Highly sensitive detection of trace gases using the time-resolved frequency downchirp from pulsed quantum-cascade lasers
A spectrometer using a pulsed, 10.25-mm-wavelength, thermoelectrically cooled quantum-cascade distributed-feedback laser has been developed for sensitive high-resolution infrared absorption spectroscopy. This spectrometer is based upon the use of the almost linear frequency down chirp of up to 75 GHz produced by a square current drive pulse. The behavior of this down chirp has been investigated in detail using high-resolution Fourier-transform spectrometers. The down chirp spectrometer provides a real-time display of the spectral fingerprint of molecular gases over a wave-number range of up to 2.5 cm-1. Using an astigmatic Herriott cell with a maximum path length of 101 m and a 5-kHz pulse repetition rate with 12-s averaging, absorption lines having an absorbance of less than 0.01 (an absorption of less than 1%) may be measured.