The dynamic compensation of nonlinearity in a homodyne laser interferometer

This paper presents a simple system for the dynamic compensation of nonlinearity in a homodyne laser interferometer that can be used for high-precision length measurement. The computer collects two phase-quadrature signals, and calculates the DC offsets, the AC amplitudes and the difference from the phase-quadrature by the elliptical fitting with a least-squares method. The control signals for adjusting these ellipse parameters are fed into the automatic control circuit through the D/A converters so that the offsets are zero, the amplitudes are same and the phase difference is 90°. As a result, the nonlinearity is eliminated electronically. The system can be used in applications requiring the real-time compensation of nonlinearity. Experimental results demonstrate that the nonlinearity of the homodyne interferometer can be reduced to sub-nanometre over the measuring range of 100 mm.