| From an optical standpoint, Earth's atmosphere is a randomly evolving and inhomogeneous medium. Transmitting light through the atmosphere results in random distortions to the light's phase and amplitude. In a free-space optical communications (FSOC) system, these random distortions manifest themselves as a fluctuating signal at the receiver. Conventional adaptive-optics (AO) systems, using steering mirrors to correct tilt aberrations and a deformable mirror to correct higher-order aberrations, have been shown reduce these signal fluctuations. However, conventional AO systems are often unsuitable for challenging platforms where requirements on weight, volume, and power consumption are strict. Recent advances in liquid-crystal spatial light modulators (SLMs) have provided a means of correcting aberrations with large dynamic ranges. Such SLMs may be used to compensate tilt and higher-order aberrations in one device, which enables the use of simplified and, thus more suitable, AO system. As a result, this research has focused on the use of such a simplified AO system in the receiver of a ground-to-air FSOC link over a range of turbulence conditions. The research methods consisted of wave-optics simulations and scaled-down laboratory experiments. The SLM compensated for tip, tilt, and the next 40 orders of aberrations. In both the simulations and experiments, the AO correction improved the variability of the received signal, thereby reducing the probability of bit errors by factors of 100,000 for the weakest turbulence and 10 for the strongest turbulence. In the best cases, the AO compensation provided an error probability less than one part in a million. |
