New methodology for measuring highly aberrated wavefronts induced by diffractive optical elements

Diffractive optical elements (DOE) have been widely used in both scientific and industrial applications. Optical wavefront measurements are one of the most important methodologies in verifying the performance of DOEs. Due to its non-destructive nature, ease of implementation, and relative short operation time, optical interferometry-based systems for wavefront measurements remain popular. The advantages and drawbacks of a non-common path and common path interferometry technique are examined first within this article. As wavefront induced by a DOE is typically highly aberrated, a new wavefront measuring system was invented to circumvent the limitations of conventional optical interferometry systems which have difficulty in handling highly aberrated wavefront systems. This newly invented system named Sparrow, which is an acronym for `Shear Phase Analyzer, Reflective/Reflective Optical Wavefront,' is discussed in detail. Within this system, a shearing interference was executed to obtain the intensity maps and the phase-shifting technique was implemented to convert the measured intensity maps into a wrapped phase difference map. As highly aberrated wavefronts can be measured by using this new instrument due to its optical and mechanical layout, a new non-iterative, path-independent phase unwrapping methodology that was developed to ensure that the phase-unwrapping tasks can be accomplished is detailed as well. More specifically, instead of using conventional Zernike polynomial fitting algorithms, an improved discrete cosine (DCT) and discrete Fourier transform (DFT) techniques, which were implemented based on the least-squares merit functions, will be examined. The simulation and experimental data obtained which verified the effectiveness and accuracy of our newly invented system is also studied.