Diode Laser Absorption Measurements Research Articles

Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N2

Spatially resolved tunable diode-laser absorption measurements of the absolute densities of He-I (23S1) metastables in a micro atmospheric pressure plasma jet operated in He/N2 and driven by ‘peaks’- and ‘valleys’-type tailored voltage waveforms are presented. The measurements are performed at different nitrogen admixture concentrations and peak-to-peak voltages with waveforms that consist of up to four consecutive harmonics of the fundamental frequency of 13.56 MHz. Comparisons of the measured metastable densities with those obtained from particle-in-cell/Monte Carlo collision simulations show a good quantitative agreement. The density of helium metastables is found to be significantly enhanced by increasing the number of consecutive driving harmonics. Their generation can be further optimized by tuning the peak-to-peak voltage amplitude and the concentration of the reactive gas admixture. These findings are understood based on detailed fundamental insights into the spatio-temporal electron dynamics gained from the simulations, which show that voltage waveform tailoring allows to control the electron energy distribution function to optimize the metastable generation. A high degree of correlation between the metastable creation rate and the electron impact excitation rate from the helium ground state into the He-I ((3s)3S1) level is observed for some conditions which may facilitate an estimation of the metastable densities based on phase resolved optical emission spectroscopy measurements of the 706.5 nm He-I line originating from the above level and metastable density values at proper reference conditions.

Heterogeneously integrated III–V-on-silicon 2.3x μm distributed feedback lasers based on a type-II active region

We report on 2.3x μm wavelength InP-based type-II distributed feedback (DFB) lasers heterogeneously integrated on a silicon photonics integrated circuit. In the devices, a III–V epitaxial layer stack with a “W”-shaped InGaAs/GaAsSb multi-quantum-well active region is adhesively bonded to the first-order silicon DFB gratings. Single mode laser emission coupled to a single mode silicon waveguide with a side mode suppression ratio of 40 dB is obtained. In continuous-wave regime, the 2.32 μm laser operates close to room temperature (above 15 °C) and emits more than 1 mW output power with a threshold current density of 1.8 kA/cm2 at 5 °C. A tunable diode laser absorption measurement of CO is demonstrated using this source.

Characterization of CO2 flow in a hypersonic impulse facility using DLAS

This work documents diode laser absorption measurements of CO2 flow in the free stream of the Longshot hypersonic impulse facility at Mach numbers ranging from 10 to 12. The diode laser sensor was designed to measure absorption of the P12 (30013) \(\leftarrow\) (00001) transition near 1.6 \(\upmu\)m, which yields relatively weak direct absorption levels (3.5 % per meter at peak Longshot free-stream conditions). Despite this weak absorption, measurements yielded valuable flow property information during the first 20 ms of facility runs. Simultaneous measurements of static temperature, pressure, and velocity were acquired in the inviscid core flow region using a laser wavelength scanning frequency of 600 Hz. The free-stream values obtained from DLAS measurements were then compared to Longshot probe-derived values determined from settling chamber and probe measurements. This comparison enabled an assessment of the traditional method of flow characterization in the facility, which indicated negligible influence from possible vibrational freezing of reservoir gases.

Data assimilation optimization for the evaluation of inverse mixing and convection flows

A data assimilation approach is developed for evaluation of fluid-thermal systems wherein a complete specification of the boundary conditions is not known a priori and experimental diagnostics are restricted to a limited region of the flowfield. The methodology is applied to the configuration of a heated jet injected into a laminar boundary layer where the jet temperature and velocity are unknowns. The closed loop method selects the initial locations for the experimental measurements of mean temperature within the flowfield using diode laser absorbance. Diode laser absorption measurements near 761 nm of molecular oxygen are recorded to characterize the time varying temperature of the two dimensional jet. The Response Surface Models built using two-dimensional unsteady Navier–Stokes simulations are used to predict the jet exit temperature and velocity, and select the second set of locations for the experimental measurements. The two sets of experimental data are used to generate the final prediction for the jet exit temperature and velocity. The jet exit velocity is correctly predicted to within the experimental uncertainty; however, the jet exit temperature is over-estimated by 9% to 23%.

Near-Surface Carbon-Dioxide Tunable Diode Laser Absorption Spectroscopy Concentration Measurements in Hypervelocity Flow

Measurements of carbon-dioxide concentration are made in the LENS-XX expansion tunnel at the Calspan—University at Buffalo Research Center to investigate the effects of surface catalysis in hypersonic flows. A tunable diode laser absorption spectroscopy setup probes the P36e absorption line of the combination band at . Numerical simulations are computed with data-parallel line relaxation using the specified reaction efficiency surface catalysis model. Absorption measurements adjacent to the surface of an aluminum cylinder correlate well with simulations indicating low catalytic efficiency at the surface. The velocity, density, freestream temperature, specific enthalpy, and stagnation point pitot pressure of the run are , , 1328.4 K, , and 31.6 kPa, respectively. Simulation of the absorption spectrum is performed using a nonhomogeneous line-by-line code using the high-resolution transmission molecular absorption database for spectroscopic parameters. Input data for the code are obtained from computational fluid dynamics simulations of the cylinder and facility. The specified reaction efficiency for the mechanism is determined inversely using detailed computational fluid dynamics and comparisons to experimental measurements. The best agreement was found for to 0.0005, indicating that the coupled line-by-line code and computational fluid dynamics approach is useful for assessing near-surface composition using tunable diode laser absorption measurements of hypersonic flows.

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.

Calibration-free sensor for pressure and H2O concentration in headspace of sterile vial using tunable diode laser absorption spectroscopy

Tunable diode laser absorption measurements of pressure and H2O concentration in the headspace of vials using a distributed-feedback (DFB) diode laser near 1.4 μm are reported. A H2O line located near 7161.41 cm(-1) is selected based on its strong absorption strength and isolation from interference of neighboring transitions. Direct absorption spectra of H2O are obtained for the measurement path as well as the reference path by scanning the laser wavelength. The pressure and H2O vapor concentration in the headspace of a vial are inferred from a differential absorption signal, which is the difference between the measured and the referenced absorbance spectra. This sensor is calibration-free and no purge gas is needed. The demonstrated capability would enable measurements of pressure and H2O concentration in the headspace of vials within 2.21% and 2.86%, respectively. A precision of 1.02 Torr and 390 ppm is found for the pressure and H2O concentration, respectively. A set of measurements for commercial freeze-dried products are also performed to illustrate the usefulness of this sensor.

TDL absorption sensors for gas temperature and concentrations in a high-pressure entrained-flow coal gasifier

We report the first tunable diode laser (TDL) absorption measurements for gas temperature and species concentrations in a pilot-scale, high-pressure, entrained-flow, oxygen-blown, slagging coal gasifier with pressures up to 18atm and temperatures to 1800K. Three locations in the gasifier flow were investigated: (1) the reactor core, (2) the exit of the reactor where a water spray quenches the reaction products (pre-quench), and (3) in the gasifier product stream before particulate clean-up (post-quench). Gas temperature was measured by three pairs of H2O transitions with distinct temperature dependence at all three locations using five distributed feedback (DFB) lasers near 1.4μm. CO, CO2 and H2O concentrations were measured at the post-quench location using DFB lasers near 2.3μm, 2.0μm and 1.4μm, respectively. Fixed-wavelength, 1f-normalized, wavelength-modulation-spectroscopy with second-harmonic detection (WMS-2f) was used for all the measurements. Usable signal-to-noise ratio (SNR)>10 was achieved with one second averaging even at gasifier conditions with large particulate loading where the non-absorption transmission loss was >99.99% from light scattering and the thermal emission was intense. The work demonstrates the capability of TDL sensors to monitor transient changes in gas temperature providing a potentially valuable control sensor for gasifier performance. In addition, measurements of CO, CO2 and H2O in the gasifier product stream showed good promise for monitoring the syngas heating value. The potential advantages of using higher WMS harmonics under high-pressure conditions were also investigated.

Modified adaptive algebraic tomographic reconstruction of gas distribution from incomplete projection by a two-wavelength absorption scheme

A modified adaptive algebraic reconstruction technique (MAART) with an auto-adjustment relaxation parameter and smoothness regularization is developed to reveal the tomographic reconstruction of H2O distribution in combustion from incomplete projections. Determinations of relaxation parameter and regularization factor are discussed with regard to the consideration of improvement in reconstruction and reduction of computational burden. A two-wavelength scheme from tunable diode laser absorption measurement, 7444.36 and 7185.59 cm ?1, is used to simplify the nonlinear solution problem for obtaining the tomographic distributions of concentration and temperature simultaneously. Results of calculations demonstrate that MAART can perform the reconstruction results more efficiently with little complex modification and much lower iterations as compared with the traditional algebraic reconstruction technique (ART) or simultaneous iterative reconstruction technique (SIRT) methods. The stability of the algorithm is studied by reconstruction from projections with random noise at different levels to demonstrate the dependence of reconstruction results on precise line-of-sight measurements.

Diode laser absorption measurements at the H α-transition in laser induced plasmas on different targets

The diode laser atomic absorption spectroscopy (DLAAS) technique has been utilized to assess the degree of optical opacity of plasma at the wavelength of the H α-line. The plasma is produced at atmospheric conditions by focusing a 6 ns Nd:YAG laser pulse at 1.064 μm on different solid target materials including aluminum, iron and titanium as major elements as well as flat pieces of plastic and wood characterized by a high content of hydrogen. The optical depth was investigated as a function of delay times ranging from 0 to 5 μs, and at laser fluences ranging from 7 to 19 J/cm 2, all at a fixed gate time of 1 μs. The results show that the plasma associated with metallic targets is almost optically thin at the H α-line over all fluences and at delay times ≥ 1 μs, but rather thick for hydrogen-rich targets (plastic and wood) over all delay times and fluences.

Diode laser absorption measurement and analysis of HCN in atmospheric-pressure, fuel-rich premixed methane/air flames

Measurements of HCN in flat, fuel-rich premixed methane/air flames at atmospheric pressure are reported. Quartz-microprobe sampling followed by wavelength modulation absorption spectroscopy with second harmonic detection was used to obtain an overall measurement uncertainty of better than 20% for mole fractions HCN on the order of 10 ppm. The equivalence ratio, φ, was varied between 1.3 and 1.5, while the flame temperature was varied independently by changing the mass flux through the burner surface at constant equivalence ratio. Under the conditions of the experiments, the peak mole fractions vary little, in the range of 10–15 ppm. Increasing the flame temperature by increasing the mass flux had little influence on the peak mole fraction, but accelerated HCN burnout substantially. At high equivalence ratio and low flame temperature, HCN burnout is very slow: at φ = 1.5 , ∼ 10 ppm HCN is still present 7 mm above the burner surface. Substantial quantitative disagreement is observed between the experimental profiles and those obtained from calculations using GRI-Mech 3.0, with the calculations generally overpredicting the results significantly. Changing the rates of key formation and consumption reactions for HCN can improve the agreement, but only by making unreasonable changes in these rates. Inclusion of reactions describing NCN formation and consumption in the calculations improves the agreement with the measurements considerably.

Foreign gas broadening and shift of the strongly “forbidden” lead line at 1278.9 nm

The collisional broadening and shift rate coefficients of the “forbidden“ 6p 2 3P 0 → 6p 2 3P 1 transition in lead were determined by diode laser absorption measurements performed simultaneously in two resistively heated hot-pipes. One hot-pipe contained Pb vapor and noble gas (Ar or He) at low pressure, while the other was filled with Pb and noble gas at variable pressure. The measurements were performed at temperatures of 1220 K and 1290 K, i.e., lead number densities of 4.8 × 10 15 cm − 3 and 1.2 × 10 16 cm − 3 . The broadening rates were obtained by fitting the experimental collisionally broadened absorption line shapes to theoretical Voigt profiles. The shift rates were determined by measuring the difference between the peak absorption positions in the spectra measured simultaneously in the heat pipe filled with noble gas at reference pressure and the one with noble gas at variable pressure. The following data for the broadening and shift rate coefficients due to collisions with Ar and He were obtained: γ B Ar = (3.4 ± 0.1) × 10 − 10 cm 3 s − 1 , γ B He = (3.8 ± 0.1) × 10 − 10 cm 3 s − 1 , γ S Ar = (− 7.3 ± 0.8) × 10 − 11 cm 3 s − 1 , γ S He = (− 6.5 ± 0.7) × 10 − 11 cm 3 s − 1 .

Tunable diode laser absorption measurement of oxygen A-band line strengths

Line strength measurements and pressure-broadening coefficients for the oxygen A-band (b1Σg+\(\leftarrow\) X3Σg-) at 13122 cm-1 using tunable diode laser absorption spectroscopy are reported. Diode lasers are scanned over individual lines and the absorption at different pressures is recorded; from these data the line strength for each transition is obtained. Average values for each transition are compared to those previously published, and in particular a comparison with the HITRAN database is presented. Line strengths reported here are 5–10% lower than the HITRAN values.

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.

Studies of site selective hydrogen atom abstractions by Cl atoms from isobutane and propane by laser flash photolysis/IR diode laser spectroscopy

The kinetics of chlorine atom abstractions from normal and selectively deuterated propane and isobutane have been measured at room temperature and 195 K using a laser flash photolysis system, and following the course of the reaction via IR diode laser absorption measurements of HCl product. In conjunction with the kinetic measurements, a comparison of the HCl signal heights from pairs of measurements on normal and selectively deuterated systems has allowed the determination of the branching fractions of the reactions at the primary, secondary (propane) and tertiary (isobutane) positions. The kinetic data (all in units of cm(3) molecule(-1) s(-1)) for the reaction of Cl atoms with propane ((1.22 +/- 0.02) x10(-10), 195 K; (1.22 +/- 0.03) x10(-10) 298 K) and isobutane ((1.52 +/- 0.02) x10(-10), 195 K; (1.25 +/- 0.04) x10(-10), 298 K) are generally in good agreement with literature data. No data are available for comparison with our measurements for the reactions of Cl atoms with CH(3)CD(2)CH(3) ((1.02 +/- 0.03) x10(-10), 195 K; (1.09 +/- 0.02) x10(-10), 298 K) or (CH(3))(3)CD ((1.32 +/- 0.03) x10(-10), 195 K; (1.12 +/- 0.04) x10(-10), 298 K). Rate coefficients at 195 K for the reactions of Cl atoms with ethane ((5.04 +/- 0.08) x10(-11) and n-butane ((2.19 +/- 0.03) x10(-10)) were also measured. The branching fractions for abstraction at the primary position increased with temperature for both propane ((40 +/- 3)% at 195 K to (48 +/- 3)% at 298 K) and isobutane ((49 +/- 4)% at 195 K to (62 +/- 5)% at 298 K). The direct measurements from this study are in good agreement with most calculations based on structure activity relationships.

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 measurement of uranium isotope ratios in solid samples using laser ablation

The isotopes 235U and 238U were simultaneously measured by absorption in a laser-induced plasma ignited by a Nd:YAG laser. Two separate diode lasers were used which probed almost the same plasma volume. The diode lasers were tuned to the absorption lines 682.6736 nm for 235U and 682.0768 nm for 238U. The 235U / 238U isotope ratio was measured on a pulse-to-pulse basis evaluating the transient absorption peaks as analytical signals. Three uranium oxide pellets were used for calibration with natural (0.714 wt.%) and depleted 235U concentration (0.407 and 0.204 wt.%). The measured 235U/ 238U mass ratio in an uranium containing mineral sample was (0.686±0.119)×10 −2 and found to be in good agreement with the mass ratio of naturally occurring isotopes (0.719×10 −2). The observed accuracy (<5%) and precision (∼17% R.S.D.) are encouraging if it is taken into account that the mineral sample was probed directly without any preceding preparation step. Spatially and temporally resolved emission spectroscopic measurements were performed to investigate the kinetics of the laser plasma and to optimize the operating conditions of the laser ablation with regard to sensitivity and selectivity of the 235U isotope detection. Optimal conditions were obtained at a distance of ∼0.3 cm from the sample surface, an argon pressure of ∼30 mbar and for 7 mJ pulse energy of the Nd:YAG laser. The limit of detection of the 235U isotope, evaluated on the basis of the 3σ criteria, was estimated to be 47 μg g −1 and improved by a factor of two compared with the sequential measurement with a single laser diode used in a previous work.

Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two separate intercomparisons of aircraft‐based carbon monoxide measurement instrumentation were conducted. On September 2, CO measurements were simultaneously made aboard the National Oceanic and Atmospheric Administration (NOAA) WP‐3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser absorption spectroscopy (TDLAS). On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluorescence method, on the NOAA WP‐3 and the U.K. Meteorological Office C‐130 Hercules. The results indicate that both of the VUV fluorescence instruments and the TDLAS system are capable of measuring ambient CO accurately and precisely with no apparent interferences in 5 s. The accuracy of the measurements, based upon three independent calibration systems, is indicated by the agreement to within 11% with systematic offsets of less than 1 ppbv. In addition, one of the groups participated in the Measurement of Air Pollution From Satellite (MAPS) intercomparison [Novelli et al., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is indicated by the variability of the ratio of simultaneous measurements from the separate instruments. This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C‐130 and the WP‐3 instruments, respectively, and indicates a precision of approximately 3.6% for the TDLAS instrument. The excellent agreement of the instruments in both intercomparisons demonstrates that significant interferences in the measurements are absent in air masses that ranged from 7 km in the midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercomparison of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularly sensitive to the details of its implementation. These intercomparisons help to establish the reliability of ambient CO measurements by the VUV fluorescence technique.

Resonance ionization spectroscopy in a thermal lithium beam by means of diode lasers

We measured two-step photoion current spectra by using a semiconductor laser and a Q-switched Nd:YAG laser with fourth-harmonic generation in an atomic lithium beam. The number of ions measured was compared with an estimate from the diode laser absorption measurement that uses a set of precisely solved rate equations. We explain the saturation effects on absorption by using transit time relaxation and detailed calculations of the ionization levels.

Diode-laser absorption measurements of hydrazine and monomethylhydrazine

Absorption measurements of pure hydrazine, N 2H 4, and monomethylhydrazine, CH 3N 2H 3 (MMH), were recorded in an optical absorption cell (3 passes, 50 cm/pass) at low pressure (0–35 Torr) using a tunable external-cavity diode laser (ECDL), operating in the range from 6350 to 6650 cm -1 (1.49 to 1.58 μm). Peak absorption cross-section measurements of hydrazine and MMH, as determined from fixed wave-length absorption measurements, were found to be α e( ν)=0.163±0.008 cm -1atm -1 at 6495 cm -1 ( λ=1539.6 nm) and α e( ν)=0.152±0.003 cm -1atm -1 at 6560 cm -1 ( λ=1524.4 nm), respectively.