Optical displacement sensor

An optical displacement sensor comprising:

• a reflective surface;
• two or more diffraction gratings spaced from the reflective surface, wherein each diffraction grating together with the reflective surface defines a respective interferometric arrangement, and wherein either i) the reflective surface or ii) the diffraction gratings is/are moveable relative to the other;
• a light source arranged to provide light to said interferometric arrangements such that, for each interferometric arrangement, a first portion of said light propagates along a first optical path via the interferometric arrangement and a second portion of said light propagates along a second different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a separation between the reflective surface and the diffraction grating of the interferometric arrangement;
• and
• for each interferometric arrangement, a respective set of one or more photo detector(s) arranged to detect at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference;
• and
• a collimating optical arrangement arranged to at least partially collimate the light between the light source and the diffraction grating(s);
• wherein each diffraction grating comprises a set of parallel grating lines extending in a respective grating line direction, and wherein the grating line direction of each diffraction grating in a set of said diffraction gratings is different from the grating line direction of each other diffraction grating in said set.