H. Dötsch

Applications of magneto-optical waveguides in integrated optics: review

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.

Magneto-optical nonreciprocal phase shift in garnet/silicon-on-insulator waveguides

We demonstrate the integration of a single-crystal magneto-optical film onto thin silicon-on-insulator (SOI) waveguides by use of direct wafer bonding. Simulations show that the high confinement and asymmetric structure of SOI allows an enhancement of approximately 3x over the nonreciprocal phase shift achieved in previous designs; this value is confirmed by our measurements. Our structure will allow compact magneto-optical nonreciprocal devices, such as isolators, integrated on a silicon waveguiding platform.

Improved design of magnetooptic rib waveguides for optical isolators

Garnet films of the following compositions (Lu,Bi)/sub 3/(Fe,Ga,Al)/sub 5/O/sub 12/ and (Tm,Bi)/sub 3/(Fe,Ga)/sub 5/O/sub 12/ are grown by liquid-phase epitaxy on [111]-oriented substrates of gadolinium gallium garnet. Double layers with opposite signs of Faraday rotation and single layers are prepared. Optical monomode rib waveguides are realized using such films. The nonreciprocal phase shift of the fundamental TM-mode is studied both theoretically and experimentally at a wavelength of 1.3 /spl mu/m. The maximum nonreciprocal effect of double layer films is about 1.7 times larger than that of similar single layer waveguides. Agreement between experiments and calculations is excellent.

Analysis of polarization independent Mach-Zehnder-type integrated optical isolator

This paper proposes, for the first time, an integrated optical isolator independent of light polarization. A Mach-Zehnder interferometer (MZI) with two nonreciprocal phase shifters, one for transverse electric (TE) modes and another one for transverse magnetic (TM) modes can be adjusted so that it blocks the fundamental modes of the waveguides constituting the interferometer propagating in one direction and is transparent for the modes propagating in the opposite direction. If the interferometer branch waveguides are in single mode regime, the performance of the device will not depend on the polarization of incoming light. The nonreciprocal phase shifters can be realized on structures with magnetization tangential to the propagation direction. Three geometries of nonreciprocal phase shifters are discussed and tolerances are estimated.

Nonreciprocal coupled waveguides for integrated optical isolators and circulators for TM-modes

Magneto-optic rib-waveguides can be utilized to construct the main component of devices like nonreciprocal Mach-Zehnder interferometers or nonreciprocal directional couplers. In this paper we present detailed investigations of nonreciprocal couplers formed by double layer rib-waveguides with alternating sign of the Faraday rotation. The performance of the proposed devices is simulated by normal mode theory and simple beam propagation calculations which include the incoming and outgoing dividing parts of the directional couplers. It is shown that the device length can be reduced by a factor of 6 as compared to former designs.

Phase-matched rectangular magnetooptic waveguides for applications in integrated optics isolators: numerical assessment

Phase matching between the fundamental TE and TM modes is an essential condition for complete polarization rotation in magnetooptic waveguides with longitudinally directed magnetization. This condition can be satisfied with embedded square waveguides or with raised strip waveguides, provided that the core dimensions are suitably chosen. Based on coupled mode theory for the vectorial modes of rectangular isotropic waveguides, we numerically simulate the performance of such devices in an experimental isolator setup, including birefringence and optical absorption. Fabrication tolerances with respect to all relevant parameters can be evaluated by simple perturbational expressions. Numerical verification shows that these formulas are accurate enough for practical purposes. The tolerances qualify the traditional polarization rotator setup as competitive to recent proposals for integrated optical isolators based on nonreciprocal interferometry.

Optical properties of bismuth and gallium substituted thulium iron garnet films

Abstract Bismuth and gallium substituted films of thulium iron garnet, grown by liquid phase epitaxy on [111] oriented substrates of gadolinium gallium garnet, are investigated for optical isolator applications. At a wavelength of λ = 1.3 μm the optical damping, the refractive index, the optical anisotropy, and the Faraday rotation are measured as function of the substitution level. It turns out that the growth induced optical anisotropy is very small, similar to the magnetic anisotropy. The observed difference between forward and backward propagation constants of TM modes is in excellent agreement with calculations.

Integrated optical isolator based on nonreciprocal-mode cut-off.

A concept of an integrated optical isolator is presented which uses the TM(0) mode propagating perpendicular to the in-plane magnetization of a planar magnetooptical waveguide. The cut-off thickness of the waveguide depends on the propagation direction. If the magnetic film has a strong Faraday rotation and is weakly guiding, the cut-off thicknesses for forward and backward propagation differ markedly. Operating close to cut-off yields a large difference between coupling efficiencies to the waveguide for forward and backward propagation.

Finite difference analysis of gyrotropic waveguides

Abstract Waveguides with gyrotropic materials are of large interest for the realization of optical isolators and circulators in integrated optical devices. This paper presents a finite difference analysis of such waveguides. The method is applicable to both planar and rib structures. From the calculated results some guidelines for optimizing the non-reciprocal effect are derived. We propose a new geometry for an effective gyrotropic waveguide.

Magnetooptical waveguides with polarization-independent nonreciprocal phase shift

Magnetooptical waveguides having nonreciprocal phase shift for both TE and TM modes can be prepared by properly adjusting the spatial variation of the Faraday rotation. Such waveguides are attractive to realize optical isolators. We investigate four concepts of magnetooptical waveguides which yield equal nonreciprocal phase shifts for the fundamental TE and TM modes. A polarization-independent Mach-Zehnder-type integrated optical isolator is presented. All the calculations are performed using material parameters typical for garnet films.