Plane-by-plane femtosecond laser inscription of first-order fiber Bragg gratings in fluoride glass fiber for in situ monitoring of lasing evolution.

We report the femtosecond laser inscription of fiber Bragg gratings (FBGs) in an Er-doped fluoride glass fiber used for lasing at a mid-infrared wavelength of 2.8 µm. The lasing evolution is discussed in terms of the FBG reflectivity, wavelength transition to the Bragg wavelength, and output power of the mid-infrared fiber laser. A first-order and short (2.5-mm-long) Bragg grating showed a reflectivity of 97%, because of a laser-induced index modulation of 1.1 × 10. This modulation was sufficient to saturate this system's output power. The laser oscillator is designed to lase in the atmospheric window of 2799-2800 nm slope. Further, this oscillator's efficiency is as high as 29.1% for the launched pump power over the range of 0.4-4.6 W and at a lasing wavelength of 2799.7 nm. This oscillator also exhibited a FWHM bandwidth of 0.12 nm.