Wavelength-division multiplexing[1]... it's what they use with fiber-to-the-home to get bidirectional signals with a single strand of fiber. Or the big-boy version is DWDM which ISPs use to stuff dozens of customers on a single strand.

You might also be interested in https://phys.org/news/2026-11-air-holes-silicon-wafers-flat....

[1] https://en.wikipedia.org/wiki/Wavelength-division_multiplexi...