Abstract The present paper focuses on development of an external cavity quantum cascade laser (EC-QCL) without anti-reflection coating that can emit at two different wavelengths. The setup is based on a double Littrow-type configuration with an intra-cavity acousto-optic frequency shifter. A widely course double-mode tuning range of 430 nm for the spectral range between 4420 and 4850 nm in pulsed operation is achieved by rotating the diffraction gratings at a fixed acoustic frequency and injection current. A fine continuous tuning range over 60 nm is reached by modulating the RF driver tuning voltage from 5 to 10 V (the acoustic frequency from 60 to 84 MHz) in a few ms without moving any part, when just the first order beam path is open (in single-mode operation). This range is extended to 80 nm, when both the first- and zero-order beam paths are open simultaneously (in double-mode operation). The line-width of the laser is estimated to be 1.7 cm-1documentclass12pt{minimal} usepackage{amsmath} usepackage{wasysym} usepackage{amsfonts} usepackage{amssymb} usepackage{amsbsy} usepackage{mathrsfs} usepackage{upgreek} setlength{oddsidemargin}{-69pt} begin{document}$$^{-1}$$end{document}. These figures are convenient for the fast solid and liquid spectroscopy. To the best of our knowledge, it is the first demonstration of a such double-mode EC-QCL.
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