Fibre-optic interferometer

Abstract

The invention relates to a fibre-optic Mach-Zehnder Interferometer for measuring such as change in length. Two light conducting fibres (1,2) are connected to a coherent light source (5). A measurement converter (6), actuated by a measurement quantity (P) changes the length of the optical path and thereby the phase angle for the light wave in one fibre (2). The phase angles for the light waves in the fibres (1, 2) are compared and a corresponding output signal (I) is fed back to compensate for the phase angle change in the measurement converter (6). To make the comparison result (I) independent of the light intensities in the light conducting fibres (1,2) these are connected to an opto-electronic directional coupler (7). This is selected such that the output signal (I) varies as the phase displacement .phi., according to the relationship: I=2.times.A.times.B.times.sin .phi., where A and B are the amplitudes of the light waves in the fibres (1,2). By using the directional coupler (7), the interferometer can be made simple and accurate. The opto-electronic coupler ( 7) allows the use of polarized light, whereby accuracy may be further improved.