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In this work, a variable-pulse-oscillator is developed and coupled with a burst-mode amplifier for generation of high-energy laser pulses with width of 100 ps to 1 ms and near-Gaussian temporal pulse shape. Pulse energy as high as 600 mJ is demonstrated at 1064 nm, with a super-Gaussian spatial profile and beam quality as good as 1.6 times the diffraction limit. A time-dependent pulse amplification model is developed and is in general agreement with experimentally measured values of output pulse energy and temporal pulse shape of the amplified pulses. Key performance parameters (pulse energy, temporal pulse shape, and spatial beam profile and quality) are analyzed as a function of pulse width across seven orders of magnitude. Additionally, the model is used to elucidate deviations between the simulated and experimental data, showing that the relationship between pulse width and output pulse energy is dominated by the variable-pulse-width oscillator performance, not the burst-mode amplifier.

Simultaneous multi-point multi-parameter flow measurement using Interferometric Rayleigh scattering (IRS) at 100-kHz repetition rate is demonstrated. Using a burst-mode laser and an un-intensified high-speed camera, interferograms are obtained that contain spatial, temporal and scattered light frequency information. The method of analysis of these interferograms to obtain simultaneous multi-point flow velocity and temperature measurements is described. These methods are demonstrated in a 100-kHz-rate study of a choked, under-expanded jet flow discharged by a convergent nozzle. Measurement results and uncertainties are discussed. The 100-kHz IRS technique with un-intensified imaging is applicable in large-scale wind tunnels for the study of unsteady and turbulent flows.

Two-dimensional, Rayleigh-scattering-based temperature measurements utilizing a turbulent jet flame were performed in this study at 100-kHz frequency. This tenfold increase in measurement speed—compared to the 10-kHz frequency considered previously—facilitated identification and tracking of several highly dynamic flow features. Findings of this study demonstrate that flow-feature dynamics become uncorrelated qualitatively and quantitatively prior to an elapse of 100 μs between successive measurements, thereby necessitating the temperature-measurement frequency to exceed 10 kHz. At the proposed 100-kHz measurement frequency, resolution of the Taylor microscale and integral scales have been demonstrated in both space and time for this flow.

Rotationally resolved, broadband absorption spectra of the fundamental vibrational transition of the asymmetric C–H stretch mode of methane are measured under single-laser-shot conditions using time-resolved optically gated absorption (TOGA). The TOGA approach exploits the difference in timescales between a broadband, fs-duration excitation source and the ps-duration absorption features induced by molecular absorption to allow effective suppression of the broadband background spectrum, thereby allowing for sensitive detection of multi-transition molecular spectra. This work extends the TOGA approach into the mid-infrared (mid-IR) spectral regime, allowing access to fundamental vibrational transitions while providing broadband access to multiple mid-IR transitions spanning ∼150 cm−1 (∼160 nm) near 3.3 μm, thereby highlighting the robustness of this technique beyond previously demonstrated electronic spectroscopy. Measurements are conducted in a heated gas cell to determine the accuracy of the simultaneous temperature and species-concentration measurements afforded by this single-shot approach in a well-characterized environment. Application of this approach toward fuel-rich methane–nitrogen–oxygen flames is also demonstrated.

We report the high-speed imaging of multi-species and multi-parameter combustion diagnostics for turbulent non-premixed jet flames using a three-legged burst-mode laser system. Simultaneous OH/CH2O planar laser-induced fluorescence and Rayleigh-scattering imaging measurements at a 10-kHz rate are obtained. OH and CH2O concentrations, flame temperatures, and heat-release rates are simultaneously acquired in two-dimensions at 10 kHz.

Picosecond laser electronic-excitation tagging (PLEET) was demonstrated in a Mach-6 Ludwieg tube at a repetition rate of 100 kHz using a 1064 nm, 100 ps burst-mode laser. The system performance of high-speed velocimetry in unseeded air and nitrogen Mach-6 flows at a static pressure in the range of 5–20 torr were evaluated. Based on time-resolved freestream flow measurements and computational fluid dynamics (CFD) calculations, we concluded that the measurement uncertainty of 100 kHz PLEET measurement for Mach 6 freestream flow condition is ∼1%. The measured velocity profiles with a cone-model agreed well with the CFD computations upstream and downstream of the shockwave; downstream of the shockwave the discrepancy between the CFD and experimental measurement could be attributed to a slight nonzero angle of attack (AoA) or flow unsteadiness. Our results show the potential of utilizing 100 kHz PLEET velocimetry for understanding real-time dynamics of turbulent hypersonic flows and provide the capability of collecting sufficient data across fewer tests in large hypersonic ground test facilities.

Two-dimensional gas-phase coherent anti-Stokes Raman scattering (2D-CARS) thermometry is demonstrated at 1 kHz in a heated jet. A hybrid femtosecond/picosecond CARS configuration is used in a two-beam phase-matching arrangement with a 100-femtosecond pump/Stokes pulse and a 107-picosecond probe pulse. The femtosecond pulse is generated using a mode-locked oscillator and regenerative amplifier that is synchronized to a separate picosecond oscillator and burst-mode amplifier. The CARS signal is spectrally dispersed in a custom imaging spectrometer and detected using a high-speed camera with image intensifier. 1-kHz, single-shot planar measurements at room temperature exhibit error of 2.6% and shot-to-shot variations of 2.6%. The spatial variation in measured temperature is 9.4%. 2D-CARS temperature measurements are demonstrated in a heated Osub2/subjet to capture the spatiotemporal evolution of the temperature field.

Single-shot, tomographic imaging of the three-dimensional concentration field is demonstrated in a turbulent gaseous free jet in co-flow using volumetrically illuminated laser-induced fluorescence. The fourth-harmonic output of an Nd:YAG laser at 266 nm is formed into a collimated 15 × 20 mmsup2/supbeam to excite the ground singlet state of acetone seeded into the central jet. Subsequent fluorescence is collected along eight lines of sight for tomographic reconstruction using a combination of stereoscopes optically coupled to four two-stage intensified CMOS cameras. The performance of the imaging system is evaluated and shown to be sufficient for recording instantaneous three-dimensional features with high signal-to-noise (130:1) and nominal spatial resolution of 0.6-1.5 mm at x/D = 7-15.5.

Laser-induced breakdown spectroscopy (LIBS) evaluates the emission spectra of ions, radicals, and atoms generated from the breakdown of molecules by the incident laser; however, the LIBS signal is unstable at elevated pressures. To understand the cause of the signal instability, we perform simultaneous time-resolved measurements of the electron density and LIBS emission signal for nitrogen (568 nm) and hydrogen (656 nm) at high pressure (up to 11 bars). From correlations between the LIBS signal and electron number density, we find that the uncontrollable generation of excess electrons at high pressure causes high instability in the high-pressure LIBS signal. A possible method using ultrafast lasers is proposed to circumvent the uncontrolled electron generation and improve signal stability at high pressure.

High-speed, laser-based tomographic imaging of the three-dimensional time evolution of soot volume fraction in turbulent jet diffusion flames is demonstrated to be feasible at rates of 10 kHz or higher. The fundamental output of a burst-mode Nd:YAG laser with 1 J/pulse is utilized for volumetric impulsive heating of soot particles with a laser fluence of 0.1 J/cmsup2/sup, enabling signal-to-noise ratios of ~100:1 in images of the resulting incandescence. The three-dimensional morphology of the soot distribution is captured with a spatial resolution of1.5 mm using as few as four viewing angles, with convergence of the soot volume fraction to within ~95% occurring with seven or more viewing angles. Uniqueness of the solution is demonstrated using two sets of eight images captured at the same time instant, with agreement to90% in peak values between the two sets. These data establish parameters for successful high-speed, three-dimensional imaging of the soot volume fraction within highly transient combustion environments.

This paper describes a novel laser diagnostic and its demonstration in a practical aero-propulsion engine (General Electric J85). The diagnostic technique, named hyperspectral tomography (HT), enables simultaneous 2-dimensional (2D) imaging of temperature and water-vapor concentration at 225 spatial grid points with a temporal response up to 50 kHz. To our knowledge, this is the first time that such sensing capabilities have been reported. This paper introduces the principles of the HT techniques, reports its operation and application in a J85 engine, and discusses its perspective for the study of high-speed reactive flows.

Spectral focusing using broadband femtosecond pulses to achieve highly selective measurements has been employed for numerous applications in spectroscopy and microspectroscopy. In this work we highlight the use of spectral focusing for selective excitation and detection of gas-phase species. Furthermore, we demonstrate that spectral focusing, coupled with time-resolved measurements based upon probe delay, allows the observation of interference-free coherence dynamics of multiple molecules and gas-phase temperature making this technique ideal for gas-phase measurements of reacting flows and combustion processes.

Rotational-level-dependent dephasing rates and nonresonant background can lead to significant uncertainties in coherent anti-Stokes Raman scattering (CARS) thermometry under high-pressure, low-temperature conditions if the gas composition is unknown. Hybrid femtosecond/picosecond rotational CARS is employed to minimize or eliminate the influence of collisions and nonresonant background for accurate, frequency-domain thermometry at elevated pressure. The ability to ignore these interferences and achieve thermometric errors of

High-repetition-rate, single-laser-shot measurements are important for the investigation of unsteady flows where temperature and species concentrations can vary significantly. Here, we demonstrate single-shot, pure-rotational, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) thermometry based on a kHz-rate fs laser source. Interferences that can affect nanosecond (ns) and ps CARS, such as nonresonant background and collisional dephasing, are eliminated by selecting an appropriate time delay between the 100-fs pump/Stokes pulses and the pulse-shaped 8.4-ps probe. A time- and frequency-domain theoretical model is introduced to account for rotational-level dependent collisional dephasing and indicates that the optimal probe-pulse time delay is 13.5 ps to 30 ps. This time delay allows for uncorrected best-fit N2-RCARS temperature measurements with ~1% accuracy. Hence, the hybrid fs/ps RCARS approach can be performed with kHz-rate laser sources while avoiding corrections that can be difficult to predict in unsteady flows.

The use of femtosecond-laser sources for the diagnostics of combustion and reacting-flow environments requires detailed knowledge of optical dispersive properties of the medium interacting with the laser beams. Here the second- and third-order dispersion values for nitrogen, oxygen, air, carbon dioxide, ethylene, acetylene, and propane within the 700–900 nm range are reported, along with the pressure dependence of the chromatic dispersion. The effect of dispersion on axial resolution when applied to nonlinear spectroscopy with ultrabroadband pulses is also discussed.

A novel technique has been developed to obtain simultaneous tomographic images of temperature and species concentration based on hyperspectral absorption spectroscopy. The hyperspectral information enables several key advantages when compared to traditional tomography techniques based on limited spectral information. These advantages include a significant reduction in the number of required projection measurements, and an enhanced insensitivity to measurements/inversion uncertainties. These advantages greatly facilitate the practical implementation and application of the tomography technique. This paper reports the development of the technique, and the experimental demonstration of a prototype sensor in a near-adiabatic, atmospheric-pressure laboratory Hencken burner. The spatial and temporal resolution enabled by this new sensing technique is expected to resolve several key issues in practical combustion devices.

We demonstrate the design and implementation of a fiber-optic beam-delivery system using a large-aperture, tapered step-index fiber for high-speed particle-image velocimetry (PIV) in turbulent combustion flows. The tapered fiber in conjunction with a diffractive-optical-element (DOE) fiber-optic coupler significantly increases the damage threshold of the fiber, enabling fiber-optic beam delivery of sufficient nanosecond, 532-nm, laser pulse energy for high-speed PIV measurements. The fiber successfully transmits 1-kHz and 10-kHz laser pulses with energies of 5.3 mJ and 2 mJ, respectively, for more than 25 min without any indication of damage. It is experimentally demonstrated that the tapered fiber possesses the high coupling efficiency (~80%) and moderate beam quality for PIV. Additionally, the nearly uniform output-beam profile exiting the fiber is ideal for PIV applications. Comparative PIV measurements are made using a conventionally (bulk-optic) delivered light sheet, and a similar order of measurement accuracy is obtained with and without fiber coupling. Effective use of fiber-coupled, 10-kHz PIV is demonstrated for instantaneous 2D velocity-field measurements in turbulent reacting flows. Proof-of-concept measurements show significant promise for the performance of fiber-coupled, high-speed PIV using a tapered optical fiber in harsh laser-diagnostic environments such as those encountered in gas-turbine test beds and the cylinder of a combustion engine.