Recent development of waveguide nonreciprocal devices, i.e. , isolator and circulator, is reviewed. Two major difficulties exist for their fabrication. First, form birefringence must be overcome to obtain high TE-TM mode conversion efficiency. Secondly, both of input and output light must be either TE or TM mode. A variety of methods and device structures have been proposed to solve these difficulties. However, experimental demonstration of waveguide isolator has been limited. After a brief discussion on advantages and drawbacks of proposed methods and device structures, a new fabrication process of waveguide isolator is described. A device structure which ensures TE mode operation was achieved by making adjoining nonreciprocal and reciprocal mode converters by using laser annealing technique. Faraday and Cotton-Mouton effects of Bi-substituted iron garnet film were used for nonreciprocal and reciprocal mode converters, respectively. Growth-induced and stress-induced birefringences of the film were used to cancel the form birefringence. An isolation ratio of 12.5dB was achieved at wavelength of 1.15μm with external magnetic field of 100 Gauss.
"Nonreciprocal Devices For Integrated Optics", Proc. SPIE 1126, Electro-Optic and Magneto-Optic Materials and Applications, (14 December 1989); doi: 10.1117/12.961381; https://doi.org/10.1117/12.961381