Superchannel Fiber Optical Transmission Technique With High Spectral Efficiency

The ever-increasing bandwidth demand driven by new Internet applications including cloud computing,3-D video,sharing economy and virtual reality(VR)stimulates ultra-large capacity requirement for future fiber optical transmission.Therefore,given the limited bandwidth of optical fibers with low attenuation and optical amplifiers,high spectral efficiency(SE)transmission becomes a key technology for the future fiber optical transmission.Based on high-order modulation formats and pulse shaping technique,the superchannel transmission is an effective solution to increase the SE.However,the ultra dense wavelength division multiplexing(DWDM)may bring the performance penlaty from inter-carrier interference(ICI)and severe fiber nonlinearity induced impairments.Besides,the transmission distance of high-order modulation formats is limited,due to its low tolerance of nonlinear interference.Therefore,in addition to focus on increasing the SE with superchannel transmisison,we also carry out comprehensive inestigations of nonlinear impairment compensation for the superchannel transmission.With extremely low computational complexity,our research fruits can extend the transmission of superchannel by using high-order modulation formats,leading to higher transmission capacity.Based on the projects of 863 High Technology Plan and National Natural Science Fundation,this thesis aims to increase the SE of superchannel and compensate the nonlinear impairments with low complexity.We conduct comprehensive investigations and propose several innovative transmission schemes as follows.(1)We propose an ICI mitigation for dual carrier superchannel transmission.Using the modified decision directed least mean square(DD-LMS)based multi-input multi-output(MIMO)processing,we can simultaneously realize both polarization division de-multiplexing and ICI mitigation for the PDM-m-PSK and PDM-m-QAM formats.Thanks to the innovative modification of frequency shift operation,the MIMO equalization enables synchronous output of dual-carrier signals with robust performance against frequency offset,laser phase noise as well as polarization mode dispersion(PMD).The experimental back-to-back transmission of PDM-16QAM signal has 8.3-dB optical signal to noise ratio(OSNR)improvement by using the proposed MIMO processing.Three dual-carrier 160-Gb/s PDM-16QAM superchannels within 25-GHz grid are experimentally transmitted over 640km standard single mode fiber(SSMF),leading to net SE of6.0-b/s/Hz.(2)We propose and experimentally demonstrate a faster-than-Nyquist(FTN)transmission with a novel M&M algorithm for the ICI mitigation.To the best of our knowledge,it is the first experimental demonstration of PDM-16QAM FTN transmission over the SSMF.Due to the use of maximum likelihood sequence detection(MLSD)and MIMO processing for severely overlapped Nyquist-PDM-16QAM carriers,a net SE up to7.7-b/s/Hz is achieved by transmission of 15-GHz spaced 2×16-GBd PDM-16QAM dual-carrier superchannel over the 960-km SSMF,considering hard decision forward error correction(HD-FEC).Furthermore,with the help of soft-decision FEC(SD-FEC),the FTN superchannels with carrier spacings of 14-GHz and 15-GHz are successfully transmitted over 960-km and 1920-km SSMFs,respectively.(3)A split digital backpropagation(DBP)scheme for digital subcarrier-multiplexing(SCM)superchannel transmissions,which is denoted as SSDBP,is proposed and studied in both experiments and simulations.The implementation of the SSDBP is split at the both transmitter and receiver,leveraging existing fast Fourier transform/inversed fast Fourier transform(FFT/IFFT)to reduce computation complexity(CC).We experimentally demonstrate that the SSDBP,with a CC reduction up to 50%compared to the original receiver-side SCM-DBP,can achieve a nonlinear compensation Q~2 gain of 0.9-dB and1.3-dB for the 1920-km and 2880-km 34.94-GBd PDM-16QAM supercahnnel transmissions,respectively.The maximum SSMF reach can be extended by 31.6%using2-step SSDBP with only 27.5 complex multiplications per sample.We also numerically investigate the impact of digital-to-analog converter(DAC)resolution with the SSDBP implemented at the transmitter,and the impact of parameter uncertainties on the nonlinear compensation performance of the SSDBP.(4)We propose and experimentally demonstrate a single-step digital backpropagation(DBP)scheme for the 34.94-GBd PDM-32QAM supercahnnel transmission.With only 23additional complex multiplications per sample,the proposed single-step modified SCM-DBP(M-SCM-DBP)achieves 0.75-dB Q~2 improvement,compared to single carrier transmission with linear compensation(LC)over the 960-km SSMF.The CC of the M-SCM-DBP is only 37%of the conventional SCM-DBP and 10%of the the low pass filter assisted DBP(LDBP).The 36%SSMF reach extension to 1220-km is achieved in comparison with that using the single carrier scheme with linear compensation(LC).Based on numerical simulation,the flexible operation of the M-SCM-DBP for PDM-32QAM and PDM-64QAM superchannel transmissions is validated.