Bismuth rare‐earth iron garnet composition for a magneto‐optical wheel rotation rate sensor

A magneto‐opticalgarnet composition has been developed for use in a multimode, two‐port, fiberoptic wheel rotation rate sensor for aerospace applications. The sensor utilizes a layer of (Bi, Y, Gd, Tm, Lu, Ca)3(Fe, Si)5O12grown on a (111)‐oriented substrate of Gd3Ga5O12 by standard liquid‐phase‐epitaxy techniques. The sensor has an integral biasing magnet and lensless coupling of multimode glass fibers to a polarizer/garnet/analyzer sandwich. The sensor operates at a signal channel of 725 nm and a reference channel of 850 nm, convenient wavelengths for semiconductor emitters and detectors. The (Bi, Y, Gd, Tm, Lu, Ca)3 (Fe, Si)5O1 2 layer is grown to 25‐μm thickness on one side of the substrate. It has a saturation field of 500 Oe, Curie temperature of 530 K, and the following approximate room‐temperature optical properties at 725 nm: a Faraday rotation of 15°, an optical attenuation of 5 dB, a specific rotation of 0.65°/μm, a specific attenuation of 0.25 dB/μm, and a figure of merit of 2.5°/dB. The low figure of merit is a consequence of the strong optical absorption of iron cations in the near infrared, but it is sufficient for this device. Gadolinium and thulium incorporation onto dodecahedral lattice sites serves the dual purpose of reducing the saturation magnetization and reducing the temperature dependence of magnetization. Device operation is specified over a temperature range of −65 to 450°F (−54 to 232°C), but layers of slightly higher Curie temperature allow operation to an upper temperature limit of 550°F (288°C).