基于“凸面曝光-凹面复制”法的平场凹面光栅设计与制作关键技术研究

A flat-field concave diffraction grating is the key component of a portable grating spectrometer, because it integrates three optical properties: dispersion, imaging, and flat spectral image plane. The diffraction efficiency of a grating depends mainly on its groove shape. The common technology to accurately control the groove shape is to first produce a photoresist grating mask by using the standard holographic technique, and then to perform dry etching by using a collimated ion-beam. However, a uniform blazed angle across the whole grating area cannot be obtained because the incident angle of the ion beam is restricted by the curvature of the concave substrate. This problem, with which the researchers have confronted for a long time, severely limits the diffraction efficiency and restricts the application of concave gratings. We proposed a method to fabricate flat-field concave gratings by replicating a convex master grating fabricated directly by the holographic recording. The method translates etching a concave grating into etching a convex grating, and greatly reduces the difficulty of ion-beam etching. Furthermore, the distance between two light sources in the fabrication light system are often short, which makes it difficult to build the fabrication structure, sometimes even impossible. We proposed a method to increase the equivalent distance between the two virtual spot light sources by introducing a compensate mirror, which can avoid the interference of the space light filters. This work will give a theoretical analysis, numerical simulation and experimental test. By this work, we hope to find out a new and better solution for making flat-field concave grating with both good spectral image quality and high diffraction efficiency.

平场凹面光栅集色散、聚焦、平像场于一体，是光栅光谱仪器中的关键元件。然而受凹面基底几何形状的限制，凹面光栅很难采用离子束直接刻蚀，很多情况下甚至完全不可能，导致凹面光栅的槽型无法精确控制，衍射效率一直难以提高，成为其应用瓶颈。针对这一问题，我们提出直接曝光制作凸面母光栅，然后复制得到平场凹面光栅的方法。由于凸面光栅易于刻蚀，因而该方法为通过离子刻蚀控制凹面光栅槽型提供了一种切实可行的解决方案。用ZEMAX进行的初步仿真结果证实采用本方法与传统直接曝光法制得的凹面光栅像质性能相当。进一步，针对曝光光路中两个点光源由于受到机械结构限制不能过于靠近的问题，我们提出在光路中加入"补偿镜"的方法，拉大实际曝光点的间距，有效避免两路空间滤波器之间的干涉。本项目将对上述方法在优化设计、制作工艺、系统集成中的关键问题进行攻关，探索出一整套可行的解决方案，为未来制作闪耀平场凹面光栅打下坚实基础。

平场全息凹面光栅集色散、聚焦、平像场于一体，是便携式光栅光谱仪器中的关键元件，直接决定了光谱仪器的最终质量。衡量其质量的两个最重要的指标是成像质量与衍射效率。针对衍射效率指标，本项目提出了通过复制凸面母光栅来制作闪耀凹面光栅的方法。同时，为了进一步拓宽微型光谱仪光谱范围，同时保证其分辨率不损失，本项目还提出了两种新的光谱仪光路结构，经过仿真及实验验证，这两种新的光路结构均具有良好效果。针对成像质量指标，本项目提出了一种补偿镜法曝光凹面光栅的方法，该方法为在传统的双光束曝光光路的基础上加装补偿镜。补偿镜法曝光凹面光栅又分为单光路补偿镜法和双光路补偿镜法。项目完成了优化算法及其实现，并通过实验验证了该方法的有效性。本项目的成果为微型光谱仪的研制提供了坚实的理论基础和方法。