Femtosecond laser technique measures velocities in hypersonic arc-jet flows

Researchers have demonstrated femtosecond laser tagging velocimetry, a new optical method for measuring extremely fast, turbulent flows in hypersonic arc-jet wind tunnels without inserting disruptive probes. The technique uses ultrashort laser pulses to “tag” molecules already in the gas, creating a pattern that can be imaged after a known delay. By tracking the pattern’s displacement, scientists calculate velocity with high precision. The study, published in Communications Engineering, targets longstanding challenges in hypersonics diagnostics, where conventional tools like Pitot tubes, hot-wire anemometers, or pressure taps can fail under extreme heat, shocks, and reactive chemistry. The femtosecond approach avoids particle-lag issues seen in seeding-based particle image velocimetry. The paper describes how previous molecular tagging approaches relied on oxygen excitations or water-vapor photodissociation, and the new work extends the concept into the femtosecond regime.