Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light. This nonlinearity is typically only observed at very high light intensities such as those provided by pulsed lasers. Nonlinear optics gives rise to a host of optical phenomena.
Non-linear effects are consequence of the interaction of incident photons with the glass material of the fiber at the subatomic level. Hence for a deep understanding of the physics involved in this non-linear effects, we must deal with quantum mechanics. But let us continue further for our basic understanding of this phenomena...
Nonlinearity in an optical fiber is not a design or manufacturing defect. It is an inherent characteristic of light energy passing through a fiber. Therefore nonlinearity is a headache for design engineers of fiber optic based communication systems.
The effects of optical nonlinearity are divided into: Scattering phenomena & Refractive index phenomena.
They can loosely be gouped into atleast four types. The first type is the interaction of incident photons with molecular vibrations known as
stimulated Raman scattering. The second type occurs when photons interact with acoustic photons called as stimulated
Brillouin scattering.
Both scattering mechanisms result in a shift of photon wavelength and are referred to as inelastic scattering, because part of the incident light energy is transferred to the optical fiber material.
The third type is caused by the intensity dependent changes of the refractive index when photons interact with lattice electrons instantaneously. In isotropic SiO
2 glass this effect leads to processes known as 4-photon mixing. This is popularly known as
four wave mixing or FWM and
Self Phase Modulation or SPM. The fourth type involves the trapping of a perturbed electron, so that the refractive index chaange is long lasting. This effect is known as
photorefractive effect.
Non linear effects in optical fibers have gained a significant influence on the operation of long haul optical communication systems. In particular, the use of dense wavelength division multiplexed, DWDM, systems high power light sources, smaller spectral width lasers and small fiber cores lead to power densities which are high enough to exceed the critical threshold level for nonlinear effects like four wave mixing, stimulated Brillouin scattering and self-phase modulation.
The threshold power levels are in the order of 190mW for the SBS and 30mW for the SPM resp. The other nonlinear effects such as stimulated Raman scattering require higher power densities and thus are not as harmful in practical applications. FWM, SBS and SPM can lead to severe signal distortions.
Four-wave mixing can be one of the most damaging nonlinear effects in wavelength division multiplexed communication systems. When the intensity of the laser signal reaches a critical value, ghost channels appear, some of which may fall within the true channels. The result of FWM is that several wavelengths mix to produce new wavelengths.
In the SBS process a significant proportion of the optical power travelling through the fiber may be converted into a second light wave, shifted in frequency, travelling backwards towards the transmitter. SBS can be detrimental in an optical communications system in a number of ways by introducing severe additional signal attenuation, by causing multiple frequency shifts in some cases and by introducing a high intensity backwards coupling into the transmission optics. It will be necessary therefore to operate at power levels below the threshold for SBS, and this could place a severe limitation on the launch power and thus also the repeater spacing.
Are you getting tired? Well let me take a coffee and come back.
As you know, taking coffee is not so good for health, but when we deal with Non-linear effects, things will start appearing non-linear !! OK, back to the class,
Self phase modulation leads to an intensity dependent broadening of the signal spectrum. This greater frequency width increases pulse spreading through group velocity dispersion. The process can be understood as a differential phase shift between the center and the tails of the optical pulse due to the intensity dependent refractive inded. The index change is extremely small but the corresponding phase modulation can be significant when added up in a long fiber.
If SPM and chromatic dispersion are well adjusted to each other (correct light intensity and negative dispersion) pulse narrowing instead of pulse broadening can occur, which can lead to optical solitions.
Solitions are light pulses that are not subject to deformation by dispersion in optical fibers and therefore in principle do not experience any limitation in bit rates over long transmission lengths. Thus they seem to be well suited for telecommunication along transoceanic transmission lines. Solition transmission has been demonstrated successfully in labs upto a fiber lengths of 9000 km.
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Published: July 21, 2008
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