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.

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.

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.

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.

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.

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.

Fibre Bragg gratings (FBGs) are obtained through a permanent and periodic refractive index modulation in the core of the single-mode optical fibre. For many years, they have been employed in telecommunication industry as a passive device for wavelength division multiplexing and dispersion compensation components, or in laser apparatus for laser fibre stabilization, Erbium amplifier gain flattening device and amplifier pump reflectors. In aerospace structures, FBGs are used as sensors for structural health monitoring of composite materials as they are able to perform measurements of several parameters inside the material in an elegant and low intrusiveness way. Based on the Bragg and optical fibre structure many kind of customizations can be applied on FBG sensors during the manufacturing process. Each of them gives to the FBG sensor different proprieties and sensing abilities. In this work, we address the numerical simulation of the reflected spectrum by a special FBG sensor called a tilted FBG (TFBG), in which the core refractive index modulation is performed in way to obtain a tilted Bragg super-structure. By considering the classic Coupled-Mode theory for weakly-guided waveguides, we solved the mode propagation equations with the Transfer Matrix method (TMM) obtaining the TFBG reflectivity for different tilt angles.