<title>Phase locking in multichannel laser systems</title>

Abstract

It is well known way to increase the radiation power by combining beams from many lasers. However, the great increase in the intensity on the target may be obtained only if all beams are mutually coherent. The problem of making radiation of multichannel (multi-element) laser coherent was widely discussed. Several review articles were published recently [1-3] . The idea of coherent combining of a large number of phase locked beams is used in several concepts of super-high-power laser system: for chemical HF/DF lasers [4], for free electron lasers [5), for semiconducter lasers [6). The following methods to phase locking for laser array were considered: 1. frequency and phase locking of laser array to the radiation of master oscillator (MO) by adaptive optics method [7]; 2. the scheme master oscillator and power amplifier array [8; 3. phase locking of slave laser array to the injection signal from MO [9); 4. optical coupling between resonators (channels) of laser array [10]. Method of adaptive optics is attractive because it may have universal applications. However it needs design and production of many detectors, modulators, electrical serve loops and so on. Finally, it would be very complex system. The schemes 2 and 3 are at very initial stage of development. It was shown experimentally that the HF multiline radiation after the power amplifier (PA) may be coherently combined on a target with the radiation from MO [8] . The questions are leaving open of oscillator isolation and of choosing parameters of PA and MO to achieve high power and efficiency simultaneously. The amplifier array concept was not tested experimentally. There is a successful attempt at coherent combining of MO radiation with the radiation of injection locked Nd-YAG laser [11] . However, the transfer of this method to the array of injection locked lasers seems to be highly problematic due to tight tolerances for resonator length detunings. The concept of optically coupled laser array evolved very fast. So, this paper will be devoted to new results and ideas that made a progress in the concept of optically coupled lasers after the overviews [1-3] . These results relate to the problem of phase locking for two lasers and for the array of N lasers and concern the spectrum of collective modes and their stability. The dynamics of optically coupled lasers due to the resonator lengths detunings will be considered, too.