Losses in Optical Fiber: Attenuation | Dispersion | Tube Buffering


What is the Attenuation? How many types of Dispersion? How many types of Fiber Buffering Loose? What is the Tube Buffering?

ATTENUATION : loss of optical power

Attenuation is defined as the loss of optical power over a set distance, a fiber with lower attenuation will allow more power to reach a receiver than fiber with higher attenuation.

Attenuation may be categorized as intrinsic or extrinsic.

INTRINSIC ATTENUATION

It is loss due to inherent or within the fiber. Intrinsic attenuation may occur as

(i) Absorption – Natural Impurities in the glass absorb light energy.

(II) Scattering – Light rays travelling in the core reflect from small imperfections into a new pathway that may be lost through the cladding.

  1. Absorption – Natural Impurities in the Glass Absorb Light Energy.

 

Absorption Scattering

Or

(2) Scattering – Light Rays Travelling in the Core Reflect from small Imperfections into a New Pathway that may be Lost through the cladding.

EXTRINSIC ATTENUATION

It is loss due to external sources. Extrinsic attenuation may occur as –

  • Macrobending – The fiber is sharply bent so that the light travelling down the fiber cannot make the turn & is lost in the cladding.

Extrinsic attenuation

(II) Microbending – Microbending or small bends in the fiber caused by crushing contraction etc. These bends may not be visible with the naked eye.

Attenuation is measured in decibels (dB). A dB represents the comparison between the transmitted and received power in a system.

DISPERSION

It is defined as the spreading of light pulse as it travels down the fiber. ecause of the spreading effect, pulses tend to overlap, making them unreadable by the receiver.

 

BANDWIDTH

It is defined as the amount of information that a system can carry such that each pulse of light is distinguishable by the receiver.

System bandwidth is measured in MHz or GHz. In general, when we say that a system has bandwidth of 20 MHz, means that 20 million pulses of light per second will travel down the fiber and each will be distinguishable by the receiver.

NUMBERICAL APERTURE

Numerical aperture (NA) is the “light – gathering ability” of a fiber. Light injected into the fiber at angles greater than the critical angle will be propagated. The material NA relates to the refractive indices of the core and cladding.

NA  =  n12 – n22

where n1 and n2 are refractive indices of core and cladding respectively.

NA is unitless dimension. We can also define as the angles at which rays will be propagated by the fiber. These angles form a cone called the acceptance cone, which gives the maximum angle of light acceptance. The acceptance cone is related to the NA

Æ  = arc sing (NA) or

NA = sin Æ

where Æ  is the half angle of acceptance.

The NA of a fiber is important because it gives an indication of how the fiber accepts and propagates light. A fiber with a large NA accepts light well, a fiber with a low NA requires highly directional light.

In general, fibers with a high bandwidth have a lower NA. They thus allow fewer modes means less dispersion and hence greater bandwidth. A large NA promotes more modal dispersion, since more paths for the rays are provided NA, although it can be defined for a single mode fiber, is essentially meaningless as a practical

Single mode multimode

characteristic. NA in a multimode fiber is important to system performance and to calculate anticipated performance.

Total Internal Reflection

(Summary)

* Light Ray A : Did not Enter Acceptance Cone – Lost

* Light Ray B : Entered Acceptance Cone – Transmitted through the Core by Total Internal Reflection.

NA = 0.275 (For 62.5 mm Core Fiber)

DISPERSION : Types of dispersion in a fiber

Dispersion is the spreading of light pulse as its travels down the length of an optical fiber. Dispersion limits the bandwidth or information carrying capacity of a fiber. The bit-rates must be low enough to ensure that pulses are farther apart and therefore the greater dispersion can be tolerated.

There are three main types of dispersion in a fiber –

(I) Modal Dispersion

(II) Material dispersion

(III) Waveguide dispersion

MODAL DISPERSION

Modal dispersion occurs only in Multimode fibers. It arises because rays follow different paths through the fiber and consequently arrive at the other end of the fiber at different times. Mode is a mathematical and physical concept describing the propagation of electromagnetic waves through media. In case of fiber, a mode is simply a path that a light ray can follow in travelling down a fiber. The number of modes supported by a fiber ranges from 1 to over 100,000. Thus a fiber provides a path of travels for one or thousands of light rays depending on its size and properties. Since light reflects at different angles for different paths (or modes), the path lengths of different modes are different. Thus different rays take a shorter or longer time to travel the length of the fiber. The ray that goes straight down the center of the core without reflecting, arrives at the other end first, other rays arrive later. Thus light entering the fiber at the same time exist the other end at different times. The light has spread out in time.

The spreading of light is called modal dispersion. Modal dispersion is that type of dispersion that results from the varying modal path lengths in the fiber. Typical modal dispersion figures for the step index fiber are 15 to 30 ns/ km. This means that for light entering a fiber at the same time, the ray following the longest path will arrive at the other end of a 1 km long fiber 15 to 30 ns after the ray, following the shortest path. Fifteen to 30 billionths of a second may not seem like much, but dispersion is the main limiting factor on a fiber’s bandwidth. Pulse spreading results in a pulse overlapping adjacent pulses as shown in figure. Eventually, the pulses will merge so that one pulse cannot be distinguished from another. The information contained in the pulse is lost Reducing dispersion increases fiber bandwidth.

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