Integrated magnetooptic cross strip isolator

A bimodal planar waveguide segment of specific length and thickness between two thinner single mode sections can serve as an interferometer. Depending on the phase gain of the two modes in the thick region, these fields can interfere destructively or constructively at the transition from the bimodal to the single mode section. We employ this geometry to realize a simple magnetooptic isolator configuration, using a wide strip that is etched into a double layer in-plane magnetized magnetooptic film. The magnetization is oriented parallel to the strip; the light traverses the strip perpendicularly. Then the magnetooptic effect causes the phase velocities of TM polarized waves to be different for opposite directions of light propagation, resulting in a nonreciprocal power transfer across the strip. For a properly selected geometry one can expect isolator performance. If the strip width varies slightly, then adjusting the beam in-coupling position means to change the distance which the light travels in the two mode segment. This offers a convenient tuning possibility, which may be a means to overcome the strict fabrication tolerances that apply usually to interferometric integrated isolator concepts.