https://en.wikipedia.org/wiki/Optical_time-domain_reflectometer https://en.wikipedia.org/wiki/Optical_time-domain_reflectome... OTDR basically uses light to me

https://en.wikipedia.org/wiki/Optical_time-domain_reflectometer https://en.wikipedia.org/wiki/Optical_time-domain_reflectome... OTDR basically uses light to measure a fiber cable's performance. A lot of these devices have break locators in them. and DF I imagine OP is talking about dark fiber. https://en.wikipedia.org/wiki/Dark_fibre https://en.wikipedia.org/wiki/Dark_fibre