This thesis is concerned with the development and application of laser induced thermal grating spectroscopy (LITGS) as a tool for thermometry in reacting and non-reacting flows. LITGS signals, which require resonant excitation of an absorbing species in the measurement region to produce a thermal grating, are acquired for systematic measurements of temperature in high pressure flames using OH and NO as target absorbing species in the burned gas. The signal obtained in LITGS measurements appears in the form of a time-based signal with a characteristic frequency proportional to the value or the sound speed of the local medium. With knowledge of the gas composition, the temperature can be derived from the speed of sound measurement. LITGS thermometry using resonant excitation of OH in the burned gas region of in oxygen enriched CH4/O2/N2 and CH4/air laminar flames was performed at elevated pressure (0.5 MPa) for a range of conditions. Measurements were acquired in oxygen enriched flames to provide an environment in which to demonstrate LITGS thermometry under high temperature conditions (up to 2900 K). The primary parameters that influence the quality of LITGS signal were also investigated. The signal contrast, which acts as a marker for the strength of the frequency oscillations, is shown to increase with an increase in the burnt gas density at the measurement point. LITGS employing resonant excitation of NO is also demonstrated for quantitative measurements of temperature in three environments – a static pressure cell at ambient temperature, a non-reacting heated jet at ambient pressure and a laminar premixed CH4/NH3/air flame operating at 0.5 MPa. Flame temperature measurements were acquired at various locations in the burned gas close to a water-cooled stagnation plate, demonstrating the capability of NO-LITGS thermometry for measuring the spatial distribution of temperature in combustion environments. In addition, the parameters that in influence the local temperature rise due to LITGS were also investigated in continuous vapour flows of acetone/air and toluene/air mixtures at atmospheric conditions. Acetone and toluene are commonly targeted species in previous LITGS measurements due to their favourable absorption characteristics. Results indicate that LITGS has the potential to produce accurate and precise measurements of temperature in non-reacting flows, but that the product of the pump intensity at the probe volume and the absorber concentration must remain relatively low to avoid significant localised heating of the measurement region.

<p>Ultrafast laser-based diagnostic techniques are powerful tools for the detailed understanding of highly dynamic combustion chemistry and physics. The ultrashort pulses provide unprecedented temporal resolution along with high peak power for broad spectral range−ideal for nonlinear signal generation at high repetition rate−with applications including next-generation combustors for gas turbines, plasma-assisted combustion, hypersonic flows and rotating detonation engines. The current work focuses on advancing (i) femtosecond (fs) two-photon laser-induced fluorescence, and (ii) hybrid femtosecond/picosecond vibrational and rotational coherent anti-Stokes Raman scattering (fs/ps RCARS and VCARS) to higher pressures for the first time. </p><p>Quantitative single-laser-shot kHz-rate concentration measurements of key atomic (O-atom) and molecular (CO) species is presented using femtosecond two-photon laser-induced fluorescence (TP-LIF) for a range of equivalence ratios and pressures in diffusion flames. A multitude of signal-interfering sources and loss mechanisms−relevant to high-pressure fs TP-LIF applications−are also quantified up to 20 atm to ensure high accuracy. The pressure scaling of interferences take into account degradation, attenuation and wave-front distortion of the excitation laser pulse; collisional quenching and pressure dependent transition line-broadening and shifting; photolytic interferences; multi-photon ionization; stimulated emission; and radiation trapping. </p><p>Hybrid fs/ps VCARS of N₂ is reported for interference-free temperature measurement at 1300-2300 K in high-pressure, laminar diffusion flames up to 10 atm. A time asymmetric probe pulse allowed for detection of spectrally resolved CARS signals at probe delays as early as ~200-300 fs while being independent of collisions for the full range of pressures and temperatures. Limits of collisional independence, accuracy and precision of the measurement is explored at various probe-pulse delays, pressures and temperatures. </p><p> </p><p>Additionally, a novel all diode-pumped Nd:YAG amplifier design is presented for generation of time-synchronized ps-probe pulses for hybrid fs/ps RCARS of N₂. High-energy, nearly transform-limited, single-mode, chirp-free ps probe-pulses are generated at variable pulsewidths. The detailed architecture and characterization of the laser is presented. kHz-rate RCARS thermometry is presented up to 2400 K. Excellent spatial, spectral, and temporal beam quality allowed for fitting the theoretical spectra with a simple Gaussian model for the probe pulse with temperature accuracies of 1-2%. </p> <p><br></p>