MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Long-lived nitric oxide molecular tagging velocimetry with 1 + 1 REMPI

The successful demonstration of long-lived nitric oxide (NO) fluorescence for molecular tagging velocimetry (MTV) measurements is described in this Letter. Using 1 + 1 resonance-enhanced multiphoton ionization (REMPI) of NO at a wavelength near 226 nm, targeting the overlapping Q1(7) and Q21(7) lines of the A-X (0, 0) electronic system, the lifetime of the NO MTV signal was observed to be approximately 8.6 µs within a 100-Torr cell containing 2% NO in nitrogen. This is in stark contrast to the commonly reported single photon NO fluorescence, which has a much shorter calculated lifetime of approximately 43 ns at this pressure and NO volume fraction. While the shorter lifetime fluorescence can be useful for molecular tagging velocimetry with single laser excitation within very high-speed flows at some thermodynamic conditions, the longer lived fluorescence shows the potential for an order of magnitude more accurate and precise velocimetry, particularly within lower speed regions of hypersonic flow fields such as wakes and boundary layers. The physical mechanism responsible for the generation of this long-lived signal is detailed. Furthermore, the effectiveness of this technique is showcased in a high-speed jet flow, where it is employed for precise flow velocity measurements.

OH planar laser-induced fluorescence imaging system using a kilohertz-rate 283 nm UV Ti:sapphire laser

A narrow linewidth Ti:sapphire laser is developed and characterized for the generation of an ultraviolet nanosecond laser pulses for the planar laser-induced fluorescence (PLIF) imaging of hydroxyl (OH). With a pump power of 11.4 W at 1 kHz, the Ti:sapphire laser produces 3.5 mJ at 849 nm with pulse duration of 17 ns and achieves a conversion efficiency of 28.2%. Accordingly, its third-harmonic generation outputs 0.56 mJ at 283 nm in BBO with type I phase match. An OH PLIF imaging system has been built; a 1 to 4 kHz fluorescent image of OH of a propane Bunsen burner has been captured based on this laser system.

Recent progress in high-speed laser diagnostics for hypersonic flows [Invited]

The recent progress in high-speed (≥100k H z) laser diagnostics for hypersonic flows is reviewed. Owing to the ultrahigh flow speed, a laser frequency of 100 kHz or higher is required for hypersonic diagnostics. Here, two main laser diagnostic techniques are discussed: focused laser differential interferometry (FLDI) and pulse-burst laser-based diagnostics. Single- and multiple-point FLDI measurements have been widely applied to hypersonic flows for flow velocity and density fluctuation measurements. The progress of pulse-burst laser-based hypersonic diagnostics, including flow velocity measurements and 2D flow visualization, is also discussed.

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.

O2 based resonantly ionized photoemission thermometry analysis of supersonic flows

Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O₂ RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of the C³Π, (v = 2) ← X³Σ(v^' = 0) absorption band of O₂. The efficient ionization of O₂ liberates electrons that induce electron avalanche ionization of local N₂ molecules generating N₂ ⁺, which primarily deexcites via photoemissions of the first negative band of N2+(B ² Σ u+-X ² Σ g+). When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O₂ RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique's fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.

One-dimensional air temperature measurements by air resonance enhanced multiphoton Ionization thermometry (ART)

In this work, a detailed calibration study is performed to establish non-intrusive one-dimensional (1D) rovibrational temperature measurements in unseeded air, based on air resonance enhanced multiphoton ionization thermometry (ART). ART is generated by REMPI (resonance enhanced multi-photon ionization) of molecular oxygen and subsequent avalanche ionization of molecular nitrogen in a single laser pulse. ART signal, the fluorescence from the first negative band of molecular nitrogen, is directly proportional to the 2-photon transition of molecular oxygen C³Π (v = 2) ← X³Σ (v'=0), which is used to determine temperature. Experimentally, hyperfine structures of the O₂ rotational branches with high temperature sensitivity are selectively excited through a frequency-doubled dye laser. Electron-avalanche ionization of N₂ results in the fluorescence emissions from the first negative bands of N₂ ⁺ near 390, 425, and 430nm, which are captured as a 1D line by a gated intensified camera. Post processing of the N₂ ⁺ fluorescence yields a 1D thermometry line that is representative of the air temperature. It is demonstrated that the technique provides ART fluorescence of ∼5cm in length in the unseeded air, presenting an attractive thermometry solution for high-speed wind tunnels and other ground test facilities.

Noncolinear optical parametric oscillator for broadband nanosecond pulse-burst CARS diagnostics in gases

Demonstration of broadband nanosecond coherent anti-Stokes Raman scattering (CARS) using a burst-mode-pumped noncolinear optical parametric oscillator (NOPO) has been achieved at a pulse repetition rate of 40 kHz. The NOPO is pumped with the 355-nm output of a burst-mode Nd:YAG laser at 50 mJ/pulse for 45 pulses and produces an output centered near 607 nm, with a bandwidth of 370 cm^-1 at energies of 5 mJ/pulse. A planar BOXCARS phase matching scheme uses the broadband NOPO output as the Stokes beam and the narrowband 532-nm burst-mode output for the two CARS pump beams for single-laser-shot nitrogen thermometry in near adiabatic H₂/air flames at temperatures up to 2200 K.

10 kHz 2D thermometry in turbulent reacting flows using two-color OH planar laser-induced fluorescence

10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate ∼5mJ/pulse at 282 nm and 286 nm to excite the Q₁(5) and Q₁(14) transitions of the A²Σ⁺←X²Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.

CH and NO planar laser-induced fluorescence and Rayleigh-scattering in turbulent flames using a multimode optical parametric oscillator

An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of ∼430nm when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at ∼215nm, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed CH₄-H₂-NH₃-air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent CH₄-H₂-NH₃ diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.

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.

100 kHz krypton planar laser-induced fluorescence imaging

Krypton planar laser-induced fluorescence (Kr PLIF) was demonstrated at a repetition rate of 100 kHz. To achieve this increased rate, a custom injection-seeded optical parametric oscillator was built to efficiently convert the 355 nm output of a high-energy, high-repetition-rate nanosecond burst-mode laser to 212.56 nm to excite Kr from the ground to the 5p[1/2]₀ electronic state. Successful tracking of flow structures and mixture fraction was demonstrated using detection speeds 100 times greater than previously attained with a femtosecond laser source. The increase in repetition rate makes time-resolved Kr PLIF relevant for high-speed flows in particular.

Tunable narrow-linewidth 226 nm laser for hypersonic flow velocimetry

We report on the development and application of a novel, to the best of our knowledge, all-solid-state tunable narrow-linewidth 226 nm UV laser system. The laser system consists of three parts: a tunable single-frequency Ti:sapphire 787 nm laser, a single-frequency long-pulse-width flattop-shaped 532 nm laser, and a nonlinear frequency transformation system. The 532 nm laser is a sum-frequency mixed with the second harmonic of the 787 nm laser to produce the 226 nm laser. The maximum output pulse energy at 226 nm is 3 mJ. Nitric oxide planar laser-induced fluorescence velocimetry is demonstrated in the China Aerodynamics Research and Development Center's FD14 hypersonic shock tunnel using this 226 nm laser system. It is proven that this laser is convenient for high-resolution molecular tagging fluorescence spectroscopy.

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.

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.

100-kHz Interferometric Rayleigh Scattering for multi-parameter flow measurements

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.

Simultaneous high-speed imaging of temperature, heat-release rate, and multi-species concentrations in turbulent jet flames

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.

10 kHz simultaneous PIV/PLIF study of the diffusion flame response to periodic acoustic forcing

Response of a laminar diffusion dimethyl-ether flame forced by an acoustic field is provided. A forcing frequency of 100 Hz, which is chosen based on the typical thermo-acoustic instability frequency in a practical combustor, is applied to the flame at a Reynolds number of 250. The development of the forced vortical structures present in this flame has been investigated utilizing a burst mode laser with a repetition rate of 10 kHz. Flame/vortex interaction is visualized by planar laser-induced fluorescence (PLIF) of formaldehyde, which is used to identify the early-stage fuel decomposition in the flame. The flame structure is also correlated with the velocity field, which is obtained utilizing particle imaging velocimetry (PIV). The resulting phase-resolved and time-averaged velocity and vortex images indicate that the amplitude of excitation has pronounced effects on the flame via modifying the local heat release.

Simultaneous high-speed SO2 PLIF imaging and stereo-PIV measurements in premixed swirling flame at 20 kHz

Interactions between flow structures and premixed swirling flame were investigated using simultaneous sulfur dioxide (SO₂) planar laser induced fluorescence (PLIF) and stereoscopic particle imaging velocimetry (PIV) with high temporal resolution at 20 kHz. In this work, a premixed swirling flame was operated with methane and air doped with 0.5% (volume fraction) SO₂ at ambient pressure under different equivalence ratios (ϕ=0.7-1.2). The results show that global SO₂ PLIF signal shows a consistent response to the density ratio with the change of equivalence ratio, making it a good indicator for the high temperature zone and a very useful tool to study the global effect of equivalence ratio. In addition, the three-component flow structure is affected by the varying equivalence ratio and the structure of the inner recirculation zone changes accordingly. The transient results show that the circumferential velocity of some vortices outside the flame zone is inconsistent with that of the main flow and these vortices cause local flame contour curling and shedding. The high temporal resolution measurements provide more details for the study of the evolution of some isolated flame isles.

Experimental investigation on an acoustically forced flame with simultaneous high-speed LII and stereo PIV at 20 kHz

An ethylene-air diffusion flame was acoustically forced with a frequency of 100 Hz at four amplitudes ranging from 40% to 140%. The average bulk velocity of the fuel was 0.6 m/s. The soot distribution and velocity fields were measured by simultaneous two-dimensional laser-induced incandescence (LII) and stereo particle image velocimetry (PIV) at 20 kHz laser repetition rate. The LII signal was calibrated by pulse-to-pulse laser energy variation, and it was observed that the soot regions extended along the central axis of the flame and shrank radially under acoustic forcing compared with the steady flame. The volume fraction of soot in the acoustically forced flame decreased with increased acoustic driving. In addition, the PIV results revealed that the resident time was strongly associated with the formation of an oval-shaped soot region, which was induced by external acoustic forcing.

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. Velocity measurements are assessed for accuracy and precision. Measurement errors vary in the range of 0.6-1.2%, while the instrument precision is found to lie between 1.2 m/s and 2 m/s and exhibits little variation over the full operating range of the facility. An examination of the signal intensity reveals little to no thermodynamic dependence, and the signal lifetimes exhibit an inverse dependence on both pressure and temperature. The PLEET signal is demonstrated to be largely unaffected by buoyancy despite the large temperature rise. The velocity dynamic range of the measurements is found to be a factor of at least 200 in these experiments with the capacity to measure much higher velocities as well. The spatial resolution of the velocity measurements is found to lie between 2 and 2.7 mm, and the maximum frequency response is 12.5 kHz with the ability to resolve up to 50 kHz with the current measurement system. Overall measurement uncertainties in the streamwise velocity are found to lie between 4% and 4.8% for high to low velocities, while the uncertainty in the transverse velocity is less than 6 m/s. The measurement uncertainties are found to be dominated by systematic errors in the calibration procedure, which could be improved in future experiments.

Generation of high-energy, kilohertz-rate narrowband tunable ultraviolet pulses using a burst-mode dye laser system

Typical commercial pulsed dye laser systems used in the generation of narrowband, tunable ultraviolet radiation for planar laser-induced fluorescence (PLIF) imaging are optimized for either high (∼5-10  kHz) repetition rates at comparatively low ultraviolet pulse energies (hundreds of microjoules) or high-output pulse energies (>10  mJ) at comparatively low repetition rates (∼10  Hz). In this work we use a frequency-doubled Nd:YAG burst-mode laser to pump a custom dye laser system for high pulse energies and repetition rates of 7.5, 10, and 20 kHz at 566 nm. The frequency-doubled output of over 2.2 mJ/pulse at 283 nm, which can be used for PLIF imaging of combustion radicals, is an order of magnitude higher per pulse energy as compared with continuously pulsed dye laser systems and is ∼3× higher in overall efficiency than a burst-mode optical parametric oscillator at similar wavelengths. The influence of repetition rate, pump energy, and dye concentration on the output conversion efficiency and pulse-to-pulse stability of the current system is discussed.

Compact burst-mode Nd:YAG laser for kHz-MHz bandwidth velocity and species measurements

A compact-footprint (0.18  m²) flash-lamp-pumped, burst-mode Nd:YAG-based master-oscillator pulsed-amplifier laser is reported with a fundamental 1064 nm output of over 14 J per burst. A directly modulated diode laser seed source is used to generate 10 ms duration arbitrary sequences of 500 kHz doublet or MHz singlet pulses for flow-field velocity or species measurements, respectively. Flexible pulse widths are used to balance the energy distribution of pulse doublets and achieve second-harmonic conversion efficiencies up to 42%. Burst-mode laser performance characteristics, measurement accuracies in turbulent flows, and prospects for kHz-MHz flow-field diagnostics are discussed.

Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.

1-kHz two-dimensional coherent anti-Stokes Raman scattering (2D-CARS) for gas-phase thermometry

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 O₂ jet to capture the spatiotemporal evolution of the temperature field.

Regenerative amplification and bifurcations in a burst-mode Nd:YAG laser

An Nd:YAG-based burst-mode regenerative amplifier laser was developed that offers high extraction efficiency at high repetition rates with low seed energies. The regenerative amplification technique, combined with the burst-mode laser technology, shows promise as an efficient method for amplification of femtojoule-nanojoule pulses up to millijoule energies at repetition rates exceeding 100 kHz. Output energies at repetition rates near the inverse upper state lifetime are limited by bifurcations in the pulse energies of the burst. A model is developed and advantages and limitations are discussed.

100-kHz-rate gas-phase thermometry using 100-ps pulses from a burst-mode laser

Temperature measurements based on gas-phase coherent anti-Stokes Raman scattering (CARS) spectroscopy are demonstrated in reacting flows at a rate of 100 kHz employing a burst-mode laser with a pulse duration of ∼100  ps. The recently developed picosecond-duration, high-energy burst-mode laser is used to pump an optical parametric generator/optical parametric amplifier that produces broadband light centered at ∼680  nm to provide the Stokes beams for excitation of the rovibrational Raman transitions of H(2). The 532-nm output of the picosecond burst-mode laser is then utilized as a pump beam for the CARS process that generates 100 single-shot spectra at a rate of 100 kHz during the 1-ms duration burst. Coherent spectroscopy-based temperature measurements at 100 kHz will significantly aid the understanding of transient and unsteady flow phenomena related to turbulent combustion, transonic and hypersonic flows, high-enthalpy flows, and the dynamics of energetic materials.

Effects of repetitive pulsing on multi-kHz planar laser-induced incandescence imaging in laminar and turbulent flames

Planar laser-induced incandescence (LII) imaging is reported at repetition rates up to 100 kHz using a burst-mode laser system to enable studies of soot formation dynamics in highly turbulent flames. To quantify the accuracy and uncertainty of relative soot volume fraction measurements, the temporal evolution of the LII field in laminar and turbulent flames is examined at various laser operating conditions. Under high-speed repetitive probing, it is found that LII signals are sensitive to changes in soot physical characteristics when operating at high laser fluences within the soot vaporization regime. For these laser conditions, strong planar LII signals are observed at measurement rates up to 100 kHz but are primarily useful for qualitative tracking of soot structure dynamics. However, LII signals collected at lower fluences allow sequential planar measurements of the relative soot volume fraction with a sufficient signal-to-noise ratio at repetition rates of 10-50 kHz. Guidelines for identifying and avoiding the onset of repetitive probe effects in the LII signals are discussed, along with other potential sources of measurement error and uncertainty.

100-ps-pulse-duration, 100-J burst-mode laser for kHz-MHz flow diagnostics

A high-speed, master-oscillator power-amplifier burst-mode laser with ∼100  ps pulse duration is demonstrated with output energy up to 110 J per burst at 1064 nm and second-harmonic conversion efficiency up to 67% in a KD*P crystal. The output energy is distributed across 100 to 10,000 sequential laser pulses, with 10 kHz to 1 MHz repetition rate, respectively, over 10 ms burst duration. The performance of the 100 ps burst-mode laser is evaluated and been found to compare favorably with that of a similar design that employs a conventional ∼8  ns pulse duration. The nearly transform-limited spectral bandwidth of 0.15  cm(-1) at 532 nm is ideal for a wide range of linear and nonlinear spectroscopic techniques, and the 100 picosecond pulse duration is optimal for fiber-coupled spectroscopic measurements in harsh reacting-flow environments.

100 kHz thousand-frame burst-mode planar imaging in turbulent flames

High-repetition-rate, burst-mode lasers can achieve higher energies per pulse compared with continuously pulsed systems, but the relatively few number of laser pulses in each burst has limited the temporal dynamic range of measurements in unsteady flames. A fivefold increase in the range of timescales that can be resolved by burst-mode laser-based imaging systems is reported in this work by extending a hybrid diode- and flashlamp-pumped Nd:YAG-based amplifier system to nearly 1000 pulses at 100 kHz during a 10 ms burst. This enables an unprecedented burst-mode temporal dynamic range to capture turbulent fluctuations from 0.1 to 50 kHz in flames of practical interest. High pulse intensity enables efficient conversion to the ultraviolet for planar laser-induced fluorescence imaging of nascent formaldehyde and other potential flame radicals.

Investigation of optical fibers for high-repetition-rate, ultraviolet planar laser-induced fluorescence of OH

We investigate the fundamental transmission characteristics of nanosecond-duration, 10 kHz repetition rate, ultraviolet (UV) laser pulses through state-of-the-art, UV-grade fused-silica fibers being used for hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) imaging. Studied in particular are laser-induced damage thresholds (LIDTs), nonlinear absorption, and optical transmission stability during long-term UV irradiation. Solarization (photodegradation) effects are significantly enhanced when the fiber is exposed to high-repetition-rate, 283 nm UV irradiation. For 10 kHz laser pulses, two-photon absorption is strong and LIDTs are low, as compared to those of laser pulses propagating at 10 Hz. The fiber characterization results are utilized to perform single-laser-shot, OH-PLIF imaging in pulsating turbulent flames with a laser that operates at 10 kHz. The nearly spatially uniform output beam that exits a long multimode fiber becomes ideal for PLIF measurements. The proof-of-concept measurements show significant promise for extending the application of a fiber-coupled, high-speed OH-PLIF system to harsh environments such as combustor test beds, and potential system improvements are suggested.

Large-aperture, tapered fiber-coupled, 10-kHz particle-image velocimetry

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.

Fiber-coupled, 10 kHz simultaneous OH planar laser-induced fluorescence/particle-image velocimetry

Planar laser-induced fluorescence (PLIF) and particle-image velocimetry (PIV) techniques that employ free-standing optics face severe challenges when implemented in harsh environments associated with practical combustion facilities because of limited optical access and restrictions on operation of sensitive laser systems. To circumvent this problem, we have developed and implemented a fiber-coupled, high-speed ultraviolet (UV) PLIF/PIV system for measuring hydroxyl radical (OH) concentration and velocity in a realistic 4 MW combustion rig. This system permits delivery of high-power, 10 kHz, nanosecond-duration OH-PLIF excitation pulses (283 nm) and PIV pulses (532 nm) through a common 6 m long, 600 μm core, deep-UV-enhanced multimode fiber. Simultaneous OH-PLIF and PIV imaging at a data-acquisition rate of 10 kHz is demonstrated in turbulent premixed flames behind a bluff body. The effects of delivering high-repetition-rate, intense UV and visible beams through a long optical fiber are investigated, and potential system improvements are discussed.

50-kHz-rate 2D imaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectral tomography

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.

All-diode-pumped quasi-continuous burst-mode laser for extended high-speed planar imaging

An all-diode-pumped, multistage Nd:YAG amplifier is investigated as a means of extending the duration of high-power, burst-mode laser pulse sequences to an unprecedented 30 ms or more. The laser generates 120 mJ per pulse at 1064.3 nm with a repetition rate of 10 kHz, which is sufficient for a wide range of planar laser diagnostics based on fluorescence, Raman scattering, and Rayleigh scattering, among others. The utility of the technique is evaluated for image sequences of formaldehyde fluorescence in a lifted methane-air diffusion flame. The advantages and limitations of diode pumping are discussed, along with long-pulse diode-bar performance characteristics to guide future designs.

Nitric-oxide planar laser-induced fluorescence at 10 kHz in a seeded flow, a plasma discharge, and a flame

This study demonstrates high-repetition-rate planar laser-induced fluorescence (PLIF) imaging of both cold (~300 K) and hot (~2400 K) nitric oxide (NO) at a framing rate of 10 kHz. The laser system is composed of a frequency-doubled dye laser pumped by the third harmonic of a 10 kHz Nd:YAG laser to generate continuously pulsed laser radiation at 226 nm for excitation of NO. The laser-induced fluorescence signal is detected using a high-frame rate, intensified CMOS camera, yielding a continuous cinematographic propagation of the NO plume where data acquisition duration is limited only by camera memory. The pulse energy of the beam is ~20 μJ with a spectral width ~0.15 cm(-1), though energies as high as 40 μJ were generated. Hot NO is generated by passing air through a DC transient-arc plasma torch that dissociates air. The plasma torch is also used to ignite and sustain a CH(4)/air premixed flame. Cold NO is imaged from a 1% NO flow (buffered by nitrogen). The estimated signal-to-noise ratio (SNR) for the cold seeded flow and air plasma exceeds 50 with expected NO concentrations of 6000-8000 parts per million (ppm, volume basis). Images show distinct, high-contrast boundaries. The plasma-assisted flame images have an SNR of less than 10 for concentrations reaching 1000 ppm. For many combustion applications, the pulse energy is insufficient for PLIF measurements. However, the equipment and strategies herein could be applied to high-frequency line imaging of NO at concentrations of 10-100 ppm. Generation of 226 nm radiation was also performed using sum-frequency mixing of the 532 nm pumped dye laser and 355 nm Nd:YAG third harmonic but was limited in energy to 14 μJ. Frequency tripling a 532 nm pumped dye laser produced 226 nm radiation at energies comparable to the 355 nm pumping scheme.

Ultrahigh laser pulse energy and power generation at 10 kHz

This Letter presents results from a new master-oscillator, power-amplifier pulse-burst laser system demonstrating ultrahigh pulse energies greater than 2.0 J/pulse at 1064 nm with interpulse separations of 100 μs (10 kHz) for burst durations of 100 pulses. Each pulse generates peak powers exceeding 130 MW and an average power of approximately 20 kW is generated over a 100-pulse-burst. Pulse energies decrease by less than 10% over a 100 sequential pulses, demonstrating negligible "droop" over long-duration pulse trains. Second-harmonic generation of 532 nm with conversion efficiency greater than 50% is demonstrated for 100-pulse-burst durations.

Investigation of optical fibers for gas-phase, ultraviolet laser-induced-fluorescence (UV-LIF) spectroscopy

We investigate the feasibility of transmitting high-power, ultraviolet (UV) laser pulses through long optical fibers for laser-induced-fluorescence (LIF) spectroscopy of the hydroxyl radical (OH) and nitric oxide (NO) in reacting and non-reacting flows. The fundamental transmission characteristics of nanosecond (ns)-duration laser pulses are studied at wavelengths of 283 nm (OH excitation) and 226 nm (NO excitation) for state-of-the-art, commercial UV-grade fibers. It is verified experimentally that selected fibers are capable of transmitting sufficient UV pulse energy for single-laser-shot LIF measurements. The homogeneous output-beam profile resulting from propagation through a long multimode fiber is ideal for two-dimensional planar-LIF (PLIF) imaging. A fiber-coupled UV-LIF system employing a 6 m long launch fiber is developed for probing OH and NO. Single-laser-shot OH- and NO-PLIF images are obtained in a premixed flame and in a room-temperature NO-seeded N(2) jet, respectively. Effects on LIF excitation lineshapes resulting from delivering intense UV laser pulses through long fibers are also investigated. Proof-of-concept measurements demonstrated in the current work show significant promise for fiber-coupled UV-LIF spectroscopy in harsh diagnostic environments such as gas-turbine test beds.

Characterization of a CH planar laser-induced fluorescence imaging system using a kHz-rate multimode-pumped optical parametric oscillator

The performance characteristics of a new CH planar laser-induced fluorescence (PLIF) imaging system composed of a kHz-rate multimode-pumped optical parametric oscillator (OPO) and high-speed intensified CMOS camera are investigated in laminar and turbulent CH4-H2-air flames. A multi-channel Nd:YAG cluster that produces up to 225 mJ at 355 nm with multiple-pulse spacing of 100 μs (corresponding to 10 kHz) is used to pump an OPO to produce up to 6 mJ at 431 nm for direct excitation of the A-X (0, 0) band of the CH radical. Single-shot signal-to-noise ratios of 82:1 and 7.5:1 are recorded in laminar premixed flames relative to noise in the background and within the flame layer, respectively. The spatial resolution and image quality are sufficient to accurately measure the CH layer thickness of ~0.4 mm while imaging the detailed evolution of turbulent flame structures over a 20 mm span. Background interferences due to polycyclic-aromatic hydrocarbons and Rayleigh scattering are minimized and, along with signal linearity, allow semi-quantitative analysis of CH signals on a shot-to-shot basis. The effects of design features, such as cavity finesse and passive injection seeding, on conversion efficiency, stability, and linewidth of the OPO output are also discussed.

Kr-PLIF for scalar imaging in supersonic flows

Experiments were performed to explore the use of two-photon planar laser-induced fluorescence (PLIF) of krypton gas for applications of scalar imaging in supersonic flows. Experiments were performed in an underexpanded jet of krypton, which exhibited a wide range of conditions, from subsonic to hypersonic. Excellent signal-to-noise ratios were obtained, showing the technique is suitable for single-shot imaging. The data were used to infer the distribution of gas density and temperature by correcting the fluorescence signal for quenching effects and using isentropic relations. The centerline variation of the density and temperature from the experiments agree very well with those predicted with an empirical correlation and a CFD simulation (FLUENT). Overall, the high signal levels and quantifiable measurements indicate that Kr-PLIF could be an effective scalar marker for use in supersonic and hypersonic flow applications.

High-speed CH planar laser-induced fluorescence imaging using a multimode-pumped optical parametric oscillator

We report on high-speed CH planar laser-induced fluorescence (PLIF) imaging in turbulent diffusion flames using a multimode-pumped optical parametric oscillator (OPO). The OPO is pumped by the third-harmonic output of a multimode Nd:YAG cluster for direct signal excitation in the A-X (0,0) band of the CH radical. The lasing threshold, conversion efficiency, and linewidth are shown to depend on the number of pump passes in the ring cavity of the OPO. Single-shot CH PLIF images are acquired at 10 kHz with excitation energy up to 6 mJ/pulse at 431.1 nm. Signal-to-noise ratios of ~25-35 are the highest yet reported for high-speed CH PLIF.