Study on Optical Property of Fabricating Free-Form Micro-Optical Element by Varying Dose Exposure Method

Abstract

Free-form lens is a special element with non-rotating body and aspheric structure. The surface can’t be expressed by stationary analytic formula, and generally it expressed by discrete list point. Free-form lens can distribute light intensity freely,and control light angle、optical path difference etc. Specific light spot is formed in illumination surface, meanwhile the light energy utilization ratio is improved greatly. There are two kinds of methods about Free-form fabrication, and the first kind of method is that free-form is grinded by a small grinding head ,which is controlled by computer; its disadvantage is low precision、high waster rate，especially for optical material; The second kind fabricating method is that multi-degree-of-freedom single-point diamond turning equipment; its disadvantage is that plenty of sub-micron crack formed when brittle material fabricated. In addition, micro-lens array element can’t be fabricated by the two kinds of methods. In allusion to the feature of free-form without stationary analytic formula ,which varying dose exposure lithography method for free-form micro-optical element is presented for free-form micro-lens and array. The method is from the theory that the relation between different exposure energy and different developing depth, adopting laser direct writing equipment free-form photoresist structure is formed in material surface, then the free-form photoresist structure is transfered on the substrate by ion etching method. Any surface figure can be fabricated by controlling every point laser energy for this method, and surface roughness can’t be effected because ion etching belongs to nanofabrication. In this paper, that low Distribution of light amplitude and relation of between exposure energy and exposure time was analysised on base of optical propagation theory and mathematical model of light distribution law was built and simulated by computer program.The results indicate that light energy distributes in accordance with specific law; the exposure depth increases with exposure time. The experiment of exposure and developing finished, used the BOL-500 laser direct writing system in Changchun University of Science and technology,american Futurrex62A photoresist, 412nn He-Ne laser device, 5‰NaOH developing solution. The experimental data coincide with simulation result via comparative analysis. Meanwhile，a set of free-form micro-lens array testing system was established and focal spot shape,energy distribution,focal distance,F parameter were tested about the micro-lens array.