Not optical, but piezo electrical, usually with a crystal or air as the medium instead of mercury. Optical is much the same principle, a feedback loop incorporating the delay line, so the same bits get re-injected over and over again and can only be read out at specific points in time.
Like most things, the concept is much simpler than making one work — and I was hoping I could find some papers on the applied side of photon delay lines. (Since OP commented it was related to his/her PhD.)
I thought the mercury delay lines were the craziest thing I’d heard about in computer evolution until I learned about using a cathode ray tube as memory.
In a way that is a delay line too, the phosphor decay time allows you to read out the bits a bit later than you put them in. The big advantage is that it is theoretically random access.
Would be much appreciated.