Abstract: To permit operation of the light guide under widely varying temperature conditions, for example varying between -50.degree. C. to +70.degree. C., plug elements (5,7), for example of synthetic rubies or sapphires and retained within a hard metal sleeve (4) are formed, essentially throughout their entire cross-sectional area, with facing matching engagement surfaces. The exit or coupling plug element (5) is formed with a polished surface (15) at the side remote from the entrance plug element (7), the polished surface extending at right angles to the axis of a central duct or channel (9) which retains an optical fiber light guide. The exit or coupling plug element (5) is so positioned in the holding sleeve (4) that the polished surface (15) is located at a projecting portion beyond the terminal end (16) of the sleeve, to define a projecting distance (17). Two couplings are connected together by a housing (20).