Optimizing pump threshold and conversion efficiency of whispering gallery optical parametric oscillators by controlled coupling

Threshold and efficiency of optical parametric oscillation in whispering gallery resonators have been predicted to depend strongly on the coupling strength. Our experiments, varying the coupling strength continuously over 3 orders of magnitude, confirm these predictions. The pump threshold changes by a factor of 20, allowing the adaptation of the system to a wide range of input powers. The minimum threshold of 0.20 mW occurs for significant pump undercoupling. The efficiency increases monotonically with coupling, spanning 4 orders of magnitude with a maximum efficiency of 30% in contact yielding output powers exceeding 1 mW.