外差激光干涉仪周期非线性误差形成机理与补偿方法

Periodic nonlinear error and its compensation method in heterodyne laser interferometer

针对纳米级周期非线性误差制约外差激光干涉仪使其不能适应下一代超精密装备制造与重大科学工程提出的亚纳米乃至皮米测量精度需求的问题,分析了外差激光干涉仪中两类周期非线性误差的形成机理,并对周期非线性误差的补偿方法进行了研究.结果表明:第1类周期非线性误差是由于双频激光不能完全分离引起双频激光交叉混叠,进而导致的周期非线性误差,该误差幅度可从数纳米到数十纳米;第2类周期非线性误差是由于测量光束在光学界面产生了具有多阶多普勒频移特征的虚反射光束,进而引入的周期非线性误差,该误差幅度可从数皮米至数纳米.对于第1类周期非线性误差,现有误差补偿方法,如椭圆拟合法等,可将其抑制至0.1 nm量级,特别是空间分离式外差激光干涉仪则从原理上完全消除了这一类误差;而对于第2类误差,通过降低虚反射率和空间滤波可以将第2类误差降低至数十皮米到数百皮米,剩余误差尚不能完全满足皮米测量的精度需求,亟待发明新的误差抑制或补偿技术.

Due to the periodic nonlinear error, heterodyne laser interferometer cannot meet the requirements of the sub-nanometer and even picometer measurement accuracy of the next generation of ultra-precision equipment manufacturing and major scientific engineering. Aiming at the problem, this paper analyzes two types of periodic nonlinear errors in heterodyne laser interferometer and investigates their compensation methods. Results show that the first type of periodic nonlinear error is caused by optical mixing due to the incomplete separation of dual-frequency lasers, whose amplitude ranges from several nanometers to tens of nanometers. The second type of periodic nonlinear error is induced by multi-order Doppler frequency shift(DFS) ghost beam generated by the ghost reflection of measurement beam at the optical interface, whose amplitude ranges from several picometers to several nanometers. For the first type of periodic nonlinear errors, the current nonlinear error compensation methods, such as ellipse fitting method, can suppress them to 0.1 nm level. In particular, the spatially separated heterodyne laser interferometers proposed in recent years can completely eliminate the first type of nonlinear error in principle. As for the second type of error, by reducing ghost reflectivity and spatial filter, the error can be reduced to tens of picometers or hundreds of picometers, while the residual error is still too large to meet the accuracy requirements of picometer measurement. Thus, it is urgent to develop new error suppression or compensation technologies.