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Homodyne Phase-Shift-Keying Systems:Past Challenges and Future OpportunitiesLeonid G.Kazovsky'Departmen1 of Electrical Engineering, Stanford lJniversi@ Packard 362. Stanford, CA 94305, USA.E-mail: kccovsh&stunlbrd. edit*mrrent!r:on sabburicnlwith Scuolu Superiore Sunt Anna, Pisa, Italy

Abs tract: Homod yne phase-shift-keying systems can achieve the best sensitivity and the longesttransmission distance, This paper reviews past researc h efforts and ex amines future possibilities inthis field.02005 Optical Society of AmericaOCIS codes: (060.1660) oherent communications;(060.2920) omodyning; (060.45 10) Optical communications.

1. IntroductionHomodyne phase-shift-keying (PSK) systems remain one of the most promising ways to approach theoreticalreceiver sensitivity, excellent spectral efficiency, and longest transmission distance for both free-space and fiber-based optical comm unication systems [1,2]. Homodyne PSK attracted considerable attention in late eighties andearly nineties, and early experiments employed optical phase-lock loops (OPLLs) to lock the phase of the localoscillator to that of the incoming signal [3-51.The main emphasis of the research efforts was receiver sensitivitysince coherent homod yne PSK systems can, in principle, achieve the b est receive r sensitivity. Th e interest in PSKsystems dro pped significantly after the development and d eployment of Erbium-doped fiber amplifiers (EDFAs) inthe 90's. It has been revived recently in the co ntext of differential phase-shift keyin g (DPSK). DPSK ystems useself-homodyning p rovided by a Mach-Zehnder delay interferometer (MZDI) and balance detection 161. Therefore, alocal oscillator is not necessary, and implementation is simpler than with phase-locked coherent receivers. A partfrom receiver sensitivity, the emp hasis is now on spectral efficiency and tolerance to fiber nonlinearities, two issueswith increasing significance in ultra-long haul com mun ication systems .Coherent homodyne PSK systems can be used in conjun ction with op tical amplifiers or in systems where in-lineoptical amplifiers cannot be used, such as satellite-to-satellite and ground-to-satellite laser communications. Theycan also serve an important role for fiber communications in wavelengtb regions where optical amplifiers are notavailable. A coheren t receiver provides very narrowband filtering, an d as a result the optical channels can be packedmore closely together. This improved receiver selectivity combined with advanced modulation formats candrastically increase spectral efficiency. It has also been shown that coherent anaIog links have superior dynamicrange aver direct detection systems in some cases [7].2. Past Research EffortsPSK optical homodyne detection offers the best sensitivity for optical coherent detection. It also requires smallerreceiver bandwidths as compared to heterodyne detection. However, it imposes stringent requirement on thelinewidth of the lasers used. In the past this has been a serious challenge for homodyne PSK systems. Figure 1summarizes linewidth requ iremen ts and theoretical sensitivity limits for com mon co herent techniq ues. Inspection ofPig. 1 reveals that linew idth is no longer a bottleneck problem for most coh erent techniques.PLL design is critical for phase-locked coherent receivers. Several PLL architectures have been proposed andexperimentally demo nstrated, including the balanced pilot-carrier OPLL [9 ] and the decision-driven OPLL (DD-OPLL) [XI. The balanced OPLL architecture has the advan tage of suppressing the excess intensity noise o ft he lasersused, but imposes the most stringent requirem ents on the laser linewidths. Its implementation, on the other hand, issimpler that of the DD-OPLL. An experim ental balanced OPLL has been demonstrated in Ref. [3]. Using thatOPLL, receiver sen sitivities of 25 photonshit at 140 Mbps over 28.6-km of SMF and 332 photonsibit at 2 Gbpswere achieved [3]. In another experiment, a 4 Gbps pilot-camer homodyne system achieved a sensitivity of 72photonshit 141. An experimental DD-OPLL was demon strated with 297 photonsibit sensitivity at a bit rate of 10Gbps 1171.Phase-diversity homodyne systems have been designed to be used in conjunction with ASK, and can be used inconjunction with DPSK as well. T hey don't need an OPLL and are able to tolerate much wider laser linewidths,even of the order of the bit rate [lo]. These receivers have the same sensitivity and linewidth requirements asheterody ne receivers, but require a much smaller receiver bandwidth since they operate in the baseband. Their main

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disadvantages are their complexity and sensitivity to implementation imperfections. An experimental three-branchphase-diversity homodyne DPSK receiver using HeNe h e r s and achieving sensitivity of47.8dEim at 320 Mbpswas demonstrated in [181.

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mismatches, power consumption, and bandwidth. Advances in high-speed electronics enable the possibility toemploy m odern DS P techniques in cohe rent receiver design. O ne such possibility is the loo p filter design, which is acritical part of a coherent OPLL. A multi-pole digital filter that is adaptive for optimized locking and trackingperform ance, and can equalize the laser FM response, would enhance the receiver performance and stability. In arecent paper [30] new coherent detection scheme using DSP or demodulation s proposed. The method is based onphase diversity detection employing new high-speed DS P techniques. It has most advan tages of coheren t detection,but withou t the need for analog OPLL. Post-detection signal processing perform s synchron ous demo dulationusing adigital PLL. In addition, it is possible to eliminate the chromatic dispersion penalty (that cannot be accom plishedusing direct detection, since the phase information of the signal is lost in the photodetection process). The principlewas d emon strated at 10 Gbps using off-line signal processing.Applications of coherent systems in a relatively remote future might inv olve access or metropolitan networks.They could provide a large number of available channels dynamically allocated and reused due to the highselectivity and tunability of the co herent receivers. Use o f optical amplifiers could also be avoided due to the highsensitivity of those receivers. Low-cost, polarization-insensitive integrated transceivers are a must for thedeploy men t of such systems and networks.PSK homodyne systems, in particular, could find applications in free-space op tical communications, inwhich asecure sateltite-to-satellite and ground-to-satellite optical link could be established. Since in-line optical amplifierscannot be used in such a link, the high sensitivity of PSK homodyne receivers makes them appealing for thisapplication. Low power consumption integrated transceivers and narrow-linewidth efficient lasers need to bedevelop ed to make such a system practical. Other ch allenges that need to be investigated and addressed towards thatgoal are the impact of atmospheric turbulence, D oppler shift and pointing and tracking subsystems.Self-homodyne RZ-DPSK format seems to be attractive for deployment in long-haul transmission systems.Non-linear phase noise, caused by conversion of ASE-to-phase noise through SPM and XPM, can cause severeperform ance degradation in long-h aul systems employing some kind of PSK format [3 I]. Compensation of the non-linear phase jitter may be needed for enhancing their performance. Several schemes have been proposed to mitigatethe penalty induced by this effect, both in the optical and electronic domain [21,32]. All these schemes apply anintensity-derived correction to the phase of the received signal, effectively reducin g the variance of the accumulatednonline ar phase jitter.Coh erent system s provide exception al sensitivity, high spectral efficiency, and lo nger transmission reach comparedto TMiDD systems. Past research efforts were successfid in designing and implem enting poIarization independentreceivers, optical PLLs, nd sub-MHz linewidth lasers. Several spectacular experiments were carried out. Futureresearch might focus on novel m oduIation formats (e.g. QAM) for increas ed spectral efficiency, nonlinear phasenoise comp ensation, receiver tun ability, implementation of low-cost and low po wer co nsum ption transceivers, andfield trials for both fiee-space and fiber-based applications.

I 4. Summary

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