Laser beam shaping and transformation using freeform surface design

Abstract

Lasers are used in many areas including material processing (lithography, semiconductor manufacturing), welding, cutting, drilling, medical procedures (eye surgery and cosmetic skin treatments), and optical data processing. Laser diodes are the most developed lasers in recent years, due to their multiple features like low cost, compactness, electronic compatibility, broad range of wavelengths, and high pulse prepetition frequency values. On the other hand, the irradiance distribution of a laser-diode is elliptical Gaussian, not circular. In order to be widely used commercially, the beams from the laser diodes must be shaped to a desired shape through optical elements by the redistribution of energy profile into other beam irradiance profiles [3]. This work focuses on redistribution of Gaussian profile laser beam irradiance to other forms propagating through an optical media. In this work, single lens system is designed by getting surface parameters which are the coordinates of the surface points through solving the first order partial differential equations based on Snell’s law and energy conservation principle. Meantime, a lens array is applied to the multi-laser source with the Gaussian distribution. The simulation results show that this method can be very well used for beam shaping. For the two lens system, the surface parameters are solved by the same method used in one freeform lens design. Meanwhile, another new method based on the separated variable mapping is also used in acquiring the surface parameters. This separated variable mapping means that the surface parameters of the source plane and target plane can be numerically specified based on “source-to-target” [4] or “target-to-source” [5,6]. And the mapping is calculated based on energy iii conservation principle, Snell’s law, and constancy of optical path length. According to comparison of the simulation results from two different methods, the method based on solving partial differential equations is more effective and efficient to transform a collimated Gaussian laser beam into a rectangular “flat-top” one, and the uniformity is almost 90%. Also, a lens array is applied to the multi-laser source with Gaussian distribution. The simulation results again show that the method based on solving partial differential equations is better than the other one.