Filled Aperture Coherent Beam Combining using Multi-Plane Light Conversion and Microcontroller-Based Hill Climbing for Optical Feeder Links

Authors: Clément Jacquard, Claire Autebert, Raphaël Piccon, Pierre Vernaz-Gris, Gauthier Trunet, Julien Bayol, Pu Jian, Guillaume Labroille 

Cailabs, Rennes, FRANCE

Abstract

Coherent combination of laser beams has become a key technology for the development of high-power lasers for various applications, in particular for very high throughput Optical Feeder Links for ground to satellite communications. In this paper, we present the implementation of a filled-aperture coherent beam combiner composed of polarization-maintaining fiber components, Electro-Optic Modulators (EOMs) as phase control devices, erbium-doped fiber amplifiers, a Multi-Plane Light Converter (MPLC) and a feedback loop to achieve coherent combining. The feedback loop is based on a Hill Climbing method implemented on a consumer microcontroller executing the Stochastic Parallel Gradient Descent (SPGD) algorithm.

The MPLC combiner achieves coherent combination efficiency largely beyond tiled aperture coherent beam combiners: experimentally measured efficiency ~80% compared to a maximal theoretical efficiency of 67% for tiled aperture. Moreover, the reflective design of the MPLC enables high power handling, with 120W handling achieved with the uncooled combiner used in this paper, and multi-kW handling achievable with a cooled design.

The SPGD algorithm enables precise control of the EOMs to achieve a stable optical output power. Our experimental results demonstrate the effectiveness of the proposed approach, achieving coherent combination of up to 6 input channels with impulse response equivalent bandwidth sufficient to compensate the vast majority of thermal and mechanical perturbations, negligible power loss duration due phase wrapping interruptions and a combining efficiency above the currently known state-of-the-art.