Rapid scan (Doppler-limited) absorption spectroscopy using mid-infrared quantum cascade lasers

Laser characteristics have been evaluated for mid-infrared quantum-cascade (QC) lasers operating in a continuous mode at cryogenic temperatures. These tests were performed to determine the suitability of QC lasers for use in various spectroscopic applications, including Doppler-limited molecular absorption spectroscopy and pressure-limited LIDAR instrumentation. Using rapid-scanning techniques, direct absorbance measurements of nitric oxide (NO), ammonia (NH3) and nitrous oxide (N2O) have been performed with QC lasers, operating at either 5.2 or 8.5 mu m. Measured Doppler-limited absorption profiles show no distortion with increased averaging (up to 10(3)-10(4) samples averaged), thereby minimizing the need for sophisticated data acquisition systems which re-register successive data streams to accommodate for laser frequency jitter and drift. Additionally, the high tuning rates (2.5 cm(-1) in 0.6 milliseconds; 5-10 kHz sweep repetition rate) achieved with the QC lasers allow for the measure of relatively rapid transient phenomena or a high degree of signal averaging in a short time. Noise-equivalent absorbances of 3 x 10(-6) have also been obtained without optimizing the optical arrangement.