The Application Of Freeform Optics In Laser Beam Shaping

Due to their good directivity, mono-chromaticity and high brightness, lasers are widely used in material processing, laser welding, photolithography and other fields. In these applications, the energy distribution of laser is required to be uniform. However, in most cases, the original energy distribution of the laser beam is Gaussian distribution, which restrict the application of the laser in many fields. Therefore, it is necessary to design laser beam shaping system to transform the Gaussian beam into uniform beam.At present, a variety of laser beam shaping methods have been developed,but most of them are based on collimated lasers. In these previous methods, the laser beam need to be collimated before entering the shaping system. Therefore, the collimated system is necessary for divergent beam shaping. The collimated system not only complicates the laser beam shaping system, but also reduces the optical transmission efficiency. In the thesis, the freeform lens is designed to transform the Gaussian beam into uniform beam. Both the collimated and divergent beam can be shaped by the freeform lens directly. In addition, we designed a laser beam shaping system made up of freeform lens and prism groups, which transform a Gaussian beam with square aperture into a uniform beam with rectangular aperture.This thesis is organized as following:Firstly, a single freeform lens is designed, which realizes the shaping of the collimated laser beam. In the design process, the energy mapping between the input and output beam is established at the beginning. Based on the energy mapping relationship and Snell's law, the point cloud data on freeform surface are calculated. Then, solid model of the freeform lens is constructed in Rhino software. By the algorithm, rotational symmetry and non-rotational symmetry freeform lens are designed respectively for the collimated laser beam shaping. The collimated Gaussian beam with circular aperture is transformed into the uniform beam with irradiance uniformity of 90% by the rotational symmetry freeform lens. And the collimated Gaussian beam with rectangular aperture is transformed into the uniform beam with irradiance uniformity of 90.5% by the non-rotational freeform lens.Secondly, we propose the inverse feedback optimization and three-dimensional interaction optimization design method which can be used to design freeform lens for the divergent laser beam shaping. In the case of freeform lens design for divergent beam shaping, the initial layout of lens should be designed based on the collimated beam. With the initial layout of lens, the inverse feedback optimization and three-dimensional interaction optimization design method are used to optimize the initial freeform lens, respectively, when the diverging beam is applied. The key of the reverse feedback optimization method is to adjust the size of the energy grids in the cross section of the input beam according to the feedback coefficient. By the method, a divergent Gaussian beam with divergent angle of 2.5 °(half angle) is transformed into uniform beam with the irradiance uniformity of 90.4% by 3 times of reverse feedback optimization. Also, a three-dimensional interactive optimization method is proposed. Before optimization, the initial freeform surface sample points are fitted as a polynomial. Thus, the coefficients of the polynomial are optimized by using 3D interactive optimization. By employing the 3D interactive optimization method, the irradiance uniformity of the output beam passing the optimized freeform lens is improved by 5.6% than that passing the initial freeform lens. The machining error analysis and the assembly tolerance analysis of the freeform lens are also performed. It is shown that the uniformity is almost unchanged while the process error varies from-5um to 5um. And the longitudinal deviation error dz has little influence on the irradiance uniformity, but the lateral deviation error dx or slant angle error d? has a significant influence on the irradiance uniformity.Finally, we designed a laser beam shaping system made up of freeform lens with double freeform optical surfaces and prism groups, which transform a Gaussian beam with square aperture(whose beam waist is 10mm) into a uniform beam with rectangular aperture(160mm×10mm). The irradiance uniformity of the shaped beam is 93.2%.