Measurement Accuracy Improvement of Homodyne Laser Diode Interferometer by Real-Time Nonlinear Error Suppressing

Compact homodyne laser diode interferometers (HLDIs) can not only be applied as an instrument for off-machine measurement, but also be integrated into an equipment as a sensor for on-machine measurement. However, nonlinear errors, which not only caused by manufacturing and installation errors of optical components, but also motion errors of the detector, will affect the displacement measurement accuracy of the HLDI. Therefore, a real-time suppression method of nonlinear errors, in which nonlinear errors of the whole period of the quadrature signals is considered, using a field programmable gate array (FPGA) is proposed in this article. A signal contrast is defined to evaluate the quality of quadrature signals. An investigation of the effects of the nonlinear errors caused by the detector motion errors on the signal contrast of the quadrature signals and the measurement accuracy of the HLDI are performed, from which an acceptable range of the signal contrast during the displacement measurement is evaluated. Finally, a demonstration application of the proposed methods to a miniature laser diode interferometer (MLDI), which is developed by the authors’ group, is carried out. The displacement measurement accuracy of the MLDI is improved to be ±23.05 nm.