Dual-frequency laser comprising a single fiber ring cavity for self-injection locking of DFB laser diode and Brillouin lasing.

Low-noise lasers are a powerful tool in precision spectroscopy, displacement measurements, and development of advanced optical atomic clocks. While all applications benefit from lower frequency noise and robust design, some of them also require lasing at two frequencies. Here, we introduce a simple dual-frequency laser leveraging a ring fiber cavity exploited both for self-injection locking of a standard semiconductor distributed feedback (DFB) laser and for generation of Stokes light via stimulated Brillouin scattering. In contrast to the previous laser configurations, the system is supplied by a low-bandwidth active optoelectronic feedback. Importantly, continuous operation of two mutually locked frequencies is provided by self-injection locking, while the active feedback loop is used just to support this regime. The fiber configuration reduces the natural Lorentzian linewidth of 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 recorded with beating of two laser outputs. Translating the proposed laser design to integrated photonics will dramatically reduce cost and footprint for many laser applications such as ultra-high capacity fiber and data center networks, atomic clocks, and microwave photonics.

Stabilizing DFB laser injection-locked to an external fiber-optic ring resonator.

Self-injection locking to an external fiber cavity is an efficient technique enabling drastic linewidth narrowing and self-stabilization of semiconductor lasers. The main drawback of this technique is its high sensitivity to fluctuations of the configuration parameters and surroundings. In the proposed laser configuration, to the best our knowledge, for the first time the self-injection locking mechanism is used in conjunction with a simple active optoelectronic feedback, ensuring stable mode-hopping free laser operation in a single longitudinal mode. Locking to 4-m length fiber resonator causes a drastic narrowing of the DFB laser linewidth down to 2.8 kHz and a reduction of the laser phase noise by three orders of magnitude. We have explored key features of the laser dynamics with and without active feedback, revealing stability and tunability of the laser linewidth as an additional benefit of the proposed technique.