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1Gbps Wireless Network Made With Red and Green Laser Pointers 157

Posted by samzenpus
from the macgyver-network dept.
MrSeb writes "Back in the olden days, when WiFi and Bluetooth were just a glimmer in the eye of IEEE, another short-range wireless communications technology ruled supreme: Infrared Data Association, or IrDA for short. IrDA was awful; early versions were only capable of kilobit-per-second speeds, and only over a distance of a few feet. Trying to get my laptop and mobile phone to link up via IrDA was, to date, one of the worst tech experiences I've ever had. There's a lot to be said for light-based communications, though. For a start, visible (and invisible) light has a frequency of between 400 and 800THz (800 and 375nm), which is unlicensed spectrum worldwide. Second, in cases where you really don't want radio interference, such as hospitals, airplanes, and other sensitive environments, visible light communication (VLC), or free-space optical communication, is really rather desirable. Now researchers at the National Taipei University of Technology in Taiwan have transmitted data using lasers — not high-powered, laboratory-dwelling lasers; handheld, AAA-battery laser pointers. A red and green laser pointer were used, each transmitting a stream of data at 500Mbps, which is then multiplexed at the receiver for a grand total of 1Gbps."
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1Gbps Wireless Network Made With Red and Green Laser Pointers

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  • by Anonymous Coward on Thursday May 03, 2012 @12:38AM (#39875801)

    I thought IrDA was a famous Starcraft player...

  • Not new (Score:5, Informative)

    by Anonymous Coward on Thursday May 03, 2012 @12:42AM (#39875815)

    This is old hat:
    http://www.airlinx.com/products.cfm/product/19-0-0.htm

    It's stuff you can just go buy in a shop, we've used it here for around 15 years to connect across a street to the other office. We have a laser interlink.

    • Re:Not new (Score:5, Informative)

      by symbolset (646467) * on Thursday May 03, 2012 @01:35AM (#39876051) Journal
      And a screaming deal at only $28,000 for a pair. [neobits.com]
      • Re: (Score:3, Funny)

        by Anonymous Coward
        That's cheaper than my speaker cables!
    • Yup. The first time I saw a laser link across a highway between two buildings was in the 1980s. So this is 30 year old technology already.
      • Re:Not new (Score:5, Informative)

        by Midnight Thunder (17205) on Thursday May 03, 2012 @06:08AM (#39876885) Homepage Journal

        While the technology is old, the implementation seems to be new. Also, the form it has taken means that we are likely to see cheaper commercial solutions coming out or a whole bunch of hobbyists implementing this themselves - or both. $100 vs $4000+. I can just imagine mesh networks based on this.

        If these can be coupled with solar power and are of low energy use, then I can imagine these being alternative solutions to laying cables in remote areas.

        • by skids (119237)

          For enterprise purposes, the equipment costs these days are lower than they used to be and still falling. Eventually, though, an "industrial quality" system of this type will hit a pointy where the hardware cost is inconsequential, because the actual cost of operating such a unit is really in the support contracts, unless you are big enough to have a trained FSO/RF specialist on staff and stock backup units. (Many of these FSO units these days are hybrid units that use microwave and FSO to provide technol

        • by mcgrew (92797) *

          If these can be coupled with solar power and are of low energy use, then I can imagine these being alternative solutions to laying cables in remote areas.

          Interesting idea. However, I don't think it would work well for that. You need line of sight and nothing between the transmitter and reciever.

          You mentioned mesh networks, I don't see why we couldn't make mesh networks with wi-fi.

          TFS said "Trying to get my laptop and mobile phone to link up via IrDA was, to date, one of the worst tech experiences I've ever

          • by Catbeller (118204)

            For mesh networks, we really need one of the former TV channels. Fat chance they let us do it; we could subvert the entire system they've built on the dark backend which they use to monitor everyone. A true Little Brother TV channel mesh network wouldn't need the internet backbones, not if it were cheap, easy, and used its own protocols. So it will be illegal. But, as I said elsewhere on the talkback, don't let that stop you.

        • by jklovanc (1603149)

          If these can be coupled with solar power and are of low energy use, then I can imagine these being alternative solutions to laying cables in remote areas.

          The range is only 25 meters. it is much cheaper to lay 25 meters of cable and a repeater every few kilometers than a laser repeater every 25 meters. If a single repeater goes out the whole line is down. Basically one would have 40 failure points every kilometer. Say one is 10km from the nearest hard line that means 400 repeaters and 400 possible points of failure.

    • Re:Not new (Score:5, Informative)

      by dark12222000 (1076451) on Thursday May 03, 2012 @01:47AM (#39876089)
      You're missing the point. The summary clearly states that the interesting point here is that it was done with cheap 10$ laser pointers that you can buy from Amazon. Yes, this was old tech - if you were willing to shell out 15k for high end gear. The fact that it can be reproduced for a much lower price (maybe a few hundred at most by the time you get integrated units and pay for research?) and therefore more likely to see more widespread usage, is the point.
      • by julesh (229690)

        The reason the available commercial equipment for this stuff is expensive has nothing to do with the quality of the laser, though (the site GP linked to specifies the laser in their entry level device as being a 7mW laser diode, so probably about 50% more powerful than the lasers used in the OP's article). The point is that it's expensive because the only application it's viable for is inter-building linkage, which *almost nobody wants to do*. You can't use it to replace ordinary wireless networks, becaus

        • Re:Not new (Score:4, Insightful)

          by khipu (2511498) on Thursday May 03, 2012 @04:29AM (#39876593)

          I'm pretty sure that if you got the price down to about $50, people would find a lot more uses for this, including sharing network connections with friends (in particular in rural areas), secure communications, and distributing access points. Not everybody lives in cities with otherwise excellent coverage.

          • by julesh (229690)

            I'm pretty sure that if you got the price down to about $50, people would find a lot more uses for this, including sharing network connections with friends (in particular in rural areas), secure communications, and distributing access points. Not everybody lives in cities with otherwise excellent coverage.

            I don't think it'd be useful for those applications. Reading TFA, this device has a useful range of approximately 10m, which is somewhat limiting. Even without this limit, houses in rural areas are unlikely to have undisturbed line of site. Birds will be a problem (current commercial systems solve this by using redundancy to allow routing around birds, I believe, which makes your $50 an unrealistic target price). To make these things work properly, they really need to be on top of tall buildings to ensu

          • Even at $100 it may still be cheaper than digging up the ground and laying fibre optics in certain cases. I think what would really change things is if these were easily installable by someone who isn't a specialist.

          • by arth1 (260657)

            I'm pretty sure that if you got the price down to about $50, people would find a lot more uses for this, including sharing network connections with friends (in particular in rural areas)

            People do that? I know they share wives and sisters with friends (in particular in rural areas), but bandwidth? C'mon. Pull the other one.

        • by iamhassi (659463)

          But then these guys can mostly just dig up the land between their buildings and lay cables, which will give higher capacity and more reliability (one of the universities has an issue because one of their buildings is separated from the rest by a public road... they might benefit from this).

          You could spend $50 a lot of times vs digging up land between buildings and laying cables, so if capacity and reliability is a problem, just buy 10x for greater capacity and reliability, or even 100x would probably still be cheaper than digging and laying cables.

          • Hardware is just a small part of the cost. There is installation and maintenance. The cost of going more than 25M includes installing a pole ever 25m, getting power to the poles and maintaining the repeater on top of the poles. Even solar panels need to be cleaned regularly. Would you rather maintain 2 switches inside buildings and 100M of cable or 16 laser nodes( for redundancy) of which 12 are on top of poles with attached solar panels? All it takes to bring this network down is for a birds with long tai

        • I can think of another use:
          Temporary in-room networking where security or bandwidth conjestion are a concern. I could envision a server room issue where you needed to understand what was happening at multiple points in your network that aren't normally tapped. You use something like a vampire tap and a raspberry pi to get copy off the data, analyze, and send back to something like splunk. However, rather than running temporary wires all over, instead send them by laser to the central monitor. Then when

      • Re: (Score:2, Interesting)

        by Anonymous Coward

        Radio hams have been experimenting with point to point communication by light. It's been mountain-top to mountain-top so needs quite precise alignment. Also the data rates have been quite low - more voice. But the technique is quite old. We've known about modulating laser diodes for some time.

        A quick search reveals this site reporting a 104 mile link using LEDs. http://www.bluehaze.com.au/modlight/

    • by Ihmhi (1206036)

      1) Go to a rave with a laser light show.
      2) Run a bootleg wireless.
      3) ????
      4) PROFIT

    • by tkohler (806572)
      30 Mbps in ~1970

      from the very entertaining: Birth of the Laser Printer [computerhistory.org]

      "The problem is, the bits are all coming out a kilometer away, and the printer’s down here at the other end, so how do we get the data to this thing?

      So, we sat down one time and said, "So why don’t we make an optical link?" Because we looked at doing microwave, but those were only three megahertz, and you’ve got to get enough FCC permission to do that, even then. So the interesting thing is there are no communication

  • by TWX (665546) on Thursday May 03, 2012 @12:42AM (#39875817)
    Now I have another thing to implement for Bring Your Own Device...

    This does make me wonder, however, if we could see fiber optic gbics that don't cost thousands of dollars each if the technology that makes this free-air communication possible can be adapted to fiber optic applications.
  • Efficiency? (Score:2, Funny)

    by Anonymous Coward

    How does the Mbit/mW compare to a 802.11b/g/n pringles cantenna?
    Which can achieve further distance assuming LOS?

    • by julesh (229690)

      802.11n uses 250mW transmit power to achieve 600Mbits = about 2.4mW/Mb
      A laser pointer typically uses about 5mW, so 2 of them will be about 10 mW, for 1Gbit = about 0.01mW/Mb.

      But then 802.11n is omnidirectional, whereas a laser is unidirectional, so this is really an apples to htcs comparison.

      • by c0lo (1497653)

        But then 802.11n is omnidirectional, whereas a laser is unidirectional, so this is really an apples to htcs comparison.

        I think you're missing something from the GP post

        How does the Mbit/mW compare to a 802.11b/g/n pringles cantenna?

        A cantenna [wikipedia.org] is essentially a wave-guide, making the transmission highly directional.

  • They demonstrated one way data transmission over a very short distance, not a network.
  • by Anonymous Coward on Thursday May 03, 2012 @12:49AM (#39875865)

    Laser based FSO isn't exactly a new field.
    1Gbps data rate with a diode laser isn't that hard to achieve even with pretty simple drivers and 1-bit amplitude modulation.
    Neither is using wavelength multiplexing some revolutionary new idea.
    So... huh?

    • Cost?
      • by julesh (229690)

        OK, you try turning this into a viable commercial product at a lower cost than the competition. The problem is, this is a niche market because these things are really hard to find a suitable application for. You'll be setting up a manufacturing base and then selling maybe 1000 units per year, so you need to offset the cost of manufacturing, support staff, sales staff, development.... hence you'll be selling each unit for $1000 or more. Probably much more, because to make it useful you'll need precision m

    • Re: (Score:2, Interesting)

      by sveinb (305718)
      Well, that's what I thought until I tried. Connected a laser pointer to a signal generator and measured its light output. As the frequency increased above about 1 MHz, the modulation level decreased to a non-usable level.
  • by emag (4640) <slashdotNO@SPAMgurski.org> on Thursday May 03, 2012 @01:00AM (#39875917) Homepage

    Ok, the real question is... how does this apply to /.'s new BI focus? Can I use this instead of spreadsheets or specialized software to properly align my Business Intelligence with the synergies of the corporation for maximization of profitability?

    Ouch, that hurt...

  • by drwho (4190) on Thursday May 03, 2012 @01:11AM (#39875957) Homepage Journal

    This was done years ago. I remember seeing the story, I think it was on gbppr. The problem is, these laser pointers aren't designed to be used constantly and they wear out.

    • by thegarbz (1787294) on Thursday May 03, 2012 @04:00AM (#39876469)

      Got a source on that? Laser diodes don't "wear out" as far as I am aware. They may be damaged by thermal runaway in the short term or long term by poor design but the only critical factors here is the stability of the current source, choice of bias point, and thermal design. They certainly don't get tired over time.

      • The manual for my green laser pointer cautions you against using it constantly - I'm guessing the heatsink arrangements are not what they could be in there.

        The researchers probably took them apart and made sure they were properly cooled though.

      • I assume that laser diodes are no more fundamentally resistant to degradation over time than ordinary LEDs are. Not a huge problem(LEDs are usually specced to be something like 80% of original output after 100,000 hours); but solid state devices only look immortal compared to their mechanical counterparts.
        • by thegarbz (1787294)

          Indeed but you need to take take into consideration that 100000 hours is 11 years of continuous use. That will exceed the typical useful life of product such as this.

    • by dbIII (701233)
      The "done years ago" could apply to anything.
      I was using a homemade microwave data link in 1988 but the wireless and laser stuff now is still interesting.
  • "A red and green laser pointer were used, each transmitting a stream of data at 500Mbps, which is then multiplexed at the receiver for a grand total of 1Gbps"

    That made me think of blue lasers, which would have even better rates.

    But, how about longer waves, such as infrared or even radio? Are there any radio lasers around? THAT would do for long distance calls, and proabably be enough for E.T. to phone home.

    • Re:radio lasers (Score:4, Informative)

      by mpoulton (689851) on Thursday May 03, 2012 @01:16AM (#39875979)

      Are there any radio lasers around?

      That would be a MASER (microwave, not light), and they predate lasers. However, a maser holds no advantage over a regular microwave transmitter for terrestrial communications. The distance of point to point microwave links with standard radio technology is limited by the curvature of the earth, not power or beam divergence. Even with tall towers, it's very hard to obtain a line of sight path between two points on earth more than about 50 miles apart.

      • by symbolset (646467) *
        It may, however, be handy in space. Like Planetary Resources is planning to do. Because for high data rate communications over long distances in space with low power it's pretty awesome.
        • Beam divergence [wikipedia.org] is a bitch at low frequencies. EM signals don't travel in a straight line, a ray of them tends to get wider over distance. This effect is stronger at low frequencies. For space you need the highest frequencies you can get if you want to have some usable distance. Gamma lasers would be preferable, if it were possible to make those.
          Or you'd need a very wide beam and thus a very large laser/maser.
      • The distance of point to point microwave links with standard radio technology is limited by the curvature of the earth, not power or beam divergence.

        Perhaps there is a way to mitigate this limitation with strategically spaced Hot Pockets®.

      • Even with tall towers, it's very hard to obtain a line of sight path between two points on earth more than about 50 miles apart

        That was a big disadvantage with MASERs. However, what I propose, is data transfer using high-powered LASERs. Initial handshaking will be done by "creating" a line of sight path before communications can start in lower-power mode :D

    • Yeah, using laser pointers is a good idea, but what do you do when the cats jump on your data?
    • by julesh (229690)

      That made me think of blue lasers, which would have even better rates.

      But, how about longer waves, such as infrared or even radio?

      A typical 1.5mW near-IR laser diode can emit at 2.5Gb/s and costs only about 3 times as much as a laser pointer, so is more economical.

  • by guttentag (313541) on Thursday May 03, 2012 @01:37AM (#39876061) Journal
    It's only a matter of time before the MPAA/RIAA gets this outlawed because pirates could be using it to broadcast entire ripped DVDs to each other in mere seconds using sharks with frickin' multiplexin' red and green lasers attached to their heads! You laugh, but it will happen. [slashdot.org]
  • A similar project (Score:5, Informative)

    by ard (115977) on Thursday May 03, 2012 @02:00AM (#39876123)

    A proof of concept on laser pointer networking was done two years ago, if you are interested see
    http://www.diva-portal.org/smash/record.jsf?searchId=4&pid=diva2:325270 - Fulltext at
    http://www.diva-portal.org/smash/get/diva2:325270/FULLTEXT01

  • by Grayhand (2610049) on Thursday May 03, 2012 @02:07AM (#39876139)
    is getting the sharks to hold still.
  • by aaronb1138 (2035478) on Thursday May 03, 2012 @02:20AM (#39876165)

    I'm surprised the cell phone companies haven't implemented something similar on their towers to reduce backhaul. Have dozens of towers in a given area relay optically to a super node tower with amazing backhaul. Have them relay to a few others in a standard mesh network layout for redundancy. Might even reduce their spectrum need if they are using channels to talk tower to tower. May have some issues with rain I suppose though, but that could be mitigated if laser wavelengths for which water is not refractive exist. Or just use laser arrays with heavy multiplexing and parallel signal reinforcement.

    • by Hadlock (143607)

      I'm pretty sure they do this already using microwave

    • Re:Cell Towers? (Score:5, Informative)

      by adolf (21054) <flodadolf@gmail.com> on Thursday May 03, 2012 @04:04AM (#39876489) Journal

      Visible line-of-sight issues ruin the possibility in many applications. Rain is murderous to low-power visible light connections, as is fog and snow. Even wind will affect a laser-based length over any substantial distance as the end-points sway (and yes, all towers sway in the breeze).

      Meanwhile, cell towers quite commonly already link with microwave: The big parabolic reflectors covered with fiberglass radomes that you see on many (perhaps most, or nearly all) cellular towers are not for subscriber usage, but to link neighboring towers together. This is often done using licensed frequencies, though unlicensed bands are also used.

      There are generally also redundant backhauls using copper or fiber or both, but I guess the point I'm trying to make is that cell towers -already- use wireless RF backhauls...and that the tech described in the article isn't likely to change that.

      As it stands, resistance to rain-fade and other weather seems to be excellent, at least anecdotally: I've never experienced it, and I've carried a cell phone for at least 1.5 decades.

      (Disclaimer: I work with RF and wide-area long-range wireless networking as part of my day job, though not necessarily with back-end cellular systems in particular. Just because optical networking seems like a general non-starter to me doesn't mean that it's unsuitable for the uses that you suggest.)

      • by Lumpy (12016)

        But the new cell towers seem to be bad a building fade. 100% signal in my yard from the cell tower 5 blocks away. 20% signal inside the house near a window.

      • by qubezz (520511)

        This must be why air-laser consumer tech never came out of Silicon Valley or the UK; instead we have fiber optics. They knew about fog, so they pointed their lasers through glass tubes.

        This seems just another "maker fair" type story, the type of which gets old and annoying - (undereducated person) discovers (old technology) made from (cheap new technology) thanks to (smarter people who understand new technology). It's a step above my research paper on popsicle-stick bridges though.

        Now do some high school s [wisc.edu]

  • by burnttoy (754394) on Thursday May 03, 2012 @02:21AM (#39876169) Homepage Journal

    For a start, visible (and invisible) light has a frequency of between 400 and 800THz (800 and 375nm), which is unlicensed spectrum worldwide.

    My God! They're broadcasting my movies over an unlicensed, unregulated carrier! This MUST be stopped! This "visible" light will aid paedophiles, piracy, terrorists, drug dealers and all manner of criminality!

    • by artor3 (1344997)

      No worries, the visible light spectrum is already being regulated. If you don't believe me, feel free to set up a 1500 watt spotlight pointing towards oncoming traffic on your street tonight. Let us know how it turns out.

  • by pbjones (315127) on Thursday May 03, 2012 @02:21AM (#39876171)

    not good for 20% of males that have red/green defective colour vision, you desensitised clod...

  • by subreality (157447) on Thursday May 03, 2012 @02:33AM (#39876219)

    10Mbit, 1200-1400 meter range, GFDL-licensed open designs: https://en.wikipedia.org/wiki/RONJA [wikipedia.org]

    Instead of lasers they use LEDs with relatively inexpensive lenses.

  • by silentcoder (1241496) on Thursday May 03, 2012 @03:35AM (#39876393) Homepage

    A group of students at The University of Pretoria in South Africa did exactly this while I was still studying there, this was circa 2001.
    A large part of their motivation was to help build a technology for high-speed networks that were not subject to the state protected telecoms monopoly.

    They used almost exactly the same technology, lazer-pointers for sending streams, but I believe they used solar-cells for receivers.

    I remember they boasted speeds of over 1mbs which (back then) was incredibly fast (in fact faster than the internal buffers of the P2 computers they used - so that the data actually slowed DOWN after being received) but I don't believe they ever went beyond a single point-to-point connection.

    Maybe one of the students who were involved is on slashdot and can give more details ?

    • by amorsen (7485)

      1Mbps in 2001 was dead slow, sorry. 100Mbps ethernet was everywhere and 1Gbps ethernet available. Computers could easily fill a 100Mbps connection while 1Gbps was more of a challenge.

      • Did you miss the location ? It was NOT slow if you were in South Africa. At that stage it still tended to take a while for US technology to reach the bottom of Africa. Hell, in some cases, it still does.

        Hell most corporates here were still running on 10Mbps ethernet at the time.

        • by amorsen (7485)

          The Pentium II should have no trouble filling 100Mbps ethernet, even at 233MHz.

  • I don't agree with the original poster about the deficiencies in IrDA.

    This was in a time when dial-up access was the norm, if you were lucky you had ISDN, and your cellphone gave you patch 9600 baud connectivity. IrDA was fine in this situation.

    I used my Nokia 6320 phone and Palm V to restart servers whilst on call from the comfort of restaurants, and even to make changes to Perl scripts from a different country. The range was poor, but fine, the performance was limited by the cellphone not IrDA.

    Kids of tod

    • by deroby (568773)

      I remember copying files from one laptop to another via the IR ports. There was an option in the BIOS (Dell) to chose between 'Normal IR' and 'Fast IR' and the latter gave something like 6Mbit I seem to remember, not sure, but it surely was fast enough to copy setups and iso's etc. Sure we were not allowed to move the laptops around in the meanwhile, but copying things was much faster over that link than using the 10Mbit network that was shared with the entire floor.
      Eventually we found out about using a dir

  • Sorry but this has existed as a communications system in the middle east and india for well over a decade now. People over there have been doing this with laser pointers and LED's for a very long time.

    Granted it was only 100bt as it used existing transciver chips, but the jump to 1Gps is not that hard.

    and why red and green? The existing designs all use RED for alignment and then IR for data comms so that it's not visible from acute angles.

  • by nurb432 (527695)

    Worked great for me. For extended range or in 'noisy' environments id stick an optical fiber between the 2 devices.

  • Those with a (or even an) historical bent may be interested in the first outdoor optical communication system, the heliograph [wikipedia.org], which used reflected sunlight for long-distance communication via Morse code. The record distance covered was 183 miles (295 km), between Mount Ellen, Utah, and Uncompahgre Peak, Colorado on 17 September 1894.

    To my knowledge, this record for terrestrial (i.e., non-moonbounce) optical communication has never been broken, even by modern laser and LED systems. The closest attempt [reast.asn.au] of

  • ... move around? Or do you have to stay in the same spot where the lasers were aligned?

  • My experience, on windows: enable IrDA on phone. Put phone next to laptop. Phone recognized, systray icon pops up. Send files.

    IrDA was the closest we ever came to solving the still-unsolved problem of how to transfer files wirelessly between two machines sitting next to each other. It's telling that the de-facto standard now is to carry around USB flash drives: god help you if you've lost whatever cereal-box prize you were using.

    Compared to the dicking around we have to do with bluetooth - which, incidental

    • I had a BJC-55 for a while when I was using a laptop in my later high school years (back when - I presume - laptops were rare). Although it wasn't particularly practical, the fault had nothing to do with IrDA - which connected and setup on Windows pretty flawlessly.

      I do remember it being quick enough that if I slid a device past the port, they'd link up and promptly delink when out of range pretty commonly.

      So yeah - I'm in full agreement. It would be awesome if we could have a light-based gigabit data proto

  • employing red and green laser pointer lasers (LPLs).

    Well that's fucking clever of them.

  • Man, after ten years of watching the internet slide into being something resembling a privately-run prison system, a light glimmers.

    Damn it, yes. Use da lasers. We need alternative cheapnets that go around the massively centralized and monitored leylines that the commercial and government spooks have utterly co-opted.

    Lasers on the roofs, encrypted comm, backbones sneakily flashed around the countryside. Move the switches and bridges into software on commodity hardware, so they can't be controlled by manufac

  • Me and a buddy of mine cobbled together a Super Sekret Spi Laser Telephone System in highschool. It was a couple of cheap laser pointers and photovoltaic cells wired to the transistors in radio kits with microphones. They only worked well at night, over shortish distances, and were a complete bitch to aim. Still.. SUPER SEKRET SPI LASER PHONE!

  • I went to the paper, expecting to find a fairly high powered laser that is not a pointer, and expecting to call someone out on calling them pointers. However, they're only 5 mW, which is indeed a pointer. Cool that they can use such low powered lasers for this.

    The FDA has regulations [fda.gov] in the U.S. saying that no laser products over 5 mW may be marketed as "pointers":

    Class IIIb lasers cannot legally be promoted as laser pointers or demonstration laser products.

  • I don't know why this is being hyped so much... from my brief look it seems pretty dodgy.
    I'm not an expert in data transmission, but I have reviewed quite a few papers.

    Two main points stand out:

    1. They have two lasers of different wavelengths just so they can use the phrase "wavelength division multiplexing", but the lasers point at separate photodiodes! The lasers could be the same wavelength and it would make no difference.
    Doing this adds nothing to their paper and lowers my impression of the research qua

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