Boris S. Leonov

This Letter describes, to the best of our knowledge, the first application of an Nd:YAG-pumped diamond Raman laser (DRL) for OH planar laser-induced fluorescence (PLIF). DRL was used to convert 532 nm pump radiation into 573 nm output, corresponding to the 1 ^st Stokes Raman shift, via the stimulated Raman scattering process in a diamond crystal, with an achieved conversion efficiency of 52%. The 573 nm output was then doubled to generate the UV radiation targeting the Q ₁ (14) OH transition near 286.5 nm. The overall conversion efficiency from 532 nm pump to 286.5 nm UV output was recorded at 5%. This demonstration was performed in methane/air combustion produced by a Hencken burner. Single-shot OH PLIF images were acquired in premixed “flat" flame and diffusion flame configurations with 160 µJ UV pulse energy and 5 cm sheet height. Overall, the DRL proved to be a promising and easy-to-implement alternative for combustion diagnostics and OH spectroscopy.

Using only a pulsed laser and an imaging system, the molecular-based optical diagnostic of femtosecond laser electronic excitation tagging (FLEET) provides a simple and yet powerful tool for monitoring and quantifying high-speed flow dynamics, without requiring the introduction of particles or foreign species into the flow. FLEET is typically operated for measurements at continuous repetition rates up to 1 kHz, limited by the trade-off between pulse energy and repetition rate of commercially available Ti:sapphire femtosecond lasers delivering millijoule pulse energies at kHz rate. Higher temporal sampling, up to the MHz range, has only been achieved using burst-lasers or high-repetition rate imaging systems, albeit with an inherently discrete temporal coverage at the much lower burst repetition rate. Herein, using a Ludwieg tube facility as a benchmark case, we demonstrate 100 kHz continuous repetition rate FLEET velocimetry in supersonic nitrogen flow across the entire operating envelope. This capability provides a two order of magnitude increase in temporal sampling of flow dynamics over previous reports. The microjoule-range pulse energy delivered by the 100 kHz optical source was efficiently harvested for generating and tracking tracer molecules in the flow and high precision velocimetry, by using higher-energy photons, thereby avoiding plasma filamentation and preserving the non-intrusive nature of measurements. The comparison with 500 Hz measurements performed under the same tunnel operating conditions highlights the benefit of the higher temporal sampling in capturing the steady and unsteady flow dynamics throughout the wind tunnel operating time of ≈ 700 ms , producing unique experimental data for Ludwieg tube characterization. The substantial enhancement of FLEET flow velocimetry capability paves the way for high-resolution dynamic measurements of turbulence, shock-waves, and boundary-layer transitions, providing insight into the fundamental physics and computational models validation, crucial for many aerospace applications.

This Letter presents the first, to the best of our knowledge, use of the Nd:YAG fifth harmonic for quantitative absorption line broadening measurements via frequency-scanning planar laser-induced fluorescence (FS-PLIF). This measurement is performed in a combustion environment generated with an oxy-propane torch. Using a harmonic of the Nd:YAG for PLIF removes the need for complex wavelength conversion equipment and greatly simplifies the optical setup while providing higher pulse energies than other conventional systems. Frequency scanning is performed for measurement rates of up to 20 kHz with a 500 kHz laser repetition rate and an average pulse energy of 1.7 mJ at 213 nm. The measured average linewidth of absorption features equals 0.366 cm ⁻¹ , corresponding to an estimated temperature of 2200 K. Predicted 2D temperature profiles show good agreement with CFD-simulated results.

This article presents the design and detailed characterization of a new supersonic wind tunnel at the Aerospace Laboratory for Lasers, ElectroMagnetics, and Optics of Texas A&M University, tailored for optical diagnostic development and sub-scale fundamental compressible fluid dynamics research. A Ludwieg tube tunnel architecture was selected due to its robustness, versatility, and low operational costs. The tunnel consists of a 50-foot-long driver tube constructed from modular Tri-Clamp spools, a Mach 4 nozzle with 3 in. exit diameter configured as a free jet, and a fast-acting valve with 14 ms opening time for high-duty-cycle operation. Such construction proved to be a robust, compact, and affordable solution for academic applications. Characterization methods consisted of simultaneous high-speed dot-schlieren, total and static pressure measurements, and femtosecond laser electronic excitation tagging. Average flow velocity for the first steady-state test time was measured via FLEET at (668.0 ± 5.7) m/s. The Mach number was calculated based on the angles of the attached oblique shocks formed near the 30° cone model. Calculated Mach number was repeatable from run to run and had small oscillations near the average value of 3.96 ± 0.03. Based on the simultaneously measured velocity and Mach number, the static temperature was calculated to be between (68.6 ± 0.3) K and (66.3 ± 0.3) K throughout the 400 ms test time, completely defining the thermodynamic state of the generated freestream flow.

In this article, we introduce high-repetition-rate planar laser-induced fluorescence of sodium using a UV/IR scheme in which the 3s-4p transition at 330 nm is excited, followed by detecting fluorescence from the 3d-3p intermediate transition at 819 nm for qualitative imaging of flame structure. This approach avoids background interference from strong sodium emission from the flame at 589 nm by using strong long-pass filters. A flame was rendered in three dimensions (3D) by sweeping the laser sheet across it, highlighting the utility of the UV/IR excitation/detection scheme in resolving complex structures. The ethanol-based flame was mixed with water, and NaCl salt was dissolved to provide the atomic sodium necessary for the flame fto be excited by the laser. The success of filtering for background removal assisted the production of high-resolution 3D renderings of a "frozen" flame, which were possible due to the 250 kHz repetition rate of the laser.

This article presents the first demonstration of the tunable 5th harmonic of a Nd:YAG pulse-burst laser used for single-shot velocity and acceleration measurements via Nitric-oxide ionization induced Flow Tagging and Imaging. This demonstration was performed in a highly transient hypersonic flowfield created by injecting a supersonic jet perpendicularly to the main hypersonic flow in the Actively Controlled Expansion Tunnel at Texas A&M University. Overall, this new method of resonant UV-light excitation leading to the generation of IR and near IR emission for velocimetry purposes performed well, with spatially resolved single-shot velocities measured at 200 kHz at multiple locations. Additionally, the demonstration was successfully completed in a low-enthalpy hypersonic flow with a static temperature of 60 K, suggesting effective 5th harmonic energy coupling despite the low population of high rotational states of nitric oxide at those temperatures via quasi-resonant two-photon ionization.

This article presents the first demonstration of the tunable 5th harmonic of a Nd:YAG pulse-burst laser used for single-shot velocity and acceleration measurements via Nitric-oxide ionization induced Flow Tagging and Imaging. This demonstration was performed in a highly transient hypersonic flowfield created by injecting a supersonic jet perpendicularly to the main hypersonic flow in the Actively Controlled Expansion Tunnel at Texas A&M University. Overall, this new method of resonant UV-light excitation leading to the generation of IR and near IR emission for velocimetry purposes performed well, with spatially resolved single-shot velocities measured at 200 kHz at multiple locations. Additionally, the demonstration was successfully completed in a low-enthalpy hypersonic flow with a static temperature of 60 K, suggesting effective 5th harmonic energy coupling despite the low population of high rotational states of nitric oxide at those temperatures via quasi-resonant two-photon ionization.

This article presents, to the best of our knowledge, the first demonstration of burst-mode nitric oxide (NO) planar laser-induced fluorescence measurement of a reacting environment via Nd:YAG 5th harmonic excitation. A tunable pulse-burst laser generated 1 ms bursts of resonant 213 nm radiation at 200 kHz, 500 kHz, and 1 MHz repetition rates with 16 mJ, 3.5 mJ, and 0.35 mJ average pulse energies, respectively. Nitric oxide, naturally formed in the vicinity of an oxy-acetylene flame, was targeted via three excitation schemes: γ(1, 0), γ(1, 0) + β(5, 1), and β(4, 1). The generated fluorescence was collected in two separate spectral windows to demonstrate the selectivity of the electronic excitation and explore new filtering approaches for high background luminosity environments: 240 nm–260 nm and 275 nm–375 nm. Overall, this new method of UV-light generation for NO excitation performed very well with a substantial amount of collected signal even with modest energy levels accessible at a 1 MHz rate. The primary advantage of utilizing the 5th harmonic to access NO resonances was its simplicity and robustness through direct burst mode laser up-conversion and simplified master oscillator, power amplified (MOPA) frequency control, obviating the need for an optical parametric oscillator.