提出并验证了一种基于矢量叠加原理对输入射频信号进行差分编码的光子模数转换方案.该方案采用两路不同波长脉冲源通过一个相位调制器对输入射频信号进行调制，调制输出接入延时线干涉仪从而产生两路具有固定相位差的差分调制信号.利用矢量叠加模块对两路差分调制信号的强度进行不同程度的衰减和组合，从而获得系统量化所需的各路传递函数之间的相移量.该方案能够对输入射频信号实现差分编码，有效提高了系统的比特精度.同时该方案结构简单，只需要一个相位调制器，一个延时线干涉仪和一个矢量叠加模块即可构建.另外，各路传递函数之间的相移量是通过衰减两路差分调制信号的强度来实现，避免了传统光子模数转换方案中调制器对温度、环境敏感而引入的相位漂移问题.本文对4比特模数转换系统进行了原理性验证，实验结果证实了基于矢量叠加原理的光子模数转换方案的可行性.

In this paper, a novel photonic analog-to-digital conversion scheme with differential encoding based on vector superposition is proposed and demonstrated. Two pulse sources with different center wavelengths are employed to modulate the input signal by a phase modulator, the modulated signal is sent to a delay-line interferometer to achieve two differential modulated signals with a specific phase difference. By adjusting and combining the intensity of two differential modulated signals with a vector superposition module, the desired phase shifts among different transfer functions can be obtained. Compared with most existing photonic analog-to-digital conversion schemes, the proposed scheme can differentially encode the input signal with improved bit resolution; furthermore, this scheme features its relative simpler configuration, because only one phase modulator, one delay-line interferometer and a vector superposition module are required; in addition, since the desired phase shifts of transfer functions are realized by attenuating the signal intensities, the proposed scheme can effectively alleviate the problem of phase bias-drift induced by modulators. Proof-of-concept experiment of a 4-bit photonic analog-to-digital conversion system based on the proposed scheme is successfully carried out, which demonstrates the feasibility of the approach.