Infrared Linewidth and Line Position Measurements Using Diode Lasers with Internal Calibration

Abstract

Measurements in the infrared region using semiconductor lasers, particularly as applied to atmospheric pollution monitoring, depend critically on the availability of reliable line positions and linewidth information. We demonstrate a unique internal calibration technique such that spectral locations can be determined accurately, to the order of the monochromaticity of the laser. This is based on low-pressure absorption measurements and subsequent fitting of a Voigt profile and determining the Doppler component. With this technique, we are able to determine the self- and the N2-broadening parameters of the NH3 line at 852.7 cm−1 to be 26.2 ± 0.4 MHz Torr−1 and 5.2 ± 0.1 MHz Torr−1, respectively. Also, the spectral locations for several SO2 lines in the 1180.1 cm−1 region were determined accurately.

Photoacoustic spectroscopy in gases based on wavelength modulation

“…It is possible to use the Stark or the Zeeman effect to modulate the laser wavelength or the absorption wavelength to provide selectivity (13)(14)(15). Typical modulation extents are fairly limited because of the magnitudes of these molecular constants, so that the gas sample must be at low pressures to reduce the effect of pressure broadening (16). The PAS signal, however, in certain cases is dependent on the total pressure (17), and detectability may thus be compromised.…”

Trace detection in gases using photoacoustic spectroscopy and Fabry-perot interferometry

“…Hinkley and Freed (66) reported linewidths as narrow as 54 (1.8 X 10"* cm"4 by heterodyning a Pbg ggSn^ j^^^e laser, operating cw at 10.6 urn, with a stabilized single frequency COg gas laser. Because of this character istic diode lasers have been successfully used for line parameter studies of gaseous pollutants (67). Other applications Include the Figure 2, is a device that uses a fixed-frequency laser (COg,, GO, or HF gas laser) to pump a semlconAjctor crystal in a magnetic field (62).…”