Mid-infrared trace-gas sensing with a quasi-continuous-wave Peltier-cooled distributed feedback quantum cascade laser

• Published in: Applied physics. B, Lasers and optics Lasers and optics, 2004-11, Vol.79 (7), p.907-913
• Main Authors: WEIDMANN, D, TITTEL, F. K, AELLEN, T, BECK, M, HOFSTETTER, D, FAIST, J, BLASER, S
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Ammonia; Applied sciences; Biological and medical applications; Detection; Exact sciences and technology; Feedback; Fundamental areas of phenomenology (including applications); General equipment and techniques; Global environmental pollution; Instruments for environmental pollution measurements; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Laser spectroscopy; Lasers; Optics; Physics; Pollution; Quantum cascade lasers; Semiconductor lasers; laser diodes; Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing; Spectroscopy; Tuning
• ISSN: 0946-2171; 1432-0649
• DOI: 10.1007/s00340-004-1634-z

A recently developed distributed feedback quantum cascade laser (QCL) capable of thermoelectric-cooled (TEC) continuous-wave (cw) operation and emitting at {/content/HYYCMHAL9V91GT8V/xxlarge8764.gif}9 {/content/HYYCMHAL9V91GT8V/xxlarge956.gif}m is used to perform laser chemical sensing by tunable infrared spectroscopy. A quasi-continuous-wave mode of operation relying on long current pulses ({/content/HYYCMHAL9V91GT8V/xxlarge8764.gif}5 Hz, {/content/HYYCMHAL9V91GT8V/xxlarge8764.gif}50% duty cycle) is utilized rather than pure cw operation in order to extend the continuous frequency tuning range of the quantum cascade laser. Sulfur dioxide and ammonia were selected as convenient target molecules to evaluate the performance of the cw TEC QCL based sensor. Direct absorption spectroscopy and wavelength-modulation spectroscopy were performed to demonstrate chemical sensing applications with this novel type of quantum cascade laser. For ammonia detection, a 18-ppm noise-equivalent sensitivity (1 {/content/HYYCMHAL9V91GT8V/xxlarge963.gif}) was achieved for a 1-m absorption path length and a 25-ms data-acquisition time using direct absorption spectroscopy. The use of second-harmonic-detection wavelength-modulation spectroscopy instead of direct absorption increased the sensitivity by a factor of three, achieving a normalized noise-equivalent sensitivity of 82 ppb{/content/HYYCMHAL9V91GT8V/xxlarge8201.gif}Hz for a 1-m absorption path length, which corresponds to 2X10 cm{/content/HYYCMHAL9V91GT8V/xxlarge8201.gif}Hz.