I think you can also use a prism to just split 1 fiber into 2 or more, much like a mirror, similar to how some laser systems work. So in essence the original be

I think you can also use a prism to just split 1 fiber into 2 or more, much like a mirror, similar to how some laser systems work. So in essence the original beam goes on it's merry way unaffected and the duplicate(s) goes into a black box. Though I know very little about this subject I would assume the splitter has to be exact and completely lossless. I also think there are tools to measure any interference. For example a device like this is used to reflect fibers: http://www.ozoptics.com/ALLNEW_PDF/DTS0095.pdf http://www.ozoptics.com/ALLNEW_PDF/DTS0095.pdf