A proposed opto- electronic oscillator (OEO) would generate a microwave signal having degrees of frequency stability and spectral purity greater than those achieved in prior OEOs. The design of this system provides for reduction of noise levels (including the level of phase noise in the final output microwave signal) to below some of the fundamental limits of the prior OEOs while retaining the advantages of photonic generation of microwaves. Whereas prior OEOs utilize optical amplitude modulation, this system would utilize a combination of optical phase modulation and suppression thereof. The design promises to afford, in the opto-electronic domain, the low-noise advantages of suppression of carrier signals in all-electronic microwave oscillators.
OEOs that utilize suppression of radio-frequency carrier signals have already been demonstrated to reject amplifier flicker noise. However, a second important advantage of microwave carrier suppression — reduction of the effects of thermal noise or shot noise (photon-counting noise) — has not previously been realized in OEOs. In microwave applications, realization of this advantage is made possible by (1) use, in oscillators or interferometers, of power levels higher than can be tolerated by a low-noise follower amplifier, combined with (2) means for reducing power levels at detectors while preserving sensitivity. In the proposed system, realization of this advantage would be made possible by notable aspects of the design that would enable the use of high optical power levels to reduce shotnoise- induced variation in the frequency of an OEO.