All-Fiber Optical Nonreciprocity Based on Parity-Time-Symmetric Fabry-Perot Resonators

Abstract

\n Nonreciprocal light transmission in an all-fiber device with remotely tunable isolation ratio and switchable isolation direction is proposed and demonstrated. Three cascaded fiber Bragg gratings (FBGs) are inscribed in an erbium-ytterbium co-doped fiber (EYDF) to form two mutually coupled Fabry-Perot (FP) resonators. The two FP resonators have an identical geometry, but the gain and loss of the FP resonators, and their mutual coupling are manipulated by controlling the pumping power to realize broken parity-time (PT) symmetry. Strong optical nonreciprocity is achieved due to gain saturation nonlinearity that is enhanced by the broken PT symmetry. The proposed device is fabricated, and its operation is experimentally evaluated. Nonreciprocal light transmission with an isolation ratio of 8.58 dB at 1550 nm and a 3-dB bandwidth of 125 MHz is experimentally demonstrated. Our study also indicates, by optimizing the design, an isolation ratio up to 33 dB can be achieved. The proposed approach provides an all-fiber solution for a remotely tunable and optically controlled isolator, which may find great applications in software-defined optical networks.