Reconstruction of dynamical perturbations in optical systemsby opto-mechanical simulation methods

摘要

High-performance objectives pose very strict limitations on errors present in the system. External mechanical influencescan induce structural vibrations in such a system, leading to small deviations of the position and tilt of the opticalcomponents inside the objective from the undisturbed system. This can have an impact on the imaging performance,causing blurred images or broadened structures in lithography processes. A concept to detect the motion of thecomponents of an optical system is presented and demonstrated on a simulated system. The method is based on acombination of optical simulation together with mechanical simulation and inverse problem theory. On the optical sideraytracing is used for the generation of wavefront data of the system in its current state. A Shack-Hartmann sensor isimplemented as a model to gather this data. The sensor can capture wavefront data with high repetition rates to resolvethe periodic motion of the vibrating parts. The mechanical side of the system is simulated using multibody dynamics.The system is modeled as a set of rigid bodies (lenses, mounts, barrel), represented by rigid masses connected by springsthat represent the coupling between the individual parts. External excitations cause the objective to vibrate. The vibrationcan be characterized by the eigenmodes and eigenfrequencies of the system. Every state of the movement during thevibration can be expressed as a linear combination of the eigenmodes. The reconstruction of the system geometry fromthe wavefront data is an inverse problem. Therefore, Tikhonov regularization is used in the process in order to achievemore accurate reconstruction results. This method relies on a certain amount of a-priori information on the system. Themechanical properties of the system are a great source of such information. It is taken into account by performing thecalculation in the coordinate system spanned by the eigenmodes of the objective and using information on the spectrumof frequencies present in the current vibration as a-priori data. The position of the individual lenses as a function of timeis then calculated from several frames of the wavefront data and extrapolated to future timesteps. Information on thesystem gathered with this method can be useful for applying and controlling countermeasures against the vibrationsduring use of the objective or for designing new systems that are less influenced by vibrations.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.