Chap 18 Optical Fiber Communication

8/14/2019 Chap 18 Optical Fiber Communication

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By:- Gaurav Kumar

ZEPHYR SYSTEM PVT. LTD

We Think and operate globally


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Optical Fiber Communication


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Topics to be covered in this session :-

Transmission Medium

Optical Fiber Cable OFC Types

Optical Fiber Advantages

Total Internal Reflection

Optical losses Numerical Aperture

Session Objective


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Transmission Media

The means through which data is transformed from one place to another

is called transmission or communication media. There are two categories

of transmission media used in computer communications.

Guided Media

Unguided Media


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Guided Media

Bounded media are the physical links through which signals are confined

to narrow path. These are also called guide media.

Three common types of bounded media are used of the data

transmission. These are

Coaxial CableTwisted Pair Cable

Optical Fiber Cable


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Optical Fiber Cable

Based on the principle that light in a glass medium can carry more

information over longer distances than electrical signals.

Transmit digitized light signals beyond 100 km without amplification.

Transmission speeds of as high as 9.9 Gb/s.

Fiber GeometryOptical fiber material is usually silica or borosilicate glass surrounded by

a cladding of the same material with a lower refractive index and have

exceedingly small diameters.

Core(m) Cladding(m)

8 125

50 125

62.5 125

100 140


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OFC Structure

Core

62.5

Cladding

125



OFC Types

Refractive index profile

Step index fiber has a core with uniform index throughout.

Graded index fiber has a nonuniform index, highest at the center and

gradually decreases until it matches with that of the cladding.

Classification of Fibers

Multimode Step Index Fiber (Step Index Fiber).

Multimode Graded Index Fiber (Graded Index Fiber).

Singlemode Step Index Fiber (Single Mode Fiber).



Single Mode & Multi Mode- Cross section



Step Index Multi Mode Fiber

All rays within a certain angle will be totally reflected at the Core

cladding boundary.

Rays striking the boundary at angles greater than the critical angle

will be partially reflected and partially transmitted out through the

boundary.

After many such bounces the energy in these rays will be lost fromthe fiber, which affects the fiber's bandwidth. .



Graded Index Multi Mode Fiber

Fiber has refractive index with larger values towards the center.

Instead of being sharply reflected, the light is now bent or

continuously refracted in an almost sinusoidal pattern.

This fiber is used in applications requiring a wide bandwidth



Single Mode Fiber

SM fiber has an exceedingly small core diameter of only 5 to 10 m.

Standard cladding diameter is 125 m.

SM fibers easily have a potential bandwidth of 50 to 100 GHzkm.

Core diameter is so small, splicing technique and measuring

techniques are more difficult



Fiber Optical Construction

SIECOR Fiber Optic Cables have the following parts : Optical Fiber Buffer tube

Strength member Jacket

Cable buffer tube types : Loose buffer or Tight buffer



Optical Fiber System

Optical Fiber Transmission System uses light waves as carrier of

the information signals.

Principle: Total Internal Reflection

Voice, Data or Video is transmitted through a glass or plastic fiber,

in the form of light

Transmission sequences :1. Information is encoded into electrical signals.2. Electrical signals are converted into light signals.

3. Light travels down the fiber.4. A detector changes the light signals into electrical signals.5. Electrical signals are decoded into information



Transmission Sequence



(i) Optical Fibers are non conductive (Dielectric)

Grounding and surge suppression not required.

(ii) Electromagnetic immunity

Immune EMI, No radiated energy, No unauthorized tapping

(iii) Large Bandwidth (> 50 GHz for 1 km length)

Future upgradability, One time cable installation costs.Low Loss (5 dB/km to < 0.25 dB/km typical)

Long, unrepeated links (> 70 km is operation).

(v) Small, light weight cables

Easy installation and handling, Efficient use of space.

(vi) Available in Long lengths (> 12 KMs)(vii) Security

(viii) Universal medium

Optical Fiber- Advantages



Light consists of an electric field and a magnetic field that oscillate to

very high rates, on the order of 1014 Hz.

Speed of light changes when it travels from one material to another,

resulting in light changing its direction of travel.

This deflection of light is called refraction.

Different wavelengths of light travel at different speeds in the samematerial.

Refractive index denoted by n, is a dimensionless and is the ratio of the

velocity of light c in free space to its velocity v in a specific material :

n = (c / v)

Even when light passes from one index to another, a small portion isalways reflected back into the first material. This reflection is known as

Fresnel reflection.

For light passing from air to glass, reflection loss is about 0.17 dB.

Light propagation in Optical Fiber




Propagation of light through fiber is governed by the indices of the coreand cladding by Snell's law.

Light injected into the fiber and striking core to cladding interface at

greater than the critical angle, reflects back into core, since the angle

of incidence and reflection are equal, the reflected light will again be

reflected.The light will continue zigzagging down the length of the fiber.



Optical Fiber Characteristics

Optical Wavelength

It is a characteristic of light that is emitted from the light source and

is measured in nanometers (nm).

In visible spectrum, wavelength is described as the colour of light.

Example: Red light has a longer wavelength than Blue light.

Typical Wavelengths: 850nm,1300nm and 1550nm, all are invisible.

Frequency

It is the number of pulse per second emitted from a light source.

Frequency is measured in units of hertz (Hz).It terms of optical pulse 1 Hz = 1 pulse/sec.



Optical Window

Window is defined as the range of wavelengths at which a fiberbest operates.

Typical Window

Window Operational Wavelength

800nm-900nm 850nm

1250nm-1305nm 1300nm

1500nm-1600nm 1550nm



Attenuation

Defined, as the loss of optical power over a set distanceFiber with a lower attenuation, will allow more power to reach a

Receiver

Intrinsic Attenuation

Loss within the fiber due to Absorption & Scattering

Extrinsic Attenuation

Loss due to external sources

Attenuation CategoriesIntrinsic AttenuationExtrinsic Attenuation



Attenuation vs Wavelength

Three principal windows of operation: 850-nm, 1310-nm & 1550-nm

wavelength bands.

These are the regions in which attenuation is low and matched to the

capability of a transmitter to generate light efficiently and a receiver to

carry out detection.



Macro bendingIf fiber is sharply bent, light traveling in fiber

cannot make the turn and is lost in the cladding.

Micro bendingSmall bends in the fiber caused by crushing,

contraction, etc., bends may not be visible with the naked eye.

Attenuation- Micro bending & Macro bending

Macro bending Micro bending


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Spreading of light pulse as it travels down the length of a fiber.Dispersion limits the bandwidth of a fiber.

Types of dispersion

Chromatic Dispersion

Polarization Mode Dispersion

Dispersion


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Chromatic Dispersion

CD is the spreading of a light pulse as it travels down a fiber.

During the propagation of light, all of its spectral components propagate

accordingly.

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.Effect of chromatic dispersion is pulse spread.


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Polarization Mode Dispersion

PMD is caused if the fiber is not truly a cylindrical waveguide.

Mechanical stress upon the fiber, bends during cabling, imperfectionsduring fiber manufacturing are the reasons for the variations in the

cylindrical geometry.

PMD is not an issue at low bit rates but becomes an issue at bit

rates in excess of 5 Gbps.


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Numerical Aperture

Numerical aperture (NA): NA= (n12n22)1/2

Typical NA values are 0.1 to 0.4 which correspond to acceptance

angles of 11 degrees to 46 degrees

Acceptance angle of a fiber: a= sin-1

NA

Light that enters at an angle equal to or less than the acceptance

angle will be guided

NA is more means more light gathering power


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A fiber with a large NA accepts light well, a fiber with a low NArequires highly directional light.

Fibers with a high bandwidth have a lower NA.

Numerical Aperture


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REVIEW

In this session we discussed about the:-

Transmission Medium

Optical Fiber Cable

OFC Types

Optical Fiber Advantages


Optical losses Numerical Aperture


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Documents

Chap 18 Optical Fiber Communication