A Parity-Time-Symmetric Optoelectronic Oscillator Based on Non-Reciprocal Electro-Optic Modulation

In this paper, we propose and experimentally demonstrate a parity-time- (PT-) symmetric optoelectronic oscillator (OEO) based on non-reciprocal electro-optic modulation for the generation of a microwave signal with a low phase noise. In the proposed OEO, two mutually coupled optoelectronic loops with one having a gain and the other a loss are implemented based on the copropagating and counterpropagating modulation in a traveling-wave electro-optic Mach-Zehnder modulator (MZM), to achieve PT symmetry. Due to the non-reciprocity of the travelling-wave MZM, the modulation indices are different for the copropagating and counterpropagating light waves in relative to the applied microwave signal, which is then interpreted as different microwave round-trip gains for the mutually coupled optoelectronic loops. Once the gain/loss coefficient is greater than the coupling coefficient, the PT symmetry is broken, and single-mode oscillation is achieved. The proposed OEO is experimentally demonstrated. A 10-GHz microwave signal with a phase noise of -110 dBc/Hz at an offset frequency of 10 kHz and a sidemode suppression ratio of 42 dB is generated. The work provides a new solution to overcome the mode selection problem in a long-cavity OEO. Since a single physical loop is employed, the proposed PT-symmetric OEO has the simplest architecture as compared to any existing PT-symmetric OEOs.