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Highly sensitive detection of trace gases using the time-resolved frequency downchirp from pulsed quantum-cascade lasers

McCulloch, Michael T. and Normand, Erwan L. and Langford, Nigel and Duxbury, Geoffrey and Newnham, D.A. (2003) Highly sensitive detection of trace gases using the time-resolved frequency downchirp from pulsed quantum-cascade lasers. Journal of the Optical Society of America B, 20 (8). pp. 1761-1768. ISSN 0740-3224

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Abstract

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^-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.