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.

@en

The present dissertation covers the development of ultrabroadband femtosecond/picosecond coherent Raman spectroscopy (CRS) to measure temperature and species concentrations in gas-phase chemically reacting flows. Since its first demonstration in 1965, CRS has been vastly employed as a non-linear optical spectroscopic technique to quantify scalars in gas-phase chemically reacting flows, and it is presently regarded as a benchmark to measure temperature and concentrations of major species in combustion environments. The commercial availability of ultrafast regenerative laser amplifiers has brought forth an astounding amount of advancements over the past ten years, with the development of time-resolved CRS techniques able to perform measurements on a timescale shorter than that of molecular collisions in gas-phase media. Hybrid femtosecond/picosecond (fs/ps) CRS in particular represents the current state-of-the-art for gas-phase thermometry with unprecedented accuracy and precision, achieved with remarkable spatial and temporal resolution. The high peak power provided by amplified fs laser systems enables spectroscopy to be realised in one- and two-dimensional imaging configurations acquiring single-shot images of the relevant scalar fields. Furthermore, the broad spectral bandwidth of fs laser pulses allows for a great simplification of the fs/ps CRS instrument. In two-beam fs/ps pure-rotational CRS a single broadband fs laser pulse coherently excites the whole rotational energy manifold of the target molecules, resulting in the coherent scattering of a spectrally narrow ps probe probe pulse. Moreover, the introduction of spectral broadening techniques prompted the development of ultrabroadband fs/ps CRS, where a single temporally-compressed supercontinuum pulse can excite, in principle, all the Raman-active modes of the target molecules. Ultrabroadband fs/ps CRS thus allows for the simultaneous investigation of the rotational and the vibrational motion of all the major species present in the probed volume, and could become the laser diagnostic tool for scalar determination in gas-phase chemically reacting flows, both in thermal equilibrium and in non-equilibrium conditions. For this to become a reality, however, a robust experimental protocol is needed for the implementation of ultrabroadband fs/ps CRS, which could be reliably employed behind the thick optical windows present in many practical experiments, such as those involving pressurised combustors and enclosed chemical reactors. In this respect, the present thesis revolves around two main experimental developments. The first one concerns the implementation of fs laser-induced filamentation as the supercontinuum generation mechanism to perform ultrabroadband fs/ps CRS. The research demonstrated that fs laser-induced filamentation can be employed in situ to compress the excitation pulse directly behind thick optical windows and inside the chemically reacting flow under study. This ultrabroadband coherent light source is employed throughout the present research to perform single-shot fs/ps CRS measurements, over a spectral region ranging ∼500-2000 cm-1, the so-called "vibrational fingerprint region". Single-shot detection of four major combustion species –hydrogen, oxygen, carbon dioxide, and methane– is demonstrated in this region of the Raman spectrum, and fs/ps CRS thermometry based on each one of them is validated in a number of laboratory flames. The influence of the combustion environment on the non-linear optical phenomena underpinning fs laser-induced filamentation and on the resulting pulse self-compression is furthermore investigated, evaluating the impact of the local composition and temperature of the gas-phase optical medium.The second experimental advancement addresses the need for an accurate quantification of the resulting spectral excitation bandwidth, with the development of a novel CRS experimental protocol. The conventional protocol entails the measurement of the nonresonant (NR) CRS signal ex situ in a non-resonant gas (typically argon), sequential to the CRS experiment, to map the spectral excitation profile. The novel protocol, on the contrary, is based on the generation of the NR CRS signal in situ in the combustion environment, simultaneous to that of the resonant CRS signal, thus removing a source of systematic bias in the spectral referencing. In order to practically implement this protocol, a polarisation-sensitive coherent imaging spectrometer is developed, which can simultaneously record the cross-polarised resonant and NR CRS signals in two distinct detection channels. The required polarisation angle to generate the resonant and NR CRS signals with orthogonal polarisation is theoretically determined, and the same angle is proven to realise the in situ referencing of any completely depolarised Raman transition. This referencing protocol is firstly applied to pure-rotational CRS thermometry on N2 and O2 in the pure-rotational region of the Raman spectrum, up to ∼500 cm-1. Thereupon the protocol is employed to realise ultrabroadband CRS on H2, whose pure-rotational spectrum spans more than 1500 cm-1 at flame temperatures. The adoption of the in situ referencing protocol proves essential to perform accurate H2 CRS thermometry behind the thick optical window. The novel protocol is also demonstrated on the ro-vibrational Raman spectrum of second vibrational mode (𝜈2) of CH4, which is completely depolarised, as are the Raman spectra associated to the least symmetric vibrations of more complex polyatomic molecules (e.g. heavier hydrocarbons). In this respect, ultrabroadband fs/ps CRS with in situ referencing of the spectral excitation efficiency could be employed not only to perform accurate thermometry in chemically reacting flows, but also to measure the concentrations of all the major molecular species in the probed volume.In parallel to these experimental developments, the present research also involves the development of time-domain models for the pure-rotational and ro-vibrational CRS signals detected in the spectral window up to 2000 cm-1. In particular the CH4 𝜈2 model is, to the best of the author’s knowledge, the first of its kind to include more than 10 million spectral lines, proving the suitability of this modelling approach to complex polyatomic molecules, which could pave the way to the future application of quantitative ultrabroadband fs/ps CRS to investigate a broader set of chemically reacting flows.All in all, the results collected in the present dissertation provide a basis for the direct use of ultrabroadband fs/ps CRS for scalar measurements in numerous and diverse practical applications in the applied science and engineering domain. The possibility of simultaneously measuring temperature and the concentrations of major species in chemically-reactive flows is paramount to understanding the physical and chemical processes at the base of many propulsion and power generation technologies. To name one, the in situ generation of the compressed excitation pulse provides a straightforward path to the use of ultrabroadband fs/ps CRS to perform spatially-resolved measurements of all the relevant scalar fields in high pressure combustion chambers. On the other hand, the ability of performing quantitative spectroscopy on complex polyatomic molecules is of great interest to many chemical engineering platforms, such as chemical reactors for the reforming of CH4 in commodity hydrocarbons and carbon-neutral H2.@en

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.

@en

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.

@en

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.

@en

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.

@en

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.

@en

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.

@en

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.

@en

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.

@en