Widely tunable optoelectronic oscillator based on selective parity-time-symmetry breaking

Abstract

Optoelectronic oscillators (OEOs) are promising solutions for generating microwave signals with low phase noise and wideband tunability, and they can be applied to converging systems such as communications, radars, and electronic warfare systems. However, a significant challenge remains in ensuring a low phase noise, wideband tunability, and ultra-high side mode suppression ratio (SMSR) simultaneously. Parity-time (PT) symmetry breaking provides an excellent tool for single-mode oscillation by exploiting the interplay between the gain and loss. The oscillation mode was previously fixed because the breaking of the PT symmetry cannot be accurately manipulated. Herein, we propose an OEO with selective PT-symmetry breaking showing a wideband tunability and ultra-high SMSR. The tunability of the proposed OEO is attributed to the selection of different modes to break the PT symmetry using a widely tunable microwave photonic filter (MPF). The large roll-off of the MPF significantly enhances the gain difference between the selected and competing modes. Consequently, both the output power and SMSR of the OEO increase. During the experiment, the measured oscillation frequency is tuned from 2.6 to 40 GHz. The output power of the selected mode is enhanced by 12.9 dB, and the maximal SMSR reaches up to 71.4 dB. Further, the measured phase noise of the OEO at 17.74 GHz reaches −129 dBc/Hz at a 10-kHz offset frequency. Exploration of the selective PT-symmetry breaking provides the possibility of developing classes of widely tunable OEOs with an ultra-high SMSR and excellent low phase noise simultaneously.