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Optical Design & Engineering

Beating the Grind

Improved technologies make the manufacture of aspherical optics practical.

From oemagazine August 2001
31 August 2001, SPIE Newsroom. DOI: 10.1117/2.5200108.0004

Imagine specifying a mechanical part with 0.6 millionth of an inch surface accuracy and 40 billionths of an inch rms microroughness. Any machine-shop purchasing agent, after recovering, would show you the door. Without appropriate material, manufacturing tools, or measurement devices, this request is absurd.

Now imagine specifying a conventional optical lens to λ/4 surface accuracy and 10 Å rms microroughness. Any optician would want to know two things: how many you need and how soon you need them. Surprisingly, these are the same specifications as given to the machine shop above. Opticians in this case, however, have a number of advantages.

First, they are working with glass, one of the few materials that can hold such difficult tolerances. Further, they have manufacturing tools and methodologies, as well as metrology tools, that can easily achieve such precise specifications.

Now imagine asking for an aspheric lens with the same specifications. This request would generate a much less enthusiastic response. Manufacturing tools for aspheres are completely different from spherical or flat optics, and every aspect of the specification (low-, mid-, and high-frequency) requires hard work. Meanwhile, standard methods for asphere metrology do not exist.

challenges of aspheres

Spherical, ring-tool generating dates back to the ancient Romans, who used it for shaping gemstones. With proper alignment, this simple geometry results in a nearly perfect and smooth spherical surface (see figure 1). It is relatively insensitive to machine accuracy, and since the advent of sophisticated computer control, smoothing is not required at all; a part can go directly into the final polishing step.

Figure 1. Aspheric surfaces can be created using peripheral grinding geometry.

In the case of aspheres, the only choice for grinding is some form of point-contact contouring (see figure 2). In contouring mode, however, the final accuracy becomes much more sensitive to the manufacturing environment, with strong dependence on the positioning accuracy of the machine, the condition of the grinding wheel, and vibrations in the system. Moreover, aspheric surfaces created with point-contact contouring generally need significant smoothing prior to final polishing.

Figure 2. This example of fabrication of a lens with more than 400 μm of aspheric departure to λ/5 p-v uses the Schneider ALG200 (20 minutes), the Q22-X MRF System (20 minutes), and a Diffraction International computer-generated hologram.

To overcome the extreme sensitivity in point-contact asphere contouring, manufacturers have used precision machine tools with high-speed spindles and fine grinding wheels. The goal is to achieve final form accuracy on a generating machine while introducing minimal surface and subsurface damage. Nearly any polishing process that follows generation will degrade the form even as it improves finish, so minimizing polishing results in less figure degradation. This strategy is sound but difficult to implement. Optimal grinding depends on a variety of ever-changing variables, most notably the condition of the grinding wheel.

A generating strategy offering ease of use, flexibility, minimal dedicated tooling, and stable operation for a wide range of materials has recently been developed. The basic philosophy for the ALG200 platform from Schneider Opticmachines (Steffenberg, Germany) is to use a two-spindle machine with an automatic tool changer to cluster a number of grinding wheels and smoothing pads. This machine uses a controlled grinding technique that combines rough spherical generating with rough and fine contouring, followed by pad smoothing and final centering. Integrating bound abrasive pad smoothing on the machine overcomes the traditional shortcomings of contour mode grinding, because the pads can smooth most of the mid-frequency errors. A final integrated centering step is essential for an asphere, because it automatically places the optical axis at the mechanical center of the aspheric surface. This integrated machining center yields an easy-to-use, robust asphere generating capability.

a better polish

Spherical surface polishing, the next step in lens fabrication, has changed little since it was first developed more than 300 years ago. A typical polishing tool is approximately the same size as the lens surface and is the perfect mate for the lens. Cushioned by a slurry of water and soft polishing abrasives, the two surfaces are rotated and oscillated against each other to yield a very smooth and specular optical surface. With the proper setup parameters and operator experience, it is easy to achieve λ/4 p-v figure with 10 Å rms surface finish and λ/10/cm slope (mid- frequency error).

For an asphere, this simple lapping model does not work. As soon as the lap begins to oscillate, the lap and the workpiece no longer have the proper mating surfaces and begin to destroy the aspheric shape. Although opticians have used a number of tricks to overcome this classic dilemma in asphere manufacturing, the only successful approaches have been very complex, expensive to implement, and custom made.

The inherent benefits of aspheres, coupled with the difficulty of polishing them, have made aspheric polishing the Holy Grail for many optics manufacturers for decades. Many advanced manufacturing technologies have been developed, including computer-controlled subaperture polishing processing techniques. Asphere fabrication requires subaperture approaches because of the misfit between tool and workpiece. Use of a polishing tool that is smaller than the workpiece, mechanically flexible, or even actively controlled reduces misfit. Most of such techniques are proprietary technologies developed for the larger optical manufacturers. Recently, commercial subaperture pad polishers have entered the marketplace.

All of these approaches suffer from the same problem: they are not deterministic because the polishing tools change uncontrollably with time. Other innovative, deterministic subaperture material-removal processes have been developed over the years. Ion beam figuring, in particular, can be successful in certain niche applications, but it has not been universally deployed.

Magnetorheological finishing (MRF) is making inroads into the marketplace because it is a deterministic polishing process that easily fits into the normal optical production environment. Its key attribute is that it employs magnetorheological fluid as a polishing tool. The removal rate of this tool correlates very well with the viscosity of the fluid, which is controlled in situ to ±1%. The extreme stability of the polishing tool, coupled with nearly infinite compliance to an aspheric workpiece, makes the MRF technology a significant advance in deterministic polishing of all optics—spheres, flats, and aspheres. MRF machines have been deployed at more than 50 optical fabrication facilities all over the world, which makes this a mainstream and accessible technology.

a measure of quality

Spherical surfaces are easy to measure using general-purpose Fizeau interferometers or other equipment that is found in most optics shops. A normal Fizeau interferometer, however, cannot measure most aspheres without specialized null optics, which are often cumbersome, expensive, and difficult to use. Furthermore, null optics are normally specific to only a particular aspheric shape, and in some cases, the null optic can be as difficult to manufacture as the asphere itself. More flexible surface profiling systems overcome many of these problems but are generally either less precise with less resolution or are extremely expensive.

There are various ways to test aspheres, but most of them are expensive, time-consuming, difficult to use, and difficult to qualify. For a normal optics shop, the computer-generated hologram (CGH) has become the standard for asphere metrology. Diffraction International (Minneapolis, MN) has developed a CGH for the industry that is easy to use, accurate, and works with nearly any Fizeau interferometer. Several aspheric analogs to the Fizeau interferometer are under development, but the CGH is currently the best way to measure general aspheric lenses and mirrors.

It is now possible to affordably produce high-precision glass aspheres. These technologies will not become pervasive, however, until a significant market for aspheres emerges, which will require new awareness within the optical design and program management communities. Although optical designers have appreciated the benefits of aspheres for many years, an "asphere averse" culture has become gospel over the past decades for visible optics—and justifiably so. Many program managers have colorful war stories about the perfect aspheric design that was never delivered on time, on budget, or within specification.

With the availability of new tools, however, asphere manufacturing has matured. Aspheres can now be manufactured in volume, on deadline, and for a reasonable cost. Bring on the aspheres, designers; the manufacturers are ready for you. oe

Dan Golini

Don Golini is president of QED Technologies, Rochester, NY.