the operating position that is to say with the movable casing part 1b slid back against the fixed casing part 1a and with the optical fibre held within the mechanism fig3 shows the internal working parts of the mechanism of fig1 and 2 the casing part 1a houses a secondary waveguide 2 and a pair of detectors 3 the secondary waveguide 2 is a silica rod having a curved portion 2a and two straight portions 2b the detectors 3 are large area germanium photodiodes together the secondary waveguide 2 and the detectors 3 form part of an optical coupling device of the type described in the specification of our copending patent application no 8706929 the contents of which are incorporated herein by way of reference the casing part 1a also houses a pair of fixed locators 4 made of polytetrafluorethylene and a pair of spring loaded rubber faced guide wheels 5 the locators 4 each carry a v shaped jaw 4a the casing part 1b houses a sliding plunger assembly including an operating bar 6 which carries a pair of sliding locators 7 made of polytetrafluorethylene the operating bar 6 is mounted within the casing part 1b and the locators 7 each carry a v shaped jaw 7a a brass sliding plunger 8 is slidably mounted on the operating bar 6 for movement in the directions of the arrows b and c an operating lever 9 is fixed to the plunger 8 the operating lever projecting through and being slidably relative to an aperture 10 formed in the top surface of the casing part 1b see fig1 and 2 movement of the operating bar 6 in the direction of the arrow c is.