Phase-locking of rapidly changing phase fluctuations in a strongly pumped fibre amplifier using a stochastic parallel gradient descent controller

Abstract

We demonstrate phase-locking of the rapidly changing phase fluctuations in a strongly pumped fibre amplifier using a stochastic parallel gradient descent (SPGD) controller. The rapidly changing phase fluctuations are measured using a multi-dithering technique. The bandwidth requirement for an active phase controller is estimated by computing the structure function of the phase fluctuations. It is revealed that in our experimental setup, the phase fluctuation rates of λ/10 averaged amplitude are 830 Hz, 3.1 kHz and larger than 10 kHz under total pump powers of 71 W, 181 W and 230 W, respectively. A DSP-based SPGD controller with an iteration rate of 16 500 times per second is developed. Phase-locking is demonstrated under the pump power of 181 W. The amplified fibre laser beam interferes with a reference beam at the far-field plane to diagnose the phase-locking performance. It is shown that the peak intensity is increased by a factor of 1.57 and the visibility of the far-field pattern is higher than 0.56 when the system is in closed loop. The residual error is computed to be 0.07 waves from the far-field fringe pattern. The control error of the SPGD controller is restricted to be less than 1/25 wavelength by analysing the video document.