Objective Wireless laser communication technology has advantages such as high communication speed, good security, and portability, making it widely applicable in both military and civilian communication fields. Aircraft, as one of the crucial platforms for wireless laser communication research, can effectively enhance the reliability and applicability of laser communication, playing a significant role in the future integrated aerospace network. However, the practical application of airborne laser communication faces various challenges. Apart from common atmospheric laser communication issues like atmospheric absorption, scattering, platform vibrations, and cloud cover, aerodynamic optical effects pose a significant constraint on the application of airborne laser communication.
Methods This paper addresses the issue of selecting the shape of the optical window for the airborne laser communication optical terminal. It designs streamlined fairings with conformal and planar optical window shapes (Fig.3). Fluent is used to simulate and analyze the flow field around the fairings under different flight altitudes, speeds, and azimuth angles. The refractive index distribution at the front end of the optical window is obtained based on the density and refractive index relationship (Fig.7). Then, using the ray tracing method, the wavefront distortion of the two windows under different conditions is obtained (Fig.6), verifying the independence of the optical grid and tracing length in ray tracing (Fig.8-9). Finally, the far-field free diffraction spot changes are analyzed using phase screen analysis (Fig.13).
Results and Discussions The results show that, at different azimuth angles, RMS increases first and then decreases, reaching a minimum at 90°, and rapidly increasing after exceeding 90°, reaching the maximum at 180° (Fig.10). Overall, the RMS caused by the spherical window is slightly larger than that caused by the planar window. When changing the pitch angle, the spherical window exhibits good flow field consistency, while the planar window has some influence on the flow field near the window (Fig.11). Flight altitude and speed are also crucial factors affecting window aerodynamic optical effects. Increasing flight altitude and decreasing flight speed can weaken aerodynamic optical effects, and the RMS change caused by the planar window is smaller than that caused by the conformal window (Fig.12). To gain a deeper understanding of the performance of the optical system under actual working conditions and observe the impact of wavefront distortion on the optical system, we conducted far-field diffraction analysis of the calculated wavefront distortion. With increasing azimuth angle, the changes in intensity and offset become more drastic, resembling the trend of RMS changes. However, for the planar window, after azimuth angle exceeds 150°, its wavefront distortion becomes more severe compared to the conformal window (Fig.14). Increasing flight altitude can weaken the window's impact on aerodynamic optics, and increasing flight speed results in noticeable differences between the two, with speed having a more significant impact on aerodynamic optical effects than altitude (Fig.15).
Conclusions When the azimuth angle is less than 90°, there is little difference in the optical transmission performance between the two window shapes. However, beyond 90°, the RMS value of the conformal window consistently exceeds that of the planar window. Nevertheless, after wavefront aberration and far-field diffraction, the distortion of the spot from the conformal window is noticeably smaller than that from the planar window, especially at azimuth angles of 180°, where the planar window's far-field spot shows significant peak intensity reduction and spreading. Changes in pitch angle demonstrate better stability for the conformal window compared to the planar window. Variations in flight altitude and speed significantly affect the peak intensity of the far-field diffraction spot, while having a smaller impact on the spot's displacement. Increasing flight altitude weakens the influence of the window shape on aerodynamic optical effects, while increasing flight speed exacerbates the differences between the two window shapes, with flight speed having a stronger impact on aerodynamic optical effects compared to flight altitude. The wavefront aberrations caused by aerodynamic optical effects are highly complex, and relying solely on wavefront aberration RMS values cannot fully define their impact on the optical system. For instance, although the RMS value of the planar window is smaller than that of the conformal window at azimuth 180°, the spreading degree of the spot during far-field diffraction is stronger than that of the conformal window. Overall, the conformal window exhibits more stable flow fields during variations in pitch, altitude, and speed, showing a more consistent trend and performing better in far-field diffraction. Conversely, the planar window's RMS value is lower than that of the conformal window under conditions of large azimuth angles and low-speed flight.
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