Active coherent combining method based on multi-aperture wave front sensors

Abstract

Phase synchronization methods of several lasers are widely applied in the systems with high average power and narrowness. Among different approaches of coherent combining the most significant ones are methods based on active parameters control of each laser in the system. The control is usually conducted with distributed adaptive optical systems. In most practical cases it is an adaptive system of aperture sensing which is based on optimization (maximization) of image sharpness function [1]. The alternative system is the one with reference beam and wave front sensor (WFS) based on holographic [2] and heterodyne [3] principles. However, such systems are more difficult to construct. It is also impossible to use traditional adaptive optics wave front sensors in this multi-aperture system, because its work is based on the principle of building a continuous chart of phase aberrations. This paper considers the system of coherent combining with active feedback of multi-aperture wave front sensors (MWFS) without supporting beam group based on Gerchberg-Saxton algorithm [4, 5]. This algorithm allows reconstructing the complex fields on lens aperture and on its focal plane by measuring its intensity distributions. Structure scheme of active phase-locking system based on the Gerchberg-Saxton algorithm is shown on Fig.1. Optical signal from master oscillator splits to N fiber amplifiers and after passing through phase modulators the main beam focuses on object. The second beam forms control signals for phase modulators and after the compression it divides into two subbeams. The first one goes to photodetector PD1, and the second one focuses on the surface of photodetector PD2. Lenses and photodetectors are arranged in a manner where field distribution of main beam coincides with field distribution in front plane of photodetector PD1; it is connected by fast Fourier transform with field in front plane of photodetector PD2. Measured amplitude distributions are sent to computing device where iterated algorithm defines phase distribution. The control system based on this distribution forms control signals for phase modulators. The typical peculiarity of using this algorithm in phase information restoration is the presence of stagnation, which is often associated with local extremums [6]. In this work the strategy of phase information restoration based on Fig. 1. Block diagram of active phase-locking system, based on the Gerchberg-Saxton algorithm.