The average power of solid-state and fiber lasers has considerably increased during the last ten years. The 10 kW power is not record-breaking any longer, and a topical problem nowadays is to create lasers with a power of 100 kW. Therefore, the study of thermal effects caused by absorption of laser radiation in the bulk of optical elements becomes ever more important. The Faraday isolator (FI) strongly depends on these effects because its magnetooptical elements (MOEs) are relatively long and its absorption α0 is 10−3...10−2 сm−1, see Table 1. As a result, heat release power is at least tenths of percent of transmitted laser power P0.