What are the most commonly used optical transmitters?
Remote controls are perhaps the most commonly used optical transmitters.
The question can be interpreted in many ways, but let's assume that "most commonly used" means that there are perhaps the more of these devices than other optical devices and that most people have used them.
Therefore, we argue that the most common optical transmitter is the TV remote control - or any similar remote for a radio, PVR, VCR, DVD player, etc. These devices use a Light Emitting Diode, or LED, to send information from the hand-held remote to the device being controlled. The light is in the infrared range which cannot be seen by the human eye. The information is encoded onto the light by turning it on and off to represent binary data which are zeros and ones. This is called intensity modulation.
Other interpretations could be "most commonly used fiber-optic transmitters" for this one could refer to this answer:
What is an optical transmitter?