Optical Flats

Optical flats are plates with one or two surfaces of particularly high optical quality and flatness. The degree of flatness is quantified by the distance between two parallel imaginary planes, where one is tangential to the highest point of the surface and the other one to the lowest point. That flatness deviation is normally specified as λ/10 or better (i.e., a smaller value), and sometimes even substantially better — e.g. down to a few nanometers. The surface roughness is also usually very low.

Optical flats are mainly used as highly flat reference surfaces in interferometers for checking the flatness of optical elements such as optical windows, laser mirrors, prisms, optical filters or laser crystals. For example, one may place an optical flat close to an inspected surface, but slightly tilted against that surface, so that surfaces with ideal quality would result in a regular pattern of straight interference fringes when the assembly is illuminated with monochromatic light. Any deviations from that pattern then indicate deviations from perfect flatness.

Optical flats are fabricated in several steps, essentially in the same way as mirror substrates.

With further production steps, such as the application of a dielectric coating, optical flats also be turned into high-quality optical elements themselves. For example, a Twyman–Green interferometer requires a very flat reference mirror, which can be made from an optical flat.

For checking the quality of optical flats, one may inspect them in interferometers, ideally using an even better reference surface. In some cases, one uses reference surfaces based on a liquid like mercury, which can be extremely flat but is difficult to handle.

Typically, an optical flat has a cylindrical shape, a diameter of a couple of centimeters, a thickness of a few millimeters, and is made of fused silica or some other clear optical glass. It thus looks like a mirror substrate. However, different types of optical flats are available:

• Some of them are very large, with diameters of tens of centimeters, suitable for testing very large surfaces.
• Different optical materials can be used. Fused silica is very common, since it is mechanically quite hard and chemically stable, and it exhibits a relatively small coefficient of thermal expansion (0.55 · 10⁻⁶ K⁻¹). However, one can also use glass ceramics (e.g. Zerodur) with even several times lower thermal expansion coefficients, if that is of interest. For testing infrared optics, for example, various other materials can be used, for example germanium, silicon, zinc selenide (ZnSe) and sapphire.

Different degrees of surface quality can be obtained. A typical flatness specification would be λ/10, but even higher qualities are available. Usually, the surface quality should be substantially better than the specifications of the objects to be tested, so that obtained deviations from flatness are largely dominated by the quality of those objects.

To maintain their specified flatness during use, optical flats must possess sufficient mechanical stiffness to resist gravitational sagging or deformation from mounting forces. A common rule of thumb is a thickness-to-diameter ratio (aspect ratio) of at least 1:6 for larger flats, although 1:5 is preferred for highest precision. Thinner flats may be cheaper but require careful support to avoid bending that invalidates the flatness specification.

Even optical flats made from a robust material such as fused silica should be handled with great care to avoid any surface damage. For example, one should always store optical flats in a suitable box, in which it is wrapped in a soft material.

During interferometric measurements, both the optical flat and the tested surface should be very clean; see the article on cleaning of optics.