A Toolbox for Isophase-Curvature Guided Computation of Metrology Holograms
Ulf Griesmann, Johannes A. Soons, Gufran S. Khan
We describe the algorithmic foundations of an open-source numerical toolbox, written in the Octave language, for the creation of computer-generated binary and multi-level holograms used in interferometric form error measurements of complex aspheric and free-form precision surfaces and wavefronts. In a typical measurement setup for this type of surface, a hologram is used to generate a test wavefront that has the design shape of the surface, which is then compared to a fabricated part using an imaging laser interferometer. The optical function of the hologram in the measurement is generally modeled with optical ray-tracing software and it can be encapsulated by a scalar optical phase function φ : R 2 → R. The toolbox converts phase functions into equivalent binary holograms that generate the desired test wavefronts for an interferometric form error measurement. The algorithms in this toolbox take advantage of the relationship between the local properties of phase functions and the local geometry (curvature) of isophase lines. It forms the core of an efficient algorithm for the computation of optical holograms. Holograms are created in a format that can be processed by most laser- or e-beam lithography systems. While the toolbox is chiefly aimed at the creation of hologram layouts needed for measurements of precision surfaces and wavefronts, we show that the isophase following algorithm is easily extended to phase functions with singularities and discontinuities. Such phase functions result in holograms with zone bifurcations, and they can be used to generate helical wavefronts. Light beams with helical wavefronts have applications beyond surface- and wavefront metrology. The toolbox also includes a family of functions for the efficient estimation and evaluation of Zernike polynomials, which are widely used in optical applications.