Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging velocimetry

Abstract

Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging (STARFLEET), a nonseeded ultrafast-laser-based velocimetry technique, is demonstrated in reactive and nonreactive flows. STARFLEET is pumped via a two-photon resonance in N₂ using 202.25 nm 100 fs light. STARFLEET greatly reduces the per-pulse energy required (30 μJ/pulse) to generate the signature FLEET emission compared to the conventional FLEET technique (1.1 mJ/pulse). This reduction in laser energy results in less energy deposited in the flow, which allows for reduced flow perturbations (reactive and nonreactive), increased thermometric accuracy, and less severe damage to materials. Velocity measurements conducted in a free jet of N₂ and in a premixed flame show good agreement with theoretical velocities, and further demonstrate the significantly less intrusive nature of STARFLEET.

Krypton Tagging Velocimetry (KTV) Investigation of Shock-Wave/Turbulent Boundary-Layer Interaction

“…In contrast to the limitations of implementing PIV techniques in high-speed facilities, the implementation of tagging velocimetry is not limited by timing issues associated with tracer injection 41 or reduced particle response at Knudsen and Reynolds numbers 42 typical of high-speed wind tunnels. Noted methods and tracers of tagging velocimetry include VENOM, [44][45][46][47][48] APART, 49-51 RELIEF, [52][53][54][55][56] FLEET, 57,58 STARFLEET, 59 PLEET, 60 iodine, 61, 62 acetone, [63][64][65] and the hydroxyl group techniques, 66-68 among others. [69][70][71][72][73] In this paper, we are attempting to establish KTV as a viable method of measurement in the complex flow of shock-wave/turbulent boundary-layer interaction.…”

Rayleigh Scattering Density Measurements from Ultrafast Lasers in High-Pressure, Cryogenic Wind Tunnels

“…Another strategy, though more complicated, could be to use a different harmonic of the fs-laser beam; the (usual) fundamental wavelength of 800 nm is close to several of the FLEET emission lines, invariably leading to contamination of the FLEET signal. If, alternatively, a much shorter wavelength were used for excitation (for instance, 202 nm used for the STARFLEET technique 18 ), much of this secondary emission could be avoided and the scattering signal could potentially be isolated from the FLEET signal, allowing a coincident measurement to be made. Although the differential Rayleigh scattering cross-section is significantly larger at this wavelength, the quantum efficiency of many image sensors decreases precipitously below 300 nm, so the increase in signal intensity would likely be mitigated to an extent.…”

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

“…However, FLEET is a non-resonant process, as shown by the red transition of Figure 8. FLEET process requires absorption of at least eight photons to overcome the 9.8 eV [135] dissociation energy of N 2 . As a result, in order to induce multi-photon dissociation, the femtosecond laser with an energy of several mJ is needed, which can photo-dissociate many other species, heat the probing volume, or alter local chemistry.…”

“…One way to avoid these problems is to use low laser pulse energy. Therefore, selective two-photon absorptive resonance FLEET (STARFLEET) approach was developed by Jiang et al [135][136][137][138], which significantly reduces the per-pulse energy to 30 µJ. As shown by the blue transition in Figure 8, STARFLEET is designed to exploit the resonant excitation of the N 2 a 1 Σ + g ← X1Σ + g transition via two-photon absorption at ∼202.25 nm.…”