Study of Non-Linear Effects in Optical Fiber Communication


Kerr and scattering effects are the nonlinear effects present in the Optical fiber communication system. In nonlinear effects, an optical signal gets reduced in power strength. While it gets reduced in strength, it gets spread while propagating through the optical fibers. During high optical intensities, it becomes much stronger. But, they are very weak at low power. This is the definition of nonlinear effects.

The various nonlinear effects present in the optical fiber are Kerr effects and Scattering effects. There are two types of scattering effects. They have Stimulated Raman scattering and Stimulated Brillouin scattering effects. And, there are three types of Kerr effects. They are Self-phase modulation, Cross-phase modulation, and Four-wave mixing.

Nonlinear effects in the Optical fiber communication link

Scattering effects- 
In scattering effects, an optical wave may be scattered by exciting molecular vibrations at a shifted frequency. Therefore, the scattered photon emerges above or below the incident photon frequency with the difference between the two photons. These are deposited and later extracted from the scattering medium.

There are two terms for the scattered wave. The formerly scattered wave is called the Stokes component and the other scattered wave is called as an anti-stokes component. The main important outcome of this scattered wave is that it provides optical gain at the shifted frequency. Also, the incident optical frequency is called the pump frequency. It hence gives stokes and anti-stokes components of the scattered radiation.

Kerr effects-
Kerr nonlinear effects can be defined by the intensity-dependent refractive index of the optical fiber. The various processes produced by the Kerr effects are mentioned above. The are SPM (Self-phase modulation), CPM (Cross-phase modulation) and FWM (Four-wave mixing). In Self-phase modulation, the optical light results in a different transmission phase for the peak of the pulse. It is then compared with the leading and trailing pulse edges.

It causes modulation to the pulse spectrum. Similarly, in cross-phase modulation, phase modulation of the overlapping pulses is caused. It is due to the reason that variations in the intensity of one pulse will modulate index of the fiber.

And, the Four-wave mixing is a weak effect when the multichannel signals remain in phase with each other over long transmission distances. In the four-wave mixing, the fiber chromatic dispersion is very close to zero.

Types of Attenuation on Optical fiber

The various types of attenuation caused by the optical fiber are Material absorption loss and Material scattering loss. The material absorption loss is of two types. They are intrinsic and extrinsic. And, the factors responsible for the material scattering loss are Compositional variation, density variation, and structural variation.

Optical power budget on the fiber communication link

It is generally found in the fiber communication link where it is usually specified in decibels. It is usually the allocation of available optical power among different loss producing mechanisms. Here, the loss-producing mechanisms are coupling loss, fiber attenuation loss, connector loss, etc. The optical power budget is allocated to ensure that enough signal strength is available at the receiver.

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