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Laser-optical force sensor

Bernd Rudolf Arminger

Bernd Rudolf Arminger

This thesis describes the function, structure and some measurements of a laser-optical force sensor. The force sensor was built for subsequent investigations of this measurement principle and is based on the work of Prof. Dr.-Ing. Wolfgang Holzapfel (University of Kassel).

The measurement principle is based on the photoelastic effect within a Nd:YAG-laser which is optically pumped by a laser-diode.

Over a force-application-system the force to be measured is introduced directly into the laser-crystal, which leads to mechanical stress.

Due to the tensile state within the crystal the refractive index changes, depending on the oscillation plane of the light (birefringence). This results in a minimum and a maximum optical path length within the laser-resonator.

The phenomenon leads to the formation of two laser modes, whose polarization planes are perpendiculary to each other and whose associated frequencies differ by a certain force dependent value. This effect is highly linear over several orders of magnitude of the applied force.

To be able to measure the frequency difference, the two laser modes are projected onto a 45°-plane with assistance of a polarising filter, developing a binaural beating of the electromagnetic wave, whose envelope is modulated with the difference frequency which can thus be picked up with a photodiode.

Since time and frequency can be measured very exactly, the accuracy of the measuring procedure is very high.

Conventional systems for force measurement are usually based on the deformation of a probe element. In that respect the laser-optical measuring principle differs, because here not the deformation, but the mechanical stress is directly measured. For this reason it is suitable also for the measurement of highly dynamic forces.

September 2004