Communicating with a beam of light

Last weekend, ARRL, the national association of amateur radio, held a contest called, “10 GHz and up” with the goal of communicating via radio or microwaves over long distances. [KA7OEI] and a few friends decided to capitalize on the “and up” portion of the ’10 GHz and up” contest by setting up a full-duplex voice link over a distance of 95 miles. They used the 478 THz band, also known as red LEDs and laser pointers.

With [Ka7OEI]’s friends [Ron] and [Elaine] perched atop a 5700 foot-high mountain near Park City, Utah, [Gordon], [Gary] and [KA7OEI] trudged up a hill about 10 miles north of Salt Lake City. With the help of a pair of 500,000 candlepower spotlights, the two teams found each other and began pointing increasingly higher power LEDs at each other.

The teams started off with 3 Watt red LEDs before moving up to 30 Watt LEDs and a photodetector at each end. Even though the teams weren’t working with a true line-of-sight – refraction of the atmosphere allowed them to transmit this far – they were able to transmit tone-modulated Morse and even full-duplex voice.

Not bad for a transmission that bends the FCC’s “275 GHz and up” amateur band to its breaking point.

36 thoughts on “Communicating with a beam of light

    1. Nope; recreating Alexander Graham Bell,who was the first to use wireless telephony by using light over a century ago. Everyone who used electrons to modulate photons since Bell are variations of his efforts. Who knows where the experiments by amateurs of all sorts(not just radio amateurs) will lead this to. Sadly if history is a guide will end up being restricted in the use of what was previously deemed “unusable”,until the amateurs started using it

    2. Fun modern tech recreation’s of Bell’s first wireless telephony transmissions. Unfortunately the curvature of the Earth limits distance on the Plains. However in my area there are high points that would allow the amazing distance of at least 25 miles:), but it’s all on private property.

  1. Interesting stuff research wise, although it should be noted that if you’re after the longest distance then radio waves is what you need.
    In the QRP world, a thousand miles with one watt or less aren’t that uncommon. Anyway, a somewhat similar project is the one at which describes an optical data link over visible/IR light.

      1. In germany we use the legal max of about 750W with stacked Yagi antennas to get enought gain to overcome the ~170db transmission loss on it’s way. Then you might go with CW.

      1. Not really? Nah!

        They had equipment to modulate the led in “baseband”. That is they just had the led flicker with the analog value of the audio signal. Better encoding and bandwidth along the signal path, and you can easily go up to MHz, and GHz (and the accompanying bitrates) because you’re using light.

        Light has enormous bandwidth. Fiber optics nowadays carry signals at gigabit, 10gigabit and 100gigabit. Still this pales in comparison with the theoretical bandwidth that fiber provides…..

        1. You can’t do that with just a beam of light through the air. You can do that with fiber as different entry angles show up on the other side, because the light can bounce off the medium edges.

          Not so with open-air.

      2. The limiting factor is the RECEIVER, not the air column between the TX and RX. As you increase the gain of the RX amplifier, you increase the impedance to the photo-detector. Since it’s a PN junction, capacitance plays a major role when your impedance approaches megohms range. Your RX bandwidth is limited to about 4kHz in most cases (just wide enough for audio) because of that pesky PN junction. Even PIN diodes can’t help here; they merely defer the inevitable.

      1. now that i think about it, barry might have been partailly right, … -> INTERFERENCE could affect it.

        but that interference would only be on the frequencies that (the) various light sources are flickering at

        you’d still have resonable bandwidth between 1 and 100 mbps, with small “gaps” of time where the low frequencies affect the most and cause loss of signal, but the overall thouroput (transfer/second) would still be huge compared to copper wire

        …interference like:
        60hz bulb
        25khz CFL
        100khz LED PWM
        moving car headlights… along bumpy road
        flashing light up signs (billboard ect)
        strobe-lights (emergency services)

        that still leaves a fair bit of bandwidth for non-low-freq applications. like ethernet XD

        granted this laser-link would be near useless for alalog NTSC, it WOULD however be execellent (i think) for ethernet and digital webcam HD videoconferencing, provided your brain doesnt melt every time an ambulance strobe causes your framerate to drop ONE frame each time the strobe flashes

        30 f/s – 5flashes/s = 25 f/s … still better then cableinternet, still better then my webcam! lol

      2. There was something like this written about in Scientific American (or maybe it was Elektor). Infra-red LEDs in the office ceiling transmit very high speed data to receivers attached to computers, printers etc.

        More free from interference than Wifi, and more bandwidth. Has the [un]desirable property of only working within one room, line-of-sight. Could be good for offices where there’s lots of equipment and only limited radio bandwidth.

    1. As someone who works for a company that does free-space optics communications, let me assure you that it’s more than a little difficult to move the gap between analog 4kHz and digital data.

      If you have ideal conditions, sufficient power of transmission (which finding a fast-data rate, modulation stable laser is not an easy task), ideal optics (which are definitely non-trivial to design for anything more robust than an engineering project), a discriminating enough reception design (we’ve used PINs and APDs; the only thing that’s been reliable enough at gigabit speeds at any distance more than 1km with reasonable and legal power levels has been an APD), then yes, you certainly could go at full fiber-optics speeds.

      There are a few differences in the physics of how the light propagates through a fiber versus through the air. The simple free-space optics loss is quite tough to overcome, not to mention that the wavefront of the light beam can alter significantly depending on the optics design. A small skew can grow rapidly. At slower speeds, it’s not as much of a problem. A single bit in a ten megabit optical signal is approximately 30 meters long. A single bit in a gigabit signal is .3 meters long. The skew of a signal, the aberrations present from just transmitting through air can easily lengthen and shorten the receiver’s perceived bit. It’s a lot easier to discriminate a 30M long bit than a .3M one.

      That said, it’s a fun thing to work with (and I’ve actually built an operational Ronja clone; the stuff I work with and have cast offs from is a bit better though ;) ), but let me assure you that the leap from a long-distance analog optical audio connection to a high data rate digital signal are non-trivial. It’s still real neat and awesome to see these kinds of setups and pushing the bounds of communications!

  2. FTA: “As soon as we brought our transmitters up to full power, we reduced them again to 1/4-1/15th as each other’s signals were strong enough that there was noticeable distortion in the received audio” … “As it turned out, speech was copiable – with some difficulty – down to the 40-50 milliamp range with the old receivers but the APD receivers extended this down to around 20 milliamps” That’s pretty amazing.

    Also, they tested the link using cheapo laser pointers. It worked. The blog has a write up on how to build a similar

  3. Also, I have some experience with
    using LEDs in high frequency mode
    where you reverse bias the emitter
    at about -1V so the positive 3V
    pulses fully saturate the junction.
    This increases the bandwidth over
    a cheap diode such as a phlatlight
    to well over 100 MHz :-)

  4. I have just successfully bid on the 470-790 Thz band, and once my FCC licenses are in place, I will be enforcing my rights through my legal department.

    Anyone found to be broadcasting in my spectrum will be subject to legal action, especially corporations with public broadcasting once called “billboards” and you pirates whose shirts broadcast in “green” without paying me the proper royalties.

  5. “Your RX bandwidth is limited to about 4kHz in most cases (just wide enough for audio) because of that pesky PN junction. Even PIN diodes can’t help here; they merely defer the inevitable.”

    I have found through tests, regular phototransistors go up to a bit over 100.khz, PIN diodes go up to ~ 4.mhz

    ..actual working circuits (optically coupled IrDA @ 115.2 kbps)

    “this laser-link would be near useless for alalog NTSC”

    I’ve done this too, by modulating the light using wideband FM at 3.mhz (VCR record signal), color-under (688.khz) and sound (4.5 mhz)

    heavy snow one day caused some picture quality loss. link was ~ 200′ across a street with 6 and 8 ” lenses

    “let me assure you that the leap from a long-distance analog optical audio connection to a high data rate digital signal are non-trivial.”

    I’ve done 3.2 mhz baseband digital so far but lost interest.

    “you reverse bias the emitter at about -1V so the positive 3V pulses fully saturate the junction.
    This increases the bandwidth”

    you can just clamp the diode to ground between pulses (bi-directional drive)

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