Optics & Laser Technology 39 (2007) 155–156 Improved pinhole spatial filter P. Hariharan , M. Roy Physical Optics Laboratory, School of Physics, University of Sydney, NSW 2006, Australia Received 24 November 2004; received in revised form 9 March 2005; accepted 21 April 2005 Available online 5 July 2005 Abstract If the pinhole placed at the focus of a microscope objective to spatially filter a laser beam is replaced by an annular phase mask, it is possible to obtain a clean beam providing very nearly uniform illumination over an extended area. We now show that even better results can be obtained by adding an absorbing layer to the mask. r 2005 Elsevier Ltd. All rights reserved. Keywords: Laser beams; Beam expansion; Uniform illumination 1. Introduction The beam from a laser operating in the TEM 00 mode typically has a diameter of only 1 or 2 mm and a Gaussian amplitude profile. To obtain reasonably uni- form illumination over an extended area, it is necessary to expand the beam using a microscope objective with a pinhole spatial filter placed at its focus to eliminate random diffraction patterns due to scattered light (spatial noise) [1], and use only the central part of the expanded beam. It has been shown [2] that, by replacing the pinhole by an annular phase mask, it is possible to obtain a clean beam providing very nearly uniform illumination over an extended area, with a minimal loss of light. We now show that even better results can be obtained by adding an absorbing layer to the mask. 2. Theory If the pinhole is replaced with a mask having a radially varying complex amplitude transmittance tðrÞ, the amplitude in the expanded beam at a radial distance r from the axis is given by the integral [3] AðrÞ¼ Z 1 0 aðrÞtðrÞJ 0 ð2prpÞ2pr dr, (1) where aðrÞ is the complex amplitude of the beam incident on the mask. We consider a pinhole containing an annular mask with a complex amplitude transmittance tðrÞ where tðrÞ¼ 1 for rpr m ; t for r4r m ; 0 for rX1: 8 > < > : (2) Such a mask could be produced, as before, in two steps by electron-beam etching of thin films deposited on a glass substrate. In the first step, an opaque film is deposited on the substrate and a pinhole (radius r) is etched in it. In the second step, as shown in Fig. 1,a transparent film with a refractive index n and a thickness d ¼ l=2ðn  1Þ, which produces a phase shift of p, and a thin absorbing film with an amplitude transmittance t, are deposited on the same substrate, and a concentric hole with a radius r m is etched in them. We will assume that this mask is placed in the focal plane of the microscope objective, and the beam incident on the mask has a Gaussian amplitude profile. We will also assume that the outer diameter of the mask ðr ¼ 1Þ is equal to that of the beam waist. We can then optimize ARTICLE IN PRESS www.elsevier.com/locate/optlastec 0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.04.002 Corresponding author. E-mail address: hariharan_optics@hotmail.com (P. Hariharan).