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Introduction to Fiber Optic Communication

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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


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

38 ph/bit taking into account more sophisticated OA statistics

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


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