中心博士生秦睿恒的工作——Signal-to-noise ratio enhancement for digitalizing low amplitude wideband signals in photonic analog-to-digital converters（光子模数转换系统中宽带低幅度信号数字化的信噪比增强技术）的相关成果近期被OPTICS EXPRESS期刊接收发表，该工作得到了国家自然科学基金（T2225023，62205202）的部分资助。光子模数转换技术（PADC）作为解决传统电子接收系统中带宽和时钟抖动问题的有效技术途径，近年来发展迅猛。然而，低幅度信号在实现满量程高精度数字化领域仍然面临挑战，从而制约了低幅值输入信号的信噪比（SNR）。本研究从理论上建立了一种基于光载波-边带比（OCSR）的采样模型，实现了调制、光放大、以及光电探测过程的最佳组合。根据理论模型构建的的基于OCSR的采样器实现了高信号增益、低噪声系数以及极宽的工作频率范围。具体而言，我们对调制器的低偏置区域进行了深入分析，并将OCSR为3dB的工作点选定为马赫-曾德调制器（MZM）的传递函数，通过载波抑制以有效提高射频信号边带的增益。光电探测器拍频后，有效信号能量达到了最大值，从而充分利用了后端量化器的满量程精度，增强了整个PADC系统的信噪比。在实验验证方面，我们搭建了采样率为20GSa/s的4通道PADC，相较于传统工作在正交偏置的采样器，所提出的基于OCSR的采样器在小信号输入的精度具备显著优势，将PADC的信噪比提高了5dB，有效位数提升了0.8bit，并验证了该方法在8 GHz瞬时带宽的线性频率调制（LFM）信号输入下同样有效。本研究所提出的方法使得PADC成为低幅值高分辨率宽带信号采集领域的有竞争力的解决方案，并有望应用于下一代雷达、通信等多种应用场景。
摘要: 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 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 to be achieved. 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.