Do you believe that an ordinary laser pen can travel 100 miles in the air? We did it, and I believe you can do it.

1 semiconductor laser

The light source of general optical fiber communication is laser, but when communicating in the atmosphere, the incoherent light produced by the diode is sometimes better than the laser. The main reason is that changes in atmospheric density can destroy the phase correlation of the laser and lead to random fluctuations in brightness, which may damage the information contained in the beam. Second, it takes a large diameter beam to communicate at a distance, but it's not easy to beam coherent light, which requires very sophisticated lenses and adds weight and cost.

Even with these weaknesses, with common high-tech laser pointers for optical communication is still a very attractive things, they are cheap, readily accessible, low power consumption, safe, and easy to rely on the PWM modulation. Last but not least, laser communication is so cool!

Price is the cheapest red laser pen is very ideal, they produce a collimating beam even if only a few millimeters in diameter, but also can be in great distances observed with the naked eye, the climate good visual range can be as far as 100 km.

USB Interface Laser Pen

2 Laser modulation - demodulation circuit

The simple method of transmitting audio signals is to modulate and pulse width modulation. A laser-based amplitude modulation (PWM) circuit, designed by Ron (K7RJ), is designed to transmit sound with a single laser pointer as far as possible. The circuit includes a photodiode based optical receiver. However, because the main purpose of the circuit is to demonstrate the modulation circuit, without the sensitivity or the focus of the lens, the communication distance is very short, only about 100 meters.