We provide a general technique for evaluating the performance of an optical sensor for the detection of chemical warfare agents (CWAs) in realistic environments and present data from a simulation model based on a field deployed discretely tunable 13CO2 laser photoacoustic spectrometer (L-PAS). Results of our calculations show the sensor performance in terms of usable sensor sensitivity as a function of probability of false positives (PFP). The false positives arise from the presence of many other gases in the ambient air that could be interferents. Using the L-PAS as it exists today, we can achieve a detection threshold of about 4 ppb for the CWAs while maintaining a PFP of less than 1:106. Our simulation permits us to vary a number of parameters in the model to provide guidance for performance improvement. We find that by using a larger density of laser lines (such as those obtained through the use of tunable semiconductor lasers), improving the detector noise and maintaining the accuracy of laser frequency determination, optical detection schemes can make possible CWA sensors having sub-ppb detection capability with <1:108 PFP. We also describe the results of a preliminary experiment that verifies the results of the simulation model. Finally, we discuss the use of continuously tunable quantum cascade lasers in L-PAS for CWA and TIC detection.
We present an analytical model evaluating the suitability of optical absorption based spectroscopic techniques for detection of chemical warfare agents (CWAs) and toxic industrial chemicals (TICs) in ambient air. The sensor performance is modeled by simulating absorption spectra of a sample containing both the target and multitude of interfering species as well as an appropriate stochastic noise and determining the target concentrations from the simulated spectra via a least square fit (LSF) algorithm. The distribution of the LSF target concentrations determines the sensor sensitivity, probability of false positives (PFP) and probability of false negatives (PFN). The model was applied to CO2 laser based photoacosutic (L-PAS) CWA sensor and predicted single digit ppb sensitivity with very low PFP rates in the presence of significant amount of interferences. This approach will be useful for assessing sensor performance by developers and users alike; it also provides methodology for inter-comparison of different sensing technologies.
We report sensitive and selective detection of Diisopropyl methylphosphonate (DIMP) - a decomposition product of Sarin and a common surrogate for the nerve gases - in presence of several gases expected to be interferences in an urban setting. By employing photoacosutic spectroscopy with broadly tunable CO2 laser as a radiation source we demonstrate detection sensitivity for DIMP in the presence of these interferences of better than 0.5 ppb in 60 second long measurement time, which satisfies most current homeland and military security requirements and validates the photoacoustic spectroscopy as a powerful technology for nerve gas sensing instrumentation.
A novel trace-gas sensor system has been developed based on resonant photoacoustics, wavelength modulation spectroscopy, near-infrared diode lasers and optical fiber amplifiers that can achieve parts-per-billion sensitivity with a ten centimeter long sample cell and standard commercially-available optical components. An optical fiber amplifier with 500 mW output power is used to increase the photoacoustic signal by a factor of 25, and wavelength modulation spectroscopy is used to minimize the interfering background signal from window absorption in the sample cell, thereby improving the overall detection limit. This sensor is demonstrated with a diode laser operating near 1532 nm for detection of ammonia that achieves an ultimate sensitivity of less than 6 parts-per-billion. The minimum detectable fractional optical density, αminl, is 1.8x10-8, the minimum detectable absorption coefficient, αmin, is 9.5x10-10 cm-1, and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5x10-9 Wcm-1/&sqrt; Hz. These measurements represent the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.
A multiplexed diode-laser absorption sensor system, comprised of two distributed feedback (DFB) InGaAsP diode lasers and fiber-optic components, has been developed to non-intrusively measure gas temperature and H2O concentration over a single path in the combustion region of a 50-kW purposed annular dump combustor. The wavelengths of the DFB lasers were independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm and 1392 nm. Temperature and water vapor concentration were determined from the measured absorbances. In addition, measurements of CO, C2H2, and C2H4 concentrations in the exhaust were determined from absorption spectra recorded using a fast-sampling probe, a multi-pass absorption cell, an external cavity diode laser (ECDL), and a distributed feedback diode laser (DFB). The ECDL was tuned over the CO R(13) transition near 1568 nm and the C2H2 P(17) transition near 1535 nm, and the DFB laser was tuned over selected C2H4 transitions near 1646 nm. A correlation was established between the magnitude of the measured temperature oscillations in the combustion region and measured concentrations of CO and hydrocarbons in the exhaust. Adaptive control strategies were applied to maximize the coherence of the temperature oscillations and thus optimize the combustor performance. The closed-loop control system was able to adaptively optimize the phase and amplitude of the applied forcing within 100 ms, and the forcing frequently within 10 seconds. These results demonstrate the applicability of multiplexed diode-laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, in high-temperature environments.
A multiplexed diode-laser absorption sensor system, comprised of two distributed feedback (DFB) InGaAsP diode lasers and fiber-optic components, has been developed to non-intrusively measure gas temperature and H2O concentration over a single path in the combustion region of a 50-kW model pulsed incinerator. The wavelengths of the DFB lasers wee independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm. Temperature was determined from the ratio of measured peak absorbencies and used for closed-loop control of the combustor. In addition, measurements of CO, CO2, and C2H4 concentrations were determined from absorption spectra recorded in the incinerator exhaust using a fast-sampling stainless steel, water-cooled probe and a multi-pass absorption cell. An external cavity diode laser was tuned over the CO R(13) transition near 1568 nm and the CO2 R(16) transitions near 1572 nm, and a DFB laser was tuned over selected C2H4 transitions near 1646 nm. A correlation was established between the magnitude of the observed temperature fluctuations and the measured CO concentration in the exhaust. The amplitude of temperature fluctuations was controlled in a feedback loop by adjusting the relative phase between the primary and secondary forced air flows. The results obtained demonstrate the applicability of multiplexed diode laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, in high- temperature combustion environments.