Stabilizing Brillouin fiber laser for applications in distributed BOTDA sensing

Self-injection locking to an external fiber cavity is an efficient technique enabling drastic linewidth narrowing and self-stabilization of semiconductor lasers. We introduce a simple dual-frequency laser that employs the same external ring fiber cavity for self-injection locking of a standard semiconductor DFB laser and for the generation of the Stokes light via stimulated Brillouin scattering. In contrast to the previous Brillouin laser configurations, the system spliced from standard telecom components is supplied by a low-bandwidth active optoelectronic feedback that helps to maintain the self-injection locking to provide both the DFB laser line narrowing and permanent coupling between the DFB laser and the fiber ring cavity thus enabling the dual-frequency laser operation. The laser performance characteristics are well superior to the on-board laser modules commonly used with BOTDA. In particular, the configuration reduces the natural Lorentzian linewidth of the light emitted by the laser at pump and Stokes frequencies down to 270 Hz and 110 Hz, respectively, and features a stable 300-Hz-width RF spectrum characterizing beating between two laser outputs. In a direct comparison with the commercial BOTDA, we explore the utilization of our low-cost solution for the BOTDA sensing demonstrating distributed measurements of the Brillouin frequency shift in 10-km sensing fiber with 1.5m spatial resolution.