Laser-active whispering-gallery resonators as versatile platform for optical three-wave mixing

While laser sources are limited in their emission spectra by their gain-medium, non-linear frequency conversions allow for generation of coherent light in almost any spectral region. Optical whispering-gallery resonators (WGRs), fabricated from non-linear crystals, were successfully applied as miniaturized, high-efficiency frequency converting devices [1]. Frequency conversion in WGRs is usually induced by pumping optical resonances of the WGR by means of an external narrow-linewidth and frequency tunable laser. Hence, the effective technological overhead of the overall system is rather high, hindering this technology from spreading into everyday life's applications. One approach to overcome this major drawback is to incorporate laser-active ions into the resonator's material. This turns passive WGRs into narrow-linewidth lasers themselves, capable of inducing non-linear processes directly within the same crystal. With this, requirements on pump sources are lowered considerably, as laser-active ions can be excited even with incoherent light. The non-linear optical processes of interest can be selected by tailoring the phase-matching properties of the crystal by periodic domain inversion. With these tools at hand, laser-active whispering-gallery resonators may emerge as a versatile platform for continuous-wave frequency conversion. In principle, coherent light of any frequency within the transparency range of the non-linear crystal can be generated.