Method and apparatus for absolute measurement of force by use of

Abstract

The present invention discloses a method and an apparatus which enables one to obtain definite non-modulo-2.pi.-dependent measurements wherein a method for the fiber-optic measurement of force, which is based upon the strain-induced birefringence of a single-mode light waveguide, provides a compensation method for the phase differences of two linear modes of the exiting light waves, which are caused by the force acting on the light waveguide and the consequent change in birefringence. In one embodiment of the method, a liquid crystal cell, the birefringence effect of which can be controlled electrically, is used as a compensation element. The maximum of an autocorrelation function, which occurs as an image, and the respective voltage at the liquid crystal cell, are evaluated as a measurement for the birefringence which is to be compensated for and, accordingly, for the effective force. In an alternate embodiment of the method, the two partial waves are directed transversely relative to the propagation direction of the wave fronts by an optical component with linearly space-dependent birefringence. The autocorrelation function can then be obtained as a spatial coordinate and can be plotted electronically by a line detector such as a line array. The location of the maximum of the autocorrelation function is, under certain defined conditions, a measurement for the birefringence which is caused in the light waveguide by mechanical tension from which the magnitude of the force, which acts on the force sensor, can be calculated.