Isn't it just "distance/c"? (or something like 0.999c since light is a bit slower in the atmosphere)
E.g. "light travels approximately 1.5x slower through optical fiber than in a vacuum"
https://www.commscope.com/globalassets/digizuite/2799-latenc...
Turns out, it functions differently. Instead of total internal refraction it has to rely on weird physics like photonic crystals. The pictures are absolutely wild.