Herriott Cell Optical Isolators and Amplifiers - Rayleigh Length of the Beam Envelope

Recently, we demonstratad a Faraday isolator made of fused silica in a Herriott cell [1]. The polarization of a 5 W laser beam with a wavelength of 532 nm was rotated by an angle of 45° after 30 passes through two fused silica plates with a a thickness of 6.35 mm, which were placed in a magnetic field of about 0.5 T. The weak Faraday effect in fused silica was accumulated over the total propgation distance of about 38 cm. We reproducibly achieved a depolarization ratio of about -27 dB. In principle, this type of isolator could be realized with various materials. It might be useful for the development of isolators at wavelengths, where Faraday media with a high Verdet constant are not available.