A spectrometer using a pulsed, 10.25-μm-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 downchirp of up to 75 GHz produced by a square current drive pulse. The behavior of this downchirp has been investigated in detail using high-resolution Fourier-transform spectrometers. The downchirp spectrometer provides a real-time display of the spectral fingerprint of molecular gases over a wave-number range of up to 2.5 cm<sup>−1</sup>. 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.
© 2003 Optical Society of America
(010.1120) Atmospheric and oceanic optics : Air pollution monitoring
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(300.6260) Spectroscopy : Spectroscopy, diode lasers
(300.6340) Spectroscopy : Spectroscopy, infrared
(300.6390) Spectroscopy : Spectroscopy, molecular
Michael T. McCulloch, Erwan L. Normand, Nigel Langford, Geoffrey Duxbury, and D. A. Newnham, "Highly sensitive detection of trace gases using the time-resolved frequency downchirp from pulsed quantum-cascade lasers," J. Opt. Soc. Am. B 20, 1761-1768 (2003)