Practical coupling device based on a two-core optical fiber. 
A practical coupling device that relies on a dual-core fiber in a loop configuration is presented.  Its coupling properties are analyzed in terms of the optical path difference between the cores, which is controlled by the rotation of the fiber about its axis and by a small twist applied to it along the loop.  The device actually acts as an anisotropic coupler, and the coupled power can be perfectly controlled from 0-100% by proper adjustment of the loop.  A simple implementation of the device was used in the fabrication of a compact single-fiber Michelson interferometer.