INTERFEROMETRIC DISTANCE SENSING DEVICE AND METHOD

Distance to a target is sensed using a common path interferometer, wherein a first fraction of light from a light source is collected after reflection by a partially reflective element together with reflection from a target of a second fraction of light from the light source that has been transmitted by the partially reflective element. The collected light is split in two parts, both containing a part of the first fraction and part of the reflection from the target. The parts are fed through a first and second optical branch path to a first side of a three-way optical coupler respectively. Light from at three terminals on a second side of the N way coupler is fed to respective light intensity detectors. Information representing an excess distance traveled by the first fraction from detection signals is determined from the least three light intensity detectors.