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A novel, to the best of our knowledge, method for multipoint hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering measurements is presented. The pump/Stokes and probe beams are each split into 16 discrete points with 90 and 24 µJ/pulse, respectively, using simple diffractive optical elements, which are used in combination with a focusing lens and narrowband spectral amplifier for 1 kHz excitation along a linear array of probe volumes. Single-shot and averaged temperature and O2/N2 profile measurements are demonstrated along a line with 1 mm spacing in room temperature and heated N2 flows. This enables measurements over varying spatial extents for 1D profiles and potentially 2D grids in a simple and compact optical arrangement.

Laser illumination is preferred for visualization of hypersonic flows, but coherent speckle typically limits the spatial resolution. This work develops a burst-mode non-collinear optical parametric oscillator to realize a broadband signal for speckle-free imaging.

Uncertainty analysis was performed for kHz-rate hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering thermometry. A high-energy probe pulse and simultaneous nonresonant background acquisition improved the precision of the technique for low-pressure applications.

High spectral purity of the laser source in Filtered Rayleigh Scattering (FRS) is necessary to remove stray scattering. In this study, the spectral purity of a burst-mode laser was improved to 0.9999987 using an etalon

A burst-mode nitrogen (

This paper presents the possibility of using coherent microwave scattering (CMS) for temporally resolved measurements of the electron momentum-transfer collision frequency in small plasma objects. Specifically, the electron collision frequency is inferred via phase information from microwave scattering off microplasmas operating in the mixed collisional-Thomson scattering regime. We further suggest the combination of phase and amplitude measurements to derive total electron counts and temperatures in small plasmas. An experimental validation of this concept is performed by 10.5 GHz CMS off laser-induced, variable-pressure oxygen and air plasmas.

Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde (

The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency microwave scattering off laser induced air-based microplasmas with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via intensified charge-coupled device (ICCD) photography, and measurements of relative phases, total scattering cross sections, and total number of electrons $N_e$ in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of the diagnostic in the Thomson "free-electron" regime - where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract $N_e$ from the scattered signal., Comment: 27 Pages, 5 Figures

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.

A molecular tagging method for velocity measurements in reacting environments such as propulsion devices and high-temperature combustion-assisted wind tunnels is described. The method employs a femtosecond (write) laser to photodissociate H 2 O , a common combustion product, into a locally high concentration of OH radicals. These radicals are tracked by planar laser-induced fluorescence (PLIF) from the A 2 Σ − X 2 Π (1–0) vibrational band excited by a time-delayed 284 nm (read) laser sheet. As a variant of hydroxyl tagging velocimetry, the source laser can also be used to dissociate nitrogen for femtosecond laser electronic excitation tagging velocimetry to mark the time-zero location of the write laser for velocimetry in non-reacting regions using the same imaging system without OH PLIF. The OH tracer lifetime is studied in a hydrogen-air Hencken burner operating at Φ = 0.5 − 1.8 to evaluate the tracking capability for velocimetry over a range of conditions. Effects of changing read laser wavelength, excitation energy, and influence of background flame emission are also studied. The data processing methodology and results are described for tracking displacements with 9–25 µm uncertainty in a hydrogen diffusion flame. This method presents several advantages in operational convenience and availability of laser sources, and it provides an avenue for improvements in the repetition rate, precision, and applicability over previously demonstrated hydroxyl tagging schemes.

A burst-mode laser system is developed for hybrid femtosecond/picosecond (fs/ps) rotational coherent anti-Stokes Raman scattering (RCARS) at megahertz rates. Using a common fs oscillator, the system simultaneously generates time synchronized 1061 nm, 274 fs and 1064 nm, 15.5 ps pulses with peak powers of 350 MW and 2.5 MW, respectively. The system is demonstrated for two-beam fs/ps RCARS in N 2 at 1 MHz with a signal-to-noise ratio of 176 at room temperature. This repetition rate is an order of magnitude higher than previous CARS using burst-mode ps laser systems and two to three orders of magnitude faster than previous continuously pulsed fs or fs/ps laser systems.

Simultaneous dual-plane stereoscopic particle-imaging velocimetry (DS-PIV) and hydroxyl planar laser-induced fluorescence (OH-PLIF) was performed at 10 kHz repetition rate in a highly turbulent swirling jet flame. The burst-mode laser system provides sufficient 532-nm dual-pulse energy enabling high-speed large-area dual-plane stereo-PIV measurement. The scattered light from the two planes was separated onto respective cameras using orthogonal polarizations. A diode-pumped solid-state Nd: YAG/dye laser system produced the narrowband excitation 283-nm beam for the 10-kHz OH-PLIF imaging. The combined diagnostics produced simultaneous, temporally resolved two-dimensional fields of OH and two-dimensional, three-component velocity fields, facilitating the observation of the interaction of fluid dynamics with flame fronts.

Holography is a powerful tool for three-dimensional imaging. However, in explosive, supersonic, hypersonic, cavitating, or ionizing environments, shock-waves and density gradients impart phase distortions that obscure objects in the field-of-view. Capturing time-resolved information in these environments also requires ultra-high-speed acquisition. To reduce phase distortions and increase imaging rates, we introduce an ultra-high-speed phase conjugate digital in-line holography (PCDIH) technique. In this concept, a coherent beam passes through the shock-wave distortion, reflects off a phase conjugate mirror, and propagates back through the shock-wave, thereby minimizing imaging distortions from phase delays. By implementing the method using a pulse-burst laser setup at up to 5 million-frames-per-second, time-resolved holograms of ultra-fast events are now possible. This technique is applied for holographic imaging through laser-spark plasma-generated shock-waves and to enable three-dimensional tracking of explosively generated hypersonic fragments. Simulations further advance our understanding of physical processes and experiments demonstrate ultra-high-speed PCDIH techniques for capturing dynamics., Shock-waves in explosive, supersonic or ionizing environments impart phase distortions to holographic imaging. Here, the authors report an ultra-high-speed phase conjugate digital in-line holography technique where a laser passes through the shock-wave and is reflected back through the phase distortion, thus correcting phase delays.

A diode pumped Nd:YAG narrowband spectral amplifier was demonstrated to generate high energy picosecond probe pulses for hybrid femtosecond/picosecond rotational coherent antiStokes Raman scattering thermometry measurements in a calibration flame and rotating detonation engine exhaust.

Broadband femtosecond (fs) two-photon laser-induced fluorescence (TP-LIF) of the B1Σ+←X1Σ+, Hopfield-Birge system of carbon monoxide (CO) is believed to have two major advantages compared to narrowband nanosecond excitation. It should (i) minimize the effects of pressure-dependent absorption line broadening and shifting, and (ii) produce pressure-independent TP-LIF signals as the effect of increased quenching due to molecular collisions is offset by the increase in number density. However, there is an observed nonlinear drop in the CO TP-LIF signal with increasing pressure. In this work, we systematically investigate the relative impact of potential deexcitation mechanisms, including collisional quenching, forward lasing, attenuation of the source laser by the test cell windows or by the gas media, and a 2+1 photoionization process. As expected, line broadening and collisional quenching play minor roles in the pressure-scaling behavior, but the CO fs TP-LIF signals deviate from theory primarily because of two major reasons. First, attenuation of the excitation laser at high pressures significantly reduces the laser irradiance available at the probe volume. Second, a 2+1 photoionization process becomes significant as the number density increases with pressure and acts as a major deexcitation pathway. This work summarizes the phenomena and strategies that need to be considered for performing CO fs TP-LIF at high pressures.

A novel, to the best of our knowledge, optical arrangement is evaluated for performing single-shot femtosecond laser electronic excitation tagging in a 16-point grid (Grid-FLEET) with single-ended optical access. The optical arrangement includes a diffractive optical element beam splitter to produce a grid of laser beams in a simplified, flexible, and efficient manner for tracer-free multi-component molecular tagging velocimetry in a two-dimensional field. Analysis of the optical element with respect to beam forming is described, and Grid-FLEET measurements are evaluated relative to the precision of previously described single-point FLEET measurements using Lagrangian tracking for flow in a laminar jet and around a sharp corner. Utilizing a conventional 1-kHz laser source coupled to a high-speed intensified camera, it is also feasible to achieve measurement rates of 100 kHz or higher by mapping the Lagrangian grid to one or more Eulerian measurement points. The data further indicate that enhancement of the instantaneous vector fields and spatial velocity gradients can be analyzed to enhance the understanding of multi-dimensional flow physics in applications in which the use of tracers may be difficult and where multi-directional optical access may be limited.

A burst-mode nitrogen ( N 2 ) picosecond vibrational coherent anti-Stokes Raman scattering (ps-VCARS) system is presented for accurate flame thermometry at 100 kHz repetition rate. A frequency-tripled ps burst-mode laser is used to pump a custom optical parametric generator/amplifier to produce 607 nm broadband Stokes pulses with 120 c m − 1 bandwidth, along with a narrowband 532 nm pump/probe beam. A simultaneous shot-to-shot nonresonant background (NRB) measurement is implemented to account for Stokes spectral profile and beam overlap fluctuations. The 100 kHz ps-VCARS data are benchmarked in a near-adiabatic C H 4 / a i r Hencken calibration flame with an accuracy of 1.5% and precision of 4.7% up to peak flame temperatures. The use of N 2 VCARS and simultaneous NRB measurements enables high-speed thermometry for a wide range of fuels and combustion applications.

Spatio-temporally resolved measurements of temperature using hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman scattering (fs/ps RCARS) are evaluated for characterizing the highly dynamic exhaust flow of a non-premixed hydrogen-air rotating detonation combustor (RDC). The RCARS system utilizes a recently developed kHz-rate probe-pulse amplification system that enables high probe-pulse energies and sufficient sensitivity to track RDC exhaust gas temperatures during the short ~1.5 s run time with a precision of ~2%. Because of the potential for high spatial gradients in temperature and pressure in the RDC exhaust, estimation of bias errors due to spatial averaging in the 700-µm-long RCARS probe volume is conducted by employing the results of a reactive three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) model with a structured grid of 48.5 million cells. This results in a potential bias error of ~1.5% due to exhaust temperature gradients and underscores the need for high spatial resolution. The experimental and predicted exhaust temperature histograms show good correspondence with a statistically similar skew-normal distribution relevant to the flow's local dynamical features. By utilizing a high-speed camera synchronized with the RCARS system, it was possible to compare the numerical simulation results with the measured exhaust temperature profile obtained from knowledge of the instantaneous detonation-wave azimuth position. Similar azimuthal spatial variations of ~300 K were observed in the experimental and computed temperatures, indicating a relatively well-mixed exhaust flow. The temperature pattern factors of 0.19 and 0.20 obtained from the experimental and numerical data, respectively, are relatively close to isobaric combustors in modern gas turbine engines. These results illustrate the ability of the fs/ps RCARS and numerical modeling approaches to evaluate characteristics of the RDC exhaust flow for future development in propulsion and power generation systems.

Chemical imaging with sufficient spatial resolution to resolve microparticles in tablets is essential to ensure high quality and efficacy in controlled release. The existing modalities have the following disadvantages: they are time-consuming or have poor spatial resolution or low chemical specificity. Here, we demonstrate an epi-detected mid-infrared photothermal (epi-MIP) microscope at a spatial resolution of 0.65 μm. Providing identical spectral profiles as conventional infrared spectroscopy, our epi-MIP microscope enabled mapping of both active pharmaceutical ingredients and excipients of a drug tablet.

Ignition processes in scramjet pilot cavities are highly transient events that are influenced by factors including freestream Mach number and inlet geometry, turbulence intensity, cavity geometry, ignition source, and fueling composition and flow rate. In particular, the location of the flame kernel and associated propagation rate of the flame front throughout the cavity can significantly influence the end state of the ignition process. In this work formaldehyde (CH2O) was used as a flame marker to track ignition progress in a plane throughout the span-wise width of the cavity, while chemiluminescence imaging provided path-integrated flame location along the span-wise and axial directions. Planar laser-induced fluorescence (PLIF) excitation utilized the 355 nm frequency-tripled output of an Nd:YAG burst-mode laser operating at 50–100 kHz over 10 ms with available pulse energy up to 80 mJ. Simultaneous CH* chemiluminescence imaging from the top of the cavity was obtained with a high-speed complementary metal-oxide semiconductor camera. A freestream Mach number of 2 with ethylene fuel rates from 55–90 standard liters per minute were examined along with two different ignition sources: a spark discharge and pulse detonator. The resulting formaldehyde PLIF and chemiluminescence images indicate a strong correlation between fueling rate and the delay between the onset of ignition and stable combustion. More importantly, the span-wise propagation rate and structure of the flame front is highly dependent on the fueling rate, burning region, and ignition source.

Three-dimensional imaging of the tracer concentration field is demonstrated at a rate of 20 kHz in a turbulent gaseous free jet. The acetone tracer species that is seeded into the jet is excited by the fourth-harmonic output of a Nd:YAG burst-mode laser. The laser beam is expanded and re-collimated to a 35-mm-diameter beam and passed through a diffuser to reduce spatial variations within the beam profile. Seven high-speed cameras coupled to high-speed intensifiers with vertical stereoscopes collect fourteen simultaneous views of the fluorescence emission. The images are processed and a linear relationship between signal intensity and acetone concentration was verified. Eight of the fourteen processed views are then reconstructed into volumes using a simultaneous multiplicative algebraic-reconstruction technique, limited by the software. The mixing behavior is characterized and compared with turbulent-jet scaling laws and data from the literature at similar conditions. The accuracy, precision, and limitations of the technique are discussed, and potential strategies for implementation in reacting flows are considered.

Methane is one of the very few substances that show rotation of individual molecules in the crystalline phase. Here we explore the evolution of the rotation spectrum of methane from single molecules to clusters containing up to about 4 × 10(3) molecules. The clusters were assembled in He droplets at T = 0.38 K and studied via infrared laser spectroscopy in the ν3 region of the methane molecules. Well-resolved rotational structure in the spectra was observed in clusters containing up to about 50 molecules. We have concluded that in distinction to the crystals molecular rotation in methane clusters is confined to the surface and is enabled by low coordination of the molecules. On the contrary the molecules in the cluster's interior are in amorphous state wherein the rotation is quenched. These results demonstrate that even at very low temperature the surface of the methane clusters remains fluxional due to quantum effects.