Magnetooptical garnets combine high Faraday rotation with low optical losses in the near infrared region where optical communication via glass ?ber 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.

Applications of magneto-optical waveguides in integrated optics: review

https://iopscience.iop.org/article/10.1088/1468-6996/15/1/014602/pdf

Magneto-optical non-reciprocal devices in silicon photonics

A compact waveguide optical isolator whose input and output light are TE or TM mode has been realized by making adjacent nonreciprocal and reciprocal TE‐TM mode converters in a (BiGdLu)3(FeGa)5O12 film. Designed configurations of the magnetization directions were produced by making use of local laser annealing of the film grown on a Gd3Ga5O12 substrate of which the face normal is tilted from the [111] axis. The laser‐annealed and nonannealed regions worked as the nonreciprocal and reciprocal mode converters, respectively. The observed isolation agreed with theoretical calculation. An isolation ratio of 12.5 dB was achieved at a wavelength λ=1.15 μm.

Waveguide magneto‐optic isolator fabricated by laser annealing

This letter describes the use of a compact sputter-deposited thin-film magnet that is directly integrated onto a waveguide isolator fabricated in magnetic thin-film garnets. Film magnets of different thicknesses are sputtered onto the waveguides, and isolation ratios of 29 dB are obtained with negligible excess loss. Stress-birefringent effects due to the ferromagnetic film overlayer are eliminated by the use of an aluminum compliant buffer.

Integrated optical isolators with sputter-deposited thin-film magnets

The miniaturization of optical isolators and their on-chip integration with various optoelectronic devices has been pursued for a number of years. However, reducing the standard paradigmatic construction used in commercial isolators to waveguide form requires stringent fabrication tolerances and special materials that tend to render the process difficult and costly. Thus, novel approaches in device design and materials technology are called for and are being pursued. This paper discusses the application of the Faraday and nonreciprocal phase shift effects in waveguide isolator operation, the complications that arise in the miniaturization of these devices, and the novel solutions being pursued.

The on-chip integration of magnetooptic waveguide isolators

The many advantages of thin‐film waveguide magneto‐optic devices have been difficult to realize because of the linear birefringence of these films, which makes it very difficult to achieve the required Faraday rotation. In epitaxial garnet films, this linear birefringence (the difference between the refractive indices of the TE and TM modes) can be reduced to small values by (1) growing single‐mode multilayer films to minimize the shape effect, (2) growing the films in compression to control the photoelastic effect, and (3) annealing at high temperatures to eliminate the growth‐induced effect. The remaining birefringence can be reduced to zero by growing the top active layer too thick so that the shape effect is smaller in magnitude than the stress effect, and then thinning it by chemical etching until the effects exactly cancel each other at a particular wavelength. An alternative method is to grow the top layer too thin and then to deposit a dielectric layer such as silicon nitride of the proper thickness to reduce the shape effect so that it exactly cancels the stress effect. Theoretical results on multilayer magneto‐optic waveguides and successful etch‐tuning experiments at 1.32 and 1.51 μm are presented. These tuning procedures can produce polarization‐independent waveguide devices such as isolators or polarization rotators which may replace bulk or thick‐film devices.

Thin‐film garnet materials with zero linear birefringence for magneto‐optic waveguide devices (invited)

The linear birefringence of magneto‐optic garnet thin films, which interferes with the Faraday rotation in waveguide devices, can be reduced to small values by growing multilayer films to minimize the shape effect, annealing at high temperatures to reduce the growth‐induced effect, and growing the films in compression to control the photoelastic effect. The remaining birefringence can be reduced to zero by chemically etching the film until the effects exactly cancel each other at a particular wavelength. This etch‐tuning procedure is demonstrated on a two‐layer bismuth‐yttrium iron garnet (Bi‐YIG) film at wavelengths of 1.51 and 1.32 μm. Extinction ratios of 100 to 1 have been achieved at 45° and 90° Faraday rotations at both wavelengths.

Elimination of birefringence in garnet films for magneto‐optic waveguide devices

Epitaxial films of modified yttrium iron garnet which are used as optical waveguides for magneto‐optic devices have nominally planar anisotropy so that their magnetization can be aligned by small in‐plane fields. Films grown on (111) substrates usually have some residual cubic anisotropy that is difficult to measure in the active layer of the double‐ or triple‐layer films used in single‐mode waveguides. A new method to determine the magnetic properties of the optically active layer itself has been developed. It is based on magneto‐optical hysteresis loops in which the Faraday rotation of the waveguided light is plotted against one component of a rotating in‐plane magnetic field. Steps in these hysteresis loops are directly related to the sixfold symmetry of the magnetocrystalline anisotropy of these films. The influence of linear birefringence on the measured Faraday rotation, the magnetostrictive effects associated with the pressure of the prism used for optical coupling into the waveguide, and the advan...

M. McGlashan‐Powell

Papers

Thin‐film garnet materials with zero linear birefringence for magneto‐optic waveguide devices (invited)

Elimination of birefringence in garnet films for magneto‐optic waveguide devices

Magneto‐optic waveguide hysteresis loops of ‘‘planar’’ magnetic garnet films