Machining Advances Lead to Multifunctional Optics

Bernd Reis

Advanced computer numerical control processing allows optics designers to combine several mechanical and optical functions within one monolithic component.

Today’s optical systems usually serve a variety of functions simultaneously, resulting in ever-more-complex subsystems with increasingly critical requirements related to performance, precision, robustness and miniaturization. More functionality often means more subcomponents, which limits miniaturization. The solution may lie in combining several mechanical and/or optical functions within one component to produce multifunctional monolithic optics (Figure 1).

Figure 1. Compared with a conventional optical system (top), the benefits of a multifunctional monolithic approach (bottom) could include using fewer components, minimizing unwanted environmental effects and enhancing optical parameters.

Overall, advanced computer numerical control manufacturing systems have enhanced optics production in several ways:

• For conventional optics. Machining optics in one setup or chucking with a computer numerical control machining system can reduce scrap rates and processing time, as well as boost productivity and precision.

• For complex 3-D glass and ceramic components. Computer numerical control machining enables manufacturing of parts as large as 800 x 800 x 600 mm with micron precision. Production speed and parts quality also may increase. Building optomechanical systems with components made from the same or similar material also means that individual components will have fewer problems related to different rates of thermal expansion.

• For multifunctional and monolithic optics. Combining optical and mechanical functions within one component enables designers to miniaturize the optical system without reducing its performance, precision or robustness. Components also reap the same computer numerical control machining benefits as the other optics categories. One example is a monolithic prism that is a central part of a tilt sensor. The prism combines three conventional optical components: a lens, a rhombus, and a prism- and plane-parallel plate.

Figure 2. A multifunctional monolithic prism in an electronic-distance-measurement sensor integrates two rangefinders.

Simplifying optical system design

The goal of designing and building multifunctional monolithic optics is not only to reduce the number of components, but also to incorporate overall and concurrent efficiency in design, manufacturing and assembly. For instance, although the optics’ surface shapes may be very complex, the computer numerical control machining process can often accommodate these designs without a trade-off in cost. It also allows designers to minimize or eliminate unwanted environmental effects that are common to multicomponent systems.

First, take the example of a multifunctional monolithic scan mirror. Eliminating glued surfaces can reduce adverse temperature effects on the mirror’s dimensions. Designers often can avoid the effects of extreme temperature gradients caused by heat expansion or diffusion coefficients that could lead to temporary or permanent misalignment. Another benefit may be the elimination of extreme pressure differences caused by outgassings from adhesives or permeation of liquids into a multicomponent system. Even the aging of adhesives is no longer an issue.

From a mechanical standpoint, designers also can eliminate problems related to vibration and shock. There also will be less risk of unwanted resonance effects compared with a system having a number of components, each with its own specific resonance frequency.

Now consider the problem of tolerance stacks common in any multicomponent assembly. In traditional design of such systems, engineers often must use a variety of techniques to calculate the effect of both coordinate (positional) and geometric (including mechanical and optical) tolerance accumulations on individual optical parts and assemblies — their goal being to ensure proper fit and function among all parts within the assembly. By designing multifunctional monolithic systems, they can eliminate sources of potential error during assembly, such as adhesive layers, as well as eliminate or minimize tolerance stack-up. This can significantly improve the cost-quality relationship.

Optical properties also may improve with the use of a monolithic optic. In general, when comparing monolithic elements and classical multicomponent systems, engineers at SwissOptic AG have found that the monolithic component often has better wavefront properties as well as other enhanced optical parameters, such as luminosity.

Perhaps one of the major issues driving the use of multifunctional monolithic optics is the trend toward miniaturization. Increasingly, OEMs and other end users expect smaller footprints for optical systems without any compromise in performance. These demands are coming from a variety of sources, including the sensor, life sciences, information and telecommunications industries.

For sensor manufacturers, combining micro-optic elements with optomechanical components offers a new level of integration. For example, a prism that is a main component of an electronic-distance-measurement sensor incorporates two rangefinders, including an IR sensor that measures cooperative targets such as retroreflectors, and a sensor with a collimated diffraction-limited beam that measures any natural surface, even one with low reflection.

Potential benefits of this design include: the ability to optimize required interfaces between different subassemblies or materials; simplified assembly because the alignment and positioning tolerances of the micro-optic diffractive elements are accounted for during the design stage; and size reduction — in this case, a tenfold reduction compared with conventional solutions, because of direct integration of the micro-optic element into the glass prism. The lens elements are less than 1 mm.

The next step in furthering the integration level of multifunctional and monolithic optics will involve the use of segmented dichroic coatings. These would facilitate the inclusion of refractive and diffractive features, as well as other wavelength-selective functions such as partial mirrors and filters on one monolithic component. For example, it is now possible to produce structured optical interference filters with defined patterns of any spectral sensitivity — on one planar surface (Figure 3). Designers now can structure filters with a lithographic process on planar surfaces down to less than or equal to 8 x 8 μm (i.e., high-end CMOS).

Figure 3. With a threefold liftoff coating process, it is possible to produce a red, green, blue filter pattern to a size less than or equal to 8 x 8 μm on a single surface. Current technology allows structuring any dichroic coating to provide spectral characteristics specific to each application.

One of the next steps will be to develop the lithographic process further for the partial coating of etched or replicated microstructures on the surface of the substrate for spectrally sensitive diffractive and refractive micro-optics.

Practical considerations

SwissOptic machines multifunctional and monolithic optics only in glass and does not use a molding process. Therefore, the most significant cost advantages will come from high-tech applications and lot sizes of less than 50,000 parts per year. When feasible, molding will probably have the advantage in the serial production of higher volumes.

Meet the author

Bernd Reiss is project manager for process and product development at SwissOptic AG in Heerbrugg, Switzerland.