Optical three-port circulators made with ring resonators.

We propose a circulator consisting of a ring resonator coupled to three waveguides with Bragg reflectors at one end of each waveguide. A magneto-optically active material placed inside the ring resonator causes the two counter-propagating modes to split in resonance frequency, which can be exploited for perfect circulation by properly adjusting the coupling between the three waveguides and the ring. Such a device features a transmission spectrum that is similar to three-port photonic crystal circulators but is much simpler to build as it only contains elements that have already been experimentally realized.