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The Photonics Spotlight

Lasers Attract Dust to Cavity Mirrors

Dr. Rüdiger Paschotta

At least in visible laser beams, one can often easily observe dust particles captured in the beam focus. They are dancing around, and cannot escape from the focus as long as no air current blows them away. A few hundred milliwatts e.g. in a green laser beam are sufficient. The effect results from light forces on the dust particles, creating a sufficiently deep potential, combined with friction in air, which dissipates the excess energy. The particles experience a dipole force toward the regions with highest optical intensity, i.e. not only toward the beam axis, but also toward the beam waist, even though the gradient in that direction is normally much smaller, so that it can take a while to get there.

The less than funny aspect of this phenomenon is of course that this effect can systematically push dust particles to cavity mirrors and capture them there until they are burned in. This leads to a steady performance degradation or even failure of the laser. Note that the effect also occurs in infrared lasers, even though it may not be as obvious there.

The best solution is of course to have the laser setup in an air-tight enclosure, and to do the assembly and any maintenance (with opened enclosure) in a clean room, or at least below a flow box. Particularly for maintenance, this can of course be hard to arrange, if the laser is operated in a not very clean environment.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page, because its location is permanent. See also the Encyclopedia of Laser Physics and Technology.

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