Phase-noise and stability comparison of fiber-loop and integrated-waveguide coupling in optomechanical crystal oscillators.

Optomechanical crystal cavities (OMCCs) display appealing features to function as on-chip photonic microwave oscillators (PMOs) when operated in the phonon lasing regime. Silicon OMCCs have so far demonstrated the best performance in terms of phase noise in a free-running configuration. Some preliminary experiments have also demonstrated that the generated microwave tone can be used for mixing wireless signals compliant with 5G standards. Still, more research is needed to identify the primary noise sources and determine the path towards improvement. Here, we report experiments on the realization of optomechanical PMOs operating at 4 GHz under two different driving conditions of the cavity: light coupling via a tapered fiber loop versus using an adjacent integrated waveguide illuminated by a lensed fiber. We performed measurements of the phase noise and the frequency stability in time of the detected tone for both cases and observed an improvement when the light is coupled by an adjacent integrated waveguide. This can be explained by the fact that the ambient-induced motion and changes due to perturbations in the refractive index of the medium surrounding the fiber loop result in variations of the coupling, which in turn affect the stability of the PMO. These results emphasize the relevance of a mechanically stable coupling technique when using such optomechanical devices in real applications, which can be of particular importance in environments where large vibrations are expected.