Download What are Fiber Optics

1
NET 535 :
NEW TECHNOLOGIES
DR .HANAA ABDALAZIZ ABDALLAH
Fiber
Optic
Technology
Contents:
Introduction
Construction
Principle
Working
Classifications
Application
Advantages
Disadvantages
Introduction:
We hear about fiber-optic cables whenever people
talk about the telephone system, the cable TV system
or the Internet.
We are increasing ability to transmit more
information, more quickly and over longer distances.
An optical fiber (or fiber) is a glass or plastic fiber
that carries light along its length
What are Fiber Optics?
Fiber optics (optical fibers)
are long, thin of very pure
glass about the diameter of a
human hair.
They are arranged in bundles
called optical cables and used
to transmit light signals over
long distances.
Construction:
Optical Fiber consists of 3 regions
1.Core – thin glass center of the
fiber where light travels.
2.Cladding – outer optical
material surrounding the core
3.Buffer Coating – plastic
coating that protects
the fiber, and make it strong .
Principle:
The principle behind the transmission of the light waves in
an Optical Fiber is “Total Internal Reflection”
The angle of refraction at the interface between two media is
governed by Snell’s law:
n1 sin θ1  n2 sin θ2
Refraction, Critical angle , Total internal reflection
Angle of incidence
Angle of incidence = critical angle
< critical angle
Angle of incidence > critical angle
Working:
When light traveling in a dense medium hits a boundary
at a steep angle (larger than the "critical angle “for the
boundary), the light will be completely reflected. This
phenomenon is called total internal reflection.
Total internal reflection occurs when light enters from
higher refractive index to lower refractive index
material,
Acceptance Cone
Acceptance
Cone
qa
n2 cladding
n1 core
n2 cladding
-If the angle too large  light will be lost in cladding
- If the angle is small enough  the light reflects into
core and propagates
Acceptance Angle
Acceptance
Cone
n2 cladding
n1 core
n2 cladding
qa
Acceptance angle, qa
, is the maximum angle in which
external light rays may strike the air/Fiber interface
and still propagate down the Fiber with <10 dB loss.
Note: n1 belongs to core and n2 refers to cladding)
qa  sin
1
n  n2
2
1
2
Transmission of signal:
The light in a fiber-optic
cable travels through the core
by constantly bouncing from
the cladding and the principle
called total internal
reflection.
Hence the cladding does not
absorb any light from the
core.
Types of Optical Fibers:
Material
used
Glass
Fiber
Plastic
Fiber
Mode of
transmission
Single
mode
Multi
mode
Refractive
index profile
Step
Index
Graded
Index
Classification based on “Material Used”
On the basis of number of modes:on the basis of number of modes of propagation the optical
fiber are classified into two types:
(i) Single mode fiber (SMF) and
(ii) Multi-mode fiber (MMF)
Single-mode fibers – in single mode fiber only one mode
can propagate through the fiber. This type of fiber has small
core diameter(5um) and high cladding diameter(70um) and
the difference between the refractive index of core and
cladding is very small. There is no dispersion i.e. no
degradation of signal during travelling through the fiber.
The light is passed through the single mode fiber through
laser diode.
Multi-mode fiber :Multi mode fiber allows a large number of modes for the
light ray travelling through it.
The core diameter is (40um) and that of cladding is(70um)
The relative refractive index difference is also larger than
single mode fiber.
There is signal degradation due to multimode dispersion.
They are not suitable for long distance communication due
to large dispersion and attenuation of the signal.
Classification based on “Modes of Transmission”
STEP INDEX FIBER
The refractive index of core is constant
The refractive index of cladding is also constant
GRADED INDEX FIBER
In this type of fiber core has a non uniform refractive index
that gradually decrease from the centre towards the core
cladding interface.
The cladding has a uniform refractive index.
The light rays propagate through it in the form of skew rays
or helical rays.
Classification based on “Refractive Index profile”
Advantages:
Less signal degradation
Less Expensive
Light weight
Digital Signals
Non-Flammable
Higher carrying Capacity
Thinner
data security
Free From Electromagnetic Interference
High Bandwidth Over Long Distances
Fiber optics have a large capacity to carry
high speed signals over longer distances
without repeaters than other types of cables
Disadvantages
Higher initial cost in installation
Interfacing cost
Strength
Lower tensile strength
More expensive to repair/maintain
Tools: Specialized and sophisticated
LOSSES IN OPTICAL FIBER CABLES
 The predominant losses in optic Fibers are:
 absorption losses due to impurities in the Fiber material
 wavelength dispersion because of the use of a non-
monochromatic source
 radiation losses caused by bends and kinks in the Fiber
 pulse spreading or modal dispersion due to rays taking
different paths down the Fiber
 coupling losses caused by misalignment & imperfect
surface finishes

Dispersion is referred to widening the pulse as the light travels
through the fiber optics
COUPLING LOSS
DISPERSION LOSS
OPTICAL FIBER ARCHITECTURE
TX, RX, and Fiber Link
Input
Signal
Transmitter
Coder or
Light
Converter
Source
Source-to-Fiber
Interface
Fiber-optic Cable
Fiber-to-light
Interface
Light
Detector
Receiver
Amplifier/Shaper
Decoder
Output
OPTICAL FIBER ARCHITECTURE –
COMPONENTS

Light source:



Input
Signal
Amount of light emitted is
proportional to the drive current
Two common types:
 LED (Light Emitting Diode)

 ILD (Injection Laser Diode)
Source–to-fiber-coupler (similar to
a lens):

A mechanical interface to couple
the light emitted by the source
into the optical fiber
Coder or
Converter
Light
Source
Source-to-Fiber
Interface
Fiber-optic Cable
Fiber-to-light
Interface
Light
Detector
Amplifier/Shaper
Decoder
Output
Receiver
Light detector:



PIN (p-type-intrinsic-n-type)
APD (avalanche photo diode)
Both convert light energy into current
LIGHT SOURCES (MORE DETAILS…)
 Light-Emitting Diodes (LED)
 made from material such as AlGaAs or GaAsP
 light is emitted when electrons and holes recombine
 either surface emitting or edge emitting
 Injection Laser Diodes (ILD)
(LASER) Light Amplification
by Stimulated Emission of Radiation
 similar in construction as LED except ends are highly polished
to reflect photons back & forth
A LIGHT SOURCES
LED (Light emitting diode)
ILD (injection laser diode)
DETECTORS
•Detector is the receiving end of a fiber optic link.
There are two kinds of Detectors
1. PIN (Positive Intrinsic Negative)
2. APD (Avalanche photo diodes)
PIN
APD
PIN PHOTODETECTOR
w
The high electric field present in the depletion region causes photo-generated carriers to
Separate and be collected across the reverse –biased junction. This give rise to a current
Flow in an external circuit, known as photocurrent.
HOMEWORK
QUESTION 1: The refractive indices for core and
cladding for a step index fibre are 1.52 and 1.41
respectively Caculate (1) Critical angle (2)
Numerical Aperture (3) The maximum incidence
angle
THANKS