Effect of Laser Coupling and Active Stabilization on the Phase Noise Performance of Optoelectronic Microwave Oscillators Based on Whispering-Gallery-Mode Resonators

We investigate the effect of laser coupling efficiency and active stabilization on the phase noise performance of optoelectronic oscillators based on monocrystalline whispering-gallery-mode disk resonators. We show that the resonator's intrinsic optical quality factor is not the only parameter to optimize in order to get a good phase-noise performance. In addition, the oscillator setup may involve many noise conversion processes that have to be evaluated and understood in order to optimize the spectral purity of the output microwave. We study the effect of the laser lightwave coupling into and out of the optical resonator on the spectral purity of the oscillator, and we evidence the role of this critical parameter on the phase noise performance. We also investigate the upconversion of baseband noise, even originating from marginal components in the oscillator setup, onto the output microwave and the resulting degradation of the spectral purity. Our experimental results are obtained using two ultrahigh-Q disk resonators manufactured with calcium and magnesium fluoride, respectively.