So You Think You Know Micro-Optics…
Posted by Chris Toomey, Sales and Marketing Manager at Ross Optical, a division of Precision Optics ● April 23, 2020 at 11:00 AM
Keep in mind that a human hair is about 100 um (0.100 mm) diameter! When most people think about specifying micro-optics (< 1 mm in size), they assume they are just very small versions of standard optical components and all the manufacturing methods and specifications are the same. This is not the case! Modern manufacturing techniques for larger optics have evolved to the point where you typically do not have to think much about them when designing an optical system and many of the potential manufacturing issues are no longer considered. In the realm of micro-optics, many of the old limitations and considerations still apply.
Conventional tooling and CNC machinery rarely can be used for optics smaller than 1 mm, so the select few companies manufacturing micro-optics have typically developed their own tooling, equipment, and processes to be able to offer such small optics. Because of this, micro-optics harken back to the old days in that they are typically only available with a spherical or plano surface profile. Depending on the radius of curvature needed for your lens, the manufacturer might be limited to making them one at a time or could possibly block them in larger quantities. There is not a conventional mass production method for cylinder or aspheric micro-optics, but they can be molded in glass or plastic with size limitations of a few mm, typically. We are one of only a handful of manufacturers who can manufacture cylinder micro-optic (< 1 mm) surfaces, our methods are proprietary, and these also have radius of curvature limitations. Molding requires materials of sufficiently low melting point, which significantly limits available glass types and the upfront mold costs can be very high. There will also be a limitation on the glass types available for custom micro-optics, so check with us to verify the feasibility of your optic with our processes, or we can provide design services.
MIL-PRF-13830 cosmetic specifications do not apply to micro-optics, but the general concepts and definitions can provide guidance in formulating requirements. As an example, the qualitative definition of a scratch versus a dig is useful for even smaller imperfections. However, surface quality and many other specifications do not scale with clear aperture size, and allowable defects can occupy a significant portion of the available clear aperture. Experts agree that the specification does not make sense for optics below 5 mm in size. The Mil-Spec was originally developed for visible-use optics (binoculars, rifle scopes, etc.) and over the decades has been used to specify many other types of optics. For this reason, the inspection conditions are specifically defined and highlight the need for defining the inspection conditions for specific micro-optics.
The ISO 10110 specification is applicable and useful for parts down to 2 mm in size since that is the minimum scratch length, and on a 500 micron diameter optic, an allowable scratch can run across the entire clear aperture. For smaller optics, it is critical for the supplier, who is familiar with the capabilities and limitations of its proprietary manufacturing techniques, to work closely with the customer, who understands the functional requirements of the optic, to define an appropriate inspection technique and acceptance criteria for specific parts.
An important factor in guiding quality is use, since an optic used solely for illumination can have looser specifications than one used for imaging. For optics smaller than 1 mm, the typical approach is to avoid any visible imperfections under a certain magnification. Because there is no official specification for micro-optics (yet!), it is best to work with a manufacturer to ensure you get an optic which will work for your application.
Ever thought about what a 90% clear aperture means for a 500 um diameter lens? This type of specification means that the clear aperture is 450 um, which leaves only 25 um outside the clear aperture on each side to accommodate room for edge chips generated during manufacturing, outer diameter (OD) tolerance specifications, and space to hold the optic for coating. Considering OD tolerance on the optic, inner diameter tolerance on the coating tool, thermal expansion during coating, as well as the need for clearance to load the optics in coating tools, this makes coating micro-optics much more challenging than coating larger-sized optics. Yes, there are some novel ways and materials which can be used to hold the optic, but even those have temperature range limitations which then limit materials and deposition methods used for any given coating. You may not be able to get that super-low-reflection coating you want because the design must change. When you get to even smaller-sized optics, like a 100 um right-angle prism, coating options become very limited. Even if a coating supplier has great designs, make sure they know how to handle micro-optics, or if they even can. While very tiny micro-optics can have AR coatings, the process of manufacturing and coating may require additional handling and fabrication steps which can add to the cost of your part. It may be beneficial to understand and budget for added losses and ghosting in your design as it may result in lower overall production cost. For broadband systems, it is best to verify what coating performance is available for your specific size and geometry of micro-optic.
When you are ready to delve into the realm of micro-optics, contact us so we can help you up front to avoid the expense of fixing things after your design is finalized.