Voice over laser

So now that I have the laser, I might as well try fun stuff out with it. Like modulating the beam. I know hams have experimented with long-range laser communication, so maybe it could even work.

So I built a poor girl's beam modulator. It is, in essence, a sophisticated optical apparatus which I call a "Laser". It consists of a piezo buzzer stolen from a defunct fire alarm, a piece of a makeup mirror I broke, and lots of Blu-Tack, among other things. The piezo buzzer is directly connected to my laptop's audio Line Out, so I can make it buzz arbitrary signals. The piezo makes the mirror vibrate, which causes deviations in the beam's direction. Kind of like a DMD, except not digital nor micro.

The receiver side is situated a short distance away. It is just a simple wide-spectrum photodiode. It connects to another sound card's Line In. When the beam deviates a little, it won't directly hit the diode any more, and this causes the diode voltage to fluctuate. This should allow us to send information over the channel.

I found a piece of audio with a folk tune in the beginning followed by a lady reading out numbers. I applied a suppressed-carrier single-sideband modulation at 6 kHz, so as to avoid the hum caused by lighting in the room. It's painfully trivial using SoX and Perl, by the way:

open(S,"sox salakalastaja.wav -t .raw -e signed -|");
open(U,"|sox -b 16 -c 1 -e signed -t .raw -r 44100 ".
       "- ssb.wav sinc 6000 -n 4096");
 
while(not eof(S)) {
  read(S,$a,2);
  print U pack("s",unpack("s",$a) *
          cos(($n++ * 2 * 3.141592653589793 * 6000) / 44100);
}
 
close(U);
close(S);

I still need to make some amplifiers. The piezo could take a lot more voltage, but line level only goes so far. Also, the unamplified output of the photodiode is very weak at -90 dB, so we're almost hitting the 16-bit quantization floor. Here's a sine wave being transmitted over the laser:

However, audio from initial crude testing is actually pretty intelligible. In the beginning, you can hear the 50 Hz buzz caused by flickering lamps, and then me tuning the heterodyne to 6.000 kHz. The noise mostly comes from 16-bit quantization/dithering.