Mid-infrared external-cavity quantum-cascade laser

Single-frequency operation of a $10.4‐\mathit{\mu }\mathrm{m}$ room-temperature multimode quantum-cascade laser was achieved by use of a 17-mm short Littrow-type external-cavity configuration. The spectral intensity of the external-cavity laser was increased as much as 20-fold compared with that of a multimode quantum-cascade laser without an external cavity. A single-frequency tuning range of 76 nm $\left(7 {\mathrm{cm}}^{-1}\right)$ was achieved. The results were obtained without antireflection coating of the laser output facet.