Frequency modulation spectroscopy with infrared lasers

Abstract

Frequency modulation spectroscopy (FMS) in the IR spectral region using a carbon dioxide laser source has been recently reported.1 Here we report on the results of subsequent experiments aimed at increasing the sensitivity and versatility of the IR FMS technique for the ultrasensitive detection of trace molecular species. FM spectroscopy can be implemented using either fixed frequency or continuously tunable laser sources. For cases in which the laser carrier frequency must remain fixed, as with a carbon dioxide laser, a molecular absorption feature may be probed with a single isolated sideband by either Stark shifting the absorption feature into coincidence with the sideband or by continuously tuning the sideband through the absorption feature. For sources in which the laser carrier can be continuously tuned in frequency, such as a lead-salt diode laser, the most convenient method is clearly to fix the sideband frequency and tune the laser carrier. We have investigated experimentally all three approaches using a carbon dioxide laser and a semiconductor lead-salt diode laser.