Ray tracing, wavefronts, and caustics: use of Shack-Hartmann wavefront sensor for analyzing light propagation

Abstract

ABSTRACT Wavefront measurements are a key point in the development of imaging techniques. Nowadays, a common tool for these measurements is the Shack-Hartmann sensor, where the results are often given in terms of the Zernike polynomials. The interpretation of the results, as one moves the Shack-Hartmann sensor in the axial zone, is sometimes difficult as it involves the task of visualizing the geometrical propagation of the wavefront. We present a numerical tool based on ray tracing that visualizes wavefronts and caustics as the beam propagates and enables the calculation of the Zernike polynomials at any intermediate stage. Keywords: Wavefront measurements. Shack-Hartmann sensor. Zernike polynomials. 1. INTRODUCTION The knowledge of the wavefront at any st age is the basis for the development of optical instruments. For example, for the calculation of the point spread function (PSF). A comm on device for the measurement of wavefronts is the Shack-Hartmann (SH) sensor, where an array of microlenses performs a wavefront sampling on a particular plane. The interpretation of the results given by the SH is related (among other things) to the mode of operation of the device. Anyway it is convenient to develop computational tools for analyzing the evolution of wavefronts as light propagates, since the changes in wavefront’s shape are not easy to visualize. In fact, near the focal zones this shape prevents the use of the SH sensor, as we will illustrate. Our aim is to present the theoretical basis for the development of this numerical tool, able to visualize wavefronts and caustics as the beam propagates and also enabling the calculation of the Zernike polynomials at any inte rmediate stage.