Vibrational Coherent anti-Stokes Raman Scattering Research Articles

Dual-Pump Vibrational Coherent Anti-Stokes Raman Scattering System Developed for Simultaneous Temperature and Relative Nitrogen–Water Vapor Concentration Measurements

Simultaneous gas phase temperature and water vapor concentration measurement are important to understand reacting flows such as combustion or gas reforming processes. Here, coherent anti-Stokes Raman scattering (CARS) offers the possibility for non-intrusive measurements with a high temporal and spatial resolution. Therefore, this work demonstrates the simultaneous measurement of temperature and relative water vapor–nitrogen concentrations by using dual-pump vibrational coherent anti-Stokes Raman scattering (DPVCARS). A calibration procedure is developed for a temperature range of 473 K to 673 K and a water vapor concentration of 24% to 46% at ambient pressure. This setup is tested with 500 CARS single pulse spectra taken in a gas cell at a known temperature and concentration. Based on these results, information about precision and accuracy can be delivered.

Temperature measurement utilizing the vibrational CARS signal from the ground level X2Σg+ of N2+ in high-temperature flames

We analyze the generation of the vibrational coherent anti-Stokes Raman scattering (CARS) signal from the ground level X2Σg+ of N2+ and demonstrate the feasibility for gas-phase thermometry utilizing the vibrational CARS signals of N2+ in high-temperature flames. The vibrational CARS signal of N2+ is obtained using the filamentation-based and two-beam hybrid femtosecond/picosecond CARS system and is employed to extract the fitting temperatures in high-temperature flames. The accuracy and precision of the fitting temperatures from the 1000 consecutive single-shot vibrational CARS spectra of N2+ at a target temperature of 1706 K are 1.23% and 5.76%, respectively, which are similar to the fitting results of N2 in previous filamentation-based CARS thermometry and could also demonstrate that the measured temperature in the filamentation is indeed not affected by the ionization. We further introduce the CARS signal of N2+ in multiple species measurement to obtain more details about the CARS process during filamentation. This work would provide an optional target molecule for CARS thermometry and help researchers further understand the molecular dynamics of N2+ ions during the filamentation.

Time-resolved CO2, CO, and N2 vibrational population measurements in Ns pulse discharge plasmas

Time-resolved CO2 and N2 vibrational populations and translational-rotational temperature are measured in a CO2–N2 plasma sustained by a ns pulse discharge burst in plane-to-plane geometry. Time-resolved, absolute number density of CO generated in the plasma is also inferred from the experimental data. CO2 and CO vibrational populations are measured by mid-IR, tunable quantum cascade laser absorption spectroscopy, and N2 vibrational populations are measured by the ns broadband vibrational CARS. Transient excitation of N2 and CO2 asymmetric stretch vibrational energy modes is detected during the discharge burst. The time-resolved rate of CO generation does not correlate with N2 or CO2(ν 3) vibrational temperatures, indicating that CO2 dissociation via the vibrational excitation is insignificant at the present conditions. The rate of CO generation decreases gradually during the discharge burst. The estimated specific energy cost of the CO product is close to that of N atoms in pure nitrogen, measured previously at similar operating conditions. Comparison of the experimental data with the kinetic modeling analysis indicates that CO2 dissociation in collisions with electronically excited N2 molecules is the dominant channel of CO generation at the present conditions, although the inferred CO yield in these processes is significantly lower than 1. The effect of vibrational energy transfer between N2 and CO2 on the plasma chemical processes is insignificant. The kinetic model underpredicts a rapid reduction of the N2 and CO2(ν 3) vibrational temperatures during the later half of the discharge burst and in the afterglow. V–T relaxation of N2 by N and O atoms generated in the ns pulse discharge plasma does not affect the vibrational relaxation rate in a significant way. However, rapid V–T relaxation of CO2 by O atoms has a significant effect on the relaxation rate. The difference between the experimental data and the modeling predictions may be due to the unknown scaling of the CO2–O V–T rates with the vibrational quantum number.

Filamentation-based, two-beam, 1-kHz, and single-shot N2 vibrational CARS thermometry in high-temperature flames

We achieve two-beam, 1-kHz, and single-shot N2 vibrational coherent anti-Stokes Raman scattering (CARS) thermometry using hybrid femtosecond/picosecond CARS in high-temperature flames. Ultrabroadband pump/Stokes pulses are generated employing the filamentation of high-power femtosecond pulses and can excite the Raman shift up to ∼3000 cm−1. This configuration can avoid the need of a long hollow-core fiber and an additional supercontinuum compressor to obtain ultrabroadband pulses. The quasi-common-path second harmonic bandwidth compression system is utilized to generate narrow-band and sideband-free picosecond pulses, which are employed as the probe pulses in the CARS process. The accuracy and the precision of the fitting results from 1000 consecutive single-shot spectra at a target temperature of 1706 K are 0.66% and 4.47%, respectively. The results demonstrate that the simplified two-beam system has the potential for dynamic and accurate temperature measurement in practical applications.

Simultaneous Temperature and Pressure Measurements in Compressible Flow Using Nanosecond O2 Coherent Anti-Stokes Raman Spectroscopy.

Simultaneous pure-rotational coherent anti-Stokes Raman spectroscopy (PRCARS) and vibrational O2 CARS spectroscopy (VCARS) were performed at elevated pressure and lowered temperature conditions in non-reacting compressible flow. We applied dual-pump CARS in a three-laser, three-color configuration to simultaneously acquire the PRCARS and VCARS spectra of O2. PRCARS spectra provide excellent sensitivity to temperature at relatively low temperatures. Pressure was extracted using the differential response of collisional effects in the PRCARS and the VCARS spectra. We used an under-expanded jet outside a choked converging nozzle as the compressible flow-field. We numerically analyze the pressure sensitivity of the combined CARS technique. Finally, we compare the collisional narrowing lineshape models of rotational diffusion narrowing and modified-exponential-gap model, for fitting the experimental spectrum.

N2 vibrational excitation in atmospheric pressure ns pulse and RF plasma jets

Time-resolved N2 vibrational temperature and translational–rotational temperature in quasi-two-dimensional atmospheric pressure plasma jets sustained by ns pulse and RF discharges in nitrogen/noble gas mixtures are measured by the broadband vibrational Coherent Anti-Stokes Raman Scattering (CARS) . The results indicate a much stronger vibrational excitation in the RF plasma jet, due to the lower reduced electric field and higher discharge power. In a ns pulse discharge in N2/He, N2 vibrational temperature is significantly lower compared to that in N2/Ar, due to the more rapid vibration–translation (V–T) relaxation of nitrogen by helium atoms. In the RF plasma jets in N2/Ne and N2/Ar, the vibrational excitation increases considerably as the nitrogen fraction in the mixture is reduced. The experimental data in the RF plasma jet in N2/Ar jet are compared with the kinetic modeling predictions. The results indicate that nitrogen vibrational excitation in N2/Ar plasma jets with a small N2 fraction in the mixture (several percent) is controlled primarily by electron impact, anharmonic vibration–vibration (V–V) pumping, and V–T relaxation by N atoms. In comparison, V–V energy transfer from the vibrationally excited molecules in the first excited electronic state, N2(A3Σu +, v), which are generated primarily by the energy transfer from the metastable Ar atoms, has a minor effect on the vibrational populations of the ground electronic state, N2(X1Σg +, v). Although the discharge energy fraction going to electronic excitation is significant, the predicted quasi-steady-state N2(A3Σu +) number density, controlled by the energy pooling and quenching by N atoms, remains relatively low. Because of this, the net rate of N2(X1Σg +) vibrational excitation by the V–V energy transfer from N2(A3Σu +) is much lower compared to that by the direct electron impact. The results show that atmospheric pressure RF plasma jets can be used as sources of highly vibrationally excited N2 molecules and N atoms.

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: I. Measurement procedure and results

The non-equilibrium ro-vibrational distribution functions of molecules in a plasma can heavily influence the discharge operation and the plasma-chemistry. A convenient method for measuring the distribution function is coherent anti-Stokes Raman scattering (CARS). CARS spectra are measured in a ns-pulsed plasma between two parallel, 1 mm spaced molybdenum electrodes in nitrogen at 200 mbar with pulse durations of 200 ns/250 ns and a repetition rate of 1 kHz. The CARS spectra are analyzed by a fitting routine to extract information about the vibrational excitation of the nitrogen molecules in the plasma. It is found that during the discharge the vibrational distribution for can be described by a vibrational two-temperature distribution function. Additionally, the electric field is measured by the electric field induced second harmonic generation method during the discharge pulse. It is found to be constant in time after the initial ionization wave with values close to 81 Td for the investigated conditions. During the afterglow between two discharge pulses a more general fitting approach is used to obtain the population differences of two neighboring vibrational states. This allows to capture the more complex vibrational dynamics in that time period. The measurement results are discussed in more detail and compared to simple plasma models in a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305205).

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: II. Analysis

The understanding of the ro-vibrational dynamics in molecular (near)-atmospheric pressure plasmas is essential to investigate the influence of vibrational excited molecules on the discharge properties. In a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), results of ro-vibrational coherent anti-Stokes Raman scattering (CARS) measurements for a nanosecond pulsed plasma jet consisting of two conducting molybdenum electrodes with a gap of 1 mm in nitrogen at 200 mbar are presented. Here, those results are discussed and compared to theoretical predictions based on rate coefficients for the relevant processes found in the literature. It is found, that during the discharge the measured vibrational excitation agrees well with predictions obtained from the rates for resonant electron collisions calculated by Laporta et al (2014 Plasma Sources Sci. Technol. 23 065002). The predictions are based on the electric field during the discharge, measured by electric field induced second harmonic generation Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), Lepikhin et al (2020 J. Phys. D: Appl. Phys. 54 055201) and the electron density, which is deduced from the field and mobility data calculated with Bolsig+ Hagelaar and Pitchford (2005 Plasma Sources Sci. Technol. 14 722–33). In the afterglow a simple kinetic simulation for the vibrational subsystem of nitrogen is performed and it is found, that the populations of vibrational excited states develop according to vibrational-vibrational transfer on timescales of a few microseconds, while the development on timescales of some hundred microseconds is determined by the losses at the walls. No significant influence of electronically excited states on the populations of the vibrational states visible in the CARS measurements () was observed.

Dynamic and sensitive hybrid fs/ps vibrational CARS thermometry using a quasi-common-path second-harmonic bandwidth-compressed probe

We demonstrate a 1-kHz, single-shot, dynamic, and sensitive flame temperature measurement using hybrid femtosecond/picosecond vibrational coherent anti-Stokes Raman scattering. This benefits from a 7-cm−1, 240-μJ, and sideband-free picosecond pulse out of a broadband 35-fs pulse through a quasi-common-path second harmonic bandwidth compressor system. Measurements around optimal time delay exhibit superior performance with a temperature inaccuracy of less than 1% and a precision of ∼2% at 1790 K. These results demonstrate the advantages and potential for the accurate and fast temperature measurement in practical or harsh environments.

Investigation on wall and gas temperatures inside a swirled oxy-fuel combustion chamber using thermographic phosphors, O2 rotational and vibrational CARS

Wall and gas phase temperatures inside a swirled oxy-fuel combustion chamber are important to characterize the combustion process. Wall temperatures were measured by thermographic phosphors and discussed in combination with gas phase temperatures. For gas phase temperatures an O2 vibrational coherent anti-Stokes Raman scattering (CARS) approach was compared to a N2/O2 rotational CARS (RCARS) system. The vibrational CARS (VCARS) setup was favorable due to higher signal strength at high temperatures. With this system gas phase temperature profiles inside a swirled oxy-fuel combustion chamber were measured and discussed for different operation conditions. The location of intermittent reaction zones could be determined. In order to provide a measurement tool for gas-assisted pulverized solid fuel flames the developed O2-VCARS system was successfully tested in such a harsh environment. Possible error sources related to particles within the probe volume are discussed.

CARS thermometry in laminar sooting ethylene-air co-flow diffusion flames with nitrogen dilution

Temperature measurements were performed in nonpremixed, sooting jet flames using coherent anti-Stokes Raman scattering (CARS). The jet fluid was ethylene diluted with different concentrations of nitrogen, and the co-flow fluid was dry air. The flames were stabilized on a Yale burner, also known as Smooke/Long burner, and the operating conditions were chosen to match previous experiments on the burner by the Yale group and other research groups. The selected flames have also been identified as target flames by the International Sooting Flame Workshop. CARS measurements of temperature provide insight into the structure of these sooting flames and serve as benchmark data for comparison with computations. A dual-pump vibrational CARS system was used here in order to avoid soot interferences. We were able to obtain high quality CARS spectra in all regions of the flame including the highly sooting regions and near the exit of the fuel nozzle, where the nonresonant modulation of the nitrogen CARS spectrum is strong due to the high ethylene concentrations. The measurements are compared with CFD results in literature and discussed in context of measured soot volume fraction, also, from literature.

Interference-free hybrid fs/ps vibrational CARS thermometry in high-pressure flames.

Interference-free hybrid femtosecond/picosecond vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen is reported for temperature measurements of 1300-2300K in high-pressure, laminar H2-air and CH4-air diffusion flames up to 10bar. Following coherent Raman excitation by 100fs duration pump and Stokes pulses, a time-asymmetric probe pulse is used for the detection of spectrally resolved N2 CARS signals at probe delays as early as ∼200-300 fs. This allows for full rejection of nonresonant contributions while being independent of collisions for single-shot precision of ±2% at elevated pressures. The effects of collisions at longer probe-pulse delays are also investigated to determine the feasibility of varying the detection timing from 200fs to 100ps.

Dual-pump vibrational CARS measurements of temperature and species concentrations in turbulent premixed flames with CO2 addition

A dual-pump vibrational coherent anti-Stokes Raman scattering (DPVCARS) system was applied for simultaneous temperature and species concentration measurements in jet flames on a newly developed piloted axisymmetric reactor assisted turbulent (PARAT) premixed flame burner. Turbulent lean premixed methane/air round jet flames with varying CO2 dilution were stabilized using an annular flow of hydrogen at the flame base. Carbon dioxide was injected into the fuel stream to simulate effects of dry exhaust gas recirculation (EGR). The flames were operated at a nearly constant Reynolds number, Lewis number, and adiabatic flame temperature to minimize thermal and transport effects. Velocity boundary conditions at the burner exit were measured using particle image velocimetry. Simultaneous temperature and major species concentrations (O2 and CO2) were measured along the axial direction at the burner centerline and along the radial direction at representative axial locations above the burner. Progress variables calculated based on the temperatures were used to study the mean flame brush thickness and the turbulent burning velocity. Probability density functions (PDFs) of temperature at various locations are presented and discussed. The DPVCARS measurements provide an experimental database including temperature and turbulent burning velocity for combustion model evaluation and validation and demonstrated a strong potential for flame structure investigation with future improvement on spatial resolution and signal to noise ratio. The merits and limitations of vibrational CARS for diagnostics in turbulent premixed combustion are discussed. The results show that flames with CO2 addition depart from the universal mean flame brush structure, while their RMS fluctuation values at an identical mean temperature collapse with the undiluted flame. The CO2 addition increases the combustion intensity with a decrease in turbulent burning velocity when minimizing the thermal and transport effects.

Comparison of Raman‐active crystals as a narrowband probe light source for picosecond three‐color vibrational CARS thermometry

The feasibility of N2‐based vibrational three‐color coherent anti‐Stokes Raman scattering (CARS) thermometry employing narrowband stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman‐active crystal to provide the wavelength‐shifted probe radiation has been investigated. This shift of the probe beam wavelength is aimed to enable temperature measurements in sooting flames by using the established technique of N2 vibrational CARS spectroscopy in the range free from the interference of the detected radiation with the absorption/emission bands of the flame intermediate radicals C2. The use of solid‐state Raman wavelength shifters may be an inexpensive solution providing reliable temperature measurements. The comparative study of the employment of two high SRS gain Raman‐active crystal materials [namely, Ba(NO3)2 and KGd(WO4)2] pumped by a 100‐ps frequency‐doubled Nd:YAG laser pulse has been performed. To test and compare the operation of each of the crystals in a three‐color CARS experiment, we have performed temperature measurements at ambient conditions and in a fuel‐rich sooting ethylene/air flame. Despite the comparable conversion efficiencies into the first‐order Stokes SRS radiation provided by both crystals, the achievable pulse energies were four to five times larger for the KGd(WO4)2 crystal, and the vibrational CARS spectra at flame temperatures had a significantly better signal‐to‐noise ratio. Copyright © 2017 John Wiley & Sons, Ltd.

Three-color vibrational CARS thermometry of fuel-rich ethylene/air flames using a potassium gadolinium tungstate Raman-active crystal as a source of narrowband probe radiation.

Three-color broadband vibrational coherent anti-Stokes Raman scattering (CARS) temperature measurements were carried out in laminar fuel-rich sooting ethylene/air flames. Stimulated Raman scattering (SRS) of a picosecond pump laser pulse in a Raman-active potassium gadolinium tungstate [KGd(WO4)2] crystal was employed as a source of narrowband probe radiation. In the three-color CARS experiment, this wavelength-shifted radiation enables N2-based vibrational CARS temperature measurements in sooting flames free of the signal interference with the absorption/emission bands of the flame intermediate radicals C2. Spatial temperature profiles for different fuel-rich atmospheric pressure ethylene/air flames are presented in comparison with the results of two-color broadband vibrational and dual-broadband pure rotational CARS temperature measurements. The comparison shows the suitability of the three-color CARS measurement technique employing the KGd(WO4)2 crystal for accurate, C2 interference-free, temperature measurements in sooting flames.

Nonresonant background suppression in wide-field Coherent anti-Stokes Raman scattering microscopy with transport of intensity equation based phase imaging

Coherent anti-Stokes Raman scattering (CARS) microscopy is a valuable tool for label-free imaging of biological samples, since it enables providing contrast via vibrational resonances of a specific chemical bond. However, in a conventional CARS image the Raman resonant anti-Stokes radiation is often superimposed by a nonresonant contribution arising from the electronic part of the polarization. The situation becomes worse if a sample is composed of a significant amount of water, where a strong nonresonant background over the whole image is obtained.To date, various approaches including Epi, polarization sensitive, time-resolved, and CARS phase imaging have been implemented to suppress the undesirable nonresonant background in CARS microscopy. Notably, optical heterodyne based phase imaging schemes are of particular interest due to their intrinsic ability to retrieve Im(χ(3)), which is proportional to the Raman resonant signal. Nevertheless, all the reported phase imaging methods that require an independent reference wave lead to an increase in the setup complexity, thus making the measurement sensitive to external perturbations. In order to simplify the setup, single-beam scheme has also been utilized for vibrational CARS imaging by using wave-front sensors to acquire the phase of the complex anti-Stokes amplitude. However, this method demands highly accurate wave-front sensors.In this paper we present a reference-less CARS phase imaging technique to suppress nonresonant CARS background based on transport of intensity equation (TIE). Resonant CARS radiation ECARSR can be obtained when the frequency difference between the pump and Stokes beams is tuned to match a molecular vibration frequency (Raman resonant mode). In contrast, the nonresonant background ECARSNR can be obtained when the frequency difference between the pump and Stokes beams does not match a molecular vibration frequency (Raman resonant mode). Considering the fact that there is a phase shift of π/2 between the resonant and non-resonant CARS field, the phase imaging of both resonant and nonresonant CARS field can provide a background-free image. In implementation, three intensity images of the CARS field under resonant mode are recorded at three neighboring planes by moving the CCD camera along the axial direction. In the meantime, three images of the CARS field under non-resonant mode are also recorded. Considering the fact that the TIE links the intensity distributions in three neighboring planes (through which a beam transverses) with the phase distribution of the field, the phase images of the CARS field under both resonant and nonresonant modes are reconstructed from the recorded intensity images. The phase difference φχ between the resonant CARS field and the non-resonant CARS field is calculated. Eventually, the CARS background is efficiently suppressed by using the relation ICARSbf≅ICARSR·sin2φχ.Compared with conventional CARS background suppression techniques, the proposed method is robust against environmental disturbance, since it does not require an additional reference beam. Furthermore, the proposed method is easy to incorporate in a conventional CARS configuration. Therefore, the proposed method has the potential to become a versatile technique to image deep tissue with low background signal.

High temperature O2 vibrational CARS thermometry applied to a turbulent oxy‐fuel combustion process

An O2 vibrational coherent anti‐Stokes Raman scattering system for the temperature analysis of industrially relevant staged oxy‐fuel jet burners with a thermal load of 400 kW is presented. The equivalence ratio for both investigated burners was set to ϕ = 0.9 using a mixture of pure oxygen and natural gas H (high calorific value). Temperature profiles were achieved in the mixing region of the turbulent diffusion type burners at a selected downstream position. Temperature distributions and scatter plots show local temperature variations between 447 and 3085 K and demonstrate the turbulent behavior of the combustion process. Copyright © 2016 John Wiley & Sons, Ltd.

Combined spontaneous Stokes and coherent anti-Stokes Raman scattering spectroscopy

The simultaneous determination of multiple parameters is the key in the characterization of processes and materials that change with time. In combustion environments, the combined measurement of temperature and chemical composition is particularly desirable. In the present work, possible approaches for the simultaneous application of spontaneous Raman scattering (RS) and coherent anti-Stokes Raman scattering (CARS) spectroscopy are proposed and analyzed. While RS provides concentration information of all major species, vibrational CARS is a highly accurate thermometry tool at flame conditions. Five experimentally feasible CARS–RS schemes are identified and discussed with respect to signal intensity, measurement volume, and experimental complexity. From this analysis, one scheme was found to be the best option. It utilizes a broadband dye laser centered at 852 nm as a pump and the fundamental 1064-nm radiation of the Nd:YAG as Stokes laser. The third harmonic is used as CARS probe and RS laser. The experimentally most elegant scheme replaces the third harmonic in the above scheme by the second harmonic hence involving the smallest number of optical components in the setup.

Vibrational CARS thermometry and one-dimensional simulations in laminar H2/air counter-flow diffusion flames

Broadband N2 vibrational coherent anti-Stokes Raman scattering (VCARS) thermometry was performed along the nozzle centerline of counter-flow diffusion flames. Fuel mixtures consisted of varying percentages of H2 in N2 and the oxidizer was air. We investigated the influence of varying the percentage of H2 in the fuel stream on the temperature profiles along the nozzle centerline. Measurements were performed, with varying global strain rates, both near and far from flame extinction. One-dimensional simulations, performed using Combustion Simulation Lab (COSILAB) software, show that the temperature profiles are relatively insensitive to the nozzle exit centerline velocity; implying that the 1-D simulations provide an excellent approximation of the real temperature profiles along the nozzle centerline. Various chemical mechanisms were employed within the COSILAB framework for comparison against VCARS temperature measurements. Simulations were also performed to evaluate the influence of thermal diffusion and nitrogen chemistry on nozzle centerline temperature profiles.

Vibrational CARS thermometry and one-dimensional numerical simulations in CH4/H2/air partially-premixed flames

Temperature measurements were obtained along the nozzle centerline of CH4/H2/air partially premixed flames, in a counter-flow burner, using N2 vibrational coherent anti-Stokes Raman scattering (VCARS). Measurements were performed for flames with no H2 and varying percentages of CH4 in the fuel stream and for flames with nearly constant CH4 content and varying H2 content in the fuel stream. VCARS measurements were compared against 1-D numerical simulations using different chemical mechanisms within the frame work of COSILAB software. All the mechanisms compared well against experimental data except for flames at low equivalence ratio where the San Diego mechanism performed the best. Interestingly, the peak flame temperature remained nearly constant with increasing percentage of H2 in the fuel stream. The presence or absence of thermal diffusion made little difference in the comparison between numerical simulations and experiments. The influence of radiation on nozzle centerline temperature profiles was also investigated.