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Structured laser beam shines light on aerosol particles

27 Jun 2007

A new optical technique measures both the velocity and trajectory of particles in an air stream, allowing real-time detection of contaminants or chemicals.

The method, developed by William Herzog and a team from Lincoln Laboratory at MIT, uses a spatial pattern of light projected by a structured laser beam (SLB). The four separate beams of the illumination pattern are precisely oriented, so that as a particle moves across the beams both its trajectory and velocity can be measured (Applied Optics 46 3150).

The structured beam is generated by shining a 100 mW single-mode fiber-coupled 808 nm laser onto a 0.050 mm thick air-slot mask made from NiCo. The mask has four slits, each of which is 0.15 mm high and 1.5 mm wide, resulting in four individual beams of carefully controlled orientation (see diagram).

Two beams are parallel and separated by 300 µm, allowing measurement of a particle's time-of-flight between them and calculation of its velocity. The other two beams are angled so as to have a variable spacing relative to the parallel beams. One is created by an angled slit in the mask and is positioned between the parallel beams, registering the x axis position of a particle, while the other is deviated by 2° using a microprism and lies above the other three beams, registering the z axis. A photomultiplier aligned at 90° to the SLB collects the elastically scattered light from the particles as they cross each beam, sampled at 1 MHz.

"This approach is a development of a well-established technique," Herzog explained to optics.org. "It's already known that a continuous-wave laser can tell you whether a particle is present or not in an air stream, but this goes beyond that. It combines particle velocimetry with position detection, and the structured beam is designed such that all possible particle trajectories result in a unique signature."

There are some assumptions involved. The velocity measurement is only strictly valid for particles moving parallel to the air stream at a constant speed, and is also affected by the accuracy with which the spacing between the two parallel beams is known. The data acquisition system and its signal-to-noise ratio also directly influences all the SLB measurements.

Experiments so far have been conducted using 3 µm polystyrene latex spheres, although the team recognize that in the real world aerosol particles can be more complex. "The test spheres are uniform and precisely shaped, which real aerosols frequently are not, but this meant we could be sure that any effect we observed was an authentic experimental result rather than a fluctuation in the material," said Herzog.

The technique could easily be extended to measure additional kinetic variables by using a SLB with additional features. The dynamic motion of a particle, including acceleration from external forces, could be measured by collecting data from more than one location along a single beam.

"The method is fairly simple to operate and certainly adaptable," said Herzog. "Moving the relevant beam spacing will change the accuracy with which velocity is measured, for example. This SLB approach has changed our group's approach to particle detection, and it's relatively inexpensive too."

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