Fiber tapering with precise mode control
Careful tapering of a fiber allows for part of the light that travels in the core to couple to mode that travels in the cladding. There it can interact with the outside world where the refractive index of the surrounding medium will influence the propagation (phase and amplitude). When the taper ends, the light recombined with the light in the core to create an interference that can be detected at the output. In this way interferometric sensing can be achieved without tapering all the way down to the point that all of the light senses the outside and the fiber becomes fragile.