A mid-infrared tunable diode laser (TDL) system for atmospheric applications

Abstract

A tunable diode laser (TDL) system operating in the mid-infrared region has been used for studies related to several aspects of atmospheric applications. To characterize the operation of the laser diode, the mode structure was mapped over the full temperature tuning range. For use as a reference, a segment of the broadband OCS spectrum was obtained from 1997.5 cm-1 to 2000.5 cm-1 at a pressure of 1.8 Torr. The spectrum is rich and has previously been reported in detail, so it is well suited as a standard reference for the TDL system in this spectral window. Many rare isotopic lines of OCS appeared in natural abundance, indicating a high sensitivity of the system. Experiments were then carried out in which OCS was mixed with air at varying pressures from 0 Torr to atmospheric conditions. By monitoring a 160 13C34S transition appearing in natural abundance at 2000.1467 cm-1, the detection limit of the system was estimated to be about 4 ppm by volume at an optical path length of 4 m. Pressure broadening and shift of OCS and H20 transitions were also investigated. Finally a sample of car exhaust was obtained and introduced into the sample cell as a direct atmospheric application. Absorption lines due to H20 and OCS were observed as well as more than 15 additional "unknown" lines. By comparison of their wavenumbers against the HITRAN standard atmospheric database (1), some of the unknown lines are identified with H2S.