All-fiber oscillators by femtosecond laser direct-writing fiber Bragg gratings breaking through kilowatts

Femtosecond laser direct writing of fiber Bragg gratings (FBGs) offers mask-free flexibility, but high insertion loss and unfavorable thermal slope have limited its use in high-power all-fiber oscillators. Based on our previously proposed femtosecond laser line-guided plane-by-plane direct-writing method, we refine the grating-inscription technique for the oscillator cavity mirrors. This optimization maintains the target reflectivity while achieving more uniform refractive index modulation with a modulation depth below 1×10 −3 , thereby reducing the thermal slope. High-reflectivity ( HR ∼99 % ) and low-reflectivity ( LR ∼8 % ) FBGs are written directly into 20/400 μm fibers without stripping the coating. The optimized HR-FBG exhibits a thermal slope of only 11.2°C/kW, reduced by 89% compared to the previous cavity mirror fabricated by femtosecond laser direct-writing HR-FBGs. Using these FBGs, we construct an all-fiber oscillator that achieves an output power of 3147 W at 1080.7 nm with an optical-to-optical conversion efficiency of 72.4%. The output power is 23.5 times the highest results reported previously, while both gratings retained the coating and were implemented without a cooling package for improved mechanical reliability. We reached a new milestone in high-power all-fiber oscillators based on femtosecond laser direct-writing FBGs, substantially reinforcing laser stability and potentially benefiting advanced manufacturing.