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A two-temperature (2T) model is proposed for estimating the temperature of a spatially inhomogeneous hot zone using single-beam diode laser absorption spectrometry. The proposed algorithm is based on fitting experimental absorption spectra by a linear combination of two database-simulated single-temperature spectra with different temperatures. The model efficiency is experimentally demonstrated when determining the temperature in different flame zones of a Wolfgard – Parker slot burner. The proposed 2T algorithm is used to find the maximum and minimum temperatures in different flame sections, which are compared with local temperatures in the same flame zones measured by the method of coherent anti-Stokes Raman scattering (CARS). It is shown that the maximum temperatures determined by the proposed 2T algorithm are in good agreement with CARS data in flame zones where high-temperature regions prevail. A good agreement between the minimum temperatures obtained by these two methods is also observed for predominantly cold zones.

A review of the works that we have been carried in the last 5 years on the use of the method of absorption spectroscopy with diode lasers for the diagnosis of hot zone parameters is given. A spectrometer with two lasers operating in different spectral ranges with sufficiently strong absorption lines has been developed for the case of high pressures and gas temperature. The results of the search for optimal lines in different spectral ranges have been presented. The results of laboratory measurements and tests at the experimental power stand at TSAGI Named after Prof. N.E. Zhukovsky have been presented. Approaches to determining the maximum temperature for spatially inhomogeneous hot zones have been discussed. The method of difference spectra and method of fitting the experimental spectrum with the sum of one-temperature spectra have been proposed. The efficiency of the proposed processing algorithms for specific types of hot zones has been shown experimentally.

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).

We have developed a prototype of a diode-laser absorption spectrometer for noncontact determination of the temperature and concentration of water vapor in the combustion zone of mixed supersonic flows. The method is based on fast scanning of the radiation frequencies of two diode lasers in different spectral ranges containing several absorption lines of water molecules, determining the integral intensities of selected lines, and calculating the medium temperature from the intensity ratio of these lines. The described version of the spectrometer allows one to determine the parameters of the gas medium at high pressures (up to 3 atm) and temperatures (up to 2000 K). The criterion of selection of the spectral scanning ranges of the lasers is described, which is based on minimizing the error of determining the temperature at the expected experimental measurement accuracy of the integral intensities of lines. A program for selection of appropriate combinations of lines from spectroscopic databases has been developed. An algorithm for obtaining the temperature by fitting experimental spectra and theoretical spectra simulated using databases has been developed. Errors of determining the temperature for different fitting algorithms have been estimated. We show that, for the temperature estimation to be more accurate, it is necessary to take into account maximal possible number of lines lying in the scanning ranges of the two lasers.

An absorption spectrometer based on diode lasers is developed for measuring two-dimension maps of temperature and water vapour concentration distributions in the combustion zones of two mixing supersonic flows of fuel and oxidiser in the single run regime. The method of measuring parameters of hot combustion zones is based on detection of transient spectra of water vapour absorption. The design of the spectrometer considerably reduces the influence of water vapour absorption along the path of a sensing laser beam outside the burning chamber. The optical scheme is developed, capable of matching measurement results in different runs of mixture burning. A new algorithm is suggested for obtaining information about the mixture temperature by constructing the correlation functions of the experimental spectrum with those simulated from databases. A two-dimensional map of temperature distribution in a test chamber is obtained for the first time under the conditions of plasma-induced combusion of the ethylene – air mixture.

Tunable diode laser absorption spectroscopy (TDLAS) has become a proven method of rapid gas diagnostics. In the present review an overview of the state of the art of TDL-based sensors and their applications for measurements of temperature, pressure, and species concentrations of gas components in harsh environments is given. In particular, the contemporary tunable diode laser systems, various methods of absorption detection (direct absorption measurements, wavelength modulation based phase sensitive detection), and relevant algorithms for data processing that improve accuracy and accelerate the diagnostics cycle are discussed in detail. The paper demonstrates how the recent developments of these methods and algorithms made it possible to extend the functionality of TDLAS in the tomographic imaging of combustion processes. Some prominent examples of applications of TDL-based sensors in a wide range of practical combustion aggregates, including scramjet engines and facilities, internal combustion engines, pulse detonation combustors, and coal gasifiers, are given in the final part of the review.

Determination of a gas medium temperature by diode laser absorption spectrometry (DLAS) is based on the measurement of integral intensities of the absorption lines of a test molecule (generally water vapor molecule). In case of local thermodynamic equilibrium temperature is inferred from the ratio of the integral intensities of two lines with different low energy levels. For the total gas pressure above 1 atm the absorption lines are broadened and one cannot find isolated well resolved water vapor absorption lines within relatively narrow spectral interval of fast diode laser (DL) tuning range (about 3 cm −1 ). For diagnostics of a gas object in the case of high temperature and pressure DLAS technique can be realized with two diode lasers working in different spectral regions with strong absorption lines. In such situation the criteria of the optimal line selection differs significantly from the case of narrow lines. These criteria are discussed in our work. The software for selection the optimal spectral regions using the HITRAN-2012 and HITEMP data bases is developed. The program selects spectral regions of DL tuning, minimizing the error of temperature determination δТ/T , basing on the attainable experimental error of line intensity measurement δS . Two combinations of optimal spectral regions were selected – (1.392 & 1.343 μm) and (1.392 & 1.339 μm). Different algorithms of experimental data processing are discussed.

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.

We report a procedure for temperature and water vapour concentration measurements in an unsteady-state combustion zone using diode laser absorption spectroscopy. The procedure involves measurements of the absorption spectrum of water molecules around 1.39 μm. It has been used to determine hydrogen combustion parameters in M = 2 gas flows in the test section of a supersonic wind tunnel. The relatively high intensities of the absorption lines used have enabled direct absorption measurements. We describe a differential technique for measurements of transient absorption spectra, the procedure we used for primary data processing and approaches for determining the gas temperature and H2O concentration in the probed zone. The measured absorption spectra are fitted with spectra simulated using parameters from spectroscopic databases. The combustion-time-averaged (~50 ms) gas temperature and water vapour partial pressure in the hot wake region are determined to be 1050 K and 21 Torr, respectively. The large signal-to-noise ratio in our measurements allowed us to assess the temporal behaviour of these parameters. The accuracy in our temperature measurements in the probed zone is ~40 K.

Experimental version of diode laser absorption spectrometer (DLAS) for contactless measurements of temperature and water vapor concentration in supersonic gas flows is developed. The spectrometer can be used for the measurements of temperature up to 2500 K and total pressure up to 3 atm. The technique is based on the registration of the transient absorption spectra of a target molecules and fitting of the experimental spectra by the simulated ones constructed using the spectroscopic databases. The temperature is inferred from the ratio of the intensities of the absorption lines with different low energy levels. In gas media with the above parameters the absorption lines are broadened which demands the use of two diode lasers (DL) working in different spectral ranges. The software for selection of the optimal line combinations was developed. The combination of two strong lines in the spectral ranges 1.39 μ and 1.34 μ was selected as the optimal one. The efficiency of the developed technique was exemplified in the first set of the experiments in conditions of real propulsion in Zhukovsky Central Aerohydrodynamic Institute (TsAGI) for the temperatures within (500-2200) К range and total pressure up to 3 atm.

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.

The example of detection of Cl atoms in a glow discharge was used in an investigation of the ultimate capabilities of the method of low-absorption measurement with the aid of tunable diode lasers. The excess laser noise and low-frequency fluctuations of the transmission of the optical channel were compensated by a double-beam optical system with logarithmic subtraction of the measuring and reference beam signals. Simultaneous modulation of the laser radiation wavelength and of the glow discharge current at frequencies ~1 — 10 kHz, and detection of the difference-frequency signal made it possible to reach a detection threshold limited by the laser-radiation shot noise. The limit reached in this way corresponded, for a lock-in detection time constant 1 s, to a minimum detectable absorption of 2 ×10-7, which for a discharge 15 cm long was equivalent to a Cl concentration of 6000 atoms cm-3 in a metastable excited state.

A review of recent works made in the Diode Laser Spectroscopy Group of the Institute of Spectroscopy in Troitsk, Russia, is presented. The qualitative modifications of the rovibrational energy level structure under rotational excitation (so called critical phenomena) were studied for the v 1 , v 3 bands of SnH 4 and the v 3 band of the CF 4 molecule. The parameters of molecular supersonic jets have been determined for various pulsed valves. The dynamics of collisionless SF 6 and NH 3 molecules' excitation by TEA CO 2 laser radiation were investigation. Possibilities of intracavity detection of molecules by means of pulsed diode lasers with an external resonator have been analyzed.

The excitation of ammonia by one- and two-photon transitions by TEA CO2-laser radiation has been studied under collisionless conditions. It has been shown that the experimental results of two-photon excitation of 14NH3 molecules in the 2v 2 a(1, 1) ← a(1, 1) transition by the 10P(24) CO2-laser line, the frequency offset of which is ≈ 0.02cm−1, can be described well by modelling the laser radiation as a broadband chaotic field. The experimental results of one-photon excitation in 14NH3 and 15NH3 [4], with the offset values exceeding the CO2-laser linewidth, can be also described well with the use of a chaotic field with a continuous spectrum. In the case of a nearly exact one-photon resonance, however, the excitation efficiency of molecules depends critically on the real mode structure of the CO2-laser radiation.

A new technique to measure spectral linewidths of pulsed IR lasers is described. The results for the linewidth Γ of a TEA CO 2 laser are presented. The value of Γ =0.013±0.002 cm -1 obtained experimentally is in a good agreement with the results of model calculations.

A method of diode laser differential absorption spectrometry (DLDAS) is proposed. The method is based on the detection of absorption spectra variations caused by the changes of a parameter of a condensed media (temperature, composition of the components of a mixture, pH, etc.). Some simple theoretical background of the proposed technique is presented. The potentialities of the method are demonstrated in the 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.

An investigation was made of the current-voltage characteristics of injection lasers with isoperiodic PbSnSeTe layers and of double-heterostructure lasers with a controlled carrier-density profile, based on lead selenide. The investigation was carried out in the temperature range 20–80 K. The methods used in the fabrication of lasers and in measurements under pulsed conditions are described. It is shown that at 20 K the tunnel component of the current becomes important. An increase in temperature increases the role of the thermal current and already at 80 K the diffusion component becomes the dominant one. The tunnel component of the threshold current at 80 K is ~0.25.

A method for measuring the line width Γ of the emission spectrum of a TEA CO2 laser is described and the results of such measurements are given. The experimental value Γ = 0.013 ± 0.002 cm − 1 is compared with the results of a simple modeling calculation. A good agreement is obtained between the measured and calculated results.

A new technique to measure spectral linewidths of pulsed IR lasers is described. The results for the linewidth Gamma of a TEA CO2 laser are presented. The value of Gamma = 0.013/cm +/- 0.002/cm obtained experimentally is in a good agreement with the results of model calculations.

The excitation of ammonia via one- and two-photon transitions by TEA CO 2 -laser radiation has been studied under collisionless conditions. It has been shown that the experimental results of two-photon excitation of 14 NH 3 molecule in the 2 (nu) 2 a(1, 1) $IMP a(1, 1) transition by the 10P(24) CO 2 -laser line, the frequency offset of which is approximately equals 0.02 cm -1 , can be described well by modeling the laser radiation as a broadband chaotic field. The experimental results of one-photon excitation in 14 NH 3 and 15 NH 3 , 4 with the offset values exceeding the CO 2 - laser linewidth, can be also described well with the use of a chaotic field with a continuous spectrum. In the case of a nearly exact one-photon resonance, however, the excitation efficiency of molecules depends critically on the real mode structure of the CO 2 -laser radiation.

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.

The dynamics of excitation by CO2 laser pulses of SF6 molecules from individual rotational sublevels J″ of the ground vibrational state have been investigated in a pulsed molecular jet under essentially collisionless conditions. At different energy fluences of exciting laser pulse Φ = 10−5 + 10−1 J/cm2, the dependences of the fraction fJ of excited molecules on the rotational quantum number J″ have been obtained, when SF6 molecules were excited on the 10P(16) CO2 laser line, as well as on the 10P(18) and 10P(14) lines at Φ = 0.1 J/cm2. Comparison of experimental results with theoretical predictions18–20 shows rather strong disagreement between theory and experiments both in the population of the υ3 = 1 state and in the role of multiphoton transitions. It is shown that the narrowing of the exciting laser pulse spectrum (the 10P(16) line, Φ ≲ 10−2 J/cm2) results in drastic decrease of the fraction of excited molecules from the (υ3 = 0, J″ = 4) state which is most resonant with laser radiation.

A new approach to the spectroscopy of highly excited vibrational states of polyatomic molecules has been elaborated. The molecules of CrO 2 Cl 2 were prepared in states with a vibrational energy of the ground electronic term A 1 of ≈ 19000 cm −1 by means of internal conversion of electronic energy from the electronic state B 1 excited by laser radiation. The spectroscopy of the vibrationally excited molecules has been carried out in the region of the ν 6 and ν 1 bands with diode and CO 2 lasers. The fwhm of the obtained spectrum was ≈ 15 cm −1 . The intermode interaction in CrO 2 Cl 2 has been theoretically analyzed, and the calculated spectrum compared with that measured experimentally. The time evolution of the spectrum of vibrationally excited CrO 2 Cl 2 molecules has been studied. The average energy transferred per one collision with unexcited CrO 2 Cl 2 molecules was equal to 〈δ E 〉 ≈ 1200 cm −1 .

The Zeeman effect in the infrared absorption spectrum of a molecule without an electronic magnetic moment was observed using a linear spectroscopic technique with a tunable diode laser. In the magnetic field B = 500 kG weak splittings of the asR(4, 1) and asR(4, 0) lines in the v 2 -band of NH 3 were recorded. The values of the Zeeman splittings in these cases were −1×10 -2 cm -1 . The picture of the Zeeman splittings observed is shown to be connected with the difference between the g -factors for the excited (υ 2 = 1) and the ground (υ 2 = 0) vibrational states of the NH 3 molecule. This difference is about (-10)-(15)% of the g -factor for the lower vibrational level of the transition.

The results of recording of the diode-laser spectrum of GeH4, having a natural distribution of isotopes, within the range 932·4-937·5 cm-1 are presented. Taking into account the Coriolis interaction between (0100) and (0001) vibrational states, the rotational and centrifugal constants are obtained.

A comparison is made of the results of a theoretical description of the excitation of a "two-level molecule" using various models of a laser field and the results of experiments on collisionless excitation of NH3 and SF6 by multimode CO2 laser radiation. The importance of the role of the mode structure of the laser field is demonstrated.

A pulsed frequency tunable diode laser was used to investigate the IR multiphoton-induced depletion of rotational sublevels of the ground vibrational state of SF6 molecules cooled in a pulsed free jet at exciting energy densities between ≈10-2 and 2.3 J cm-2. The depletion of all rotational sublevels was effective at considerable (≈5–11 cm-1) pumping frequency detunings from the linear absorption spectrum (LAS) of the molecule the width of which under the conditions of experiment (Trot ≈ 18 K) was ≈2–3 cm-1. The fraction of molecules excited by a pumping pulse from individual rotational sublevels was measured and its dependence on the exciting pulse frequency and energy density investigated. The effect of collisions on the depletion of the rotational sublevels was studied.

The absorption spectra of 14NH3 and 15NH3 molecules from 930 cm-1 to 1220 cm-1 have been obtained with CW and pulsed Pb1–xSnxSe diode lasers. The laser emission frequency has been tuned by varying crystal composition, diode temperature, hydrostatic pressure, or injection current. The registration of the absorption spectra with CW PbSe laser continuously tuned by varying hydrostatic pressure has been accomplished. The possibility of gas isotope abundancies measurements by diode lasers is considered.

The work was aimed at the study of collisionless excitation of molecules by intensive laser infrared (IR) radiation. Molecules of SF6 and NH3 were cooled in a pulsed supersonic jet. Excitation was provided by various lines of a TEA CO2 laser. A tunable diode laser was used for probing of molecules. The dependences have been obtained for the fraction f of molecules excited from individual rotational levels on the density Φ of the excitation energy [1-5]. A model of the laser field is proposed which gives a good description of f (Φ) in the case of one- and two-photon excitation of NH3 [5]. It is believed that the present model will describe appropriately IR excitation of molecules of SF6 type in the region of lower discrete vibrational levels.