Sampling bandwidth expansion in channel-interleaved photonic analog-to-digital converters based on optical spectral convolution

Optical sampling pulses generated by cascade modulators have been widely studied for broadband signal acquisition in photonic analog-to-digital converters (PADCs). In this paper, we establish a comprehensive theoretical optical sampling model that incorporates the photodetection-induced optical spectral convolution, which in turn affects PADCs' bandwidth. Under sufficient bandwidth conditions of the sampling Mach-Zehnder modulators (MZMs), the bandwidth of the PADC is determined by the electrical spectrum, i.e., the optical power spectrum of the optical sampling pulses, rather than by the optical spectrum. Leveraging the flexibility and adjustability of dual parallel Mach-Zehnder modulators (DPMZMs), we achieve precise control over each optical comb lines. The optimal optical spectrum for the cascaded DPMZM is derived by optimizing the variance of the optical power spectrum, and 11 electrical frequency comb lines with a flatness of 3.5 dB can be obtained. In the experiment, a two-channel PADC is configured and 11 linear-phase electrical frequency comb lines with a flatness of 3.9 dB are achieved. With an input frequency of 4 GHz, a PADC frequency response degradation of 3.9 dB is achieved in a range of 0 to 42 GHz. Furthermore, our model enables large-scale integration of the PADC as the absence of dispersion compensation fiber, making PADC a competitive solution to ultra-wideband signal acquisition in the future.