Whispering gallery optical parametric oscillators become mature

Continuous-wave optical parametric oscillators (cw OPOs) combine narrow-linewidth emission with a large wavelength tuning range far beyond that of lasers. Conventionally, these devices are based on a nonlinear-optical crystal surrounded by a mirror cavity. In this configuration, cw OPOs are nowadays operated from the visible to the mid-infrared with watt-level output powers. They are ideally suited not only for high-resolution spectroscopy. Enabled by the pioneering work [1] of researchers from the Jet Propulsion Lab, a new OPO configuration was demonstrated some years ago [2] - whispering gallery optical parametric oscillators (see Fig. 1a). Here, the light is guided by total internal reflection in a spheroidally-shaped nonlinear-optical crystal. These monolithic devices do not require any reflective or anti-reflection coating, and they are easily miniaturized down to sub-millimeter diameters. Furthermore, due their low oscillation thresholds, they can be pumped by compact laser diodes. However, they are intrinsically triply-resonant, i.e. all three interacting waves are simultaneously circulating the cavity. It might be assumed that the wavelength tuning of WGR OPOs is much more difficult to control compared with that of conventional mirror-based singly-resonant systems. Hence, it was questionable if they are applicable in fields beyond fundamental science.