Real time evaluation and correction of nonlinear errors in single frequency interferometers

Nonlinear errors in single frequency interferometers are mainly caused by inaccuracies in quadrature signals. A novel design for real time correction of these errors has been developed using a technique based on the use of a microcontroller and computational analogue circuits. The former is responsible for the detection of these nonlinear errors while the latter is responsible for their correction. As a result, errors arising from nonlinearity are reduced to about 0.3% of an interference fringe over a bandwidth for the quadrature signals ranging from d.c. to 100 kHz. Moreover, a simplified method for evaluating the degree of these nonlinear errors without resorting to a length standard is also presented. Finally, a demonstration application of this new correction technique to a laser interferometer was carried out and experimental results are given.