“…In general, the molecules of hydrocarbon gas, toxic gas, and harmful gas have a strong absorption characteristic spectrum in this band. The conventional 3–5 μm mid‐infrared lasers mainly include: chemical oxygen‐iodine lasers (COILs) [5], free‐electron lasers (FELs) [6], gas lasers (GLs) [7], quantum cascade lasers (QCLs) [8], optical parametric oscillator (OPO) or optical parametric amplifier (OPA) lasers [9], directly pumped mid‐infrared solid lasers [10], and so on. Recently, a rare‐earth‐doped fibre laser at 3.92 μm has been successfully developed [11].…”