Achieving precise optical surfaces: manufacturing processes

Optical manufacturing covers a wide range of fields and provides a multitude of optical functions:

• reflective functions: light is reflected on an optical surface ;

• refractive functions: light passes through an optical surface ;

• diffractive functions: light is diffracted by an optical surface.

In this article, we will confine ourselves mainly to new technologies for the production of optical components associated with reflective and refractive functions.

A simple optical component has several aspects:

• an optical material sourced from the world's leading glassmakers: Schott, Ohara, Hoya, Hikari, CDGM. The quality of this material is an important issue for refractive components, and increasingly limits their performance. For reflective components, the material serves as a "support" for the optical function and also provides an important mechanical function;

• active surfaces for refractive components, a single surface for reflective components ;

• Each of these surfaces has a geometric shape that can be flat, cylindrical, spherical, aspherical or free-form. These surfaces are roughed and polished, and their final shape guarantees the optical performance of the component. The outer shape is then machined;

• to improve the optical or spectral performance of the component, a thin-film treatment is applied to the surfaces: antireflective, dichroic, reflective, polarizer, separator, etc;

• to improve stray light performance, it may be necessary to apply a black varnish to the component edges or to part of the surface.

New technologies have emerged over the last twenty years to produce the entire optical component, and in this article we'll look at modern manufacturing processes for optical surfaces (excluding coating, bonding and varnishing). After an essential review of the "great classics" of optical technology, still operational in industry and mandatory in some cases, we'll take a look at the various machining and polishing techniques. The use of these technologies is possible or not, depending on the material, the desired geometric function and the need for precision. We will continue with a review of the metrology required to implement these new technologies, and conclude with their application in industry, current limitations and current avenues of development.