Phasing of seven-channel fibre laser radiation with dynamic turbulent phase distortions using a stochastic parallel gradient algorithm at a bandwidth of 450 kHz

We have demonstrated an experimental setup for the coherent phasing of a seven-channel fibre laser system (λ = 1064 nm) in a scheme comprising a master oscillator and a set of parallel amplifiers with lithium niobate-based fibre-optic phase modulators. Using a stochastic parallel gradient algorithm, an instrumental phase modulator control unit ensures a bandwidth of the system up to 450 kHz. The effectiveness of phasing of light transmitted through a turbulent medium with a characteristic time scale τ_turb has been studied experimentally as a function of phasing time τ_ph. The results demonstrate that the average Strehl ratio begins to rise at τ_turb/τ_ph ≥ 2 and that the effectiveness of compensation for dynamic phase distortions in the beam propagation path rises sharply at τ_turb/τ_ph ≈ 20. For τ_turb/τ_ph ≥ 30 – 40, the average Strehl ratio remains constant at the level reached.