The mode of spatial optical in wireless laser communication systems directly affects the coupling efficiency of spatial optics to single-mode fibers. The mode conversion model of a free-spatial optical path based on a liquid crystal spatial light modulator is studied to improve the content of fundamental modes. According to the spatial spectrum filtering relationship between the conversion mode and the target mode, the corresponding conversion transfer function (CTF) is obtained. Combined with the simulated annealing algorithm, the CTF is accelerated locally, and its optimal value is obtained. Therefore, the conversion efficiency from higher-order modes to fundamental mode is generally improved. In addition, an experimental platform is established to verify the rationality of the simulation results. Further, it is indicated that the coupling efficiency can be improved by mode conversion.

In wireless laser communication, the effects of atmospheric turbulence cause laser spots to appear as irregular speckles in the far-field plane, which may make automatic acquisition, pointing, and tracking systems unable to locate a spot position. We propose an innovative and precise anti-interference, spot-detection algorithm for far-field irregular speckles based on the intensity characteristics of the far-field spots. Before obtaining the spot center coordinates through two-dimensional vector space circle fitting, the proposed algorithm employs computer graphics combined with mathematical morphology to fill the connected domains and extracts light-spot edges using the Canny operator, thus resulting in an accurate detection of far-field spots. The algorithm validation, using an automated experimental platform (atmospheric turbulent refractive-index structure parameter Cn2 is 10^−  15  m^−  2/3), yields a detection threshold accuracy of 30 to 32 pixels (each pixel represents 3  μrad) for 10.2 km far-field spot images. The corresponding deviation satisfies the two-dimensional aiming platform for a minimum step distance of 17.45  μrad, verifying the feasibility and effectiveness of the algorithm.

To improve the performance of wireless laser communication (WLC) systems, wavefront sensorless adaptive optics based on deformable mirror (DM) eigenmodes was used to correct the distorted far-field spot. A set of DM eigenmodes conforming to the derivative–orthogonal relationship is derived, and the system simulation model under different atmospheric turbulence intensities is established by combining Fresnel diffraction and multiphase screen transmission principle. The influence of mode bias on the correction effect is analyzed. The ability of the proposed method to correct the distorted far-field spot was studied by using Strehl ratio (SR) as the evaluation index. The influence of the proposed method on the coupling efficiency of single-mode fiber and bit error rate performance of the WLC system is discussed. Indoor and outdoor experimental systems were built. The indoor experiment results show that the proposed method can increase the SR of the system from 0.11, 0.32, and 0.56 to 0.57, 0.71, and 0.88, respectively. The outdoor experiment results show that the proposed method can effectively correct the wavefront distortion caused by dynamic atmospheric turbulence.