Amplitude-modulated Laser Transmitter

Back in high school, I built a very simple circuit that was capable of transmitting sound across a room using a laser beam. With slow-scan TV software on two PCs, images could also be sent over the laser beam!

June 26, 2006

Today, I built a simple laser transmitter and receiver!

The parts are simple: the transmitter consists of a laser diode I stole out of a laser pointer wired to a 1k to 8 ohm audio transformer, with the other end connecting to the input. What this does is "modulate" the laser, encoding the beam with the input signal by varying the laser's strength as the input varies. At the other end, you have a photocell and a battery acting as the receiver. Very simple.

This setup works well with sound. It sounds about as good as an AM radio transmission. You can also send data across the beam, much like how is done with modems. I used some software called MultiMode to send some text using Morse code across the beam of light, which was pretty neat. In theory, one could link two dial-up modems using this and make a (rather slow) optical network link!

Next, I looked to send images across the laser beam. I did a little research and read about slow scan television, an image transmission method used by amateur radio operators. This encodes an image into sound, with the length being anywhere from ten seconds to several minutes, depending on the quality setting. I downloaded a program called MMSSTV which is used for this purpose, and used it to send some pictures over the laser beam. I was rather surprised with the results:

*160x120 black and white transmission (8 seconds)*

*160x120 color transmission (24 seconds)*

I also took a look at a sonogram of the transmission in Amadeus II:

Below is a schematic of this setup. An audio transformer is placed in series with the power source for the laser. On the other end of that audio transformer is the audio input. This way, as the incoming sound changes, the amplitude of the laser changes, hence amplitude modulation. Then, on the receiver end, simply place a photocell in series with a battery, and hook that up to an audio amplifier.