non linearity in optical fibres

Discussion in 'General Electronics Chat' started by u-will-neva-no, Jun 22, 2012.

  1. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
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    Hey everyone!

    I have been reading more on using optical fibres for communication systems and have come across two analytical expressions to model non linearity in the fibre (Poggiolini and chen equations). I was hoping that someone could explain why non-linearity is an issue. Thanks!
     
  2. steveb

    Senior Member

    Jul 3, 2008
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    Can you provide background on these equations? I'm not familiar with these names and what effects their equations deal with. There are many nonlinear effects in optical fiber including stimulated Raman scattering, Raman amplification, soliton propagation, self-phase modulation, cross-phase modulation, stimulated Brillouin scattering, four wave mixing and others.

    In general, non-linearity is an issue because optical fiber confines light to such a small area that even relatively low power/energy levels result in very high optical intensity and triggering of nonlinear effects that are hard to see in bulk media. Also, long distance high speed communications systems are sensitive to even small effects. Generally, nonlinearity degrades communications system performance and creates cross-talk between channels, but sometimes it can be used to provide benefits.
     
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  3. u-will-neva-no

    Thread Starter Member

    Mar 22, 2011
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    Could you explain what benefits please?

    The poggiolini and Chen equations were from two papers on the IEEE. Search for "Information spectral efficiency and launch power density limits due to fiber nonlinearity for coherent optical OFDM systems" and "Analytical modeling of nonlinear propagation in uncompensated optical transmission links"

    They basically are two similar analytical expressions to model the non linear power (That is my understanding anyway). Thanks for the reply :)
     
  4. steveb

    Senior Member

    Jul 3, 2008
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    Well, I don't have direct access to IEEE published papers and have to pay for them to see them. Hence, I won't comment in detail unless you want to post some detailed questions here.

    As far as benefits, the best example I can think of is soliton propagation. Solitons are special nonlinear pulses that travel without dispersion. The high power of the light pulse actually modulates the refractive index and allows trailing parts of the pulse to speed up and leading parts of the pulse to slow down. Hence, the pulses act a little bit like particles and can be used for very long distance communications.

    Since fiber has loss, these solitons would eventually decay to the point where they no longer have enough intensity to generate the nonlinear effect. So, another nonlinear effect called Raman amplification is used to amplify the soliton pulse signal. This requires using a high power laser with shorter wavelength than the signal. The Raman effect allows the pump laser energy to be transferred over to the soliton signal. Hence the soliton communications allows elimination of dispersion and loss and is a very good way to communicate over ultra-long distance. Of course, the engineering of such systems is very far from trivial.
     
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  5. Wendy

    Moderator

    Mar 24, 2008
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    There is a factor called power dispersion. If the laser is not pure it will have sidebands. The speed of light varies in a fiber optic, similar to how a prism works. The lower frequency sidebands will trail behind, while the higher frequency sideband move ahead. Eventually the slower and faster pulses overlap, pretty much eliminating the digital modulation you want to keep.

    The answer has been purer lasers, with fewer sidebands. A benefit to this approach is you can put lasers that are different colors (just a little) in the same fiber, increasing the bandwidth dramatically. Last I heard you can fit 80 channels into a single FO, each channel 40 gigabit or better.

    Erbium Doped amps are lasers that use the amplification part of the LASER acronym. Most LASERs people see are oscillators. With the amplifier the signal goes in at -30db, and comes out +10db, to the point where LASER safety glasses are required.
     
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  6. steveb

    Senior Member

    Jul 3, 2008
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    This is all true and good information Bill. However, it's important to point out that dispersion of this type is normally considered a linear process.

    However, there can be interactions that yield nonlinear behavior. Years ago I was involved in doing analog communications (cable TV application) with optical fibers and erbium doped amplifiers. When you modulate a laser, often the frequency of the laser will change with the applied modulating current. This is called frequency "chirp". A chirping laser can interact with the fiber dispersion to create a nonlinear arrival time for the signal at the receiver. For analog systems, this is a big problem since the analog signal is distorted nonlinearly (harmonic distortion results).
     
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