The whispering-gallery mode (WGM) optical microresonator facilitates the continuous propagation of light waves with minimal loss, owing to total internal reflection. This characteristic can considerably enhance the interaction between light and matter, increase the efficiency of nonlinear effects, and remarkably reduce the threshold for nonlinear effects. Various microresonator-based nonlinear optical effects, such as stimulated Raman scattering, stimulated Brillouin scattering, and four-wave mixing, have been extensively researched. Studies of microresonator-based nonlinear optics have been applied in several research avenues, including optical switches, nonlinear optical devices, and precision measurement. Indeed, the study of nonlinear optics in WGM optical microresonators is of considerable importance. Crystalline optical microresonators offer unique benefits over silica-based WGM optical microresonators. One major example is the calcium fluoride (CaF₂) crystalline microresonator, which has emerged as an ideal platform for studying nonlinear optics due to its high nonlinear coefficient, low absorption coefficient, and suitability for long-term storage after processing. Researching the nonlinear effects based on CaF₂ crystalline microresonators entails excellent prerequisites. However, in the current scientific landscape, research into CaF₂ crystalline microresonators has not been extensively pursued. Additionally, the study of CaF₂ crystalline microresonator-based nonlinear optics is not widespread. In light of the above discussion, this study aims to further explore the potential of CaF₂ crystalline optical microresonators, particularly in the research of stimulated Brillouin scattering, stimulated Raman scattering, and other nonlinear effects in microresonators. Additionally, it aims to provide a preliminary foundation for subsequent nonlinear applications in CaF₂ crystalline microresonators.

The fabrication of CaF₂ crystalline microresonators with an ultrahigh quality factor up to 3.6 × 10⁸ was achieved using an ultraprecision polishing technique. This provided the prerequisite foundation for the study of nonlinear optics. We designed and constructed an experimental platform for studying nonlinear optics, where the pump laser's wavelength was manipulated using a tunable laser. The pump laser was amplified until its laser power approached the threshold; this amplification was achieved by employing an erbium-doped fiber amplifier. Thereby, CaF₂ microresonator-based nonlinear effects were excited. In the study of stimulated Brillouin scattering, Brillouin lasers and low-noise Brillouin cascade lasers were efficiently generated by modifying the pump wavelength and increasing the pump laser power. In order to acquire a Brillouin optical frequency comb, the pump wavelength was adjusted to scan from short to long wavelength. Consequently, we achieved a first-order Brillouin optical frequency comb with a perfect comb tooth state. Given that stimulated Raman scattering exhibited an ultrawide gain range, in our study, four-wave mixing assisted by stimulated Raman scattering can generate optical frequency combs at longer wavelengths. Different pump-wavelength detuning and pump power can be adjusted to optimize the signal-to-noise ratio of Raman lasers and the output of Raman combs. Furthermore, in the experimental demonstration of ultrawide Raman spectra, the coupling and interaction among numerous modes in the resonator may cause asymmetric comb tooth spacing distribution and power distribution.

We fabricate CaF₂ optical microresonators using ultraprecision machining techniques with a custom-built machining system. The quality factor of the microresonator attains a value of 3.6×10⁸, providing an appropriate platform for nonlinear optics (Fig. 3). We acquire substantial nonlinear experimental results, including a signal-to-noise ratio of 56.23 dB for the first-order stimulated Brillouin laser (Fig. 5) and 60 dB for the stimulated Raman laser (Fig. 7). Even for the fourth-order stimulated Brillouin laser in cascaded Brillouin systems, a signal-to-noise ratio of 26 dB (Fig. 5) is preserved. Furthermore, we manage to satisfy both the phase and energy requirements of stimulated Brillouin scattering and four-wave mixing by employing precise control mechanisms. The coupled-Brillouin optical frequency comb achieves a perfect state, resulting in an optical frequency comb with a single multiple of the free spectral range (Fig. 6). The four-wave mixing assisted by Raman laser generates an optical frequency comb with a bandwidth of 900 nm (Fig. 8), extending the comb tooth range into the visible light spectrum. Noteworthily, our results of CaF₂ microresonator-based nonlinear optics are more comprehensive compared with the results of early studies on CaF₂ microresonators. Additionally, certain experimental results were not demonstrated in early studies on CaF₂ microresonators. Our results provide strong evidence for CaF₂ microresonators being an ideal platform in nonlinear optics research.

Targeted investigations are conducted to explore the remarkable performance of CaF₂ crystalline optical microresonators in the realm of nonlinear optics. The CaF₂ microresonator, obtained using ultraprecision machining techniques, exhibits an ultrahigh quality factor of 3.6×10⁸. We have succeeded in exciting nonlinear effects. In the experiments, we simultaneously excite stimulated Brillouin scattering, stimulated Raman scattering, and the four-wave mixing effect in the CaF₂ microresonators at submilliwatt power levels. The results demonstrate the high-efficiency generation of stimulated Raman laser and Brillouin lasers. In addition, a fourth-order cascaded Brillouin laser is achieved by satisfying the frequency shift condition of Brillouin scattering. Particularly, Brillouin-coupled four-wave mixing, Raman-assisted Kerr effect, and ultrabroadband Raman optical frequency comb have been achieved by satisfying the corresponding phase-matching and energy-conservation conditions. The four-wave mixing process, with assistance from the Raman laser, yields optical frequency combs with a bandwidth of 900 nm. The spectral range of the optical frequency comb has been extended to the visible light wavelength regime. This achievement provides a foundation for subsequent research and development of applications such as lasers in the visible light wavelength range.