Adaptive compensation of atmospheric effects with a high-resolution micro-machined deformable mirror

Abstract

ABSTRACT We present experimental results for the adaptive compensation of atmospheric turbulence effects on a free-space laser communication links at near horizontal propagation paths over 2.5 km and 5 km lengths. A high-resolution micro-machined piston type mirror array (12x12 elements) and a fast beam steering mirror were used in an adaptive optics laser communication system based on the model-free stochastic parallel gradient descent (SPGD) optimization wavefront control technique. Control of the mirror was performed by a VLSI SPGD micro-controller. The experimental results demonstrate the improvement of the receiver performance (fiber coupling efficiency) on a summer day with a refractive index structure constant in the order of 2 10 14 m -2/3 C n . Keywords: Adaptive optics, free-space optical communication, MEMS mirror, stochastic parallel gradient descent 1. INTRODUCTION Free-space optical (FSO) communication is considered as a valuable technology for many different areas, e.g. as a cost-effective alternative to short-distance fiber-optical links, for mobile high-bandwidth data transfer as needed in military and industrial applications, or for long-range ground-to-satellite communication. Wavelength division multiplexing (WDM) is implemented in current fiber-optic communication systems to increase the data transfer rate. To use the WDM technology in FSO systems a direct fiber-to-fiber connection without conversion of optical signals into electrical and back is desired. In general, the performance of FSO communication links is derogated through beam wandering induced by atmospheric turbulence, thermal expansion and vibrations of mounts and buildings as well as through beam spreading by higher-order atmospheric turbulence effects. An adaptive optics (AO) system is expected to compensate such wavefront distortions at least partially. Micro-machined deformable mirrors (µDMs) are promising candidates for the active element in FSO links because of prospective low cost, compactness and low operation power. Strong scintillations due to long horizontal path propagation cause problems for conventional AO systems using wavefront reconstruction from Shack-Hartmann measurements. A model-free optimization strategy, i.e. the maximization of a scalar system-performance metric (the received power in case of FSO links) can be considered as reasonable alternative approach: A novel efficient algorithm (stochastic parallel gradient descent