Magnetooptical garnets combine high Faraday rotation with low optical losses in the near infrared region where optical communication via glass fiber is established. In this spectral range garnets are the only materials discussed to realize nonreciprocal devices as optical isolators and circulators. Although such devices are available as microoptical components, practical versions of their integrated counterparts are still lacking. Numerous concepts have been developed theoretically many of which are tested experimentally. This paper presents an overview of the state of the art of the applications of garnet films in integrated optics. Also the technique of combining garnets with semiconductor materials is shortly discussed.

A waveguide optical isolator based on nonreciprocal interference is demonstrated. Ridge waveguides are fabricated in a Mach–Zehnder configuration on a single film of bismuth-, lutetium-, neodymium-iron garnet. With this design, no polarizers are required to achieve extinction in the backward propagation direction. This isolator exhibits a 19 dB extinction ratio at λ=1.54 μm. A flat wavelength dependence, to within 2 dB, has been observed in the range between 1.49 and 1.57 μm.

A thick, bimodal segment of specific length and height between two single mode sections of a planar waveguide can serve as an integrated optical interferometer. It is realized by etching a wide strip from a guiding film. A -vertically guided, laterally unguided -beam of light is then made to traverse the strip perpendicularly. For a wide range of materials the structure can be dimensioned such that it shows the proper behaviour of an interferometer: Depending on the phase gain of the two modes in the thick region, the guided light interferes either almost completely destructively at the transition to the output segment, i.e. the power is radiated away into the substrate and cover regions, or constructively, i.e. most of the power passes the device. We believe that for certain applications structures of this kind can be a simple substitute for instruments like Mach-Zehnder interferometers or directional couplers. This is illustrated by two numerically simulated examples: A polarizer constructed from silicon based waveguides, which offers ¿¼ dB polarization discrimination and ¼ ½ dB insertion loss with a total length of only ½¼ micrometers, and a proposal for an integrated magnetooptic isolator experiment, where the freedom in the lateral direction can be exploited for a proper tuning of the device.

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Magnetic garnet films of composition (Nd,Lu,Bi)3(Fe,Al,Ga)5O12 are grown by liquid-phase epitaxy on [111] oriented substrates of gadolinium gallium garnet. They have positive uniaxial anisoptropy and support lattices of parallel stripe domains. A monomode optical rib waveguide is prepared by photolithography and ion-beam etching parallel to the (112̄) direction, which is the preferred direction of the stripe domains. A magnetic domain pattern can be created such that a domain wall is located at the rib center while the adjacent walls stick to the flanks of the rib. At a wavelength of 1.3 μm a nonreciprocal phase shift of the guided fundamental transverse electric mode is observed.