In recent years, an optical frequency comb (OFC) has shown great advantages and application prospects in the fields of optical atomic clocks, laser frequency measurement, and precision spectral measurement. Limited by the resolution and response time of current optical measuring equipment, OFCs with a frequency interval of at least 20 GHz are required to calibrate the spectral measurement equipment in the field of planetary exploration. Although an OFC with an output power of mW level is sufficient to realize gas detection in a confined space, higher power OFCs in specific transmission bands are required in the application scenarios such as outside detection of hazardous chemicals and space detection. Therefore, the development of high-power OFCs with large frequency intervals in specific wavelengths is of great significance, which is also the hotspot and difficulty in the field of current laser technologies.

A scheme to achieve an OFC in a free-space optical cavity by using the stimulated Raman scattering as the intermediate process is proposed (see Fig. 1). Firstly, the pump light is injected into a Raman oscillator to form a high-power density Raman field in the cavity. Then, the Raman laser with a high power density in the cavity reaches the first-order stimulated Brillouin scattering (SBS) threshold under the action of the acoustic field in the gain medium. Finally, with the further increase of intra-cavity power density, the cascaded Stokes and anti-Stokes with equal frequency intervals are generated under the joint action of SBS and four-wave mixing.

The power and spectral characteristics of the output laser are studied by using a 1 μm quasi-continuous-wave laser as the pump. With a limited pump power, an OFC in the 1.2 μm band with up to 101 W is demonstrated with a conversion efficiency of 41% as shown in Fig. 2. Moreover, the output beam quality is also significantly improved compared with the pump beam quality (inset of Fig. 2). By optimizing the resonator parameters and pump conditions, the output spectra with single Raman frequency, first-order SBS, and OFC are obtained, respectively. As shown in Fig. 3, an OFC with a frequency interval of 71 GHz and 23 spectral lines is obtained in the 1.2 μm band corresponding to the overall bandwidth of 1.55 THz.

Here, we propose and verify the possibility of using the Raman field as an intermediate process to excite the Brillouin OFC in a free-space oscillator. As far as we know, this is the highest reported power for any Brillouin OFCs, which is four orders of magnitude higher than that of the micro-resonator-based OFC. This free-space approach provides a new path for realizing high-power OFCs in specific wavelengths.