Magneto-Optical Properties of (GdBi)3(FeAl)5O12 Bulk Single Crystals and Their Application to a 1.3 μm Optical Isolator

A compact optical isolator has been developed for use at a wavelength of 1.3 μm, using a Bi-substituted gadolinium iron garnet (Gd_3-xBi_xFe₅O₁₂) single crystal as a Faraday rotator. The optical isolator features a 0.35 dB insertion loss, 34 dB backward loss, and approximately 33.6 dB isolation ratio at 1.3 μm. The insertion loss and the magneto-optical figure of merit in Gd_1·8Bi_1·2Fe₅O₁₂ at 1.3 μ are about 0.025 dB and 3000 deg/dB, respectively. The temperature and magnetic field dependences of the Faraday rotation and the optical absorption have been measured for Gd_3-xBi_xFe_5-yAl_yO₁₂ single crystals, grown by an improved flux method in which Bi₂O₃ is used as a flux. The saturation field of the Faraday rotation decreases with Al substitution. The magneto-optical figure of merit of the Fe³⁺ absorption region at about 0.9 μm was approximately independent of the Al content for y ≪ 0.5. The Faraday rotation in Gd_3-xBi_xFe₅O₁₂ (0.85 ≪ x ≪ 1.2) changes by about 7.5%/50°C as the temperature rises above 25°C.