V.V. Liger

Diode-laser atomic-absorption spectrometry by the double-beam-double-modulation technique

Abstract The limitations of absorption measurements in atomic-absorption spectrometry with tunable diode lasers are investigated. It is shown that the double modulation technique (diode-laser wavelength modulation and sample modulation) with detection at the sum or difference frequency suppresses spurious etalon effects, background absorption, residual diode-laser-amplitude modulation and the noise which accompanies these effects, and enables measurement of detection limits determined by the laser excess noise. Detection limits in absorption, defined as absorption equal to the root-mean square value of noise, as low as 1 × 10 −6 AU (absorption units) were achieved for metastable Cl atoms in a modulated low-pressure microwave-induced plasma with a time constant of 1 s. In order to eliminate laser excess noise and signal variations due to changes of optical transmittance, a double-beam arrangement with logarithmic subtraction of sample and reference detector currents was developed. It enables suppression of variations of the laser radiation power outside the detection pass-band and the achievement of a detection limit of about 2 × 10 −7 AU determined by shot noise only.

Dynamic range improvement and background correction in diode laser atomic absorption spectrometry

The peculiarities of background correction and linearization of a calibration curve in atomic absorption spectrome- try with semiconductor diode lasers have been investigated. The logarithmic conversion of a signal was shown to be a very efficient tool for signal processing. The linearity of the calibration curve up to a concentration nearly three . orders of magnitude above the characteristic concentration absorbance 1.7 was obtained. Background absorbances up to 1.4 did not influence the value of the analytical signal in a graphite furnace, nor did it cause a significant increase in the noise. An absorbance of 4 = 10 y5 could be measured with a background absorbance of 1.2. Q 1999 Elsevier Science B.V. All rights reserved.

Development of Diode Laser Absorption Spectroscopy Method for Determining Temperature and Concentration of Molecules in Remote Object

We develop a method of diode laser absorption spectroscopy for noncontact measurements of the temperature and pressure of a gas mixture under unsteady-state conditions at low signal-to-noise ratios. The method is based on the measurement of the absorption spectra of water molecules and approximation of experimental spectra by spectra simulated based on spectroscopic databases. Different approximation algorithms of experimental spectra are tested, such as fitting by individual contours and fitting by a part of the simulated spectrum. We reveal that, at small signal-to-noise ratios, the approximation of experimental data by a simulated spectrum yields more correct data on the temperature of the mixture compared to the fitting by individual contours. For the examined temperature range of 300–1300 K, the determination error of the gas temperature in the test cell proved to be approximately three times lower than upon fitting by individual contours. The developed method of recording and processing spectra is used to measure the temperature, water vapor concentration, and total pressure under the unsteady-state combustion conditions of an air-hydrogen mixture in a supersonic flow.

Dual wavelength method for molecular difference absorption measurements in turbid media

A dual wavelength differential absorption method for the detection of inclusions in turbid media is proposed. The method is based on the measurement of the transmitted intensity differences of the two diode lasers tuned to selected wavelengths within a broad absorption band of an inclusion of interest. The strategy for the optimum selection of the diode laser wavelengths and initial adjustment of the detection scheme is developed. It is shown that small deviations in the wavelengths from an optimum position result in a considerably deteriorated background suppression. The method is demonstrated by the detection of two dyes in a small volume inside a phantom filled with a turbid medium with scattering and absorption coefficients similar to those of biological tissues. The type and relative concentrations of the dyes were chosen so that the optical properties at the laser wavelengths are similar to those of a mixture of oxy- and deoxy-hemoglobin. The proposed method is insensitive to inclusions in the test object with spectrally independent scattering and absorption but detects the changes of the dye composition of a few percent inside a small sub-volume. The technique can be useful for the detection of the physiological status and changes in biological tissues.

Dynamics of H2O Temperature and Concentration in Zone of Plasma-Assisted High-Speed Combustion

The paper is focused on study of particular mechanisms of plasma impact on high-speed combustion. A tunable diode laser absorption spectroscopy (TDLAS) technique and appropriate instrumentation was developed for the measurement of temperature and water vapor concentrations in reacted gases. The technique is based on the detection of the spectra of H2O absorption lines with different energies of low levels. Spectra were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. Under conditions of high level of flow disturbances an optimal technique for fitting the experimental spectra was developed based on presentation of the transient absorption spectra as 2D images in the first step of data processing. In the most cases the high signal-to-noise ratio enabled the reconstruction of the temporal behavior of temperature with a resolution of �] 1ms. The validated TDLAS technique was applied for detection of temperature and H2O concentration in the combustion zone of a supersonic (M = 2) air-fuel flow. Direct wallinjected hydrogen and ethylene were used as the fuels. The combustion process was initiated and sustained by near-surface electric discharge. The data of DLAS measurements have proved the idea of two-stage mechanism of plasma-assisted combustion.

Temperature Measurement in Plasma-Assisted Combustor by TDLAS

The paper is focused on measurements of gas temperature distribution in plasma-assisted supersonic combustor. A tunable diode laser absorption spectroscopy (TDLAS) technique was applied for that. Spectra of H2O absorption were recorded using fast frequency scanning of a single distributed feedback (DFB) laser. Under conditions of high level of flow disturbances an optimal technique for fitting the experimental spectra was developed based on visualization of the transient absorption spectra as 2D images in the first step of data processing. In the most cases the high signal-to-noise ratio enabled the reconstruction of the temporal behavior of temperature with a resolution of ∼1ms. Direct wall-injected ethylene was used as the fuel in two geometrical configurations: fuel orifices upstream or downstream of the electrode system of near-surface electric discharge. Conformity of the data of DLAS measurement with the two-stage mechanism of plasma-assisted combustion is discussed. I. Introduction variety of optical diagnostic techniques are used for gaseous medium characterization at presence of electrical discharge plasma– optical emission spectrometry (OES), Thomson scattering (TS), coherent anti-stocks Raman scattering (CARS), Doppler line broadening, etc. These techniques, providing valuable data for specific types of plasma and specific range of plasma parameters, have some limitations. Tunable diode laser (DL) absorption spectroscopy (TDLAS) is a widely used spectroscopic technique for the detection of gas parameters in reacting flows [1–8]. This technique provides remote, non-intrusive measurements of the parameters of a hot zone with time resolution in the µs-ms range depending on the specific experimental conditions. The technique is usually based on the measurements of the ratio of the absorption line intensities of a test molecule. If a Boltzmann distribution of the energy levels is established, the ratio of the line intensities depends only on the kinetic temperature of the object. DLAS possesses many attractive features as the diagnostic technique. The spatial coherence of DLs enables to deliver a narrow laser beam to a probing zone without noticeable divergence, which makes it possible remote sensing. In contrast to any version of optical emission-fluorescence technique, DLAS does not need a large solid angle of light collection – all information, which can be deduced from absorption signal, is “frozen” in a narrow laser beam of low divergence. Therefore the thermal emission of an investigated hot zone and laser stray light can be easily eliminated by a set of diaphragms and lenses. As a consequence, the hottest and most dense zones of an object can be probed by DLAS.

Measurement of Non-Stationary Gas Flow Parameters Using Diode Laser Absorption Spectroscopy at High Temperatures and Pressures

The layout of an absorption spectrometer with diode lasers for contactless measurement of the temperature and water-vapor concentration in gas flows with mixture pressures of up to 3 atm and temperatures of 300–2000 K has been designed. The technique is based on the rapid tuning of the radiation wavelength of two lasers, the registration of the absorption lines of water molecules that are located in the tuning range, and the fitting of the experimental absorption spectra by theoretical ones that have been simulated using spectroscopic databases. The original components of the spectrometer and different algorithms of the processing of experimental spectra are described. The performance of the spectrometer and processing methods were tested in the laboratory with a cuvette at a pressure of 1 atm and temperatures of 300–1500 K. The different processing algorithms give a reasonable coincidence of data on hot zone parameters that were obtained by the method of diode laser absorption spectrometry, and the temperature that was measured using standard sensors. The designed layout of the spectrometer passed the first tests on the T-131 experimental stand at the TsAGI (Central Aerohydrodynamics Institute).

Determination of the Parameters of Condensed Media by the Dual-Wavelength Differential Absorption Method

A method of dual-wavelength differential absorption with the use of diode lasers is proposed. The method makes it possible to detect changes in parameters of media that cause broad absorption lines of selected molecular components of these media either to shift or to deform. The potentialities of the method proposed are demonstrated in experiments on remote contactless measurement of the temperature of aqueous solutions and measurement of the deviations of the composition of a mixture of dyes from the equilibrium state. These investigations show ways of increasing the sensitivity of the method proposed in solving particular experimental tasks.

Determination of the Maximum Temperature in a Non-Uniform Hot Zone by Line-of-Site Absorption Spectroscopy with a Single Diode Laser

A new algorithm for the estimation of the maximum temperature in a non-uniform hot zone by a sensor based on absorption spectrometry with a diode laser is developed. The algorithm is based on the fitting of the absorption spectrum with a test molecule in a non-uniform zone by linear combination of two single temperature spectra simulated using spectroscopic databases. The proposed algorithm allows one to better estimate the maximum temperature of a non-uniform zone and can be useful if only the maximum temperature rather than a precise temperature profile is of primary interest. The efficiency and specificity of the algorithm are demonstrated in numerical experiments and experimentally proven using an optical cell with two sections. Temperatures and water vapor concentrations could be independently regulated in both sections. The best fitting was found using a correlation technique. A distributed feedback (DFB) diode laser in the spectral range around 1.343 µm was used in the experiments. Because of the significant differences between the temperature dependences of the experimental and theoretical absorption spectra in the temperature range 300–1200 K, a database was constructed using experimentally detected single temperature spectra. Using the developed algorithm the maximum temperature in the two-section cell was estimated with accuracy better than 30 K.

Data Processing Algorithm for Diagnostics of Combustion Using Diode Laser Absorption Spectrometry

A new algorithm for the evaluation of the integral line intensity for inferring the correct value for the temperature of a hot zone in the diagnostic of combustion by absorption spectroscopy with diode lasers is proposed. The algorithm is based not on the fitting of the baseline (BL) but on the expansion of the experimental and simulated spectra in a series of orthogonal polynomials, subtracting of the first three components of the expansion from both the experimental and simulated spectra, and fitting the spectra thus modified. The algorithm is tested in the numerical experiment by the simulation of the absorption spectra using a spectroscopic database, the addition of white noise, and the parabolic BL. Such constructed absorption spectra are treated as experimental in further calculations. The theoretical absorption spectra were simulated with the parameters (temperature, total pressure, concentration of water vapor) close to the parameters used for simulation of the experimental data. Then, spectra were expanded in the series of orthogonal polynomials and first components were subtracted from both spectra. The value of the correct integral line intensities and hence the correct temperature evaluation were obtained by fitting of the thus modified experimental and simulated spectra. The dependence of the mean and standard deviation of the evaluation of the integral line intensity on the linewidth and the number of subtracted components (first two or three) were examined. The proposed algorithm provides a correct estimation of temperature with standard deviation better than 60 K (for T = 1000 K) for the line half-width up to 0.6 cm−1. The proposed algorithm allows for obtaining the parameters of a hot zone without the fitting of usually unknown BL.