Optical Metrology for Transmission Interferometric Testing

Abstract

The degree of sophistication and customization available in optical components has been driven by advances in lens design and fabrication. The optical testing of these components remains a challenge. In particular, the precision measurement of the properties of transmissive optics such as transmitted wavefront, surface figure, and index of refraction continues to require new methodology tools.A method of surface figure measurement is described based on the transmitted wavefront of an optical element obtained from a Mach-Zehnder interferometer. Given known values for the refractive index and center thickness, along with the sample's transmitted wavefront, the unknown surface profile is reconstructed in a deterministic way. The technique relies on knowledge of one of the surfaces of the element, such as an easy to measure plano or spherical surface, and is well-suited for testing aspheric surfaces. Additionally, this method has the advantage of making measurements on multiplexed surfaces, such as a lenslet array.Since the index of refraction of materials varies with wavelength, the test sample undergoes chromatic effects with wavelength. Chromatic aberration is an important concern whenever optics are designed for use in the visible spectrum. A method has been presented for obtaining the longitudinal chromatic aberration of a test part from the transmitted wavefronts at 5 different wavelengths. The longitudinal chromatic aberration measurements on a plano-convex lens and an achromat are presented.Injection molding is becoming a popular manufacturing method for optical plastic elements because of low cost and mass production. During injection molding, the plastic lenses undergo large pressure and temperature changes so that the resulting lens has a spatially-varying index. Since the index is assumed to be a single number in the design stage, except for index-gradient lenses, an inhomogeneous index of the sample can cause a decrease in optical performance. The surface reconstruction algorithm can be modified to find two dimensional index values over the test aperture. In this case, both surfaces are measured by an external interferometer and one unknown parameter is the index value.