Quantum-cascade laser measurements of stratospheric methane and nitrous oxide

Abstract

A tunable quantum-cascade (QC) laser has been flown on NASA's ER-2 high-altitude aircraft to produce the first atmospheric gas measurements with this newly invented device, an important milestone in the QC laser's future planetary, industrial, and commercial applications. Using a cryogenically cooled QC laser during a series of 20 aircraft flights beginning in September 1999 and extending through March 2000, we took measurements of methane (CH(4)) and nitrous oxide (N(2)O) gas up to ~20 km in the stratosphere over North America, Scandinavia, and Russia. The QC laser operating near an 8-mum wavelength was produced by the groups of Capasso and Cho of Bell Laboratories, Lucent Technologies, where QC lasers were invented in 1994. Compared with its companion lead salt diode lasers that were also flown on these flights, the single-mode QC laser cooled to 82 K and produced higher output power (10 mW), narrower laser linewidth (17 MHz), increased measurement precision (a factor of 3), and better spectral stability (~0.1 cm(-1) K). The sensitivity of the QC laser channel was estimated to correspond to a minimum-detectable mixing ratio for methane of approximately 2 parts per billion by volume.

Wavelength Modulation Spectroscopy: Combined Frequency and Intensity Laser Modulation

“…WMS was originally developed by use of lead-salt laser diodes emitting in the mid-infrared. [1][2][3][4] It was then applied to optical communication lasers in the near infrared 8 -13 and more recently to the novel quantum-cascade lasers in the mid-infrared [5][6][7] and using visible laser diodes. 14 WMS is based on the modulation of the light emitted by a laser that is slowly tuned through an absorption feature of the species to be detected.…”

“…Wavelength modulation spectroscopy ͑WMS͒ is a widely used technique for trace-gas detection by use of semiconductor lasers in the mid-infrared [1][2][3][4][5][6][7] and near infrared. 8 -13 This method takes advantage of several properties of semiconductor lasers, such as their tunability and modulation capabilities, to perform absorbance measurements in the 10…”

Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] This sensitive technique is based on sinusoidal modulation of the light emitted by a laser diode that is slowly tuned through an absorption feature of the species to be detected. The interaction of the modulated light with the absorption feature leads to the generation of signals at the different harmonics of the modulation frequency, which can be detected with a lock-in amplifier.…”

Compact gas cell integrated with a linear variable optical filter

“…The most significant system aspect is whether the spectral selectivity is implemented in the light source or in the detector. Scanning the spectrum with a tunable light source -either in narrowband with tunable diode-lasers [14] or in wideband using quantum cascade lasers [15]-and selecting one specific wavelength, as is the case in the nondispersive IR (NDIR) gas sensor [16], are two main methods used for implementing spectral selectivity in the light source. The second option is implemented in the detector, where the spectrum is scanned using a wideband source and a tunable narrowband filter at the detector [17].…”