Measured performance of a refractive Gauss-to-flattop reshaper for deep-UV through near-IR wavelengths

Abstract

A system of two plano-aspheric lenses which transforms a collimated, radially symmetric Gaussian beam to a flattop is described. To minimize diffraction of the output beam, the lenses were designed to accept essentially all (99.7%) of the input beam, and the output intensity distribution was chosen to have a controlled roll-off, given by the Fermi-Dirac function. Both aspheric surfaces were convex, simplifying fabrication by the technique of magnetorheological figuring. The optics were made of fused silica and, with suitable antireflection coatings, a single prescription can be used at any wavelength from 250 to 1550 nm. Measurements of the output intensity distribution were made by directly illuminating a CCD sensor with the flattop beam, and these results were compared quantitatively to the theoretical design. At the 8 mm diameter output aperture, 78% of the total beam power is contained in a region with 5% rms intensity variation, representing a fourfold improvement in power utilization over the Gaussian input. The beam propagates approximately 0.5 m without significant change in the intensity profile. Both the intensity uniformity and the propagation range are believed to be limited by the accuracy of the aspheric surfaces. It was verified that expanding the linear dimensions of the beam by the factor m increases by m2 the range over which the beam retains its shape as it propagates. The optics have been successfully used in a holographic data storage test stand and for deep-UV interferometric lithography.