Objective   Brillouin laser is an important technological approach for achieving high coherence and low noise lasing, among which Brillouin lasers in free space have been proven to generate high-power single-frequency laser radiation. However, unlike the widely studied guided-wave-based Brillouin lasers, no studies on the linewidth properties have been reported for Brillouin lasers in free space. In this paper, a series of research works have been conducted on the generation, parameter regulation, and performance optimization of the Brillouin lasers in free space using diamond as gain media. We experimentally studied the feasibility of realizing linewidth narrowing of the Brillouin laser in free space.

  Methods  The structure of the spatial Brillouin laser and the corresponding linewidth measurement device is shown respectively (Fig.1(a), (b)). The Brillouin laser uses a diamond crystal as the Brillouin gain medium, which has the highest known thermal conductivity and transmission range. A directly pumped ring cavity structure is used for the experiments, where the linewidth of the pumped light is 7.36 kHz. The linewidth behavior of the Stokes light is comparatively investigated by choosing three different sets of coupled mirror reflectivity: R₁ = 96%, R₁ = 97% and R₁ = 98.5% for the experiments.

  Results and Discussions   The measurement results (Fig.2) show that the Stokes linewidth becomes narrower as the coupler reflectivity increases. The Stokes linewidths corresponding to three sets of coupler reflectivity are 3.2 kHz, 2.43 kHz and 1.77 kHz, respectively, and all of them realize linewidth compression compared with the pump, with the highest compression ratio of 4.1. Theoretically, the output efficiency and linewidth compression can be improved at the same time by decreasing the insertion loss of the intracavity element, and the analysis shows that, at the pump power of 60 W and coupled-mirror reflectivity of 96%, the linewidth of 1.6 kHz and up to 80% can be achieved by decreasing the insertion loss of the intracavity element. The analysis shows that at a pump power of 60 W and a coupling mirror reflectivity of 96%, a linewidth of 1.6 kHz and a Stokes output with an optical conversion efficiency of up to 80% can be realized by reducing the insertion loss of the intracavity components. In the future, when realizing ultra-narrow linewidth laser radiation, the technical noise introduced in the system will be the main obstacle limiting the further reduction of the fundamental linewidth.

  Conclusions   For the first time, we have verified the feasibility of realizing linewidth-narrowed Brillouin laser output in a free-space optical transport structure. The study provides a feasible technical solution for obtaining high-power, narrow-linewidth lasing with a wide wavelength range. The result is of great significance for promoting the development of diamond laser technology and advancing the application of highly coherent light sources.