Abstract

In this paper, the use of a single and compact pulsed infrared CO₂-TEA laser Dial system for range-resolved remote sensing of atmospheric urban pollution is presented. The Use of CO₂-TEA lasers in this context requires operation at maximum output energy or peak power throughout a wide spectrum of CO₂ transitions in the 9 to 11 μm bands. This lidar system uses a CO₂ transversely excited atmospheric (TEA) frequency agile laser that achieves rapid tunability by using a small, flat copper minor mounted on a non-resonant galvanometer. The copper mirror folds the laser mode onto a grating at discrete angles to select each wavelength. The laser is capable of switching reliably between lines at a rate of 200 Hz. The most dominant time constant attributed to CO₂-TEA laser pulses whose nitrogen tail can extend for several microseconds, limiting the range resolution of the lidars to some hundreds of meters unless pulse-deconvolution techniques¹ are adopted, is circumvented in this work by the use of a plasma shutter. Of course, electro-optic crystals such as CdTe and CdSe could be used between polarisers to achieve the same results. However, the minimum permissible clear aperture due to the large laser beam diameter is approximately 9 to 30 cm², requiring an excessively large and expensive electro-optic crystal; telescopic reduction of the beam diameter could be performed to reduce the required aperture at the expense of the beam divergence. This sealed plasma-shutter pulse clipper filled with helium to an optimum pressure of 500 mbars is shown in Fig. 1.

© 1998 IEEE