  Optomechanical oscillators (OMOs) combine the co-existing high quality factor
mechanical and optical resonances in an integrated device to realize low phase
noise RF oscillations. While several attempts have been demonstrated towards
modeling the phase noise in such oscillators, the close-to-carrier phase noise
models in literature do not account for $1/f^3$ (pink noise) and higher order
slopes in the phase noise spectra. Here we present a phase noise model,
corroborated with experimental characterization of phase noise of two
monolithic integrated silicon OMOs, accounting for contributions to the phase
noise due to thermomechanical, and adsorption-desorption (AD) noise. The model
shows good agreement with experimental data and provides further insights into
the mechanisms underlying the noise processes contributing to different slopes
in the phase noise spectra in OMOs.
