Application Briefs

Two quantities are perennially in short supply on engineering projects: time and money. When it comes to producing custom optical components in less time, for less money, one feasible approach is to modify off-the-shelf components to meet customers’ unique requirements. This tactic is particularly valuable in the prototyping stage or for one-of-a-kind instruments, when a full production run would be unjustifiably expensive and take an unreasonably long time.

Modifying off-the-shelf components can be a cost-effective approach to creating custom optics.
Optical fabricators have several options for modifying stock optics. For example, fabricators can quickly modify off-the-shelf optics to fulfill specific customer performance and schedule requirements by changing the diameter of an optic — a process known as “edge down.” Other common processing options improve surface quality or modify surface figure. But the possibilities don’t stop there. Additional final touches like cementing, edge blackening, or coating can open up almost limitless possibilities for customization.

Optical Fabrication

Optical fabricators don’t quote long lead times to be recalcitrant — producing high-quality optical components takes time. Fabricating an optical element from start to finish is no small task, whether the component is as simple as a window or as complex as a two-sided asphere. The general process more or less involves three basic steps: grinding, polishing, and centering, but the techniques used to execute them can vary.

The first step is usually to roughly shape the surface by grinding away material from a blank — a glass cylinder that is either pressed or cut into a shape that approximates the final form. The blank is ground one side at a time with diamond-impregnated grinding tools which are gradually stepped down from coarse to fine grit. Most optics go through two to four separate grind steps to create the desired surface shape and roughness.

Figure 1. The above demonstrates how an edge down can be used to correct beam deviation in a curved surface. At left, the optical and mechanical axes are not aligned. By edging the optic down, these axes can be realigned.
Next, the surfaces are polished with a loose abrasive in suspension (vs. a bound abrasive in grinding) that flows between the polishing tool and workpiece in the form of a slurry. Polishing mechanically removes glass and also chemically modifies the surface. Smoother surfaces require an abrasive with smaller particles, which increases fabrication time. Fabricators select the abrasive necessary to reach the desired surface irregularity and roughness in a reasonable time. The large majority of optics only need a single polishing process, but extremely accurate or smooth surfaces can require two polishing steps, similar to grinding.

The final step in the fabrication process is centering or edging the optic. This process brings the element in to its designed outer diameter and, if the surfaces are curved, also aligns the optical axes of each surface to the mechanical axis. For centering, the optic is either clamped or temporarily adhered to a stem, and a grinding wheel cuts down the outer diameter of the part. This stage also introduces any required bevels, chamfers, or sag flats.

All those steps are required whether the part has a production run of 1,000 or of 10, but when your custom optic can be produced by modifying an off-the-shelf part many of those steps are already done. Of course, a custom optic – by definition – requires some special fabrication, but using a stock part as a starting point leverages the time already invested in production. Your modification might be as simple as reducing the diameter of an existing filter or lens, or improving the surface accuracy beyond the λ/4 of an off-the-shelf part. You can even save time and money with custom aspheric lens fabrication by starting from a “close” spherical lens. To leverage stock parts you need access to a large catalog of existing optical components and the experience and capability to make the appropriate modifications.

Edge Down

Figure 2. The above represents how aspheric departure is commonly defined. Note that the sphere (in black) represents the best fit sphere (BFS) as it intersects the asphere (in red) at the center and edge of the part.
If you’re in the situation where a stock optic meets all your requirements except for outer diameter, then you can benefit from the most basic modification: an edge down. For example, one customer was prototyping a free-space optical communications system, and stock filters met their performance requirements. But they were producing a compact system, and needed smaller components. This alteration didn’t require any modification of the optical surfaces, but it did require special care because the surfaces were coated. In cases like this, the optic must be held properly so as not to damage the coated surface, and sometimes a temporary protective layer is applied to preserve very fragile coatings.

For any optic with a curved surface, be it on one side or both, a characteristic known as beam deviation (Figure 1) must also be taken into account. Beam deviation is the angular mismatch between the optical axis and the mechanical axis of an optic. When a curved optic is edged down these axes are aligned as best as possible before the cut is made. After the cut, the final beam deviation can then be measured.

Beyond Edge Down

A glass saw can make the same kind of dimensional adjustment as the standard grinding wheel, but it also offers the flexibility to produce components with arbitrary shapes. A glass saw is a computer numerical control machine with a four- to seven-inch blade mounted on a vertically translating arbor. The machine holds the optic on a three-axis table that rotates and moves in x and y directions. Selecting the proper blade, the correct rotational speed, and the optimum machining pattern leads to final parts with clean edges, minimal edge chipping, and intact coatings.

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