This paper proposes a non-common-view axis alignment method for the alignment requirements of airborne laser communication systems. The system consists of a ground transmitting end and an airborne relay terminal. The ground transmitting end uses a camera and a pan-tilt for image tracking, while the airborne relay end uses a two-dimensional mirror to control the beam to achieve non-common-view axis alignment between the transmitting and receiving sides. The working principle and process of both the transmitter and receiver of the non-common-view axis alignment system for airborne laser communication were compared with traditional wireless optical alignment methods. The design process of the two-dimensional mirror used in this paper is introduced, the scanning trajectory of the two-dimensional mirror is simulated and analyzed according to the beam scanning principle, and the field experiment link is set up to carry out the airborne laser communication experiment. The experimental results show that when the link distance is 10 m, the tracking errors of the system in the azimuth and pitch directions are 19.02 µrad and 22.35 µrad respectively, and the amplitude of the electrical signal output by the signal detector is 84.0 mV; When the link distance is 20 m, the tracking errors of the system in the azimuth and pitch directions are 39.66 µrad and 33.94 µrad respectively, and the amplitude of the electrical signal output by the signal detector is 23.0 mV. Using this method, the alignment can be completed without data return, and the establishment of the reverse link can also be realized while the transmission link is quickly established, and there is no need for an air stability platform. The feasibility of the application of the non-common-view axis alignment method to the airborne laser communication system is verified.

In free-space optical communication, the transmission of signal light and beacon light of differing wavelengths through the same atmospheric channel encounters variations in how the atmospheric refractive index absorbs and scatters light. This leads to distinct degrees of wavefront aberrations between the signal and beacon lights. In this study, we employed statistical optics to derive wavefront phase structure functions for both signal and beacon lights under conditions of strong turbulence. We explored how wavefront distortion varies among beams of different wavelengths after propagation through such turbulent conditions. Our findings revealed that as the turbulence outer scale escalates, the difference in wavefront distortion between signal and beacon lights stabilizes after an initial increase, assuming constant wavelengths. Furthermore, we observed significant changes in the relative wavefront aberrations when the inner scale of turbulence surpasses the separation between two points on the receiving apertures. As the disparity in wavelength decreases, so does the difference in wavefront aberrations. Finally, we propose a method for correcting wavefront aberrations based on coefficients of Zernike polynomials corresponding to beams with different wavelengths. This approach is validated through simulation and experimentation, demonstrating an 11% enhancement in the signal-to-optical Strehl ratio and a 0.072 increase in spot energy after the addition of correction coefficients compared with before their inclusion. These results solidify the efficacy of our method in improving adaptive optics correction accuracy.

Wavefront distortion caused by atmospheric turbulence can be described as different types of aberrations, such as piston, tilt, defocusing, astigmatism, coma and so on. The operation of dual deformable mirrors can have mutual coupling effects, which affect the correction effect of wavefront distortion. This study combines a fast-steering mirror (FSM) and a deformable mirror (DM) to form a dual-deformable-mirrors wavefront correction system, and proposes a decoupling algorithm that can correct any specified aberration. In this decoupling algorithm, both the FSM and the DM are controlled using the mode method, and the specific corrected aberrations are obtained based on a limited matrix. The compensation ability of the DM is directly characterized by the mode coefficients of the aberrations, which can achieve independent correction of any order of aberrations and effectively reduce the coupling effect of the dual-deformable-mirrors wavefront correction system. An adaptive optical dual-deformable-mirrors wavefront correction system experiment was built to verify the decoupling algorithm. When the DM corrects the 3rd-, 10th-, and 25th-order aberrations, and the FSM only corrects the 1st- and 2nd-order aberrations, the coupling coefficients are approximately $1.17\times 10^{-3}$, $1.814\times 10^{-2}$ and $7.81\times 10^{-3}$, respectively, and their magnitude reaches 10⁻² and below 10⁻², respectively. The experimental results show that the decoupling algorithm can effectively suppress the coupling effect between the FSM and the DM.

Atmospheric turbulence-induced random fluctuations in the refractive index can lead to the degradation of the polarization of polarized light. In accordance with the unified theory of coherent polarization, a comprehensive investigation was undertaken to explore the variation in the degree of polarization (DOP) of laser beams propagating through atmospheric turbulence channels under diverse weather conditions. This investigation involved both theoretical analyses and experimental validations, providing a multifaceted approach to understanding the dynamics of laser beam propagation in atmospheric turbulence. To this end, numerical simulations were performed to analyze the polarization-maintaining characteristics of laser beams with varying wavelengths, turbulence intensities, and initial DOP values. To validate the simulation results for various weather scenarios, three experimental links with different propagation distances were constructed. The experimental results demonstrated that as the turbulence intensity increased, the average DOP of the beam continuously decreased until it reached a threshold value. Furthermore, the polarization fluctuations exhibited a distance-threshold effect, wherein the polarization parameters tended to saturate beyond a critical propagation distance.