Laser transmits audio using frequency modulation


Transmitting audio over a laser pointer using amplitude modulation is pretty easy to do, however it is susceptible to all manner of interference. The LASER transmitter/receiver project by Samer Muhi takes this to the next level by using frequency modulation to reduce the amount of interference caused by ambient light and changes in transmitter intensity.

This project demonstrates the use of laser in optical communications. the project consists of a transmitter and receiver that will support a one-way communications link between two separate locations. It is mainly built from a few ICs, operational amplifiers and transistors. It is possible to transmit music, voice and data. The laser used in the transmitter is a simply a laser diode that it used in cheap laser pointers and at the receiver, the laser sensor is a photo-diode.

[via hacked gadgets]

20 thoughts on “Laser transmits audio using frequency modulation

  1. This concept is nothing new. In fact, old movie film with sound had the ‘soundtrack’ on the side of the film that was part of the emulsion in the film. Light went through the soundtrack part, and a photocell on the other side picked up the variances of the projected light.
    Even cooler is the fact that a laser can ‘pick-up’ what is being said in another room. A beam bounced off a window into a collecter will vibrate like microphone diaphragm. The sound in the room vibrates the window.

    1. 1) The optical sound track was Amplitude Modulation, not frequency modulation.

      2) You CAN vary the frequency (and thus color) of the laser by changing the drive voltage, cavity size, temperature, etc. Not too practical as there is no wavelength analyzer in there, and just a single channel detector.

      3) Yes it does seem to be pulse modulation, followed with a LIA. Lots of spectroscopic systems use choppers (watch your fingers, they are aptly named!) to modulate the sample signal to higher frequency(away from line noise or 1/f noise) and then demodulate using a low pass filter to recover the original signal.

      Still great project, and really well executed! Cheers.

  2. It’s probably buried somewhere between the schematics and the video, but I couldn’t find it. How do you frequency modulate a monochromatic fixed frequency light source, that is, a laser? (I just know i’m going to feel stupid once/if i have the answer to this) PM… AM… that’s easy to understand with lasers, but…FM?

    1. I just thought that they meant PULSE MODULATION. But yes, frequency modulation of a laser? If you change the frequency of the laser you change the color, correct?

      1. I think what you do is modulate the laser signal at a frequency, and vary the modulation frequency to transmit an FM signal. The frequency of the laser has to stay constant (and the detector probably can’t detect it anyway)

        1. how is that distinct (if i understood your comment correctly) from PM ..pulse modulation? that is, is it FM if the carrier’s frequency doesn’t change?

          1. I could be wrong here, but I think what is going on here is that a carrier frequency is being transmitted by varying the intensity of the laser beam, and this carrier is the one that is being modulated to send FM signals. This is different from pulse modulation because it actually varies the intensity rather than keying the output of the laser.

          2. Yeah, i agree with Matt. It appears as though he is modulating (or even pulsing) the laser on and off, then varying this pulse-rate with a FM modulation, where, modulation is provided by the music.

            Very cool project! When i first watched the clip i was wondering where all the noise was until i looked at the block diagram.

  3. By making the laser intensity vary the creator achieves the transmission of analog data. It is true analog. The following may be easier to visualize. It’s simple, it works and the creator can handle hecklers well. :-)

    There are several ways to modulate a laser both analog and digital. You can vary the intensity as most solid state lasers will allow that to some extent.

    You can create a laser line with optics and then shine that through a film and get varying output.

    You can use an LCD shutter which can be either an analog representation or digital. You can use a mechanical shutter for digital. You can turn the laser on and off for digital.

    There are encoding methods to use the laser’s intensity range to stuff more bits per time interval making more efficient use of the bandwidth but that will limit transmission range the same as the analog modulation.

    Either analog method shown uses less complex parts than a similar digital setup.

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