What is Replication?
Replication is the process of manufacturing optics by transferring the optical surface of a master (or mold) to one or more copies of the surface. The copies, which only approximate the final surface before replication, then take on the precise characteristics of the master surface. Surfaces that are 5 Angstroms smooth or have gratings with 3200 L/mm are routinely reproduced.
There are several different forms of replication including: thin film epoxy replication, plastic molding, nickel electro-formed optics, and plastic casting. Each is specifically suited for a specific range of applications. Thin film epoxy replication lends itself to high or moderate accuracy optics in low to moderate, and in some cases even high production quantities. Both transmissive and reflective optics can be replicated using this process. Plastic molding is used mainly for small lenses made in large quantities. Plastic casting is somewhat uncommon and is suited only to very low quality optics. Nickel electro-formed optics are used mostly for searchlight mirrors where high temperature operation is required, as well as moderate to low quality reflective optics in the visible or infrared.
Because of the extensive range of capabilities and applications that are related to Replicated Optics, we will limit this discussion to only the area of thin film epoxy replication, and mainly to reflective replicated optics.
We will describe the replication process by using as an example, the production of a 1/4 wave concave spherical mirror on an aluminum substrate. The reflective surface will be a thin film of aluminum, over-coated with a dielectric film for protection.
A master that contains the desired optical surface is produced first. For this example, it is assumed to be a polished convex glass master made by conventional polishing methods. At the commencement of the replication process, the master is first coated with a release layer. This layer will permit successive layers of material to be easily pulled from the master later in the process. On top of the release layer is put the dielectric film followed by the aluminum coating. These elements are applied in a vacuum coater.
Aluminum blanks (or substrates) are machined conventionally and prepared for replication. A carefully chosen liquid epoxy is then poured onto either the optical surface of the coated master or the machined surface of the substrate and the two are bonded together. After the epoxy is cured, the master and replica are separated, transferring the thin film to the substrate. This now becomes the replicated mirror. The release layer is cleaned from the replica and the excess epoxy is trimmed from around the edges of the replicated surface. The master is cleaned and returned to the replication process to produce additional replicas. The mirror produced is 1/4 wave because the epoxy compensates for imperfections in the substrate providing an optical surface true to the master.
The same general technique that is used to produce spherical optics is used to produce flat optics. The master in this case is flat. It is generally made of glass and larger than the part being replicated. Standard production flat masters can be used for more than one configuration of flat mirror and more than one part can usually be put on a single master. This means that making even a single flat replica can be cost effective.
In the examples of flat and spherical mirrors, a direct master-to-replica transfer was used. For aspheric optics, this is not the typical approach. Using an off-axis parabola as an example, it is easier to fabricate and test a concave parabola than a convex one. Therefore, for concave aspheric replicas, a concave master is produced. The problem is that now both the replicas and the master are concave and the surface cannot be transferred directly.
The solution involves transferring the concave surface of the master to a convex substrate using the replication process. This replicated convex part used as a master (called a sub-master), is then coated with the release layer and mirror coating and used to make the concave replicas. This technique has the added advantage of using a single master to produce large quantities of replicas by producing many convex sub-masters that are subsequently used to produce the concave replicas.