SBS Suppression and Electronic Path-Length Matching in Coherent Beam Combining

Coherent beam combining has been long used as a method to increase the total power and directivity of laser systems. Nevertheless, traditional beam-combining methods tend to face issues, such as stimulated Brillouin scattering (SBS) downstream, which can compromise the efficiency and effectiveness of the laser system. The common approach to achieving the true time delay necessary for coherently combining phase-modulated beams involves trimming optical fibers or using optical trombones. However, these methods are not only difficult to implement precisely but also suffer from substantial insertion loss. Furthermore, fiber heating and vibration can lead to variable path-length mismatches, posing additional challenges.

Technology Description

The technology employs coherent beam combining in which each individual beam is phase-modulated with a unique pseudo-random bit sequence (PRBS). This approach prevents the occurrence of stimulated Brillouin scattering downstream, a prevalent problem within the field. For a coherent combination of these phase-modulated beams, the PRBS waveforms have to be true-time-delayed meticulously within a minimal fraction of the bit duration. The beams are generated from a kilohertz linewidth seed, shared among multiple (N>1) arms, each treated with a distinct copy of the PRBS pattern. This method differs significantly from traditional techniques utilizing optical fibers and optical trombones to achieve true time delay. Trimming fibers presents precision-related challenges, while optical trombones are known for large insertion loss. Moreover, this technology addresses the issue related to path-length mismatch varying because of heating or vibration of the fibers. The relative phase between the PRBS patterns can be stabilized using phase-locked master oscillators, enabling compensation for physical path length mismatches of the optical fibers.