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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 traveling 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.

 

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 lost due to external sources. Extrinsic attenuation may occur as –

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

 

(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.

NUMERICAL 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 a unitless dimension. We can also define 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 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 single-mode fiber, is essentially meaningless as a practical,

 

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.

 

Model dispersion can be reduced in three ways :

(I) Use a smaller core diameter, which allows fewer modes.

(II) Use a graded -index fiber so that light rays that allow longer paths also travel at a faster velocity and thereby arrive at the other end of the fiber at nearly the same time as rays that follow shorter paths.

(III) Use a single-mode fiber, which permits no modal dispersion.

MATERIAL DISPERSION

Different wavelengths (colours) also travel at different velocities through a fiber, even in the same mode, as

 

n = c/v

where n is index of refraction, c is the speed of light in vacuum and v is the speed of the same wavelength in the material. The value of V in the equation changes for each wavelength, Thus Index of refraction changes according to the wavelength. Dispersion from this phenomenon is called material dispersion, since it arises from material properties of the fiber.

Each wave changes speed differently, each is refracted differently. White light entering the prism contains all colours. The prism refracts the light and its changes speed as it enters the prism. Red light deviates the least and travels the fastest. The violet light deviates the most and travels the slowest.

The amount of material dispersion depends on two factors :

(I) The range of light wavelengths injected into the fiber. A source does not normally emit a single wavelength, it emits several. This range of wavelengths, expressed in nanometer is the spectral width of the source. An LED has a much higher spectral width than a LASER – about 35 nm for a LED and 2 to 3 nm for a LASER.

(II) The centre operating wavelength of the sources

Around 850nm, longer (reddish) wavelengths travel faster than the shorter (Bluish) ones. At 1550nm however the situation is reversed. The shorter wavelengths travel faster than the longer ones. At some point, the cross over must occur where the bluish and reddish wavelengths travel at the same speed. This crossover occurs around 1300nm, the zero-dispersion wavelength. At wavelengths below 1300nm, dispersion is negative. So wavelengths travel or arrive later. Above 1300 nm, the wavelengths lead or arrive faster.

This dispersion is expressed in Pico seconds per kilometer per nanometer of source spectral width (ps/km/nm).

WAVEGUIDE DISPERSION :

Waveguide dispersion, most significant in a single- mode fiber, occurs because optical energy travels in both the core and cladding, which have slightly different refractive indices. The energy travels at slightly different velocities in the core and cladding because of the slightly different refractive indices of the materials. Altering the internal structures of the fiber, allows waveguide dispersion to be substantially changed, thus changing the specified overall dispersion of the fiber.

BANDWIDTH AND DISPERSION :

A bandwidth of 400 MHz -km means that a 400 MHz-signal can be transmitted for 1 km. It means that the product of frequency and the length must be 400 or less. We can send a lower frequency for a longer distance, i.e. 200 MHz for 2 km or 100 MHz for 4 km.

Multimode fibers are specified by the bandwidth-length product or simply bandwidth.

Single mode fibers on the other hand are specified by dispersion, expressed in ps/km/nm. In other words for any given single mode fiber dispersion is most affected by the source’s spectral width. The wider the source spectral width, the greater the dispersion.

Conversion of dispersion to bandwidth can be approximated roughly by the following equation.

0.187

BW      =   0.187/ (Disp) (SW) (L)

Disp = Dispersion at the operating wavelength in seconds/ nm/ km.

SW = Spectral width of the source in nm.

L  =  Fiber length in km.

So the spectral width of the source has a significant effect on the performance of a single mode fiber.

OPTICAL WINDOWS :

Attenuation of fiber for optical power varies with the wavelengths of light. Windows are low-loss regions, where fiber  carry light with little attenuation. The first generation of optical fiber operated in the first window around 820 to 850 nm. The second window is the zero-dispersion region of 1300 nm and the third window is the 1550 nm region.

High loss regions, where attenuation is very high occur at 730, 950, 1250 and 1380 nm. One wishes to avoid operating in these regions. Evaluation of losses in a fiber must be done with respect to the transmitted wavelength.

Figure shows a typical attenuation curve for a low loss multimode fiber.

Making the best use of the low loss properties of the fiber requires that the sources emit light in the low loss region of the fiber. Plastic fibers are best operated in the visible light area around 650 nm. One important feature of attenuation in an optical fiber is that the constant at all modulation frequencies within the bandwidth.

 

Attenuation in a fiber has two main causes.

(I) Scattering

(II) Absorption

We can obtain losses less  than 2.5 dB/km in the first window at 850 nm. Graded index fibers in the second window with loss below 1 dB/km and in the thrid window below 0.5 dB/km are obtained. Even lower losses are regarded as feasible for monomode fibers in all the three windows. Typically minimum loss in the three windows for the multimode fiber is 2.5 dB/km, 0.44 dB, km and 0.22 dB/km respectively. The corresponding figures for a monomode fiber are 1.9 dB/km, 0.32 dB/km and 0.048 dB/km.

CABLE CONSTRUCTION

Cabling is an outer protective structure surrounding one or more fibers. Cabling protects fibers environmentally and mechanically from being damaged or degraded in performance. Important consideration in any cable are tensile strength, ruggedness, durability, flexibility, environmental resistance, temperature extremes and even appearance. Evaluation of these considerations depends on the application.

Fiber Optic Cables have the following parts in common ;

(I) Optical Fiber

(II) Buffer

(III) Strength member

(IV) Jacket

Cable Components

Component	Function	Material
Buffer	Protect fiber From Outside	Nylon, Mylar, Plastic
Central Member	Facilitate Stranding Temperature Stability Anti-Buckling	Steel, Fiberglass
Primary Strength Member	Tensile Strength	Aramid Yarn, Steel
Cable Jacket	Contain and Protect Cable Core Abrasion Resistance	PE, PUR, PVC, Teflon
Cable Filling Compound	Prevent Moisture intrusion and Migration	Water Blocking Compound
Armoring	Rodent Protection Crush Resistance	Steel Tape

 

Loose Tube Buffering

One way of isolating the Optical Fiber from External Forces is to Place an Excess Fiber Length within on Oversized “Buffer” Tube.

Siecor/ Optical Cable fills these tubes with a Jollylike Compound to Provide Additional Cushioning and Prevent the incursion of Moisture.

 

NOTE : Additional Excess Length is Achieved when the “Buffered” Fibers are Stranded together during the Cabling Operation.

It is the plastic coating applied to the coating. It protects fiber from outside stress. The cable buffer is one of two types.

(I) Loose Buffer

(II) Tight Buffer

The loose buffer uses a hard plastic tube having an inside diameter several times that of the fiber. One or more fibers lie within the buffer tube. As the cable expands and shrinks with temperature changes, it does not affect the fiber as much. The fiber in the tube is slightly longer than the tube itself. Thus the cable can expand and contract without stressing the fiber. The buffer becomes the load-bearing member.

The tight buffer has a plastic directly applied over the coating. This construction provides crush and impact resistance. It is more flexible and allows tighter turn radius. It is useful for indoor applications where temperature variations are minimum and the ability to make tight turns inside walls is desired.

 

Types of Fiber Buffering

 

Strength member :

Strength members add mechanical strength to the fiber. During and after installation, the strength members handle the tensile stresses applied to the cable so that the fiber is not damaged. The most common strength members are Kevlar, Armid Yarn, Steel and Fiber glass epoxy rods.

Kevlar is most commonly used when individual fibers are placed within their own jackets. Steel and fiber glass members find use  in multifiber cable. Steel offers better strength than fiberglass but in some cases it is undesirable when one wishes to maintain an all-dielectrical cables. Steel attracts lightening whereas fiberglass does not.

Jacket

It provides protection from the effects of abrasion, oil, ozone, acids, alkali, solvents and so forth. The choice of jacket material depends on degree of resistance required for different influences and on cost.

The outer layers are often called the sheath. The jacket becomes the layer directly protecting fibers and the sheath refers to additional layer.

MULTIFIBER CABLE :

It often contain several loose buffer tubes, each containing one or more fibers. The use of several tubes allows identification of fiber by tube, since both tubes and fibers can be colour coded. These tubes are stranded around a central strength member of steel or fiber glass rod. The stranding provides strain relief for the fibers when the cable is bent.

Typical Mini-Bundle Cable

 

Description

1    –  Blue

2    –  Orange

3    –  Green

4    –  Brown

5    –  Slate

6    –  White

7    –  Red

8    –  Black

9    – Yellow

10  –  Violet

11  –  Blue/ Black

12  –  Orange/ Black