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 μm. Measured Doppler-limited absorption profiles show no distortion with increased averaging (up to 10³-10⁴ 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 × 10^-6 have also been obtained without optimizing the optical arrangement.