Self-injection locked sub-hertz Brillouin laser enabled by a Möbius-type ring resonator

The stimulated Brillouin scattering plays a crucial role in the generation of narrow-linewidth lasers, as it can increase coherence and reduce phase noise. Traditional Brillouin lasers require complex optical frequency stabilization systems, whereas the self-injection locking technique can greatly simplify the setup and reduce costs. In this paper, we fabricate a Möbius-type ring resonator using a polarization-maintaining fiber loop with a 90^∘ splice and only one coupler. In such a cavity, the pump light, circulating twice to complete one round trip, effectively separates its modes into two sets. One set is used for self-injection locking, and the other provides a stable laser output, thereby realizing self-injection locking with just a single coupler. Using this method, we narrowed the Lorentzian linewidth of a 1310 nm DFB laser to 8.00 (±2.02) Hz and that of the generated Brillouin laser to 890 (±350) mHz, achieving a highly cost-effective approach to producing narrow-linewidth lasers.