Abstract: An optical fibre reflectometer in which the end of the optical fibre 4-port coupler for optical coupling to test fibres is not protected from damage by the use of a sapphire window because of the problems of unwanted reflections that this engenders. Instead the end is coupled to the test fibres by means of a replaceable slug (33) containing a stub length of fibre (20). This fibre stub (20) is intrinsically just as susceptible to cumulative damage when test fibres are repeatedly butted against its end, but it protects the coupler because it is itself butted against the coupler only once in its life. When the cumulative damage becomes excessive the slug can be replaced more simply and cheaply than replacing the whole coupler.