Remote displacement sensing by decoupling a compact sensing head from a heterodyne Michelson interferometer

Interferometric displacement sensors offer unparalleled sensitivity for measuring the motion of masses. Careful design of the optical layout is needed to make the sensor resistant to typical noise sources and have compact sensing elements. We present a sensor that achieves a sensitivity below 1pm/Hz between 3Hz and 800Hz and has a compact sensing head using only three optical components. We achieve this by decoupling the small sensing head from a heterodyne Michelson interferometer and using a single fiber as an optical link. Hence, the fiber has to carry both the test and the reference beam. The design performance is demonstrated and validated, as well as its limiting factors are discussed in detail. Our design is a versatile displacement sensor that can be built with off-the-shelf components. The sensing head is small enough to meet many applications in spacecraft, seismometers, and active feedback control systems such as those used in ground-based gravitational-wave detectors.