Magneto-optical Branching Waveguides and Their Application to Nonreciprocal Optical Devices

The branching waveguide principle underlyring guided-wave devices is described, and an application to nonreciprocal devices is discussed. Branching waveguides are classified into two types: a symmetric type operating as a power divider, and an asymmetric mode-splitter type. A 3-dB coupler and hybrid coupler are provided with these branching waveguides. The authors indicate a possible construction of guided-wave type nonreciprocal devices such as isolators and circulators, employing 3-dB couplers, hybrid couplers, nonreciprocal phase shifters and a reciprocal phase shifter, the principle of which is the same as that of microwave hybrid type nonreciprocal circuits. The authors also describe the fabrication of a Mach-Zender interferometer on epitaxial YIG film, which is the key element for realizing the nonreciprocal devices discussed here.