But the method I tried worked well and was very simple.
The sender unit (connected to the receiver) was just an Infrared LED with a 30 ohm resistor in series (just to limit the current. The receiver unit (connected to the servo) was a photodiode with a 1 megohm resistor in series. (The LED and photodiode were a matched pair but I can't remember where I bought them from. Ebay probably.)
The early trials were surprisingly successful with the signal being happily beamed through several mm of plastic or even water.
If you used a separate 5v supply at the receiver end then that is just one wire to transmit a servo signal.