Generally, the light source for optical fiber communication is the green laser pointer. However, when communicating in the atmosphere, the incoherent light generated by the diode is sometimes better than the laser. The main reason is that the change in atmospheric density will destroy the phase correlation of the laser and cause random fluctuations in brightness, which may destroy the information contained in the beam. Second, large-diameter beams are required for long-distance communication, but expanding coherent light is not easy. This requires the use of very precise lenses and increases weight and cost.
Even with these shortcomings, it is still very attractive to use common laser pointers for optical communication. They are cheap, readily available, low power consumption, relatively safe, and easily modulated by technology. Last but not least, how cool is laser communication!
The cheapest red laser pointers are ideal. The collimated beams they produce, even if they are only a few millimeters in diameter, can be observed by the naked eye at a considerable distance, and the visible distance can be up to 100 kilometers in good weather.
A simple method of transmitting audio signals is amplitude modulation plus pulse width modulation. In the figure you can see an AM-Pulse Width Modulation circuit based on a laser pointer. Its purpose is to transmit sound using simple ready-made parts and a laser pointer as much as possible. This circuit contains a photodiode-based optical receiver. However, because the main purpose of the circuit is to demonstrate the modulation circuit. There is no effort on sensitivity and no focusing lens. Therefore, the communication distance is very short, only about 100 meters.
It is no secret that the laser focused beam is used improperly. For this reason, the high-quality units available on the market today use so-called infrared beam filters. The role of these safety mechanisms is to prevent the laser itself from emitting invisible light. This is very important for all output power, especially for some of the combustion devices discovered today. This is because the beam is visible when using a visible laser, so you can carefully avoid reflective surfaces and avoid exposure due to experience or potential carelessness. This simple and inexpensive device that does not use cheap equipment is actually a bit dangerous to use indoors for a long time at high power. Make sure that every laser system you consider has an infrared filter to prevent unnecessary exposure.