Measuring Aspheric Optics using Binary Holograms

Aspheric surfaces are indispensable in high-performance optical systems. The ability to accurately manufacture these surfaces to the required shape depends on the ability to measure them. In this project we develop and characterize procedures that address this measurement challenge through the application of Computer Generated Holograms (CGHs). The project focuses on an innovative application of CGHs to measure the mandrels used to form mirrors for X-ray telescopes.

The application of aspheric surfaces in optical systems increases system performance while reducing complexity, weight, and cost. However, interferometric measurement of aspheric surfaces poses formidable challenges because it is difficult to obtain a reference wavefront that closely matches the desired form of the asphere. This is a measurement barrier to the development, manufacture, and subsequent application of aspheric elements. In this project we address this challenge through the application of Computer Generated Holograms (CGHs). CGHs are a class of nano-structured optics used as auxiliary optics to enable the form measurement of aspheric polished optical surfaces.

Fabrication mandrel.

The goals of this project are the development and characterization of procedures for the application of CGHs to measure aspheric optics. The project focuses on developing a test for measuring the shape of non-focusing polished optical surfaces in collaboration with the National Aeronautics and Space Administration (NASA). An important example of this type of surface are the mandrels used to form segmented mirrors for next-generation space-based X-ray telescopes. These telescopes will have hundreds of nested, wafer-thin, parabolic or hyperbolic mirrors which must be fabricated and aligned to very tight specifications. Tiny deviations in the shape and alignment of the mirrors would result in distorted and blurry images. The mirrors are made by slumping thin glass sheets on a heated, precisely formed mandrel. Measuring the shape of the mandrels is a very challenging measurement problem. Well-established optical tests for conic aspheres cannot be applied to the mandrels - the non-focusing counterpart of the focusing X-ray mirrors. In addition, the measurements have to ensure that mirrors fabricated from different mandrels have a common focal point. Our new measurement approach holds promise to address these challenges, thus facilitating the realization of a new generation of X-ray telescopes.

The technical plan of the project has the following components: 1) design of the CGH patterns and measurement setup that yields the capability to measure the aspheric optical surface to the required accuracy, 2) development of generic procedures to apply semiconductor and nano-fabrication equipment to manufacture the CGHs, 3) development of inspection methods to validate the wavefronts generated by the CGHs, 4) assessment of the relation between fabrication errors and CGH performance, 5) application of the manufactured CGHs to measure the aspheric mandrel surface, and 6) development of standards-compliant uncertainty statements for the measurement of the aspheric surface that address the effects of CGH performance and alignment errors.

• Completed, in collaboration with NASA, the design and theoretical analysis of a new measurement technique that uses two Computer Generated Holograms (CGHs) to measure both the form and dimensional errors of mandrels for the fabrication of x-ray telescope mirrors.  Completed the design of the CGHs and developed algorithms to generate the pattern layout files required to fabricate the CGHs.  Developed fiducialization scheme to align the CGHs and fix the absolute scale of the measurement.  Completed theoretical performance simulations, e.g., to minimize the effects of higher order diffractions and to estimate measurement uncertainties.
• Conducted process characterization experiments for fabricating diffractive optical elements and developed alternative approaches where necessary. Examples are a modified spin-coating procedure suitable for thick optical substrates and a modification to the Reactive Ion Etching (RIE) process to achieve uniform etching depth over large areas.