Whispering gallery optical parametric oscillators: Just a scientific oddity?

Optical parametric oscillators (OPOs) convert the frequency of laser light to almost arbitrary values. Nowadays, they serve as wavelength-agile light sources for spectroscopic applications as well as photon generators for quantum-optical experiments. Conventional OPOs are based on a nonlinear-optical crystal surrounded by a mirror cavity. Regarding the cavity, one often finds the following rule of thumb: the more waves are resonant, the lower is the oscillation threshold and the more difficult is the wavelength tuning. In whispering-gallery-resonator-based OPOs (WGR OPOs), light is guided by total internal reflection in a millimeter-sized spheroidally-shaped nonlinear-optical crystal. Thus, these devices are intrinsically triply resonant. Indeed, they provide microwatt-level oscillation thresholds, i.e., the lowest values of all OPO configurations. However, following the abovementioned rule, their applicability in fields beyond fundamental science might be questionable, because the most striking feature of an OPO, i.e., the wavelength tunability, is hampered. Nevertheless, several experimental studies revealed that the output wavelengths of WGR OPOs could be tuned in well-defined steps over hundreds of nanometers by temperature variation. Combined with strategies for mode-hop free tuning, it is possible, e.g., to tune the output wavelength in a controlled way until it meets MHz-wide resonances. This is sufficient for high-resolution spectroscopy. WGR OPOs are nowadays operated around various center frequencies, covering the visible-to-mid-infrared spectral range. These light sources - despite of their intrinsic triple resonance - might serve as compact and wavelength-agile devices for various applications.