The convergence of “new” materials, improved free-form shape fabrication, and changes in the aerospace marketplace has transformed the previously static field of custom optics into a very dynamic market segment. In this article, we examine the impact of four key trends that characterize this changing landscape.
On the technology side, there are the maturation of “new” optical materials that avoid some of the performance/ weight/cost trade-offs of traditional substrate materials, and the development of robotic methods that can deterministically create substrates with free-form shapes and low wavefront errors. On the applications/market side, the two important trends are an increased demand for volume manufacturing of custom components, and outsourced component integration, where end users are looking for subsystems and even complete systems rather than just optical elements.
New Material Capabilities
Traditionally, many of the custom optics for aerospace applications were fabricated either from some type of glass, a thermally stable ceramic (e.g., Zerodur), or beryllium. These all involved trade-offs in terms of stiffness, thermal (expansion/ contraction) sensitivity, weight and cost. For example, glass (and fused silica) is lower cost, easy to polish and has a low coefficient of thermal expansion (CTE). However, it is not particularly stiff, so components need thickness or other structural optimization, which adds weight and is undesirable for airborne or spaceborne applications.
Recently there have been important advances in two alternative materials – bare aluminum and silicon carbide – that avoid some of the traditional trade-offs. Aluminum is both light and stiff, which explains its ubiquitous use in airframes. However, aluminum alloys have historically been used only in high performance reflective optics when given a thick cladding of a harder, more polishable metal, such as nickel. In addition to their higher cost, these bimetallic optics are then notoriously sensitive to thermal deformations due to the difference in CTE for nickel and aluminum.
This has now changed thanks to the latest multi-step fabrication methods that enable optical finishes on even low-cost aluminum alloys. At Coherent this entails an initial heat treating cycle to dissipate the nanoscale particles of iron, bismuth and other materials. This is followed by using single point diamond turning (SPDT). This diamond turning inevitably imparts a grooved surface finish. The surface figure is carefully measured using Fizeau interferometry relative to a computer generated holographic (CGH) optic used as a null lens. The surface roughness may also be separately measured using a phase measuring microscope. These data are then used to program a robotic system used for “Deterministic Polishing” (see section on free-form polishing) followed by thermal stress relief, and application of a high-performance optical coating. The final rms surface figure on these optics can then be as good as λ/100 (at 633 nm) with (rms) surface roughness of just a few Å (Figure 1). These values mean that aluminum optics now can even be used for ultraviolet applications. And since aluminum is the most common metal used in mounts and housings etc., the entire assembly is thermally matched and hence provides stable alignment and focusing.
Improvements in silicon carbide represent another material advance, particularly for spaceborne applications. Silicon carbide is an excellent compromise between glass and beryllium. It is stiffer than glass but much less thermally responsive than beryllium. However, silicon carbide traditionally was a difficult material that was rarely used for larger optics. The problem was principally one of supply; producers of silicon carbide did not have the capacity and experience to reliably deliver large, high-quality blanks. Thanks to a volume demand from the semiconductor industry, the material has advanced substantially in terms of lead time, size limits, manufacturability and quality.
Robotic Manufacturing of Free-Form Shapes
Free-form optics refer to optical surfaces that are non-spherical/cylindrical and go beyond simple symmetrical asphere forms. The extra degrees of freedom provided by free-form shapes enable designers to build systems with fewer components (i.e., lower weight, size and cost) and reduced aberrations and distortions, resulting in a wider field of view, superior resolution, and so forth. The military were early users of free-form optics for applications such as heads up displays in aviation, where performance and minimum size were pre-eminent considerations. But today, as more designers are becoming aware of free-form capabilities, and the cost to produce them has decreased, freeform optics are now one of the fastest growing areas of custom optics.
Coherent is supporting this market growth by the use of cost-effective fabrication of free-form (and aspheric) optics. In particular this fabrication relies on completely new robotic production techniques, which utilize a concept called Deterministic Polishing.
Traditional (spherical, cylindrical) optics have been fabricated by the same method for hundreds of years, i.e., random (stochastic) grinding and polishing that delivers a radially symmetrical result. Conversely, aspheres are typically created in a deterministic way by fine cutting using SPDT. However, SPDT alone leaves a grooved surface that must then be polished in some way to reach an optical quality sufficient for ultraviolet or laser applications. The latest Deterministic Polishing methods combine the deterministic shape control of SPDT but with the surface finish of random polishing. In simple terms, the fabrication involves repeated shape and surface metrology and computer software that precisely controls the (non-random) motion of a robotically driven polishing pad. (Of course, this approach is contingent on proprietary tools which can efficiently test a free-form shape.)
There are two main advantages to Deterministic Polishing. First it enables the production – within certain limits – of any continuous CAD/CAM defined surface, providing the optical designer with unprecedented freedom and system capabilities (Figure 2). Second, because it is a software-controlled robotic process, it is highly repeatable. This latter advantage dovetails perfectly with a key market trend, namely the demand for higher capacity manufacturing of custom surfaces.
Historically, many custom optics for aerospace were “one-off” single units, such as used in satellites, or in space (i.e., Hubble) or ground-based telescopes, often under the auspices of NASA. But today this is changing for two reasons. First is the advent of a global commercial spaceflight industry, often referred to as New Space. Here the payloads are frequently a distributed system of several smaller satellites, e.g., based on the so-called CubeSat communications network protocol, rather than just a single, big-ticket payload. Second, both spaceborne and ground-based telescopes increasingly use a segmented primary mirror consisting of individual segments that are adaptively focused to correct for various aberrations and atmospheric distortions.
There is thus a growing need to produce custom optics using volume manufacturing. This must deliver both some economy of scale cost benefit, and the ability to repeatably create multiple identical optical elements (Figure 3). The new Deterministic Polishing methods mean that aspheric and free-form shapes can now be produced with equal or better repeatability than conventional spherical lenses and mirrors. Examples of this include the segments Coherent polished for the next-generation Webb space telescope currently scheduled for launch in 2021.
Subsystems and Systems
Another important trend in the demand for custom aerospace optics is common to several other market sectors, namely the drive to improve the performance/cost ratio of end products by outsourcing sub-systems, or even complete systems, to qualified manufacturers. This enables the primary customer to move towards specifying system performance, rather than component performance. Key fabrication, assembly and testing tasks are all collectively outsourced to a vendor. This minimizes both the direct capital costs of the overall system and also the indirect costs by reducing the need for in-house photonic expertise and the time to design, build and test internally. As with volume fabrication, this is now one of the fastest growing areas for high performance optics in aerospace.
In conclusion, custom optics for aerospace applications is a rapidly evolving market segment, following decades of relative stasis. The changes in manufacturing techniques and usable materials, as well as increased commercialization of aerospace, particularly for space flight, are all combining to provide new opportunities and exciting capabilities for vendors and customer alike.