John Carmack muses using a long fiber line as as an L2 cache for streaming AI data — programmer imagines fiber as alternative to DRAM

The article mentions delay line memory[1] and how the mercury used in them wasn't pleasant. But that's just one kind, there are many different kinds, the referenced Wikipedia article lists many types, including using long metal wires, ultrasonics and indeed optical.

[1]: https://en.wikipedia.org/wiki/Delay-line_memory