Michael E. Webber

Ammonia monitoring near 1.5 µm with diode-laser absorption sensors

We investigated ammonia spectroscopy near 1.5 mum to select transitions appropriate for trace ammonia detection in air-quality and combustion emissions-monitoring applications using diode lasers. Six ammonia features were selected for these trace-gas detection applications based on their transition strengths and isolation from interfering species. The strengths, positions, and lower-state energies for the lines in each of these features were measured and compared with values published in the literature. Ammonia slip was measured in the exhaust above an atmospheric pressure premixed ethylene-air burner to demonstrate the feasibility of the in situ diode-laser sensor.

Fiber-amplifier-enhanced photoacoustic spectroscopy with near-infrared tunable diode lasers

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be 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.

In situ combustion measurements of CO 2 by use of a distributed-feedback diode-laser sensor near 2.0 mm

High-resolution absorption measurements of CO(2) were made in a heated static cell and in the combustion region above a flat-flame burner for the development of an in situ CO(2) combustion diagnostic based on a distributed-feedback diode laser operating near 2.0 mum. Calculated absorption spectra of high-temperature H(2)O and CO(2) were used to find candidate transitions for CO(2) detection, and the R(50) transition at 1.997 mum (the nu(1) + 2nu(2) + nu(3) band) was selected on the basis of its line strength and its isolation from interfering high-temperature water absorption. Measurements of spectroscopic parameters such as the line strength, the self-broadening coefficient, and the line position were made for the R(50) transition, and an improved value for the line strength is reported. The combustion-product populations of CO(2) in the combustion region above a flat-flame burner were determined in situ to verify the measured spectroscopic parameters and to demonstrate the feasibility of the diode-laser sensor.

High-sensitivity, high-selectivity detection of chemical warfare agents

We report high-sensitivity detection of chemical warfare agents (nerve gases) with very low probability of false positives (PFP). We demonstrate a detection threshold of 1.2ppb (7.7μg∕m3 equivalent of Sarin) with a PFP of <1:106 in the presence of many interfering gases present in an urban environment through the detection of diisopropyl methylphosphonate, an accepted relatively harmless surrogate for the nerve agents. For the current measurement time of ∼60s, a PFP of 1:106 corresponds to one false alarm approximately every 23months. The demonstrated performance satisfies most current homeland and military security requirements.

Optical detection of chemical warfare agents and toxic industrial chemicals: Simulation

We present an analysis of optical techniques for the detection of chemical warfare agents and toxic industrial chemicals in real-world conditions. We analyze the problem of detecting a target species in the presence of a multitude of interferences that are often stochastic and we provide a broadly applicable technique for evaluating the sensitivity, probability of false positives sPFPd, and probability of false negatives sPFNd for a sensor through the illustrative example of a laser photoacoustic spectrometer sL-PASd. This methodology includes s1d a model of real-world air composition, s2d an analytical model of an actual field-deployed L-PAS, s3d stochasticity in instrument response and air composition, s4d repeated detection calculations to obtain statistics and receiver operating characteristic curves, and s5d analyzing these statistics to determine the sensor’s sensitivity, PFP, and PFN. This methodology was used to analyze variations in sensor design and ambient conditions, and can be utilized as a framework for comparing different sensors.

Measurements of NH 3 and CO 2 with distributed-feedback diode lasers near 2.0 µm in bioreactor vent gases

Measurements of NH(3) and CO(2) were made in bioreactor vent gases with distributed-feedback diode-laser sensors operating near 2 mum. Calculated spectra of NH(3) and CO(2) were used to determine the optimum transitions for interrogating with an absorption sensor. For ammonia, a strong and isolated absorption transition at 5016.977 cm(-1) was selected for trace gas monitoring. For CO(2), an isolated transition at 5007.787 cm(-1) was selected to measure widely varying concentrations [500 parts per million (ppm) to 10%], with sufficient signal for low mole fractions and without being optically thick for high mole fractions. Using direct absorption and a 36-m total path-length multipass flow-through cell, we achieved a minimum detectivity of 0.25 ppm for NH(3) and 40 ppm for CO(2). We report on the quasi-continuous field measurements of NH(3) and CO(2) concentration in bioreactor vent gases that were recorded at NASA Johnson Space Center with a portable and automated sensor system over a 45-h data collection window.

Diode-Laser Sensors for Real-Time Control of Pulsed Combustion Systems

A diode-laser based, closed-loop control system has been developed to nonintrusively optimize a pulsed, 50-kW dump combustor. The adaptive control system used temperature and water mole fraction measurements obtained at 10-kHz rates from the peak absorbance values of H 2 O features near 1.4 μm. In addition, measurements of CO, C 2 H 2 , and C 2 H 4 concentrations in the exhaust, determined from diode-laser absorption spectra recorded using a fast-sampling probe and a multipass absorption cell (nominal 33-m-long path), were used to evaluate the effectiveness of the control strategies. A correlation was established between the magnitude of the observed temperature oscillations and the measured CO concentration in the exhaust. Adaptive control strategies were then applied to maximize the coherence of the burning vortices in the combustion region and thus optimize the combustor performance. The closed-loop control system was able to adaptively tune the phase and amplitude of the applied forcing within 100 ms and the forcing frequency within 10 s. 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 combustion environments.

Ultrasensitive gas detection using diode lasers and resonant photoacoustic spectroscopy

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.

DIODE-LASER SENSOR SYSTEM FOR CLOSED LOOP CONTROL OF A 50-kW INCINERATOR

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.

Optical detection of chemical warfare agents and toxic industrial chemicals

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.