Dual-wavelength adaptive optical system for free-space communications performance improvement

The analysis of the efficiency of laser radiation transmission through the atmosphere from the point of view of an alternative to existing radio communication methods of information transfer is based on determining the influence of negative effects such as scattering, the absorption of radiation, and the turbulence of air flows on the optical beam. The influence of scattering and absorption in the atmosphere is usually minimized by choosing an appropriate radiation wavelength, primarily the in near-IR spectrum. Atmospheric turbulence occurs due to uneven heating of the underlying earth's surface. Such heating causes local fluctuations in the refractive index of air, which leads to the appearance of an optical path difference of propagating optical rays. As the radiation propagates along the transmission path, the number of fluctuations increases; consequently, the wavefront of the beam arriving at the receiving focusing system has a shape that is far from flat, which manifests itself as a focal spot with a shape different from the diffraction-limited one. Various modifications of adaptive optics systems are typically used to reduce turbulence effects. As a result, an optical circuit was created that simulates a wireless communication channel, with two laser beams simultaneously directed along the atmospheric path, with wavelengths of 1550 nm and 830 nm. The first beam transmits the primary radiation, while the second beam analyzes the characteristics of the introduced turbulence onto a Shack–Hartmann wavefront sensor, as the sensor's sensitivity lies in the frequency range of 400–1100 nm. Artificial turbulence was generated using a fan heater, and the resulting phase fluctuations were corrected using the bimorph wavefront corrector.