非共光路外差激光干涉测量中的光学非线性/光程差模扰动机理与优化方法

It is crucial for heterodyne interferometry to eliminate the periodic errors caused by frequency mixing and achieve a sub-nanometer or even picometer scale measurement. In order to physically negate the effect of frequency mixing, several interferometer configurations based on spatially separated laser beams have been developed, which however are still suspect to unknown nonlinearity errors varying from 10pm to 1nm, though frequency mixing and beam overlap are totally avoided. What’s more, the separated beams may contribute to some serious unequal optical path errors caused by optical disturbance. From the above, this study will firstly establishes a model based on multi-order Doppler-Frequency-Shift ghost beams to demonstrate the periodic errors in heterodyne interferometers with spatially separated beams, and then explore the suppression method for errors from unequal optical paths between measurement and reference arms. On this basis, we will further explore the methods to design and optimize novel heterodyne interferometers with spatially separated beams, which can efficiently get rid of the periodic errors and the optical path disturbances. A new prototype of heterodyne interferometer with spatially separated beams will also be developed,.which is expected to reveal little periodic errors and high optically stability, to validate the study above. This study is key to our own intellectual-property rights in heterodyne interferometry at sub-nanometer or picometer level, and will broadly support the development of nanotechnologies including semiconductor industry, surface science, metrology and big science project, etc.

突破双频激光交叉混迭引起的纳米级光学非线性误差、实现亚纳米甚至皮米级超精密测量是外差激光干涉测量新技术研究正面临的重要使命。然而，近年来被寄予厚望的非共光路外差激光干涉测量新技术，其采用非共光路结构、避免双频激光交叉混迭后仍出现作用机理不明、幅度10pm至1nm的光学非线性误差，且非共光路结构还易导致严重的光程差模扰动误差。为此，本项目首先建立一种基于多阶多普勒频移虚反射光束的光学非线性模型，揭示非共光路条件下光学非线性的产生机制和作用规律；接着，研究非共光路条件下多个测量臂/参考臂之间的光程差模扰动机制和作用规律；在此基础上，探索可同时减小光学非线性、光程差模扰动的干涉光路优化设计方法，并研制一套光学非线性低、稳定性高的原理样机以验证研究内容。本项目研究将促进形成具有自主知识产权的亚纳米/皮米级激光干涉测量技术，在下一代微电子制造、精密计量、大科学工程等领域具有广泛的应用前景。

本项目主要针对下一代亚纳米甚至皮米级非共光路外差激光干涉测量研究中的纳米级光学非线性误差这一关键基础问题开展研究。传统外差激光干涉测量研究中，一般认为光学非线性误差是由于共光路的测量光、参考光无法完全分离引起“双频激光交叉混叠”，进而导致的误差现象，然而本领域即使经过多年努力、从根源上消除双频激光交叉混叠问题之后，光学非线性误差依然存在，可见该理论不能解释所有光学非线性误差问题，阻碍了亚纳米甚至皮米级超精密外差激光干涉测量技术的发展。.本项目研究发现了光学非线性误差的第二种起源：由于测量光束的部分虚反射光束引起“多阶多普勒频移虚反射光束自混叠”，也会导致光学非线性误差。在此基础上，建立了一种基于多阶多普勒频移虚反射光束的光学非线性模型，揭示非共光路条件下光学非线性的产生机制和作用规律；研究了非共光路条件下多个测量臂/参考臂之间的光程差模扰动机制，揭示了非共光路外差激光干涉测量系统光学热漂移作用规律；探索了可同时减小光学非线性、光程差模扰动的干涉光路优化设计方法，并研制了一套光学非线性低、稳定性高的原理样机，对研究内容进行验证。实验结果表明，基于本项目方法的外差激光干涉测量系统，其光学非线性误差为5pm，热漂移1.2nm/℃(等效于1.2pm/mK)。.本项目研究将促进形成具有自主知识产权的亚纳米/皮米级激光干涉测量技术，在下一代微电子制造、精密计量、大科学工程等领域具有广泛的应用前景。