Technical Note:
Replication Justification

Replicated optics can be used for both reflective and transmissive components. The main reasons for using replicated optics are to:

  • Minimize system cost
  • Produce light weight or low inertia optics
  • Locate optical surfaces in otherwise inaccessible locations

System cost, as used here, is the cost of:

  • Generation of the optical surface
  • Structure used to mount the optical component
  • Labor necessary to align the component in the system

Flat mirrors made of Pyrex or metal from 1 to 1/10 wave accuracy is a good application for replication. Mirrors made from float glass to 2 waves/inch irregularity that can be made by cutting up sheet stock and coating the front surface, will be cheaper to produce than the equivalent mirror made by replication. We have performed a qualitative comparison between replication, conventional polishing, single point diamond machining and electroforming. The evaluation is based on the following parameters:

  • Optical performance
  • Cost
  • Alignment capability
  • Level of precision

Since most reflective replicas can serve as their own optical mount and be machined to minimize or eliminate alignment during system installation, they frequently present a tremendous cost savings over conventional methods. For reflective components, metal optics are preferred to glass optics to take full advantage of replication. Metal optics are also easier to replicate than glass optics, thus reducing the individual component cost. For refractive optics, transmissive (usually glass) substrates must be used. Since polished surfaces must be used on the replicated blanks, spherical surfaces are not cost effective. Replication of aspheric transmissive optics presents the greatest performance/cost advantages.

Diffraction Gratings and surface holograms, whether reflective or refractive are special examples of replication that are very effective but will not be covered here.

Reflective Optics in Metal

Metal replicated optics provide the ability to:

  • Include the optical mount as part of the optical component
  • Provide an accurate mounting surface aligned to the optical surface
  • Permit a kinematic adjustment mechanism to be integral to the optical element
  • Eliminate interfaces between the mirror and the mount that can go out of alignment under temperature excursions, vibration and shock
  • Have their temperature coefficient of expansion matched to the optical bench making the system less susceptible to ambient temperature changes

Flat Metal Mirrors

Flat metal mirrors do not need to be made round or rectangular the way glass mirrors typically are made. They can be machined rectangular, elliptical or irregularly shaped to meet the clear aperture space and mounting requirements of the optical system and also be part of the structure.

The mounting surface can be at an angle to the mirror surface, or put in a remote location, or allow a mechanism or optical path to traverse near either the front or back of the mirror. Scanning mirrors can integrate bearing shafts or have mirrors on both the front and back sides.

Since standard production masters are normally used (thus minimizing or eliminating special tooling costs), both prototype and production quantities of most flat mirrors can be manufactured cost effectively.

Precision of the optical surface is another criterion for selecting replication. Low precision mirrors that can be made from coated plate or float glass and used in low precision applications are not candidates for replication. The cost and complexity of these mirrors is so small that it often makes them cheaper than a comparable replicated mirror. When mirrors must be ground and polished to achieve a higher level of precision, then they become potential candidates for replication.

Large, flat replicated metal mirrors can be made routinely with precision of 1/10 wave as long as a reasonable aspect ratio is maintained. Up to 2 inch diameter mirrors can be made inexpensively. Up to 4" is practical, even in large scale production. Up to 12" is possible, but becomes more costly since yields become difficult to predict. A precision of 1/4 wave is practical for 12 in. or larger. Creating elements with proper aspect ratios and substrate designs are the keys to cost effectiveness. Weight or inertia requirements for the optical element is another advantage to be considered when choosing replicated optics. Often, the precision of the optical surface is not as critical as light weight or low inertia is. Metal optics permit the light-weighting of the substrate. Aluminum is the most cost effective of the light weight metals. If it was going to be polished, electroless nickel must be deposited over the machined part adding unwanted weight and inertia. A replica surface can be transferred directly onto the aluminum surface, without the need for an electroless nickel plating.

Spherical and Aspheric Optics

Metal optics are well suited to spherical mirrors and especially so to aspheric mirrors. The primary advantages to using replication for spherical optics are to:

  • Provide an integral mount
  • Provide a higher precision on the radius tolerance than is cost effective by conventional grinding and polishing

Obtaining the absolute radius accuracy of a spherical surface can be expensive. With replication, the master endures this expense. It is this radius, and thus its accuracy, that is reproduced on the replicas. Since the cost of the master can be divided over the long-term production of many replicas, its contribution to the cost of any individual component is small. And since replication provides a high degree of repeatability of optical surfaces, it is often the most economic choice.

Making aspheric optics using replication is also very cost effective. With conventional computer controlled (CNC) machines, aspheric substrates can be fabricated inexpensively. Mirror surfaces can be replicated onto these blanks. For aspheric surfaces, the cost for masters and tooling is significant. But since these costs can be spread out over the quantity of replicas that will be produced, their contribution to each mirror is often small.

For an experiment in space where a high precision, light-weighted, off-axis parabola is required, the quantity may be as low as three parts. For simpler parts with lower precision, larger minimum quantities may be required.

Aspheric surfaces that are desirable to replicate include (both on and off-axis sections):

  • Paraboloids
  • Ellipsoids
  • Hyperboloids
  • Toroids
  • Generalized aspherics
  • Cylinders, both elliptical and parabolic

Compound surfaces are also practical, such as a convex hyperboloid with a reference flat perpendicular to the optical axis. Typically, aspheric surfaces can be made to a precision of 1/10 wave. It is also cost effective to make aspheric optics to a precision as low as 10 waves depending on the configuration and the application.

Transmissive Optics

Transmissive optics have historically been produced less commonly than reflective optics. Aspheric refractive optics, arrays of small spherical lenses, Schmidt plates, laser disc player lenses, camera lenses and phase plates are typical examples of practical applications for replication. Optics as small as 1 mm or as large as 1 meter are feasible. Substrates made of glass or plastic can be used. The epoxy can be used to improve the surface accuracy of the substrate, or to remove the detrimental effects of diamond machining marks in plastic optics. Current replication epoxies show good transmission characteristics in the near UV through the near IR.

Fabrication techniques are very similar to those used in replication of reflective optics, except that the primary reflective coating is omitted. Of course, a suitably transmissive substrate must be chosen as well.

Retroreflectors

Hollow roof prisms and retroreflectors are very practical to produce by replication. These elements are typically used in cases where the optical path through a glass prism would be undesirable, where weight must be reduced, where the element must be used in environments with high shock and vibration levels, or where the element must be thermally stable in its mount. The masters, which are "male" alignment cubes, are made by conventionally polishing methods. In small quantities, the replica substrate can be fabricated by assembling three plates and then replicating the three surfaces simultaneously. For large quantities, either a cast aluminum or sintered substrate is effective and inexpensive.

Replicated Hollow Metal Retroreflectors have been made as large as 6.0 inches and as small as 0.5 inches in size. Smaller sizes and arrays of small retroreflectors are possible. Accuracies as high as 1 arc second are achievable; 5 arc seconds is standard, and 30 arc seconds can be relatively inexpensive for large area arrays or production quantities. Protected aluminum or gold coatings are standard.

Hollow Transfer Devices

Transfer devices produce output beams that are parallel to but linearly displaced from the incoming beam. They are retroreflectors that are "displaced" by a specified distance; they can be made as the equivalent of a flat fold mirror and a hollow roof prism. The hollow roof prism can be made by replication to accuracies up to 1 arc second, and the assembly can be aligned to 1 arc second for use in a laboratory environment or up to 5 arc seconds for use under environmental extremes. Protected aluminum or gold coatings are standard. The transfer device can be made of aluminum, with no glass elements, in order to withstand high shock/vibration levels and to remain thermally stable.