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Laser based detection of gaseous, liquid and solid residues and trace amounts has been developed ever
since lasers were invented. However, the lack of availability of reasonably high power tunable lasers in
the spectral regions where the relevant targets can be interrogated as well as appropriate techniques for
high sensitivity, high selectivity detection has hampered the practical exploitation of techniques for the
detection of targets important for homeland security and defense applications. Furthermore, emphasis
has been on selectivity without particular attention being paid to the impact of interfering species on the
quality of detection. Having high sensitivity is necessary but not a sufficient condition. High sensitivity
assures a high probability of detection of the target species. However, it is only recently that the sensor
community has come to recognize that any measure of probability of detection must be associated with a
probability of false alarm, if it is to have any value as a measure of performance. This is especially true
when one attempts to compare performance characteristics of different sensors based on different
physical principles. In this paper, I will provide a methodology for characterizing the performance of
sensors utilizing optical absorption measurement techniques. However, the underlying principles are
equally application to all other sensors. While most of the current progress in high sensitivity, high
selectivity detection of CWAs, TICs and explosives involve identifying and quantifying the target species
in-situ, there is an urgent need for standoff detection of explosives from safe distances. I will describe our
results on CO2 and quantum cascade laser (QCL) based photoacoustic sensors for the detection of
CWAs, TICs and explosives as well the very new results on stand-off detection of explosives at distances
up to 150 meters. The latter results are critically important for assuring safety of military personnel in
battlefield environment, especially from improvised explosive devices (IEDs), and of civilian personnel
from terrorist attacks in metropolitan areas.
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