Laser-based sensors, currently under development and testing at Sandia National Laboratories for process control, emissions monitoring, and pollution prevention, are discussed.

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.

MOCVD has emerged as an important technique for the manufacturing of advanced semiconducting and optoelectronic thin film materials with multilayered and graded compositional profiles. The commercialization of these materials will be limited without real-time monitoring and closed loop control of the growth chemistry. This limitation has the potential to be eliminated using a novel FT-IR with a novel micro-multipass gas cell. The cell, based on proprietary aberration corrected mirrors, is characterized by a 1-meter optical pathlength, a 2.5 cm base path, a d an internal volume of 2.8 cm³. The cell is designed to operate in cross-flow such that the gas in the cell is representative of the sample stream with < 0.5 second time delay following a step change in concentration. Preliminary measurements for trimethylgallium, trimethylindium, and bis(cyclopentadienyl)magnesium indicate detection limits of 1.3 Pa, 1.6 Pa, and 0.1 Pa, respectively, using the low volume cell and a one second scan time.

Three novel atmosphere gas composition monitoring methods are described that could significantly improve the quality control of steel heat treat processes. The proposed methods, gas chromatography, inter-cavity Raman analysis, and fiber- optic Raman analysis are in different stages of development but all have varying advantages over the current state of the art IR analysis. A summary description of the Raman principle is provided. Test results comparing the inter- cavity Raman analysis versus the gas chromatography analysis are given. The major disadvantage of the gas chromatography is the slow response. The cost-effective inter-cavity Raman device can monitor eight species simultaneously with a response in the order of tens of milliseconds. An in-situ fiber optic Raman analysis instrument is expanded and carburizing facility test with this novel device are described. Advantages of the fiber-optic instrument are the in-situ nature and the fast response. This instrument lends itself to be readily used in applications where hostile environments require monitoring.

This paper discusses recent research at NIST aimed towards the use of cavity ring-down spectroscopy as an absolute partial pressure sensor. The motivation for this work is reviewed, the concepts behind cavity ring-down spectroscopy are outlined, and recent results are summarized. The paper ends with a look at future metrological uses of this technique.

A new technology will be described which extends the cavity ring-down optical absorption technique to condensed matter by using a miniature, high-finesse, monolithic, total- internal-reflection-ring resonator. Evanescent waves that are generated by total-internal reflection permit input and output coupling by photon tunneling and probe the presence of absorbing species at a cavity facet. The TIR-ring design permits broadband cavity ring-down measurements of adsorbates, thin films, and liquids by eliminating the use of multilayer coatings. The basic sensing concept will first be reviewed by describing recent experiments employing a non-ring prototype in which a totally reflecting element was incorporated in a conventional ring-down cavity. The basic design issues for miniature TIR-ring cavities will then be briefly reviewed along with some numerical result obtained using a wave optics model that show the magnitude of different optical losses as a function of cavity size. A competition between losses results in an optimum size for chemical detection which occurs when the round-trip loss of the 'empty' cavity is minimized. The first experimental results will be presented for a square, fused-silica TIR- ring cavity for which the theoretically predicted photon decay time has been achieved.

The DFWM, used as a laser spectroscopy for gas phase trace detection, is receiving a great deal of attention for on- line applications in industrial combustion diagnostics. Low release of NOx requires a complete control of nascent NO formation, i.e. of the combustion chemistry involving the OH radical, and space resolved temperature measurements of each species. Both NO and OH have been detected on small hydrocarbon/air flames by DFWM in forward BOXCARS geometry. NO distribution has been investigated by exciting of (gamma) band, temperature and concentration have been calculated after spectral simulation and calibration on doped flames. HO spectra of the A²(Sigma) ⁺ - X²(Pi) , on the fundamental and first vibrationally excited transition, have been used to monitor the radical space distribution and its temperature. Line broadening, shifting and intensity borrowing phenomena related to saturation have been investigated in order to correctly model the spectra. The technique has been used to detect OH band in the combustion chamber of a dry low NOx 130 KW prototype burner, obtaining relative OH concentration profiles. A single shot broad band system, contemporary detection a few OH lines in the transition, has been built to operate in turbulent regime.

New sensor concepts base don rapidly tunable, room- temperature diode laser absorption are being developed for a variety of aeropropulsion testing and control applications. The sensors are compact, rugged, capable of remote operation, and sensitive to a number of important control parameters. This paper describes recent progress in the development of three specific sensor platforms: inlet air mass flux for in-flight aeroengine control; simultaneous density, temperature, and velocity measurements in high- speed propulsion test facilities; and multi-species emissions monitoring. The measurement principles and system architectures are reviewed, along with sample laboratory and field test data.

A quantitative imaging method using 2D single-pulse laser- induced predissociative fluorescence (LIPF) of OH concentrations is developed to study the flame structure in a swirling methane jet flame. A narrowband tunable KrF excimer laser is used to excite the P₂(8) rotational line of the A²(Sigma) implied by (Chi) ²(Pi) (3,0) transition at (lambda) equals 248.46 nm. Though this transition produces a relatively weak signal, LIPF is much less sensitive to collisional quenching in atmospheric flames and suitable for generating quantitative data. OH concentration data are obtained by careful calibration against the flat flame burner data of known fuel-air equivalence ratio and temperature using identical optical setup. In this experiment, the OH fluorescence signal is imaged onto an intensified CCD camera. Since the distribution of OH concentration has good correspondence with the flame, the measured 2D imaging of OH indicates the instantaneous shape of the reaction zone. In the present experiment, the flame structure is first identified from the flame visualization. In the upstream of the swirling flame, combustion is found to take place in two regions; the recirculation vortex inside the recirculation bubble and the thin layer between and the thin layer between the recirculation zone and the ambient air. Higher intensity of OH is found early in recirculation zone. It is superequilibrium OH, because its intensity is higher than that from the recirculated burnt gas and the measured temperature is low. The measured OH concentrations have a direct influence on flame stabilization and NOx pollutant formation.

In lean-premixed combustion, the narrow range of operating conditions where stable, low-emissions combustion, is achieved make it necessary for a fuel-air equivalence ratio sensor to be incorporated into the combustor. Such a sensor should be capable of determining nozzle-to-nozzle variations in the equivalence ratio, and have a reasonably fast response time so that the control mechanism can meter the flowrates accordingly. This paper describes the development of a flame chemiluminescence based equivalence ratio sensor, which can be installed in the individual nozzles of a gas turbine combustor. The first stage of the development involves studying the chemiluminescence characteristics of CH* and CO₂* in a dump combustor. It was observed that fuel-air mixedness does not affect the overall flame chemiluminescence, and inlet temperature variations over 50K have not produced discernible differences in the chemiluminescence intensities. Combustor velocities do affect the flame chemiluminescence intensities, and this parameter has to be taken into account while developing the sensor. An optical fiber based sensor, which can easily installed in a combustor, has been designed. Based on this study, a methodology for determining the equivalence ratio from the chemiluminescent intensities and flowrates is proposed for use with the sensor under development.

Advanced thermal coating materials are being developed for use in the combustor section of high performance turbine engines to allow for higher combustion temperatures. To optimize the use of these thermal barrier coatings (TBC), accurate surface temperature measurements are required to understand their response to changes in the combustion environment. Present temperature sensors, which are based on the measurement of emitted radiation, are not well studied for coated turbine blades since their operational wavelengths are not optimized for the radiative properties of the TBC. This work is concerned with developing an instrument to provide accurate, real-time measurements of the temperature of TBC blades in an advanced turbine engine. The instrument will determine the temperature form a measurement of the radiation emitted at the optimum wavelength, where the TBC radiates as a near-blackbody. The operational wavelength minimizes interference from the high temperature and pressure environment. A hollow waveguide is used to transfer the radiation from the engine cavity to a high-speed detector and data acquisition system. A prototype of this system was successfully tested at an atmospheric burner test facility, and an on-engine version is undergoing testing for installation on a high-pressure rig.

The measurement of the temporal distribution of fuel in gas turbine combustors is important in considering pollution, combustion efficiency and combustor dynamics and acoustics. Much of the previous work in measuring fuel distributions in gas turbine combustors has focused on the spatial aspect of the distribution. The temporal aspect however, has often been overlooked, even though it is just as important. In part, this is due to the challenges of applying real-time diagnostics techniques in a high pressure and high temperature environment.

The present paper describes an Electro-Optical Monitoring System developed for the real time in-situ remote sensing of H₂S over large areas, at very low concentrations in air, well below the hazardous levels.

In this communication, we discuss the development of a fiber optic probe for use in field screening of waste sites that are contaminated with heavy metals. The prove, which is deployed via cone penetrometer truck, uses the principle of laser induced breakdown spectroscopy to gather qualitative and quantitative about metal contamination in-situ.

The emission inventories of aircraft emissions are being set up using flight routing data and test rig measurements of the engine manufacturers for certification purposes which have to be extrapolated with respect to the actual parameters at cruise altitude. Precise data from in-service engines are not existing. FTIR-emission-spectroscopy as a remote sensing multi-component exhaust gas analysis method has been further developed to specify the traceable molecules in aircraft exhausts, to determine the detection limits, and to obtain reliable statements concerning its accuracy. The first measurement with the Airbus A340 engine CFM56-5C2 during run up tests at ground level showed the overall ability of the FTIR-emission system to analyze the exhausts of modern gas turbines with high bypass ratio and mixing of fan air into the exhausts before the nozzle exit. Good quality spectra were measured and analyzed with respect to the mixing rations of CO₂, H₂O, CO, NO, and N₂O, and the emission indices of CO, NO, and N₂O. Total measurement times at one thrust level should be about 15 minutes to obtain reliable result which can be compared to the ICAO data of this engine.

Tunable diode laser absorption spectroscopy is used to measure the time evolution of hydrogen fluoride concentrations produce from a series of enclosed heptane/air pan fires extinguished by FE-36 or FE-36 plus ammonium polyphosphate (APP). Recent advances in room temperature fiber coupled, near-IR diode lasers provide isolation of the laser and signal processing electronics from the hostile sampling site. For the fires studied, the change in HF gas concentration with time is dependent upon the fire fighting chemical used to extinguish the fire. The presence of APP is observed to accelerate the dissipation of HF from the fire enclosure. Visible attenuation spectroscopy is also used to measure the amount of light attenuation that occurs as a hand held fire extinguisher containing powder fire fighting agents released in the crew space of a M1-Abrams land combat vehicle. Obscuration test demonstrate that release of APP from extinguishers in an occupied space does not present a visibility challenge to the vehicle personnel.

A sensitive technique based on laser photofragmentation/fragment detection spectrometry is reported for the trace detection of energetic materials such as TNT, PETN, and RDX. A single laser operating near 227 nm is used for both the photofragmentation of the parent molecule and detection of the characteristic NO fragment via its A-X transitions near 227 nm. The fragment detection methods employed are resonance-enhanced multiphoton ionization with miniature electrodes and laser-induced fluorescence with a photodetector. Experiments are also conducted in the visible region using 454-nm radiation for photofragmentation and fragment detection. The application of this technique in the trace analysis of the above- mentioned compounds at ambient pressure is demonstrated with limits of detection in the range of sub- to low parts-per- million for a 20-sec integration time and 20-120 (mu) J of laser energy at 227 nm and approximately 5 mJ at 454 nm. An increase in detecting sensitivity is projected with an increase in laser energy and an improved system design.

Low-cost, hydrogen-gas-leak detectors are needed for many hydrogen applications, such as hydrogen-fueled vehicles where several detectors may be required in different locations on each vehicle. A fiber-optic leak detector could be inherently safer than conventional detectors, because it would remove all detector electronics from the vicinity of potential leaks. It would also provide freedom from electromagnetic interference, a serious problem in fuel- cell-powered electric vehicles. This paper describes the design of a fiber-optic, surface-plasmon-resonance hydrogen detector, and efforts to make it more sensitive, selective, and durable. Chemochromic materials, such as tungsten oxide and certain Lanthanide hydrides, can reversibly react with hydrogen in air while exhibiting significant changes in their optical properties. Thin films of these materials applied to a sensor at the nd of an optical fiber have been used to detect low concentrations of hydrogen gas in air. The coatings include a thin silver layer in which the surface plasmon is generated, a thin film of the chemochromic materials, and a catalytic layer of palladium that facilitates the reaction with hydrogen. The film thickness is chosen to produce a guided-surface plasmon wave along the interface between the silver and the chemichromic material. A dichroic beam-splitter separates the reflected spectrum into a portion near the resonance and a portion away from the resonance, and directs these two portions to two separate photodiodes. The electronic ratio of these two signals cancels most of the fiber transmission noise and provides a stable hydrogen signal.

Derivative UV-absorption spectroscopy is a powerful spectroscopic technique for multicomponent gas analysis, particularly in combustion and process controlling applications. It offers enhanced selectivity and sensitivity compared to conventional techniques. We here report on a test of a special system with optical derivative generation in a waste incineration plant. Gas analysis is performed by transmission spectroscopy. A deuterium lamp is used as UV- source. Spectroscopic filtering is provided by a special grating monochromator. The grating is mounted on a galvanometer scanner, thus allowing a computer controlled wavelength scan and modulation. Signal analysis is performed with lock-in amplifier. The is from of detection for derivative spectra with a movable optical component is the origin of the term DYnamic Derivative Spectroscopy (DDS). The performance of this spectroscopic technique was demonstrated in a measurement campaign at a municipal solid waste incineration plant. The sensitivity for relevant gases is blow ppm level with an optical cell length of 10cm. The basics of the DDS and its performance will be explained, and data on NO, SO₂ and NO₂ will be reported.

With stricter environmental regulations optimization of the combustion process for reduced pollutant emission and higher fuel efficiency is a major objective for manufacturers. The promotion of oxy-fuel combustion is one alternative technology that has been demonstrated as a means for manufacturers to meet their environmental objectives. Despite the benefits oxy-fuel combustion can offer further optimization using monitoring and control techniques are still needed. Here we present a novel method for monitoring and controlling oxy-fuel burners by strategic placement of optical sensors. The sensors are integrated into an industrial oxy-fuel capable of withstanding harsh environments. Radiation from the flame at selected wavelength regions is collected by fiber optics attached to the burner and transported to a miniaturized PC-based spectrometer. The spectral information obtained is used to construct a neural network (NN) model that relates the real- time signal collected to burner operating parameters such as, stoichiometry, power, and fueled and/or oxidizer changes. This processed information from the NN can then be used in a control-loop for adjusting and optimizing combustion parameters or alerting operators of potential burner problems. Examples of using this technology on AIr Liquide's pilot furnaces in both the US and France and from an industrial glass melting tank will be presented. The potential of the sensor and NN approach is demonstrated for both conventional burner and an advanced wide flame burner. The results show that both stoichiometry and power changes can reliably be detected by use of the optical sensors. In addition, an example demonstrating the method on oxy-fuel oil flames to monitor oil atomization quality and stoichiometry will be presented.

Experimental and chemical modeling studies of a 60-Torr Nh₃/N₂O/Ar::1.03/1.59/0.54 flame are performed in order to provide further testing and refinements of a detailed chemical mechanisms developed previously in our laboratory. This mechanism consists of 87 reactions and 20 species and is denoted VS. Flame temperatures are measured with a coated, thin-wire thermocouple and by OH and NH by laser- induced fluorescence (LIF). Species concentration profile of NH₃, N₂O, N₂, NO, O₂, NH and OH are recorded using molecular beam pass spectrometry and/or LIF. The experimental species mole fractions are then compared to PREMIX laminar flame code calculations, which use the measured temperature profile and one of two detailed N/O/H chemical mechanisms, VS or VS-modified, as input. The PREMIX calculations using the VS mechanism predict very well the shapes of the experimental NH₃, N₂O, N₂ H₂O, NO, OH and NH profiles throughout the flame. They overpredict, however, the O₂/Ar mole fraction ratio at 16.25 mm by 61 percent. Sensitivity analyses suggest refinements to the VS mechanisms. These refinements, along with rate analyses of the PREMIX results, are presented and discussed.

The production of acetaldehyde was monitored during the simulated atmospheric oxidation of ethyl-3-ethoxyproprionate (EEP), a paint component. The detection of acetaldehyde in ambient pressure air to below 1 ppbv levels was demonstrated using resonance enhanced multiphoton ionization (REMPI) via the two photon resonant 3 s implied by n Rydberg transition at 363.5 nm. The reactions were carried out at room temperature in 100 L FEP Teflon bags while the air was continuously pumped through a parallel plate REMPI cell. Banks of black lights and sun lights provided UV radiation in the 300-450 nm region to simulate atmospheric photocatalysis by the sun. The REMPI measurements confirmed previous GC/MS and FTIR measurements but in real-time with higher sensitivity. The technique can be directly applied to the detection of atmospheric pollutants and real-time monitoring of manufacturing processes.

A laser induced fluorescence (LIF)-based optical fiber probe has been developed for spatially and temporally resolved measurements of the equivalence ratio in actual gas turbine combustors. By using the probe to simultaneously detect laser induced fluorescence and Mie-scattering, its applicability has been extended to liquid-fueled combustors. The probe has been tested in a simple flow system consisting of a four-inch diameter flow tube with a spray nozzle at the center and Jet-A as the fuel. PDPA measurements are made to correlate the Mie-scattering signal with the number density of liquid droplets. The results indicate that the probe can be used for qualitative characterization of liquid fuel vaporization, and that quantitative measurement of fuel vapor concentration in liquid-fueled gas turbine combustion is possible with calibration of the temperature dependence of the fluorescence.

The copolymerized propylene-butyl of which the 'solubility parameter' almost coincides with that of hydrocarbon and organic solvent gases such as toluene, xylene, diethyether, chloroform and acetone, is chosen as a material of the sensing membrane coated on the quartz resonator. It is found that copolymerized propylene-butyl-film coated quartz resonator microbalance gas sensor exhibits high sensitivity and excellent selectivity for these gases, especially for toluene and xylene gas, suggesting that the 'solubility parameter' is effective to functional design of the sensing membrane of quartz resonator gas sensors.

Spectroscopic techniques are ideal for characterization and process control of electron beam generated beam generated vapor plumes. Absorption based techniques work well for a wide variety of applications, but are difficult to apply to optically dense or opaque vapor plumes. We describe an approach for monitoring optically dense vapor plumes that is based on measuring the group velocity delay of a laser beam near an optical transition to determine the vapor density. This technique has a larger dynamic range than absorption environment. Aluminum as chosen because of its prevalence in high performance aircraft alloys. In these applications, composition control of the alloy constituents is critical to the deposition process. Data is presented demonstrating the superior dynamic range of the measurement. In addition, preliminary data demonstrating aluminum vapor rate control in an electron beam evaporator is presented. Alternative applications where this technique could be useful are discussed.

A new technology that uses a single sensor for temperature and velocity measurements in nonisothermal fluids is presented in this paper. The system is a multi-channel scanner designed for measuring velocity and temperature in applications where single- or multi-point measurements are required. This unit uses 12-bit data acquisition system along with digital circuitry for measuring both velocity and temperature with the same sensor. These unique sensor are designed to be low profile to minimize flow disturbance. Of the salient points of the new system is elimination of measurement error as seen in dual sensor system used in the same application. Dual sensor induced errors can be in excess of 20 percent that are caused when these sensors are used in non-isothermal flows. This technology provides an accurate and easy to use vehicle for mapping the temperature and velocity of flows in multitude of environments.

We found that an intense yellow photostimulated luminescence (PSL) can be observed when UV-light or visible-light irradiated SrS:Eu,Sm and CaS:Eu,Sm phosphor ceramics are estimated with IR light with wavelength from 800 nm to 1700 nm. This phenomenon in these phosphor materials is applicable to a novel 2D IR sensor. In the paper, we describe PSL characteristics in SrS:Eu,Sm and CaS:Eu,Sm phosphor ceramics, especially in SrS:Eu,Sm phosphor ceramics, and discuss the mechanism of the PSL which is observed by stimulating the phosphor with IR light.

Circular birefringence is also called optical rotation. An experimental set-up using the photoelastic modulator (PEM) is developed in our lab for measuring small optical rotation in chiral solutions. Sugar solutions at known concentrations are used as standards to test the feasibility of the method and the sensitivity of the instrument. The sensitivity of this current instrument is determined to be 0.001 degrees. Using the same set-up, low-level linear birefringence in optical materials can also be measured at a different harmonic of the PEMs resonant frequency. Examples of monitoring residual linear birefringence for optical elements are given. Recent progress made in our lab is briefly discussed.

The in-use performance of polymer composites is highly dependent on the polymeric structure, which in turn, is highly dependent on the processing conditions. We have been developing a Fourier transform Raman system capable of high temperature measurements within curing devices through the use of fiber optic probes. The goal is to use real-time spectral data to control heat schedules and ultimately, composite properties. This presentation will describe the development of cure models based on reaction mechanisms for an epoxy resin and a polyimide using IR and Raman spectroscopy. It will also describe correlations between molecular structure and mechanical properties obtained by simultaneous Raman and rheology measurements. In addition, new spectral methods to determine cure kinetics will be presented.

New research involving determination of endpoint on low open area etches using full spectrum optical emission spectroscopy (OES) will be presented. Traditionally, monochromator based systems have been used to determine endpoint by monitoring one or two strongly emitting wavelengths. For exposed open areas of < 1.0 percent, a more sensitive approach is required for the next generation of chips. Array detectors can provide a wealth of spectral information from a variety of gas phase emitting species, which can potentially each make a contribution to the endpoint signal. Evolving windowed factor analysis (EWFA) processes al of this spectral information into a single eigenvector output representing overall variance, which is used to call endpoint. The eigenvector output, which is influenced by all spectral emissions, is more responsive to the endpoint condition than one or two individual wavelengths. EWFA results form low exposed open area studies will be presented.

Oxy-fuel technology that uses high purity oxygen in place of air has demonstrated to be a cost-effective method for improving melting operations providing benefits in fuel savings, reduction in capital investment, and reduction of NOx and particulate matter. These benefits are evident in the glass industry where an estimated 15 percent of the US production has already been converted to oxy-fuel. However conversion from air-fuel is complicated by the drastic differences between the combustion characteristics such as flame temperature, momentum, flame chemistry, and heat transfer properties. For optimum performance using oxy-fuel combustion well-characterized burners with knowledge of the temperature in the combustion space is needed. Temperature characteristics for a given burner design are useful for both validation and parameter adjustment in 3D numerical models and optimizing the flame to the process. Because of the higher temperatures and steeper gradients in oxy-fuel flames traditional measurement techniques used on industrial flames, e.g., suction pyrometer or coherent anti-stokes Raman spectroscopy have limited use. Here we present results using a modified line reversal technique to monitor the emission and transmission of oxy-fuel flames seeded with sodium. The technique provides real-time information on the line-of-sight temperature observed from industrial scale turbulent flames.

Tunable Diode Laser Absorption Spectroscopy, operating in the near IR, is the best method available for making continuous measurements of many gases. The measurements are interference free and can be made with high sensitivity in fractions of a second. Sensitivities in the parts per billion range can be achieved for a number of gases such as CO, CO₂, HF, HCl, CH₄, C₂H₄, C₂H₂, O₂, NH₃, HCN, H₂S.