Thermooptics of magnetoactive medium: Faraday isolators for high average power lasers
Federal Research Center Institute of Applied Physics of the Russian Academy of Sciences, ul. Ulyanova 46, Nizhny Novgorod, 603000, Russian Federation
The Faraday isolator, one of the key high-power laser elements, provides optical isolation between the master oscillator and the power amplifier or between the laser and its target, for example, a gravitational wave detector interferometer. However, the absorbed radiation inevitably makes the magnetoactive medium heat up and leads to thermally induced polarization and phase distortions in the laser beam — self-action process which limits the use of Faraday isolators in high average power lasers. What is unique about a magnetoactive medium thermooptics is that parasitic thermal effects arise against the background of circular birefringence rather than in an isotropic medium. Also, even an insignificant polarization distortions of the radiation result in a worse isolation ratio, which is the key characteristic of the Faraday isolator. All possible laser beam distortions are analyzed for their deteriorating effect on the Faraday isolator parameters. The mechanisms responsible for and key physical parameters associated with different kinds of distortions are identified and discussed. Methods for compensating for and suppressing parasitic thermal effects are described in detail, the published experimental data are systematized, and directions for further research are discussed based on the results achieved.