High-energy, low-jitter, narrowband ps probe laser for kHz-rate fs/ps coherent anti-Stokes Raman scattering.

Hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) thermometry often utilizes ps probe pulses derived from pulse shaping or spectrally filtering the primary laser source or by synchronization with a low repetition rate external laser. This results in limited energy, spectral resolution, and/or repetition rate of the ps probe. In this work, a master-oscillator power-amplifier (MOPA) laser was synchronized to the oscillator of a Ti:sapphire regenerative amplifier to achieve high-energy (600 µJ), narrowband (58 ps) probe pulses at kHz repetition rates. Temporal filtering allows the pulse characteristics to be adjusted for each application. At 25 Torr, relevant to high-speed flows, the kHz-rate MOPA system generated signal-to-noise ratios 3× higher in nitrogen and had improved precision relative to a 10 ps probe derived from spectral filtering and the power-amplifier. The MOPA system also enabled single-shot ro-vibrational hybrid fs/ps CARS thermometry in 650 K heated air.

High-efficiency narrow-bandwidth KTP optical parametric oscillator for kHz-MHz planar laser-induced fluorescence.

The electronic excitation of key combustion species or flow tagging of chemical species requires a narrowband tunable UV source. In this work, a potassium titanyl phosphate (KTP) burst-mode optical parametric oscillator (OPO) pumped by a 532 nm laser is developed to generate a spectrally narrow signal and an idler output with 1.48 ± 0.19 cm-1 bandwidth without the need for injection seeding. The idler (1410-1550 nm range) is further mixed with 355 or 266 nm to generate 284 or 226 nm for OH or NO planar laser-induced fluorescence (PLIF), respectively, with up to 1.9% conversion efficiency from 1064 nm to the UV. MHz-rate burst profiles are reported, and OH and NO PLIF are demonstrated in a rotating detonation combustor at rates up to 200 kHz.

Methods to improve burst-mode laser spectral purity for high-speed gas-phase filtered Rayleigh scattering.

In the filtered Rayleigh scattering (FRS) technique, Doppler or homogeneously broadened light from weak molecular scattering is separated from orders-of-magnitude stronger elastic scattering from surfaces, windows, particles, and/or droplets using a narrowband filter. In this work, high-speed detection of such weak molecular scattering is enabled by a burst-mode laser system that can achieve a spectral purity of ∼0.999999. This allows for an additional two orders of magnitude of attenuation from a narrowband iodine molecular filter for high-speed detection of gas-phase FRS in the presence of direct surface scattering at 532 nm. The methodology, system characterization, and feasibility of single-shot gas-phase FRS at 100 kHz or higher are presented and discussed.

Spatially multiplexed femtosecond/picosecond coherent anti-Stokes Raman scattering for multipoint array measurements.

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.

Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas.

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 (10.5-12 GHz) microwave scattering off laser induced 1-760 Torr air-based microplasmas (287.5 nm O2 resonant photoionization by ~ 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons [Formula: see text] in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS 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 [Formula: see text] from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement.

Burst-mode 100 kHz N2 ps-CARS flame thermometry with concurrent nonresonant background referencing.

A burst-mode nitrogen (N2) 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 120cm-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 CH4/air Hencken calibration flame with an accuracy of 1.5% and precision of 4.7% up to peak flame temperatures. The use of N2 VCARS and simultaneous NRB measurements enables high-speed thermometry for a wide range of fuels and combustion applications.

Picosecond laser electronic excitation tagging velocimetry using a picosecond burst-mode laser.

Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen (N2) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves "writing" via photo-dissociating nitrogen molecules and "tracking" by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET.

Concentration and pressure scaling of CH2O electronic-resonance-enhanced coherent anti-Stokes Raman scattering signals.

Nanosecond electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) is evaluated for the measurement of formaldehyde (CH2O) concentrations in reacting and nonreacting conditions. The three-color scheme utilizes a 532 nm pump beam and a scanned Stokes beam near 624 nm for Raman excitation of the C-H symmetric stretch (ν1) vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the 101403 vibronic transition between the ground (X~1A1) and first excited (A~1A2) electronic states. This allows detection of CH2O at concentrations as low as 9×1014molecules/cm3 (55 parts per million) in a calibration cell with CH2O and N2 at 1 bar and 450 K with 3% uncertainty. The measurements show a quadratic dependence of the signal with CH2O number density. Pressure scaling experiments up to 11 bar in the calibration cell show an increase in signal up to 8 bar. We study pressure dependence up to 11 bar and further apply the technique to characterize the CH2O concentration in an atmospheric premixed dimethyl ether/air McKenna burner flame, with a maximum concentration uncertainty of 11%. This approach demonstrates the feasibility for spatially resolved measurements of minor species such as CH2O in reactive environments and shows promise for application in high-pressure combustors.

Grid-based femtosecond laser electronic excitation tagging for single-ended 2D velocimetry at kilohertz rates.

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.

Femtosecond laser activation and sensing of hydroxyl for velocimetry in reacting flows.

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 H2O, a common combustion product, into a locally high concentration of OH radicals. These radicals are tracked by planar laser-induced fluorescence (PLIF) from the A2Σ-X2Π (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.

Generation of high-energy, Gaussian laser pulses with tunable duration from 100 picoseconds to 1 millisecond.

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.

Dual-output fs/ps burst-mode laser for megahertz-rate rotational coherent anti-Stokes Raman scattering.

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 N2 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.

Megahertz-rate OH planar laser-induced fluorescence imaging in a rotating detonation combustor.

Megahertz-rate hydroxyl radical planar laser-induced fluorescence (OH-PLIF) was demonstrated in a hydrogen/air rotating detonation combustor for the first time, to the best of our knowledge. A custom injection-seeded optical parametric oscillator (OPO) pumped by the 355 nm output of a high-energy burst-mode laser produced narrowband pulses near 284 nm for OH excitation. The system generated sequences of more than 150 ultraviolet pulses with 400 µJ/pulse at 1 MHz and 150 µJ/pulse at 2 MHz. The order of magnitude improvement in the repetition rate over prior OH-PLIF measurements and in the number of pulses over previous megahertz burst-mode OPOs enables spatiotemporal analysis of complex detonation combustion dynamics.

High-energy laser pulses for extended duration megahertz-rate flow diagnostics.

Optical diagnostics of highly dynamic supersonic and hypersonic flows requires laser sources with a combination of high pulse intensities and fast repetition rates. A burst-mode Nd:YAG laser system is presented for increasing the overall energy of 532 nm pulse trains by ∼100× and the number of high-energy pulses by 30× for extended duration megahertz-rate flow diagnostics. At a lower repetition rate of 100 kHz, unprecedented energies near 1 J/pulse are achieved at 532 nm over a 1.1 ms burst. The laser performance is characterized and demonstrated for megahertz-rate laser-induced breakdown spectroscopy in a Mach 2 turbulent jet.

Megahertz-rate shock-wave distortion cancellation via phase conjugate digital in-line holography.

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.

Pressure-scaling characteristics of femtosecond two-photon laser-induced fluorescence of carbon monoxide.

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.

Quantitative femtosecond, two-photon laser-induced fluorescence of atomic oxygen in high-pressure flames.

Quantitative femtosecond two-photon laser-induced fluorescence of atomic oxygen was demonstrated in an H2/air flame at pressures up to 10 atm. Femtosecond excitation at 226.1 nm was used to pump the 3pP3J'=0,1,2←←2pP3J''=0,1,2 electronic transition of atomic oxygen. Contributions from multiphoton de-excitation, production of atomic oxygen, and photolytic interferences were investigated and minimized by limiting the laser irradiance to ∼1011 W/cm2. Quantitative agreement was achieved with the theoretical equilibrium mole fraction of atomic oxygen over a wide range of fuel-air ratios and pressures in an H2/air laminar calibration burner.

Lifetime-filtered laser-induced exciplex fluorescence for crosstalk-free liquid-vapor imaging.

Laser-induced exciplex fluorescence is a well-established technique for liquid-vapor imaging in evaporating sprays that offers phase-dependent spectrally separated emission. However, the accuracy of this approach is limited by substantial crosstalk from the liquid to vapor phase signals. This Letter shows the use of a combination of spectral and temporal filtering to reduce this crosstalk by three orders of magnitude and eliminate the need for temperature-dependent crosstalk corrections in the N,N-diethylmethylamine/fluorobenzene system. The relative decay rates of the liquid and vapor signals are quantified and show crosstalk-free imaging for monodisperse evaporating droplets over a wide range of exciplex tracer concentrations.

Interference-free hybrid fs/ps vibrational CARS thermometry in high-pressure flames.

Interference-free hybrid femtosecond/picosecond vibrational coherent anti-Stokes Raman scattering (CARS) of nitrogen is reported for temperature measurements of 1300-2300 K in high-pressure, laminar H2-air and CH4-air diffusion flames up to 10 bar. Following coherent Raman excitation by 100 fs duration pump and Stokes pulses, a time-asymmetric probe pulse is used for the detection of spectrally resolved N2 CARS signals at probe delays as early as ∼200-300 fs. This allows for full rejection of nonresonant contributions while being independent of collisions for single-shot precision of ±2% at elevated pressures. The effects of collisions at longer probe-pulse delays are also investigated to determine the feasibility of varying the detection timing from 200 fs to 100 ps.

Burst-mode OH/CH2O planar laser-induced fluorescence imaging of the heat release zone in an unsteady flame.

The paper presents simultaneous high-speed (7.5 kHz) planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH4/O2/N2 flame. For this purpose, a dye laser designed for high-speed operation is pumped by the second-harmonic 532 nm output of a Nd:YAG burst-mode laser to produce a tunable, 566 nm beam. After frequency doubling a high-energy kHz-rate narrowband pulse train of approximately 2.2 mJ/pulse at 283 nm is used for excitation of the OH radical. Simultaneously, CH2O is excited by the frequency-tripled output of the same Nd:YAG laser, providing a high-frequency pulse train over 10 ms in duration at high pulse energies (>100 mJ/pulse). The excitation energies enable signal-to-noise ratios (SNRs) of ~10 and ~60 for CH2O and OH PLIF, respectively, using a single high-speed intensified CMOS camera equipped with an image doubler. This allows sufficient SNR for investigation of the temporal evolution of the primary heat release zone and the local flame structure at kHz rates from the spatial overlap of the OH- and CH2O-PLIF signals.