Intensity-based adaptive optics with sequential optimization for laser communications

Abstract

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.

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

“…The intensity level b l of each plane wave is calculated as the normalized amplitude of the associated speckle peak at the focal coordinates r l in the camera sensor. Then, the optimal phase shape of the DM can be estimated as [7]…”

“…which is directly proportional to the number of main intensity speckles. A detailed explanation of each step can be found in [7].…”

“…The objective of this paper is to validate in the laboratory the theoretical concept and numerical analyses shown in [7]. We aim to demonstrate that this technique increases the mean coupled power and reduces its variance, minimizing the overall bandwidth utilization.…”

“…As a result of these limitations, it is of high interest to efficiently compensate the field with minimized bandwidth utilization. For such a goal, we have proposed a sequential optimization technique based on the signal processing of the speckle pattern in the focal plane [7]. It works by iteratively updating and optimizing the phases related to individual speckles, maximizing the composite signal with a number of iterations proportional to the number of treated speckles.…”

Experimental evaluation of adaptive distributed frame repetition at 10Gbps for the satellite-to-ground optical link

“…For the first problem, site-diversity technique is a unique practical approach at this moment, in which the ground station with better condition would be selected for use from the separately-placed plural candidates on a moment-to-moment basis [5]. The second problem should be handled with the various kinds of technologies such as mechanics [6], optics [7], digital signal processing [8], communication protocols [9], and so on. Our group has proposed the use of Adaptive Distributed Frame Repetition (ADFR) to suppress the influence of fast fluctuating received optical power.…”