Working group 1.71
Universal high-precision measuring method
using a heterodyne interferometer

Mach-Zehnder heterodyne interferometers (if need be, in conjunction with specially developed diffraction gratings) are sub-systems of the standard measuring devices. Based on the Doppler effect, a frequency-modulated signal is generated, having an instantaneous frequency of the laser light, which is proportional to the instant velocity or angular velocity. The heterodyne interferometer transforms the modulated light frequency into an electrical frequency-modulated signal (carrier frequency of

40 MHz or 80 MHz). After electrical frequency conversion, the modulated and the unmodulated carrier signals are available in a frequency range appropriate for signal acquisition and signal processing (preferably around 1 MHz for sinusoidal motion quantities). Digital frequency demodulation furnishes series of measurement values of the velocity-time history and the angular velocity-time history. Alternatively, series of pairs of values of the displacement and rotation angle-time history are obtained by digital phase demodulation. Special algorithms lead from the measured displacement values to the velocity and acceleration, and from the measured rotation angle values to the angular velocity and angular acceleration. These measurement capabilities are made use of at PTB for accurate vibration and shock calibrations of motion quantity transducers and for the investigation of their dynamic behaviour.



Sketch of the heterodyne data processing


Heterodyne interferometer with frequency conversion technique and digital signal processing
© Physikalisch-Technische Bundesanstalt
Page created: 2007-07-11, last update: 2007-07-12, Susanne Gazioch