We report the first demonstration of a Raman fiber laser (RFL) emitting in the mid-infrared, above 3 μm. The operation of a single-mode ${\mathrm{As}}_2{\mathrm{S}}_3$ chalcogenide glass based RFL at 3.34 μm is demonstrated by using a low-loss Fabry–Pérot cavity formed by a pair of fiber Bragg gratings. A specially designed quasi-cw erbium-doped fluoride fiber laser emitting at 3.005 μm is used to pump the RFL. A laser output peak power of 0.6 W is obtained with a lasing efficiency of 39% with respect to the launched pump power.

Laser emission is demonstrated at a wavelength of 3.766 μm in a cascaded Raman gain device. The laser cavity is made of two nested pairs of fiber Bragg gratings inscribed in a 2.8 m length of low-loss ${\mathrm{As}}_2{\mathrm{S}}_3$ fiber. An erbium-doped fluoride glass quasi-CW fiber laser emitting at 3.005 μm is used to pump the cascaded Raman cavity, which converts the pump wavelength successively to the first and second Stokes orders, respectively at 3.340 and 3.766 μm. A laser output peak power in excess of 100 mW is obtained with a lasing efficiency of about 8.3% with respect to the launched pump power. This represents the highest emission wavelength delivered by a fiber laser operating at room temperature.

Type I fiber Bragg gratings (FBG) written through the coating of various off-the-shelf silica fibers with a femtosecond laser and the phase-mask technique are reported. Inscription through most of the common coating compositions (acrylate, silicone and polyimide) is reported as well as writing through the polyimide coating of various fiber cladding diameters, down to 50 µm. The long term annealing behavior of type I gratings written in a pure silica core fiber is also reported as well as a comparison of the mechanical resistance of type I and II FBG. The high mechanical resistance of the resulting type I FBG is shown to be useful for the fabrication of various distributed FBG arrays written using a single period phase-mask. The strain sensing response of such distributed arrays is also presented.

We report that by using 800 nm femtosecond pulses and the phase-mask technique, first-order fiber Bragg gratings with refractive index modulation in excess of $1\times {10}^{-3}$ can be written through both the acrylate and polyimide coatings of unloaded standard silica fibers without noticeable degradation of mechanical strength. We also demonstrate that the same experimental conditions can be applied for efficient FBG writing through the polyimide coating of pure silica core fibers, opening significant opportunities in the field of fiber sensors.

Optics InfoBase is the Optical Society's online library for flagship journals, partnered and copublished journals, and recent proceedings from OSA conferences.

Single-frequency laser emission from a distributed feedback all-fiber laser operating in the vicinity of 3 μm is demonstrated. The laser cavity was made of a 30 mm long $\pi$-phase-shifted fiber Bragg grating inscribed in a heavily erbium-doped fluoride fiber using infrared femtosecond pulses and the dithering phase-mask technique. A maximum CW output power of 12 mW was obtained at 2794.4 nm by using a multimode pumping scheme. The narrow linewidth was characterized to be lower than 20 kHz using a heterodyne technique. This achievement represents a significant step toward the development of active frequency references operating in the mid-infrared.