High pulse energy and quantum efficiency mid-infrared gas Raman fiber laser targeting CO absorption at 4.2 µm.

In this Letter, we demonstrate a high pulse energy and linearly polarized mid-infrared Raman fiber laser targeting the strongest absorption line of ${\rm CO}_2$CO at $\sim{4.2}\;\unicode {x00B5} {\rm m}$∼4.2µm. This laser was generated from a hydrogen (${\rm H}_2$H)-filled antiresonant hollow-core fiber, pumped by a custom-made 1532.8 nm Er-doped fiber laser delivering 6.9 ns pulses and 11.6 kW peak power. A quantum efficiency as high as 74% was achieved, to yield 17.6 µJ pulse energy at 4.22 µm. Less than 20 bar ${\rm H}_2$H pressure was required to maximize the pulse energy since the transient Raman regime was efficiently suppressed by the long pump pulses.