Fiber Optic Wiki

Intrinsic Attenuation

September 5, 2011

Intrinsic attenuation results from materials inherent to the fiber. It is caused by impurities in the glass during the manufacturing process. As precise as manufacturing is, there is no way to eliminate all impurities. When a light signal hits an impurity in the fiber, one of two things occurs: It scatters or it is absorbed. Intrinsic loss can be further characterized by two components:Material absorption.Rayleigh scattering

Chromatic Dispersion

September 5, 2011

Chromatic dispersion is the spreading of a light pulse as it travels down a fiber. Light has a dual nature and can be considered from an electromagnetic wave as well as quantum perspective. This enables us to quantify it as waves as well as quantum particles. During the propagation of light, all of its spectral components propagate accordingly. These spectral components travel at different group velocities that lead to dispersion called group velocity dispersion (GVD). Dispersion resulting from GVD is termed chromatic dispersion due to its wavelength dependence. The effect of chromatic dispersion is pulse spread.

Cross-Phase Modulation

September 5, 2011

Cross-phase modulation (XPM) is a nonlinear effect that limits system performance in wavelength-division multiplexed (WDM) systems. XPM is the phase modulation of a signal caused by an adjacent signal within the same fiber. XPM is related to the combination (dispersion/effective area). CPM results from the different carrier frequencies of independent channels, including the associated phase shifts on one another. The induced phase shift is due to the walkover effect, whereby two pulses at different bit rates or with different group velocities walk across each other. As a result, the slower pulse sees the walkover and induces a phase shift. The total phase shift depends on the net power of all the channels and on the bit output of the channels. Maximum phase shift is produced when bits belonging to high-powered adjacent channels walk across each other.

Graded-Index Multimode Fibers Solves the Problem of Modal Dispersion

September 5, 2011

Graded-index fiber’s refractive index decreases gradually away from its center, finally dropping to the same value as the cladding at the edge of the core. The change in refractive index causes refraction, instead of total internal reflection, which bends light rays back toward the fiber axis as they pass through layers with lower refractive index. No total internal reflection happens because refraction bends light rays back into the fiber axis before they reach the cladding boundary.

Modal-Dispersion and Limit on Step-Index Multimode Fibers’ Bandwidth

September 5, 2011

Take a look at the illustration for a step-index multimode fiber. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle (numerical aperture). Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than those enter at steeper angles (which reflect many more times off the core/cladding boundaries as they travel the length of the fiber). The arrival of different modes of the light at different times is called Modal Dispersion.

Optical-Power Measurement

September 4, 2011

The power level in optical communications is of too wide a range to express on a linear scale. A logarithmic scale known as decibel (dB) is used to express power in optical communications.

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