20.2  W monolithic 3.5  μm fiber laser system using actively temperature-tuned femtosecond laser inscribed FBGs

Motivated by strong demand for high-power mid-infrared sources in spectroscopic detection, minimally invasive surgery, and precision polymer processing, we report a record-breaking 20.2 W all-fiber laser operating at 3550 nm, representing, to our knowledge, the first demonstration exceeding 20 W beyond 3 μm in monolithic fiber architecture. The system incorporates a 2.5-m-long, lightly Er 3+ -doped ZrF 4 fiber (1% molar fraction) paired with a pair of femtosecond-laser-direct-written fiber Bragg gratings (FBGs), and achieves a slope efficiency of 37.2% through dual-wavelength pumping at 981 nm and 1980 nm. Precision independent thermal control effectively compensated for relative wavelength shifts between the high-reflectivity (HR) and low-reflectivity (LR) FBG pair, significantly improving spectral overlap. Comprehensive testing confirmed robust operation, demonstrating 2 h power stability with 1.36% root mean square fluctuation and 30 min wavelength stability with less than ±0.12 nm variation at 12.5 W output power.