飞秒激光通过高倍物镜加工中的全息散焦控制

Holographic Defocus Control in Femtosecond Laser Processing with High NA Objective

飞秒激光通过高NA值的物镜进行表面微加工时,过短的焦深会影响加工性能。分析了高倍物镜所带校正环对焦点的离散作用,提出了一种利用空间光调制器(SLM)进行全息散焦控制的方法。通过改变衍射轴锥镜(Axicon)计算机生成全息图(CGHs)中环形闪耀光栅的周期,来进行散焦控制以形成长焦深。焦点轴向强度分布以加工于聚酰亚胺薄膜(PI)表面微孔的形式表示,其分布与仿真结果一致。使用0.6NA的40倍镜进行散焦加工,当环形闪耀光栅最小环周期为200μm,并由内向外以400μm递增时,可获得50.90μm的均匀焦深,是直接使用高倍物镜聚焦进行加工所获得的6.15μm均匀焦深的8.28倍。此外还通过SLM与校正环进行复合散焦加工,进一步均匀化了整个焦深区域。

When the femtosecond laser is used in surface micro-structuring through a high NA objective lens,, too short of focal depth affects the processing performance. The focus discrete effects of the correction collar on the objective lens is analyzed. A method of holographic defocus control using spatial light modulator (SLM) is proposed. By varying the period of the annular blazed grating which is based on the diffractive axicon computer generated hologram (CGHs),defocus control is performed to form a long depth of focus. The distribution of focus axial intensity is expressed in the form of a row of micropores which are processed on the surface of polyimide film (PI), and the distribution is consistent with the simulation result. The defocusing process is performed using a 40× objective lens of 0.6 NA. When the center period of the annular blazed grating is 200 pm and increase progressively from inside to outside by 400 μm,a 50.90 μm uniform depth of focus can be obtained, which is 8.28 times the length of the depth of focus obtained by directly processing with high NA objective lens. In addition,the composite defocus process is performed by SLM and correction collar to further homogenize the entire region of depth of focus.