Finding The Linear In A Laser

If your path has taken you through any work with hi-fi audio, you will be aware of the effects of distortion on sound quality. The tiniest non-linearity in a component can ruin the result, and people who work at the extreme end of the hi-fi spectrum will go to impossible lengths to chase the tiniest percentages of distortion that no human could possibly hear.

[Monta Elkins] has a Boldport kit, the Lite2Sound, which as its name suggests translates a light level to an audio signal. Given a laser diode and a source of country music from his Amazon Echo then, perhaps he could transmit the sound across a beam of laser light. And given that the Lite2Sound is an all-analogue device so unless it incorporates a low-pass filter it might struggle with PWM, to achieve that feat he would have to modulate the country music directly onto the laser light.

In the video below he shows us how he characterised his laser diode by plotting its VI curve on an oscilloscope, and identified its most linear region. He was then able to supply a voltage in the middle of that region, and simply overlay the line level audio from the Echo through an RC network. The result is a successful transmission of music via laser that sounds OK, though we’d find it interesting to see what an audio analyser would make of it. We’d also be interested to know whether the VI curve also maps to the same profile in the light intensity, we suspect the answer would be “close enough”.

So laser wireless audio can be done, and anyone who points out that the same feat could have been achieved with Bluetooth is spoiling the fun. After all, what’s a hi-fi without Frickin’ lasers!

12 thoughts on “Finding The Linear In A Laser

  1. Typically the current is linearly related to luminosity, however the brightness to our eyes is the log of it. Twice the number of photons does not “look” twice as bright. The non-linearity between current and luminosity comes from thermal effects reducing the efficacy of the laser diode at higher powers.

    1. “…the current is linearly related to [luminous power]”. Well, kind of. You’re thinking of an LED, whose output power is typically proportional to the current through it, over 4 or 5 orders of magnitude. Forrest Mims (I think) described how to modulate an LED to send sound over light in 1977 or 1978 (the year I used the method for a science fair project).

      A laser is not an LED, however, and has a threshold current below which it produces no laser light. Above the threshold, laser output power increases dramatically, until the circulating power in the laser cavity gets high enough to ablate its mirror(s), and then it’s not a laser any more. The range of drive current between “no output” and “pfft” is usually (much) less than a single order of magnitude.

      [Monta], here, simply biased the laser on with a fixed current, and modulated that current over a relatively small, nearly-linear range by capacitively-coupling the audio signal into the bias current.

      The real way to linearize the laser diode output power is to monitor its output with a local photodiode, and use that in the feedback loop of the driver. You can use a separate diode and beam splitter if you want, or you could step away from the dollar store and spring for a real laser diode that has a monitor diode built in.

  2. Too bad the momentum of photons is so low. Otherwise one might be able to hear the music while the beam bounces off a tightly bound “drumhead” of shiny mylar. :-) If I had gone to the trouble of building such a device, I would still find myself trying it. Imagine if it worked. If you used a powerful enough infrared laser you could project music almost anywhere you could see. I could put that William Shatner album I’ve got to good use and drive my enemies crazy with music they can hear but can’t get rid of. Buahahahahaha!!! :-)

    ..most likely there would be too much heat and a fire would be produced before anyone heard anything. I don’t loath anyone that much.

    1. You don’t need to rely on photon momentum to transmit the sound. The photoacoustic effect works very well all by itself: the modulated energy absorbed in the target causes a time-varying heating and consequent thermal expansion, which produces audio signals directly. The “pop” you hear from a photo flash is caused by that effect. Bell, himself, experimented with it as a method to transmit sound, with his “photophone” in 1880 (!).

      If you want to transmit directly into your ears or brain, optical photons don’t penetrate very well, so it’s better to use microwaves. Search for “medusa microwave silent audio” for an interesting use for it.

      1. “medusa microwave silent audio” Wow. cool googlable search string. :-) Now I regret having empathy for every soul alive. Think of the fun I could have if I lacked ethics. Oh well, I guess I’ll just have to enjoy just thinking about it. I wonder if it would be possible suggest ideas into people’s minds that way ever so subtly so they get the idea without knowing they had heard it, bypassing the bs-filter we all throw up when we know we are listening. Think of the marketing potential.
        …. cheesburgers are delicious ….
        …. you deserve a corvette ….
        …. don’t worry, the wife won’t find out …

  3. A good few years ago, I remember playing around with methods to modulate a He-Ne laser beam with audio. The simplest was wiring the secondary of a PA transformer in series with the tube cathode, and driving the primary with the output from a hi-fi. Detection consisted of a small solar panel, capacitvely coupled to a headphone preamp. This worked well up to a mile or so distance. I also tried using a Pockels cell to modulate the laser beam – it was driven directly from the output of the same PA transformer, and gave a really high degree of modulation, but was very tricky to align.

    Diode lasers make things a heck of a lot easier!

  4. Sending the audio as OOK or manchester encoded data could be another way to do it, i.e. raw .wav, or mp3, or codec2 frames.

    Would be a good way to air gap an sdr at the tower from the shack, too, if lightning is an issue.

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