Research On Coherent-Lite Technology For Optical Interconnection Of Data Center

With the rapid development of virtual reality,meta-universe and Internet of Things（Io T）applications,short-reach optical interconnection represented by data centers（DCs）need to meet the demand of substantial traffic growth.Constrained by the dispersion-induced power fading,receiver sensitivity and bandwidth,it is difficult for direct modulation direct detection（IMDD）systems to cope with the upcoming high-capacity requirements of DCs.Superior to IMDD system,traditional coherent detection with digital signal processing（DSP）can meet the ever-increasing demand of DC.However,the high cost of narrow linewidth lasers and power-hungry DSP hinder the application of traditional coherent detection technology in DC scenarios.How to reduce the cost and power consumption of the coherent detection while satisfying the high-capacity of DC network（DCN）is a problem that needs to be solved.In order to balance the transmission capacity and cost,the traditional coherent detection system can be simplified from two different routes,one route is the coherent architecture while the other one is traditional DSP in receiver.The self-homodyne coherent detection（SHCD）system is one of the typical simplified coherent architecture.However,the polarization fading of the remote local oscillator（LO）hinders the application of SHCD system.We discuss the polarization fading problem of the remote LO and the corresponding solution of random phase modulation at the transmitter.On this basis,a simplified DSP coherent system based on adaptive polarization controller（APC）is further proposed.The system can not only solve the problem of polarization fading,but also realize a simplified DSP without multiple input multiple output（MIMO）,carrier phase recovery（CPR）,frequency offset compensation（FOC）and fixed chromatic dispersion compensation（CDC）.In terms of simplification of traditional DSP,traditional adaptive equalization（AEQ）uses large amount and fixed taps to cope with instantaneous differential group delay（DGD）,which may lead to high power consumption.We propose feedforward adaptive equalization（FFD-AEQ）based on fractional Fourier transform training sequence（Fr FT-TS）which is superimposed on the signal.The FFD-AEQ can reduce the complexity and power consumption while has the similar performance to that of traditional adaptive equalization.The main research results of this dissertation include:（1）For the polarization fading problem in SHCD system,a method of random phase modulation at the transmitter is proposed to suppress polarization fading,in which the two different schemes of uniform distribution sequence（UDS）and Gaussian distribution sequence（GDS）are used.224Gb/s dual polarization 16 quadrature amplitude modulation（DP-16QAM）signal SHCD system is proposed to explore the suppression effect of two different phase modulation schemes.Through scanning the modulation amplitude and modulation frequency,it is proved that the two schemes of UDS and GDS can alleviate polarization fading of remote LO.（2）In order to eliminate the power-hungry MIMO DSP,we propose simplified self-homodyne coherent detection（SHCD）system,in which the multiple input multiple output free（MIMO-free）DSP is achieved with the help of Bi Di-operated APC.Since the polarization wandering of the LO is relevant to that of the counter-propagating signal within the same fiber,the APC can realize the polarization tracking and compensation of the LO and counter-propagating signal simultaneously.Under the feedforward error correction（FEC）threshold of 2×10^-2,the experimental results show that the proposed SHCD system can achieve polarization rotation tolerance and linwidth tolerance up to 850rad/s and 1MHz,respectively.In our proposed SHCD system,the power-consuming 2×2 MIMO DSP,FOC,CPR and fixed CDC are no longer needed,resulting in the realizing simplified DSP while maintaining the full spectral efficiency（SE）of traditional coherent system.To further verify the feasibility of the proposed MIMO-free DSP method,we report the first successful real time demonstration of MIMO-free SHCD using 800Gb/s single-carrier DP-64QAM coherent transceivers.The APC can track the SOP of upstream LO,while the polarization-demultiplexing of downstream 91Gbaud DP-64QAM signal is achieved.The proposed system tolerates dynamic SOP rotation up to 400rad/s,which can satisfy most of the short reach DCN scenarios.Besides,the experimental results verify that real-time 800 Gb/s DP-64QAM MIMO-free transmissions can be realized using low-cost DFB with～MHz linewidth.（3）Considering the impact of DGD in the future DC application scenarios,the low complexity feedforward adaptive equalization（FFD-AEQ）based on Fractional Fourier transform（Fr FT）training sequences（TSs）is proposed.According to relationship bwtween fractional domain and time domain,the shift of aggregated peak of Fr FT TS can be utilized to jointly estimate the differential group delay（DGD）and time offset（TO）.The proposed DGD estimation has stable performance in a wide PMD range from 0 ps to 108 ps.The estimated DGD can be obtained before AEQ.By utilizing the prior information of DGD,the FFD-AEQ which can adjust the number of taps in MIMO.The experimental results show that the tap-varied AEQ has 52.6%power consumption decrease compared to the tap-fixed AEQ with 13 taps.