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Optical/IP

Optical Crossconnects

Before reading this you may find the following tutorials useful:
Optical Networks, Wavelength Division Multiplexing (WDM)

Huge amounts of information traveling around an optical network need to be switched through various points known as “nodes.” Information arriving at a node will be forwarded on towards its final destination via the best possible path, which may be determined by such factors as distance, cost, and the reliability of specific routes. The conventional way to switch the information is to detect the light from the input optical fibers, convert it to an electrical signal, and then convert that back to a laser light signal, which is then sent down the fiber you want the information to go back out on.

It all sounds unnecessarily complicated, don't you think? It's as if to give money to a friend, you would have to convert it into Euros, then back into your own currency before your friend can use it. Now, even though your information is safer in electrical form than your money may be in Euros, it still seems a strange way to work. What if we could just move the light itself around, without all this ridiculous conversion to electric signals? Well, my friend, what you need is an optical crossconnect (OXC).

The advantages of being able to avoid the conversion stage are significant. “Optical switching” should be cheaper, as there is no need for lots of expensive high-speed electronics. Removing this complexity should also make for physically smaller switches. Additionally, optical switches are relatively future-proof. An electrically based switch will have electronics designed to detect the incoming light signal. If you increase the speed at which the light signals operate (increasing the “bit-rate”) then the electronics will need to be upgraded to handle the faster speeds. If you are just rerouting light, however, it doesn't matter how fast the data is coming — so you can accommodate any future upgrades of bit-rate without needing to upgrade the switch (this is called “bit-rate transparency”). Optical crossconnects are just now coming onto the market with these benefits and more.

Optical crossconnects are very much designed with simplicity in mind. You've got some light in one fiber that you want to move to a different fiber, so just redirect the light somehow and that's all you need — it's child's play. Unfortunately, the technologies used seem to come out of science fiction rather than a child's bedtime story. There is a wide range of wild and wacky ways to switch light between optical fibers. Semiconductor amplifiers, liquid crystals, holographic crystals, and tiny moveable mirrors are just a few. Truly buttock-clenching switching developments are anticipated in the future. One of the most common techniques being developed is that of the tiny moveable mirrors known as micro-electro-mechanical systems (MEMS).

MEMS Mirror MEMS Optical Cross Connect MEMS consist of mirrors no larger in diameter than a human hair. They can be arranged on special pivots so that they can be moved in three dimensions, and several hundred such mirrors can be placed together on mirror arrays no larger than a few centimeters square. Light from an input fiber is aimed at a mirror, which is directed to move the light to another mirror on a facing array. This mirror then reflects the light down towards the desired output optical fiber. It perhaps sounds a little bizarre, but it does seem to work. MEMS mirror arrays are even used successfully in some of the modern digital projectors used for computer-based presentations.

Key Points

  • Crossconnects forward signals to their destination by specific routes
  • Traditional crossconnects convert light to electricity then back to light
  • Optical crossconnect advantages include cost, size, and bit-rate transparency
  • Redirecting light from one optical fiber to another, without electrical conversion
  • Most advanced optical switching technology is MEMS, tiny moveable mirrors


Further Reading

Fiber Bragg Gratings (FBGs), Arrayed Waveguide Gratings (AWGs), Semiconductor Optical Amplifiers (SOAs)

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dansteph 12/4/2012 | 7:31:42 PM
re: Optical Crossconnects If I'm right, is this company claiming to be able to give OOO from a DWDM link to dropping it off to legacy equipment. There is already a very well taken product on the market that already has all the capabilities except for the OOO to the drop offs, it still requires a little OEO at this point. Could someone explain how they take the info off the DWDM and drop it off to the legacy equipment without some form of electrical conversion? I'm definately not saying it's not possible, I would just like to know how it's achieved.

An all OOO cross connect is viable, as long as there is no info to be dropped off and it's just directing the info. At the end of the day, the structure of signals is still apparent in the DWDM link and therefore holds the required info of where it's supposed to go. How it gets there, the info doesn't care. After all DWDM doesn't care what it takes as long as the equipment interfacing the traffic type (ie the point at which traffic joins the link or leaves it)has the functionality.
manoflalambda 12/4/2012 | 7:37:43 PM
re: Optical Crossconnects Can?t speak to all MEMS switches, but ones I am aware of have no such limitation.

One vendor quotes in data sheets three ?bands? or windows. The data sheets are available on companies web site.
1570 to 1610 nm
1530 to 1570 nm
1290 to 1330 nm
If a DWDM signal was wider than one of the windows, perhaps you would have to do something before you switch, but I don?t think that is the case.


As Lambdaswitching says, and from I have read, the limitations in range are not the mirrors, which work for 13xx, 15xx and L band, but the components married around them. Hence the mirrors can handle DWDM streams at any currently use wavelength.

Certainly any opaque (OEO) ports would be wavelength dependent. The transparent (OOO) ports might be limited by characteristics of fiber built into the system and/or monitoring devices.

Salute,
Manoflalambda
lambdaswitching 12/4/2012 | 7:37:45 PM
re: Optical Crossconnects gladysnight saidGǥ

My understanding is that it may be even more restrictive than that. I've read in some press release or other that you have to have the dwdm manage the demux because the mirrors have small angle of reflection (not sure of specific terminology?) which cannot deal effectively with (for example) 100 different wavelengths simultaneouslyGǥ

CanGt speak to all MEMS switches, but ones I am aware of have no such limitation.

One vendor quotes in data sheets three GǣbandsGǥ or windows. The data sheets are available on companies web site.

1570 to 1610 nm

1530 to 1570 nm

1290 to 1330 nm

If a DWDM signal was wider than one of the windows, perhaps you would have to do something before you switch, but I donGt think that is the case.
gladysnight 12/4/2012 | 7:37:46 PM
re: Optical Crossconnects ".....
It's all done optically in the above example, while today you need to put one DWDM box in front of every fiber coming to an OOO OXC (Unless you don't plan to extract any lambads from any fiber at that particular OXC)." -Belzebutt
----------------------------

My understanding is that it may be even more restrictive than that. I've read in some press release or other that you have to have the dwdm manage the demux because the mirrors have small angle of reflection (not sure of specific terminology?) which cannot deal effectively with (for example) 100 different wavelengths simultaneously.

So that the OOO is ONLY a lambda switch - i.e. it is NOT a really expensive automated fibre patch panel that switches a whole fibres worth of traffic irrespective of its lambda content.

Maybe manoflalambda can discuss? . . . . .
MKTG_Hack 12/4/2012 | 7:37:47 PM
re: Optical Crossconnects Hey Mono:
This still isn't making sense. Is what you are talking about similar to an SS7 network? And if an OXC is storing preselected maps, that sounds like curcuits to me. If either of these are true then I don't get it. Where's the whoop? It might make sense if the OXC is like an old railroad tack switch that simply helps the train get on a specific track without caring where that train might want to go. Give us some more details PLEASE!
laserman 12/4/2012 | 8:14:21 PM
re: Optical Crossconnects I heard that there is a new company making optical cross connect under a $50/port. Is it true?. Sorry for my ignorance, but I am not very familiar with this subject. Can some one tell me which company is it?. Thanks

Laserman
manoflalambda 12/4/2012 | 8:14:45 PM
re: Optical Crossconnects Do you mean move the mirrors? Databases of mirror positioning info and D/A converters/Op-Amps and the like.

Salute,
Manoflalambda
nightRaider 12/4/2012 | 8:14:47 PM
re: Optical Crossconnects . . . "Most advanced optical switching technology is MEMS, tiny moveable mirrors "

But what is used to switch mems. DSPS?





manoflalambda 12/4/2012 | 8:25:56 PM
re: Optical Crossconnects http://www.lucent.com/press/05...

Global Crossing bought 1 or more LambdaRouters
from Lucent.

Salute,
Manoflalambda
shun_y 12/4/2012 | 8:27:02 PM
re: Optical Crossconnects "Optical transistors"--

Having an optical device that has some transistor-like bahavior is one thing, packing godzillions of transistors and gates into one chip is another. I didn't see their website because it was not working, but one thing I am sure of is that we won't see anything like the velio switch chip in the optical domain anytime soon.

Same story with optical memory, storing a few kilo bits in some bulk optical meterial is one thing, storing a few giga bits in a chip is totally different.

If they claim their 'optical transistor' will revolutionize the industry, be very skeptical.

-Shun
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