Chapter 10 Coherent Optical Communication Systems
Chapter 10Coherent Optical CommunicationSystemsIoannis RoudasAbstract The rapid evolution of long-haul optical communications systems,witnessed in the last five years, is due to the gradual adoption of spectrally efficient, multilevel modulation formats, in conjunction with polarization divisionmultiplexing (PDM) and coherent intradyne detection assisted by digital signalprocessing (DSP). The objective of this tutorial chapter is to briefly review theoperating principles of state-of-the-art long-haul coherent optical communicationssystems. Due to limitations in space, it focuses mainly on coherent optical systemsusing quadrature phase-shift keying (QPSK) modulation.10.1 IntroductionThe commercialization in 2008 of the first 40 Gb/s coherent optical communications systems employing polarization division multiplexing (PDM) Quadraturephase-shift keying (QPSK) and intradyne detection assisted by digital signalprocessing (DSP) marked a major milestone in long-haul transmission [1, 2].Coherent receivers were intensively studied in the eighties [3–7] because oftheir superiority to their direct-detection counterparts, mainly in terms of sensitivity and frequency selectivity. However, they were considered impractical at thetime, due to their high cost and complexity, as well as their vulnerability to phasenoise and polarization rotations. The revived interest in coherent detection islargely due to the substitution of previously proposed analog electronic andI. Roudas (&)Department of Electrical EngineeringUniversity of Patras26504 Rio, Greecee-mail: roudas@ece.upatras.grN. (Neo) Antoniades et al. (eds.), WDM Systems and Networks, Optical Networks,DOI: 10.1007/978-1-4614-1093-5 10, Ó Springer Science Business Media, LLC 2012373
374I. Roudasoptoelectronic modules (which were bulky, slow, expensive, and largely inefficient) in coherent optical receivers with relatively inexpensive, high-speed,application-specific integrated circuits (ASICs) (see recent surveys [8–11, 15] andthe references therein). The latter enable adaptive electronic equalization of lineartransmission impairments, i.e., chromatic dispersion, polarization mode dispersionand polarization-dependent loss, and to some extent, of fiber nonlinearities. Theyalso allow for adaptive electronic compensation of imperfections of the analogoptical transmitter and receiver front-ends, such as time skew of quadraturecomponents and polarization tributaries, quadrature and polarization imbalance,etc. Finally, they perform all standard digital receiver functionalities such asdigital clock recovery, intermediate frequency offset and phase-noise estimation,symbol decision, differential decoding, forward error correction, etc.There is an emerging consensus among major system vendors that coherentoptical PDM-QPSK communications systems are the most attractive candidates for100 Gb/s Ethernet1 transmission over existing terrestrial networks [2, 12, 16–19].At the moment, several companies have announced the development ofapplication-specific integrated circuits (ASICs) for DSP in coherent homodynesynchronous PDM-QPSK receivers operating at this symbol rate. Furthermore,recent field trials [21, 22] have demonstrated the practicality of long-haul 28 GBdcoherent optical PDM-QPSK systems.PDM M-ary Quadrature Amplitude Modulation (M-QAM) is actively investigated for use in next-generation long-haul terrestrial optical communicationssystems [12]. This modulation format is intended for either single carrier or multicarrier systems using orthogonal frequency division multiplexing (OFDM), inorder to achieve equivalent bit rates of the order of 400 Gb/s or even 1 Tb/s perwavelength channel [12]. PDM M-QAM allows for a nominal spectral efficiencyof 2 M b/s/Hz. Recent hero experiments using coherent optical PDM M-QAMcommunication systems achieved several world records, most notably unprecedented aggregate WDM bit rates approaching 70 Tb/s [23, 24] and a spectralefficiency close to 12 b/s/ Hz [25].Due to the proliferation of research studies on coherent optical PDM-QPSK andPDM M-QAM communication systems during the last five years, it is difficult toexhaustively cover all aspects of this topic here. The objective of this tutorialchapter is to briefly review the operating principles in long-haul PDM-QPSKcoherent optical communications systems.1Actually, it is necessary to use an effective bit rate of 112 Gb/s, which corresponds to a symbolrate of PDM-QPSK of 28 GBd, in order to achieve a net per channel 100 Gb/s data ratetransmission. The reason is that one must take into account the overhead due to current forwarderror correction (FEC) (*7%) and the Ethernet packet header (*4%). We assume a WDMchannel spacing of 50 GHz, which is compatible with the current ITU grid specifications andprovides some margin for bandwidth narrowing due to the concatenation of several reconfigurable optical add-drop multiplexers (ROADMs). Then, the spectral efficiency of these systems is2 b/s/Hz, i.e., half of the nominal spectral efficiency of PDM-QPSK [12–14].
10Coherent Optical Communication Systems375The rest of the chapter is organized as follows: In Sect. 10.2, we initiallypresent the digital M-PSK transmitter and receiver optimal architectures in a blockdiagram form. Next, we review the operating principle of coherent detection anddescribe different variants of coherent receivers. In Sect. 10.3, we describe theimplementation of the functionalities of the optimal M-PSK transmitter andreceiver using various photonic devices, i.e., a QM, a balanced receiver, a phasediversity receiver with 90 hybrid, and a polarization-diversity receiver. In Sect.10.4, we review the most prominent DSP algorithms. Finally, in Sect. 10.5, wedevelop an abstract model for the performance evaluation of an optical communication system using M-PSK modulation and synchronous homodyne detection.The details of the calculations are given in the Appendices.10.2 Multilevel Differential Phase-Shift KeyingMultilevel phase-shift keying (M-PSK) is a type of digital modulation formatwhereby information is encoded into discrete changes D/k of the phase of thecarrier at time instants equal to multiples of the symbol period [26, 29]. Sincephase changes are less affected by additive white Gaussian noise compared toamplitude changes, this modulation format exhibits higher sensitivity thanamplitude shift keying (ASK).10.2.1 Signal RepresentationThe M-PSK signal can be written as [26] sðtÞ ¼ sðtÞejxs tð10:1Þwhere f:gdenotes the real part and sðtÞ is the complex envelope sðtÞ ¼ AN 1Xck gðt kTÞð10:2Þk¼0where A is the carrier amplitude, N is the number of transmission symbols, T is thesymbol period, gðtÞis the symbol shape, and we defined the complex symbolsck ejDukð10:3ÞIn Eq. 10.3, the discrete phase changes D/k take values in the setf2pði 1Þ M þ UgMi¼1where U is an arbitrary initial phase.ð10:4Þ
376I. RoudasFig. 10.1 Geometrical representation of M-PSK signal sets on the complex planea M ¼ 2; U ¼ 0; b M ¼ 4; U ¼ 0Substituting Eqs. 10.2, 10.3 into Eq. 10.1 and using trigonometric identities, wecan express M-PSK modulation as the superposition of two carriers at the samefrequency but with 90 phase difference carrying M-ary amplitude modulations(called in-phase and quadrature components)sðtÞ ¼ IðtÞ cos xs t QðtÞ sin xs t fflfflfflfflfflfflffl{zfflfflfflfflfflfflffl} fflfflfflfflfflfflffl{zfflfflfflfflfflfflffl}in Þ ¼ AN 1Xcos D/k gðt kTÞk¼0QðtÞ ¼ AN 1Xð10:6Þsin D/k gðt kTÞk¼0From Eqs. 10.3, 10.4, we observe that symbols ck can take M discrete complexvalues. A geometric representation of this set of M complex values is shown inFig. 10.1. The symbols are represented on the complex plane as a constellation ofequidistant points on a circle.It is worth noting that random bits at the entrance of the transmitter must bemapped into the M discrete complex values that symbols ck can take prior totransmission. In Fig. 10.1, words of m ¼ log2 Mbits are associated with differentconstellation points using Gray coding, e.g., words corresponding to adjacentconstellation points differ by a single bit (see details below).
10Coherent Optical Communication Systems377Fig. 10.2 QPSK transmitter (Symbols: SPC: Serial-to-parallel converter, DAC: Digital-toanalog converter, Carrier gen: Carrier generator, p/2: 90 deg. Phase shifter)10.2.2 Transmitter and Receiver ArchitecturesFrom Eqs. 10.5, 10.6 it is straightforward to derive the block diagram of the QPSKtransmitter. Figure 10.2 shows an example of implementation of an ideal QPSKtransmitter for U ¼ p 4: The transmitter does not segment the input PRBS intowords of two bits but instead, uses a serial-to-parallel converter to alternativelysend bits to two binary pattern generators. These produce two baseband antipodalpffiffiffibinary waveforms with instantaneous amplitude 1 2 at symbol rate R ¼ 1 T;which modulate two CW carriers A cos xs t; A sin xs t: Therefore, a signaling ratereduction by one-half is achieved. The two quadrature components are added andtransmitted in the channel.The optimal synchronous QPSK receiver structure is shown in Fig. 10.3.A synchronous receiver is equipped with a carrier phase recovery circuit, whichcomputes the carrier phase change h acquired during propagation. The edwithcosðxs t þ hÞ; sinðxs t þ hÞ; respectively, and filtered using two LPFs withimpulse response gðT tÞ (matched filters). For ideal NRZ pulses, these LPFs canbe ideally implemented as integrate and dump (I&D) filters (e.g., finite timeintegrators which perform a running average of duration TÞ: In the absence ofdistortion, the waveforms at the output of the I&D filters are smoothed replicas ofthe baseband amplitude modulating waveforms IðtÞ; QðtÞ: These signals aresampled once per symbol, at the appropriate sampling instants, which are calculated using a symbol synchronization circuit, in order to minimize the errorprobability. Two binary decision devices, with thresholds equal to zero, are used torecover the bits. The decisions are made independently in the two receiver branches. Finally, the recovered binary sequences are combined into a single bit streamusing a parallel-to-serial converter.
378I. RoudasFig. 10.3 Optimal synchronous QPSK receiver. (Symbols: Carrier gen: Carrier generator, p/2:90 deg. Phase shifter, LPF: Lowpass filter, Decision: decision circuit, SPC: Serial-to-parallelconverter)Fig. 10.4 Block diagram of a representative coherent optical PDM-QPSK system. (Symbols:PRBS: Pseudo-random bit sequence, SLD: Semiconductor laser diode, CPL: 3-dB coupler, QM:Quadrature modulator, PBC: Polarization beam combiner, MUX: Optical multiplexer, VOA:Variable optical attenuator, DCF: Dispersion compensating fiber, SSMF: Standard single-modefiber, OA: Optical amplifier, DMUX: Optical demultiplexer, PBS: Polarization beam splitter,PCTR: Polarization controller, LO: Local oscillator, BRx: Balanced receivers, ADC: Analog-todigital converter, DSP: Digital signal processing module)10.2.3 Coherent Optical PDM QPSK SystemThe implementation of the aforementioned M-PSK transmitter and receiverstructures for optical communications, in the case of PDM, is shown in Fig. 10.4.More specifically, Fig. 10.4 shows the block diagram of a representative longhaul, PDM QPSK optical communications system with a polarization- and phasediversity coherent optical receiver. At the transmitter, the optical signal from a CWsemiconductor laser diode (SLD) is equally split and fed into two parallelquadrature modulators (QM). Two independent maximal-length pseudo-random
10Coherent Optical Communication Systems379bit sequences (PRBS) of period 2n 1; at a bit rate Rb each, drive two QM. Thetwo optical QPSK signals are superimposed with orthogonal SOPs, using apolarization beam combiner (PBC), to form a PDM-QPSK signal. The latter iswavelength division multiplexed (WDM) with additional channels carrying PDMQPSK signal, using an optical multiplexer (MUX), and transmitted through Namplified spans composed of standard single-mode fiber (SSMF) and, possibly,dispersion compensating fiber (DCF). An additional dispersion pre-compensationmodule, composed of a DCF and a booster optical amplifier, might be included inthe latter case.The optical receiver front-end is composed of an optical demultiplexer(DMUX), acting as an optical bandpass filter (BPF), a polarization beam splitter(PBS), a laser diode, acting as a local oscillator (LO), two 2 9 4 90 opticalhybrids, and four balanced photodetectors (BRx’s). The x- and y-polarizationcomponents of the received optical signal and the local oscillator are separatelycombined and detected by two identical phase-diversity receivers composed of a2 9 4 90 optical hybrid and two BRx’s each, at the upper and lower polarization branches, respectively. The photocurrents at the output of the fourbalanced detectors are low-pass filtered (LPF), sampled at integer multiples of afraction of the symbol period Ts ; using an analog-to-digital converter (ADC), andfed to an application-specific integrated circuit (ASIC) for DSP (see below fordetails).The aforementioned components of the M-PSK transmitter and receiver areexplained in detail in the next section.10.3 Optical ComponentsIn this section, we describe the implementation of the functionalities of the opticalM-PSK transmitter and receiver using various photonic devices, i.e., a QM, abalanced receiver, a phase-diversity receiver with 90 hybrid, and a polarizationdiversity receiver.10.3.1 Optical Transmitter: Quadrature ModulatorThe QM is shown in Fig. 10.5 [28]. It is composed of a Mach–Zehnder interferometer which contains two push–pull Mach–Zehnder modulators [43], one ineach arm, and a phase modulator at the lower arm that introduces a phase difference between the two arms.The complex envelope of the modulated electric field at the output of the s ðtÞcan be written as a function of the unmodulated input electric fieldmodulator E in ðtÞ (see Appendix A)E
380I. RoudasFig. 10.5 Quadraturemodulator pVj j2VpVp3pV1 ðtÞpV2 ðtÞ j2Vp3 33 sinsin eEin ðtÞEs ðtÞ ¼ e22Vp12Vp2ð10:7Þwhere V1 ðtÞ; V2 ðtÞ; V3 are the driving voltages and Vp1 ; Vp2 ; Vp3 are the half-wavevoltages of the two Mach–Zehnder modulators and the phase shifter, respectively.The two Mach–Zehnder modulators operate with drive voltages that take valuesin the discrete setsV1 2 f Vp1 ; Vp1 gV2 2 f Vp2 ; Vp2 gð10:8ÞThe voltage of the phase shifter is set at V3 ¼ Vp3 2; in order to introduce aphase difference of p 2 between the two arms.It is assumed that that the input into the QM is an unmodulated optical wavewhose complex envelope can be written aspffiffiffiffiffiffiffi in ðtÞ ¼ 2Ps ejusEð10:9Þwhere Ps is the average optical power and /s is the initial phase of the transmittedCW signal. In the previous formula, intensity and phase noises of the laser areneglected. Then, the complex envelope of the output electric field can be written aspffiffiffiffiffi s ðtÞ ¼ Ps ejus þjukEð10:10Þwhere uk ¼ p 4; 3p 4:10.3.2 Coherent Detection FundamentalsThe term coherent is used, in the context of optical communications, to refer toany technique employing nonlinear mixing between two optical waves on a
10Coherent Optical Communication Systems381semiconductor photodiode [4].2 The carrier frequencies of the optical waves canbe identical or different. In the former case, we have coherent homodyne detection.In the latter case, we have coherent homodyne detection (when the carrier frequency difference of the two waves is larger than or of the order of the symbolrate) or coherent intradyne detection (when the carrier frequency difference of thetwo waves is a fraction of the symbol rate).The application of coherent homodyne detection for optical frequencies datesback to 1801, when Young proposed his now famous two-slit interferenceexperiment as persuasive evidence of the wave nature of light [30]. In moderntimes, coherent heterodyne detection of electromagnetic waves has been usedsince the early days of radio communications. More specifically, heterodynedetection of radio waves was proposed by Fessenden [31], who also coined theterm heterodyne from the Greek words ‘heteros’ (other) and ‘dynamis’ (force).Heterodyning gained immense popularity with the development of the superheterodyne receiver by E. H. Armstrong in 1921 [32]. Optical heterodyning was usedfor the first time by [33] in the visible part of the electromagnetic spectrum and by[34] in the infrared.The earliest papers on coherent optical communication systems appeared in1979, in Japanese, and in 1980, in English. The revived interest in coherent opticalhomodyne receivers in combination with advanced modulation formats startedaround 2004, e.g., see early articles [35–40].10.3.2.1 Coherent Single-Ended DetectionThe operating principle of coherent detection is explained in numerous textbooks,e.g., [41–42, 99]. Consider two traveling electromagnetic waves with carrier frequencies fs andflo ; respectively, from two independent laser sources, labeled thereceived signal and the local oscillator signal, respectively. The waves propagatein the same direction with identical states of polarization (SOP). Therefore, theelectric fields of the two waves can be treated as scalars and they are denoted by Elo ðtÞ; respectively.Es ðtÞ; For simplicity, it is assumed that Es ðtÞ; Elo ðtÞ are both unmodulated (CW)sinusoidal signalspffiffiffiffiffiffiffiEs ðtÞ ¼ 2Ps cosðxs t þ us Þð10:11ÞpffiffiffiffiffiffiffiffiffiElo ðtÞ ¼ 2Plo cosðxlo t þ ulo Þ2In contrast, in the digital communications literature the term coherent is used to refer todemodulation techniques in which the absolute phase of the incoming signal is tracked by thereceiver. In optical communications, such receivers are called synchronous. In this report, we willbe interested exclusively in coherent synchronous receivers.
382I. Roudaswhere Ps ; Plo are the average optical powers, xs ¼ 2pfs ; xlo ¼ 2pflo are theangular carrier frequencies, and us ; ulo are the initial phases of the received signaland the local oscillator signal, respectively. In the previous formulae, intensity andphase noises of the lasers are neglected.The electric field of the combined signal impinging upon the photodiode, at asingle detection point, can be written as the superposition of the electric fields ofthe received signal and the local oscillatorEr ðtÞ ¼ Es ðtÞ þ Elo ðtÞð10:12ÞThe photodiode is modeled as a square-law detector which responds to thesquare of the electric fieldDEiðtÞ ¼ R Es ðtÞ2ð10:13Þwhere R is the responsivity of the photodiode and the angle brackets denote timeaveraging over an interval proportional to the response time of the photodiode.By substituting Eqs. 10.12, 10.11 into Eq. 10.13 and using trigonometricidentities, we obtain the following expression for the photocurrent in the absenceof noisepffiffiffiffiffiffiffiffiffiffiffiiðtÞ ¼ R½Ps þ Plo þ 2R Ps Plo cosðxIF t þ uIF Þð10:14Þ fflfflfflfflfflfflffl{zfflfflfflfflfflfflffl} �fflfflfflfflfflfflfflfflfflfflffl}direct detectiontermcoherent detectiontermwherexIF ¼ 2pðfs flo ÞuIF ¼ us uloð10:15ÞIt is observed that Eq. 10.14 is the sum of three terms due to the direct-detectionof the received signal and the local oscillator signal, and their mixing (coherentdetection term), respectively. The latter preserves the information transferred bythe amplitude, the frequency and the phase of the received signal. Therefore, thistype of detection can be used in conjunction with amplitude, frequency or phasemodulation formats. In addition, the amplitude of the coherent detection termdepends on the power of the local oscillator, which can be made very large. This isthe reason for the improved receiver sensitivity exhibited by coherent detection.10.3.2.2 Balanced ReceiverAn implementation of the coherent receiver with fiber-optic components is shownin Fig. 10.6 [44, 118]. This configuration uses a directional 3-dB coupler and twoidentical p-i-n photodiodes connected back-to-back. A received si
structures for optical communications, in the case of PDM, is shown in Fig. 10.4. More specifically, Fig. 10.4 shows the block diagram of a representative long-haul, PDM QPSK optical communications system with a polarization- and phase-diversity coherent optical receiver. At the transmitter, the optical signal from a CW
Part One: Heir of Ash Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26 Chapter 27 Chapter 28 Chapter 29 Chapter 30 .
TO KILL A MOCKINGBIRD. Contents Dedication Epigraph Part One Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Part Two Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18. Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26
Of the many non-linear optical techniques that exist, we are interested in the coherent Raman rl{ effect known as Coherent Anti-Stokes Raman Scattering (CNRS). The acronym CARS is also used to refer to Coherent Anti-Stokes Raman Spectroscopy. CA RS is a four-wave mixing process where three waves are coupled to produce coherent
DEDICATION PART ONE Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 PART TWO Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 .
2. EEPN in Optical Communication Systems Figure 1 describes the block diagram of a long-haul, coherent optical communication system with EDC and CPR. The LPN from the transmitter (Tx) laser passes through the optical fiber and the dispersion compensation unit, and thus the net CD experienced by the Tx LPN approaches zero. By
approach for realizing high-capacity long-haul optical communication systems [1-5]. From the beginning of this century, coherent detection combined with large-bandwidth analog-to-digital converter (ADC), digital-to-analog converter (DAC) and DSP has increased the achievable capacity of optical fiber communication systems [6-10].
Semiconductor Optical Amplifiers (SOAs) have mainly found application in optical telecommunication networks for optical signal regeneration, wavelength switching or wavelength conversion. The objective of this paper is to report the use of semiconductor optical amplifiers for optical sensing taking into account their optical bistable properties .
The modern approach is fact based and lays emphasis on the factual study of political phenomenon to arrive at scientific and definite conclusions. The modern approaches include sociological approach, economic approach, psychological approach, quantitative approach, simulation approach, system approach, behavioural approach, Marxian approach etc. 2 Wasby, L Stephen (1972), “Political Science .
