Coherent anti-Stokes Raman Scattering Thermometry Research Articles

High-energy, low-jitter, narrowband ps probe laser for kHz-rate fs/ps coherent anti-Stokes Raman scattering.

Hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) thermometry often utilizes ps probe pulses derived from pulse shaping or spectrally filtering the primary laser source or by synchronization with a low repetition rate external laser. This results in limited energy, spectral resolution, and/or repetition rate of the ps probe. In this work, a master-oscillator power-amplifier (MOPA) laser was synchronized to the oscillator of a Ti:sapphire regenerative amplifier to achieve high-energy (600 µJ), narrowband (58 ps) probe pulses at kHz repetition rates. Temporal filtering allows the pulse characteristics to be adjusted for each application. At 25 Torr, relevant to high-speed flows, the kHz-rate MOPA system generated signal-to-noise ratios 3× higher in nitrogen and had improved precision relative to a 10 ps probe derived from spectral filtering and the power-amplifier. The MOPA system also enabled single-shot ro-vibrational hybrid fs/ps CARS thermometry in 650 K heated air.

Background suppression for CARS thermometry in highly luminous flames using an electro-optical shutter.

An electro-optical shutter (EOS), comprising a Pockels cell located between crossed-axis polarizers, is integrated into a nanosecond coherent anti-Stokes Raman scattering (CARS) system. The use of the EOS enables thermometry measurements in high-luminosity flames through significant reduction of the background resulting from broadband flame emission. A temporal gating ≤100 ns along with an extinction ratio >10,000:1 are achieved using the EOS. Integration of the EOS enables the use of an unintensified CCD camera for signal detection, improving upon the signal-to-noise ratio achievable with inherently noisy microchannel plate intensification processes previously employed for short temporal gating. The reduction in background luminescence afforded by the EOS in these measurements allows the camera sensor to capture CARS spectra at a broad range of signal intensities and corresponding temperatures, without saturation of the sensor, thus enhancing the dynamic range of these measurements.

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.

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.

Improved opposition-based self-adaptive differential evolution algorithm for vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering thermometry.

We propose an improved opposition-based self-adaptive differential evolution (IOSaDE) algorithm for multi-parameter optimization in vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) thermometry. This new algorithm self-adaptively combines the advantages of three mutation schemes and introduces two opposite population stages to avoid premature convergence. The probability of choosing each mutation scheme will be updated based on its previous performance after the first learning period. The IOSaDE method is compared with nine other traditional differential evolution (DE) methods in simulated spectra with different simulation parameters and experimental spectra at different probe time delays. In simulated spectra, both the average and standard deviation values of the final residuals from 20 consecutive trials using IOSaDE are more than two orders of magnitude smaller than those using other methods. Meanwhile, the fitting temperatures in simulated spectra using IOSaDE are all consistent with the target temperatures. In experimental spectra, the standard deviations of the fitting temperatures from 20 consecutive trials decrease more than four times by using IOSaDE, and the errors of the fitting temperatures also decrease more than 18%. The performance of the IOSaDE algorithm shows the ability to achieve accurate and stable temperature measurement in CARS thermometry and indicates the potential in applications where multiple parameters need to be considered.

1‐kHz hybrid femtosecond/picosecond coherent anti‐Stokes Raman scattering thermometry of turbulent combustion in a representative aeronautical test rig

AbstractThe study of complex reactive flows met in aeronautical engines is an experimental challenge that requires the development of noninvasive and precise measurement techniques. Coherent anti‐Stokes Raman scattering (CARS) is a standard spectroscopic technique used for temperature measurements, which provides unmatched reliability and accuracy at high repetition rate thanks to its recent adaptation to femtosecond temporal regime. This paper reports the first demonstration of hybrid fs/ps‐CARS thermometry in a representative aeronautical combustor (ONERA MICADO test bench, Reynolds number ≈105). Single‐shot N2 CARS spectra have been recorded at 1 kHz, allowing temperature follow up, statistical and frequency (0.1–500 Hz) analysis of CH4/air flame at 0.34‐MPa total pressure. The achieved results demonstrate the ability of the technique to be applied in representative engines conditions and its maturity for semi‐industrial applications.

Post-detonation fireball thermometry via femtosecond-picosecond coherent anti-Stokes Raman Scattering (CARS)

Accurate knowledge of post-detonation fireball temperatures is important for understanding device performance and for validation of numerical models. Such measurements are difficult to make even under controlled laboratory conditions. In this work temperature measurements were performed in the fireball of a commercial detonator (RP-80, Teledyne RISI). The explosion and fragments were contained in a plastic enclosure with glass windows for optical access. A hybrid femtosecond-picosecond (fs-ps) rotational coherent anti-Stokes Raman scattering (CARS) instrument was used to perform gas-phase thermometry along a one-dimensional measurement volume in a single laser shot. The 13-mm-thick windows on the explosive-containment housing introduced significant nonlinear chirp on the fs lasers pulses, which reduced the Raman excitation bandwidth and did not allow for efficient excitation of high-J Raman transitions populated at flame temperatures. To overcome this, distinct pump and Stokes pulses were used in conjunction with spectral focusing, achieved by varying the relative timing between the pump and Stokes pulses to preferentially excite Raman transitions relevant to flame thermometry. Light scattering from particulate matter and solid fragments was a significant challenge and was mitigated using a new polarization scheme to isolate the CARS signal. Fireball temperatures were measured 35–40 mm above the detonator, 12–25 mm radially outward from the detonator centerline, and at 18 and 28 µs after initiation. At these locations and times, significant mixing between the detonation products and ambient air had occurred thus increasing the nitrogen-based CARS thermometry signal. Initial measurements show a distribution of fireball temperatures in the range 300–2000 K with higher temperatures occurring 28 µs after detonation.

CO2 chirped-probe-pulse femtosecond CARS thermometry

The application of CO2 chirped-probe-pulse (CPP) fs coherent anti-Stokes Raman scattering (CARS) for flame thermometry is discussed. A phenomenological model for CO2/O2 CPP fs CARS was developed and the simulations showed the potential for CO2 temperature measurements free of interferences from O2 and with no contribution from the nonresonant background by using a long time delay for the chirped probe pulse. The accuracy and precision for CO2 CPP fs CARS thermometry is compared with N2 CPP fs CARS thermometry for temperatures between 295 K and 1420 K. The measurement accuracy for the CO2 measurements is comparable with that for the N2 measurements, while the precision for the CO2 temperature measurements is significantly better at lower temperatures. We also demonstrate that accurate flame thermometry using a single set of laser parameters can be achieved using the delayed CPP approach.

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.

Rotational CARS thermometry and concentration measurements in ethane‐nitrogen mixtures using Fourier analysis

AbstractRotational coherent anti‐Stokes Raman scattering (CARS) is an established diagnostic technique for thermometry and species concentration measurements of di‐atomics and tri‐atomics in combustion processes; however, studies on hydrocarbon fuel molecules have been scarce. In this work, we have investigated the diagnostic potential for rotational CARS on ethane with emphasis on concentration measurements in mixtures with nitrogen. Rotational CARS spectra have periodic line structures, and because ethane and nitrogen spectra partially overlap, a novel Fourier analysis approach was developed and applied to facilitate evaluation of relative concentrations and temperatures. Instead of the standard procedure of spectral fitting using libraries of theoretically calculated spectra, the fitting was based on an experimental database of temperatures and relative ethane/nitrogen concentrations. The method was successfully demonstrated in an ethane diffusion flame where ethane, initially at room temperature, is heated downstream as well as mixed with increasing amounts of nitrogen. The evaluated temperatures and concentrations are in good agreement with computational fluid dynamic model simulations.

Methane-air flame thermometry using planar lase-induced fluorescence (PLIF)

Temperature measurements were performed in a central cross-section of a partially premixed atmospheric methane-air flame of a laboratory burner using two-line planar laser-induced fluorescence (PLIF) of OH radicals and coherent anti-Stokes Raman scattering (CARS) by H2 molecules. The results of a comparative analysis of 2D PLIF and single-point CARS thermometry of the flame are presented.

Development and applications of nonlinear optical spectroscopy: 15th ECONOS/35th ECW meeting in Gothenburg (Sweden)

The joint conference of the European Conference on Nonlinear Optical Spectroscopy (ECONOS) and European CARS Workshop (ECW) covers all theoretical and experimental aspects of nonlinear optical spectroscopy and microscopy. Topics include Coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy, and other forms of coherent Raman scattering, other approaches to the detection of nonlinear optical responses in time and frequency domain, and investigations of the physical and chemical processes defining the spectral signatures in different nonlinear media. Also, emerging analytical techniques and their applications in biomedical research, material science, and engineering are discussed. The 15th ECONOS conference and 35th CARS workshop were held in Gothenburg, Sweden, on 24–27 April 2016, chaired by David Sedarsky and Stina Hemdal (Chalmers University of Technology). Altogether, 50 participants shared their research in oral sessions and poster sessions and participated in lively discussions and social activities. After an invited lecture and a welcome reception in the historic Chalmerska Huset on Sunday, the regular scientific program began on Monday morning at the same place. Five invited talks were given during the course of the conference, followed by contributed oral presentations, and two poster sessions. This special section of the Journal of Raman Spectroscopy constitutes a selection of three papers presented at ECONOS/ECW 2016. Due to the broad scope of the conference, the papers cover a broad range of methods and their applications to a variety of systems. The special section starts with a contribution by Martinez et al. 1 who applied a pump-probe Raman-Raman technique to obtain high-resolution Raman spectra of vibrationally excited states in a series of acetylene isotopologues. They utilized the naturally occurring process of vibration-to-vibration collisional relaxation to populate excited states of interest. Some of these states were observed and studied for the very first time. Meißner et al. 2 carried out a feasibility study on the use of a Raman-active crystal in picosecond-CARS of nitrogen. The Raman crystal facilitated narrowband stimulated Raman scattering (SRS) to produce a coherent beam for three-color CARS thermometry. The concept was demonstrated by temperature measurements at ambient conditions and in a fuel-rich sooting ethylene/air flame. The third paper represents a theoretical study of hybrid fs/ps-CARS by Marrocco 3. Hybrid fs/ps-CARS is a time-domain and frequency-resolved method, where interaction of a Raman medium with simultaneous femtosecond (fs) pump and Stokes pulses is interrogated by means of a delayed picosecond (ps) probe. The resulting anti-Stokes signal is usually studied by means of numerical methods built on the conventional time-dependent model of Raman coherence. The paper presents analytical closed-form solutions to the time model and validates their reliability against published experimental results. The results show that the method allows a fast signal reconstruction and provides adequate spectral accuracy. This ECONOS/ECW special section highlights the continuous progress in the area of nonlinear optical spectroscopy and its applications across the science and engineering disciplines. The enthusiasm and creativity of the scientific community as well as the rapid development of novel and improved experimental equipment will ensure an exciting future of nonlinear spectroscopy research. We would like to thank all the delegates, presenters of talks and posters, helpers in the background, and sponsors, which made the 2016 ECONOS/ECW meeting in Sweden an enjoyable and memorable event. Special thanks to the Editor-in-Chief of the Journal of Raman Spectroscopy, Larry Nafie, for making this special section possible, and of course, to all the authors and reviewers of the papers for their invaluable contribution.

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.

Experimental cross-correlation nitrogen Q-branch CARS thermometry in a spark ignition engine

A purely experimental technique was employed to derive temperatures from nitrogen Q-branch Coherent Anti-Stokes Raman Scattering (CARS) spectra, obtained in a high pressure, high temperature environment (spark ignition Otto engine). This was in order to obviate any errors arising from deficiencies in the spectral scaling laws which are commonly used to represent nitrogen Q-branch CARS spectra at high pressure. The spectra obtained in the engine were compared with spectra obtained in a calibrated high pressure, high temperature cell, using direct cross-correlation in place of the minimisation of sums of squares of residuals.The technique is demonstrated through the measurement of air temperature as a function of crankshaft angle inside the cylinder of a motored single-cylinder Ricardo E6 research engine, followed by the measurement of fuel–air mixture temperatures obtained during the compression stroke in a knocking Ricardo E6 engine. A standard CARS programme (SANDIA's CARSFIT) was employed to calibrate the altered non-resonant background contribution to the CARS spectra that was caused by the alteration to the mole fraction of nitrogen in the unburned fuel–air mixture. The compression temperature profiles were extrapolated in order to predict the auto-ignition temperatures.

Single‐shot thermometry using fiber‐based picosecond coherent anti‐stokes Raman scattering (CARS) spectroscopy

Coherent anti‐Stokes Raman scattering (CARS)‐spectroscopy‐based thermometry techniques developed for laboratory flames face a stiff challenge when implemented in practical combustion environments such as engine test facilities. In addition to limited optical access, the harsh environments associated with these test facilities (i.e. uncontrolled humidity, vibration, and large thermal gradients) often restrict the operation of sensitive laser systems. To circumvent this problem, we have developed a fiber‐based, picosecond (ps) CARS system employing long‐length (up to 6 m), multimode silica fibers that permits the laser system to be isolated from the high heat and vibration of the test hardware. Proof‐of‐principle, single‐laser‐shot temperature measurements using a 3‐m‐long fiber system are demonstrated in atmospheric pressure, near‐adiabatic, laboratory H2–air flames. Limitations that are imposed by long‐length multimode‐fiber beam delivery for CARS thermometry are identified as nonlinear effects during beam propagation along the fiber, depolarization of the laser beam, and degradation of the beam quality at the output of the fiber. These effects are investigated in detail, and potential improvements are suggested. The current study shows promise for implementation of fiber‐based ps‐CARS spectroscopy in harsh environments such as those encountered in combustors and afterburners in practical gas‐turbine‐engine test facilities. Copyright © 2013 John Wiley & Sons, Ltd.

On the sensitivity of rotational O2 CARS thermometry to the Herman–Wallis factor

Contributions of vibration–rotation interaction to line intensities of pure rotational coherent anti‐Stokes Raman scattering (CARS) of oxygen are here considered in the perspective of what was remarked by us recently for nitrogen. The interaction is conventionally evaluated by means of the so‐called Herman–Wallis (HW) factor, the need of which is conceptually clear but the theoretical expression is debatable, as shown in the literature of this field. In this work, the various solutions for the anisotropic HW factor of pure rotational Raman transitions are compared in light of the implications for the corresponding CARS spectral line intensities. The results show that neglecting the use of HW factors in rotational N2 and O2 CARS thermometry leads to systematic overestimation of temperatures; on the order of 1.5% in the temperature range 300–2100 K in comparison with the use of the most plausible HW factors. The results also suggest that, in general, oxygen is more sensitive than nitrogen to the HW correction, and in particular, different S‐branch HW expressions for oxygen are responsible for thermometric uncertainties between 0.5% and 1% at flame temperatures. Copyright © 2012 John Wiley & Sons, Ltd.

Validation of a rotational coherent anti‐Stokes Raman scattering model for N2O at temperatures from 295 K to 796 K

Pure rotational coherent anti‐Stokes Raman scattering (CARS) spectra of N2O was recorded in a series of temperature calibrated cell measurements in the region 295–796 K. A theoretical rotational CARS code for N2O was implemented and temperature analysis was performed by fitting the modeled theoretical spectra to the experimental spectra. Excellent agreement between the experimental and modeled spectra was obtained and the thermometric accuracy for the measurements was evaluated to be better than 1%. Also, rotational N2 CARS spectra were recorded at the same measurement conditions and the temperature analysis performed on these spectra resulted in the same accuracy. The peak signal strength was found to be ~5 times stronger for N2O than for N2 at ambient temperature and pressure. The temperature precision was evaluated to a relative standard deviation of 2.0%–2.7% in the studied temperature range, about half of the values for N2, which is attributed to the larger number of spectral lines for N2O. Rotational N2O CARS thermometry show great potential, because of a high Raman cross‐section and large number of populated rotational states at any temperature. Copyright © 2012 John Wiley & Sons, Ltd.

Vibration–rotation interaction in time‐resolved coherent anti‐Stokes Raman scattering for gas‐phase thermometry

Different approaches to the vibration–rotation (VR) interaction in time‐resolved intensities of coherent anti‐Stokes Raman scattering (CARS) of light homonuclear diatomics (namely H2, N2 and O2) are here compared in view of the diagnostic refinement of the technique that has an acknowledged thermometric potential for studies in combustion science. From this perspective, VR interaction is treated in agreement with past works about conventional or frequency‐resolved CARS, and although the current theoretical context refers to optical nonlinearities excited by ultrashort laser pulses, the results confirm the strong sensitivity of H2 CARS to the VR interaction quantified by means of the Herman–Wallis (HW) factor. Indeed, depending on the model of the interaction, time‐resolved H2 signals can be subject to indeterminations on the order of 10% leading to relevant thermometric inaccuracies. Heavier diatomics are less sensitive, but the choice of an appropriate HW factor can contribute to improve the confidence in CARS thermometry with ultrashort laser pulses. Copyright © 2012 John Wiley & Sons, Ltd.

CARS diagnostics of the burning of H2 — O2 and CH4 — O2 mixtures at high temperatures and pressures

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to determine the parameters of gaseous combustion products of hydrogen and hydrocarbon fuels with oxygen at high temperatures and pressures. The methodical aspects of CARS thermometry, which are related to the optimal choice of molecules (diagnostic references) and specific features of their spectra, dependent on temperature and pressure, are analysed. Burning is modelled under the conditions similar to those of real spacecraft propulsion systems using a specially designed laboratory combustion chamber, operating in the pulse-periodic regime at high temperatures (to 3500 K) and pressures (to 20 MPa) of combustion products.