Femtosecond direct-writing of low-loss fiber Bragg gratings using a continuous core-scanning technique

We report the inscription of low-loss fiber Bragg gratings using focused femtosecond (fs) pulses and a continuous core-scanning technique. This direct-write technique produces high-fidelity Type I-IR gratings that share the inherent advantages of other direct-write methods, such as the point-by-point (PbP) method, for which the grating period is a free parameter. However, here we demonstrate an order of magnitude improvement in scattering loss compared to PbP gratings, to a level comparable with that of phase-mask-based fs inscription. A first-order grating was inscribed in standard telecommunications fiber with $-49\mathrm{dB}$ transmission at the Bragg wavelength and $0.1\mathrm{dB}$ broadband scattering loss. Potential application of these gratings to large-mode-area fibers and chirped grating fabrication are highlighted.