Optical displacement sensors

An optical accelerometer comprising:

• an interferometric arrangement comprising a first optical element and a second optical element, wherein the first optical element is moveable with respect to the second optical element;
• a light source;
• and
• at least one photo detector;
• wherein the light source is disposed to provide light to the interferometric arrangement such that a first portion of said light propagates along a first optical path via said 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 distance between the first optical element and the second optical element, and the photo detector is disposed to detect at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference;
• the optical accelerometer further comprising:
• a support structure having a position that is static relative to the light source and the at least one photo detector, wherein the second optical element comprises a surface of the support structure and/or is disposed on a surface of the support structure;
• and
• a dual-layer membrane that is deflectable relative to the support structure, the dual-layer membrane comprising a first membrane layer, a second membrane layer spaced from the first membrane layer, and at least one proof mass positioned between the first and second membrane layers, wherein the proof mass is attached to or integrally formed with each of the first and second membrane layers so that the first and second membrane layers are mechanically coupled to move together in response to a deflecting force acting on the proof mass, and wherein the first optical element comprises a surface of the first membrane layer or is disposed on a surface of the first membrane layer;
• wherein the second optical element and at least part of the support structure are positioned between the first and second membrane layers.