Fiber manufacturers have to keep in mind the use of CWDM and DWDM(multiple light wavelengths over a single strand of fiber) when constructing fiber cables. They can’t have lightwaves “bouncing around” inside fiber and have it work.
I think you’d need a high loss cable, or a leaky cable, so that the light hits the converter in a short distance. If you pick up your 80% of the incoming in a kilometer of coated cable you’ll go broke building a 100 KW array.
Like the guitar strings on a Hard Rock Casino sign, they are made to have strong side emissions to be visible. Um, the strings are fibers, yano.