You can tell the distance of a fibre cut by measuring time it takes for any internal reflections to come back to you. There will be faint reflections when you c

You can tell the distance of a fibre cut by measuring time it takes for any internal reflections to come back to you. There will be faint reflections when you cut the fiber unless you are extremely lucky and skilled, because of the nature of the material. Think of the edges around a chip in a drinking glass. Source: my Dad used to work for Agilent in the 90s, who made test equipment for doing exactly this (although that was mainly to find which roadworks had put a backhoe through the fiber I think, but they did undersea stuff too).