Study of the Noise Processes in Microwave Oscillators Based on Passive Optical Resonators

International audienceTwo types of optoelectronic oscillators delivering high spectral purity microwave signals are presented in this paper. These oscillators use the Pound-Drever-Hall laser stabilization technique to lock the laser carrier onto two different types of passive optical resonators featuring high quality factors: a fiber ring resonator and a whispering gallery mode monocrystalline disk-shaped micro-resonator. The different noise processes occurring inside these oscillators are discussed. A particular attention is given to the conversion of the laser's amplitude and frequency noise into RF phase noise via the laser stabilization loop and the resonator, and via the photodetector nonlinearity as well. A modeling approach using a CAD software is also proposed to qualitatively evaluate the laser noise conversion through the optical resonator. Moreover, the different contributions of the nonlinear optical scattering noise are discussed, mainly in the case of the fiber ring resonator based oscillator. When controlling these nonlinear optical effects in the case of the fiber ring resonator, a low phase noise operation of the oscillator has been achieved, with a -40 dBc/Hz noise level at 10 Hz offset frequency from a 10.2 GHz RF carrier