为了探测几十赫兹的低频水下声信号，采用一种光学测量方法，建立了低频水下声信号的光学探测系统。当未扩束的激光束照射到受水下声信号调制的液体表面时，液体表面相当于位相型光栅，观察到了稳定、清晰的衍射图样，衍射图样呈非对称分布。得到了衍射光强分布的解析表达式，解释了在斜入射的条件下衍射图样位置及强度的非均匀分布和在垂直入射条件下，衍射光场的均匀分布，理论和实验比较吻合。结果表明，在激光束斜入射条件下，衍射图样的位置及强度是非均匀分布的。

In order to detect the low-frequency underwater acoustic signal at tens of hertz, an optical measurement method was put forward and a measurement system was developed. The clear and high stable diffraction pattern was observed experimentally when laser beam illuminates on the liquid surface modulated by underwater acoustic signal. The diffraction fringes were asymmetric. The expressions of the diffraction intensity were derived theoretically. The asymmetric distribution of position and diffraction intensity with laser oblique incidence was explained and the symmetric distribution of diffraction intensity with laser normal incidence was explained. Theoretical results were in good agreement with experimental ones. The results show that the position and diffraction intensity with laser oblique incidence has asymmetric distribution.