Free-space laser communications with adaptive optics: Atmospheric compensation experiments

Abstract

Refractive index inhomogeneities of the turbulent air cause wave-front distortions of optical waves propagating through the atmosphere, leading to such effects as beam spreading, beam wander, and intensity fluctuations (scintillations). These distortions are responsible for severe signal fading in free-space optical communications systems and therefore compromise link reliability. Wave-front distortions can be mitigated, in principle, with adaptive optics, i.e., real-time wave-front control, reducing the likeliness of signal fading. However, adaptive optics technology, currently primarily used in astronomical imaging, needs to be adapted to the requirements of free-space optical communication systems and their specific challenges.In this chapter we discuss a non-conventional adaptive optics approach that has certain advantages with respect to its incorporation into free-space optical communication terminals. The technique does not require wave-front measurements, which are difficult under the strong scintillation conditions typical for communication scenarios, but is based on the direct optimization of a performance quality metric, e.g., the communication signal strength, with a stochastic parallel gradient descent (SPGD) algorithm.We describe an experimental adaptive optics system that consists of a beamsteering and a higher-resolution wave-front correction unit with a 132-actuator MEMS piston-type deformable mirror controlled by a VLSI system implementing the SPGD algorithm. The system optimizes the optical signal that could be coupled into a single-356 Thomas Weyrauch and Mikhail A. Vorontsov mode fiber after propagating along a 2.3-km near-horizontal atmospheric path. We investigate characteristics of the performance metric under different atmospheric conditions and evaluate the effect of the adaptive system. Experiments performed under strong scintillation conditions with beam-steering only as well as with higherresolution wave-front control demonstrate the mitigation of wave-front distortions and the reduction of signal fading.

Iterative correction procedure for images degraded by turbulence

“…In the last few years, the SPGD was widely used in adaptive optics and coherent beam combining for its high performance of fast convergence and wide parameter bound [6].…”

Si integrated optical phased array for efficient beam steering in optical wireless communications

“…The beam steering function can be realized in several ways, and one popular solution is using micro-electro-mechanical-system (MEMS) based steering mirrors [5,6]. However, such mechanical beam steering mechanism has several fundamental limitations, including low stability, low precision, large size, and low speed.…”

High-speed, model-free adaptive control using parallel synchronous detection

“…Several applications in RF and optical communications call for compensation of high speed variations, mainly in the propagating media, such as multi-path fading, co-channel interference [1] and atmospheric turbulence [2]. The nonlinear dynamics of the communication system, as well as the stochastic nature of the noise introduced by the propagating medium, preclude the use of conventional system identification and optimal control techniques, while speed requirements dictate real-time designs for the control.…”