Abstract

Efficiency and uniformity are desirable factors for optical array illuminators. Unfortunately, the intensity profile of lasers is Gaussian, which may not meet the uniformity requirements of lenslet array illuminators. Fourier transform elements such as phase gratings could lessen the influence of the Gaussian intensity distribution. We present an alternative way to make an optical array illuminator with high efficiency and good uniformity. The approach is first to convert Gaussian laser beams to uniform laser beams that illuminate a lenslet array. A spot array with nearly equal intensity can be generated in the focal plane of the lenslet array. A volume phase hologram recorded with two collimated Gaussian beams is an efficient way to convert Gaussian beams of light into uniform non-Gaussian beams. According to Kogelnik's coupled wave equation, the diffraction efficiency at the Bragg condition is a function of the exposure energy. By controlling the exposure energy, the diffraction efficiency is made to be close to the reciprocal of the Gaussian function, so that the intensity of the diffracted beam is nearly uniform when illuminated with a Gaussian beam. A theoretical calculation shows that a 5% variation of the diffracted beam can be obtained while the incident Gaussian beam has 20% variation of the intensity. By using the converted beam to illuminate an lenslet array, we expect to get 80% light efficiency.

© 1992 Optical Society of America