Sub-kilohertz Brillouin Fiber Laser with Stabilized Self-Injection Locked DFB Pump Laser

We report on experimental demonstration of a low-cost sub-kilohertz Brillouin fiber ring laser pumped from an actively stabilized self-injection locked distributed feedback (DFB) laser diode. Locking of the commercial DFB laser to a -11-m-length high-Q-factor fiber-optic ring cavity leads to -10,000-fold narrowing of the laser Lorentzian linewidth down to 400 Hz. Such pump laser operation inside the ring cavity forces the cavity to host Brillouin lasing enabling the laser threshold power as low as -1.5 mW. The laser operation is perfectly stabilized by active optoelectronic feedback driven by a simple microcontroller. The laser delivers radiation at Stokes frequency with the Lorentzian linewidth reduced down to -75 Hz and a phase noise less than -100 dBc/Hz (>30 kHz). The reported laser configuration is of great interest for many laser applications where a narrow sub-kHz linewidth, simple design and low cost are important.