Reversible optical isolators and quasi-circulators using a magneto-optical Fabry-Pérot cavity

Nonreciprocal optical devices are essential for laser protection, modern optical communication and quantum information processing by enforcing one-way light propagation. The conventional Faraday magneto-optical nonreciprocal devices rely on a strong magnetic field, which is provided by a permanent magnet. As a result, the isolation direction of such devices is fixed and severely restrict their applications in quantum networks. In this work, we experimentally demonstrate the simultaneous one-way transmission and unidirectional reflection by using a magneto-optical Fabry-Pérot cavity and a magnetic field strength of 50 mT. An optical isolator and a three-port quasi-circulator are realized based on this nonreciprocal cavity system. The isolator achieves an isolation ratio of up to 22 dB and an averaged insertion loss down to 0.97 dB. The quasi-circulator is realized with a fidelity exceeding 99% and an overall survival probability of 89.9%, corresponding to an insertion loss of ~0.46 dB. The magnetic field is provided by an electromagnetic coil, thereby allowing for reversing the light circulating path. The reversible quasi-circulator paves the way for building reconfigurable quantum networks.