preferred form of the invention the means 100 may be formed by a cube made up of two prisms glued together and consisting of refractive material the separation surface formed by the hypotenuse of the two prisms is treated in such a manner as to be a polarisation separator this element has a preferential optical axis the incidental rays are transmitted with a linear polarisation parallel to this axis and fed into the optical fibre f the rays coming from the fibre f are split into two components having orthogonal directions of polarisation the component having a direction of polarisation orthogonal to the preferential optical axis is wholly reflected along the direction of exit orthogonal to the direction of incidence that towards the detector 109 the cube faces may moreover have undergone a surface treatment to prevent disturbing reflections this treatment is known to those versed in the art other optical elements may be used within the scope of the invention particularly in the case of particular birefringent polarisers such as the glan prism a polariser should however be selected which transmits the rays incident in a first direction of polarisation without alteration and totally reflects the rays having a direction of polarisation orthogonal to the preceding direction finally and more commonly if the optical element providing separation by polarisation does not polarise the rays use may be made of a pair of polariser and analyser elements for example analogous to those 105 107 employed in the measuring head described with reference to fig1 finally some semiconductor lasers transmit an already.