Realizing Parity-Time Symmetry in a Single-Physical-Loop Optoelectronic Oscillator

An Optoelectronic Oscillator (OEO) for single frequency oscillation enabled by broken parity time (PT) symmetry is presented. The PT symmetry is realized based on two equivalent loops with the gain loop formed by the beating of the optical carrier with the ±1st order sidebands and the loss loop formed by the beating of the ±1st order sidebands with the ±2
^nd
order sidebands at a photodetector in a single-physical-loop OEO. Once the gain and loss coefficients in the system are made identical in magnitude and are greater than the coupling coefficient, PT symmetry is broken and a single frequency oscillation is achieved. Experimental results show that a microwave signal at 9.997 GHz with a high sidemode suppression ratio of 45 dB and an ultra-low phase noise of -142 dBc/Hz at a 10kHz offset frequency is generated.