AM Laser Transmitter

Today, I experimented with a very simple modulated laser.

The parts are very 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 – if the noise was acceptable – get some interconnectivity.

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.

The first transmission, on the left, is using a low-quality setting with black and white (160x120), the transfer time is 8 seconds. (.wav of the black & white SSTV transmission)

The second transmission, on the right, is using a higher-quality setting with color (160x120), since the transfer has four times the data for the color, the transfer time is 24 seconds. (.wav of the color SSTV transmission)

I took a look at the encoding using the sonogram feature in Amadeus II. On the left is the chirping preamble, with some of the data transmission beginning at 1.3 seconds. At the right is a closer look at the data transmission, with the sonogram viewing about 0.32 seconds.

EDIT Feb. 21st, 2008: Robert from the Philippines asked me recently how exactly everything s hooked up. Basically, place an audio transformer in series with the power source for the laser. On the other end of that audio transformer will be your audio input. This way, as the incoming sound changes, the amplitude of the laser changes, hence amplitude modulation. Then, on the reciever end, simply place a photocell in series with a battery, and hook that up to an audio amplifier.