Study On Key Techniques In High Speed Short Reach Transmission Based On PAM-N Modulation Direct Detection

Spurred by the rapid growth of cloud services,streamed video,and Internet of Things,it is necessary to focus more on high speed optical interconnect.N level pulse amplitude modulation（PAM-N）based direct detection system is popular due to its low cost and easy generation.In December 2017,IEEE802.3bs released the standard of using 50 Gbps/lane PAM4 in 50 G,200 G and400 G Ethernet.For PAM modulated direct detection system,there are still many problems to be solved,such as limited receiver sensitivity of conventional PAM-N formats,skewed eye-diagram caused by the interaction between adiabatic chirp of DML and chromatic dispersion,impact of SOA-induced pattern effect on the filter requirements in vestigial sideband（VSB）direct detected transmission,bandwidth limitation of optical transmitters,etc.Our thesis focuses on the researches of the key techniques of high speed short reach transmission based on PAM-N direct detection system.The details are discussed as follows:1.New advanced modulation formats based on conventional PAM4formatsOne main disadvantage of PAM4 format is that the sensitivity is relatively low in comparison with low-order modulation formats.To address this problem,we proposed several advanced modulation formats:two dimensional（2D）coded PAM4 format,2D honeycomb trellis coded modulation（TCM）PAM4,and 2D-PAM8-TCM.2D means two consecutive symbols in time domain.2D coded PAM4 format can be generated by simply choosing the subset of 8 symbols from the 16 symbols to increase the minimum Euclidean distance.Honeycomb 2D PAM4 is more power efficient since honeycomb format requires lower average energy to reach the same minimum Euclidean distance,then it is combined with TCM to get 2D honeycomb TCM PAM4 format.2D-PAM8-TCM is proposed to achieve high spectral efficiency of 2.5 bits/symbol with high coding gain.TCM combines encoder and modulation effectively by introducing set partitioning.2.Skew correction of eyediagram based on PWL equalizerWhen DML works at a high bias current,the interaction between adiabatic chirp and chromatic dispersion can lead to skewed eye diagram.To correct this skewed eye diagram,piecewise linear equalzier（PWL）is proposed,which consists of threshold decomposition,parallel equalizers,and addition.Then,the applications of PWL equalizer on PAM4 transmission and high bitrate over 100 Gbps 2D-PAM8-TCM transmission is analyzed and tested.After optimizing the threshold set,PWL can effectively correct the skewed eye diagram and enable the BER of 56 Gbps PAM4 signal over 40 km transmission,and 84 Gbps PAM4 signal over 20 km transmission below the hard FEC limit(3.8×10^-3).Besides,experimental results show that PWL requires lower complexity to achieve the similar BER performance to Volterra equalizer.In104 Gbps 2D-PAM8-TCM over 10 km DML-DD transmission system,both transceiver nonlinearities and the interaction between DML chirp and fiber dispersion will introduce nonlinear distortion.To compensate for the nonlinear distortion,we propose a computationally efficient PWL-Volterra equalizer.Compared to only using the conventional 2nd order Volterra equalizer with a similar BER performance,the PWL-Volterra equalizer shows 29% computational complexity reduction.3.Optical filter requirements in an SOA-amplified VSB-PAM4 modulation direct detection transmission systemVSB modulation is introduced to suppress the power fading induced by chromatic dispersion in direct detection system.SOA is used to compensate the power loss after long distance fiber transmission.The low input saturation power makes the SOA-amplified signal susceptible to the pattern effect,which causes a considerable spectral broadening,thereby influencing the design of VSB filter.Qualitative analysis is given and upper sideband（USB）is proved better than lower sideband（LSB）owing to the suppression of SOA-induced pattern effect.Experimental results of 56 Gbps PAM4 over 80 km SSMF transmission in MZM-DD system verified the above conclusion.4.100 GBd PAM6 generation and detection using optical time and polarization interleaving techniqueLimited by bandwidth limitation of optical transmitters,current equipment fails to support high bit rate over 200 Gbps per wavelength transmission.Optical time and polarization interleaving（OTPI）technique is proposed to break through the bandwidth limitation.The key idea of 100 GBd（250 Gbps）OTPI-PAM6 signal is to generate 100 GBd PAM6 signal by interleaving two 50GBd 33%PDM-RZ-PAM6 signal in time domain and polarization domain by two lower-bandwidth DACs（25 GHz）and commercial available optical modulators.With 2⁹（9）order Volterra equalizer,100 GBd PAM6（250 Gbps）signals have been successfully transmitted over 1 km non-zero dispersion shifted fiber（NZDSF）fiber below 20%-OH SD-FEC limits.