I transmitted analog sound along a HeNe laser
Not recommended as its a little dangerous.
You just put a audio transformer on the cold side and an amp on the other.
Works a lot better then you would think.
I've transmitted audio via a 5mW red diode laser using analog power modulation. Did it over several hundred feet with audio quality so high that you couldn't tell it wasn't directly plugged in to the source.
The receiver was pretty simple; a low quality laser power meter with analog output plugged into a speaker system. The transmitter circuit was a little more complicated, but basically involved combining an analog audio source and a constant DC source. The DC source was about halfway between the min and max for a the laser. The audio source was put on a potentiometer and cut down to be as weak as possible. Large variations in the laser brightness (from a powerful audio signal) lead to nonlinearites which degraded the audio.
Yeah. Once built a laser modem that could run up too 9600 baud over about 90 metres. (purchased a kit, I didnt design the thing)
Used a 5mW 660nm diode, I further upgraded the unit to a 10mW.
That was, IIRC, back in 1994. So I am pretty sure there would be vastly superior kits and/or schematics these days, if you really, genuinely, really want to get involved with this type of thing, than you might have to keep an eye on Silicon Chip magazine etc and look out for the specific articles on these devices.