Optical Fabrication

The article on optical glasses explains what kind of glass materials are typically used in optics. In this section, we assume inorganic glasses, distinguished from organic glasses as used for plastic optics.

Optical glasses are fabricated basically by mixing a number of purified compounds in the form of a powder, heating them to produce a glass melt, and slowly cooling them until solidification is achieved. For a high optical quality, highly purified materials are used. Also, they need to be thoroughly mixed to avoid inhomogeneities. The contamination with unwanted substances from any containers needs to be avoided.

A very slow and well controlled cooling process needs to be applied until complete solidification is achieved. During that cooling, the glass can anneal, which means that internal mechanical stress is largely removed. The final density and the resulting refractive index of the glass substantially depend on the annealing rate, which may e.g. be 2 K/h (fine annealed glass) or sometimes even substantially slower. The annealing rate should not only be low enough, but consistently applied for a particular type of optical glass.

Manufacturers often supply optical glasses in the forms of blocks, i.e. as block glass. Other supply forms of raw optical glasses, from which optical components such as lenses or prisms can be made, are large strips and rods. Some suppliers also offer processed forms like plates, cut prisms or pressed blanks.

A first step after glass production is the fabrication of blanks, i.e., pieces with roughly the right dimensions and some oversize of e.g. about 1 mm. For volume production, such optical blanks may be produced by embossing or molding processes. For small volumes, one often uses cutting or trepanning (core drilling) from larger glass blocks (block glass delivered by a glass manufacturer).

Special glass cutting machines are available, which typically contain a circular diamond saw. Its blade contains tiny diamonds which are incorporated in a steel core, for example. The diamond particles act as a very hard abrasive material, which is substantially more resistant than typical optical glasses. By using relatively thin blades in a carefully controlled process (e.g. with well defined linear movement of the blade), one can cut glasses with high reliability (i.e., without breaking or other otherwise damaging the glass) and with small loss of material. During the sawing process, the blade is typically cooled with clean water, which also helps to remove the glass dust and thus to avoid inhalation of tiny particles by operators.

After cutting, one often applies some kind of curve generator for preparing plane or curved surfaces, which come closer to the final shape, e.g. within 0.1 mm. The purpose of generation is essentially that of mechanical machining, but due to the brittle nature of glasses, adapted processing methods need to be applied.

The details depend on the type of optical component. For the generation of lenses, for example, one typically uses cup-shaped grinding wheels and a lip of diamond grit which is held in a metal or resin matrix. Such a process naturally produces spherical surfaces, which can be convex or concave. Our processes are required for producing aspheric lenses. For example, one may apply diamond turning.

Even a carefully controlled generation process causes some damage at and slightly below the glass surface. Therefore, and not only to reach the final surface accuracy, the top layer of the surface needs to be removed with suitable lapping methods, i.e., with refined kinds of grinding.

One typically starts with loose abrasive lapping for removing the damaged surface part and matching the required surface shape within a few micrometers (equivalent to several optical wavelengths). Here, the material removal rate is relatively high. In several following steps, mostly with pellet lapping one can then apply finer and finer lapping methods (with much lower removal rates). Essentially, and subsequent steps one uses finer and finer lapping agents.

Next, optical polishing is applied until the required optical surface quality is achieved, e.g. with deviations below λ/10 Polishing is a process similar to lapping and can be done with the same kind of machinery, but instead of a diamond grit one typically uses cerium oxide powder in a synthetic pad as a polishing agent.

In some cases, one applies special superpolishing methods for achieving a particularly high surface quality.

Optical components also often need to be edged, which means that the peripheral regions are properly processed for precise mounting. For example, the outer parts of a lens should allow the easy mounting of the lens with proper orientation of its optical axis.

In order to achieve high quality results with good efficiency, all cutting, lapping and polishing processes need to be adapted to the particular materials used. Optical glasses can have mechanical parameters varying in quite large ranges. In particular, some glasses are rather soft, while others are very hard; some intermediate level of hardness is usually ideal for processing. Some glasses, for example fluoride glasses, can be rather hard to process since they are unusually brittle. At the same time, they may be highly sensitive to humidity, which requires additional precautions.

Note that the selection of optical glasses needs to take into account many other aspects, such as refractive index, chromatic dispersion, availability and price, so that the mechanical properties cannot always be ideal for fabrication.

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