In addition to the problems of beam quality degradation and mode instability caused by thermal accumulation, the phenomenon of linewidth broadening caused by amplified spontaneous radiation noise cannot be ignored when obtaining high-power narrow-linewidth lasers using traditional technical means. Furthermore, due to the defects in the atomic energy levels of the gain medium, traditional technology cannot produce narrow linewidth laser output in some special bands (2 μm). However, the optical nonlinear effects that have emerged with the development of laser technology provide a new approach for achieving narrow linewidth laser radiation at special wavelengths. Among these effects, stimulated Brillouin scattering (SBS), as a third-order nonlinear effect, has significant advantages in producing ultra-narrow linewidth lasers. By combining the fast decay mechanism of acoustic phonons in the SBS process with the strong feedback provided by the cavity, both microwave-guided Brillouin lasers and fiber Brillouin lasers can achieve narrow linewidth laser outputs that are much lower than those of conventional single-frequency lasers. However, these demonstrations based on waveguide structures also face challenges such as higher-order Stokes light generation during power boosting, which limits their ability to further increase single-frequency power.

To address the challenges faced by waveguide Brillouin lasers in power enhancement, space-structured Brillouin lasers have made it possible to produce narrow linewidth lasers with special wavelengths. This is due to their convenient thermal management and control of phase-matching conditions at high power through the separation of the cavity from the gain medium and the versatility of the gain material. Space Brillouin lasers have increased their power output from an initial 20 mW, limited by the gain level, to more than 20 W of single-frequency power output without saturating the power curve. The output wavelength has also been extended from the visible to the near-infrared band. However, despite these impressive demonstrations, there have been no reports on the characterization of Stokes light broadening and narrowing in Brillouin lasers with spatially optical structures, which is essential for further research and development of Brillouin lasers.

Based on a theoretical analysis of the linewidth of a space-operated Brillouin laser and optimization of the measurement scheme, a single-frequency power output with a linewidth compression factor of 2.5 (3.2 kHz) and a power exceeding 20 W was obtained for a continuously operated diamond Brillouin laser. This result was published in High Power Laser Science and Engineering, vol. 11, Issue 3 (Duo Jin, Zhenxu Bai, Zhongan Zhao, Yifu Chen, Wenqiang Fan, Yulei Wang, Richard P. Mildren, Zhiwei Lü. Linewidth narrowing in free-space-running diamond Brillouin lasers[J]. High Power Laser Science and Engineering, 2023, 11(4): 04000e47).

Graphic 1 description: (a) phase matching, (b) optical path structure and (c) linewidth measurement system for diamond Brillouin lasers

The linewidths induced by the thermal quantum noise of the Stokes light at the reflectivity of three sets of coupled mirrors and the linewidths induced by the phase diffusion of the pump light are calculated as shown in the table. It can be found that the fundamental linewidth in the spatial Brillouin laser is mainly determined by the phase diffusion of the pump light due to the existence of the intrinsic loss in the spatial cavity.

Table 1 description: Fundamental linewidths of the Stokes optical output at different coupled mirror reflectivity, linewidth due to quantum noise: ∆ν_{S_T}, linewidth due to pumped optical phase diffusion ∆ν_{S_P}:

After optimizing the length of the fiber used in the linewidth measurement structure shown above, the delayed fiber length was set to 1 km. The Stokes optical output linewidth corresponding to different coupling mirror reflectivities was measured, and the results are shown in Fig. 1(a). As the coupling mirror reflectivity increases, the Stokes output linewidth gradually narrows. The Stokes linewidths corresponding to the reflectivities of the three coupled mirrors are 3.2 kHz, 2.43 kHz, and 1.77 kHz, respectively, all of which are compressed compared to the pump light. Even at a coupled mirror reflectivity of 96%, the linewidth is still compressed by a factor of nearly 2.5. The corresponding linewidth compression trends are consistent with those predicted in Table 1, but the measured linewidth values are higher than the theoretically calculated values. The difference between the experimentally measured and theoretical values can be attributed to the introduction of additional technical noise (e.g., ambient noise and electronic noise from the locking system). The pump-Stokes power curves corresponding to the three reflectances are shown in Fig. 2(b). At maximum pump power, the Stokes output powers corresponding to the three reflectances are 22.5 W, 16.9 W, and 13.6 W, with corresponding optical conversion efficiencies of 37.5%, 29.1%, and 23.4%, respectively.

Graphic 2 description: Linewidth and power output curves at different coupled mirror reflectivity

Diamond Brillouin lasers provide a new technical route for realizing high-power ultra-narrow linewidth special wavelength laser radiation. In addition, theoretical analysis shows that by reducing the insertion loss of intracavity components it is theoretically possible to realize single-frequency laser output with narrower linewidth and higher power.