Reel-to-reel femtosecond-laser inscription cladding scattering-enhanced fiber for laser linewidth narrowing

We demonstrate a novel, to the best of our knowledge, cladding-localized Rayleigh-scattering-enhanced (cl-RSE) fiber that provides broadband, low-loss self-injection feedback. Through system optimization, the reel-to-reel femtosecond-laser inscription system limits fiber jitter to <5 µm during meter-scale processing. The resulting 2-m cl-RSE fiber exhibits 10-40 dB random scattering enhancement at each point, with an overall 22 dB reflection increase and only 0.7 dB total loss. Serving as a passive external cavity for a DBR fiber laser, the cl-RSE fiber compresses the linewidth by half. Its feedback remains stable under temperature fluctuations and dynamic strain, demonstrating the practicality of cl-RSE fiber as a robust, alignment-free device for linewidth narrowing.