Generation of high-efficiency hundred-millijoule stimulated Brillouin scattering in fused silica

  Objective   Stimulated Brillouin scattering (SBS) is a powerful tool for serving as a phase conjugation mirror (PCM) due to its inherent properties of high gain, small frequency shift, and phase conjugation. Solid-state gain media offer the advantages of high stability and high repetition rate SBS, compared to liquid and gas gain media. However, solid gain media face the challenge of recovery once breakdown occurs. Currently, there is limited research on achieving high-efficiency and high-energy SBS generation in solid media, which restricts the application of solid gain media in high-energy SBS. In this study, we experimentally investigate an SBS generator based on bulk fused silica to provide guidance for the development and application of all solid-state SBS systems with high efficiency.

  Methods   The experimental setup is illustrated (Fig.1). A passively Q-switched nanosecond laser, based on a ring cavity, is used as the pump source, delivering a pulse width of 10 ns. A Fabry-Perot etalon is inserted into the cavity to control the number of longitudinal modes. Lenses L₁ and L₂ are utilized to adjust the beam diameter from 3.2 mm to 5.6 mm, while the focal length of L₃ is 250 mm. Fused silica, with a length of 200 mm, serves as the Brillouin gain medium. The output characteristics of SBS generation, including threshold, slope efficiency, damage threshold, and beam profile, are studied by varying the pump mode and pump intensity.

  Results and Discussions   Compared to a single longitudinal-mode (SLM) pump, the SBS threshold for a multi-longitudinal-mode (MLM) pump is 14% higher, and the damage threshold is only 34 mJ (Fig.2(a)). A phase-conjugate reflectivity of up to 81.0%, with a slope efficiency of 85.8%, is achieved when the pump single pulse energy is 183.1 mJ. The results indicate that MLM pulse spikes are the key factor causing optical breakdown in the SBS process, while SLM pumping can effectively prevent the optical breakdown in solid media. The narrowest Stokes pulse width of 5.5 ns is obtained at an energy reflectivity of 15%; While the waveform maintains good fidelity at the highest input pump energy (Fig.2(b)). The Stokes beam profile exhibits a good cleanup effect under low-energy pumping conditions (Fig.2(c)). However, it gradually evolves towards the pumping profile as the energy increases. This suggests that high reflectivity also leads to high beam quality fidelity of Stokes.

  Conclusions   In this study, we have demonstrated the feasibility of achieving high-efficiency and high-energy SBS output in fused silica. A Stokes energy of 183.1 mJ with a slope efficiency of 85.8% was obtained when the pump energy was 226 mJ. This research lays the foundation for optimizing the characteristics of SBS-PCM based on solid Brillouin gain media, as well as its expansion in pulse compression, Brillouin amplification, and beam combination. It has important implications for achieving high-power all-solid-state SBS lasers.