A temperature sensor (1) has a pressure sensor (10), the distal end of which is inserted in a sealed chamber (2) filled with a liquid. The pressure sensor has a light guide (4), a cavity (14) at a distal end of the light guide, a diaphragm (11) forming a wall of the cavity and being configured to deflect with applied pressure, and a detector to detect changes in light reflection due to deflection of the diaphragm. The liquid (3) which changes volume in response to temperature changes and this volume change is sufficient to change the pressure applied on the diaphragm (11), and the a interrogation system processes pressure data and/or light reflection data to generate an output indicating temperature of the fluid in the chamber.