Ultrabroadband coherent Raman spectroscopy for reacting flows

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.