Direct and wavelength modulation spectroscopy using a cw external cavity quantum cascade laser

A continuous wave external cavity quantum cascade laser (EC-QCL) operating between 1872 and 1958 cm-1 has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the v2 band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of 8.4 × 10-4 cm-1 Hz-1/2 was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of 23 to 3.7× 10-5 cm-1 Hz-1/2. © 2009 American Institute of Physics