| Heterodyne Laser interferometry provides precision displacement sensing for modern positioning technology in nanometer scale, such as semiconductor lithography and ultra-precision calibration for microscope scanning stages. To achieve the optimum measurement accuracy and repeatability from a laser interferometric device, the periodic error induced by the optical mixing effect needs to be minimized. In this thesis, we present system design and testing results for a new heterodyne interferometer setup, which is based on optical heterodyning and the general laser interferometric concept with optimized RF electronics. It employs an acousto-optic modulator instead of beam-splitter prisms to separate the optical beams. The mixing error can be minimized. Furthermore, high precision photo detection and phase demodulation are implemented to guarantee the final resolution. With the focus of periodic error, heterodyne efficiency and optical detection noise, theoretical derivation and comprehensive consideration are made for system design of the prototype setup. In addition, diagnostic scheme is created and signal abstraction for displacement is developed with the technique of LabVIEW-based virtual instrumentation. The system validation is conducted for photo detection, I&Q phase signal and final displacement measurement. A signal-to-noise ratio (SNR) of 50 dB is achieved for photo detection at a heterodyne frequency of 80 MHz. An SNR of above 35 dB is obtained for the phase signal with a 21.4 MHz bandwidth. The final displacement resolution is about 0.5 nm. Compared to a commercial heterodyne interferometer, the results of displacement measurement are demonstrated over a large dynamic range. Furthermore, two experiments are made to investigate the influence of mechanical impact and long-term thermal change, which give out some guidance for performance enhancement in the future. |
