Rare-earth-doped fiber lasers at 3–5 μm can provide a variety of applications in defense, security, medicine, and so on. Limited by the maximum phonon energy of silicate glass host at room temperature, these lasers are normally based on soft-glass materials, e.g., fluoride or chalcogenide glass. However, due to the limited transparency of these fibers, until now these systems have only achieved coverage up to 3.92 μm. Here, we report a CW fiber laser operating well beyond 4 μm with significant power based on ${\mathrm{CO}}_2$-filled silica hollow-core fibers. By pumping via a homemade 2 μm laser diode, $\sim 82\mathrm{mW}$optical power at 4.3 μm was achieved at room temperature with a maximum laser efficiency $\sim 19.3\%$. Our demonstration represents the longest-wavelength CW fiber laser to date, paving the way towards compact and high-power mid-infrared fiber lasers beyond the 4 μm wavelength.

We reported here a high efficiency, high peak-power, narrow linewidth 1.9 μm fiber gas Raman amplifier based on SRS in a H2-filled anti-resonance hollow-core fiber. A CW seed fiber laser near 1908 nm is amplified by the first vibrational Stokes wave of hydrogen molecules while pumping with a pulsed 1064 nm microchip laser. The seed significantly enhances the vibrational SRS and suppresses the rotational SRS, resulting in a record optical-to-optical conversion efficiency of ~41%, corresponding quantum efficiency of ~73.5%, for such H2 vibrational Raman experiments in hollow-core fibers, with 1.4 meter fiber and only 3 bar H2, which is more preferable in applications. The maximum average Stokes power of 74.2 mW and peak power of ~150 kW with linewidth about 1 GHz (~12pm) is obtained. This paper provides an effective method to obtain efficient 1.9 μm fiber sources of both high peak-power and narrow linewidth.

We report here the first hundred-watt continuous wave fiber gas laser in H2-filled hollow-core photonic crystal fiber (PCF) by stimulated Raman scattering. The pump source is a homemade narrow-linewidth fiber oscillator with a 3 dB linewidth of 0.15 nm at the maximum output power of 380 W. To efficiently and stably couple several-hundred-watt pump power into the hollow core and seal the gas, a hollow-core fiber end-cap is fabricated and used at the input end. A maximum power of 110 W at 1153 nm is obtained in a 5 m long hollow-core PCF filled with 36 bar H2, and the conversion efficiency of the first Stokes power is around 48.9%. This work paves the way for high-power fiber gas Raman lasers.

Fiber gas laser based on hollow-core fiber is a novel and potential method to emit mid-infrared output. Here, we firstly demonstrate the fiber gas amplifier in CO ₂ -filled hollow-core fiber. Through the research of the fiber gas amplifier, self-absorption phenomenon is observed. 4.3 μm locates near the strong absorption wavelength band of CO ₂ . As the increase of gas pressure and the injected pump power, 4.3 μm laser generated by four absorption lines from 00001-20012 can be absorbed. The strength of the absorption depends on the wavelength difference between the output wavelength and the absorption wavelength. This research proves the way for improving the conversion efficiency of fiber gas lasers filled with CO ₂ by avoiding the self-absorption effect of CO ₂ .