A Comparison of an Improved Design for Two Integrated Optical Isolators Based on Nonreciprocal Mach-Zehnder Interferometry

Nonreciprocal rib waveguide structures can be used to realize integrated optical isolators. The nonreciprocal phase shift is the difference between the forward and backward propagation constants of TM modes in magneto-optic waveguides. It can be optimized with respect to absolute value and temperature dependence if double layer waveguides with different magnetic and nonmagnetic layers are prepared. In this paper we propose an improved design for two different Mach-Zehnder interferometer isolators the nonreciprocal parts of which are formed by such double layer waveguides. One concept utilizes a nonreciprocal and a reciprocal arm. In the other case both arms are nonreciprocal but with opposite sign of the nonreciprocal phase shift. A particular property of both concepts is that the lengths of the nonreciprocal arms are well defined. The rest of the interferometer is made by reciprocal rib waveguides. Therefore, the nonreciprocal phase shift is well known. The concepts are compared with regard to isolation ratio, forward losses and fabrication tolerances. Moreover, we simulate the entire isolator by a finite difference beam propagation calculation.