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Fiber Optics

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Fibre Basics Optical fiber is information-carrying

medium made with silica-based glass.

It consists of two regions : the core andthe cladding.

The center part of the fiber is called"core" with refractive index N1 and thepart which surrounds the core with lower

refractive index N2 is called "cladding".

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Fiber Basics contd.., When the light is launched into the fiber, the cladding

confines the lights into the fiber core and the lighttravels down the fiber by internal reflection between the

boundaries between the core and the cladding.

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Fiber Basics contd.., When light is transmitted on a fibre, the most

important consideration is what kind of light? The electromagnetic radiation that we call light

exists at many wavelengths. These wavelengths go from invisible infrared

through all the colors of the visible spectrum toinvisible ultraviolet.

Because of the attenuation characteristics offibre, we are only interested in infrared lightfor communication applications.

This light is usually invisible, since thewavelengths used are usually longer than the

visible limit of around 700 nanometers (nm).

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Fiber Basics contd.., Only some portion of the electromagnetic spectrum is

visible to (can be detected by) the human eye. Electromagnetic radiation in this range of

wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths in

air from 400 to 700 nm In fiber-optic communications, dark fiber or unlit

fiber (or fibre) is the name given to individual fibersthat have yet to be used within cables that have beenalready laid.

They are hence not yet connected to any device, andare only there for future usage.

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Fiber Basics contd.., Fiber size is generally mentioned as

diameter of core followed by diameter of

cladding

50/125m (European standard) for MM

fiber 62.5/125m (North American Standard)

for MM fiber and

9/125m for SM fiber.

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Types of Fibers There are basically two kinds of fibers as

Single mode fiber

Multimode fiber

Again Multimode fiber can be of two types Multimode Step-Index fiber Multimode Graded-Index fiber

The difference between them is in the waylight travels along the fiber.

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Refraction of light As a light ray passes from one transparent medium to

another, it changes direction. This phenomenon is called refraction of light.

How much that light ray changes its direction depends onthe refractive index of the mediums.

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Refractive Index Refractive index is the speed of light in a

vacuum (abbreviated c,c=299,792.458km/second) divided by thespeed of light in a material (abbreviated v).

Refractive index measures how much a

material refracts light. Refractive index of amaterial, abbreviated as n, is defined as

n=c/v

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Snells Law In 1621, a Dutch physicist named Snell derived the relationship

between the different angles of light as it passes from onetransparent medium to another. When light passes from onetransparent material to another, it bends according to Snell's law

which is defined as:

n1sin(1) = n2sin(2)where:n1 is the refractive index of the medium the light is leaving1 is the incident angle between the light beam and the normal

(normal is 90 to the interface between two materials)n2 is the refractive index of the material the light is entering2 is the refractive angle between the light ray and the normal

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Snells Law

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Total Internal Reflection When a light ray crosses an interface from lower RI

into a medium with a higher refractive index, it bendstowards the normal.

Conversely, light traveling cross an interface from ahigher refractive index medium to a lower refractiveindex medium will bend away from the normal.
http://en.wikipedia.org/wiki/Image:Reflexion_totale_interne.png

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Critical Angle This has an interesting implication: at some angle,

known as the critical angle c, light travelingfrom a higher refractive index medium to a lowerrefractive index medium will be refracted at 90;in other words, refracted along the interface.

If the light hits the interface at any angle larger

than this critical angle, it will not pass through tothe second medium at all. Instead, all of it will bereflected back into the first medium, a processknown as total internal reflection.

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Critical Angle The critical angle can be calculated from

Snell's law, putting in an angle of 90 for the

angle of the refracted ray 2. This gives 1:Since

2 = 90

Sosin(2) = 1

Then

c = 1 = arcsin(n2/n1)

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Principle of Operation An optical fiber is a cylindrical dielectric

waveguide that transmits light along its axis,by the process of total internal reflection.

The fiber consists of a core surrounded by acladding layer.

To confine the optical signal in the core, the

refractive index of the core must be greaterthan that of the cladding. The boundary between the core and cladding

may either be abrupt, in step-index fiber, or

gradual, in graded-index fiber.

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Total internal reflection Optical fibers are based entirely on the principle of total

internal reflection. This is explained in the followingpicture.

Optical fiber is a long, thin strand of very pure glass aboutthe diameter of a human hair.

Optical fibers are arranged in bundles called optical

cables and used to transmit light signals over longdistances.

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The Structure of an OpticalFiber

Typical optical fibers are composed of core,cladding and buffer coating.

The core is the inner part of the fiber, whichguides light.

The cladding surrounds the core completely.

The refractive index of the core is higher than

that of the cladding, so light in the core thatstrikes the boundary with the cladding at anangle shallower than critical angle will bereflected back into the core by total internal

reflection.

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For the most common optical glass fibertypes, which includes 1550nm single mode

fibers and 850nm or 1300nm multimodefibers, the core diameter ranges from 8 ~62.5 m.

The most common cladding diameter is 125

m. The material of buffer coating usually is soft

or hard plastic such as acrylic, nylon and withdiameter ranges from 250 m to 900 m.

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Buffer coating provides mechanicalprotection and bending flexibility for the

fiber.

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What is Fiber Mode?

An optical fiber guides light waves indistinct patterns called modes.

Mode describes the distribution of lightenergy across the fiber.

Fibers that carry more than one mode at aspecific light wavelength are calledmultimode fibers.

Some fibers have very small diameter corethat they can carry only one mode whichtravels as a straight line at the center of thecore. These fibers are single mode fibers.

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This is illustrated in the followingpicture.

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Multimode Fiber

Fiber with larger core diameter is calledmultimode fiber.

In a step-index multimode fiber, rays of lightare guided along the fiber core by totalinternal reflection.

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When a light is injected in to a fiber there willbe only a finite number of possible paths forthe light to take.

These paths are called modes The problem with multimode operation is

that some of the paths taken by particularmodes are longer than other paths.

This means that light will arrive at differenttimes according to the path taken. Thereforethe pulse tends to disperse (spread out) as ittravels through the fibre.

This effect is called as Modal Dispersion.

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Dispersion

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In multimode step index fiber there is abrupt changein refractive index between core and cladding as aresult wave changes its direction inside the fiber

abruptly. This type of fiber costs less to the manufacturer but

has highest dispersion.,

This type is generally used for short distances and

low cost applications

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Rays that meet the core-cladding boundaryat a high angle (measured relative to a linenormal to the boundary), greater than the

critical angle for this boundary, arecompletely reflected. The critical angle (minimum angle for total

internal reflection) is determined by the

difference in index of refraction between thecore and cladding materials. Rays that meet the boundary at a low angle

are refracted from the core into the cladding,and do not convey light and hence

information along the fiber.

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As the core is in centre and has highestrefractive index, axial mode slows down and

higher order mode travels faster. So allmodes arrive at the end of the fiber closertogether.

This reduces modal dispersion allowinglonger fiber runs or higher ratetransmissions.

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Optical Fiber Types

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Fibre Refractive IndexProfiles

Following figure shows the refractive indexprofiles of some different types of fiber.

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Refractive Index Profile

RI Profile of Multimode Step-Index Fibre Here the RI of core and cladding changes abruptly.

RI Profile of Multimode Graded Index Fibre Graded index fibre has the same dimensions as step

index fibre. The refractive index of the core changes slowly

between the fibre axis and the cladding.

RI Profile of Single-Mode Fibre Single-mode fibre is characterized by its narrow core

size. This is done to ensure that only one mode can

propagate.

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evanescent wave

The waveguide analysis shows that the lightenergy in the fiber is not completely confined

in the core.

Instead, especially in single-mode fibers, asignificant fraction (around 20%) of the lightenergy in the bound mode travels in thecladding as an evanescent wave.

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The most common typeof single-mode fiber hasa core diameter of 8 mto 10 m.

Core diameter is acompromise.

We can't make the coretoo narrow because oflosses at bends in thefiber.

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Cutoff Wavelength

It's not strictly correct to talk about single-modefibre and multimode fibre without qualifying it although this generally done all the time.

A fibre is single-mode or multi-mode at a particularwavelength.

This single-mode fibre characteristic called thecutoff wavelength.

The cutoff wavelength is the shortest wavelength atwhich the fibre remains single-mode. At wavelengths shorter than the cutoff the fibre is

multimode

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Cutoff Wavelength

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Transmitting Light on aFibre

An optical fibre is a very thin strand of silica glass ingeometry quite like a human hair.

In reality it is a very narrow, very long glass

cylinder with special characteristics. When light enters one end of the fibre it travels

(confined within the fibre) until it leaves the fibre atthe other end. Two critical factors stand out:

1. Very little light is lost in its journey along thefibre. 2. Fibre can bend around corners and the light will

stay within it and be guided around the corners.

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Light Propagation inMultimode Fibre

The key feature of light propagation in a fibre is thatthe fibre may bend around corners.

Provided the bend radius is not too tight (2 cm is about

the minimum for most multimode fibers) the light willfollow the fibre and will propagate without loss due tothe bends.

This phenomena is called total internal reflection. A ray of light entering the fibre is guided along the fibre

because it bounces off the interface between the coreand the (lower refractive index) cladding.

Light is said to be bound within the fiber.

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Numerical Aperture

Multimode optical fiber will only propagate light thatenters the fiber within a certain cone, known as theacceptance cone or cone of acceptance of the fiber.

The half-angle of this cone is called the acceptance angle,max.

For step-index multimode fiber, the acceptance angle isdetermined only by the indices of refraction:

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Wheren is the refractive index of the medium light istraveling before entering the fiber

nfis the refractive index of the fiber corenc is the refractive index of the cladding

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Properties of Fibers

There are some important properties offibers as explained below.

1. Loss: The losses occur in fiber because of the

impurities present in the glass.

More accurately, impurities in the glass canabsorb light but the glass itself does notabsorb light at the wavelengths of interest.

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3. Dispersion Dispersion occurs when a pulse of light is spread

out during transmission on the fibre. A short pulse becomes longer and ultimately joins

with the pulse behind, making recovery of areliable bit stream impossible.

Axis: The center of an optical fiber.

Mainly there are many types of dispersions.

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Material Dispersion

Material dispersion is also called aschromatic dispersion.

Both lasers and LEDs produce a range of

optical wavelengths (a band of light) ratherthan a single narrow wavelength.

The fibre has different refractive indexcharacteristics at different wavelengths and

therefore each wavelength will travel at adifferent speed in the fibre. Thus, some wavelengths arrive before

others and a signal pulse disperses (orsmears out).

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Modal dispersion

When using multimode fibre, the light isable to take many different paths ormodes as it travels within the fibre.

The distance traveled by light in each modeis different from the distance traveled inother modes.

Therefore, some components of the pulse

will arrive before others. The difference between the arrival time of

light taking the fastest mode versus theslowest obviously gets greater as thedistance gets greater.

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Dispersion

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Waveguide dispersion Waveguide dispersion is a very complex

effect and is caused by the shape andindex profile of the fibre core. However, this can be controlled by careful

design

4. Numerical Aperture5. Cone of Acceptance

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Core/Clad Concentricity: Tighter core/clad concentricity tolerances

help ensure that the fiber core is centered inrelation to the cladding.

This reduces the chance of ending up withcores that do not match up precisely when

two fibers are spliced together. A core that is precisely centered in the fiber

yields lower-loss splices more often.

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Fiber Curl Fiber curl is the inherent curvature along a

specific length of optical fiber that is exhibited

to some degree by all fibers. It is a result of thermal stresses that occur

during the manufacturing process. Therefore, these factors must be rigorously

monitored and controlled during fibermanufacture.

Tighter fiber-curl tolerances reduce thepossibility that fiber cores will be misalignedduring splicing, thereby impacting splice loss

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Losses in Fiber

Losses in a fiber can be of two types Intrinsic Losses

Extrinsic Losses Intrinsic losses occurs due to something

inside or inherent to the fiber.

It is caused by impurities in the glass duringthe manufacturing process.

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Intrinsic Losses contd..,

As precise as manufacturing is, there is noway to eliminate all impurities, though

technological advances have causedattenuation to decrease dramaticallysince the first optical fiber in 1970.

When a light signal hits an impurity in thefiber, one of two things will occur: it willscatter or it will be absorbed.

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Scattering:Scattering accounts for the majority(about 96%) of attenuation in optical fiber.

Light travels in the core and interacts withthe atoms in the glass.

The light waves collide with the atoms,

and light is scattered as a result. Some scattered light is reflected back

toward the light source (input end)

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AbsorptionThe second type of intrinsic attenuation in

fiber is absorption. Absorption accounts for 3-5% of fiber

attenuation.

This phenomenon causes a light signal to be

absorbed by natural impurities in the glass.

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Extrinsic Losses

The second category of losses are extrinsiclosses.

Extrinsic losses can be caused by twoexternal mechanisms:

macro bending micro bending.

Both cause a reduction of optical power.

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Extrinsic Losses contd..,

Macro bendingIf a bend is imposed on an optical fiber,

strain is placed on the fiber along theregion that is bent. The bending strain will affect the

refractive index and the critical angle of

the light ray in that specific area. As a result, light traveling in the core can

refract out, and loss occurs.

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A macro bend is a large-scale bend that isvisible; for example, a fiber wrapped around

a person's finger. This loss is generally reversible once bends

are corrected.

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Micro bendingThe second extrinsic cause of attenuation is

a micro bend. This is a small-scale distortion, generally

indicative of pressure on the fiber. Micro bending may be related to

temperature, tensile stress, or crushingforce.

Like macro bending, micro bending willcause a reduction of optical power in the

glass.

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Micro bending is very localized, and the bendmay not be clearly visible upon inspection

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