Whispering gallery optical parametric oscillators: Coupling is the key

Optical parametric oscillators (OPOs) are versatile sources for coherent light. They convert a pump wave at wavelength λ_p to a signal at λ_s and an idler wave at λ_i fulfilling the condition 1/λ_p = 1/λ_s + 1/λ_i. Thus, the only fundamental tuning limitation is, that the frequency of the generated waves is lower than the one of the pump wave. The standard setup comprises a dielectric mirror resonator which is inherently large and coated for a specific spectral range. Recently, optical parametric oscillation has been demonstrated in optical whispering gallery resonators (WGRs). These monolithic cavities guide light by total internal reflection, combining very high quality factors and small mode volumes. This allows pump thresholds down to 10 μW pump power. Unlike conventional mirror resonators, whispering gallery resonators offer the ability to continuously vary the coupling strength. This is, e.g., possible by changing the distance between the coupler (in our case a prism) and the resonator. Our theoretical model suggests that strong overcoupling is necessary to achieve a high conversion efficiency. In contrast to this, to minimize the pump threshold the system should be weakly undercoupled. This allows to optimize the system for either high efficiency or low threshold power. Here, we experimentally proof this stated behavior.