Connectors for coaxially coupling the end of a linear optical waveguide element for a receiving surface

For coupling the end of an optical fibre or other linear optical waveguide element to a small-area receiving surface (e.g. surface of photodetector diode) in such manner as to ensure that the fibre makes contact with the surface at right angles to and substantially centrally to the surface, the receiving surface is centrally arranged at the end of a cylindrical boss, and the end of the fibre or like element is supported in a coupling member comprising a body having an end portion in which the end of the element is centered and of which at least the end portion which in the coupled position is nearest to the receiving surface is enclosed in a cylindrical holder tube which extends beyond the end of the end portion to form a collar adapted to encircle the boss and providing a cylindrical bore for sliding co-operation with the cylindrical outer surface of the boss.