Proof of concept for adaptive sequential optimization of free-space communication receivers.

In a downlink scenario, the performance of laser satellite communications is limited due to atmospheric turbulence, which causes fluctuations in the intensity and the phase of the received signal, leading to an increase in bit error probability. In principle, a single-aperture phase-compensated receiver, based on adaptive optics, can overcome atmospheric limitations by adaptive tracking and correction of atmospherically induced aberrations. However, under strong turbulence situations, the effectiveness of traditional adaptive optics systems is severely compromised. We have developed an alternative intensity-based technique that corrects the wavefront by iteratively updating the phases of individual focal-plane speckles, which maximizes the power coupled into a single-mode fiber. Here, we present the proof of concept for this method. We show how this technique offers around 4 dB power gain with fewer than 60 power measurements under strong turbulence conditions. It delivers a good performance in different turbulent regimes, and it shows robustness against severe deterioration of the signal-to-noise ratio.

Intensity-based adaptive optics with sequential optimization for laser communications.

Wavefront distortions of optical waves propagating through the turbulent atmosphere are responsible for phase and amplitude fluctuations, causing random fading in the signal coupled into single-mode optical fibers. Wavefront aberrations can be confronted, in principle, with adaptive optics technology that compensates the incoming optical signal by the phase conjugation principle and mitigates the likeliness of fading. However, real-time adaptive optics requires phase wavefront measurements, which are generally difficult under typical propagation conditions for communication scenarios. As an alternative to the conventional adaptive optics approach, here, we discuss a novel phase-retrieval technique that indirectly determines the unknown phase wavefront from focal-plane intensity measurements. The adaptation approach is based on sequential optimization of the speckle pattern in the focal plane and works by iteratively updating the phases of individual speckles to maximize the received power. We found in our analysis that this technique can compensate the distorted phasefront and increase the signal coupled with a significant reduction in the required number of iterations, resulting in a loop bandwidth utilization well within the capacity of commercially available deformable mirrors.