Phase-Modulated Standing Wave Interferometer

The actual technical implementation of conventional interferometers is quite complex and requires manual manufacturing. In combination with the required construction space defined by the optical setup, their applications are limited to selected measuring tasks. In contrast, Standing Wave Interferometers (SWIs) offer an enormous potential for miniaturisation because of their simple linear optical setup, consisting only of a laser source, a measuring mirror and two transparent standing wave sensors for obtaining quadrature signals. The two sensors are located inside the measuring beam and therefore directly influence the length measurement. To reduce optical influences on the standing wave and avoid the need for an exact and long-term stable sensor-to-sensor-distance, a single sensor configuration was developed. There, a phase modulation is superimposed to the sensor signal by a forced oscillation of the measuring mirror. When the correct modulation stroke is applied, the resulting harmonics in the sensor signal are 90° phase-shifted to each other and can hence be used for obtaining quadrature signals for phase demodulation and direction discrimination by an arctan-algorithm.