The opto-electronic oscillator is one of the most promising architecture for generating ultra-stable microwaves in the 0.1–100 GHz range. The idea consists in storing light energy instead of radio-frequency energy. This was usually achieved in a km-length fiber delay line [1,2], with the inconvenient of being bulky, energy-greedy, with a spectrum presenting spurious peaks that can be detrimental to applications. To overcome the drawbacks of fiber-based opto-electronic oscillators, it has been proposed to replace the fiber delay line with a high-Q whispering-gallery mode (WGM) resonator [3]. In such a resonator, photons are trapped inside the dielectric by total internal reflection, hence performing optical energy storage. For a Q-factor in the 109 range, the photon lifetime in the cavity Tph = Q/C0o is of the order of 1 μ8 with a corresponding linewidth of 100 kHz. According to Leeson theory, such linewidth allow to obtain superior phase noise performances than in traditional radio-frequency systems.
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