Evaluation of a cone tracking algorithm for free-space optical communication in direct mode

Abstract

The implementation of cone tracking to optimize the beam pointing in a retro-reflective link was reported in prior work [1]. One parameter required to close the loop is the phase difference between the scanning signal and the reflected modulated power, which gives the direction of the error. The phase difference is easy to measure in a retro-reflective link because the beam is reflected back to the transmitter making the feedback signal locally available. In a direct mode configuration, this phase difference is unknown. The problem was avoided in prior work [2] by using two detectors a few inches apart. This paper presents the results of the evaluation, using simulation, of an algorithm that estimates and tracks the phase difference between the two ends of the link making it possible to close the loop and insure convergence. It also shows how to implement the feedback path without using an additional beam as opposed to using an additional high-power wide-divergence beam.