Isotope effect in germanium single crystals as a promising medium for high-power mid-IR Faraday isolators

The magneto-optical and thermo-optical characteristics of high-purity germanium crystals of different isotopic compositions were studied to assess the applicability of this material as a magnetoactive medium for Faraday isolators for high-power radiation in the mid-IR range. An increase in the Verdet constant with increasing atomic mass of the isotope was observed in experiments. The physical mechanisms arising from the quantum theory of the Faraday effect associated with interband transitions, which can explain this dependence, are discussed. No isotopic variations in the thermo-optical characteristics that determine the maximum operating power of Faraday isolators of conventional design were observed. The results of measurements at a wavelength of 1940 nm demonstrated the possibility of constructing a Faraday isolator with one magneto-optical element at an operating power over 100 W and an isolator with compensation of thermally induced depolarization using a reciprocal rotator at an operating power surpassing 200 W. The prospects of using Ge crystals for developing Faraday isolators for longer wavelength radiation were discussed. The use of radiation with wavelengths greater than or similar to 2 mu m opens up broad opportunities for increasing the operating power of isolators based on various schemes and for creating broadband isolators.