Abstract: There is disclosed an optical sensor for detecting one or more measurands such as temperature or pressure, comprising a probe light source arranged to generate probe light, and a sensor head arranged to receive the probe light from the probe light source and impose on the probe light an interference signal responsive to the one or more measurands. The sensor then also comprises an interrogator arranged to receive the probe light from the sensor head, measure the imposed interference signal, and determine the one or more measurands from the measured interference signal, and an optical fibre arranged to carry the received probe light at least some of the way from the sensor head to the interrogator, wherein the optical fibre is disposed within a protective conduit. A granular material may then be packed within the conduit so as to restrict or prevent lateral movement of the optical fibre within the conduit. The optical fibre may also or instead be disposed within one or more flexible sleeves within the conduit.