Vol. 24, No. 14 | 11 Jul 2016 | OPTICS EXPRESS 15486 Faraday rotator based on TSAG crystal with <001> orientation 1* 2 2 RYO YASUHARA, ILYA SNETKOV, ALEKSEY STAROBOR, ЕVGENIY 2 2 MIRONOV, AND OLEG PALASHOV 1National Institute for Fusion Science, 322-6, Oroshi-cho, Toki, Gifu 509-5292, Japan 2Institute of Applied Physics of the Russian Academy of Sciences, 46 Ulyanov Street, Nizhny Novgorod, 603950, Russia *[email protected] Abstract: A Faraday isolator (FI) for high-power lasers with kilowatt-level average power and 1-µm wavelength was demonstrated using a terbium scandium aluminum garnet (TSAG) with its crystal axis aligned in the <001> direction. Furthermore, no compensation scheme for thermally induced depolarization in a magnetic field was used. An isolation ratio of 35.4 dB (depolarization ratio γ of 2.9 × 10−4) was experimentally observed at a maximum laser power of 1470 W. 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Reitze, “Investigation of self-induced depolarization of laser radiation in terbium gallium garnet,” IEEE J. Quantum Electron. 35(8), 1116–1122 (1999). 32. E. Khazanov, N. Andreev, O. Palashov, A. Poteomkin, A. Sergeev, O. Mehl, and D. H. Reitze, “Effect of terbium gallium garnet crystal orientation on the isolation ratio of a Faraday isolator at high average power,” Appl. Opt. 41(3), 483–492 (2002). 1. Introduction A Faraday isolator (FI) is a key optical component of many laser systems as it is used to prevent backward reflection from the laser-irradiated materials or forward optics [1,2]. This device is important for laser-driven applications that utilize recently developed high-power lasers, such as high-energy and high-repetition lasers [3–5], ultra-high-power CW laser systems [6], and high-intensity laser systems [7,8]. However, it is difficult to use this device for high-average-power laser operation because of thermally induced effects, such as thermal birefringence effects that occur in the Faraday medium. More specifically, thermal birefringence degrades the extinction ratio of FIs, which is the most important parameter of such devices. Many studies aimed at solving this problem have been performed in the past 15 years. These reports include compensation methods for FIs [9–11], material parameter control methods that use cryogenic temperatures [12,13], as well as the development of new Faraday materials, such as Tb3Ga5O12 (TGG) ceramics [14–18] and Tb3Al5O12 (TAG) ceramics [19,20]. Today, FIs for lasers with an average power of over 1 kW can be realized as the result of the studies mentioned above. The next point of interest in the development of high- average-power FIs concerns realizing FIs with ultra-high average powers (e.g., 100 kW Vol. 24, No. 14 | 11 Jul 2016 | OPTICS EXPRESS 15488 lasers) [6]. In particular, material developments are important for increasing the operational average power of FIs. That is because high-average-power FIs require low absorption coefficients, good thermo-optic properties, and high Verdet constants.