CFA 8 : OO am - 9 : 45 am Room : 101 B Laser - Based Gas and Chemical Sensing

A compact, tunable mid-IR laser source is the most critical opto-electronic component in a spectroscopic molecular sensor. Until recently, the choice of such sources was limited to cryogenically cooled Pb-salt diode lasers and lowpower devices based on difference frequency generation (DFG). The development of quantum cascade (QC) lasers and especially single-frequency devices with distributed feedback (QCDFB) provided an attractive new option for IR absorption spectroscopy. QC-DFB lasers operating at near-room temperature (i.e., temperature accessible with a thermoelectric module) are of particular interest for practical applications. Presently, the thermoelectric temperature management can be achieved only in a pulsed operation mode of a QC-DFB laser. Successful applications of pulsed QC-DFB lasers operating in the 4 pm to 10 pm range to spectroscopic detection of various molecules14 have been reported to date. We shall report the first spectroscopic quantification of CO, and other species using a long-wavelength (h = 16 pm) thermoelectrically cooled pulsed QC-DFB laser. Long-wavelength QC lasers make accessible a spectral region where some large molecules, such as benzene exhibit a rotationally resolved structure, thus facilitating their detection and quantification. The laser was driven with -20 ns long current pulses at a repetition rate of 1 MHz. The lasing threshold varied from 7A to 10A in the heat sink temperature range of 4O0C to +30”C. CO, and H,O absorption lines were used for precise calibration of the laser frequency (Fig. 1). From these data, the temperature tuning coefficient was found to be -0.048 cm-’/”C. At optimized conditions that include current pulse duration, laser current, and electric coupling, the observed FWHM of the low-pressure CO, absorption line was 210 MHz, which assumes a FWHM laser linewidth of -150 MHz. Different approaches to the laser frequency manipulation for spectroscopic data acquisition will be discussed, and the achieved gas-sensing performance characteristics reported.