| The purpose of this study is to construct, test, and verify a new heterodyne displacement measuring interferometer design that eliminates the current accuracy limitation imposed by periodic error, which appears as a cyclical oscillation of the measured displacement about the true value during motion. In the new design, the two (heterodyne) frequencies are generated and spatially separated using acousto-optic modulators. By removing the potential for overlap and frequency mixing within the interferometer, periodic error is eliminated. The new concept replaces the traditional method of 'polarization coding', where the two beams (with different frequencies) are initially coincident with orthogonal polarization states and then separated using polarization dependent optics. Experimental results are presented for two arrangements of the new design at multiple target velocities. Spectral content (collected using an analog spectrum analyzer) is analyzed to verify zero periodic error. These results are compared to data collected using a traditional polarization coded heterodyne interferometer with variations in the optical alignment to demonstrate different levels of first and second order periodic error. Again, frequency spectrum data are provided. |
