Optical Networks - _2_5 Nonlinear Effects_27

Optical Networks - _2_5 Nonlinear Effects_27 - 78...

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78 Propagation of Sign als in Optical Fiber standard single-mode fiber but has a higher refractive index. This leads to a large negative chromatic dispersion. This core is surrounded by a ring of lower refractive index, which is in turn surrounded by a ring of higher refractive index. Such a variation leads to a negative chromatic dispersion slope, an important characteristic for chromatic dispersion compensation, as we will see in Section 5.7.3. 2.5 Nonlinear Effects Our description of optical communication systems under the linearity assumption we made in Section 2.3.1 is adequate to understand the behavior of these systems when they are operated at moderate power (a few milliwatts) and at bit rates up to about 2.5 Gb/s. However, at higher bit rates such as 10 Gb/s and above and/or at higher transmitted powers, it is important to consider the effect of nonlinearities. In the case of WDM systems, nonlinear effects can become important even at moderate powers and bit rates. There are two categories of nonlinear effects. The first arises due to the interaction of light waves with phonons (molecular vibrations) in the silica medium—one of several types of scattering effects, of which we have already met one, namely, Rayleigh scattering (Section 2.1). The two main effects in this category are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). The second set of nonlinear effects arises due to the dependence of the refractive index on the intensity of the applied electric field, which in turn is proportional to the square of the field amplitude. The most important nonlinear effects in this category are self-phase modulation (SPM) and four-wave mixing (FWM). In scattering effects, energy gets transferred from one light wave to another wave at a longer wavelength (or lower energy). The lost energy is absorbed by the molecular vibrations, or phonons, in the medium. (The type of phonon involved is different for SBS and SRS.) This second wave is called the Stokes wave. The first wave can be thought of as being a “pump” wave that causes amplification of the Stokes wave. As the pump propagates in the fiber, it loses power and the Stokes wave gains power. In the case of SBS, the pump wave is the signal wave, and the Stokes wave is the unwanted wave that is generated due to the scattering process. In the case of SRS, the pump wave is a high-power wave, and the Stokes wave is the signal wave that gets amplified at the expense of the pump wave. In general, scattering effects are characterized by a gain coefficient g , measured in meters per watt, and spectral width ±f over which the gain is present. The gain coefficient is a measure of the strength of the nonlinear effect. In the case of self-phase modulation, the transmitted pulses undergo chirping.
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Institute of Technology.

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Optical Networks - _2_5 Nonlinear Effects_27 - 78...

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