Coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to provide gas-phase quantitative scalar information (e.g. temperature, density, and species concentrations) for more than 5 decades. This technique is renowned for its ability to realize non-intrusive in-situ measurements in harsh environments with excellent spatial and temporal resolution and has become an important tool in multiple energy and combustion science applications, where high-fidelity data are needed. CARS is a non-linear optical process, where the signal originates from the coupling of multiple laser fields to the internal energy states of the probed molecules. This interaction results in excellent chemical specificity, while temperature information is obtained through the direct retrieval of the population distribution on the CARS signal spectrum. CARS thus represents the state-of-the-art in gas-phase thermometry, with unmatched accuracy and precision. The strong “laser-like” signal, which can be detected remotely from where it is generated, makes it also suited for extremely harsh and luminous environments such as flames and plasmas. The present chapter summarizes the fundamentals of gas-phase CARS and discusses a number of most recent advancements: i.e. single-shot CARS imaging, new light sources for ultrabroadband CARS, and simultaneous referencing of the femtosecond (impulsive) excitation efficiency. These recent developments open for interesting possibilities of using CARS in new type of experiments, with coverage of in principle all Raman active modes, obtained with space-time correlated resolution, and improved significance in the delivered data.

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We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000 cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

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Time-resolved spectroscopy can provide valuable insights in hydrogen chemistry, with applications ranging from fundamental physics to the use of hydrogen as a commercial fuel. This work represents the first-ever demonstration of in-situ femtosecond laser-induced filamentation to generate a compressed supercontinuum behind a thick optical window, and its in-situ use to perform femtosecond/picosecond coherent Raman spectroscopy (CRS) on molecular hydrogen (H2). The ultrabroadband coherent excitation of Raman active molecules in measurement scenarios within an enclosed space has been hindered thus far by the window material imparting temporal stretch to the pulse. We overcome this challenge and present the simultaneous single-shot detection of the rotational H2 and the non-resonant CRS spectra in a laminar H2/air diffusion flame. Implementing an in-situ referencing protocol, the non-resonant spectrum measures the spectral phase of the supercontinuum pulse and maps the efficiency of the ultrabroadband coherent excitation achieved behind the window. This approach provides a straightforward path for the implementation of ultrabroadband H2 CRS in enclosed environment such as next-generation hydrogen combustors and reforming reactors.

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We present a novel diagnostic technique to probe water vapor (H₂O) concentration in hydrogen (H₂) combustion environments via the time-resolved measurement of the collisional dephasing of the pure-rotational coherent anti-Stokes Raman scattering (CARS) signal of nitrogen (N₂). The rotational Raman coherence of the N₂ molecules, induced by the interaction with the pump and Stokes laser fields, dephases on a timescale of hundreds of picoseconds (ps), mostly due to inelastic collisions with other molecules in atmospheric flames. In the spatial region of H₂ flames where H₂O is present in appreciable amount, it introduces a faster dephasing of the N₂ coherence than the other major combustion species do: we use time-resolved femtosecond/picosecond (fs/ps) CARS to deduce the H₂O mole fraction from the dephasing effect of its inelastic collisions with N₂. The proof-of-principle is demonstrated in a laminar H₂/air diffusion flame, performing sequential measurements of the collisional dephasing of the N₂ CARS signal up to 360 ps. We measure the temperature and the relative O₂/N₂ and H₂/N₂ concentrations at a short probe delay, and input the results in the time-domain model to extract the H₂O mole fraction from the signal decay, thus measuring the whole scalar flow fields across the flame front. We furthermore present single-shot simultaneous thermometry and absolute concentration measurements in the turbulent TU Darmstadt/DLR Stuttgart canonical 'H3 flame' performed by dual-probe CARS measurements obtained with a polarization separation approach. This allows us to probe the molecular coherence simultaneously at -20 and -250 ps on the basis of a single-laser-shot, and record the resulting signals in two distinct detection channels of our unique polarization-sensitive coherent imaging spectrometer. The proposed technique allows for measuring the absolute concentrations of all the major species of H₂ flames, thus providing a full characterization of the flow composition, as well as of the temperature field.

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We report on the generation of coherent emission from femtosecond (fs) laser-induced filaments mediated by ultrabroadband coherent Raman scattering (CRS), and we investigate its application for high-resolution gas-phase thermometry. Broadband 35-fs, 800-nm pump pulses generate the filament through photoionization of the N₂ molecules, while narrowband picosecond (ps) pulses at 400 nm seed the fluorescent plasma medium via generation of an ultrabroadband CRS signal, resulting in a narrowband and highly spatiotemporally coherent emission at 428 nm. This emission satisfies the phase-matching for the crossed pump-probe beams geometry, and its polarization follows the CRS signal polarization. We perform spectroscopy on the coherent N₂⁺ signal to investigate the rotational energy distribution of the N₂⁺ ions in the excited B²Σ_u⁺ electronic state and demonstrate that the ionization mechanism of the N₂ molecules preserves the original Boltzmann distribution to within the experimental conditions tested.

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We present ultrabroadband two-beam femtosecond/picosecond coherent Raman spectroscopy on the ro-vibrational spectra of CO₂ and O₂, applied for multispecies thermometry and relative concentration measurements in a standard laminar premixed hydrocarbon flame. The experimental system employs fs-laser-induced filamentation to generate the compressed supercontinuum in-situ, resulting in a ∼24 fs full-width-at-half-maximum pump/Stokes pulse with sufficient bandwidth to excite all the ro-vibrational Raman transitions up to 1600 cm^-1. We report the simultaneous recording of the ro-vibrational CO₂ Q-branch and the ro-vibrational O₂ O-, Q- and S-branch coherent Stokes Raman spectra (CSRS) on the basis of a single-laser-shot. The use of filamentation as the supercontinuum generation mechanism has the advantage of greatly simplifying the experimental setup, as it avoids the use of hollow-core fibres and chirped mirrors to deliver a near-transform-limited ultrabroadband pulse at the measurement location. Time-domain models for the ro-vibrational Q-branch spectrum of CO₂ and the ro-vibrational O-, Q- and S-branch spectra of O₂ were developed. The modelling of the CO₂ Q-branch spectrum accounts for up to 180 vibrational bands and for their interaction in Fermi polyads, and is based on recently available, comprehensive calculations of the vibrational transition dipole moments of the CO₂ molecule: the availability of spectroscopic data for these many vibrational bands is crucial to model the high-temperature spectra acquired in the flue gases of hydrocarbon flames, where the temperature can exceed 2000 K. The numerical code was employed to evaluate the CSRS spectra acquired in the products of a laminar premixed methane/air flame provided on a Bunsen burner, for varying equivalence ratio in the range 0.6–1.05. The performance of the CO₂ spectral model is assessed by extracting temperatures from 40-laser-shots averaged spectra, resulting in thermometry accuracy and precision of ∼5% and ∼1%, respectively, at temperatures as high as 2220 K.

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Cascaded coherent anti-Stokes Raman scattering (CARS) signals can be efficiently generated from CARS signals when propagating collinearly with the pump/Stokes and probe beams. This effect can be seen as the CARS beam acting as the probe beam and being inelastically scattered a ‘second time’ from the Raman coherence induced along the focus of the pump/Stokes beam axis. Although much weaker, this additionally scattered signal co-propagates with the CARS signal and may complicate the analysis of the CARS spectrum used for diagnostics in the gas phase. In particular, the occurrence of the cascaded CARS process needs to be taken into account analysing minor spectral signatures at relatively high number density of scattering molecules. Here we show how polarization control can be employed to generate CARS and cascaded CARS signals with orthogonal linear polarization and how, in this way, the cascaded CARS signals can be efficiently suppressed. However, instead of rejecting this signal, we collect both the generated CARS and cascaded CARS signals on the same detector frame, and we explore the use of these counterparts for absolute concentration measurements of the Raman-active species. The cascaded CARS signal has exponential-order higher sensitivity to the number density of the scattering molecules in the mixture. We demonstrate that the ratio of the CARS and the cascaded CARS signals is independent of the probe pulse energy in use, which can be a promising approach for wide-range absolute concentration measurements in gas-phase media.

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Simultaneous detection of resonant and non-resonant femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (CARS) signals has been developed as a viable technique to provide in-situ referencing of the impulsive excitation efficiency for temperature assessments in flames. In the framework of CARS thermometry, the occurrence of both a resonant and a non-resonant contribution to the third-order susceptibility is well known. While the resonant part conceives the useful spectral information for deriving temperature and species concentrations in the probed volume, the non-resonant part is often disregarded. It nonetheless serves the CARS technique as an essential reference to map the finite bandwidth of the laser excitation fields and the transmission characteristics of the signal along the detection path. Hence, the standard protocols for CARS flame measurements include the time-averaged recording of the non-resonant signal, to be performed sequentially to the experiment. In the present work we present the successful single-shot recordings of both the resonant and non-resonant CARS signals, split on the same detector frame, realizing the in-situ referencing of the impulsive excitation efficiency. We demonstrate the use of this technique on one-dimensional CARS imaging spectra, acquired across the flame front of a laminar premixed methane/air flame. The effect of pulse dispersion on the laser excitation fields, while propagating in the participating medium, is proved to result, if not accounted for, in an ~1.3% systematic bias of the CARS-evaluated temperature in the oxidation region of the flame.

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We report spatiotemporal pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional imaging arrangement obtained with a single ultrafast regenerative amplifier system. The femtosecond pump/Stokes photon pairs, used for impulsive excitation, are delivered by an external compressor operating on a ∼35% beam split of the uncompressed amplifier output (2.5 mJ/pulse). The picosecond 1.2 mJ probe pulse is produced via the second-harmonic bandwidth compression (SHBC) of the ∼65% remainder of the amplifier output (4.5 mJ/pulse), which originates from the internal compressor. The two pump/Stokes and probe pulses are spatially, temporally, and repetition-wise correlated at the measurement, and the signal generation plane is relayed by a wide-field coherent imaging spectrometer onto the detector plane, which is refreshed at the same repetition rate as the ultrafast regenerative amplifier system. We demonstrate 1 kHz cinematographic 1D-CARS gas-phase thermometry across an unstable premixed methane/air flame-front, achieved with a single-shot precision <1% and accuracy <3%, 1.4 mm field of view, and an excellent <20 µm line-spread function.

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In practical combustion devices featuring high power densities, the interaction between flames and walls may have a significant impact on the thermodynamic efficiency and the formation of pollutants (e.g. unburned hydrocarbons and carbon monoxide). The common understanding is that at the region near the wall ~ 1 mm, where the solid metal surface is generally much cooler than the impinging flames, steep temperature gradients exist which may result in incomplete combustion and local quenching of the flame. This has been investigated experimentally, by CARS monitoring of temperature- and major species profiles from premixed methane/air flames impinging against a cooled steel side-wall, performed at well-characterized quasi-stationary laminar conditions [1]. In these new experiments, we have employed the same generic burner and CARS imaging system [2], but now operating at significantly enhanced turbulence intensities induced by a turbulence generator grid (blockage ratio 45%, turbulence level 푢푢′/푢푢ത = 6-7%). The complex interplay between the laser diagnostic and the turbulent reacting flow in the near-wall region has been overcome, for instance, with the balanced detection between CARS signals originating from unreacted- and reacted mixtures imaged on the same detector frame, the removal of the laser beams reflecting from the surface, and the suppression of background flame luminosity within the clear aperture of the coherent imaging system. A quick-fitting routine enabling rapid convergence of two-beam femtosecond/picosecond CARS signal analysis has been developed. Correlated statistics have been produced, for instance, on the instantaneous temperature gradients near the wall (position with magnitude). This temperature data supports the recent findings of velocity measurements [3], and adds to the database on this burner system which can be used as a benchmark to improve the fidelity of numerical simulation on near-wall turbulent reacting flows.@en

We develop simultaneous ultrafast CARS thermometry and species detection as a unique tool to investigate combustion modes with reduced emissions of NOx, particulates, CO and unburned hydrocarbons. Coherent anti-Stokes Raman spectroscopy (CARS) has been developed over the last 4 decades for various gas-phase-, plasma-, flame- and combustion applications and is referred to as the “gold standard” for nonintrusive spectroscopic measurements in harsh environments. It is the combination of excellent chemical selectiveness, i.e. the ability in targeting distinct quantum states of individual molecular species, and the coherent properties of the generated signal beam, which makes this technique very powerful. For most of the gas-phase applications, with rapidly changing events ranging from μs to ms, there is a constraint in performing instantaneous measurements and is beneficially fulfilled by the inherent property of the Raman process. It is the time-duration of the probe pulse which ultimately limits the temporal resolution, where today existing techniques, implemented with ultrafast laser systems operate around or even below collision lifetimes. Still, while a reacting flow is fully characterized in three-dimensional space, a limitation of CARS has been its main applicability as a point-wise measurement technique. Establishing ultrafast CARS for single-shot imaging, however, requires designing robust schemes for both the signal generation and the signal detection. Because the physical scalars quantifying an event in a reacting flow exist only for an instantaneous moment of time, laser-scanning or sample-rastering procedures may not be employed. Here we discuss two-beam femtosecond/picosecond CARS relevant for 0D, 1D, and 2D temperature measurements in flames, and we show that single-shot quantitative measurements for major species in combustion are within reach. @en

In practical combustion-based devices, such as internal combustion (IC) engines and gas turbines for power generation, there exists significant interaction between gas-phase combustion and metal surfaces. The nature of this heterogeneous interaction is complex, involving both physical and chemical effects. The metal surface is generally much cooler than impinging flames, and significant heat flux to the metal wall ensues during the flame–wall interaction (FWI). In this work, we investigate FWI in the side-wall quenching canonical configuration obtained with two-beam femtosecond/picosecond 1D-CARS thermometry. The experiments are performed using a generic burner where the turbulence intensities are significantly enhanced by the employment of a turbulence generator grid (blockage 45%). Previously, these operating conditions have only been well-characterized at quasi-stationary conditions (Re = 5000) [Bohlin, A., Jainski, C., Patterson, B.D., Dreizler, A., Kliewer, C.J.: Multiparameter spatiothermochemical probing of flame–wall interactions advanced with coherent Raman imaging. Proc. Combust. Inst. 36, 4557–4564 (2017)]. In these new experiments, complex interplay between the laser diagnostic and the turbulent combustion in the near wall region is overcome, and data is produced, for instance, on the maximum instantaneous temperature gradients versus the position of the flame-front near the wall as determined with direct thermometry contour analysis. @en

We have developed and built a small porous-plug burner based on the original McKenna burner design. The new burner generates a laminar premixed flat flame for use in studies of combustion chemistry and soot formation. The size is particularly relevant for space-constrained, synchrotron-based X-ray diagnostics. In this paper, we present details of the design, construction, operation, and supporting infrastructure for this burner, including engineering attributes that enable its small size. We also present data for charactering the flames produced by this burner. These data include temperature profiles for three premixed sooting ethylene/air flames (equivalence ratios of 1.5, 1.8, and 2.1); temperatures were recorded using direct one-dimensional coherent Raman imaging. We include calculated temperature profiles, and, for one of these ethylene/air flames, we show the carbon and hydrogen content of heavy hydrocarbon species measured using an aerosol mass spectrometer coupled with vacuum ultraviolet photoionization (VUV-AMS) and soot-volume-fraction measurements obtained using laser-induced incandescence. In addition, we provide calculated mole-fraction profiles of selected gas-phase species and characteristic profiles for seven mass peaks from AMS measurements. Using these experimental and calculated results, we discuss the differences between standard McKenna burners and the new miniature porous-plug burner introduced here.

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Overall aim and key objectives Advances in optical imaging techniques over the past decades have revolutionized our ability to study chemically reactive flows encountered in air-breathing combustion systems. Emerging technology for unravelling clean- and efficient heat release is needed for advancing new reduced emission technology, and is on the central agenda for a wide variety of energy production- and transport industry. Combustion of fossil fuels remains our largest source of energy production in the world, and global concerns regarding energy security, environmental pollution, and anthropogenic climate change have motivated a large body of research devoted to the experimental measurement and numerical simulation of combustion systems. Clean combustion engineering is the search for improved efficiency by means of strengthen the systems fuel-economy and lowering the emission of NOx, particulates, CO and unburned hydrocarbons (incomplete combustion). New reduced emission technology, greatly rely upon the ability to control the heat release and the exhaust produced by the exothermic reactions between the fuel and the oxidizer in the chemically reactive flow. For the engineering system design, it exist a significant need to inform on the flame-physics involved based on direct observation of the combustion reaction progress and interaction, which is a demanding task for any measurement technique. Chemically reactive flows are inherently multiscale, fully characterized in three-dimensional space and evolving on rapid time-scales. The combustion environment imposes a significant challenge for diagnostics, where it needs to be collected complete information ideally with correlated-field multi-parameter measurement capabilities, exhibiting high spatial and temporal resolution and provided within a snap-shot to freeze the fast dynamics involved. Concurrent detection of major- and minor molecular species (multiplexing) and determining the three most important scalars; the temperature, the flow-field, and the mixture fraction, is vitally important in studies of the reactive flow. The temperature marks the evolution of heat release and energy transfer, while species concentration gradients provide critical information on mixing and chemical reaction. Optical imaging techniques have the advantage of being non-invasive, which means that the studied process is not significantly perturbed by the measurement technique, and allowing for the acquisition of statistics in-situ. Spectroscopy offer intrinsic chemical specificity, in that different classes of molecules have specific spectral signatures serving as unique fingerprints for their identification. Laser-based diagnostics may in general provide measurements with exceptionally high spatial- and temporal resolution, which is important in producing reliable and accurate experimental data. Coherent anti-Stokes Raman spectroscopy (CARS) is one such versatile technique, which has had a profound impact on a wide variety of fields. It was pioneered in composition- and temperature measurements almost 40 years ago, and is referred to as authoritative with the level of accuracy and precision it may provide. A limitation still, has been its main applicability as a single point-measurement technique, where the experimenter needs to raster-scan the measurement samples assembling the spatial image. Because many complex systems can be fully characterized in multidimensional space, there is a large motivation for the advancement of multidimensional CARS imaging techniques. @en