A single frequency-periodic narrow filter converts DPSK to intensity modulation in a high number of WDM channels. It also strongly enhances their tolerance to chromatic dispersion and is exploited in a 16x10 Gbit/s transmission over 240 km G.652 fibre with no chromatic dispersion compensation

CIARAMELLA, ERNESTO

CONTESTABILE, GIAMPIERO

E. CIARAMELLA

L. GIORGI

R. PROIETTI

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WDM-DPSK detection by means offrequency-periodic Gaussian filteringA. D’Errico, R. Proietti, L. Giorgi, G. Contestabile andE. CiaramellaA single frequency-periodic narrow filter converts DPSK to intensitymodulation in a high number of WDM channels. It also stronglyenhances their tolerance to chromatic dispersion and is exploited ina 16 10 Gbit=s transmission over 240 km G.652 fibre with nochromatic dispersion compensation.Introduction: Differential phase shift keying (DPSK) is an attractivemodulation format because of its high robustness to nonlinearpropagation [1]. Various solutions were proposed for detecting opticalDPSK. Among them, probably the most popular is the delay Mach-Zehnder interferometer (MZI) [2, 3], which converts DPSK into anintensity modulated (IM) signal. Using MZIs could require introdu-cing into WDM systems a number of additional bulky components,one for each WDM channel. As recently proposed, a single MZI couldalso be used to demodulate any ITU DPSK channel, but here the twointerferometer arms should be carefully designed and highly stabilisedto get a frequency-periodic filtering effect [4, 5].An alternative solution to the MZI was theoretically proposed in [6].It is based on narrow optical filtering of a DPSK signal and moreovercould strongly enhance the signal tolerance to chromatic dispersion [7].Unfortunately, owing to the criticality of realising a suitable filter (e.g.Fabry-Perot filters would not work [7]), only very preliminary experi-mental results on this scheme were presented, all limited to a singlechannel [8, 9]. Therefore, one can expect that, using this technique,many components might be needed for WDM applications.Here we experimentally assess the feasibility and the potential benefitsof this approach in a 16 10 Gbit=s WDM-DPSK transmission. Princi-pally, we introduce a single component that acts as a periodic narrow-band Gaussian filter in the frequency domain (see Fig. 1), thus convertingthe DPSK modulation into IM in all WDM channels simultaneously. Thiseffectively reduces the component count and implements the solutionproposed in [6] extending it into the WDM domain. Furthermore, thisscheme enhances so strongly the tolerance to chromatic dispersion thatwe transmit 10 Gbit=s DPSK signals with no dispersion compensationover 240 km G.652 fibre, a distance approximately three times higherthan usual.Fig. 1 Reflectivity profile of FBG arrayExperiment: The component used for the experiments is a custom-made array of unchirped fibre Bragg gratings (FBGs), all written inthe same piece of fibre. It works together with an optical circulatoras the FBGs act as selective reflecting filters. The spectral behaviourof the component reflectivity is reported in Fig. 1. On each WDMchannel of the ITU-T 100 GHz grid, we get a reflectivity profilehaving around 6 GHz full width at half maximum.As indicated in [7], the reflectivity has approximately a Gaussian shape.The gratings are chirp-free, with almost flat phase response (i.e. they haveno chromatic dispersion), as it is confirmed by the operation in the back-to-back configuration. The insertion loss at the various carrier wave-lengths ranges between4.0 and5.5 dB, mainly due to coupling losses.We used this device in the experimental setup shown in Fig. 2. We had16 CW optical carriers produced by DFB lasers combined by an arrayedwaveguide router (AWG1, 32 channels, 100 GHz spacing) and modu-lated by a LiNbO3 phase modulator at 9.953 Gbit=s (PRBS: 231 1).Note that we take advantage of the PRBS property to avoid anyelectronic differential encoder (the coded sequence is the originalone, shifted by some bits). Polarisation controllers (PC) are used tomatch the polarisation of the optical carriers to the transverse electric(TE) mode of the modulator, to produce the maximum phase modula-tion depth. The DPSK signals are then transmitted along a three-spanline. Each span is made of an erbium-doped fibre amplifier (EDFA),with ’15 dBm output power, and 80 km of G.652 singlemode fibre(D¼ 16.4 ps=nm=km). No dispersion compensation is used.DFB1DFB16PCPC AWG1PMEDFA EDFASMF(80 km)SMF(80 km)SMF(80 km)EDFA EDFA AWG2OCFBGs arrayreceiverFig. 2 Experimental setupDFB: CW lasers; PC: polarisation controllers; AWGs: WDM (de)multiplexer;PM: phase modulator. FBG array: array of unchirped FBGs (bandwidth: 6 GHzon each WDM channel)We outline that, according to the original theoretical proposal, in thelinear regime the FBG array could be positioned either at the receiver or atthe transmitter with the same practical performance. However, in a reallink including nonlinear propagation effects, we found it highly prefer-able to use it at the link end, to maintain the DPSK higher robustness tononlinear effects (namely owing to the constant envelope). After demo-dulation, all channels are demultiplexed by means of another AWG(AWG2), and then received using an APD-based receiver with variablethreshold.Figs. 3a and b show typical eye diagrams in back-to-back config-uration and after 3 80 km spans, respectively, taken at one of theAWG2 output pigtails. In Fig. 3b, despite the high accumulateddispersion, the eye is quite open. Still some intersymbol interference(ISI) is present, partly owing to non-ideal behaviour of the filter and,mostly, owing to the high accumulated dispersion (around4000 ps=nm). All the WDM channels were successfully transmittedover 240 km. Fig. 3c presents the optical spectrum at the output of theline. As can be seen, the optical signal-to-noise ratio (OSNR) values arequite good and range between 30 and 32 dB for all channels.0-15-30-45-60intensity, dBl, nm1546 1548 1550 1552 1554 1556 1558 1560 1562ca bback-to-back 240 km20 ps/div. 20 ps/div.Fig. 3 Eye diagrams and optical spectraa Typical eye diagram at receiver in back-to-back configurationb After 240 km G.652 fibrec Optical spectra taken at the line output (resolution bandwidth: 0.1 nm)We measured the bit error rate (BER) for these channels after each spanand found that the penalty increases at any span up to around 2.5 dB,showing indeed quite higher tolerance to chromatic dispersion than inusual DPSK-detection techniques, as predicted in [7]. Fig. 4a shows theBER curves taken for the worst-case channel (at 1551.72 nm) in theback-to-back condition, and after 80, 160 and 240 km, respectively. WeELECTRONICS LETTERS 19th January 2006 Vol. 42 No. 2report the final power penalty at BER¼ 109 for all WDM channels inFig. 4b. The channels do not have exactly the same performance, mainlybecause of the non-ideal response of the filters (slight deviations from theGaussian amplitude response and residual phase ripples). These measure-ments confirm that the filter can be effectively used in a WDM environ-ment.Fig. 4 BER and power penaltya BER measurements for worst-case channel (l¼ 1551.7 nm) in back-to-back andafter 80 km, 160 km and 240 kmb Power penalty (at BER¼ 109) for 16 DPSK channels after uncompensatedtransmission over 240 km G.652 fibreConclusions: We have demonstrated the feasibility of an FBG-basedcomponent capable of simultaneous demodulating a high number ofWDM-DPSK channels, furthermore increasing the system toleranceto chromatic dispersion. We tested it in a system experiment demon-strating transmission over 240 km of G.652 fibre (’ 4000 ps=nmaccumulated dispersion) with no chromatic dispersion compensation.Good system performance was obtained on all 16 channels. Owing tothe spectral filtering shape of the component it could be also used fora quite higher number of WDM channels (up to 50). We note thatalthough the system demonstration was carried out using 10 Gbit=sDPSK signals, it could be directly extended to 40 Gbit=s signals. Inthat case, one should properly redesign the filter (and, clearly, thedispersion tolerances should be lower, although higher by around afactor of 3 than in MZI-detected 40 Gbit=s DPSK).Acknowledgment: This work was partly supported by the MIURCOFIN TOSCA project.# IEE 2006 24 October 2005Electronics Letters online no: 20063758doi: 10.1049/el:20063758A. D’Errico, R. Proietti, L. Giorgi, G. Contestabile and E. Ciaramella(Scuola Superiore Sant’Anna, Via G. Moruzzi 1, 56124 Pisa, Italy)E-mail: antonio.derrico@sssup.itReferences1 Gnauck, A.H., and Winzer, P.J.: ‘Optical phase-shift-keyed transmission’,J. 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European Conf. onOptical Communications, ECOC2005, Paper Th2.2.26 Røyset, A., and Hjellme, D.R.: ‘Novel dispersion tolerant opticalduobinary transmitter using phase modulator and Bragg grating filter’.European Conf. on Optical Communication, ECOC’98, 1998, pp. 225–2267 Forestieri, E., and Prati, G.: ‘Narrow filtered DPSK implements order-1CAPS optical line coding’, IEEE Photonics Technol. Lett., 2004, 16,pp. 662–6648 Penninckx, D., et al.: ‘Optical differential phase shift keying (DPSK)direct detection considered as a duobinary signal’. European Conf. onOptical Communication, ECOC’01, 30th September–4 October 2001,vol. 3, pp. 456–4579 Calabretta, N., et al.: ‘All-optical label erasure=recognition of novelDPSK optical packets for optical packet switching’. Optical FiberCommunication Conf., 2005, Tech. Dig. OFC/NFOECELECTRONICS LETTERS 19th January 2006 Vol. 42 No. 2

"WDM-DPSK Detection by means of Frequency-Periodic Gaussian Filtering"

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