An optical sensor assembly including a coupler and an optical fibre coil. The coupler is attached to the central member of the cable at a position where outer layers of the cable been removed. A first terminal of the coupler is connected to an optical fibre traveling along the cable in a fibre conduit external to the central strength member. A second terminal of the coupler is connected to a first end of the coil and a third terminal of the coupler is connected to an optical fibre a mirrored end. The coil is supported about a tubular mandrel, which mandrel fits over the cable. The other end of the coil is attached to the fibre in the cable. The mandrel is located about the cable at a position covering the coupler and the part of the cable where the outer layers are removed.