Optical apparatus

An optical device includes a Faraday rotator made of a bismuth substituted rare earth iron garnet single crystal (BIG) having a Faraday rotation of 45 degrees, and permanent magnets arranged beside the Faraday rotator to define two or more areas of a single domain structure in the Faraday rotator. Adjacent areas are magnetized in opposite directions to cause the polarization planes of light beams passing through the adjacent areas to rotate in opposite directions. The optical device satisfies the relation expressed by Hs/Br/DH>ΔD/2>0 where Hs (Oe) is a saturation magnetic field of the BIG, DH (cm−1) is a rate of change in magnetic field in the proximity to a boundary between the adjacent areas, Br (Gauss) is a residual flux density of the permanent magnets, and ΔD is a distance between the two light beams.