection of

35. Equivalent electrical circuit for optically coherent detection
below
HOM
HET
random phase picked up by the signal over the
channel, minus the phase of the LO
photodiode
effective input
RX backend: SYN / ASYN
(absent for
locked HOM)
One-sided PSD:
Effective TX
signal
RX front-end
equivalent circuit
AWGN module

36. Equivalent electrical circuit for optically coherent detection
and passband
PSK / OOK / M-ASK / DB
SYN
ASYN
HOM / HET
M-ary PSK, BPSK and QPSK in particular

37. Comparing OADD and COH detection
for the
SYN
ASYN
HET
also OADD (ASYN)
Essentially the same substitution
for an Optical Amplifier with Direct Detection
(OADD ) with
HOM
is the number of photo-electrons
generated by the signal pulse in
an equiv. DD system
(the current system with the LO turned off)
Here is the number of photons
in the signal pulse at the OA input, normalized by
Further to the symbol SNRs, we must also consider the equivalent block diagrams.
We shall see that the following two properties hold:
HOM 3 dB better
than HET SYN
OADD and HET ASYN
will be seen to be equivalent !!

38. OADD  ASYN HET analogy
Photons
per pulse
Electrical
ENV. DET
Electrical
IF Filter
LO Mixing
gain
LO shot-noise RX
backend
AWGN
Eff. ch.
Optical
Filter (OF)
PHOTO-DET
OA gain
ASE noise
SIG. GEN. MODEL + OF
The receiver block
diagrams
are identical!
received SNRs Es/No
as functions of Ks
are also identical!
Photons
per pulse

39. Comparing OADD and COH detection
Self-study
for a received photocount

40. Quantum limit sensitivity for homodyne ASK and PSK (I)
coherent homodyne detection scheme (ASK /PSK) with a photon-counting receiver.
1/e LO
Intensity to mean-count-in-T-seconds responsivity factor
Note: “Intensity” means |E|^2

41. BER OF PAM WITH OADD AND COH DETECTION
38 ph/bit taking into account more sophisticated OA statistics

42. BER OF PAM WITH OADD AND COH DETECTION
Note: this pertains to an idealized configuration
whereby the loss entailed in combining the sig and LO
is ignored

43. DD ASK Self-study PHOTON COUNTER SLICER 0 ”0” 1,2,3…”1” peak avg
”0”
1,2,3…”1”
peak
avg
Requires negligible receiver thermal noise !!!
unattainable ideal !!!
However, with either coherent or optical amplified detection
we may get the receiver thermal noise out of the way !! 20 10
Coherent: we are left with the shot-noise of the LO
OA: we are left with the ASE

44. Comparison of receiver sensitivities for several modulation formats
HOM
HET
OADD
BPSK 9 18 -
BDPSK 10 20 DB 15 30 31
OOK 36 38
QPSK
SYN
ASYN

45. Summary: comparative ideal performance
PSK HET
ASK HET
DPSK HOM
PSK HOM
ASK HOM
Photons/bit 72
ASYN ASK HET 40
QDPSK-BAL
37.3
SYN ASK HET 36 20
4PSK-BAL
18.7
PSK HET COH
DPSK-BAL
ASK-BAL 18
DD-ASK 10
PSK-BAL 9
Super-Quantum-Limit PSK 5 DB
SYN HOM 15
SYN HET 30
ASYN HET 31

46. IT’S OVER... GOOD LUCK!