In recent years, Brillouin lasers have attracted great interest, among which the most widely studied waveguide Brillouin lasers have realized low-noise, low-threshold narrow-linewidth laser radiation and have been applied to optical clocks and gyroscopes, ultra-stable lasers and microwave photonics. However, the problems faced by the guided-wave structure, such as the difficulty in power enhancement and the difficulty in expanding the operating wavelength, have constrained the application of Brillouin lasers in the directions of gravitational wave detection and long-range high-resolution spectroscopic measurements.
In view of this, Zhenxu Bai et al. at Hebei University of Technology proposed a scheme to realize a free-space-operating Brillouin laser by using diamond crystals as the Brillouin gain medium, and reported the direct radiation of a diamond Brillouin laser for the first time in the international arena in 2020, obtaining a visible Brillouin laser output with a power of 11 W, which is an order of magnitude higher compared to guided-wave Brillouin lasers. Subsequently, in 2022, the team extended the output wavelength from the visible band to the near infrared and achieved stable single-frequency laser operation with a power of up to 22.5 W and a signal-to-noise ratio of 71.8 dB.
However, the important question of whether free-space-structured Brillouin lasers have the same linewidth narrowing characteristics as guided-wave-structured Brillouin lasers has not been answered.
Recently, the team successfully answered this question in a paper titled "Linewidth narrowing in free-space-running diamond Brillouin lasers" published in the academic journal High Power Laser Science and Engineering. The paper entitled "Linewidth narrowing in free-space-running diamond Brillouin lasers", published in the academic journal High Power Laser Science and Engineering, has successfully answered the above question, and for the first time, a single-frequency Brillouin laser with a linewidth narrowing of more than a factor of 4 has been obtained in a continuously running diamond Brillouin laser.
A directly pumped ring cavity structure was used to realize the double resonance of pump light and Stokes light by PDH cavity locking technique, in which the linewidth of the pump light was 7.36 kHz. By using three sets of coupled mirrors with different reflectivities of 96%, 97%, and 98.5% for the comparative experiments, the corresponding linewidths of the Stokes light were measured to be 3.2 kHz, 2.43 kHz, and 1.77 kHz, compared with the pump light, the linewidth compression is realized, and the highest compression ratio reaches 4.2 times. Under the maximum pump power, the corresponding Stokes light output power of the three reflectances were 22.5 W, 16.9 W and 13.6 W, with corresponding optical conversion efficiencies of 37.5%, 29.1% and 23.4%, respectively.
The research results show that, using the flexible structural characteristics of the free-space Brillouin oscillator, it is expected to realize single-frequency laser output with narrower linewidth, higher power and richer wavelengths in the future by lowering the loss of intracavity components, optimizing the oscillator design and adjusting the pumping parameters, and the research results are of great significance in promoting the development and application of high-coherence laser technology.
