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Thread: Lasers in communication fiber

  1. #1
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    Default Lasers in communication fiber

    Not really laser display... but has anyone heard anything about the use of multi color, muli phase and multi angle lasers into fiber optics for expanding communications bandwidth?

    I don't know if this is true, but supposedly, if you shoot a laser into a fiber at an angle, it will come out the other end at the complement of that angle. I guess it can be done in multiple axis and with beams of different polarization and color, so many, many different beams can be sent into one fiber and easily separated at the other end by color, phase and angle. Is this true?

    James.

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    Quote Originally Posted by James Lehman View Post
    Not really laser display... but has anyone heard anything about the use of multi color, muli phase and multi angle lasers into fiber optics for expanding communications bandwidth?

    I don't know if this is true, but supposedly, if you shoot a laser into a fiber at an angle, it will come out the other end at the complement of that angle. I guess it can be done in multiple axis and with beams of different polarization and color, so many, many different beams can be sent into one fiber and easily separated at the other end by color, phase and angle. Is this true?

    James.
    You need to send a PM to 'Wiring Monkey' on this forum. Allegedly he know a little bit about communications and optical fibers

    Jem
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    James

    What your refering to there is WDM - Wavelength Division Multiplexing

    Basically a way of using the varying wavelength of diferent colors to aggrigate the bandwidth you can pass across a single fibre

    http://en.wikipedia.org/wiki/Wavelen...n_multiplexing
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    In simple terms, a service provider can have multiple customers sharing a single fibre optic.. Each customer occupies a different wavelength...

    I work for one of the largest service providers and train engineers on this stuff (Though more on the protocols) as far as I am aware we use invisible lasers and at a relatively low power...

    Obviously being in both the provider and laser business, I have tried to see what I can get my hands on.. Unfortunatly nothing good has surfaced yet

    Mark

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    A guy was here the other day from ATT to fix up our new and faulty DSL connection. He was a techy-geek, so he immediately zeroed in on my laser rig. He said there are two kinds of fiber; solid and hollow and if you shoot a laser at a polished end of a fiber at an angle; like 20 degrees off, it will come out the other end 20 degrees off. Supposedly, the beam bounces like zig-zags through the length of the fiber and that it can go like that, thousands of miles, all the way across the ocean floor. Is that for real?

    James.

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    The phenomenon is called total internal reflection. As long as the incident beam angle is smaller than the critical angle the light will be reflected within the fiber. The critical angle depends on the refractive index of the fiber and the surrounding material.

    Unfortunately it's not possible send a signal through "thousands of miles" of fiber without amplifying it. Typically in submarine communications cables there is a repeater containing a (980nm) laser pumping a short section of erbium doped fiber every few 100km or so.

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    I suppose I should pipe up seeing as Jem thinks I know something :-)

    I've not heard of multi phase or multi angle but multi colour yes, although as Loopee say's it's called WDM.

    Basically you take a flashing light, change it's wavelength and squirt it down a fibre with a bunch of other ones. at the other end you split them up and decode the flashing light.

    Typically each channel will carry up to 10Gb/S and I know of systems capable of carrying 128 channels !!!!

    They operate around 1500 - 1550nM with a spacing of 0.77nM which equates to 100Ghz

    For longer reach systems the combined signal can be amplified with a EDFA or Erbium Doped Fibre Amplifer which from my understanding are powered by black magic :-)

    Craig

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    no matter what angle you send it in at...it will always exit out scrambled within the numerical aperture of the fiber. Excluding single mode fibers...different wavelengths of light no matter how they were put into the fiber will exit homogenized into a blob of whitelight.

    that is a basic answer for the OP. Variables can be expanded upon.
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    Multiplexing is nothing new. It allows for extremely intricate communications between multiple users. Back in the day it was called trunking but with the rise of digital communications the ability to multiplex via time division, frequency division, code division, wavelength division and using mathematics to encode onto those multiplexing schemes reduced bit symbology for larger bit-words and the increase in electronic sensitivity (i.e. ability to recognize smaller and smaller channels) allows for massive increases in data throughput. Fiber is only the physical transport means (layer 1) and is limited by physical response of the systems supporting layer 1. Electronic systems can only process so fast. This is much slower than the theoretical bit rate available in fiber optic cable. This is why there is so much interest in quantum processing to overcome many of the physical limitations. As channels become narrower and narrower noise becomes a huge problem in distinguishing signaling between the channels. Regenerative repeaters (both electronic and optical) do create some delays but the systems cannot operate without them. There will only be small, incremental increases in throughput from refining processing at the normal communcations layers up to layer four. At this point increasing bandwidth is a matter of increasing the number of fibers in the ground. I remember, again back in the day, thinking we were styling because we had a few OC48s running through our building. When I left the communciations business having twenty OC192s was no big deal. More and more fiber is the current solution. Adding more wavelengths within a fiber only works to a degree. Inherent noise tends to blur the demarcation between the signaling in a fiber even with regenerative repeaters (fiber lasers) in the mix. As such we run back into the problem of electronics being able to distinguish between the signals. There are many factors that determine this but we used to calculate capacity on almost all components using Erlang calcuations which are probabilistic/statistical methods.
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  10. #10
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    lol... i just noticed I responded to an old thread from someone who was clue by foured...


    Sorry folks...carry on
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