Trace gas sensing with pulsed, distributed feedback quantum cascade laser

Detection of ammonia (NH3) and ethylene (C2H 4) at trace levels is of considerable importance for number of applications such as industrial process control, medical diagnostics, marine sciences, environmental sciences, and others. Both ammonia and ethylene are of particular interest in breath analysis, which is emerging as a valuable, non-invasive tool for clinical diagnosis of number of pathological conditions.;The use of a pulsed, distributed feedback (DFB) QC laser centered at 970 cm-1 in combination with different detection strategies for the infrared spectroscopic detection of ammonia and ethylene has been investigated. First, the laser is coupled with the technique of cavity ring-down spectroscopy. The ring-down cavity is formed by two plano-concave minors (99.7% reflectance) with 1 m radius of curvature, separated by a distance of 0.74 m. Detection limits of ∼27 ppb for ammonia and ∼130 ppb for ethylene have been achieved. The same cell in an off-axis arrangement has been used for the cavity enhanced spectroscopic studies. Detection limits of 15 ppb and 20 ppb have been demonstrated for ammonia and ethylene respectively. Further in the project, an astigmatic Herriot cell with 150 m path length was designed for the implementation inter- and intra-pulse methods. In the inter-pulse technique, the laser is excited with short pulses (5-10 ns), and the pulse amplitude is modulated with an external current ramp which gives ∼0.3 cm-1 frequency scan. In the intra-pulse technique, a linear frequency down-chirp within a pulse is used for sweeping across the absorption line. A 200 ns long pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm-1. Detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time using the intra-pulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene using the inter-pulse technique with an integration time of ∼5s have been achieved.;The availability of mid-infrared (IR) sources and a growing number of available optical components has led to the evolution and utilization of mid-IR laser spectroscopic techniques for many technological and scientific fields. Recent developments in the applications of pulsed quantum cascade (QC) lasers along with room temperature detectors promise better detection sensitivities in a cryogen free environment.