Femtosecond Laser-Based Molecular Tagging Technology for Non-Intrusive Measurement of Gas Flow and Properties

Background

The field of gas flow diagnostics is fundamental to a wide range of scientific and engineering applications, including aerodynamics, combustion, atmospheric science, and industrial process monitoring. Precise measurement of velocity profiles and scalar properties such as temperature, pressure, and species composition is essential for understanding and optimizing fluid dynamics in these contexts. Traditional measurement techniques, such as pitot tubes and hot-wire anemometry, are often intrusive, potentially disturbing the flow and limiting their applicability, especially in high-speed or reactive environments. Non-intrusive optical methods, such as laser-based velocimetry and molecular tagging, have thus become increasingly important, offering the potential for high-resolution, real-time measurements without physical interference with the flow.

Despite advances in optical diagnostics, current approaches face several significant limitations. Many established techniques, such as particle image velocimetry (PIV) and laser Doppler velocimetry (LDV), require the introduction of seed particles into the flow, which can be problematic in clean or reactive environments and may not accurately follow the gas motion, especially in regions of high acceleration or temperature gradients. Earlier molecular tagging methods, while eliminating the need for seeding, often rely on single-photon excitation and produce relatively short-lived or weak fluorescence signals, restricting temporal resolution and measurement accuracy. Additionally, these techniques may be limited in their ability to probe scalar properties or to generate complex tagging patterns for three-dimensional flow mapping. As a result, there remains a pressing need for diagnostic solutions that can provide non-intrusive, high-fidelity, and versatile measurements of both velocity and scalar fields in a wide variety of gas flows.

Technology

Femtosecond Laser Electronic Excitation Tagging (FLEET) is an advanced diagnostic technique that utilizes ultrashort femtosecond laser pulses to create persistent fluorescent tags within a gas flow. By inducing multiphoton excitation, dissociation, and ionization in molecules such as nitrogen and oxygen, FLEET generates well-defined fluorescent lines, grids, or three-dimensional patterns directly in the gas without requiring any seeding particles. These patterns are imaged using a fast-gated camera, allowing researchers to track their displacement over microsecond timescales and thus determine velocity profiles with exceptional spatial and temporal resolution. The system can be further enhanced by applying microwave radiation, which couples energy into the tagged regions via residual ions and electrons, significantly increasing the brightness and longevity of the fluorescence. Additionally, spectral analysis of the emitted light enables simultaneous measurement of scalar properties like temperature, density, pressure, and species composition.

What differentiates FLEET is its unique combination of non-intrusiveness, high resolution, and versatility. Unlike traditional velocimetry methods that require physical probes or seeding with foreign particles, FLEET operates directly on the native gas molecules, preserving the integrity of the flow and enabling measurements in environments where seeding is impractical or impossible. The use of femtosecond laser pulses allows for multiphoton processes that produce long-lived, broadband fluorescence, making it possible to write and track intricate patterns for comprehensive flow mapping, including multi-component velocity vectors and turbulence statistics. Microwave enhancement extends the observation window and boosts signal strength, while spectral analysis adds the capability to extract detailed thermodynamic information. This integrated approach positions FLEET as a powerful tool for studying complex gas dynamics, turbulence, and reacting flows with unprecedented accuracy and flexibility.

Benefits

• Non-intrusive measurement of gas velocity profiles without the need for seeding particles
• High spatial (∼100 μm) and temporal (sub-microsecond) resolution for precise flow characterization
• Capability to create complex tagging patterns (lines, grids, 3D structures) enabling multi-component velocity and turbulence measurements
• Enhanced fluorescence brightness and extended observation time through microwave radiation coupling
• Direct measurement of scalar properties such as temperature, pressure, density, and species composition from fluorescence spectra
• Applicable to arbitrary gas mixtures, with particularly strong signals in pure nitrogen
• Allows instantaneous, high-fidelity mapping of velocity and scalar fields for advanced fluid dynamics research
• Surpasses traditional velocimetry techniques by leveraging femtosecond laser pulses for multiphoton excitation and long-lived fluorescence

Applications

• High-speed gas flow diagnostics
• Turbulence and vorticity measurement
• Non-intrusive combustion analysis
• Temperature and density mapping
• Species composition detection in gases

Additional info

Femtosecond Laser Electronic Excitation Tagging (FLEET) employs ultrashort laser pulses to induce multiphoton excitation in gas molecules, creating persistent fluorescent tags. A fast-gated camera images these tags to track their displacement, precisely determining gas velocity profiles. Fluorescence spectra also yield scalar properties. This technique operates without seeding, offers high spatial and temporal resolution, and can be enhanced by microwave radiation.