Frequency comb generation based on optical parametric oscillation with second-order nonlinear materials

There have been impressive advances in the area of microresonator-based frequency combs (microcombs) during the last decade, making their way into applications in a wide variety of fields [1]. Although microcombs are commonly generated making use of third-order ( CH (3) ) nonlinear processes, several groups have recently started to investigate frequency comb generation based on the second-order ( CH (2) ) nonlinearity as it bears potential benefits like the linear electro-optic effect for either tuning or stabilization, or the possibility to convert light into more challenging spectral regions like the UV or mid-infrared [2] , [3]. For the latter, one approach is to start with optical parametric oscillation (OPO) and to operate it close to degeneracy. Then, a series of second-order nonlinear processes takes place, generating equidistant comb lines around both the pump frequency and the sub-harmonic frequency [2] , [4] . A promising CH (2) material to move into the mid-infrared region is cadmium silicon phosphide (CdSiP 2 ) which is transparent from 1 to 6.5 µm and has a high nonlinearity of 84.5 pm/V. Furthermore, OPO has already been shown for a CdSiP 2 mm-sized whispering gallery resonator pumped at telecom wavelength [5]. Based on these observations, we investigate OPO operation close to the point of degeneracy and this way, the possibility to generate frequency combs both around 1.5 µm and 3 µm. In this work, we have observed OPO with thresholds below 100 µW and conversion efficiencies of around 17 %. We could tune the OPO output wavelengths from 2.3 to 5 µm by coupling the pump light into different modes of the resonator and by controlling its temperature. One of the pump modes investigated enabled operation near degeneracy. Measured signal and idler wavelengths for this pump mode are plotted in figure 1a ) for different temperatures. However, OPO near degeneracy is prone to instabilities, as several parametric oscillations can be excited under similar conditions. Here, by tuning the temperature in the mK range and the pump detuning with respect to the resonance in the MHz range, the minimum obtained difference between signal and idler frequencies was 120 GHz (4 FSRs). Nonetheless, in this state we have already observed sidebands generated around the pump corresponding to SHG as shown in figure 1b ). Why operation closer to the point of degeneracy is hampered, is currently under investigation. A possibility to gain further insight and control on OPO operation might be to make use the linear electro-optic effect for fine tuning of the relative position of the resonances between the pump and the sub-harmonic frequencies.