Optical isolators are non-reciprocal photonic devices that allow light propagation in only one direction. By breaking time-reversal symmetry, degeneracy of forward and backward propagating photonic modes is lifted enabling non-reciprocal wave propagation. Time-reversal symmetry is typically broken using magneto-optical materials whose permittivity tensor has non-vanishing off-diagonal components, such as yttrium iron garnet (YIG, Y₃Fe₅O₁₂) and cerium-doped YIG (Ce:YIG, Ce₁Y₂Fe₅O₁₂). In particular, Ce:YIG is considered to be a promising isolator material for ~1550 nm wavelength given its large MO figure of merit, defined as Faraday rotation divided by optical absorption.

We have made optical isolators consisting of a ring resonator coupled to a waveguide. Magnetooptical cladding made of CeYIG is grown over part of the ring and magnetized perpendicular to the light path. This causes a nonreciprocal phase shift, which changes the resonant frequency of the ring. By operating off-resonance for forward propagating light and on-resonance for backreflected light, an isolation can be accomplished. Recent devices showed e.g. insertion loss ~7.4 ± 1.8 dB, and isolation ratio 13 ± 2.2 dB.

A schematic and cross-section of an integrated optical isolator, and a resonant peak. The change in magnetic field direction leads to a change in the resonant frequency of the ring. This is experimentally equivalent to changing the direction of light propagation.