Interferometric distance sensing device and method with less dependency on environment disturbances on a fiber

An interferometric distance sensing device, comprising

• a light source;
• a partially reflecting element that:
• reflects a first fraction of light from the light source;
• transmits a second fraction of the light from the light source;
• receives light from reflection of the transmitted second fraction by a target;
• and
• returns the received light as returned light together with the reflected first fraction, a return;
• a first and second optical branch path that transmit respective parts of an intensity of a combination of the returned light and the reflected first fraction from the partially reflecting element simultaneously;
• an N-way coupler, wherein N is at least three, comprising a first side with terminals coupled to the first and second optical branch path respectively, the first optical branch path providing a larger optical path length than the second optical branch path;
• at least three light intensity detectors optically coupled to respective terminals at a second side of the N-way coupler that detect the light intensity of detection signals;
• and
• a signal processing circuit including inputs coupled to the light intensity detectors, the signal processing circuit processing information to determine an excess distance traveled by the second fraction from the partially reflecting element and back from the detection signals as detected from the at least three light intensity detectors.