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table-tops

Optical table

An optical table is a piece of equipment used for optics experiments and engineering.

In optical setups, especially those involving interferometry, the alignment of each component must be extremely accurate—precise down to a fraction of a wavelength—usually a few hundred nanometers. Even small vibrations or strain in the table on which the elements are set up might lead to complete failure of an experiment. Hence, one requires an extremely sturdy table which neither vibrates nor flexes, even under heavy loads. The surface of the table must also be quite flat, to allow precision optical mounts to make good contact with the table without rocking.

Modern optical tables are typically made of steel with a thick honeycomb lattice structure. The surface usually has a grid of 1/4"-20 or M6 threaded holes which allow the components to be bolted down so they cannot move even a few nanometers. Components may also be held to the steel surface by magnets in the base of the optical mounts. Often, the table's legs are pneumatic vibration dampers. For even more accurate setups, one also prevents air movements and temperature gradients by enclosing the surface in a box of transparent plastic such as Plexiglas. One may also use a "flowbox", a device which produces a laminar stream of air flowing downwards, kept at constant temperature by special air conditioning.

Earlier optical table tops were sometimes made of a large slab of highly polished granite. The heavy, dense material strongly damped vibrations producing a very stable surface for precision optics. Granite (like wood) damps vibrations because it is a composite material. Such tables were very heavy and expensive, however, and are not commonly available today.

The metal used to construct modern optical tables has a higher speed of sound than granite and therefore a higher frequency of the first eigenmode. Any vibration produced on the table below this frequency does not produce a resonant response, making the setup less sensitive to vibrations from motorized optics, cooling water, etc. Vibration damping may be added to tables during their construction. As with granite's composite structure, the combination of several stiff materials with different speeds of sound produces a table for which a wide range of vibrations are critically damped. Viscous fluids are used in between the stiff materials, to aid in damping.

Breadboards

A cheaper alternative to an optical table is an optical breadboard—a steel honeycomb panel with 1/4"-20 or M6 threaded holes that can be placed on an ordinary table or workbench. Breadboards are not as good as optical tables, but are much cheaper and are adequate for many optics experiments that do not require extremely low levels of vibration. The low weight enables one to support these tables on soft air springs which reduce vibrations coming from the floor, although this increases vibrations due to acoustic noise.

The honeycomb structure reduces bending due to the breadboard's own weight, so it can be tilted and forces applied via the soft spring supports accelerate the table as a whole without misalignment. Breadboards can therefore be used in mobile applications, such as on airplanes. One can bolt a breadboard onto an optical table, build up a module of the experiment on it, and then transfer the module as a whole onto another table without the need to realign the components on the breadboard. Similarly, custom-built optical devices are assembled and aligned on breadboards, which are then enclosed in a case and shipped to the customer.

Rails and benches

An optical bench or optical rail is a less sophisticated piece of equipment used for simple experiments, especially for classroom demonstrations. It is a long, straight, sturdy rail of steel onto which components such as light sources and lenses can be bolted down and easily shifted along the length of the rail.

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