**Gabriele Frigenti 1,2,3,†, Daniele Farnesi 2,†, Gualtiero Nunzi Conti 1,2 and Silvia Soria 2,\***


Received: 18 February 2020; Accepted: 10 March 2020; Published: 13 March 2020

**Abstract:** Nonlinear frequency generation requires high intensity density which is usually achieved with pulsed laser sources, anomalous dispersion, high nonlinear coefficients or long interaction lengths. Whispering gallery mode microresonators (WGMRs) are photonic devices that enhance nonlinear interactions and can be exploited for continuous wave (CW) nonlinear frequency conversion, due to their capability of confine light for long time periods in a very small volume, even though in the normal dispersion regime. All signals must be resonant with the cavity. Here, we present a review of nonlinear optical processes in glass microspherical cavities, hollow and solid.

**Keywords:** kerr nonlinearity; whispering gallery mode; optical resonators; stimulated brillouin scattering; optomechanical oscillations
