在星地激光通信中，大气湍流影响光接收波面，增加通信误码率。采用差分相移键控（DPSK）体制，通过当前码元信号光与延时1 bit的码元信号光干涉得到数据信息。在高码率通信的情况下，由于大气扰动对前后码元信号光的波面影响基本相同，可以通过干涉时波面相位相减得到差分信息。在此基础上，用菲涅耳衍射理论，分析入射光信号在自由空间差分干涉结构中的衍射传输过程。数值模拟仿真过程中，模拟探测波前像差对干涉系统性能的影响，通过对探测端零差效率的分析，验证了自由空间差分干涉结构解调光信号可以在一定程度上克服大气湍流效应对光信号相位信息的扰动。从理论上给出了自由空间差分干涉结构解调光信号的使用条件和结论。

In the satellite-to-ground laser communication, atmospheric turbulence influences the wave-front and increases bit error rate. The data information is got by an unequal arm-length interferometer based on differential phase shift key (DPSK), with the interference between the former code and the latter code of signal light. Generally, atmospheric turbulence makes the same influence on the wave-fronts of the front and rear codes in high speed rate communication, so differential information is obtained by the subtraction of the two successive wave-front phases when interference occurs. On the basis of theoretical foundation above, the diffraction propagation of incident signal light in the structure of the free-space differential interference is analyzed with Fresnel diffraction theory. In numerical simulation, the impact of detected optical wave aberrations on the performance of differential interference system is discussed. It is proved that free-space differential interference structure can overcome the disturbance of optical phase information of atmospheric turbulence effects. In addition, the applications and conclusions of optical signal demodulation with free-space differential interference structures are given.