Fabrication of perfect ultra-long fiber Bragg gratings for high-performance Raman fiber lasers

18.04.2018, 14:00

Sébastien Loranger,Ph. D., Advanced Photonic Concept Laboratory, Ecole Polytechnique de Montréal, Canada

Distributed feedback (DFB) fiber laser constructed around a phase-shifted fiber Bragg grating (FBG) can achieve outstanding performance in terms of single-frequency operation with linewidth in the kHz range. Using Raman gain as pumping mechanism allows such laser to be operated in any band, including those where rare-earth cannot reach. However, due to the low Raman gain, ultra-long FBGs in highly non-linear small-core fiber must be fabricated for operation at modest pump powers (~ 1 W or less). Such gratings were, until recently, impossible to fabricate with reproducibility due to non-uniformity in specialty fibers. Ultra-long FBGs are typically defined as gratings of lengths longer than typical phase mask, hence >20 cm long. We have recently demonstrated a technique which involves a fiber characterization prior to fabrication, and thus allows for the inscription of perfect ultra-long FBGs in any imperfect fiber. The technique will be presented as well as its results. This fabrication technique has allowed for the extensive study and optimisation of Raman DFB fiber laser. The limitations (mostly thermal gradient along the fiber) were identified and understood and outstanding performance was demonstrated (350 mW pump threshold).