Slashdot Mirror
Using IR Lasers Instead of Fiber
Artifice_Eternity writes: "Can't deal with the trouble, time or expense of digging up the street to get fiberoptic cable to your building in the big city? There's another way...infrared line-of-sight infrared lasers between your building and another one nearby. Repeaters and redundancy can keep the chain going reliably for miles, with gigabit data transmission rates."
And lets not forget how fragile the whole system would be. A mirror, heavy fog, lightening striking, etc. all come to mind. From what I've heard wireless repeaters attract lightening quite well, and a nice surge like a lightening bolt can bring makes a wireless network a little worthless.
Hopefully with all this new technology coming out, cheap wireless networks will be next on the list of things that are cool for geeks. I mean WHO DOESN'T have their own intranet in their house???
Ignore the "p2p is theft" trolls, they're just uninformed
I don't know much about laser communications, but wouldn't things like the above cause the laser to scatter making it unusable in the above conditions? I could especially see this with rain because of the refractive properties of water. I do know my school uses this to get bandwidth, and all I have known it for is unreliability.
Hmm, this is nothing new. You, yourself, can buy a several watt (yes watt) IR diode and modulate it using an AOM to at least 100's of Mbits a second and more very easily for under $1500. This would easily reach a target for miles if the reciver made use of a dichroic narrowband filter and some good ECC. However, as mentioned, line of sight must be maintained.
Divergence, not mentioned in the article, is also an issue. Especially with laser diodes, it doesn't matter what kind of miracle anamorphic lens system you have to decrease the divergence of the beam, becuase pretty soon that pencil thin dot is going to become several feet in diameter. THIS is what accounts to loss more than so called 'atmosphere' causes. Photodiodes/transistors operate at a power/cm^2 ratio, and the lower this is out of the rated area the more noise. So when the beam spreads out, the concentration of power thins out and you get noise because although all your signal is getting there, you can only sample a small fraction of its power. Having low divergence also works against you because it makes the system much more difficult to align. I would start with a very large beam, just enough to get a signal, and then progress to the smallest beam possible. Vibrations at the transmitter site will likely limit this, as tiny shifts in movement only a mm will cause the beam to jump several feet miles away.
Many here have mentioned the speed is on the slow side for this technology. Well, folks, this is optics and that means you can do things in parallel. If you need more speed, just shift the wavelength of the diode and multiplex it in. This is the same principle behind DWDM systems, only it's in freespace. You don't even need a fancy FB diode to do it -- most commercial diodes have a 30 nm linewidth, and by controlling the voltage and temperature you can easily shift up or down. In any case, adding another same-wavelength line is just as easy as adding another transmitter/receiver pair at either end. If only you could do that for fiber. Instead, you have to dig up the streets.
I have had the pleasure of working with a system from Coherent that really makes free space communications shine. The system automatically adjusts and aligns itself via electronic gyros and GPS. It tells you if the current location even has any type of line of sight and if it does it zeos in on the beam (e.g. "I'm pointing S-SW, can you see me?"). The hardest thing is you must have a current connection to the other end while performing the alignment, but this was easily accomidated for at my location with a cellular modem.
This stuff is really cool and there are definate applications for anywhere that has good line of sight. For example, cell towers frequently have good line of sight to one another, so this technology would make sense for that application. There are enough towers that the network could be constructed in a serial or star configuration, without the need for many land lines near the tower.
What would be even cooler would be somehow using the high voltage transmission towers and installing a small, low cost module on each one to jump from tower to tower, or even pole to pole. Since it's optical, you don't have to worry about interference or expensive shielding (yes, there are all-optical transceivers out there).
These are just some ideas but the technology itself finally seems to be maturing. There are lots of current applications and it seems that although most carriers have loads of dark fiber underground, so the cable isn't really the problem, but maybe these companies will help drive bandwidth prices down by enabling small yet very fast ISPs to pop up and use the technology without having to haggle over ground cable. The Internet Revolution per se, isn't going to continue until we all have true broadband (10 Mbps or more, preferably 100 Mbps) to the curb for $19.95 a month.
"I'll just chip in a bit for RedHat: I actually have that installed on my university machine." - Linus, '95
In order to get the laser printer working from their ethernet computer network (both of which they invented) they had to connect from one building to another. The "easiest" way was to use two lasers from the roof of the building. They had to bring it down as in the fog it was kind of distracting to neighbours and aircraft.
An Eye for an Eye will make the whole world blind - Gandhi
Hmmm, a homegrown version of this does sound affordable, read this article ...
http://www.cedmagazine.com/ced/0009/9001.htm
Excerpt...
TeraBeamÂ's Fiberless Optical Network system features a point-to-multipoint (hub-and-spoke) configuration using 1550 nm lasers. At the heart of the technology is a transmitter/receiver that is about the size of a small satellite dish (estimated to cost about $150 to construct) that
Granted a total network of this is gonna cost
a fair amount, but a non-redundant cross
campus link could be semi-affordable .
Ex-misltech
google "32 trillion offshore needs IRS attention"
I'm surprised nobody's brought up the do-it-yourself option:
RONJA
It's been on slashdot a couple of times.
Sure it only uses LEDs but it could use lasers rather easily. It would only up the price, and possibly increase the bandwidth.
If Mr. Edison had thought smarter he wouldn't sweat as much. --Nikola Tesla
You might want to look at London's Sohonet who have been running line-of-sight lasers around London for ages. Of course, they know what they're doing with regard to optics, weather, range, and sunlight.
They also use fibre and SDSL links, which are cheaper in most cases - they have only used lasers in the special cases where it's cheaper than fibre.
Didn't the french have a problem with this in Paris with La Grande Arche de la Defense? They couldn't figure out why their laser, from one side of the building to the other, would lose connectivity at random times durnig the day. Turns out the pigeons were flying through the beam and interrupting service. I guess the only way around that would be to build a massive network of glass tubes all over the city.