Diode laser absorption sensors for gas-dynamic and combustion flows

Demonstration of temperature imaging by H₂O absorption spectroscopy using compressed sensing tomography

This paper introduces temperature imaging by total-variation-based compressed sensing (CS) tomography of H2O vapor absorption spectroscopy. A controlled laboratory setup is used to generate a constant two-dimensional temperature distribution in air (a roughly Gaussian temperature profile with a central temperature of 677 K). A wavelength-tunable laser beam is directed through the known distribution; the beam is translated and rotated using motorized stages to acquire complete absorption spectra in the 1330-1365 nm range at each of 64 beam locations and 60 view angles. Temperature reconstructions are compared to independent thermocouple measurements. Although the distribution studied is approximately axisymmetric, axisymmetry is not assumed and simulations show similar performance for arbitrary temperature distributions. We study the measurement error as a function of number of beams and view angles used in reconstruction to gauge the potential for application of CS in practical test articles where optical access is limited.

Fast spatially resolved exhaust gas recirculation (EGR) distribution measurements in an internal combustion engine using absorption spectroscopy

Exhaust gas recirculation (EGR) in internal combustion engines is an effective method of reducing NOx emissions while improving efficiency. However, insufficient mixing between fresh air and exhaust gas can lead to cycle-to-cycle and cylinder-to-cylinder non-uniform charge gas mixtures of a multi-cylinder engine, which can in turn reduce engine performance and efficiency. A sensor packaged into a compact probe was designed, built and applied to measure spatiotemporal EGR distributions in the intake manifold of an operating engine. The probe promotes the development of more efficient and higher-performance engines by resolving high-speed in situ CO2 concentration at various locations in the intake manifold. The study employed mid-infrared light sources tuned to an absorption band of CO2 near 4.3 μm, an industry standard species for determining EGR fraction. The calibrated probe was used to map spatial EGR distributions in an intake manifold with high accuracy and monitor cycle-resolved cylinder-specific EGR fluctuations at a rate of up to 1 kHz.

Optical absorption measurements of hydrogen chloride at high temperature and high concentration in the presence of water using a tunable diode laser system for application in pyrohydrolysis non-ferrous industrial process control

A tunable diode laser (TDL) was used to measure hydrogen chloride (HCl) spectra at 5747 cm(-1) (1.74 μm) and temperatures of 25-950 °C in a quartz cell. The purpose was to evaluate the capability of monitoring HCl concentration under pyrohydrolysis conditions using a near-infrared (NIR) laser. These conditions are characterized by 20-40% HCl, 2-40% H2O, and the presence of metal chloride vapors at temperatures of 600-1000 °C. Spectral peak area measurements of HCl-N2 mixtures at atmospheric pressure and a path length of 8.1 cm showed linear absorption behavior between concentrations of 5-95% and temperatures of 25-950 °C. Results from the addition of 2-40% water (H2O) indicate that the HCl peak area relationships are not affected for temperatures of 350-950 °C. Evaporating NiCl2 within the cell did not show spectral interference effects with HCl between 650 and 850 °C. The results from this work indicate that a near-infrared optical sensor is capable of measuring high HCl concentrations at high temperatures in the presence of high H2O content during pyrohydrolysis process conditions.

Mid-infrared supercontinuum generation in suspended core tellurite microstructured optical fibers

We report the fabrication of a tellurite optical fiber with a suspended core design, formed on a 220-nm-wide filament of glass. The fiber was pumped at two different wavelengths (1500 and 2400 nm) using femtosecond pulses generated from an optical parametric oscillator (OPO) in order to produce mid-infrared supercontinuum (SC). We observed that SC spectra extending to 3 μm were readily generated. To further optimize the design, detailed numerical study was performed, which revealed how the fiber structural characteristics dramatically influence the spectral broadening because of the changes in the dispersion profile and in turn, the interplay of nonlinear effects that give rise to SC generation. We found that an accurate control of the core shape can be employed to contain the generated SC spectra within well-defined spectral regions or to provide a broad extension of the continuum to beyond 4 μm.

Ultra-fast and calibration-free temperature sensing in the intrapulse mode

A simultaneously time-resolved and calibration-free sensor has been demonstrated to measure temperature at the nanosecond timescale at repetition rates of 1.0 MHz. The sensor benefits from relying on a single laser, is intuitive and straightforward to implement, and can sweep across spectral ranges in excess of 1  cm⁻¹. The sensor can fully resolve rovibrational features of the CO molecule, native to combustion environments, in the mid-infrared range near λ=4.85  μm at typical combustion temperatures (800-2500 K) and pressures (1-3 atm). All of this is possible through the exploitation of chirp in a quantum cascade laser, operating at a duty cycle of 50%, and by using high bandwidth (500 MHz) photodetection. Here, we showcase uncluttered, spectrally-pure Voigt profile fitting with accompanying peak SNRs of 150, resulting in a typical temperature precision of 0.9% (1σ) at an effective time-resolution of 1.0 MHz. Our sensor is applicable to other species, and can be integrated into commercial technologies.

Digital signal processor-based high-precision on-line Voigt lineshape fitting for direct absorption spectroscopy

To realize on-line high-accuracy measurement in direct absorption spectroscopy (DAS), a system-on-chip, high-precision digital signal processor-based on-line Voigt lineshape fitting implementation is introduced in this paper. Given that the Voigt lineshape is determined by the Gauss full width at half maximum (FWHM) and Lorentz FWHM, a look-up table, which covers a range of combinations of both, is first built to achieve rapid and accurate calculation of Voigt lineshape. With the look-up table and raw absorbance data in hand, Gauss-Newton nonlinear fitting module is implemented to obtain the parameters including both the Gauss and Lorentz FWHMs, which can be used to calculate the integrated absorbance. To realize the proposed method in hardware, a digital signal processor (DSP) is adopted to fit the Voigt lineshape in a real-time DAS measurement system. In experiment, temperature and H2O concentration of a flat flame are recovered from the transitions of 7444.36 cm(-1) and 7185.6 cm(-1) by the DSP-based on-line Voigt lineshape fitting and on-line integral of the raw absorbance, respectively. The results show that the proposed method can not only fit the Voigt lineshape on-line but also improve the measurement accuracy compared with those obtained from the direct integral of the raw absorbance.

Baseline correction for stray light in log-ratio diode laser absorption measurements

Log-ratio detection is a convenient technique for making temperature and concentration measurements using sensors based on tunable diode laser absorption spectroscopy. In many practical sensing applications, it is difficult to avoid stray light falling on the signal photodiode of the sensor. This stray light acts as noncommon-mode interference and introduces a systematic error in absorption measurements, which is not removed by baseline subtraction. This paper analyzes the factors that determine this systematic error and also presents a calibration method that can correct for it. This correction method is verified using a simple experiment.

Widely tunable frequency conversion in monolithic semiconductor waveguides at 2.4 μm

We report on the generation of continuous-wave widely tunable light between 2360 and 2530 nm using difference-frequency generation with a pump tuned between 938 and 952 nm and a signal tuned between 1490 and 1590 nm in a type-II phase-matched monolithic semiconductor waveguide. The device internal conversion efficiency is estimated to be 0.29%  W(-1)  cm(-2). This design which uses a single-sided Bragg reflection waveguide has the potential for on-chip spectroscopy, as well as environmental monitoring applications, where a tunable source of coherent radiation tuned between 2 and 3 μm wavelength is desired.

Fitting of calibration-free scanned-wavelength-modulation spectroscopy spectra for determination of gas properties and absorption lineshapes

The development and initial demonstration of a scanned-wavelength, first-harmonic-normalized, wavelength-modulation spectroscopy with nf detection (scanned-WMS-nf/1f) strategy for calibration-free measurements of gas conditions are presented. In this technique, the nominal wavelength of a modulated tunable diode laser (TDL) is scanned over an absorption transition to measure the corresponding scanned-WMS-nf/1f spectrum. Gas conditions are then inferred from least-squares fitting the simulated scanned-WMS-nf/1f spectrum to the measured scanned-WMS-nf/1f spectrum, in a manner that is analogous to widely used scanned-wavelength direct-absorption techniques. This scanned-WMS-nf/1f technique does not require prior knowledge of the transition linewidth for determination of gas properties. Furthermore, this technique can be used with any higher harmonic (i.e., n>1), modulation depth, and optical depth. Selection of the laser modulation index to maximize both signal strength and sensitivity to spectroscopic parameters (i.e., gas conditions), while mitigating distortion, is described. Last, this technique is demonstrated with two-color measurements in a well-characterized supersonic flow within the Stanford Expansion Tube. In this demonstration, two frequency-multiplexed telecommunication-grade TDLs near 1.4 μm were scanned at 12.5 kHz (i.e., measurement repetition rate of 25 kHz) and modulated at 637.5 and 825 kHz to determine the gas temperature, pressure, H2O mole fraction, velocity, and absorption transition lineshape. Measurements are shown to agree within uncertainty (1%-5%) of expected values.

Numerical investigation on high power mid-infrared supercontinuum fiber lasers pumped at 3 µm

High power mid-infrared (mid-IR) supercontinuum (SC) laser sources in the 3-12 µm region are of great interest for a variety of applications in many fields. Although various mid-IR SC laser sources have been proposed and investigated experimentally and theoretically in the past several years, power scaling of mid-IR SC lasers beyond 3 μm with infrared edges extending beyond 7 μm are still challenges because the wavelengths of most previously used pump sources are below 2 μm. These problems can be solved with the recent development of mode-locked fiber lasers at 3 μm. In this paper, high power mid-IR SC laser sources based on dispersion engineered tellurite and chalcogenide fibers and pumped by ultrafast lasers at 3 µm are proposed and investigated. Our simulation results show that, when a W-type tellurite fiber with a zero dispersion wavelength (ZDW) of 2.7 µm is pumped at 2.78 μm, the power proportion of the SC laser beyond 3 µm can exceed 40% and the attainable SC output power of the proposed solid-cladding tellurite fiber is one order of magnitude higher than that of existing microstructured tellurite fibers. Our calculation also predicts that a very promising super-broadband mid-IR SC fiber laser source covering two atmospheric windows and molecules' "fingerprint" region can be obtained with a microstructured As₂Se₃ chalcogenide fiber pumped at 2.78 μm.

Measurement of nonuniform temperature and concentration distributions by combining line-of-sight tunable diode laser absorption spectroscopy with regularization methods

Regularization methods were combined with line-of-sight tunable diode laser absorption spectroscopy (TDLAS) to measure nonuniform temperature and concentration distributions along the laser path when a priori information of the temperature distribution tendency is available. Relying on measurements of 12 absorption transitions of water vapor from 1300 to 1350 nm, the nonuniform temperature and concentration distributions were retrieved by making the use of nonlinear and linear regularization methods, respectively. To examine the effectiveness of regularization methods, a simulated annealing algorithm for nonlinear regularization was implemented to reconstruct the temperature distribution, while three linear regularization methods, namely truncated singular value decomposition, Tikhonov regularization, and a revised Tikhonov regularization method, were implemented to retrieve the concentration distribution. The results show that regularization methods not only can be used to retrieve temperature and concentration distributions closer to the original but also are less sensitive to measurement noise. When no sufficient optical access is available for TDLAS tomography, the methods proposed in the paper can be used to obtain more details of the combustion field with higher accuracy and robustness, which are expected to play a more important role in combustion diagnosis.

High-power efficient generation of visible and mid-infrared radiation exploiting four-wave-mixing in optical fibers

We report on the generation of 17.6W of visible radiation at 650 nm using four-wave-mixing in an endlessly single-mode silica fiber. The conversion efficiency was as high as ~30%. This high efficiency could be obtained by exploiting the natural absorption of silica for the mid-infrared radiation >2.5 µm. In a separate experiment 1.6 W of mid-IR radiation at 2570 nm were generated simultaneously with 14.4 W at 672 nm. These power levels of picosecond red radiation are among the highest reported so far for a diffraction limited beam quality in this wavelength region.

Simultaneous measurements of multiple parameters at elevated temperature using a frequency-division multiplexing scheme with tunable diode lasers

A multiplexed diode-laser sensor system based on second harmonic detection of wavelength modulation spectroscopy (WMS) is developed for application at elevated temperatures with two near-infrared diode lasers multiplexed using a frequency-division multiplexing scheme. One laser is tuned over a H(2)O line pair near 7079.176 and 7079.855 cm(-1), and another laser is tuned over a pair of CO(2) and CO lines near 6361.250 and 6361.344 cm(-1). Temperature and concentrations of H(2)O, CO(2), and CO could be measured simultaneously by this system. In order to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling, the WMS-1f normalized 2f method is used. Demonstration experiments are conducted in a heated static cell. The precision of temperature and the concentrations for H(2)O, CO(2), and CO are found to be 1.57%, 3.87%, 3.01%, and 3.58%, respectively. These results illustrate the potential of this sensor for applications at high temperatures.

Simultaneous measurements of multiple flow parameters for scramjet characterization using tunable diode-laser sensors

This paper reports the simultaneous measurements of multiple flow parameters in a scramjet facility operating at a nominal Mach number of 2.5 using a sensing system based on tunable diode-laser absorption spectroscopy (TDLAS). The TDLAS system measures velocity, temperature, and water vapor partial pressure at three different locations of the scramjet: the inlet, the combustion region near the flame stabilization cavity, and the exit of the combustor. These measurements enable the determination of the variation of the Mach number and the combustion mode in the scramjet engine, which are critical for evaluating the combustion efficiency and optimizing engine performance. The results obtained in this work clearly demonstrated the applicability of TDLAS sensors in harsh and high-speed environments. The TDLAS system, due to its unique virtues, is expected to play an important role in the development of scramjet engines.

Diode Laser Induced Fluorescence for Gas-Phase Diagnostics

We highlight the capabilities and potential of diode laser induced fluorescence for measurements in gas-phase reacting flows. Many applications of diode lasers in practical sensing are based on absorption spectroscopy. Fluorescence-based diagnostics possess similar advantages in terms of practicality and implementation-cost but additionally are capable of achieving excellent spatial resolution. Diode laser fluorescence instruments have been employed for high-sensitivity trace gas monitoring in applications ranging from plasma physics to atmospheric chemistry. This article begins by describing the UV-visible diode laser technology used to perform fluorescence. The principles of diode laser induced fluorescence are then reviewed and a comparison is made with absorption spectroscopy. Examples are given of concentration measurements of both atomic and molecular trace gases. Recent work on using diode laser induced atomic fluorescence for precision measurements of flame temperature is also reviewed. We conclude by a discussion of future opportunities for diode laser fluorescence spectroscopy drawing attention to interesting potential target species as well as novel application areas, such as monitoring of synthesis processes for nanomaterials.

High-precision diode-laser-based temperature measurement for air refractive index compensation

We present a laser-based system to measure the refractive index of air over a long path length. In optical distance measurements, it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlén equations, where the dominant uncertainty component is in most cases the air temperature. The method developed in this work utilizes direct absorption spectroscopy of oxygen to measure the average temperature of air and of water vapor to measure relative humidity. The method allows measurement of temperature and humidity over the same beam path as in optical distance measurement, providing spatially well-matching data. Indoor and outdoor measurements demonstrate the effectiveness of the method. In particular, we demonstrate an effective compensation of the refractive index of air in an interferometric length measurement at a time-variant and spatially nonhomogeneous temperature over a long time period. Further, we were able to demonstrate 7 mK RMS noise over a 67 m path length using a 120 s sample time. To our knowledge, this is the best temperature precision reported for a spectroscopic temperature measurement.

Methodology for detection of carbon monoxide in hot, humid media by telecommunication distributed feedback laser-based tunable diode laser absorption spectrometry

Detection of carbon monoxide (CO) in combustion gases by tunable diode laser spectrometry is often hampered by spectral interferences from H2O and CO2. A methodology for assessment of CO in hot, humid media using telecommunication distributed feedback lasers is presented. By addressing the R14 line at 6395.4  cm(-1), and by using a dual-species-fitting technique that incorporates the fitting of both a previously measured water background reference spectrum and a 2f-wavelength modulation lineshape function, percent-level concentrations of CO can be detected in media with tens of percent of water (c(H2O)≤40%) at T≤1000 °C with an accuracy of a few percent by the use of a single reference water spectrum for background correction.

Numerical investigation of mid-infrared supercontinuum generation up to 5 μm in single mode fluoride fiber

We numerically investigate mid-infrared supercontinuum generation in single mode fluoride fiber pumped by 1.56 μm picosecond fiber lasers. To get high energy conversion efficiency in mid-infrared region, the ratio of power generated in 2.5 ~5 μm range to the total input power for supercontinuum generation is optimized by varying the pulse width, peak power and fiber length. The long wavelength edge of the supercontinuum spectrum can be extended to 4.8 μm in a 100 cm long fluoride fiber pumped by a 1.56 μm fiber laser with a pulse width of 4 ps and a peak power of 100 kW, and the corresponding ratio of power generated in 2.5 ~5 μm range to the total input power is about 44.6%. The spectral broadening is mainly caused by self-phase modulation, stimulated Raman scattering and four-wave mixing. The simulated results show that high average power supercontinuum light source in 2.5 ~5 μm range could be obtained in fluoride fibers pumped by 1.56 μm picosecond fiber lasers.

An integrated laser Raman optical sensor for fast detection of nitrogen and oxygen in a cryogenic mixture

An integrated fiber optic Raman sensor was designed for real-time, nonintrusive detection of liquid nitrogen (LN(2)) in liquid oxygen (LO(2)) at high pressures and high flow rates. This was intended to monitor the quality of LO(2) in oxidizer feed lines during the ground testing of rocket engines. Various issues related to optical diagnosis of cryogenic fluids (LN(2)/LO(2)) in supercritical environment of rocket engine test facility, such as fluorescence from impurity in optical window of feed line, signal-noise ratio, and fast data acquisition time, etc., are well addressed. The integrated sensor employed a frequency doubled 532-nm continuous wave Nd:YAG laser as an excitation light source. The other optical components included were InPhotonics Raman probes, spectrometers, and photomultiplier tubes (PMTs). The spectrometer was used to collect the Raman spectrum of LN(2) and LO(2). The PMT detection unit was integrated with home-built LABVIEW software for fast monitoring of concentration ratios LN(2) and LO(2). Prior to designing an integrated sensor system, its optical components were also tested with gaseous nitrogen (GN(2)) and oxygen (GO(2)).

Improved approach for ultra-sensitive detection of NO

An improved approach has been developed for ultra-sensitive detection of the concentration of NO using Faraday Modulation spectrometry (FAMOS) combined with the strong electronic transition. By changing the modulating magnetic field attributing to linear absorption and refraction of gas sample, the polarized laser was rotated and absorbed by the complex refraction index of NO. We confirm the relation between the magnitudes of absorption and the optimum modulation magnetic field. Also, the accuracy and the precision of the technique have been evaluated at different pressures. It is shown that the system is capable of detecting NO concentration down to 0.34 ppb·m.

Simultaneous measurements of CO2 and CO using a single distributed-feedback (DFB) diode laser near 1.57 μm at elevated temperatures

A sensor using a single distributed-feedback (DFB) diode laser at 1.57 μm for the simultaneous measurement of CO(2) and CO concentration at elevated temperatures is developed. A proper line pair near 6361.250 and 6361.344 cm(-1) is chosen based on absorption strength, separation of the two lines, and isolation from interference of neighboring transitions of the major combustion gases. The concentrations of CO(2) and CO are inferred from their wavelength modulation spectroscopy (WMS) 1ƒ-normalized absorption-based WMS-2ƒ signal peak heights. The CO(2) and CO concentration measurements are within 3.3% and 5% of the expected values over the full temperature range.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: II. Experiment

A first demonstration of Faraday modulation spectrometry (FAMOS) of nitric oxide (NO) addressing its strong electronic X(2)Π(ν″ = 0) - A(2)Σ(+)(ν(') = 0) band is presented. The instrumentation was constructed around a fully diode-laser-based laser system producing mW powers of ultraviolet light targeting the overlapping Q(22)(21/2) and R(12)Q(21/2) transitions at ∼226.6 nm. The work verifies a new two-transition model of FAMOS addressing the electronic transitions in NO given in an accompanying work. Although the experimental instrumentation could address neither the parameter space of the theory nor the optimum conditions, the line shapes and the pressure dependence could be verified under low-field conditions. NO could be detected down to a partial pressure of 13 µTorr, roughly corresponding to 10 ppb·m for an atmospheric pressure sample, which demonstrates the feasibility of FAMOS for sensitive detection of NO addressing its strong electronic band.

DFB lasers between 760 nm and 16 μm for sensing applications

Recent years have shown the importance of tunable semiconductor lasers in optical sensing. We describe the status quo concerning DFB laser diodes between 760 nm and 3,000 nm as well as new developments aiming for up to 80 nm tuning range in this spectral region. Furthermore we report on QCL between 3 μm and 16 μm and present new developments. An overview of the most interesting applications using such devices is given at the end of this paper.

Technique of laser calibration for wavelength-modulation spectroscopy with application to proton exchange membrane fuel cell measurements

A diode laser sensor was developed for partial pressure and temperature measurements using a single water vapor transition. The Lorentzian half-width and line intensity of the transition were calibrated for conditions relevant to proton exchange membrane (PEM) fuel cell operation. Comparison of measured and simulated harmonics from wavelength-modulation spectroscopy is shown to yield accuracy of +/-2.5% in water vapor partial pressure and +/-3 degrees C in temperature despite the use of a single transition over a narrow range of temperatures. Collisional half-widths in air or hydrogen are measured so that calibrations can be applied to both anode and cathode channels of a PEM fuel cell. An in situ calibration of the nonlinear impact of modulation on laser wavelength is presented and used to improve the accuracy of the numerical simulation of the signal.

Measurements of CO(2) concentration and temperature at high pressures using 1f-normalized wavelength modulation spectroscopy with second harmonic detection near 2.7 microm

Tunable diode lasers (TDL) near 2.7 mum are used to measure high-resolution direct absorption and wavelength modulation with second harmonic (WMS-2f) spectra at high pressures for two CO(2) transitions near 3633.08 and 3645.20 cm(-1), belonging to the nu(1)+ nu(3) vibrational band. Important factors influencing the design of a high-pressure TDL sensor and the variation of WMS-2f line shape with changes in pressure and laser parameters are discussed. Measurements of line strength and line broadening parameters are carried out for the 3645.20 cm(-1) transition in an atmospheric-pressure, high-temperature cell. A room-temperature high-pressure cell is then used to measure the pressure shift for both CO(2) transitions. Deviation of the direct absorption and wavelength modulation spectroscopy (WMS) spectra from the Lorentzian profile is studied in a high-density (9.2 amagats) CO(2)-Ar mixture. The WMS spectra are shown to be negligibly affected by non-Lorentzian effects up to 10 atm and room temperature, in contrast with direct absorption. Measurements of CO(2) concentration and temperature are carried out in nonreactive shock-tube experiments (P approximately 8-12 atm, T~800-1200 K) to validate the accuracy and precision of wavelength-modulation-spectroscopy-based sensing. CO(2) time histories are then measured in heptane ignition experiments and compared with reaction kinetics mechanisms to demonstrate the use of this sensor in high-pressure combustion systems.

Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

Spektroskopischer Einsatz neuer langwelliger (bis 2 μm) Diodenlaser (VCSEL) für schwierige Bedingungen (Spectroscopic Application of Long-Wavelength (< 2 μm) VCSEL Diode Lasers)

Oberflächenemittierende Diodenlaser (engl. VCSEL, Vertical-cavity surface-emitting laser) werden zur raschen direkten In-situ-Molekülspektroskopie eingesetzt. Nach dem Verfahren der Absorptionsspektroskopie mittels durchstimmbarer Diodenlaser wird Sauerstoff bei 760 nm, Ammoniak bei 1540 nm, Methan bei 1680 nm sowie Chlorwasserstoff und Wasser bei 1810 nm detektiert. Druckverbreiterte und hochaufgelöste Spektren werden gezeigt und das Prinzip eines langzeitstabilen Spektrometers vorgestellt. Die Wellenlängenmodulation der VCSEL mit der Temperatur und dem Strom wird untersucht. Während der Temperaturkoeffizient in etwa derselbe ist wie für herkömmliche Diodenlaser im nahen Infrarot (DFB-Laser), lassen sich VCSEL deutlich weiter mit dem Strom durchstimmen. Darüber hinaus können VCSEL thermisch wesentlich schneller moduliert werden als konventionelle Kantenemitter. Repetitionsraten bis 5 MHz werden demonstriert. Die neu eröffneten Anwendungsfelder im Hinblick auf den weiten, modensprungfreien Durchstimmbereich (Messung bei hohem Druck, mehrere Spezies, Temperaturverteilungen) und die rasche Modulierbarkeit (Messung extrem transienter Prozesse) werden diskutiert. Weiter werden spektroskopisch interessante Eigentümlichkeiten der VCSEL (geringer Schwellstrom und Strombedarf als Vorteil für batteriebetriebene mobile Geräte, Austestmöglichkeit auf der Waferebene) beleuchtet. Die langwelligen VCSEL mit λ > 1 μm auf InP-Basis existieren noch nicht lange. Es wird angenommen, dass diese demnächst verstärkten Einzug in die Molekülspektroskopie halten werden und das Einsatzgebiet von auf Diodenlasern basierenden Geräten beträchtlich nach höheren Drücken und schwierigen Bedingungen hin erweitern werden.

Harmonic wavelet analysis of modulated tunable diode laser absorption spectroscopy signals

Wavelet analyses of tunable diode laser absorption spectroscopy signals were performed. The absorption spectroscopy data were obtained by repeatedly scanning the beams from a tunable diode laser operating in the near infrared across absorption lines of gaseous NH(3) contained within a windowed glass tube. The laser was modulated and wavelet analyses of the absorption data were performed. It was observed that harmonic wavelets could simultaneously extract the 1f and 2f harmonics as well as higher-order harmonics from the direct absorption data.

Numerical investigation of hyperspectral tomography for simultaneous temperature and concentration imaging

We investigate the simultaneous tomographic reconstruction of temperature and species concentration using hyperspectral absorption spectroscopy. Previous work on absorption tomography has relied on a small number of wavelengths, resulting in the requirement of a large number of projections and limited measurement capability. Here we develop a tomographic inversion method to exploit the increased spectral information content enabled by recent advancement in laser technologies. The simulation results clearly demonstrate that the use of hyperspectral absorption data significantly reduces the number of projections, enables simultaneous mapping of temperature and species concentration, and provides more stable reconstruction compared with traditional absorption tomographic techniques.

Flexible lock-in detection system based on synchronized computer plug-in boards applied in sensitive gas spectroscopy

We present a flexible and compact, digital, lock-in detection system and its use in high-resolution tunable diode laser spectroscopy. The system involves coherent sampling, and is based on the synchronization of two data acquisition cards running on a single standard computer. A software-controlled arbitrary waveform generator is used for laser modulation, and a four-channel analog/digital board records detector signals. Gas spectroscopy is performed in the wavelength modulation regime. The coherently detected signal is averaged a selected number of times before it is stored or analyzed by software-based, lock-in techniques. Multiple harmonics of the modulation signal (1f, 2f, 3f, 4f, etc.) are available in each single data set. The sensitivity is of the order of 10(-5), being limited by interference fringes in the measurement setup. The capabilities of the system are demonstrated by measurements of molecular oxygen in ambient air, as well as dispersed gas in scattering materials, such as plants and human tissue.

High bandwidth absorption spectroscopy with a dispersed supercontinuum source

An optical gas sensor is presented, making use of a dispersed supercontinuum source, capable of acquiring broad bandwidth spectra at ultrahigh wavelength sweep and repetition rates. Wavelength sweeps from 1100 nm to 1700 nm can be performed in 800 ns at a spectral resolution of 40 pm. This is comparable to line-widths of molecular spectra at atmospheric pressure. Quantitative measurements are presented of CH(4) employing 80 nm wide sweeps over the P- Q- and R-branches of the 2nu(3) transition near 1665 nm, at rates exceeding 100 kHz. The effective acquisition rate is determined by the amount of averaging required, and the effect of this averaging on observed precision is investigated.

Diode laser differential absorption spectrometry for measurements of some parameters of condensed media

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.

Determination of Carbon Monoxide Concentration and Total Pressure in Gas Cavities in the Silica Glass Body of Light Bulbs by FT-IR Spectrometry

Fourier transform infrared (FT-IR) spectroscopy has been adapted to control the quality of light bulbs made from silica glass. Such light bulbs contain a molybdenum accessory which, if contaminated with carbon, during the melting procedure of bulb fabrication, can cause the production of carbon monoxide. This CO can be trapped in small gas cavities in the silica glass body of the bulb. A method has been developed for the detection of CO and the total pressure within these gas cavities by traditional FT-IR spectrometry using a spectral resolution of 0.5 cm(-1). The concentration of CO was determined by using a classical least-squares (CLS) method, and the accuracy of concentration determination is reported for the case with sample and reference spectra recorded at different pressures. The total pressure in the cavities was established by two different methods: either by CLS fitting of reference spectra to sample spectra or fitting a Voigt line shape function to the spectral lines within the CO fundamental stretching band. In the latter method, the width of the lines was determined and pressure-broadening coefficients are given and compared with high-resolution data from the literature. According to the measurements, 0.55-0.80 atm total pressure and 0.8-4.0% (v/v) CO was determined in the gas cavities. This method can also be applied to determine the total pressure in similar enclosed spaces in which an appropriate indicator gas component exists.

Extension of wavelength-modulation spectroscopy to large modulation depth for diode laser absorption measurements in high-pressure gases

Tunable diode laser absorption measurements at high pressures by use of wavelength-modulation spectroscopy (WMS) require large modulation depths for optimum detection of molecular absorption spectra blended by collisional broadening or dense spacing of the rovibrational transitions. Diode lasers have a large and nonlinear intensity modulation when the wavelength is modulated over a large range by injection-current tuning. In addition to this intensity modulation, other laser performance parameters are measured, including the phase shift between the frequency modulation and the intensity modulation. Following published theory, these parameters are incorporated into an improved model of the WMS signal. The influence of these nonideal laser effects is investigated by means of wavelength-scanned WMS measurements as a function of bath gas pressure on rovibrational transitions of water vapor near 1388 nm. Lock-in detection of the magnitude of the 2f signal is performed to remove the dependence on detection phase. We find good agreement between measurements and the improved model developed for the 2f component of the WMS signal. The effects of the nonideal performance parameters of commercial diode lasers are especially important away from the line center of discrete spectra, and these contributions become more pronounced for 2f signals with the large modulation depths needed for WMS at elevated pressures.

10 kHz detection of CO2 at 4.5 microm by using tunable diode-laser-based difference-frequency generation

A compact, high-speed tunable, diode-laser-based mid-infrared (MIR) laser source has been developed for absorption spectroscopy of CO2 at rates up to 10 kHz. Radiation at 4.5 microm with a mode-hop-free tuning range of 80 GHz is generated by difference-frequency mixing the 860 nm output of a distributed-feedback diode laser with the 1064 nm output of a diode-pumped Nd:YAG laser in a periodically poled lithium niobate crystal. MIR absorption spectroscopy of CO2 with a detection limit of 44 ppm m at 10 kHz is demonstrated in a C2H4-air laminar diffusion flame and in the exhaust of a liquid-fueled model gas-turbine combustor.

Toward in-cylinder absorption tomography in a production engine

Design requirements for an 8000 frame/s dual-wavelength ratiometric chemical species tomography system, intended for hydrocarbon vapor imaging in one cylinder of a standard automobile engine, are examined. The design process is guided by spectroscopic measurements on iso-octane and by comprehensive results from laboratory phantoms and research engines, including results on temporal resolution performance. Novel image reconstruction techniques, necessary for this application, are presented. Recent progress toward implementation, including details of the optical access arrangement employed and signal-to-noise issues, is described. We present first cross-cylinder IR absorption measurements from a reduced channel-count (nontomographic) system and discuss the prospects for imaging.

Near-infrared diode laser absorption diagnostic for temperature and water vapor in a scramjet combustor

Tunable diode laser absorption measurements of gas temperature and water concentration were made at the exit of a model scramjet combustor fueled on JP-7. Multiplexed, fiber-coupled, near-infrared distributed feedback lasers were used to probe three water vapor absorption features in the 1.34-1.47 microm spectral region (2v1 and vl + v3 overtone bands). Ratio thermometry was performed using direct-absorption wavelength scans of isolated features at a 4-kHz repetition rate, as well as 2f wavelength modulation scans at a 2-kHz scan rate. Large signal-to-noise ratios demonstrate the ability of the optimally engineered optical hardware to reject beam steering and vibration noise. Successful measurements were made at full combustion conditions for a variety of fuel/air equivalence ratios and at eight vertical positions in the duct to investigate spatial uniformity. The use of three water vapor absorption features allowed for preliminary estimates of temperature distributions along the line of sight. The improved signal quality afforded by 2f measurements, in the case of weak absorption, demonstrates the utility of a scanned wavelength modulation strategy in such situations.

Diode laser absorption spectroscopy of water vapor in a scramjet combustor

A sensor based on tunable diode laser absorption spectroscopy was constructed for time-resolved temperature and water vapor concentration measurements in a scramjet combustor. The sensor probed two absorption lines near 1390 nm with two time-multiplexed lasers used to measure temperature and water vapor concentration at up to 20 kHz. A demonstration experiment was performed in the supersonic, expanding exhaust region of the combustor, showing the measurement to be repeatable, able to resolve temporal trends during tunnel operation, and sensitive to changes in combustor operating conditions.

Detection of high-temperature water vapor at 940 nm with vertical-cavity surface-emitting lasers

A vertical-cavity surface-emitting laser was used to study the absorption of water vapor in the 940 nm region. Measurements for several absorption lines within the 2 v1 + V3 vibrational band were performed. Line strengths at room temperature and in a heated absorption cell over the temperature range of 420-970 K were obtained. The line strength values were in good agreement with simulations based on the values of the HITRAN 2004 database. The measurements also showed that water vapor transitions near 940 nm are suitable for sensitive temperature determination.

Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules

A cavity ringdown spectrometer, based on a continuous-wave swept-frequency laser, enables efficient, rapid recording of wide-ranging absorption spectra as characteristic spectral signatures of airborne molecules. The rapidly swept laser frequency resonates with the longitudinal modes of the ringdown cavity, effectively sampling the absorption spectrum of an intracavity gas at intervals defined by the cavity's free spectral range and generating a full absorption spectrum within a single rapid sweep of the widely tunable laser frequency. We report a new analog detection scheme that registers a single data point for each buildup and ringdown decay event without logging details of the full signal waveform; this minimizes demand on digitizer speed and memory depth, reducing the time scale of data processing. This results in a compact, robust, easy-to-use instrument that offers fresh prospects for spectroscopic sensing of trace species in the atmosphere.

Wide-bandwidth mode-hop-free tuning of extended-cavity GaN diode lasers

We present a new approach for extended-cavity diode-laser tuning to achieve wide mode-hop-free tuning ranges. By using a multiple piezoactuated grating mount, the cavity length and grating angle in the laser can be adjusted independently, allowing mode-hop-free tuning without the need for a mechanically optimized pivot-point mount. Furthermore, synchronized diode injection-current tuning allows diode lasers without antireflection coatings to be employed. In combination these two techniques make the construction of a cheap, efficient, and easily optimized extended-cavity diode laser possible. A theoretical analysis is presented for optimal control of piezoactuator displacements and injection current to achieve the widest possible mode-hop-free tuning ranges, and a comparison is made with measurements. The scheme is demonstrated for blue and violet GaN lasers operating at approximately 450 nm and approximately 410 nm, for which continuous tuning ranges exceeding 90 GHz have been achieved. Examples of applications of these lasers are also given.

Multipass open-path Fourier-transform infrared measurements for nonintrusive monitoring of gas turbine exhaust composition

The detection limits for NO and NO2 in turbine exhausts by nonintrusive monitoring have to be improved. Multipass mode Fourier-transform infrared (FTIR) absorption spectrometry and use of a White mirror system were found from a sensitivity study with spectra simulations in the mid-infrared to be essential for the retrieval of NO2 abundances. A new White mirror system with a parallel infrared beam was developed and tested successfully with a commercial FTIR spectrometer in different turbine test beds. The minimum detection limits for a typical turbine plume of 50 cm in diameter are approximately 6 parts per million (ppm) for NO and 9 ppm for NO2 (as well 100 ppm for CO2 and 4 ppm for CO).

Combustion exhaust measurements of nitric oxide with an ultraviolet diode-laser-based absorption sensor

A diode-laser-based sensor has been developed for ultraviolet absorption measurements of the nitric oxide (NO) molecule. The sensor is based on the sum-frequency mixing (SFM) of the output of a tunable, 395-nm external-cavity diode laser and a 532-nm diode-pumped, frequency-doubled Nd:YAG laser in a beta-barium borate crystal. The SFM process generates 325 +/- 75 nW of ultraviolet radiation at 226.8 nm, corresponding to the (v' = 0, v" = 0) band of the A2Sigma+-chi2II electronic transition of NO. Results from initial laboratory experiments in a gas cell are briefly discussed, followed by results from field demonstrations of the sensor for measurements in the exhaust streams of a gas turbine engine and a well-stirred reactor. It is demonstrated that the sensor is capable of fully resolving the absorption spectrum and accurately measuring the NO concentration in actual combustion environments. Absorption is clearly visible in the gas turbine exhaust even for the lowest concentrations of 9 parts per million (ppm) for idle conditions and for a path length of 0.51 m. The sensitivity of the current system is estimated at 0.23%, which corresponds to a detection limit of 0.8 ppm in 1 m for 1000 K gas. The estimated uncertainty in the absolute concentrations that we obtained using the sensor is 10%.

Large-modulation-depth 2f spectroscopy with diode lasers for rapid temperature and species measurements in gases with blended and broadened spectra

A method that uses tunable diode lasers is developed for rapid temperature and concentration measurements of gases with highly broadened and congested spectra. Wavelength modulation absorption spectroscopy with 2f detection is utilized, because this derivative method offers benefits in dealing with blended spectral features. The 2f signal depends critically on the modulation depth of the laser alpha, which is increased to values above those typically achieved when wavelength modulation spectroscopy with diode lasers is performed. The 2f method with large modulation depths is validated by using near-IR diode lasers to probe pressure-broadened water-vapor features in the 1.4-microm region over a range of temperatures from 296 to 800 K and at pressures as high as 20 atm. Modulation depths as high as alpha = 0.8 cm(-1) are attained at modulation frequencies of 50 kHz and measurement bandwidths of 15 kHz. Comparisons of experimental results with 2f simulations, based on the HITRAN spectral database, provide confirmation of the capability of this method for rapid measurements of gas temperature and species concentration.

Tunable single-frequency diode laser at wavelength lambda = 1.645 microm for methane concentration measurements

This paper deals with the development of a novel single-frequency tunable diode laser with fiber-optic output for gas-analysis applications. The approach we propose is a convenient, simple and cheap solution for spectroscopy of single absorption lines of any gases having absorption bands in the optical fiber transparency window (0.7 microm/1.7 microm). The presence of fiber-optic output is an additional advantage for remote sensing applications. The laser operation is demonstrated as applied to R7 line of 2 nu(3) methane absorption band at lambda = 1.645 microm. The mode-hop-free tuning range of 35 GHz (1.2 cm(-1)) has been achieved.

A compact NIR fiber-optic diode laser spectrometer for CO and CO(2): analysis of observed 2f wavelength modulation spectroscopy line shapes

A compact fiber-optic diode laser spectrometer for the measurement of CO and CO(2) gas concentrations in the near infrared around 1580 nm is described. By use of a balanced receiver to suppress diode laser intensity noise a sensitivity of 6.4 x 10(-7) at 1 Hz system bandwidth was achieved. At a reduced pressure of 80 hPa this equals to a detection limit of 5.1 ppm CO and 9.1 ppm CO(2) with 1m absorption path length. The observed line shapes of the 2f wavelength modulation spectroscopy (WMS) scheme are analyzed theoretically and experimentally. Accurate measurements of magnitude and phase of the diode laser frequency and intensity modulation responses were found critically for modeling the observed line shapes. In situ measurements of gas dissociation processes inside of a medium-power carbon dioxide laser are presented as an application example.