Fig. 1. Schematic of the experimental setup to measure sum-frequency signals. An arbitrary function generator (AFG) is used to precisely control the output wavelength of the pump laser and to trigger the oscilloscope. The pump light passes through a fiber polarization controller (PC) and a beam splitter, and then couples into the LN WGM microcavity via a tapered fiber. The transmission of the pump is monitored by a photodetector connected to an oscilloscope. The tapered fiber that is used to couple the pump collects the nonlinear optical signals as well. The nonlinear optical signals are detected by a spectrometer.

Fig. 2. Nonlinear optical signals. Peak 1, marked in red, corresponding to the second harmonic generation (SHG) of the pump laser at 1521.36 nm. Peaks 2–9 in blue are the sum-frequency generation (SFG) signals of the pump laser and its background.

Fig. 3. Transmission spectra of the pump laser and its background. (a) A typical broad transmission spectrum of an LN WGM microcavity coupled to a tapered fiber. (b) The enlarged view of the yellow highlighted part of (a) showing the pump laser background in detail. The peaks marked in blue represent the WGMs associated with the sum-frequency processes.

Fig. 4. Dependence of the conversion efficiency of the typical nonlinear optical signals on that of the pump laser. (a) and (b) show the data for the second harmonic signal and that for the sum-frequency signal marked as Peak 7, respectively.