Fibre optic communication system design guidelines - Part 16: Coherent receivers and transmitters with high-speed digital signal processing

IEC TR 61282-16:2022 is a technical report on coherent optical receiver and transmitter technologies that are employed in fibre optic communication systems as well as in optical test and measurement equipment. This document describes the principle of operation and functional capabilities of coherent optical receivers as well as the operation of optical transmitters used to generate complex vector-modulated signals. It is intended to serve as a technical foundation for other IEC documents and standards related to coherent optical transmission techniques.

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Status
Published
Publication Date
09-May-2022
Current Stage
PPUB - Publication issued
Completion Date
10-May-2022
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IEC TR 61282-16:2022 - Fibre optic communication system design guidelines - Part 16: Coherent receivers and transmitters with high-speed digital signal processing
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IEC TR 61282-16
Edition 1.0 2022-05
TECHNICAL
REPORT
Fibre optic communication system design guidelines –
Part 16: Coherent receivers and transmitters with high-speed digital signal
processing
IEC TR 61282-16:2022-05(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 61282-16
Edition 1.0 2022-05
TECHNICAL
REPORT
Fibre optic communication system design guidelines –
Part 16: Coherent receivers and transmitters with high-speed digital signal
processing
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.01 ISBN 978-2-8322-0094-0

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 61282-16:2022  IEC 2022
CONTENTS

FOREWORD ........................................................................................................................... 6

INTRODUCTION ..................................................................................................................... 8

1 Scope .............................................................................................................................. 9

2 Normative references ...................................................................................................... 9

3 Terms, definitions, and abbreviated terms ....................................................................... 9

3.1 Terms and definitions .............................................................................................. 9

3.2 Abbreviated terms ................................................................................................... 9

4 Background ................................................................................................................... 12

5 Coherent transmission of vector-modulated signals ....................................................... 13

5.1 Typical receiver architecture ................................................................................. 13

5.2 Typical transmitter architecture ............................................................................. 14

5.3 Vector-modulated signals ...................................................................................... 15

5.3.1 Mathematical description ............................................................................... 15

5.3.2 Binary amplitude and phase modulation ......................................................... 16

5.3.3 Quadrature amplitude modulation .................................................................. 17

5.3.4 Polarization multiplexing ................................................................................ 18

5.3.5 Higher-dimensional coding and constellation shaping .................................... 18

6 Coherent receiver architectures and functional capabilities ............................................ 19

6.1 Basic principle of coherent detection ..................................................................... 19

6.1.1 General ......................................................................................................... 19

6.1.2 Homodyne and heterodyne detection ............................................................. 19

6.1.3 Intradyne detection ........................................................................................ 21

6.1.4 Polarization dependence ............................................................................... 21

6.1.5 Frequency dependence ................................................................................. 22

6.1.6 Phase and polarization diversity .................................................................... 22

6.2 Single coherent mixer with balanced detection ...................................................... 23

6.2.1 Principle of operation ..................................................................................... 23

6.2.2 Common-mode rejection ................................................................................ 24

6.2.3 Polarization dependence ............................................................................... 26

6.2.4 Homodyne detection ...................................................................................... 27

6.2.5 Heterodyne detection ..................................................................................... 28

6.3 Dual coherent mixer with phase diversity .............................................................. 29

6.3.1 Principle of operation ..................................................................................... 29

6.3.2 Intradyne detection with frequency offset removal.......................................... 31

6.3.3 Compensation of chromatic dispersion........................................................... 33

6.3.4 Compensation of I-Q skew and phase offset .................................................. 40

6.3.5 Spectral shaping and frequency equalization ................................................. 42

6.4 Quadruple mixer with phase and polarization diversity .......................................... 46

6.4.1 Principle of operation ..................................................................................... 46

6.4.2 Polarization demultiplexing ............................................................................ 48

6.4.3 Compensation of polarization-mode dispersion .............................................. 51

6.4.4 Compensation of polarization-dependent loss and residual CD ...................... 53

6.4.5 Carrier phase recovery .................................................................................. 54

6.4.6 Impact of laser phase noise ........................................................................... 55

6.5 High-resolution spectral analysis with coherent receivers ...................................... 62

6.5.1 Measurement methods .................................................................................. 62

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IEC TR 61282-16:2022  IEC 2022 – 3 –

6.5.2 Dual mixer with polarization diversity ............................................................. 63

6.5.3 Examples of high-resolution spectral analysis ................................................ 64

7 Digital signal processing in coherent receivers .............................................................. 66

7.1 Basic features of digital signal processing ............................................................. 66

7.2 Real-time DSPs for fibre optic communication systems ......................................... 70

7.2.1 Basic functions .............................................................................................. 70

7.2.2 Timing recovery ............................................................................................. 71

7.2.3 Cycle slip detection ....................................................................................... 71

7.2.4 Compensation of nonlinear transmission effects ............................................ 71

7.2.5 FEC decoding and performance monitoring ................................................... 72

7.3 Software-based DSPs for optical modulation analysers ......................................... 73

8 Transmitters for vector-modulated signals ..................................................................... 75

8.1 Generation of vector-modulated signals ................................................................ 75

8.2 Single Mach-Zehnder modulator ........................................................................... 77

8.2.1 Principle of operation ..................................................................................... 77

8.2.2 Modulator extinction ratio .............................................................................. 81

8.2.3 Adaptive bias control in Mach-Zehnder modulators ........................................ 83

8.3 Dual Mach-Zehnder modulators ............................................................................ 85

8.3.1 Quadrature-amplitude modulation .................................................................. 85

8.3.2 Compensation of finite extinction ratio ........................................................... 86

8.3.3 Adaptive control of I-Q phase ........................................................................ 87

8.4 Quadruple modulators for polarization-multiplexed signals .................................... 91

9 Digital signal processing in transmitters for vector-modulated signals ............................ 94

9.1 Pre-distortion of optical signals ............................................................................. 94

9.1.1 General ......................................................................................................... 94

9.1.2 Pre-compensation of linear transmitter impairments....................................... 95

9.1.3 Determination of transmitter frequency response ........................................... 96

9.1.4 Determination of transmitter skew ................................................................ 101

9.1.5 Pre-compensation of modulator nonlinearity ................................................ 103

9.2 Symbol mapper, FEC encoding and framing ........................................................ 105

10 Implementation and typical performance specifications ................................................ 106

10.1 Coherent receiver ............................................................................................... 106

10.1.1 Implementation ............................................................................................ 106

10.1.2 Typical performance specifications .............................................................. 107

10.2 Optical transmitter .............................................................................................. 108

10.2.1 Implementation ............................................................................................ 108

10.2.2 Typical performance specifications .............................................................. 109

10.3 Integrated coherent receiver and transmitter ....................................................... 111

10.4 Tuneable laser assemblies .................................................................................. 111

10.4.1 Implementation ............................................................................................ 111

10.4.2 Typical performance specifications .............................................................. 111

Bibliography ........................................................................................................................ 113

Figure 1 – Coherent optical receiver ..................................................................................... 13

Figure 2 – Optical transmitter for coherent transmission ....................................................... 14

Figure 3 – Generation of vector modulated signals ............................................................... 16

Figure 4 – Examples of modulation formats for coherent communication ............................... 17

Figure 5 – Signal and local oscillator frequencies for homodyne detection ............................ 20

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– 4 – IEC TR 61282-16:2022  IEC 2022

Figure 6 – Signal and local oscillator frequencies for heterodyne detection ........................... 20

Figure 7 – Electrical spectra of homodyne and heterodyne beat signals ................................ 21

Figure 8 – Single balanced mixer for coherent reception ....................................................... 24

Figure 9 – Balanced heterodyne mixer with electrical down-mixing ....................................... 28

Figure 10 – Dual coherent mixer with phase diversity ............................................................ 30

Figure 11 – Intradyne beat spectrum with 2 GHz frequency offset ......................................... 32

Figure 12 – Differential phase shifts introduced by 3 000 ps/nm GVD ................................... 33

Figure 13 – Transfer function for GVD of 3 ns/nm in frequency domain ................................. 34

Figure 14 – Inverse transfer functions for GVD of 3 ns/nm and 10 ns/nm in time domain ...... 36

Figure 15 – Fractionally spaced equalizer with a tapped delay line ....................................... 37

Figure 16 – Processing steps for CD compensation in the frequency domain ........................ 38

Figure 17 – Data processing for CD compensation in the frequency domain ......................... 39

Figure 18 – Dual coherent mixer with optical phase offset and signal skew ........................... 40

Figure 19 – Skew and phase offset removal in a DSP ........................................................... 41

Figure 20 – Example of an I-Q skew and phase error measurement ...................................... 42

Figure 21 – Fifth-order Bessel filter emulated with a 5-tap FSE ............................................. 43

Figure 22 – Shape of inverse Bessel filter generated with a 9-tap FSE ................................. 44

Figure 23 – Amplitude and phase of FSE-generated inverse Bessel filter .............................. 45

Figure 24 – Root-raised-cosine filter emulated with a 33-tap FSE ......................................... 46

Figure 25 – Quadruple coherent mixer with phase and polarization diversity ......................... 47

Figure 26 – 2 × 2 matrix operation for adaptive polarization demultiplexing ........................... 49

Figure 27 – Constellation points of QPSK signal after polarization demultiplexing ................. 50

Figure 28 – 16QAM signal before and after polarization demultiplexing ................................. 50

Figure 29 – FSE-based compensator for polarization-mode dispersion ................................. 52

Figure 30 – QPSK signal constellations for various amounts of PMD .................................... 52

Figure 31 – QPSK signal constellations for various amounts of PDL ..................................... 53

Figure 32 – QPSK signal constellations for various amounts of GVD .................................... 53

Figure 33 – Optical phase noise of two narrow-linewidth lasers ............................................ 56

Figure 34 – Optical frequency noise spectra of two lasers ..................................................... 58

Figure 35 – Optical phase noise spectra of two lasers .......................................................... 59

Figure 36 – Laser phase noise measurement with optical bandpass filter .............................. 60

Figure 37 – Dual coherent mixer with polarization diversity ................................................... 63

Figure 38 – High-resolution optical spectrum of a 32 GBd QPSK signal ................................ 65

Figure 39 – QPSK signal measured with coherent OSA and with grating-based OSA ............ 65

Figure 40 – Typical digital signal processing steps in a coherent receiver ............................. 67

Figure 41 – 100 Gbit/s PM-QPSK signal before and after fibre transmission ......................... 68

Figure 42 – De-convolution of a 100 Gbit/s PM-QPSK signal at various DSP stages ............. 69

Figure 43 – Block diagram of specially designed integrated circuit with DSP ......................... 70

Figure 44 – Block diagram of OMA with software-based DSP ................................................ 73

Figure 45 – Typical arrangement for generation of vector-modulated signals ........................ 76

Figure 46 – Differentially driven Mach-Zehnder modulator .................................................... 77

Figure 47 – Mach-Zehnder modulator with adaptive bias control ........................................... 78

Figure 48 – MZM operation for intensity modulation .............................................................. 79

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IEC TR 61282-16:2022  IEC 2022 – 5 –

Figure 49 – Nonlinear MZM operation for binary PSK/ASK .................................................... 80

Figure 50 – Linear MZM operation for quaternary ASK signals .............................................. 81

Figure 51 – Optical power variations in an NRZ-OOK signal with finite extinction ratio .......... 82

Figure 52 – Optical and RF output power versus bias voltage for linear operation ................. 83

Figure 53 – Dual MZM with adaptive I-Q phase control ......................................................... 86

Figure 54 – RF output power of I-Q modulator versus I-Q phase ........................................... 88

Figure 55 – Error signal for I-Q phase control derived from MZM bias dither ......................... 90

Figure 56 – I-Q phase error resulting from offset in feedback signal ...................................... 91

Figure 57 – Dual I-Q modulators for polarization multiplexing................................................ 92

Figure 58 – Adaptive bias and phase control in dual-polarization I-Q modulator .................... 93

Figure 59 – Typical digital signal processing steps in the transmitter .................................... 95

Figure 60 – Spectrum and samples of a white test vector ...................................................... 96

Figure 61 – Magnitude of transmitter frequency response ..................................................... 97

Figure 62 – Phase of transmitter frequency response ........................................................... 98

Figure 63 – Example of magnitude and phase of a pre-compensation filter ........................... 99

Figure 64 – Tap coefficients and effect of a 9-tap pre-compensation filter ............................. 99

Figure 65 – Impact of pre-compensation on signal waveform in DSP and DAC ................... 101

Figure 66 – I-Q skew in a single-sideband modulated signal ............................................... 102

Figure 67 – Non-linear pre-distortion of a Mach-Zehnder modulator .................................... 103

Figure 68 – Linearized MZM operation for quaternary ASK ................................................. 105

Table 1 – Typical receiver specifications ............................................................................. 108

Table 2 – Typical modulator specifications .......................................................................... 110

Table 3 – Additional specifications for integrated driver-modulator ...................................... 110

Table 4 – Typical specifications for tuneable laser assemblies ............................................ 112

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– 6 – IEC TR 61282-16:2022  IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SYSTEM DESIGN GUIDELINES –
Part 16: Coherent receivers and transmitters
with high-speed digital signal processing
FOREWORD

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent

rights. IEC shall not be held responsible for identifying any or all such patent rights.

IEC 61282-16 has been prepared by subcommittee 86C: Fibre optic systems and active

devices, of IEC technical committee 86: Fibre optics. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
86C/1776/DTR 86C/1782/RVDTR

Full information on the voting for its approval can be found in the report on voting indicated in

the above table.
The language used for the development of this Technical Report is English.
---------------------- Page: 8 ----------------------
IEC TR 61282-16:2022  IEC 2022 – 7 –

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in

accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available

at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are

described in greater detail at www.iec.ch/standardsdev/publications.

A list of all parts in the IEC 61282 series, published under the general title Fibre optic

communication system design guidelines, can be found on the IEC website.

Future documents in this series will carry the new general title as cited above. Titles of existing

documents in this series will be updated at the time of the next edition.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
---------------------- Page: 9 ----------------------
– 8 – IEC TR 61282-16:2022  IEC 2022
INTRODUCTION

Coherent optical receivers are widely used in long-haul fibre optic communication systems,

especially in systems that transmit optical carriers at data rates of 100 Gbit/s and higher. While

the principle of coherent detection is very similar to that of super-heterodyne (or homodyne)

detection in radio and microwave receivers, its implementation is significantly more challenging.

The main reason is that optical frequencies are substantially higher than radio frequencies, so

it becomes more difficult to match the local oscillator frequency in the coherent receiver to the

frequency of the transmitted optical signal. Furthermore, optical signals tend to be highly

polarized, which means that the amplitude of a coherently received signal can be substantially

reduced or even vanish if the polarization state of the local oscillator light does not match the

polarization state of the received optical signal. This polarization matching is particularly difficult

to achieve in fibre optic communications systems, which usually do not preserve the launched

state of polarization of the transmitted signal. To overcome these problems, modern coherent

receivers typically consist of four parallel coherent optical mixers that provide phase and

polarization diversity, and they rely on high-speed digital signal processors to retrieve the

transmitted data from the four received electrical signals.

This rather complex coherent receiver architecture is further justified by the fact that it allows

the receiver to mitigate various types of signal impairments introduced in the fibre optic link (or

in the receiver itself) simply by means of additional electronic signal processing. Most notably,

it is possible to substantially reduce the signal distortions caused by polarization-mode

dispersion (PMD) or uncompensated chromatic dispersion (CD) in the fibre link, without

requiring additional optical PMD or CD compensators. For this reason, coherent optical

communication systems generally allow signal transmission at much higher data rates than

communication systems using direct-detection receivers. Furthermore, coherent detection with

subsequent digital signal processing facilitates the decoding of complex vector-modulated

signals, such as quadrature-amplitude modulated signals (QAM) and polarization-multiplexed

(PM) signals, and thereby the transmission of higher data rates.

Aside from fibre optic communications systems, coherent optical receivers are also used in

various test and measurement instrumentation. Most notable examples are optical modulation

analysers (OMAs) and high-resolution optical spectrum analysers (HR-OSAs). Optical

modulation analysers are high-performance optical reference receivers and are used to assess

the signal quality of complex vector-modulated optical signals. They are typically composed of

a carefully calibrated coherent receiver and a high-speed real-time digitizing oscilloscope to

record the coherently received signals, which are then analysed with the help of a software-

based digital signal processor.
High-resolution optical spectrum analysers a
...

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