Research On Dual-Drive Fast-Axis Adjustable Elastic-Optical Modulation Technology In Ellipsometry

Ellipsometry is a technique used to obtain structural information and optical constants of materials by measuring changes in the polarization state of light before and after reflection from a surface.It is commonly used in fields such as thin film thickness detection and material microstructure detection due to its high measurement accuracy,non-contact measurement,and the absence of a need for a vacuum environment.However,as technology advances,the requirements for measurements also become higher,and traditional ellipsometry often uses a mechanical rotation compensator,resulting in slow modulation speeds.Therefore,a self-designed fast-axis adjustable elastic-optic modulator is used to replace the mechanical rotation compensator.By using the elastic-optic modulator as the core device,polarization adjustment can be realized with a higher modulation frequency.A dual-drive fast-axis adjustable elastic-optic modulator control system was developed for this application,and the main research content and work are as follows:First,the dual-rotation compensator model principle of ellipsometry was analyzed,and its advantages and disadvantages were introduced.The theoretical analysis and calculation of replacing the rotation compensator with a fast-axis adjustable elastic-optic modulator were conducted.The theoretical basis for controlling the fast-axis azimuth modulation through the amplitude and phase difference of the dual-drive signal was obtained.The judgment condition for the elastic-optic modulator working in pure traveling wave state was deduced through theoretical derivation,laying the foundation for the subsequent control system design.Second,the overall structure of the dual-drive fast-axis adjustable elastic-optic modulator control system was determined based on theoretical analysis and actual requirements in ellipsometry.Hardware design and software design were carried out accordingly.The hardware part includes the FPGA core control circuit,A/D conversion acquisition circuit,and dual-drive circuit design and production based on LC resonance.The software part includes the dual-drive signal source program in the FPGA,A/D acquisition program,serial communication program,LabVIEW upper computer program,and data processing program design.Finally,key modules such as the dual-drive signal source program and LC resonant dual-drive circuit were tested.Based on this,a test system for the dual-drive fast-axis adjustable elastic-optic modulator control was established.According to the test results,the control system modulation frequency reached 60kHz,and the generated dual-drive signal could be adjusted through the upper computer.The root mean square error of the output phase difference of the dual-drive signal was 0.58°,and the mean values of the output amplitudes were 492.84V_PPand 482.63V_PP,respectively.The spectrum analysis of the sampled signal of the system test showed that the designed control system could realize the elastic-optic modulator working in a pure traveling wave modulation state,meeting its control requirements in fast ellipsometry measurement.