Optical-parametric oscillators (OPOs) enjoy an ever increasing popularity as tunable, monochromatic light sources. They convert a single-frequency pump field to signal and idler waves. Typically, cavities based on dielectric mirrors provide the necessary feedback. Their wavelength specific coatings and inherently large footprints can be overcome using whispering-gallery resonators (WGRs) . These miniature, monolithic devices combine a high finesse with mechanical stability. Furthermore, the light is guided by total internal reflection making dielectric, wavelength selective coatings obsolete. In a WGR, all three interacting waves have to be resonant simultaneously due to the broadband total internal reflection. OPOs based on WGRs have been demonstrated in the past years with pump thresholds below 10 µW . Unlike their mirror-based counterparts, all WGR OPOs demonstrated so far, employed a tunable pump source, scanning forth and back across a resonance and generating light flashes. In order to achieve true continuous-wave operation, the pump source has to be locked to a whispering-gallery resonance. This requires active stabilization of the pump laser, which is impeded by distorted pump resonances during the presence of a nonlinear process . Furthermore, the temperature of the resonator has to be stabilized to ensure the triply resonance of pump, signal and idler waves.
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