1 Tb/s Nyquist-WDM PM-RZ-QPSKSuperchannel Transmission over 1000 km SMF-28 with MAPEqualization

Ze Dong,Jianjun Yu,and Hung-Chang Chien

(ZTE USA Inc.,Morristown,NJ 07960,USA)

AbstractIn this paper,we evaluate transmission in a 1 Tb/s(10×112 Gb/s)Nyquist-WDM PM-RZ-QPSK superchannel over a widely-deployed SMF-28 fiber with and without maximum a-posteriori(MAP)equalization.Over 1000 km can be reached with BERbelow the HD FEC limit and with a spectral efficiency of 4 b/s/Hz.

Keyw ords

1 Introduction

T erabit superchannel technology is attracting much interest because of its unparalleled capacity,and because it can fulfil the fast-growing demand for bandwidth[1]-[8].Coherent optical OFDM(CO-OFDM)is a candidate superchannel technology that has high spectral efficiency and superior tolerance of inter-symbolinterference(ISI).However,its performance is affected by misalignment of symbol transitions,power mismatch between subcarriers,and insufficient receiver bandwidth[2].As an alternative,Nyquist-WDM is a practical means of delivering a bundle of standardized 100G channels with partial spectral overlapping.Subcarrier crosstalk is suppressed by aggressive optical filtering,which means ISIis the dominant impairment to be tackled at the receiver[4]-[7].Transmission of a 100 Gb/s Nyquist-WDM signalover large core fiber and short erbium-doped fiber amplifier(EDFA)span with nonlinear equalization has been realized.However,this transmission is mainly for subsea applications[4]-[7].In this paper,we describe transmission of a 1 Tb/s Nyquist-WDM superchannel over widely deployed SMF-28 fiber within an 80 km-span EDFA system.The Nyquist-WDM superchannel comprises ten subcarriers with 25 GHz frequency spacing,and each carries a 112 Gb/s PM-RZ-QPSK signal so that the combined bit rate is 1.12 Tb/s.

2 Testbed Setup for Terabit Nyquist-WDM Superchannel Transmission

Fig.1 shows the experiment setup for generation and transmission of 1 Tb/s optical Nyquist-WDM PM-RZ-QPSK superchannel.The superchannel comprises 12 subchannels spaced at 25 GHz.Each subchannel is powered by an external cavity laser(ECL)with a linewidth smaller than 100 kHz.The subchannels are divided into even(ECL2-12)and odd(ECL1-11)groups.ECL2-12and ECL1-11are combined by a polarization-maintaining optical coupler(PM-OC)and are then modulated by an I/Q modulator(I/Q MOD)driven by two sets of 28 Gb/s pseudorandom binary sequences(PRBSs)with word lengths of 213-1.The RZ carver is realized using a single-arm Mach-Zehnder intensity modulator(IM)driven by a 28 GHz RFsignal multiplexed from a 14 GHz RFsource.The duty cycle of the signal after the IM is about 45%.After polarization multiplexing(PM),each subchannel carries a 112 Gb/s PM-RZ-QPSKsignal.The odd and even subcarriers are lunched separately into two independent 25/50 GHz optical interleavers(ILs)and are then combined using a rear 25/50 GHz IL.Such cascading and aggressive optical filtering mitigates interchannel interference(ICI)between Nyquist-WDM subchannels at the cost of increasing the ISI.The aggregated terabit Nyquist-WDM superchannel is then launched into a recirculating loop comprising five 80 km SMF-28 spans with an average loss of 16.7 d B and chromatic dispersion of 17 ps/km/nm at 1550 nm.The loop has no optical dispersion compensation modules.For each span,an EDFA with midstage adjustable-tilt filter is used to provide flat gain.A tunable optical bandpass filter is also used to remove amplified spontaneous emission(ASE)noise.

▲Figure 1.Testbed setup for 1Tb/s Nyquist-WDMsuperchannel transmission.

The total launch power into the transmission fiber is 10.7±4 d Bm(-4 to approximately 4 d Bm per subchannel)at 112 Gb/s.After that,a subcarrier is selected using a tunable opticalfilter(TOF)for coherent detection.At the receiver,an ECL with a linewidth less than 100 kHz is used as the fiber laser local oscillator(LO).Apolarization-diverse 90 degree hybrid is used for polarization and phase-diverse coherent detection of the LO and received opticalsignal before balance detection is performed.Analog to digital sampling and digitization occurs in the digital scope,which has a 40 GSa/s sample rate and 16 GHz electrical bandwidth.The captured data is processed through an offline DSP.First,the clock is extracted using a square and filter method,and the digital signal is resampled at twice the baud rate of the recovery clock.Second,a T/2-spaced time-domain finite impulse response(FIR)filter is used to compensate for chromatic dispersion.Third,two complex-valued,13-tap,T/2-spaced adaptive FIRfilters,based on classic constant modulus algorithm(CMA),are used to retrieve the modulus of the QPSK signal.

Carrier recovery is then performed.The feed-forward fourth power is used to estimate the frequency offset between the LO and received optical signal.Then,the Viterbi-Viterbialgorithm is used to estimate the carrier phase.To improve the transmission performance of a Nyquist-WDM PM-RZ-QPSK signal subject to tight optical filtering and crosstalk,we propose MAP equalization with high data-pattern dependence.First,a sequence of data symbols with BERless than 3×10-4is decided before averaging so that the symbols can be arranged into a data dependence pattern(64 kinds in the case of QPSK)to be a decision reference.Then,the data received after DSPis calculated by correlating with the decision reference to the maximum extent and mapping the QPSKdata dependence decisions.

3 Experiment Results

▲Figure 2.Opticalspectra(0.1 nm)(a)before RZ,(b)after RZ,(c)after two ILs for ECL7,and(d)after two ILs for ECL1-11.

▲Figure 3.Optical spectra(0.1 nm)of 1Tb/s signal in(a)back-to-back transmission and after(b)1000 km,(c)2000 km,and(d)2400 km SMF-28 transmission.

The CWlight waves(ECL1-12)range from 1541.5 nm to 1543.7 nm with wavelength spacing of 25 GHz.An IM with appropriate DC bias and electricalamplifier power control are used to create an RZ pulse with 44%duty cycle.The Vpp of the 28 GHz RFsignalis 17 V.Fig.2(a)shows a single-carrier 28 Gbaud QPSKsignal before the RZcarver,and Fig.2(b)shows a single-carrier 28 Gbaud QPSK signal after the RZ carver.Fig.2(c)shows the optical spectrum of the subchannel(ECL7)after two stages of 25/50 ILs.The ratio of in-band signal power to out-of-band signal leakage is about 18 d B.Such out-of-band leakage can cause ICIto spread to adjacent even channels that are 50 GHz apart.However,this is not an issue in a practical system where each subchannel is independently modulated and then combined through an arrayed waveguide grating(AWG)with the subchannel bandwidth highly confined.Fig.2(d)shows the optical spectrum of subchannels ECL1-11after two stages of ILs.To simulate a practical system,we turn the fifth and ninth subchannels off to mitigate crosstalk and assess the performance of the seventh subchannel after transmission.Fig.3 shows the optical spectra of ten subcarriers with and without transmission.We then characterize the back-to-back performance of the seventh subchannel by aligning its subwavelength to that of the LO and turn allthe other subchannels off(Fig.4a).The required OSNRfor a BERof 3.8×10-3is 14.5 d Bwith aggressive filtering and 19.5 d Bwithout aggressive filtering for 0.1 nm resolution.The filtering is performed by two stages of interleavers(two ILs),and 650,000 symbols are counted as BER.When allthe odd subchannels are turned on,the OSNR penalty caused by ICIbetween 50 GHz spaced subcarriers is 0.9 d B.For a 1 Tb/s Nyquist-WDM signal,the OSNRpenalty caused by 25 GHz adjacent subcarriers and tight filtering is about 13 d B.For 1000 km SMF-28 Nyquist-WDM transmission(Fig.4b),the BERof the seventh subcarrier can barely reach the hard-decision(HD)pre-FEC limit of 3.8×10-3at any launch power level unless MAPequalization is used.With MAP equalization,the BERcan be brought well below the FEC limit at subchannel launch powers of-1 and-2 d Bm.Similarly,for 2000 km SMF-28 Nyquist-WDM transmission(Fig.4c),BERbelow soft-decision(SD)pre-FEC limit of approximately 1×10-2can be obtained with MAPequalization at input power of-1 d Bm.(Fig.4c).The bit rate should be 120 Gb/s or higher if SD FEC overhead is considered.

4 Conclusion

We have described the generation,transmission,and coherent detection of a 1 Tb/s Nyquist-WDM superchannel where each subchannel carries a 112 Gb/s PM-RZ-QPSK signal on a 25 GHz grid.The robustness of MAPnonlinear equalization against tight Nyquist-WDM filtering and crosstalk has also been shown in a widely deployed SMF-28 fiber with an 80 km-span EDFA system.With subchannel launch power of approximately-1 d Bm,the obtained BERs are below the HD FEC limits for Nyquist-WDM transmission over 1000 km SMF-28 with EDFA-only amplification.

▲Figure 4.(a)BERcurve of seventh subchannel(back-to-back),(b)BERcurve for 1000 km SMF-28 transmission,and(c)BERcurve for 2000 km SMF-28 transmission.