谐衍射光学设计理论和应用研究

【作者】 娄迪；

【导师】 杨国光；

【作者基本信息】 浙江大学 ， 测试计量技术及仪器， 2008， 博士

【摘要】 普通衍射光学元件的设计以及加工在红外波段已经十分成熟,利用其特殊的负色散以及负热差特性可以有效地进行光学系统的消色差和无热化设计,因此衍射光学元件以及折/衍射混合光学系统在现代光学领域的应用也越来越广泛。目前,光学系统的发展正向着增大数值孔径、缩短入射波长以及增加光谱宽度的目标而前进。但随着数值孔径的增加、波长的缩短,普通衍射光学元件的加工工艺难度显著增大,其最小加工线宽有时已经超出了现在的微细加工能力,这也成为衍射光学发展的一个瓶颈。再加上普通衍射光学元件的等效阿贝数通常较小,其负色散绝对值较大;波长偏离中心波长时,其衍射效率下降严重,非设计级次衍射光形成杂散光,影响光学系统成像质量,这些也限制了衍射光学元件在宽光谱及多波段成像光学系统中的应用。针对上述普通衍射光学元件本身的局限性,提出了应用谐衍射来增大最小加工线宽,降低工艺难度,使得加工制作大数值孔径、短波长的混合光学系统成为可能;同时,谐衍射的应用从理论上解决了普通衍射光学元件在宽光谱和多波段光学系统中的大色散以及衍射效率下降问题。通过理论设计与实际加工测试,在谐衍射光学元件的应用特性等方面进行了实践探索。本文首先对衍射光学基础理论进行了系统研究,包括标量衍射模型、矢量衍射模型以及光线理论模型。基于标量衍射模型,对具有谐衍射表面微结构的光学元件的物理特性进行了详细的理论推导,并对二元光学多台阶近似对混合光学系统衍射效率带来的影响做出了定量分析。谐衍射是一种一般化的衍射光学元件,它通过增加普通衍射元件表面微结构的刻蚀深度,改变其相位调制函数,使其在相邻环带的相位差等于2π的整数倍。利用谐衍射在多个分立谐波长能够实现100%的理论衍射效率,且不同谐波长在不同衍射级次实现相同光焦度的特点,提出了在可见光宽波段内实现谐衍射消色差以及在光学系统中应用谐衍射来实现多波段共路共焦的思想。进一步分析了谐衍射应用多个高级次衍射光对系统色散所带来的影响,并对不同系统要求下如何选择衍射级次进行了说明,对高衍射级次的系统成像质量进行了理论探讨。深入分析了谐衍射光学元件各衍射级次的色散以及其衍射效率分布对各个波段的能量分配。对适合于可见光宽波段的单片谐衍射光学元件,以及适合于多波段共焦的谐衍射混合光学系统分别给出设计思路和详细流程。本文的主要创新有:为了替代数字波面干涉测试仪中的标准球面镜组,设计并加工完成了一套折/谐衍射混合光学系统,该谐衍射系统简化了原镜组的光学结构,并且通过测试具有小于λ/20的均方根波面测量精度,达到了系统要求。该研究对谐衍射混合光学系统在可见光波段的应用可行性进行了验证。根据上述实验结果设计制作了光焦度为+6.0D,具有分段连续表面微结构的谐衍射微光学眼镜,进行了表面轮廓测量和不同波长下的光焦度测量,其测量数据与理论设计相符合。结合863课题“三波段混合光学系统研究”,设计制作了可见光、中波红外、长波红外三波段十六台阶谐衍射混合光学系统,在该系统中利用谐衍射使用不同衍射级次的特点,实现了中波4.5um在+2级衍射和长波9.0um在+1级衍射两个红外波段的谐振共焦,对其各个波段进行了光学传递函数MTF的测试,验证该系统在各个波段的成像质量,其实际测试结果与理论计算值十分接近,达到了良好的预期效果。更多还原

【Abstract】 The conventional DOE with the +1^st diffracted order used in IR spectrum has been matured both in design and fabrication. The reversed chromatic dispersion and athermal properties of DOE make it very effective to design achromatic & athermal optical systems. Diffractive optical element （DOE） and refractive/diffractive hybrid optical systems have been widely used in the field of contemporary optics. Optical systems are currently developing towards the aim of larger NA, shorter wavelengths and broader spectrum. But it makes the DOE fabrication much more difficult for larger NA and shorter wavelengths. This disadvantage bottlenecked the development of diffractive optics. In addition, the effective Abbe number of DOE is usually very small and it means the amount of reversed dispersion is very large; and the diffractive efficiency drops dramatically when the wavelength is shifted from the center wavelength, resulting that the stray lights from the unexpected diffracted orders adversely affect the imaging performance of optical system. It also prevents DOE from being applied in the broadband/multiband optical systems.In order to solve the limitations above, we propose that harmonic diffractive elements （HDOE） be applied to increase the critical linewidth of DOE micro-structure, and to facilitate the fabrication. Thus, it will make the fabrication of hybrid optial systems with larger NA and shorter wavelengths possible. At the same time, HDOE can also solve the problems of diffractive efficiency drop at the shifted wavelength and the large amount of reversed dispersion in the broadband/multiband optical systems. Through theoretical design and practical fabrication & test, we have made some exploration in HDOE.In this dissertation, we firstly research into the principal theories of DOE, including scalar diffractive theory, vector diffractive theory and geometrical optics theory. Based on scalar diffractive theory, detailed analyses on HDOE and binary optical element （BOE） have been implemented. Harmonic diffractive optical element （HDOE） is the generalized form of DOE. It increases the etching depth of DOE surface relief, making the phase shift 2mn. HDOE can theoretically diffract 100% of energy to the different diffracted order at its correspondent harmonic wavelength, and all these diffracted orders have the same optical power. According to this feature, we can design a harmonic achromat in broadband or a harmonic multiband hybrid optical system. Further analysis indicates that, lights of higher diffracted order have different imaging properties from the 1^st diffracted order. Also, the selection of diffracted order is discussed.After the theoretical analysis of HDOE, we provide the design procedures of HDOE-used systems, including single-piece harmonic achromats and harmonic multiband hybrid optical systems.The novel ideas and works of this dissertation are as below:We designed and fabricated a refractive/harmonic diffractive optical system to substitute the standard spherical lenses in the digital wavefront interferometer. This new optical system structurally simplifies the old one and has the root mean square （RMS） wavefront measurement precision of A/20. Experimentally, we testified the practicability of using HDOE in the visible spectrum. To further the research, we fabricated a harmonic micro-optical spectacle with continuous surface relief and optical power of +6.0D. The optical power test data show good agreements to theoretical design. Another important application of HDOE is based on the "863 subject" of "Triple-band Optical System Research and Development". The adoption of HDOE with 16-step surface relief made two IR bands (4.5um-band at the 2^nd order and 9.0um-band at the 1^st order) optically coincident and focused to the same detecting plane. The MTF tests indicate that the harmonic triple-band optical system has very good imaging performance in each band.更多还原