Ultra-low-loss directly written fiber Bragg gratings for high-power random Raman fiber lasers

Ultrafast laser direct writing provides unmatched flexibility for fabricating fiber Bragg gratings (FBGs), yet its application in high-power fiber lasers has been limited by broadband scattering loss and parasitic cladding mode coupling. Here, a fast, scanning focus plane-by-plane inscription technique is proposed and demonstrated that enables the fabrication of ultra-low-loss FBGs suitable for high-power core-pumped laser operation. Short processing times are realized by combining air-bearing translation stages for fiber positioning with simultaneously scanning the focus of the inscription laser beam perpendicular to the fiber axis. A refractive index modification pattern consisting of 60 stacked lines, each 40 µm long, can be generated in just 1.4 seconds. Using this inscription technique and by optimizing the pulse energy, an unprecedented suppression of broadband and cladding mode losses has been achieved, combined with reflectivity values sufficient for random Raman laser applications. Expressed in terms of the broadband loss coefficient α, the coupling coefficient κ, and the loss coefficient γ-which is a measure of cladding mode losses-record values of κ/α = 5286 and κ/γ = 7463 were achieved. The optimized FBGs are validated in a random Raman fiber laser, where they sustain 30 W of pump power at 1480 nm, directly propagating through the FBG, showing excellent thermal behavior. These results demonstrate that the scanning focus plane-by-plane direct writing technique is a promising wavelength-agile FBG fabrication method for high-power random Raman laser applications.