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.

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.

Laser Synthesis of Cerium-Doped Garnet Nanoparticles.

The application of a pulsed laser ablation technique for the generation of cerium-doped garnet nanoparticles in liquids is investigated. The morphological and optical properties of the obtained nanoparticles are demonstrated. Features introduced by the single crystals of GdAlGaO:Ce, LuAlO:Ce, and YAlGaO:Ce from which the nanoparticles are generated, as well as the parameters of a liquid media on the garnet nanoparticle generation are experimentally studied using TEM and UV-Vis spectroscopy methods.

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.

Mach 18 flow velocimetry with 100-kHz KTV and PLEET in AEDC Tunnel 9.

Krypton Tagging Velocimetry (KTV) and Picosecond Laser Electronic Excitation Tagging (PLEET) velocimetry at a 100-kHz rate were demonstrated in Mach 18 flow conditions at the Arnold Engineering Development Center (AEDC) Tunnel 9 employing a burst-mode laser system and a custom optical parametric oscillator (OPO). The measured freestream flow velocities from both KTV and PLEET agreed well with the theoretical calculation. The increase in repetition rate provides better capability to perform time-resolved velocimetry measurements in hypersonic flow environments.

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 O/N profile measurements are demonstrated along a line with 1 mm spacing in room temperature and heated N flows.

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.

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.

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

A burst-mode nitrogen () 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 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.

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

Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen () 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.

Concentration and pressure scaling of CHO 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 () 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 () vibrational mode; further, a 342 nm resonant probe is tuned to produce the outgoing CARS signal via the 101403 vibronic transition between the ground (~) and first excited (~) electronic states. This allows detection of at concentrations as low as 9×10/ (55 parts per million) in a calibration cell with and at 1 bar and 450 K with 3% uncertainty.

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.

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 , a common combustion product, into a locally high concentration of OH radicals. These radicals are tracked by planar laser-induced fluorescence (PLIF) from the - (1-0) vibrational band excited by a time-delayed 284 nm (read) laser sheet.

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.

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.

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.

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.

4D spatiotemporal evolution of liquid spray using kilohertz-rate x-ray computed tomography.

Four-dimensional (x,y,z,t) x-ray computed tomography was demonstrated in an optically complex spray using an imaging system consisting of three x-ray sources and three high-speed detectors. The x-ray sources consisted of high-flux rotating anode x-ray tube sources that illuminated the spray from three lines of sight. The absorption, along each absorption path, was collected using a CsI phosphor plate and imaged by a high-speed intensified CMOS camera at 20 kHz.

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.

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 H/air flame at pressures up to 10 atm. Femtosecond excitation at 226.1 nm was used to pump the 3pP3←←2pP3 electronic transition of atomic oxygen.

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 H-air and CH-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 N 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.

Unseeded Velocimetry in Nitrogen for High-Pressure Cryogenic Wind Tunnels, Part 2: Picosecond-Laser Tagging.

Picosecond laser electronic excitation tagging (PLEET) is implemented in a large-scale wind tunnel for the first time. High-speed, unseeded velocimetry is performed in the NASA Langley 0.3-m Transonic Cryogenic Tunnel; repetition rates up to 25 kHz are tested.

Femtosecond, two-photon, laser-induced fluorescence (TP-LIF) measurement of CO in high-pressure flames.

Quantitative, kiloherz-rate measurement of carbon monoxide mole fractions by femtosecond two-photon, laser-induced fluorescence (TP-LIF) was demonstrated in high-pressure, luminous flames over a range of fuel-air ratios. Femtosecond excitation at 230.1 nm was used to pump CO two-photon rovibrational XΣ→BΣ transitions in the Hopfield-Birge system and avoid photolytic interferences with excitation irradiance ∼1.

Burst-mode OH/CHO 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 (CHO) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH/O/N 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.