Optical Beam Dumps
An optical beam dump is an optical element used to absorb a beam of light. Major design concerns in a beam dump typically include the management and reduction of back reflections and scattering as well as the dissipation of heat generated by absorption. For low-power systems and less demanding applications, the device can be as simple as a piece of black velvet or flock paper glued onto a stiff backing, but higher-power beam dumps must often incorporate more elaborate features to avoid back-reflection, overheating, or excessive noise. Dumping the beam with a simple flat surface may scatter unacceptably large amounts of light for some applications, even though the direct reflection may be effectively reduced. To minimize scattering, it is common to use deep, dark cavities lined with an absorbing material to dump the beam. A particularly simple and relatively inexpensive approach is to use a stack of razor blades with the sharp edges facing the beam, so that the spaces between the blades form very deep cavities from which little light escapes.
A commonly available type of beam dump suitable for most medium-power lasers is a cone of aluminum with greater diameter than the beam, anodized to a black color and enclosed in a canister with a black, ribbed interior. Only the point of the cone is exposed to the beam head-on; mostly, incoming light grazes the cone at an angle, which eases performance requirements. Any reflections from this black surface are then absorbed by the canister. The ribs both help to make light less likely to escape, and improve heat transfer to the surrounding air.
Extremely high-power beam dumps have been made using water with controlled amounts of colored salts (e.g., copper(II) sulfate) to give a moderate absorbance of the beam. The water is circulated through a long pipe with a window at one end, and chilled using a heat exchanger.
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