Tunable semiconductor lasers can monitor air pollution

Abstract

Global atmospheric pollution can pose serious health threats and endangers our quality of life. It is imperative to reverse anthropogenic damage to the atmosphere by tightly controlling and reducing the release of pollutants. However, the lack of rugged automatic instruments for profiling gas concentration currently makes monitoring and enforcing compliance virtually impossible. Optical remote sensing, a technology capable of mapping gas concentration profiles in real time, promises to fill this void. Range resolved monitoring of pollutant concentrations in ambient air enables the identification and characterization of sources and facilitates air pollution control. Light detection and ranging (LIDAR) with searchlights was adopted for atmospheric profiling long before the laser era. Two powerful methods became available with the advent of the laser: differential absorption LIDAR (DIAL) and tunable diode laser spectroscopy.1 DIAL systems are suitable for profiling gas concentration over distances up to a few tens of kilometers but they are highly complex, fragile, bulky, and can harm vision. A compact and rugged tunable laser sensor, on the other hand, cannot perform range-resolved measurements. To incorporate the advantages of both approaches, we have implemented the DIAL method with tunable semiconductor lasers using a group of retroreflectors (RRs).2 In our approach, retroreflectors divide the absorption path into range intervals similar to those of the DIAL method. Since the path length dependence of the DIAL signal is a function of the absorbing gas concentration, a series of path-integrated measurements allow for the calculation of the gas concentration in each range interval. The spacing between the RRs determines the spatial resolution. To extract fast-changing gas concentration profiles, the measurements need to be completed on a time scale in which major changes in the gas concentrations will not develop. Short laser pulses can be used to reFigure 1. The scalable model of a laser system for fenceline monitoring of industrial air pollution sources.