Brillouin scattering and single-longitudinal-mode lasing in long-length, SF ₆ -filled anti-resonant hollow-core fibers

We present a kilometer-scale, splice-free anti-resonant hollow-core fiber (AR-HCF) platform filled with SF 6 gas for reconfigurable Brillouin photonics. This system enables, to our knowledge, the first distributed observation of gas diffusion over 2.4 km and a controllable transition from spontaneous to stimulated Brillouin scattering (SpBS to SBS) within a single fiber. The Brillouin gain coefficient is tunable over four orders of magnitude—from ∼10 −5 to 0.1 m −1 W −1 —by varying gas composition and pressure, yielding a record SBS threshold above 1 kW in a practical as-drawn fiber, linewidth narrowing from 40 MHz (SpBS) to 2.5 MHz (SBS), and further to 1.1 kHz in a single-longitudinal-mode Brillouin laser with 188 mW output. These results confirm gas-filled AR-HCFs as a versatile and power-scalable platform for nonlinear optics, distributed gas spectroscopy, and narrow-linewidth laser generation.