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激光直写技术是一种快速制造技术,于20世纪80年代随着大规模集成电路的发展而兴起。如图1(a)所示,这种技术在制备过程中不需要掩模版,首先在处理后的材料基板表面涂覆一层光刻胶,预先计算出光学元件各点的浮雕深度数据,然后利用计算机辅助设计(Computer Aided Design,CAD)等软件绘制出所需图形,导入程序软件输入参数,之后通过控制激光束或者三维运动平台的移动,在光刻胶层上直接扫描曝光,最后经显影、化学蚀刻和去胶等步骤,制备出微细的连续位相浮雕结构,简化了步骤,缩短了生产周期[18-20]。经过几十年的发展为精密元器件的制备提供了新的途径,可用于各种掩模版[21-22]、二元光学元件[23-24]的制作和非球面的检测[25-26]等,具有成本低、写入速度快、操作简单等优点,被视为具有巨大发展潜力的光刻技术[27]。影响DOE表面质量的主要因素如图1(b)所示,因此,考虑激光直写技术中工艺的合理性和设备的先进性对元件的加工质量具有重要意义,文中将主要针对聚焦系统、激光能量、扫描速度阐述对制备衍射光学元件的影响。
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对于激光直写技术,要获得高分辨率必须对焦点处的能量进行极端压缩,使聚焦在样品上的光斑尺寸超过光学衍射极限,从而实现亚波长光刻分辨率[28-29]。直写光束的焦点位置对衍射光学元件的高精度制备有着至关重要的影响,因此,有必要对曝光焦平面进行精确检测和控制。Du等[30]研究了焦平面的检测方法,利用环形DOE和四分之一挡板对入射激光束的光斑形状进行调制,随着样品位置的改变,由探测相机采集到反射激光束的信息(能量和位置分布),得出样品离焦量的大小和方向。该方法的线性检测范围至少可以达到76 µm,灵敏度可以达到100 nm,检测精度可以达到100 nm。这项研究结果非常适合在激光直写过程中进行大范围焦平面的高精度检测。
通常激光直写系统在设计时也会引入自动对焦模块,以消除环境振动的影响,但直写系统中固有的离焦误差(包括光源折射率差异和对准精度等)仍然会对DOE的特征尺寸精度产生重要影响。针对此问题,Zhu等[31]利用像散聚焦技术原理开发了一种自动聚焦子系统,见图2(a)。光纤激光器(波长为650 nm)发出的红色探测光束经扩束和偏振分束器(PBS)的反射后,最终通过显微物镜聚焦于只可被蓝光写入的样品上;利用光电探测器接收被样品反射的红色光束,随着试样与显微镜物镜z轴位置的变化,光电探测器上的焦点形状发生变化,并返回一个与距离相关的电压信号,输入到用于调整显微镜位置的闭环反馈电路。使用直写系统制备了尺寸100 mm×100 mm、周期为2 μm熔融石英光栅,其分辨率可以达到亚纳米级,如图2(b)所示,通过实验的误差确定和预补偿技术可以有效消除偏差,极大地提高了制造精度。
图 2 (a) 自动对焦子系统示意图(左)和焦点对准光束和倾斜光束放大图(右)[31];(b) 周期为2 µm的铬光栅的SEM照片[31];(c) 激光直写制备曲面衍射结构系统示意图[32];(d) 自动对焦系统示意图[32];(e) 具有二元菲涅耳波带片结构的球面透镜[32]
Figure 2. (a) Schematic diagram of the autofocus subsystem (left) and magnification of the in-focus and oblique beams (right)[31]; (b) SEM photo of chromium grating with period of 2 µm[31]; (c) Schematic diagram of laser direct writing system for preparing curved diffraction structures[32]; (d) Schematic diagram of autofocus system[32]; (e) Spherical lens with binary Fresnel zone plate structure[32]
Häfner等[32]提出了一种改进型的激光直写系统,可以在旋转对称曲面上制备任意的衍射结构。图2(c)显示了直写系统的示意图,该制备过程可概括为三个步骤,首先通过集成的线性空气轴承台进行直写点的定位,根据基板表面凹陷度不断调整直写头的高度;然后利用图2(d)开发的自动对焦系统在倾斜表面上进行对焦;最后在垂直线性空气轴承平台上控制压电执行器不间断地曝光整个基板,制备了具有二元菲涅耳波带片结构的球面透镜,如图2(e)所示,最小结构周期为2.4 μm,这种自动对焦系统能够制备表面坡度高达15°的旋转对称曲面,显示了激光直写在制造复杂表面的技术优势。
Ai等[33]利用动态聚焦透镜研制了一种新型激光直写系统,其特点是聚焦透镜可以在z方向上连续线性地改变焦点位置,始终将激光聚焦在曲面上,可移动镜头的精度可以达到±1 μm。使用波长为355 nm,功率为20 mW的激光器,配合三维位移平台在40 s内制作了线宽和间距分别为12.5 μm和25 μm的同心圆形光栅。表明开发的聚焦系统能够在不同曲率半径的基板上实现微米级精度的快速制造。因此,保证焦点位置的准确性是制备高质量表面微结构的重要前提,科学合理的自动对焦系统结合焦平面检测方法是保证DOE获得高分辨率的重要手段。
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激光直写制备衍射光学元件是将聚焦的激光束作用于光刻胶的表面,利用光刻胶的曝光显影特性获得设计的结构元件的过程。利用直写技术制造DOE时,所获得的结构分辨率不仅受复杂化学(光反应、暗反应、扩散和链生长)的影响,而且还与直写系统的选定参数有关,例如激光功率和写入速度[34]。一方面,光刻胶层需要吸收能量到达一定的阈值才可以进行有效的曝光,当能量密度低于曝光阈值时,直写图案就无法显现;但是随着激光功率的增大,由于激光的强度分布特性会使线宽也随之增大。另一方面,扫描速度也会对光刻胶的能量吸收产生重要影响。当扫描速度过快时,光刻胶的部分面积无法吸收充足的能量达到曝光阈值;而当扫描速度过慢时,在写入路径上会因为能量的累计而造成过烧蚀现象;通常写入速度至少为每秒几十微米。
Jwad等[35]研究了激光能量密度和扫描速度对制造不同纳米级厚度薄膜的影响,利用仿真和实验使用纳秒激光器在覆有钛涂层的基板上制备了二级相位型菲涅耳波带板(FZP)。通过控制扫描速度改变激光累积的能量密度,将TiO2的厚度控制在纳米级,而且调节能量密度和扫描速度还能够有效提高生产效率。华中科技大学的艾俊[18]研究了激光直写光刻的能量密度分布与光刻胶的作用关系,使用激光功率为40 mW、聚焦光斑半径为5 μm的355 nm紫外激光垂直入射到光刻胶表面,建立了光刻胶层内不同深度处的曝光能量密度分布,如图3(a)所示(选择其中的z=0、0.5、1 μm)。结果表明,不同深度处的曝光能量分布遵循高斯分布,且随着胶层内深度的增加,曝光能量急剧下降。因此对不同厚度的光刻胶,需要采用不同的曝光能量密度才能彻底曝光。
中国科学院长春光学精机械与物理研究所的李凤有[36]对激光直写光刻中的激光功率和扫描速度进行实验探讨,研究了两者对直写线宽的影响。实验采用波长为442 nm的He-Cd激光器,在保持激光输出功率一定的情况下,如图3(b)所示,发现随着扫描速度的增快,对应的曝光量逐渐减小,光刻得到的线条宽度也随之变小;保持扫描速度恒定,如图3(c)所示,发现写入线宽随着激光输出功率的增加而变宽;当曝光量与光强的乘积值低于曝光阈值时,显影后的光刻胶将不会出现直写痕迹。事实上,不同种类和不同膜厚的光刻胶,需要的能量密度和刻写速度均是不同的,两者对光刻胶是共同作用的关系,在基于不同应用的特定结构制造中,需要对这两个参数进行综合分析和实验确定。
图 3 (a) 光刻胶层内不同深度处的曝光能量密度分布图[18];(b) 扫描速度与扫描线宽关系图[36];(c) 激光功率与扫描线宽关系图[36]
Figure 3. (a) Exposure energy density distribution at different depths in the photoresist layer[18]; (b) Schematic diagram of the relationship between scanning speed and scanning line width[36]; (c) Schematic diagram of the relationship between laser power and scan line width[36]
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随着光学产业的不断发展,人们对光学元件在尺寸、结构和加工精度等方面提出了更高的要求,普通的激光直写技术已经满足不了精细化的现代需求,而飞秒激光的出现为微光学元件的制备提供了新的工具。飞秒激光是指脉冲宽度极窄的超短脉冲激光,与材料发生作用时可利用多光子吸收特性将加工区域精确地控制在激光焦点处,在极短的时间内诱导材料快速电离,产生的热影响小,加工边缘整齐,可以实现微纳结构的高精度制备[37-39]。
高斯等[40]针对蓝宝石难加工的问题,提出利用飞秒激光双光子吸收效应对蓝宝石衬底进行精细加工,并系统研究了激光功率和扫描速度对加工分辨率的影响。在激光功率为0.96 mW、扫描速度为0.1 mm/s的参数下制备了单条线宽约为61 nm的直线结构,实现了超越光学衍射极限的加工分辨率,为高硬度材料的微纳结构制备提供了参考。
刘培元等[41]利用飞秒激光搭建的双光子聚合系统,在直径为6.9 μm的微纳光纤上制作了8个周期约为95 μm、平均厚度和宽度约为2.9 μm和4 μm的光栅块,成功获得了长周期光纤光栅,制造误差均在要求的范围内。王荣荣[42]、廖常锐[43]、苏亚辉[44]等人纷纷研究了基于飞秒激光的双光子聚合技术在微纳米器件制备中的应用,结果均表明,这项技术不仅可以实现三维微结构的加工,还可以达到高效精准的快速制备目的。文中对不同结构类型的飞秒激光直写系统进行了总结比较,如表1所示,下文将对基于直角坐标系和极坐标系的飞秒激光直写系统进行重点介绍。
表 1 不同类型飞秒激光直写系统的比较
Table 1. Comparison of different types of femtosecond laser direct writing systems
Femtosecond laser direct
writing systemSubstrate surface
structurePlatform movement
directionProcessing
characteristicsBased on cartesian coordinate system Piezo platform Linear symmetry plane structure Linear motion in X/Y/Z direction High machining accuracy;
Low processing efficiencyScanning mirror Linear symmetry plane structure;
Centrosymmetric surface structureLinear motion in Z direction High processing efficiency;
High machining accuracy
(with high numerical aperture objective)Linear motor Linear symmetry plane structure Linear motion in X/Y/Z direction Large processing range;
Low processing efficiencyScanning mirror
Linear motorLinear symmetry plane structure;
Centrosymmetric surface structureLinear motion in X/Y/Z direction High processing efficiency;
Large processing rangeBased on polar coordinate system Centrosymmetric surface
structureLinear motion in X/Y direction
Rotational movement in the Z directionHigh machining accuracy and efficiency;
Large processing range -
飞秒激光直写系统主要有两种类型:一是基于直角坐标系,二是基于极坐标系。目前,基于直角坐标系的飞秒激光直写系统应用最为广泛。图4(a)所示为飞秒激光直写加工系统示意图[45],基本原理是将激光焦点固定,三个压电平台构成三维直角坐标系,样品基板固定在平台上,通过计算机控制驱动器实现压电平台的移动完成激光的直写扫描。
图 4 (a) 飞秒激光直写微纳加工系统示意图[45];(b) 不同脉冲能量下获得的VPG显微放大图像[46];(c) 拓扑电荷数分别为1、3、16的HOVML的SEM图[47];(d) 不同拓扑数下HOVML的聚焦特性[47]
Figure 4. (a) Schematic diagram of the femtosecond laser direct writing micro-nano processing system[45]; (b) VPG microscopic magnification images obtained under different pulse energies[46]; (c) SEM images of HOVML with topological charge numbers of 1, 3, and 16, respectively[47]; (d) Focusing properties of HOVML under different topological numbers[47]
基于压电平台的激光直写系统是直角坐标系中常用的加工技术之一。Ma等[46]利用基于压电平台的直写系统在硫系玻璃内部制作了体相位光栅(VPG),如图4(b)所示,实验采用波长为800 nm的飞秒激光器,移动平台的位移精度为0.1 μm,以150 μm/s的移动速度在玻璃内部制造了宽度为1000 μm的VPG,周期为5 μm。结果表明,基于压电平台的飞秒激光直写技术可以制备出具有清晰衍射图案和高衍射效率(评价DOE性能的重要指标)的光栅结构。Tian等[47]在压电平台上进行了混合折衍射元件(HOVML)的制备实验研究。实验使用波长为780 nm、波长为6 mW的飞秒激光,经NA=1.4的物镜聚焦到光刻胶中,在压电平台的移动下通过控制双电流镜组进行扫描,利用原子力显微镜测量样品,如图4(c)所示,观察到样品表面光滑,粗糙度小于10 nm。如图4(d)所示,从绿色箭头开始到黄色箭头结束,相位逐渐减少2π,该系统所加工的元件可以独立产生光学涡旋,展现了在聚焦方面的独特光学特性。Liu等[48]提出一种将飞秒激光直写与离子刻蚀技术相结合的方法。通过60倍放大物镜将波长为790 nm的飞秒激光聚焦到聚合物中,利用压电平台和双电流镜组控制激光的焦点,制备了高度为2.1 μm、周期为10 μm的闪耀光栅,此闪耀光栅可以将激光强度分布调整为不同的衍射级,这种方法为加工具有更复杂结构的微型光学器件提供了基础。
基于压电平台的激光直写技术加工精度高,但是加工尺寸受压电平台的行程限制,不能制备大尺寸的衍射光学结构[49-50]。基于直线电机的飞秒激光直写系统加工范围大(可达到厘米级),可以加工大尺寸三维结构[51],但是位移平台存在较大的惯性,会影响加工的效率和精度。因此,研究人员开发了基于扫描振镜的飞秒激光直写系统,该系统是通过控制两个相互垂直反射镜的偏转角度和压电平台的升降来实现对样品基板的三维扫描,利用这种方法可以有效地提高加工效率和精度,但是受到聚焦物镜的放大倍数和数值孔径的影响,加工范围有限[52-53],而且使得光路更加复杂。
针对此类问题,吉林大学的研究团队[54]提出利用高速扫描振镜系统与气浮平台相配合的方法,将气浮平台与数字扫描振镜集成到一个系统。加工系统如图5(a)所示,进行加工时,振镜在XY平面内负责扫描加工阵列中点每个微结构,直线电机驱动气浮平台三轴移动来扩大加工范围,最终拼成阵列。使用波长为780 nm的飞秒激光经100倍物镜(NA=1.35)聚焦于光刻胶,成功制得填充因子接近100%的菲涅耳波带板(FZPA),如图5(b)所示,制造面积超过了物镜的视场角,其衍射效率达到89%。该团队进一步利用直写系统在9 mW的激光功率下,制备了直径为100 μm,每层衍射层的微结构高度为4000 nm、1064 nm和4000 nm的多层衍射光学元件[45],获得结果如图5(c)所示,用此系统制备的多层衍射光学元件都具有很好的加工精度,微结构也得到了很好的表征,通过此方法改善了线性平台小范围加工时由于系统振动造成的加工误差,同时实现了单元结构面积大的高精密微结构加工。
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上文所述的激光直写系统由于移动平台本身的结构特点,衍射元件的加工效率和加工范围总会受到限制。配合扫描振镜和直线电机的激光直写系统虽然在元件尺寸和加工效率方面有所改善,但系统的结构变得复杂。将极坐标系引入激光直写系统是一种更加灵活的方法,极坐标式飞秒激光直写系统改进了位移平台,集成了直线运动和回转运动,加工范围和加工效率与直角坐标式相比有了极大的改善[18,55]。
姜俊等[56]搭建了一种由水平位移台和高速旋转台组成的极坐标飞秒激光直写系统,如图6(a)所示。采用中心波长为800 nm、功率为13 mW的飞秒激光,扫描速度控制为20 mm/s,加工了单层高为5 μm、宽为10 μm的四阶台阶结构,扫描时间约为10 min,并在曲面透镜上制备了直径为10 mm、周期为5 μm的衍射圆光栅结构,扫描过程约为45 min,制作的衍射结构和观察的图案如图6(b)所示。这项工作有效解决了直角坐标式直写系统在加工范围、加工精度以及加工效率三者间的矛盾,可以实现飞秒激光直写技术大尺寸、高精度、高效率地制备三维结构。
图 6 (a) 极坐标飞秒激光直写系统示意图[56];(b) 透镜曲面上圆光栅图像与激光扫描共聚焦显微镜图像[56];(c) 衍射微光学元件图案化的系统配置[57];(d) 超薄衍射光学阵列的图案化程序[57]
Figure 6. (a) Schematic diagram of polar femtosecond laser direct writing system[56]; (b) Image of the circular grating on curved surface of the lens and LSCM image of the circular grating[56]; (c) System configuration for patterning of diffractive micro-optics[57]; (d) Patterning procedure for ultra-thin diffractive optics array[57]
Low等[57]提出了一种不同的极坐标飞秒激光直写系统,利用扫描仪路径的改变进行角度的变换扫描,如图6(c)所示,并研究了直写系统的平均功率、脉冲重复频率和扫描速度三种参数对制备的影响,成功制备了具有更佳光学性能的超薄衍射光学元件,如图6(d)所示。表明飞秒激光直写系统拥有更高的设计自由度和灵活性,同时该系统还为超薄衍射光学阵列的简单高效制备提供了一种实用方法,对微光学器件的广泛应用提供了有力支持。Hua等[58]借助基于气浮旋转平台的飞秒激光直写系统制备了混合折衍射透镜(HDRL)。结果表明,这种衍射元件具有优异的消球差聚焦和成像性能,此研究为实现飞秒激光直写系统制造HDRL提供了新的途径。因此基于极坐标系的飞秒激光直写系统在制备衍射光学元件方面具有很大的应用空间。
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近年来,随着激光技术的不断发展,其在材料加工领域的应用越来越广泛,上文所述的激光直写技术大多是以单光束形式进行扫描,优点是光束更容易控制。然而大多数激光器特别是飞秒激光器输出的能量过大而不适合直接进行加工,需要经过衰减才能进入光路,因此存在能量利用率低和加工效率差的问题,对此研究者提出了基于激光直写技术的多光束并行加工方法。
Sola等[59]采用双光束激光直写技术在聚合物中成功制得了周期性衍射光栅图案,其装置如图7(a)所示。结果表明,这种双光束并行加工的方法不仅可以制备高衍射效率的光学元件,也使得加工效率与单光束相比提高了两个数量级以上,在柔性材料的高效率制备方面有着潜在的应用。Poleshchuk等[60]通过与干涉光刻法比较,提出利用达曼光栅将写入激光束分为多光束形成光点阵列的新方法,实验使用五光点阵列成功制备了周期为1.6 µm的具有规则圆形结构(衍射轴棱锥)的衍射光学元件。根据达曼光栅的结构不同,光点的数量可以从几个到几十个或几百个不等。与干涉图案法相比,这种方法在制备DOE时可以按照光点数量成倍地提高加工效率,同时还可以生成更高质量的光束,从而实现DOE的高效精细加工。Winfield等[61]提出可以采用多点双光子聚合的方法,通过衍射光学元件将激光束转换为由四个等强度光点组成的线性阵列,一次性制备了周期性的透射光栅。结果表明,这种制造方法将单光束分为等强度的多光束,不仅可以提高激光能量的利用率,同时也显示了多点双光子聚合技术在扫描效率上的显著优势。
图 7 (a) 双光束激光直写干涉装置示意图[59];(b) 不同激光功率下制造的5×5光斑DOE邻近效应示意图[62];(c) 飞秒激光加工系统光路图[63];(d) 1×11光栅阵列的相位全息图和CCD观测图[63]
Figure 7. (a) Schematic diagram of the double-beam laser direct writing interference device[59]; (b) Schematic diagram of the proximity effect of 5×5 spot DOE fabricated under different laser powers[62]; (c) Light path diagram of femtosecond laser processing system[63]; (d) Phase hologram of 1×11 grating array and CCD observation image[63]
多光束激光直写技术是快速制备光学元件的有利工具,但在写入空间相近的结构时会产生“邻近效应”,最终会影响光学成像质量[64]。Arnoux等[62]研究了多点并行加工制备DOE时邻近效应的依赖性,如图7(b)所示,展示了不同数量的点和间距下的邻近效应结果,研究发现使用更大的写点间距来规避这些邻近效应是可能的,为大规模并行双光子激光直写提供了新的见解。
空间光调制器(Spatial Light Modulator,SLM)是一种可以在外部信号的控制下改变入射光振幅、偏振以及相位的动态元器件[65],可以把单个焦点调制成强度近乎相同的多焦点,实现并行多点加工,也可以改变光场强度分布实现并行面加工,极大地提升了加工效率和加工精度[66-67],在制备微结构时更加灵活。闫高宾[68]将空间光调制器引入到激光直写系统中,通过逐面积曝光方法,制备了菲涅耳透镜,利用SLM制备的衍射光学元件的分辨率可达微米级,证明该系统可以满足微光学结构的加工制作要求。周立强等[63]利用空间光调制器加载全息图将入射光束调制成多焦点阵列,如图7(c)所示为结合SLM的加工系统光路图,利用此加工系统可以同时制备11条纳米级宽度的光栅结构,如图7(d)所示,结果表明使用SLM的直写系统可以有效提高光栅的加工效率和加工精度。使用SLM的加工系统其加工效率可根据相位全息图的设计进行控制,这为研究飞秒激光制备大规模光栅的效率问题提供了新的思路。需要注意的是,目前基于空间光调制器的多光束并行加工系统基本使用的是三维位移平台,因此设计加工的光学元件也多为线型光栅结构,导致加工环形对称结构的DOE位移平台系统还需进一步的优化,同时还要考虑光路的设计和调整问题,在之后的应用过程中还需要进一步地研究。
Research and application of diffractive optical element fabricated by laser direct writing
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摘要: 衍射光学元件作为一种典型的微光学元件,其体积小、质量轻、设计自由度多、成像质量良好,在光学成像、光学数据存储、激光技术、生物医学等领域具有广阔的应用前景。随着现代光学系统的不断发展,对衍射光学元件的加工效率和制备精度提出了更高的要求。激光直写技术凭借加工精度高、工艺简单、灵活性好等优势,成为制备高精密仪器中关键光学元件所必需的一种加工方式。针对不同的加工需求,开发了多种激光直写系统,并在应用过程中不断地改进升级。另外,突破衍射极限的飞秒激光微纳结构制造技术,能够获得更高的加工精度和更好的分辨率,为微光学元件的制备提供了新的方法。首先介绍了激光直写技术的特点;其次综述了衍射光学元件直写加工技术的研究进展,包括直写技术的影响因素、激光直写系统和多光束加工技术;接着介绍了衍射光学元件的典型应用,如红外成像、色差校正、光束整形、图像显示;最后,对激光直写技术制备衍射光学元件存在的问题和未来发展趋势做出了总结。Abstract:
Significance Micro-optics theory is a new discipline for the study of the design and manufacturing of micron-sized and nano-sized optical components, as well as the use of such components to achieve the theory and technology development of light waves. As a research field of optics, diffractive optics is based on the diffraction principle of light waves developed microoptics. Diffractive optical technology is of great significance in realizing lightweight, miniaturization, integration, high efficiency and low cost of optoelectronic systems, and the development of diffractive optical technology has also become one of the important ways to develop modern optical systems. As a typical micro-optical element, diffractive optical elements have broad application prospects in industrial and civil fields such as optical imaging, laser technology, and biomedicine due to their small size, light weight, multiple degrees of design freedom and good imaging quality. The processing methods of optical element can be summarized into two types of mechanical processing and optical processing, both of which have their own advantages and disadvantages. The advent of laser provides a new idea for the preparation of diffractive optical elements. Laser processing is a non-contact wear-free technology with high precision and high flexibility, which can process complex contours and has the characteristic of environmental friendliness and simple production process, so the study of laser processing technology in the application of diffractive optical elements is of great significance. Progress With the continuous development of modern optical systems, higher requirements are put forward for the processing efficiency and preparation accuracy of diffractive optical elements. Laser direct writing technology does not need mask plate in the process of preparing diffractive optical elements, simplifies the steps, shortens the production cycle (Fig.1(a)). There are many factors affecting the preparation quality of diffractive optical elements, the article summarizes the main factors affecting the surface quality of diffractive optical elements (Fig.1(b)), and explains the influence of focusing system (Fig.2), laser energy (Fig.3) and scanning speed on the preparation of diffractive optical elements, which is very important for improving the preparation accuracy and surface quality of optical components. Different types of laser direct writing systems should also be considered in the preparation of diffractive optical element with different structures (Tab.1). From the aspects of process and system, the research progress of femtosecond laser direct writing system based on Cartesian coordinate system and polar coordinate system in processing diffractive optical element is discussed (Fig.4, Fig.6). Besides, in order to solve the problems of low energy utilization and poor processing efficiency in the process of laser preparation of diffractive optical element, a multi-beam parallel processing method based on laser direct writing technology is proposed (Fig.7). Diffractive optical elements have a variety of functions in optical systems due to their unique characteristics, and the article summarizes the typical applications of diffractive optical elements, such as infrared imaging (Fig.8), chromatic aberration correction, beam shaping, laser processing (Fig.9), image display, etc. Conclusions and Prospects In the field of optics, the development of micro-optics theory technology continues to promote the advancement of diffractive optics theory. The application of diffractive optical element has also been expanded in more fields. As a high-precision, programmable, short cycle and flexible processing method, laser direct writing technology has incomparable advantages in the preparation of diffractive optical element. But in the actual processing process, there are problems of limited processing materials, insufficient utilization of laser energy, and the complexity of the system caused by the alignment mechanism in the preparation of curved element, so the research on expanding materials, simplifying equipment, optimizing processes and seeking applications is a continuous and important topic. -
图 2 (a) 自动对焦子系统示意图(左)和焦点对准光束和倾斜光束放大图(右)[31];(b) 周期为2 µm的铬光栅的SEM照片[31];(c) 激光直写制备曲面衍射结构系统示意图[32];(d) 自动对焦系统示意图[32];(e) 具有二元菲涅耳波带片结构的球面透镜[32]
Figure 2. (a) Schematic diagram of the autofocus subsystem (left) and magnification of the in-focus and oblique beams (right)[31]; (b) SEM photo of chromium grating with period of 2 µm[31]; (c) Schematic diagram of laser direct writing system for preparing curved diffraction structures[32]; (d) Schematic diagram of autofocus system[32]; (e) Spherical lens with binary Fresnel zone plate structure[32]
图 3 (a) 光刻胶层内不同深度处的曝光能量密度分布图[18];(b) 扫描速度与扫描线宽关系图[36];(c) 激光功率与扫描线宽关系图[36]
Figure 3. (a) Exposure energy density distribution at different depths in the photoresist layer[18]; (b) Schematic diagram of the relationship between scanning speed and scanning line width[36]; (c) Schematic diagram of the relationship between laser power and scan line width[36]
图 4 (a) 飞秒激光直写微纳加工系统示意图[45];(b) 不同脉冲能量下获得的VPG显微放大图像[46];(c) 拓扑电荷数分别为1、3、16的HOVML的SEM图[47];(d) 不同拓扑数下HOVML的聚焦特性[47]
Figure 4. (a) Schematic diagram of the femtosecond laser direct writing micro-nano processing system[45]; (b) VPG microscopic magnification images obtained under different pulse energies[46]; (c) SEM images of HOVML with topological charge numbers of 1, 3, and 16, respectively[47]; (d) Focusing properties of HOVML under different topological numbers[47]
图 6 (a) 极坐标飞秒激光直写系统示意图[56];(b) 透镜曲面上圆光栅图像与激光扫描共聚焦显微镜图像[56];(c) 衍射微光学元件图案化的系统配置[57];(d) 超薄衍射光学阵列的图案化程序[57]
Figure 6. (a) Schematic diagram of polar femtosecond laser direct writing system[56]; (b) Image of the circular grating on curved surface of the lens and LSCM image of the circular grating[56]; (c) System configuration for patterning of diffractive micro-optics[57]; (d) Patterning procedure for ultra-thin diffractive optics array[57]
图 7 (a) 双光束激光直写干涉装置示意图[59];(b) 不同激光功率下制造的5×5光斑DOE邻近效应示意图[62];(c) 飞秒激光加工系统光路图[63];(d) 1×11光栅阵列的相位全息图和CCD观测图[63]
Figure 7. (a) Schematic diagram of the double-beam laser direct writing interference device[59]; (b) Schematic diagram of the proximity effect of 5×5 spot DOE fabricated under different laser powers[62]; (c) Light path diagram of femtosecond laser processing system[63]; (d) Phase hologram of 1×11 grating array and CCD observation image[63]
表 1 不同类型飞秒激光直写系统的比较
Table 1. Comparison of different types of femtosecond laser direct writing systems
Femtosecond laser direct
writing systemSubstrate surface
structurePlatform movement
directionProcessing
characteristicsBased on cartesian coordinate system Piezo platform Linear symmetry plane structure Linear motion in X/Y/Z direction High machining accuracy;
Low processing efficiencyScanning mirror Linear symmetry plane structure;
Centrosymmetric surface structureLinear motion in Z direction High processing efficiency;
High machining accuracy
(with high numerical aperture objective)Linear motor Linear symmetry plane structure Linear motion in X/Y/Z direction Large processing range;
Low processing efficiencyScanning mirror
Linear motorLinear symmetry plane structure;
Centrosymmetric surface structureLinear motion in X/Y/Z direction High processing efficiency;
Large processing rangeBased on polar coordinate system Centrosymmetric surface
structureLinear motion in X/Y direction
Rotational movement in the Z directionHigh machining accuracy and efficiency;
Large processing range -
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