Abstract: A fiber optical system for pressure measurement has a pressure sensor element (4) with at least two parallel partially reflecting surfaces (5, 7), one of which is arranged on a diaphragm (6) movable with respect to another fixed said surface as a consequence of pressure differences across said diaphragm. Said surfaces are arranged so as to cause interference phenomena of light inciding substantially perpendicularly onto and reflected by the two surfaces depending upon the actual 10 distance between these surfaces. The pressure sensor element is made of material being stable at a continuous temperature up to at least 800° C. At least said diaphragm (6) of the sensor element is made of Si C, and at least a part (23) of the optical fiber (3) connecting to said sensor element is made of a material able to withstand a continuous temperature of at least 800° C.