Go Wireless with This DIY Laser Ethernet Link

Most of us have Ethernet in our homes today. The real backbones of the Internet though, use no wires at all. Optical fibers carry pulses of light across the land, under the sea, and if you’re lucky, right to your door. [Sven Brauch] decided to create an optical link. He didn’t have any fiber handy, but air will carry laser pulses over short distances quite nicely. The idea of this project is to directly convert ethernet signals to light pulses. For simplicity’s sake, [Sven] limited the bandwidth to one channel, full-duplex, at 10 Megabits per second (Mbps).

The transmit side of the circuit is rather simple. An op-amp circuit acts as a constant current source, biasing the laser diode. The transmit signal from an Ethernet cable is then added in as modulation. This ensures the laser glows brightly for a 1 bit but never shuts completely off for a 0 bit.

The receive side of the circuit starts with a photodiode. The diode is biased up around 35 V, and a transimpedance amplifier (a current to voltage converter) is used to determine if the diode is seeing a 1 or a 0 from the laser. A bit more signal conditioning ensures the output will be a proper differential Ethernet signal.

[Sven] built two identical boards – each with a transmitter and receiver. He tested the circuit by pointing it at a mirror. His Linux box immediately established a link and was reported that there was a duplicate IP address on the network. This was exactly what [Sven] expected. The computer was confused by its own reflection – but the laser and photodiode circuits were working.

Finally, [Sven] connected his PC and a Raspberry Pi to the two circuits. After carefully aligning the lasers on a wooden board, the two machines established a link. Success! (But be aware that a longer distances, more sophisticated alignment mechanisms may be in order.)

Want to know more about fiber and networking? Check out this article about wiring up an older city. You can also use an optical link to control your CNC.

38 thoughts on “Go Wireless with This DIY Laser Ethernet Link

    1. There are plenty of optical link devices around, it is not anything new or that you couldn’t buy off the shelf. My alma mater used to have a laser link to bring network to a detached building on the campus back in 1996 or so. However, these commercial devices are not exactly cheap and quite hard to find these days, being mostly replaced by microwave links.

      The point of the article is that you can cheaply and easily build one as an alternative to wifi for your home installation, such as to bring network to a distant garden shed or something similar. With wifi you would be limited by legal requirements (it is generally not allowed to modify/add antennas or increase power to commercial equipment), laser/optical links are essentially unregulated as long as you don’t make it a health hazard.

      1. Nobody cares if you put a focused antenna attached to a ~100mw consumer wifi router on your garden shed pointing at your house.

        Go to a thrift store and buy a pair of 802.11n routers for ~$10. Hell, these days you can use a Raspberry Pi with a home-made reflector, though the limitation will almost certainly be the crappy bus on the Pi.

  1. 50% of the signal is carrier but hey it works! and it has lots of room for higher transfer rates. It almost looks like a Kansas city modulation scheme.

    BUT!!! What really grabs me is that huge huge fuse/holder wow it looks so big on a SMD board. Polly switch perhaps?

    I have two bags of those old fuse – two sizes and every current rating, fast acting and slow blow. I haven’t been able to find a purpose for them because … well … who uses fuses anymore.

    Top project all the same, now it’s time to see how far it will go.

    1. :D yes, I will use a different fuse holder next time. It’s quite pointless anyways, because the used power supply shuts off before the fuse blows.

      About the carrier-to-signal ratio, I’m actually not sure if it’s possible to do it do much better. You cannot really move the laser diode out of its lasing region electrically with the required speed; so you would need an external modulator (EOM, …), which is much more complicated and expensive.

      1. 10,000 semiconductors can blow in the time it takes for a fuse to blow. You can get semiconductor fuses if you really want to go with fuses. I tend to use a reverse diode for reverse polarity, a Zener for over voltage, a Polly-switch for over current and a thermal fuse for over temp if I am making something that gets hot.

        The FSK modulation is simple and it works so I would stick with that unless you need higher speed.

  2. Sounds to me like an implementation of fiber optic networking, just only without the fiber.

    P.S. At work there has been half a 1m^2 scrap-recycling container of said fiber optic cables cleared out.
    Something that I thought about but have yet no use for them, so they’ll stay there until taken off for recycling/reuse.

  3. What’s the maximum optical power output in normal operation and under fault conditions? How does that compare with eye safety regulations?

    The last time I investigated laser diode characteristics, they had a very sharp V-vs-I-vs-Pout characteristics. That meant it was easy to overdrive them (until the facets disintegrated!) and they would emit dangerous amounts of light.

    You’ve only got one pair of eyes, and they can be destroyed in an instant.

    As people who work with lasers are repeatedly taught, “do not look into laser with *remaining* eye”.

    1. Who cares, if they get injured its their own damn fault. You you must be a hit at parties.
      with all the the loud music.

      Maybe go fill some gaps between the grass and the sidewalk with dirt so you can sleep at night
      knowing ur squad actually dumb as inorganic corn won’t twist an anKle?

      1. I really don’t care if the designer blinds themself; that’s evolution in action. But I do care if they blind an innocent third party passing by.

        If somebody blinded me, I would commit suicide, since I’m very deaf. But not before I had done everything in my power to ensure the perpetrator couldn’t pass on their genes. Seriously.

      2. Out of curiosity, how many people have you watched die in accidents?
        How many of your friends have died in accidents?
        How many times have your children been in imminent danger of death, such that when the tale is told grown men literally have ashen faces?

        For me, >0 for all of those. Hence safety isn’t an abstract concern to me.

    2. These are cheap laser pointer diodes, they won’t deliver more than ~5mW or so and that is without modulation unless you want to destroy them really quickly. If these are the cheap modules from China I have seen as well, there is an SMD current limiting resistor in the casing too.

      The modulation from the ethernet will make the effective output power drop. They also aren’t really colimated all that well without additional optics neither – the beam will spread out with distance, reducing the amount of power you get per unit of surface. And finally, this kind of device is likely to be installed in a place where nobody could easily stare into the beam.

      However, if there isn’t a series current limiting resistor in the diode casing, then after the adjustment I would replace the bias trimpot with a fixed resistor. If that trimpot moves, the laser diode could be easily overdriven and burn out.

      So I wouldn’t be too worried about eye safety with these, unlike another project that was posted yesterday.

      1. They probably can deliver >>5mW for sufficient time to damage someone’s eyes. The designer has admitted on his website that he doesn’t know.

        I very much doubt he has considered fault conditions, e.g. PSU glitches, component failure etc.

        For comnparison, even 0.5mW classroom lasers state “do not stare into beam”.

        1. If they’re labeled as Class 2 lasers or below, they’re “safe” because the human blink reflex will kick in before damage is done to the eye. And if the lasers are being driven beyond their power ratings, I’m guessing they’ll fail before they get the opportunity to put someone’s eye out.

        2. You shouldn’t stare into any laser. Except maybe LASIK but that’s being pedantic. Doesn’t mean that a glancing reflection will blind you. It’s just not wise to spend a long time pointing the thing into your (remaining) eye.

          A simple resistor in series will limit it’s maximum possible power, based on the supply voltage.

          1. Regarding safety?
            That’s new.. Especially for allot of these youtubers with a brain-dead audience to recognize the hazards
            they place themselves in with the chemicals/voltages they’re dealing with…

            I’m talking to you RhionaSuper(dumbass)Genius, Cody’s Lab.

            There seems to be an uprising of electronic “enthusiast” wannabees where their projects are funded
            by ad revenue collected. Usually these people piss away hundreds of dollars worth of materials on
            the likes of pointless projects just for the sake of entertaining their audience.

  4. I was thinking: an ADC & laser encoder, on a HF long wire antenna (plus optic fiber to the house), equals lightning isolation.
    Just need to figure out how to power that ADC and laser encoder.

    1. Huge carbon rod to ground through a hyperinsulated tank of glaubers salt solution, which is the hot side of a stirling engine generator… You put your antenna up way high so lightning strikes every few months are a given, this passes through the carbon rod making it glow red hot, this heat is transferred to the glaubers salt solution and stored…

        1. I was just trying to be “different” … but on reflection, more chance of significant energy capture if you have two huge plates top and bottom and absorb current through the solution…. otherwise it will likely just flash boil and insulate the rod with superheated steam, also possibly leading to electrolytic dissociation of the water component and an ensuing hydrogen explosion.

    1. They’ve been working on that for the last few years, now that LED lighting is getting popular. You modulate the LED’s power by a few percent at a high speed. Human eyes don’t notice, but you get an extremely fast last-few-metres link.

    2. Popular Mechanics or/and Popular Science had a really great article about it. Can’t seem to find it right now though, have to sort through lots of CFL vs. LED fluff.

  5. Wow. I’d never thought of doing this entirely with an analog circuit rather than just re-encoding the digital bitstream via modulated laser. Quite a hack, only usable with 10Mbit, but very cool.

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