Optical fibre sensors

An optical sensor assembly such as a hydrophone attached to a cable 3 comprises a coupler 25 and an optical fibre coil 9 supported by a tubular mandrel 5 fitting over the cable. The coupler is attached to the steel core 11 acting as the central strength member by a coupler fixing 27 at a position where outer layers of the cable have been removed. The three terminals of the coupler are spliced to an optical fibre 17a travelling along the cable, a first end of the coil 33 and an optical fibre 29 having a mirrored end. The second end of the coil is attached to a fibre in the cable 17b. The hydrophone coil and the coupler are contained in a single housing, ensuring the flexibility of the cable, and sealed with a waterproof layer. The cavity between the mandrel 5 and core 11 may be filled with shock absorbing gel.