Low-threshold, high SMSR tunable external cavity quantum cascade laser around $$4.7\,\upmu \hbox {m}$$ 4.7 μ m

Abstract

Room-temperature pulsed and continuous-wave (cw) operation of a tunable external cavity (EC) quantum cascade laser (QCL) at an emitting wavelength of \(4.7\,\upmu \hbox {m}\) was presented. The effect of different external cavity lengths and grating angles of the EC–QCL system were analyzed numerically. A wide tuning range greater than \(131\,\hbox {cm}^{-1}\) was obtained in pulsed mode at room temperature. Without the anti-reflection coating procedure, single-mode cw operation with a side-mode suppression ratio (SMSR) above 20 dB and a wide tuning range greater than \(116\, \hbox {cm}^{-1}\) were achieved. Near the center region, SMSR about 30 dB was also realized through designing the external cavity length. Strain-compensation combined with two-phonon resonance in an active region design and the high-reflection coating promised low threshold current density. A record low threshold current density of \(0.901\,\hbox {kA/cm}^{2}\) for an EC–QCL operated in cw mode was realized.