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Laser-based sensor probes velocity

16 May 2005

A high-resolution velocity profile sensor developed in Germany could improve the design of airplane turbines.

Scientists have developed a compact, laser-based velocity profile sensor with sub-micron resolution to study turbulent flow. The team from the Technical University of Dresden use two superposed fan-like fringe systems, one convergent and the other divergent, to boost the resolution of their laser Doppler apparatus by two orders of magnitude. (Applied Optics 44 2274)

Conventional laser Doppler anemometry systems measure an averaged velocity in a small probe volume, where the spatial resolution is defined by the amount of overlap between two intersecting laser beams. Typically, the measurement volume is 0.1 x 0.1 x 1 mm with a resolution of around 50 µm.

"Our method is different [because] we additionally measure the position of tracer particles along the optical axis inside this volume of intersection," researcher Lars Büttner told Optics.org. "This enhances the spatial resolution to the submicrometer range."

Featuring two fiber-coupled laser diodes emitting at 660 and 825 nm, the sensor uses a special arrangement of highly-dispersive and low-dispersive optics to create two superposed interference fringe systems. Analyzing the light scatter from tracer particles reveals their velocities. "By evaluating a number of particles, the complete velocity profile in the probe volume can be recovered," explains Büttner.

Using its sensor, the team achieved a spatial resolution of 650 nm in the center of the measurement volume. According to Büttner, the device is potentially very convenient and compact with all the components fitting into a 1 cm diameter tube.

In the near future, the researchers plan to use their system, which suits the investigation of turbulent boundary layers, to probe the flow around a turbine blade.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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