Research On DAC Clock Leakage Compensation Algorithms In Short-reach High-speed Optical Fiber Transmission Systems

Discrete multi tone(DMT)and four level pulse amplitude modulation(PAM4)are two commonly used advanced modulation technologies.Due to their high spectral efficiency,they have been widely used in short-reach and high-speed optical fiber transmission systems.At present,the capacity requirement of data center has increased from 100 G to 400 G or even higher.Wavelength division multiplexing is an effective method to achieve signal transmission at the data rate up to 400 G.To realize DMT/PAM4 signal with 100 G or more per wavelength,high sampling rate and bandwidth digital-to-analog converter(DAC)are usually employed in transmitters.However,clock tone leakage(CTL)often exists in high-speed DAC.In DMT and PAM4 systems,the signal interfered by CTL may lead to the deterioration of the bit error rate(BER)performance.To solve this problem,this paper aims to investigate low-cost digital signal processing algorithms to accurately estimate and effectively compensate DAC-CTL.The research contents and achievements of this paper are as follows.Firstly,two low-complexity DAC-CTL compensation algorithms are proposed for optical fiber DMT system.One is time-domain averaging-based CTL compensation algorithm(TDA-CTLC);the other is frequency-domain CTL compensation algorithm(FD-CTLC).We verify the feasibility of TDA-CTLC and FD-CTLC algorithms in optical fiber DMT system by offline experiments,and compare the compensation performance and stability of the two algorithms.In addition,the feasibility of TDA-CTLC algorithm based on fourth-order digital interpolation in the presence of sampling frequency offset(SFO)is explored,and the impact of SFO and its estimation error on BER performance are discussed.The results show that the proposed algorithm has low implementation complexity and can effectively estimate and compensate the DAC-CTL in optical fiber DMT system.Secondly,the compensation performance of an improved TDA-CTLC algorithm is studied in the optical PAM4 transmission system.Firstly,we analyze the damage of DAC-CTL in optical PAM4 transmission system,and theoretically explain the operation principle of DAC-CTL estimation and compensation.The convergence rate and compensation performance of DAC-CTL estimation are also discussed.Secondly,the effects of fiber dispersion,interference-to-signal power ratio and ADC vertical resolution on the proposed algorithm are studied by numerical simulation.Finally,the compensation performance of the algorithm is verified in the short-reach PAM4 transmission system based on directly modulated laser.The compensation performance of the CTL compensation algorithm based on adaptive narrow band filter and the improved TDA-CTLC algorithm is compared.Numerical simulation results show that the proposed algorithm can effectively resist the impact of fiber dispersion,CTL interference and ADC quantization noise.Experimental results show that the proposed TDA-CTLC algorithm can achieve compensation performance similar to ANF-CTLC algorithm.