Hybrid Frequency-Tunable Parity-Time Symmetric Optoelectronic Oscillator

An optoelectronic oscillator (OEO) is a microwave photonic system consisting of an amplified optoelectronic feedback loop to generate a microwave signal. To reduce the phase noise, the feedback loop must be long to ensure a high Q factor, which is usually implemented by incorporating a long optical fiber in the loop. The key limitation of using a long fiber is that a large number of closely spaced longitudinal modes exist in the feedback loop, which makes single-frequency oscillation difficult to achieve. In this article, we propose to incorporate a photonic integrated microdisk resonator (MDR) in an OEO to implement parity-time (PT) symmetry as well as frequency tuning. By employing the reciprocity of light propagation in the MDR, two mutually coupled optoelectronic loops having an identical geometry with one having a gain coefficient and the other a loss coefficient, identical in magnitude, to form a PT-symmetric OEO that support single-frequency oscillation, are implemented. The tuning of the frequency is realized by thermally tuning the MDR. Experimental results show a microwave signal with a frequency that is tunable from 2 to 12 GHz is realized. The phase noise of the generated microwave signal at a frequency of 11.5 GHz is -117.3 dBc/Hz at a 10-kHz frequency offset. The use of a photonic integrated microdisk resonator makes the entire PT symmetric system have high potential for full photonic integration.