AM Laser Transmitter

So I've been playing around with this laser transmitter thing that I built a little while ago.

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, basically encoding the beam with the input 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 very well with sound, but I also tried transmitting other media across it.

You can 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 possibly get some interconnectivity.

Next, I looked to send images across the laser beam. I did a little research and read about slow scan television, a picture transmission method used by amateur radio operators. Basically, it 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 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.

So yeah, thats it. Pretty cool. This is basically how fiber optic cable or IR transmitters work. The only difference is that I am using an analog encoder, and not a digital encoder.

EDIT Feb. 21st, 2008: Robert from the Philippines asked me recently how exactly everything is 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, to pick this up, simply place a photocell in series with a battery, and hook that up to an audio amplifier.