摘要
本论文围绕国家重大项目,结合高功率激光器装置需求,对衍射光学元件的理论设计、加工工艺、测试应用等方面进行了系统的论述,其主要研究内容及获得的成果如下:
一、基于平面靶设计精细化采样的插值迭代算法
用传统的输入-输出算法(I-O:Input-Output Algorithm)进行束匀滑器件的迭代设计时,算法仅对输出面上采样点的光强进行了强制反馈,在设计结束后,在输出面采样点上能够获得比较好的束匀滑效果。如果将输出采样加密,在原采样点集之外的抽样信号,其光强值会有较大的起伏。在数值计算上,利用信号采样的内插定理,可以由原采样点计算得到这些非采样点的抽样信号,基于这一原理提出的插值迭代设计方法在迭代的过程中加入了插值点的反馈,在输出平面上做波阵面的恢复时,其振幅、位相分布能获得更好的反馈影响,就使得输出面上采样点的光强分布在迭代过程中能够获得更好的均匀性。利用这种方法不但可以提高设计结果的评价,而且能比较稳定的提供一个较好的初始位相解,为进行下一步的模拟退火优化奠定了较好的基础。
二、提高衍射光学元件的真实衍射效率
在之前的束匀滑器件设计中,通常采用的是较少的子单元数量,主光斑占整个计算窗口的面积比例是比较大的,且计算窗口的面积比较小。在随后的离线测试中发现,输出光场有比较明显的高级次衍射斑,主斑的能量利用率较低。分析其原因,很可能是由于迭代计算控制的旁瓣区域过小,使得高级次斑与主斑的距离比较小,其衍射能量就变得比较高,造成了主斑能量利用率有明显的下降。随后扩大计算点阵的子单元数量重新进行设计,在数值模拟结果中观察到高级次衍射斑与主斑的距离增大,且强度有较明显的下降,主斑的能量利用率的损失很小。相较而言,这种设计思路对于保障束匀滑器件的真实衍射效率是有帮助的。
三、离子束刻蚀工艺的误差分析模型
在70mm口径的多台阶位相分布DOE测试中,观察到目标光斑的中心出现了一个较明显的毛刺亮点,在束匀滑中,这个中心亮点的产生对于目标光斑的整体均匀性是一个很严重的破坏。利用台阶仪对刻蚀后的DOE样片进行表面扫描采样后,通过数据拟合的方式,提出了刻蚀误差的表面分布模型;在经过数值分析计算后发现,由于刻蚀误差分布具有了类球面的形态,就使得最终的束匀滑结果在光斑中心出现了亮点。在与随机误差分布的比较中发现,在同样的方差下,具有规则分布的误差形态会对于束匀滑产生更明显的影响。通过对刻蚀误差的系统分析,进行了离子束工艺的经验累积,这为后续的大口径束匀滑器件的工艺制作提供了较好的技术支持。
四、固定刻蚀系统误差下的工艺优化方法
在后续研究中,将刻蚀误差分成了两个主要部分:由刻蚀环境的随机扰动形成的正态随机误差及离子源强度在径向分布的不均匀产生的系统误差。从之前的研究得知,随机分布的误差形式对于束匀滑器件并不会产生太大影响。对于刻蚀系统误差,借助于误差分析中误差偏度系数的概念,提出了一个G参量来描述系统误差的整体分布情况。在数值模拟中发现,对于不同的离子源径向强度分布,随着刻蚀时间的变化,当G参量变化到零附近时,束匀滑效果会向较好的趋势发生变化。由此提出了一个针对离子源强度分布进行的一个工艺优化方法,对刻蚀时间进行优选。随后对两个相同系统误差下不同的刻蚀时间的DOE样片进行了数值模拟和实际测试,结果表明,在固定的系统误差下,通过控制刻蚀时间来调整工艺的方法是可行的,且容易实现。
The research devoted to the thesis focuses on the design and applications of large aperture diffractive optical elements(DOE) that will be used to realize uniform illumination on focal plane for high power laser system.The primary research and achievements are listed as follows:
1.When the DOE were designed with Input-Output(I-O) Algorithm,on focal area, the algorithm had the control of intensity feedback only in sampling locations.But in non-sampling locations,numeric calculations showed that Output intensity would surge sharp and fiercely.In numeric calculus,basing on the Nyquist-Shannon theory, intensity in non-sampling locations can be regenerated from those in sampling locations with interpolation formula.According to the interpolation method,we reform the I-O algorithm by appending the feedback control of non-sampling intensity,which can improve the effects of beam shaping because of the enhancement in illumination uniformity.Additionally,the reformed algorithm can give a better initial phase distribution of DOE stably.
2.Previously,we devised DOE with less subcells,and the calculative extent was smaller.Subsequently,the target experiment validated that on focal plane,except for the primary facula,there also had faculas of other orders,which depress the diffractive efficiency of DOE obviously.According to the numeric analysis,the lack of DOE unit and the shortage of calculative extent are possibly the main causation that other orders faculas were engender obviously.To solving this issue,we increase the quality of DOE unit properly,and extend the computing area of output beam.The numeric analysis results show clearly that the facular intensity of other diffractive orders are reduced by this means.It is helpful to avoiding the expense of diffractive efficiency.
3.We have tested a DOE of 70mm diameter with multi-step phasic distribution in the target experiment.The test results indicated that there was a sharp intensive protrusion produced in the center of primary diffractive facula,which was harmful to the uniformity of beam shaping.We processed the metrical data measured by step-scanner with surface fitting,and suggested the distribution model of etching error with spherical surface.The numeric analysis testified that such error distribution was the main reason that caused the intensive protrusion.Comparatively,in the same unbiased variance,random error influence DOE little.It is supporting to our following research and Ion Beam Etching(IBE) technique.
4.When the etching error was investigated,it was separated into two parts.One was the random error,caused by etching environments,nonsignificantly.The other was etching system error,generated from intensive nonuniformity of ion beam,weightily and mainly researched.We suggested a parameter G to describe the system error deflection overall.Analysis attested that the purpose of DOE in beam shaping would meliorate evidently when G was converted to zero.According to this,we suggested a technical optimization for DOE in IBE progress by revising the etching time in specific intensive distribution of ion beam.The experiment showed that this method was apt to actualize.
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