3 ms·
I know that even cheap LEDs have rise and fall times of easily under 5 ns with the right circuitry[0]. This means that a transmitter of 100 MHz can be easily bu
by wyc 12y ago
I know that even cheap LEDs have rise and fall times of easily under 5 ns with the right circuitry[0]. This means that a transmitter of 100 MHz can be easily built with COTS parts, so the speeds they mention make sense to me.
However, when I built a cheap optical communications system with LEDs and $0.10 diodes, the tough part was always getting the photodiode capacitance low enough to receive high frequencies while still sensitive enough to detect the signal accurately[1]. This is why lasers are very popular for communications: a concentrated beam can deliver a very high signal-to-noise ratio. With LEDs, it's tougher because the beam is dispersed and you don't want to blind people with high power output, so the signal is a lot harder to receive. It looks like they went with nice avalanche diodes instead of cheap ones, and I'm sure that have a well-designed integrated circuit helps a lot too. I'm very excited to see how this plays out.
[0]: http://arxiv.org/ftp/arxiv/papers/1011/1011.1954.pdf http://arxiv.org/ftp/arxiv/papers/1011/1011.1954.pdf
[1]: http://www.thorlabs.us/tutorials.cfm?tabID=31760 http://www.thorlabs.us/tutorials.cfm?tabID=31760
- Sniffnoy 12y agoA note -- if you're linking to arXiv, it's better to link to the abstract (http://arxiv.org/abs/1011.1954 http://arxiv.org/abs/1011.1954) rather than directly to the PDF. From the abstract, one can easily click through to the PDF; not so the reverse. And the abstract allows one to do things like see different versions of the paper, search for other things by the same authors, etc.
- JadeNB 12y agoThank you for advocating this practice! This kind of 'deep linking' seems to be the norm even among scholars, and I've never understood why.
- Luc 12y ago> This is why lasers are very popular for communications: a concentrated beam can deliver a very high signal-to-noise ratio. With LEDs, it's tougher because the beam is dispersed and you don't want to blind people with high power output I suppose I am missing something obvious... but laser beams are shaped into a beam by way of a lens at the end of a tube, right? The laser diodes I've seen emit light in all directions. You could just as well stick an LED at the bottom of that tube.
- dfox 12y agoClassic lasers directly emit coherent beam without the need for additional optics. Output aperture of most lasers might seem to emit light into all directions, but most of the output power is concentrated into the direction of the resonant cavity. Due to manufacturing tolerances low power laser diodes tend to produce somewhat irregular output beam (both diverging and uneven) which is then compensated for using some external optics that also serve to set required width of the final output beam. It is impractical to convert light from some arbitrary "point-like" source (ie. LED) into beam that has low divergence by lenses or similar optics.
- jarvist 12y agoLasers generate coherent radiation, the frequency of the light is much more monochromatic (set by the optical cavity) than a bare LED (set by the thermally smeared band gap of the material). Even a very cheap laser diode can have the output collimated with a single lens, resulting in a beam that diverges a fraction of a percent. Hence cheap laser pointers. Try and do the best you can with an LED and you will end up with a torch.
- raverbashing 12y agoAbout the issue of capacitance and detecting the signal (with visible light interfering with it, unless they work in IR only) you can modulate the signal in some way then filter for that