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I found that a guitar string can modulate a light beam if the beam is focused. You need a light source and a receiver (photocell),coupled to an op amp, for each string. I did this years ago, with incandescent lamps, before lasers were inexpensive. Lasers would probably work better, as they are much more precise.
A lot of musical instrument FX stuff used light as the modulator. Some wahwah pedals used light.
I got the parts from Radio Shack.
Have fun!