Iterative compensation of nonlinear error of heterodyne interferometer.

In order to compensate the nonlinear error of a heterodyne interferometer caused by both frequency mixing and phase demodulating electronics in real time, a novel iterative algorithm with a digital lock-in phase demodulator is proposed in this paper. By using iterative translating and scaling transforms, the phase diagram of the two output signals from phase demodulator is corrected from an ellipse with center offset to a circle at origin. As a result, the correct phase can be obtained and the nonlinear error is compensated. The nonlinear error in heterodyne interferometer is analyzed, the digital lock-in phase demodulator is designed and the iterative compensation algorithm is presented. Simulation and displacement measurement experiments were performed to verify the effectiveness of the proposed method. The experimental results demonstrated that proposed method is able to reduce the nonlinear error obviously and realize nanometer displacement measurement.