Communication is the key to buying optics

Telling vendors exact application requirements ensures an effective, economical solution for meeting optical component needs.

Oct 1st, 1997
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Communication is the key to buying optics

Telling vendors exact application requirements ensures an effective, economical solution for meeting optical component needs.

The majority of optical components used in research laboratories and in commercial products are stock elements from catalog houses. Ordering from a catalog house can be intimidating when nothing in 350 pages of catalog text appears to meet your needs and you are on a low budget with time constraints. Frequently, however, the solution to your problem is in the catalog--if you understand exactly what problem needs to be solved and what performance stock products can offer.

I recently had a conversation with an engineer who requested information on a short-pass filter. He had found a standard catalog filter that would transmit the visible wavelengths he needed, but it could not block far enough into the infrared spectral region (see figure on p. S14). To compound his problem, his system design called for a single element for the filter, so it appeared he needed a custom filter. However his budget didn`t allow for special fabrication.

After a long talk about the nature of the problem and possible trade-offs, I learned that the engineer actually needed to block only one infrared wavelength, not a full range, which meant that he didn`t need a custom filter after all. An off-the-shelf laser mirror that transmitted visible wavelengths while reflecting (rejecting) the unwanted infrared wavelength satisfied his needs nicely. Standard product, standard price, off-the-shelf--the engineer had the part in his hands the next morning.

This vignette illustrates the most important advice that you can follow when specifying optical components: discuss details of your situation with the vendor`s representative so that he or she can guide you to the most appropriate component. For high-energy or high-power applications, define required pulse length, pulse energy, repetition rate, average power, and beam diameter. Also, define the desired transmission and reflection at each wavelength involved. We frequently see requests for mirrors with impossibly wide bandwidths. Usually, all that the customer needs is a mirror that reflects at two widely separated wavelengths--a stock item available at an economical price.

Vendor representatives have broad knowledge of the products they sell and the systems that use them. For example, the most common cause of laser damage in optical components is contaminated or improperly cleaned surfaces. This can be easily avoided with proper training from catalog house personnel, who can show you how to set up on-site cleaning and inspection stations and help you establish procedures for the care of the components.

Special requirements

In today`s increasingly competitive environment, almost any vendor is willing to fabricate special components to customer-defined specifications. A common misconception is that such custom modifications are expensive. In reality, the de mand for simple modifications to standard products is a significant market force to day, and many vendors are willing and able to fabricate such com ponents on short notice and for a relatively small additional charge.

Core-drilled and D-cut mirrors are examples of modified components that can be made from existing stock items in under a week, for only a small setup charge (see photo on p. S13). Ideally, the manufacturer performs any cuts or drilling before the coating is applied. When cuts are made after the coating pro cess, the modification can potentially damage the coated surface in the neighborhood of the cut. By cutting first, then coating, manufacturers ensure that quality is maintained, though generally at the cost of an additional week in delivery time.

When time is of the essence, however, such cuts can be made on finished products. The resultant surface damage is rarely excessive, but the finished part may not meet the original specifications for the unmodified version.

Defining tolerances for modifications can result in expensive overengineering if not done properly. By specifying a tolerance of an arc second on the angle at which a hole is drilled through a mirror or lens, for example, you can make an economical part prohibitively expensive. Look in the catalog at the standard tolerances provided and discuss it with one of the vendor`s applications engineers. Time and again they see needlessly tight tolerances on a chamfer or edge bevel--a costly oversight. If you can use a standard tolerance, do so.

Before contacting the vendor, decide where you can be flexible. To return to the core-drilling example above, most vendors have a large stock of core drills. If you can accept one of the available diameters, you will save extra tooling charges and a delay time of two or three weeks while another drill is machined.

High quality is the norm

In the last ten years, the demand for high-quality optical components has rapidly expanded. Here `high quality` is defined as optical components with a surface quality specification of 10-5 scratch-dig or better and a surface figure specification of l/10 or better, measured at 633 nm (see "The language of optics," p. S8).

In a discussion at CLEO `97 (Baltimore, MD) about producing large quantities of optics with scratch-dig specifications better than 10-5, I heard a number of engineers say they thought this was either improbable or prohibitively expensive. But in fact, if a vendor`s specification is 10-5, then the majority of its stock is likely to be far better.

The economics of quality control explain why this is so. To produce 10-5 optics at CVI, we inspect 100% of all surfaces, cleaning and examining them at each of four critical stages in the production process. Most substrates are specified to meet 10-5 on both sides, and there are typically eight inspection operations per substrate to verify specifications. For vendors that produce hundreds of thousands of components per year, surfaces must be inspected in a matter of seconds or else the company would need to employ a veritable army of QC technicians.

Yet there is no way to inspect a surface quickly when it must be compared to the complex, time-consuming military specification MIL-O-13830A. We solve the problem by producing the majority of components with zero visible defects. The result is that a large portion of the company`s stock exceeds specification. So if your application requires parts meeting specifications better than 10-5, chances are good that suitable elements have already been fabricated.

Spread modifications out

For a typical manufacturer, special requirements are a way of life, especially when purchasing elements to incorporate into a product being built for customers. Few standard parts will exactly meet your needs, driving you to specify custom elements. Custom does not necessarily mean expensive, however. For typical original-equipment-manufacturer (OEM) purchase volumes, the required quantities are often sufficient to bring the price of a special product down to a level that is comparable to standard products at quantity discounted prices.

A common nightmare in the OEM world is having production come to a standstill because a single vendor failed to ship a component. The probability of such a crisis occurring rises with the number of modifications required for each element. The possibility that an alternate plan can be put into action is inversely proportional to the number of special requirements per element.

In other words, do not incorporate all of the special requirements for a system into one component. If a delivery problem arises, and the component you need is essentially a standard product with only one or two relatively minor modifications, then a vendor may be able to modify existing stock in short order, producing a temporary and viable solution. Spreading unusual or special requirements throughout the components in a system shortens the time it takes to prepare and ship them.

Nothing beats a good relationship with vendors. Get vendors involved early, make them a part of your design and problem-solving teams. Manufacturers of catalog optics are happy to make that knowledge base available to customers. o

Click here to enlarge image

Special designs are not always expensive. Core-drilled (bottom right) and D-cut mirrors (center) are easily modified from existing stock items for a minimal setup charge.

Click here to enlarge image

Transmission characteristics of a stock laser mirror allow it to function as a short-pass filter that transmits most visible wavelengths while rejecting some infrared wavelengths.

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