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 refere

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 https://en.wikipedia.org/wiki/Delay-line_memory