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Remote detection of chemical and biological warfare agents and toxic gases in the atmosphere is of current interest to both the military and civilian agencies. Out of all currently available techniques, no single technique provides efficient detection against such threats at significant standoff distances. Light detection and ranging (LIDAR) technologies, based on the transmission of laser pulses and analysis of the return signals, have demonstrated impressive capabilities in remote detection of such toxic chemicals. LIDAR is a highly sensitive tool to detect the extremely low concentrations of various toxic agents present in the form of thin clouds at distances of few kilometer. The detection of these toxic clouds is based on the approach of first detecting and measuring the range of the clouds using the scattering phenomena and subsequently identifying the composition of toxic clouds using absorption and fluorescence phenomena. Laser Science and Technology Centre (LASTEC), Delhi has been working on the design and development of LIDAR systems for detection of chemical and biological warfare (CBW) agents. In this paper, theoretical analysis of differential absorption LIDAR (DIAL) for detection of chemical agents and fluorescence LIDAR for detection of biological agents has been discussed. For some typical parametric conditions, the received power levels from different ranges to detect specific concentrations of chemical or biological clouds have been computed and discussed. The technical details of the indigenously developed backscattering LIDAR, which detects and measures the distance of cloud layers up to 5 km is also presented.

Differential absorption lidar (DIAL) techniques are advantageously used these days for detecting and monitoring traces of toxic agents located at several kilometer in the atmosphere. A theoretical study has been carried out to simulate the performance of a multiwavelength DIAL sy stem. Clouds of hy drazine, unsy mmetrical dimethy lhy drazine (UDMH), and monomethylhydrazine (MMH), located at various ranges up to 5 km in the atmosphere, have been taken as examples of the toxic agents. It has been shown that a given lidar system cannot detect any of these agents with a specific cloud thickness if the concentration of that agent is below a certain value (N min ). It has also been shown that if the concentration level of a given agent is above a certain value (N max ) at a particular distance, this value cannot be quantified for a given lidar system although the identity as well as the location of that agent can still be determined. Further, for some typical parametric conditions, the required energy levels of the laser to detect specific concentrations of these agents at different distances have been computed. Power levels of the return signals and the SNR values from different ranges have also been calculated for each of these toxic agents for a given value of the laser transmitter energy.

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