This process demonstrates its value for customers like the one producing a surface metrology instrument prototype. They were looking for a three-quarter by half-inch mirror. Starting from a stock one-inch diameter λ/20-wave enhanced-aluminum mirror, the glass saw produced a final part with the right dimensions and the required coating — with very little setup time.
The same rapid turnaround is possible with glass coring. For example, a customer developing a semiconductor testing machine needed a 35-mm diameter ND filter. Glass coring modified a 50-mm square element to the right diameter without requiring any recoating.
Speaking of recoating, however, many customers are unaware that standard or custom coatings can be deposited both rapidly and economically. For example, one customer was developing a gas monitoring system for detecting chemical compounds in industrial environments. They needed a moderate-size focusing mirror — a stock component — but they also needed a deep-ultraviolet coating. The ability to produce very small coating runs meant the customer got their enhanced-aluminum-coated six-inch focusing mirror in record time. And, of course, various modifications can be combined, as in the case of a customer prototyping a laser rangefinder. Their custom requirements were met by edging down a sapphire window and then applying a custom coating.
Surface Improvement and Modification
Surface improvement and modification are other classes of time-saving methods that produce custom optics starting from stock components. Surface improvement starts with a stock optic of the right radius, but with insufficient surface quality. For example, a λ/2 surface quality might be just fine for a perimeter sensor, but for a high-power laser application you might need λ/20. Surface improvement uses subaperture polishing techniques to improve both surface irregularity and roughness in a short amount of time. This means there’s no need for grinding, which is a big time saver.
Surface modification refers to a change of the surface curvature — changing the prescription of an optical component. Although it’s not always the best path, you might find it advantageous, for example, to convert a stock spherical surface into an aspheric surface. For this modification to be worthwhile you need to select a spherical optic that comes close to the best fit sphere (BFS) of your custom asphere (Figure 2). Starting sphere fabrication from a blank requires large amounts of material removal. Manufacturing an asphere from an already polished optic can bypass up to 75% of the standard manufacturing process. This approach can also be used to transform a spherical surface of one radius to a spherical surface of a different radius.
If you find yourself in a situation where your design appears to be full of custom components, you may be tempted to modify your requirements because you’re worried about the time and expense of producing those components. When you work with a vendor with a large catalog of stock components, you may find they can use some of the techniques outlined here to produce your optics with minimal lead time and at reasonable cost (Table A). It’s worth at least taking the time to talk with an application engineer to see if you can start your custom design from a stock component and dramatically reduce your lead time.
This article was written by Andrew Fisher, Optical Engineer, Edmund Optics (Barrington, NJ). For more information, contact Dr. Mr. Fisher at AFisher@edmundoptics. com or visit http://info.hotims.com/55585-201.