| The laser interferometry is widely used in the field of precision measurement technology. But there is a common problem in the nanometer measurement of laser interferometer. When using a flat mirror as the measurement mirror, the rotation of the measured object will make the measurement laser beam deviated. And when using a corner cube prism as the measurement mirror, the horizontal movement of the object will also make the measurement laser beam deviating from the original returning path. The problem that the measurement beam can’t correctly reverse itself back will result in a large measurement error and limit the improvement of measurement precision. In this dissertation, the method of reversing the measurement laser beam back based on the Faraday Effect is described, and the heterodyne interferometer for nanometer measurement by using this method is optimally designed and analyzed in detail.On the basis of analyzing domestic and foreign current status of nanometer measurement technology, the structure of the laser heterodyne interferometer for nanometer measurement which based on Faraday Effect is well designed. For the fixed prism structure, by emphatically analyzing the geometry relationship of optical path structure when the measurement mirror is in clockwise and counterclockwise rotations, the requirements to realize nanometer displacement measurement is then assured. The position relation of the measuring plane mirror, the wave plate and the prism is analyzed when the prism is fixed, The measurement ranges of displacement and angle and the regulation how the changes of system parameters (incident angle a, the vertical distance Ls from the point of incidence on the measuring plane mirror to prism) affect on them are solved. Then, the system parameters are optimized. For the passive prism structure, the geometry relationship of optical path structure when the measurement mirror is in clockwise and counterclockwise rotations is analyzed in detail. The analytical result shows that the measurement range of displacement is infinite and the measurement range of angle is-22.69°to16.53°when (Α, Ls)=(45°,18mm), which is obvious better than prism fixed system. After theoretical deduction, the passiv e prism system is processed, assembled and debugged and several experiments are carried out to prove it.Several experiments have been carried out as follows:1. System stability experiment, the standard deviation is10.27nm in two hours.2. Displacement measurement experiments of0°and2°angles, in the range of15μm, standard deviations are26.78669nm and17.83051nm when the steps are30nm and100nm.3. When the measurement mirror is fixed and the displacement is1μm, the experimental results of angle measurement are consistent with theoretical analysis. The angle ranges between-8°and3.5°when the system parameters are (45°,38.15mm), which is close to the theoretical result of-11.72°to5.135°. The deviation derived from the laser beam which is simplified as a line in theoretical deduction. And it illustrates that the angle range of the passive prism system doesn’t depend upon the value of displacement.4. The angle measurement experiment on the double optical path system proves that the measurement range of angle varies with the value of Ls. Because the second incident light beam changes its position on the measurement plane mirror, which brings out that system parameter Ls changes over the value of the angle, and the angle measurement range changes, too. The above experiments have shown that the designed laser heterodyne interferometer based on Faraday Effect can achieve large-range and high-precision in the measurements of displacement and angle. |
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