An optical displacement sensor for sensing an environmental stimulus is disclosed. The optical displacement sensor exhibits an optical design that has suppressed back reflection and improved alignment tolerance between its optical components. The sensor is based on an optically resonant cavity whose cavity length is affected by the environmental stimulus. An embodiment of the present invention utilizes a lens to redirect a first light signal toward an optically resonant cavity such that the light is incident on the cavity at a non-normal angle of incidence.