A microelectromechanical systems (MEMS) device comprises an optical directional coupler comprising: a first waveguide having a first and a second end, wherein a light beam is introduced into the first end; a second waveguide having a third and a fourth end, wherein the light beam is evanescently coupled between the two waveguides in the central region; a first photodetector to detect first optical power in the light beam at the second end; and a second photodetector to detect second optical power in the light beam at the fourth end; a vibrating proof mass adjacent to the coupler in a first direction from the coupler, wherein when inertial forces are applied to the MEMS device in a second direction, the proof mass moves in the first direction; a processor to determine the displacement of the proof mass from the coupler as a function of the first and the second optical power.