From optical parametric oscillation to frequency-comb generation in whispering gallery resonators made of CdSiP2

Microresonator frequency combs have seen remarkable progress in the past decade. In this work, we investigate optical parametric oscillation in a mm-sized cadmium silicon phosphide (CdSiP2) whispering gallery resonator for X(2) frequency comb generation in the mid-infrared region. The comb formation starts with optical parametric oscillation (OPO) near degeneracy followed by a series of internally pumped second-order nonlinear processes. The first step of OPO has been achieved with thresholds below 100 mW, efficiencies of up to 17 % and wavelength tunability from 2.3 to 5 mm. The control of the wavelength tuning is hampered when the OPO is operated close to degeneracy. Nevertheless, we have observed sideband generation around the pump wavelength, indicating that frequency combs can be generated using this scheme. However, improved control of the wavelength tuning near degeneracy is needed to increase the number of sidebands, leading to broadband frequency combs in the mid-infrared.