One of the most promising ways of creating high-power lasers (both high-average-power and high-peak-power lasers) is the coherent combining of laser beams in multi-channel laser systems. By coherently combining laser beams from N amplifying laser channels it is possible to increase the power of low-divergence beams proportionally to the number of channels (the total intensity of the paraxial optical beam grows as N2). The coherent beam combining was employed in the recently built solid-state laser systems providing a time average power of more than 100 kW with high beam quality, the world's most powerful solid-state lasers.
Based on the investigations of the mechanisms of refractive index changes in laser crystals and glasses O. L. Antipov and colleagues have implemented a new method of coherent combining of fiber laser systems. It relies on the effect of changes in the refractive index of aluminum or phosphorus silica waveguides doped with Yb³⁺, Tm³⁺, Nd³⁺ (used as the active medium of fiber lasers) under the action of the resonance wave of pump or saturation (eliminating the population inversion). Radiation from two erbium-doped fiber amplifying channels, in which the controlled element was an additional Yb³⁺-doped laser fiber, was coherently combined. The refractive index of the ytterbium fiber was controlled by pump signal (980 nm) and saturation signal (at 1064 nm). An algorithm for amplitude modulation of the control signal and synchronous detection in the feedback circuit provided high-speed perfor-mance, allowing noise compensation at a more than 10 kHz bandwidth and resulting in coherent beam combining of two fiber amplifiers with an efficiency of about 95%. The proposed method of optical control of the resonance ensures a lower noise level and faster performance (compared to the method of mechanical deformations of the fiber) and, in contrast to the electro-optical control, allows creating a relatively low cost all-fiber laser system.

Schematic diagram of a two-channel erbium amplifier with coherent combining of channels by resonance control of the refractive index of ytterbium amplifier

Another method of coherent beam combining currently under study at IAP RAS is the use of electro-optical modulation of the refractive index of the LiNbO3 crystal, which has unprecedented electro-optic susceptibility. High-speed waveguide-type electro-optic modulators coupled with the fiber laser system permit monitoring very rapid changes in the optical path of high-power laser beams, and eventually compensating for broadband noise distortions.

The Institute of Applied Physics jointly with the Institute of Atmospheric Optics SB RAS model the propagation of high-power radiation from multi-channel laser systems in the atmosphere. It is found that the coherent combining technique can provide focusing of intense laser beams on a remote target, even under conditions of strong atmospheric distortions of the laser beam.
IAP RAS researchers participate in the international program ICAN (International Coherent Amplification Network) aimed at creating a unique laser system with high average and peak power. Using the coherent beam combining technique in a multichannel laser system, including fiber and solid-state amplifiers, it is expected to reach under the ICAN program the megawatt average power level with high beam quality. This laser system, being far superior in output parameters to all existing lasers, is designed to accelerate elementary particles and achieve record-breaking energy levels in colliders of new generation.

Intensity distribution of the laser beam from 8-channel laser system in the plane of remote target (with atmospheric distortions) in the absence (left) and in the presence (right) of coherent addition