A pressure sensor (1) includes a crystalline material (2) and a pressure transmission device (30) to transmit the ambient pressure at the location of the pressure sensor (1) to the crystalline material (2). An input coupling device (4, 50) couples a source (11) of electromagnetic radiation into the crystalline material (2). The input coupling device comprises a polariser (50). An output coupling device (5, 51) couples the electro magnetic radiation out of the crystalline (2) to a detector (12, 15-18) and the output coupling device (5, 51) comprises an analyser (51). The polariser (50) has an electric vector orientated at an angle to the X-Y co-ordinates of the crystalline material (2).
