It goes back to the late 70s with Stephen LaBerge's and Keith Hearne's research, but as far as I know, those experiments were always involving one-way communication.

A few years ago, the author of the awesome blog lucidcode, trained himself to communicate in Morse code with the outside world: https://lsdbase.org/2012/05/11/hello-dream-world/

I also recall someone doing a full-on two-way communication experiment involving eye signals and light flashes a couple years ago, but I don't remember the source.