Efficient mid-infrared cascade Raman source in methane-filled hollow-core fibers operating at 2.8 μm.

We report here for the first time, to the best of our knowledge, a novel and efficient cascade Raman laser source operating at 2.8 μm by two stages of methane-filled hollow-core fibers (HCFs). In the first stage, a commercial 1064.6 nm laser is used as the pump source, and an efficient first-order Stokes wave of 1543.9 nm is obtained with a quantum conversion efficiency of ∼87% in 2 m ice-cream HCF filled with 2 bar methane gas. In the second stage, efficient 2.8 μm laser emission is also generated by the first-order stimulated Raman scattering of methane, while the pump source is the Stokes wave at 1543.9 nm. A maximum quantum conversion efficiency of ∼75% is obtained with 2.2 m node-less HCF filled with 11 bar methane gas, resulting in a record total quantum efficiency of ∼65%, which is 1.6 times the previous similar result. This work provides a significant efficient method to obtain a wide wavelength range of mid-infrared, even far-infrared fiber laser sources from conveniently available 1 μm band lasers with proper HCFs and different active gases.