A coherent optical time domain reflectometer of the kind including a light source (10), an optical coupling means (13) whereby light from the source is coupled into an optical transmission medium (14) and light reflected from the transmission medium is mixed together with a reference optical signal and is coupled to a detector (18). The coupler (13) is a 3 x 3 coupler with the mixed reflected light and reference signal being coupled out via two adjacent outputs to two respective inputs (16, 17) of a balanced detector (18).
COHERENT HOMODYNE REFLECTOMETER