Signal-to-noise ratio enhancement for digitalizing low amplitude wideband signals in photonic analog-to-digital converters

Photonic analog-to-digital converters (PADCs) have been investigated for nearly five decades as a promising approach to overcome the bandwidth and jitter problem and bring ADC performance to new levels. However, low-amplitude signals often struggle to achieve full-scale quantization accuracy, posing a basic challenge for achieving high signal-to-noise ratio (SNR) digitization. Here, we established an optical carrier-to-sideband ratio (OCSR) based sampler model to achieve the optimal combination of the modulation, loss compensation, and photoelectric detection processes. The OCSR-based sampler features the advantages of high useful signal gain, low noise figure, and the ability to function over a very wide frequency range. The low-bias region is investigated, and the corresponding OCSR is selected as the transfer function for the Mach-Zehnder modulator (MZM). The OCSR-based sampler enables a higher gain of the radio frequency (RF) information signal sidebands. After the beating at the photodetector, the useful signal power reaches the digitizer's full scale to fully utilize the quantization accuracy, thereby enhancing the SNR of the whole system. In the experiment, a 20 GSa/s PADC with 4 interleaved sub-channels is configured out. Considerable advantages of the proposed OCSR-based sampler over conventional quadrature-biased sampler are demonstrated in comparative tests. A ∼5 dB enhancement in SNR and an increase of ∼0.8 effective number of bits (ENOB) are achieved under sinusoidal signals, and linear frequency modulation (LFM) signals with 8 GHz instantaneous bandwidth as well.