基于正交双向迭代的自由曲面激光光束整形设计(特邀)

Freeform surface design of laser beam shaping by iteration in two orthogonal directions(invited)

激光光束整形技术在现代激光工业领域具有重要作用,为了克服传统激光光束整形自由曲面构造中存在法向矢量偏差的局限性,文中提出一种正交双向迭代的自由曲面设计方法。该方法基于能量守恒和斯涅尔定律建立光源与目标面的能量映射关系,通过正交方向坐标点迭代获得自由曲面数据点,同时利用相邻坐标点的均值进行曲面法向矢量修正,从而提高自由曲面的构造精度。设计实例表明,准直入射的激光光束经自由曲面整形反射镜,在特定目标区域内可形成高均匀度光斑。进一步从自由曲面的连续性、拟合精度与面形误差三个方面,与传统设计方法进行了详细对比分析。结果表明,所提设计方法得到的激光光束整形自由曲面连续性与平滑度更好,拟合精度更高;且在加工误差允许范围内,目标面的辐照度均匀性更稳定。

Objective In many laser industrial applications,laser beam shaping is an important process to redistribute the laser energy,which is highly essential to obtain uniform or prescribed spatial energy distribution with high efficiency.At present,there are different methods for laser beam shaping,including the grouped aspheric lenses,microlens arrays,diffractive optical elements,liquid crystal spatial light modulator and freeform optical technology.Compared with other laser beam shaping methods,the method using freeform surfaces is beneficial to make the shaping optical system more simplified and more compact.In the current freeform surface construction method for laser beam shaping,the seed curve extension algorithm in one direction has the non-negligible normal vector deviation during the generation of surface sampling points.Therefore,in this paper,a method is proposed to reduce the normal errors to improve the construction precision of freeform reflector for obtaining the highly uniform spatial energy distribution in the target plane.Methods There are mainly three steps for constructing the freeform surface(Fig.2-3).At first,the incident beamand the target plane are divided in grids according to equal energy and equal area.The main purpose of this step isto obtain the one-to-one energy mapping between the light source and the target plane,which is based on theconservation of energy and Snell's law.Then,the initial constraint conditions are set according to therequirements,which are used to calculate the sampling data points of horizontal and vertical curves on thefreeform surface.Finally,according to the results of previous two steps,the sample data points on the unknownfreeform surface can be calculated by iteration together with normal error correction.The averaging approach ofcoordinates of adjacent sampling points in the orthogonal direction is applied to relieve the normal deviations,which is very useful for reducing the normal errors to obtain smooth freeform surface relatively.Results and Discussions The proposed freeform surface construction method can effectively regulate acollimated Gaussian laser beam into the square or rectangular intensity distribution with high uniformity.On thetarget plane for a square pattern(Fig.5),the normalized irradiance uniformity is about 88.70%in the globalregion.Along the lines x=0 mm and y=0 mm on the target plane,the irradiance uniformity is about Ux=88.18%and Uy=86.67%respectively.Besides,the irradiance uniformity of local region(70 mm×70 mm)is about 92%.Inthe similar way,for a rectangular pattern on the target plane(Fig.6),the corresponding normalized irradianceuniformity is as high as about 94.30%as well as Ux=92.96%and Uy=94.07%along the lines x=0 mm andy=0 mm,which realize the laser beam shaping with high performance.On the other hand,the irradianceuniformity can also reach about 90%when the target plane has different distances for the square pattern(Fig.7-8).This indicates that the proposed method keeps robust elegantly.Further,in the aspect of surface smoothness,fitting precision and irradiance uniformity stability over a certain manufacturing error range(Fig.10-12),thefreeform surface constructed by the proposed method shows great performance compared with that constructed bythe traditional design method.Conclusions The freeform surface design by iteration in two orthogonal directions with surface normalcorrection is proposed,which can effectively regulate a collimated Gaussian laser beam into the square orrectangular intensity distribution with high uniformity.The feature sampling points of the freeform reflector arecalculated iteratively in two orthogonal directions based on energy conservation and Snell’s law.Meanwhile,theaveraging approach of coordinates of adjacent sampling points in the orthogonal direction can relieve the normaldeviations effectively.Therefore,it is very helpful for constructing the freeform surface more precisely.Thecapabilities of the presented method are demonstrated and verified by examples.Moreover,for the target planewith different distances within a certain range,the energy uniformity maintains 90%well.At the same time,thefreeform surface designed by the proposed method not only has high fitting accuracy,but also has a more stableirradiance uniformity on target plane within the allowable machining error range.It means that the proposedfreeform surface construction method keeps robust elegantly,which is very necessary and critical for laser beamshaping.