Raman Com emission in a micro optical resonator via induced Raman scattering

A micro optical resonator is an element that confines light in a small volume for a long period of time, and the light energy in the resonator can be increased by inputting continuous light (CW). By inputting continuous light (CW), the light energy in the resonator can be increased. By causing four-wave mixing of nonlinear optical effects here, comb-like spectral light with a frequency component matching the resonance frequency can be generated (optical Kercomb). This optical Kercomb has a wide mode spacing from 10 GHz to several THz on the frequency axis and is expected to be used for optical communications, microwave oscillators, dual comb spectroscopy, and optical frequency comb calibration light sources for planetary exploration. In addition to the method using four-wave mixing, Raman comb generation via induced Raman scattering (SRS) has also been reported.

SRS is a phenomenon in which the interaction between light and the molecular vibration of a material generates light with a frequency lower than the carrier frequency. Since the shift frequency is determined by the intrinsic vibrational modes of the molecules in the medium, the shape of the Raman gain spectrum differs depending on the medium. This SRS is used for optical amplification and laser oscillation, but relatively high input power, such as optical pulse excitation, is required for laser oscillation. However, the high optical confinement performance of micro optical resonators allows the threshold pump power to be greatly reduced, making it easy to generate an SRS from a CW laser. Under conditions where the broadband Raman gain excites multiple resonant modes, a multi-frequency Raman comb is generated that matches the resonant frequency.

In a four-wave mixing optical Kerr comb, the spectra are broadened while satisfying the phase matching condition, so the phases of each frequency component can be aligned. On the other hand, in a Raman comb, the spectrum broadens regardless of the phase matching condition, so the phases of each frequency component are usually not aligned. However, several research groups have reported the generation of phase-locked Raman combs, which are expected to be applied to compact pulsed laser sources, microwave oscillators, sensors, and optical coherence tomography. However, the process of Raman comb generation, spectral shape control, and its parameter dependence are not well understood. In this study, we utilize the broadband Raman gain spectrum of silica glass to clarify the parameter dependence of the Raman comb and its generation process in a silica rod micro optical resonator.