https://www.shieldenchannel.com/blogs/solar-panels/fiber-opt...
Examples of such shelters: - general purpose shleter Denis (capacity op to 2000 people): https://podzemibrno.cz/en/places-underground/cover-denis-und... - army headquarters shelter in the Vypustek cave (capacity 100+ people): https://vypustek.caves.cz/en - 10-Z shelter, Brno area civil defense headquarters (capacity 100+ people): https://10-z.cz/en/
Smaller shelters that could be found under many 50s era building were much more rudimentary, usually without idepedent source of electricity and just simple hand cranked filtration system.
Also not bad for high efficiency net zero type homes to supplement natural light without too many windows compromising the thermal envelope. I personally like the variability of natural light for how it keeps you connected to the outdoors. You know when a cloud is overhead or the sun is rising or setting. I've simulated sunset and sunrise with LEDs through color temp and timing but I've always wanted to experience the solar fiber type.