Angle-of-arrival variability of retroreflected lasers despite atmospheric reciprocity

Abstract

Corner cube retroreflectors are commonly used as cooperative targets in free-space laser applications. The previous literature suggests that due to path reciprocity, a retroreflected beam is self-corrected across a turbulent atmosphere and should show no angle-of-arrival variability in the near field. This is at odds with recent experiments that rely on angle-of-arrival measurements in retroreflected beams for effective tip/tilt correction. In this Letter we investigate the mechanism behind observed angle-of-arrival variability using numerical field propagation to model various transceiver and retroreflector geometries. We determine that asymmetric truncation of a curved wavefront at the retroreflector, transceiver, or both, results in a difference in tip/tilt between the transmitted and reflected wavefronts. This difference propagates as angle-of-arrival variation at the transceiver despite reciprocity, providing the error signal necessary for adaptive optics tip/tilt correction without a remote beacon.

Demonstration of 100 Gbps coherent free-space optical communications at LEO tracking rates

“…For this work we used the retroreflected beam as its own tip/tilt beacon, which provides an angle-of-arrival error signal despite atmospheric reciprocity due to the truncation of the Gaussian beam at the CCR on the drone payload 22 . This was convenient as it minimised the size and weight requirements of the drone.…”

“…Our work uses the retroreflected signal serving as its own tip/tilt beacon 21 , 22 , which due to atmospheric reciprocity 23 allows our terminal to simultaneously demonstrate correction of the “downlink” beam to maintain fiber coupling as well as pre-compensation of the “uplink” to maintain pointing on target. An overview of the experiment is depicted in Fig.…”

Demonstration of 100 Gbps Coherent Free-Space Optical Communications at LEO Tracking Rates

“…Our work uses the retroreflected signal serving as its own tip/tilt beacon 21,22 , which due to atmospheric reciprocity 23 allows our terminal to simultaneously demonstrate correction of the "downlink" beam to maintain fiber coupling as well as pre-compensation of the "uplink" to maintain pointing on target. An overview of the experiment is depicted in Figure 1.…”