This set of two circuits from the basis for a very simple light wave transmitter. A LASER beam is modulated and then aimed at a receiver that demodulates the signal and then presents the information (voice, data, etc..). The whole thing is very easy to build and requires no specialized parts execpt for the LASER itself.
In the transmitter schematic, no ballast resistor is shown because most small LASER power supplies already have one built in. Yours may differ, and a resistor may be needed.
The receiver should be kept away from bright lights. You may want to put a pi