David M. Sonnenfroh

Measurements of CO, CO2, OH, and H2O in room-temperature and combustion gases by use of a broadly current-tuned multisection InGaAsP diode laser.

A new laser technology that achieves nearly 100-nm quasi-continuous tuning with only injection-current control in a four-section grating-coupler sampled-reflector laser was used to detect CO and CO(2) simultaneously in room-temperature gas mixtures. The same grating-coupler sampled-reflector laser was used to perform in situ measurements of CO, H(2)O, and OH in the exhaust gases of a CH(4)-air flame. This laser is being evaluated for inclusion in a multispecies combustion-emissions exhaust-analysis sensor, and its operational characteristics as they have an impact on gas sensing are described. Preliminary results suggest that this single laser can be used to replace multilaser sensor configurations for some combustion-emissions monitoring applications.

Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors

A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10(-7) Hz(-½) in free-space configurations and 5 × 10(-6) Hz(-½) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, AlGaAs, and AlGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO(2), O(2), and H(2)O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I(2) with a sensitivity of 10(-5) are also reported.

Absorption measurements of the second overtone band of NO in ambient and combustion gases with a 1.8-µm room-temperature diode laser

We describe the development of a room-temperature diode sensor for in situ monitoring of combustion-generated NO. The sensor is based on a near-IR diode laser operating near 1.8 mum, which probes isolated transitions in the second overtone (3, 0) absorption band of NO. Based on absorption cell data, the sensitivity for ambient atmospheric pressure conditions is of the order of 30 parts in 10(6) by volume for a meter path (ppmv-m), assuming a minimum measurable absorbance of 10(-5). Initial H(2) -air flame measurements are complicated by strong water vapor absorption features that constrain the available gain and dynamic range of the present detection system. Preliminary results suggest that detection limits in this environment of the order of 140 ppmv-m could be achieved with optimum baseline correction.

Observation of CO and CO 2 absorption near 1.57 µm with an external-cavity diode laser

Near-IR and visible room-temperature diode lasers in broadly tunable external-cavity configurations are becoming commercially available for gas-sensing applications. Near 1.57 mum, a coincidence of overtone and combination-band transitions from CO, CO(2), OH, and H(2)O is particularly interesting for combustion and combustor emissions monitoring. We report initial observations of the room-temperature absorption of CO and CO(2) made with a commercial external-cavity diode laser.

Ultrasensitive, visible tunable diode laser detection of NO 2

Recent advances in room-temperature visible diode lasers and ultrasensitive detection techniques have been exploited to create a highly sensitive tunable diode laser absorption technique for in situ monitoring of NO(2) in the lower troposphere. High sensitivity to NO(2) is achieved by probing the visible absorption band of NO(2) with an AlGalnP diode laser at 640 or 670 nm combined with a balanced ratiometric electronic detection technique. We have demonstrated a sensitivity of 3.5 × 10(10) cm(-3) for neat NO(2) in a 1-m path at 640 nm and have estimated a sensitivity for ambient operation of 5 ppbv m (l0 ppbv m at 670 nm), where ppbvm is parts in 10(9) by volume per meter of absorption path length, from measured pressure-broadening coefficients.

Freezing Behavior of Single Sulfuric Acid Aerosols Suspended in a Quadrupole Trap

The freezing properties of sulfuric acid droplets were studied by suspending single 20- to 30-μm-diameter particles in a quadrupole trap and cooling them to stratospheric temperatures (≥191.5 K). Each particles dc balance voltage was measured to determine the particle composition as a function of temperature and map out the particles trajectory relative to the sulfuric acid phase diagram. Angularly resolved optical scattering patterns were monitored to detect freezing events. Particles cooled through the sulfuric acid tetrahydrate region (35–70 wt % H2SO4) did not freeze and remained spherical liquid droplets for several hours. Only particles cooled through the ice-liquid equilibrium region (<35 wt% H2SO4) showed evidence of freezing. This supports previous experimental and field observations that stratospheric sulfuric acid aerosols are likely to remain liquid to within a few degrees of the ice frost point.

Quantum cascade laser sensor for SO2 and SO3 for application to combustor exhaust streams.

We have demonstrated a high-sensitivity, room-temperature quantum-cascade (QC) laser sensor for detection of SO2 and SO3 under conditions relevant to aircraft test combustor exhaust. Two QC lasers probe infrared absorption features at 7.50 and 7.16 microm for SO2 and SO3, respectively, with a common dual-beam detection system. We inferred a noise-equivalent absorbance of approximately 1 x 10(-4) Hz(-1/2). We have demonstrated detection limits for both SO2 and SO3 of 1-2 ppmv m/Hz(1/2) (where ppmv is parts in 10(6) by volume) for 300 torr, elevated temperature, and path lengths near 1 m. This level of sensitivity permits measurement of < 1 ppmv of SO2 and SO3 at these conditions with modest signal averaging.

Multifunctional industrial combustion process monitoring with tunable diode lasers

12 To address the inherent issues with extractive sampling, Air Liquide and PSI are collaborating on the development of an in-situ multi-functional near-IR tunable diode laser system. The system is specifically targeted for application in harsh combustion environments with flue gas temperatures > 1600 degree(s)C and high particle densities. The multiplexing capability of the diode laser system allows near simultaneous detection of CO, O2, and H2O. These are essential species in characterizing the combustion state of the process, i.e., fuel-rich or fuel-lean, and the flue gas temperature. Sensor development and testing are conducted on a 700 kW oxy-fuel pilot furnace to evaluate the performance under simulated industrial conditions. Here we present pilot test results for dynamic stoichiometry changes, effect of particle entrainment, and air infiltration monitoring.

Demonstration of a rapidly strain tuned Er 3+ -doped fiber laser for sensitive gas detection

We describe a novel combination of a diode-pumped, wavelength-modulated Er(3+)-doped fiber laser light source with a sensitive noise cancellation circuit for detection of acetylene and ammonia. The laser tuning element, a fiber Bragg grating, was mounted in such a way that it could be strained controllably and rapidly, allowing noise cancellation techniques to be applied to the wavelength-modulated output of the fiber laser. The experimental setup is relatively simple and can be extended to other fiber laser wavelengths for which semiconductor lasers are not readily available by selection of a different fiber Bragg grating and gain medium.

Spark-induced breakdown spectroscopy and multivariate analysis applied to the measurement of total carbon in soil

Identifying and implementing techniques for carbon management has become an important endeavor in the mitigation of global climate change. Two important techniques being pursued are geologic and terrestrial carbon sequestration. With regard to terrestrial sequestration, in order to accurately monitor changes in soil carbon potentially induced by sequestration practices, rapid, cost-effective, and accurate measurements must be developed. Spark-induced breakdown spectroscopy (SIBS) has the potential to be used as a field-deployable method to monitor changes in the concentration of carbon in soil. SIBS spectra in the 248 nm region of eight soils were collected, and the neutral carbon line at 247.85 nm was compared to total carbon concentration determined by standard dry combustion techniques. Additionally, Fe and Si emission lines were evaluated in a multivariate statistical model to evaluate their impacts on the models predictive power for total carbon concentrations. The preliminary results indicate that SIBS is a viable method to quantify total carbon levels in soils, obtaining a correlation of (R(2)=0.972) between measured and predicated carbon in soils. These results show that multivariate analysis can be used to construct a calibration model for SIBS soil spectra.