Title: Chapter 9 Introduction to Fiber Optics
1Chapter 9Introduction to Fiber Optics
- Information Technology in Theory
- By Pelin Aksoy and Laura DeNardis
2Objectives
- Understand the importance of fiber-optic
technologies in the information society - Identify the fundamental components of a
fiber-optic cable - Understand the principles by which light travels
within a fiber-optic cable, including refraction
and total internal reflection
3Objectives (continued)
- Understand how a single fiber can carry multiple
signals through wavelength division multiplexing - Learn about the advantages and disadvantages of
fiber optics as a transmission medium for various
applications - Gain exposure to cutting-edge fiber-optic
approaches
4Background on Optical Communications
- Fiber-optic communication
- Trapping light inside an optical fiber
- Can carry any form of information
- Fiber is an optical medium, which means it is
capable of transmitting light - Based on total internal reflection (TIR)
5Tyndalls Experiment
6Structure of Fiber-Optic Cables
7General Structure of Fiber-Optic Cables
8General Structure of Fiber-Optic Cables
(continued)
9Structure of Fiber-Optic Cables Cladding
- Cylindrical material made of glass or specialized
plastic - Central portion of the fiber
- Light signal carrying the information travels
through the core - The diameter of the core can range from a couple
of micrometers (µm-one millionth of a meter) to a
couple of millimeters (mm-one thousandth of a
meter)
10Structure of Fiber-Optic Cables Jacket
- Surrounds the cladding
- Insulates and protects the fiber from physical
damage and environmental effects, such as
moisture, that might interfere with the inner
workings of the cable - Usually made of opaque plastic or another type of
material
11How Light Travels Through Fiber
- TIR is the basis of fiber-optic communication
- TIR may be considered to be an extreme case of
refraction - When a light ray strikes a boundary of two
materials with different RIs, it bends, or in
other terms, refracts to an extent that depends
on the ratio of the RIs of the two materials
12Refraction
- The phenomenon that causes a spoon inside a clear
glass of water to appear shorter and bent to an
observer looking from the outside - Light rays that strike the water/air boundary
bend to create an image that is shorter than the
actual height of the spoon in the water, because
water has a higher RI than air - TIR is the phenomenon that makes the side of an
aquarium act as a mirror when viewed at an
appropriate angle
13Refractive Index
- Refractive index of an optical medium Speed of
light in a vacuum (300,000,000 meters per
second)/speed of light in the optical medium
14Refractive Index (continued)
15Total Internal Reflection
16Total Internal Reflection (continued)
17Total Internal Reflection (continued)
18Types of Fiber-Optic Cables
- Single-mode fiber
- Multi-mode fiber
- Step index fiber
- Graded index fiber
19Single-Mode Fiber
- Very small core diameter, on the order of a few
micrometers (µm) less than that of a human hair - The cladding diameter is tens of micrometers,
making it much larger than the core - Single-mode fiber can transmit information at
very high data rates across large distances - The small diameter restricts only a single mode
of light to carry all of the information
20Multi-Mode Fiber
- Has a much larger core diameter, ranging from
tens to hundreds of micrometers - Used for lower-bandwidth, shorter-distance
applications than single-mode fibers - Information-carrying capacity is less than that
of single-mode fibers due to the physical effects
they impose on the signals traveling through them
21Step Index Fiber
- A fiber-optic cable with a uniform refractive
index throughout its core is classified as a step
index fiber - Both single-mode and multi-mode
22Graded Index Fiber
- A cable whose core refractive index is
non-uniform and varies gradually is classified as
a graded index fiber - The value of the RI of a graded index fiber is
highest in the center of the core and gradually
diminishes towards the cladding
23Refractive Index Profiles
24Fiber-Optic Communication Systems
- Pulses of electricity corresponding to the bits
arrive at the input transducer - A device that converts one form of energy to
another - Devices similar to light bulbs are used as
optical transducers at the input of fiber-optic
cables to convert electricity into light - Light emitting diodes (LED)
- Laser diodes (LD)
25Fiber-Optic Communication Systems (continued)
- Electrical signals arriving at the input of
optoelectronic devices are used to modulate the
light source - The modulated optical signal is emitted by the
source and coupled into the cable - Once the light is trapped inside the cable, it
travels to the other end, where it is demodulated
and an output transducer - Photodiode (PD) or phototransistor
26Fiber-Optic Communication Systems (continued)
27Wavelength Division Multiplexing
- Fiber-optic cables can carry multiple signals due
to their large bandwidths - Dense WDM (DWDM)
- A large number of channels are multiplexed onto a
single fiber-optic cable - Course WDM
- A smaller number of channels multiplexed
28Wavelength Division Multiplexing (continued)
A fiber-optic communication system employing WDM
29Fiber-Optic Technology Benefits/Drawbacks
- High transmission rate
- Immunity to electromagnetic interference
- Low attenuation
- High security
- Small weight and size
- Low power consumption
- High installation cost
- Difficulty in splicing
30High Transmission Rate
- Fiber supports extremely high transmission rates
- Many applications require these rates
- Video
- Music sharing
- Transmission of medical images
- Satellite imagery
- Cable TV broadcasting
- Internets transmission backbone
- A fiber optic communication system with 160 WDM
channels could transmit close to 26 terabits per
second (i.e. 26,000,000,000,000)
31Immunity to Electromagnetic Interference
- Light rather than electricity
- Immune to electromagnetic interference (EMI)
- The presence of EMI directly affects the
transmission rate - Thus, the immunity to EMI enables the large
capacity of fiber optics - Fiber used to replace copper in noisy
environments - A safer option in an industrial environment,
where the presence of an electrical signal could
present a fire hazard
32Low Attenuation
- Low attenuationon the order of fractions of a
dB/km in some transmission systems - Glass used for fiber-optic cables is much purer
than the glass we find in a car windshield - The purer the glass, the lower the signal
degradation - Low attenuation translates directly to efficient
long-distance communications, such as
transatlantic or other suboceanic cable routes
33High Security
- Unlike wireless or copper-based transmission
systems, fiber-optic systems more difficult to
crack - You cannot tap into a fiber cable by merely
slicing the cable jacket and clipping a crocodile
clip on the cable, as you can with a copper cable - Banks, embassies, and many governmental
departments that require private communications
use heavy-duty, robust fiber-optic cabling
systems due to its high level of security
34Small Weight and Size
- Because fiber cables are either made of glass or
plastic, they are much more lightweight than a
metal like copper - A fiber as thin as a strand of hair has much more
capacity than a thick, heavy coaxial copper cable - In applications used on aircraft and satellite
systems, where weight is a major issue, and in
applications where space is a concern, fiber
optics has obvious advantages
35Low Power Consumption
- Because fiber-optic cables have low attenuation
characteristics, the amount of power that must be
supplied in the cable is much lower than in
copper-based systems across the same distance - Efficient LEDs and LDs as well as efficient light
detectors are readily available to reduce the
overall power consumption of an optical link
36High Installation Cost
- High planning, installation, and maintenance cost
- To properly install, lay, and align these cables,
personnel usually require extensive training,
because these cables can be sensitive to
stretching or other physical effects - Elaborate installations are often required for
important applications to minimize these effects - The cost of installing fiber-optic systems is
decreasing because more efficient components and
techniques are being developed to make
installation easier
37Difficulty in Splicing
- Because fibers are made of either plastic or
glass, they are more difficult to splice (i.e.
bring together) than copper cables - Sophisticated splicing techniques, such as fusion
or using a connector, are needed to bring two
pieces of fiber together - Splicing fiber-optic cables is generally more
difficult than soldering or crimping two pieces
of copper wire together
38Commercial Cables
- Fiber optics used in a wide array of
applications - Medicine
- Telecommunications
- Computing
- Computer networking
- Transoceanic cables
39Commercial Cables (continued)
- Fiber optics used in a wide array of medical
applications - Endoscopy
- The process of inspecting various organs of the
body using a device called an endoscope - A fiber-optic cable as part of an endoscope can
be used to provide the light and transport the
imagery of various organs of the body such as the
stomach
40Structure of Commercial Cables
41Summary
- Fiber-optic cables are made up of a few basic
components, including the core, cladding, and
coating or jacket - Light within a fiber-optic cable is carried from
one end to the other based on a phenomenon called
total internal reflection - The core and the cladding have different
refractive indices, so that light that traverses
the core and hits the core-cladding boundary
undergoes refraction - Light rays that strike the core-cladding boundary
at an angle greater than the critical angle
undergo total internal reflection
42Summary (continued)
- Lasers and LEDs are the primary sources that are
used to insert light into a fiber-optic cable - Photodiodes and phototransistors are transducers
that convert light back into electricity - Fiber-optic cables may be classified as either
single-mode or multi-mode single-mode fibers
have a smaller core diameter and can support
higher transmission rates across longer distances
than multi-mode fibers - Fiber-optic cables may also be classified
according to the nature of their cores
refractive index - Step index fibers have a uniform refractive index
throughout their core, whereas graded index
fibers have a refractive index that varies
gradually through their core
43Summary (continued)
- WDM is a technique used to multiplex multiple
channels of information and transmit them across
a single fiber-optic cable the 1.3-?m and
1.55-?m wavelengths are the most popular, due to
the low attenuation that optical signals undergo
at these wavelengths - Advantages of fiber-optic cables include high
transmission rates, immunity to EMI, low
attenuation, high security, small weight and
size, and low power consumption - Disadvantages include higher installation and
maintenance costs and difficulty in splicing
44Summary (continued)
- Commercial fiber-optic cables are sold with
multiple strands of fibers packaged as a single
cable with extra components that increase its
strength - Established but unused fiber-optic systems are
called dark fiber - Various commercial applications of fiber-optic
cables include transoceanic communications,
computer networking, computing, and medicine