An End-to-End Digital Postprocessing Linearization Method for Optical Sampling Link

Optical sampling is seen as one of the potential means to solve the challenges of digitizing high frequency broadband microwave signals. However, the high power-induced nonlinearity has greatly hindered the progress of system performance. Specifically, the nonlinearity of the modulator and photodetector respectively decreases the precision of the sampling phase and the holding phase. Typically, a small fraction of the frequency components are cancelled from each other with the constructed symmetric coefficients, and linearization over the entire frequency band could not be achieved. To solve this problem, an end-to-end digital postprocessing linearization method is proposed to perform a full link nonlinearity correction within the entire operating bandwidth, including the modulator and photodetector nonlinearity process. The precise correction functions of the modulator and photodetector are established respectively, and combined to linearize the sampling results in the digital domain. An experimental optical sampling link is set up to verify the proposed end-to-end linearization method. To induce nonlinearity, the photodetector operates at saturation region with a constant voltage of 0.26 V for the unmodulated optical pulses, and a single tone signal of 50% modulation index is applied to a quadrature-biased MZM. The harmonics are effectively suppressed from 22.8 dB to 59.7 dB. In the case of bandwidth-limited signal reception, the nonlinearity condition remains unchanged, and the error vector magnitude (EVM) of a 64-QAM signal is improved from 15.23% to 1.79%. The performance of the optical sampling link with the end-to-end linearization in terms of the operating point is also discussed.