Technique Aids Testing of Aspheres

Dan Drollette

Deep aspherical surfaces may become easier to measure, thanks to a novel testing technique developed by researcher Yee-Loy Lam and his colleagues at Nanyang Technological University.

Their solution? Immerse the optics in a container of liquid crystal.

Aspheres offer several advantages to optical designers struggling with problems of lens aberration. These precision optics have high resolution and wide area coverage. A single asphere can replace several optical components at once, creating more compact and lightweight optical systems. When the Hubble Space Telescope's vision needed correcting, NASA installed aspheric mirrors.

Limitations abound

Unfortunately, testing aspheres is laborious and time-consuming. Their surfaces are hard to measure without the aid of auxiliary optics such as Hindle spheres or diffractive null correctors, both of which are complex and expensive to fabricate. Because of these difficulties, the use of aspheres has largely been limited to big-budget specialty applications such as defense, astronomy and semiconductor manufacturing.

But in the April 4 issue of Optical Engineering, Lam and his colleagues described an easier way to measure aspherical surfaces, using a variation of the polarizing interference technique. They placed the test element in a container of uniaxial liquid crystal, to which they applied an external DC electrical field, aligning the liquid crystal's molecules in one plane. The whole apparatus is placed between a light source, a polarizer and an analyzer.

After a beam of light passes through the polarizer, it enters the liquid crystal, where the linearly polarized light divides into ordinary and extraordinary beams, each moving at a different speed. The resulting phase difference is proportional to the difference between the pathways taken with and without the inserted lens. When the beams hit an analyzer placed at the far end of the crystal container, their two perpendicularly polarized electric fields interfere. The amplitude and the phase of the two interfering waves produce maximum and minimum intensities shown as interference fringes on a charge-coupled device camera. Conventional techniques can then interpret the resulting interferogram.

Lam said the technique could also be used for testing spherical optics. The team's immediate goal is to improve its analysis software to make this approach easier to use.