Introduction
Fiber Optic Communications
• Title: Fiber Optics Communication Systems
• Credit Hours: 3+0
• Text Book:
Savera Tanwir
– Fiber Optic Communications by Joseph C. Palais, 4thEdition
• Reference Books:
– Fiber Optic Communications by Peter K. Kaiser, 4thEdition
– Fiber Optic Communications by John Senior, 2ndEdition
– Optical Switching and Networking Handbook (McGraw-Hill
Telecommunications) by Regis J Bates
Grading Policy
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Historical Perspective
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Quiz:10%
Assignments:5%
Term Project/Paper:10%
OHTs:30%
Final Paper:45%
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Hand signals
Smoke signals
Lamps
1960 Laser : A major breakthrough
1970 Fiber
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1
The Basic Communication System
Fiber Optic Communication System
Message
Origin
TRANSMITTER
Modulator
Carrier Source
Channel Coupler
Fiber
Transmitter
Information
Channel
INFORMATION
CHANNEL
Receiver
Fiber
Optical Amplifier
Fiber
Repeater
Optical Amplifier
Fiber
RECEIVER
Detector
Amplifier
Processing
Message
Output
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Modulator
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Carrier Source
• Functions
• Generates the carrier
• Optical Oscillators
– Coverts message to proper format
– Impresses the signal on the wave
generated by carrier source
– Laser Diode (LD)
– Light Emitting Diode (LED)
• Type
• Intensity Modulation
– Analog
– Digital
– Information contained in variation of optic
power
Current
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Optical
Power
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2
Channel Coupler
Information Channel
• Path between transmitter and receiver
• Feeds power into the information
channel just like an antenna
• Problems
– Glass or plastic fiber
• Desirable Characteristics
– Low attenuations
– Large light acceptance cone angle
– Small size of fiber 50 millionth of meter
– Fibers can catch light at limited angles
Fiber Acceptance Cone
Un trapped rays
• Optical Amplifier
– Needed for long links (hundreds and thousands
of km)
Fiber
Fiber
• Repeaters
Trapped Rays
– What makes it diff from optical amplifiers???
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Information Channel
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Detector
• Propagation Time
• Demodulation
• Photodetector
– Frequency
– Path
– Current produced -> optic wave power
– Properties
• Distortion
• Small size, economy, long life, low power
consumption, high sensitivity to optical
signals
– Spreading and deformation
– Overlap -> Errors
– How to fix that?
– Solution: Send pulses at a lower rate
– Limit of Information Rate
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3
Signal Processor
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Message Output
Amplification
Filtering of noise
Desired SNR
Bit Error Rate (BER)
• If heard or viewed
– Electrical signal -> sound wave or image
– Transducers
• Loud speaker
• Cathode ray tube
• Directly Used
– Electrical connector from the signal
processor to succeeding system
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Numbers
• Units appearing frequently in this course are listed
in following table for convenient reference. This
course uses the MKSC (meter-kilogram secondcoulomb) system as much as possible.
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Prefixes
Frequency and Period
• The frequency unit, the hertz, is equivalent
to one cycle of oscillation per second .
• The time between successive peaks of an
oscillation is called the period and is given
by the reciprocal of the wave frequency.
• If f is the wave frequency and T is its period
then T=1/f.
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In FOCS we encounter
frequencies from a few
Hertz to 1014 Hertz
Lengths from kilometer
to micro meter (e.g
wavelengths of light)
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Bandwidth Requirements
Sampling
• When analog signals are transmitted digitally, the bit
rate depends on the
1.rate at which the analog signal is sampled
2.the coding scheme.
• Why the standard 4-kHz telephone channel is sampled
8000 times a second?
According to the sampling theorem, an analog signal can
be accurately transmitted if sampled at a rate of at least
twice the highest frequency contained in that signal. For this
reason the standard 4-kHz telephone channel is sampled
8000 times a second.
• 8 bits/sample -> 64 Kbps
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5
Digital Transmission Rates of
Telephone System
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SONET
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SONET
• SONET stands for synchronous optical
network
• It has been developed as a newer
transmission standard for worldwide use.
• The basic SONET transmission rate is
OC-1 at 51.8 Mbps the electrical
equivalent is STS-1
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FDDI
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FDDI
Fiber distributed data interface (FDDI) provides a standard for data
transmission in a local area network that can extend in range up to 200
kilometers (124 miles).
The relationship between bandwidth and message type has now been
established for common music, voice, and video communication
networks. Such transmission requires bandwidths that depend on the
desired rate of information transfer.
For higher speed data transfer, the fiber distributed data interface (FDDI)
has been specified to operate at a data rate of 100 Mbps.
An even faster LAN, the high performance parallel interface (HPPI),
operates at 800 Mbps.
There is a steady evolution toward higher transmission rates a data
transfer becomes more important to business and industry.
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Computing power levels
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Decibels (dB) Calculation
• If the power is P1 watts at one pint in the
system and P2 watts at some point farther
along the link, then P2/P1 is the fraction of
the power transmitted between the two
locations
• A major part of system design involves keeping
an account of the optic power along the
communications links.
• This account is usually necessary to ensure
that the wave incident on the detector has
sufficient strength to be clearly and correctly
recognized
• In other instances the received power may
even be too large for the receiver.
• The designer must be certain this does not
occur
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• Fiber systems operating below 100 kbps have low
transmission rates. Such systems can be readily, and
cheaply, constructed from available optic and electronic
components.
• Rates from 100 kbps to 10 Mbps are only somewhat
more costly and difficult to implement. This is moderate
range of information rates.
• Rates from 10 Mbps to a few thousand Mbps (several
Gbps), improved circuits, light emitters, and light
detectors must be used.
• The range above several Gbps is very high and
requires added expense and care.
• dB=10 log10P2/P1
• P2=P1 10dB/10
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Decibels (dB) Calculation
P1
P2
P3
Example 1
• What is the total loss if the three
elements in last figure have -11, -6
and -3 dB losses?
P4
P4/P1=P4/P3*P3/P2*P2/P1
dB = 10log10P4/P1
=10log10(P4/P3*P3/P2*P2/P1)
=10log10(P4/P3)+10log10 (P3/P2)+10log10
(P2/P1)
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Example 2
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dBm
• Compare power to a fixed referenced
value.
• Convenient reference level is 1
milliwatt.
• The value of power in decibels relative
to 1mW is denoted by the term dBm.
If we set P1 = 1mW
• A system has -23dB loss. Compute
the efficiency?
• dBm = 10log10P2
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Nature of Light
Wave Nature of Light
• Light is an electromagnetic wave having a very high
oscillation frequency and a very short wavelength.
• Sometimes Light behaves as a Wave
and sometimes as a Particle
• Although light waves have much higher frequencies than
radio waves, they both obey the same laws and share
many characteristics.
• All electromagnetic waves have electric and magnetic
waves associated with them and they all travel very
quickly.
– In free space electromagnetic waves travel at a
velocity of 3x108m/s
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Particle Nature of Light
Wave Nature of Light
• Sometimes Light behaves as if it were made up of
very small particles called photons. The energy of
a single photon is
• Wp= hf
• The wavelength of a light beam is given by
• λ= v/f
– where h = 6.626 x 10-34J x s and is called Planck’s
constant
– Where v is the beam velocity and f is its frequency
• Particle theory explains generation of light by
sources such as LEDs, Lasers and Laser Diodes.
• The frequency is determined by the emitting
source and does not change when the light
travels from one material to another
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• Particle theory also explains detection of light by
conversion of optic radiation to electric current
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Advantages of Fibers 1/3
•
The basic material for glass fibers is Silicon Dioxide or transparent
plastic, both of which is plentiful and readily available.
•
The comparison with other mediums is made in cost/information
transfer. One Fiber cable developed for the telephone applications
has more than 4 times the data carrying capacity as compared to a
twisted pair cable and the diameter is 30 times less.
•
For long paths, fiber cables are cheaper to transport and easier to
install than metal cables as fibers are smaller and lighter.
•
Fiber cables are very strong and flexible and can be wrapped
around tight curves.
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Advantages of Fibers 2/3
• No electric current flows through the fiber, therefore,
fibers have excellent rejection of radio-frequency
interference (RFI) and of electromagnetic interference
(EMI).
• We can pack numerous fibers together in a cable to
transmit many channels of information along a single
path without any cross talk
• Security and privacy. The fiber needs to be physically
violated to obtain the signal. In that case power drops at
the receiver and intrusion is detected.
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Applications of Fiber Optic
Advantages of Fibers 3/3
Communications
• Telephone System
• Fiber optic systems can easily be incorporated into
systems originally designed for wire transmission.
• Glass fibers can withstand extreme temperatures e.g.
800 Celsius leaves fiber unaffected.
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– Fiber is alternative to conventional copper twisted pair cables in
telephone systems.
– In one of the first installed systems, fiber trunk lines connected
telephone offices in Chicago. Operating at the T3 rate, each of
the 24 fibers in the cable had a capacity to carry 672 voice
messages.
– With repeaters, messages can be sent over thousands of
kilometers of fiber. Because of low attenuation, separation
between repeaters in a fiber system can be greater than in a
coaxial link.
– The savings in installation and maintenance costs can be
considerable when large
repeater spacings are feasible
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Applications of Fiber Optic
Applications of Fiber Optic
Communications
Communications
• Higashi Higashi-Ikoma Optical Visual Information
System (Hi Hi-OVIS)
• TAT-8
– TAT-8, underwater fiber links, covers about 6000 km
between the east coast of the United States and
Europe.
– With repeater spacing of 50 km, just over 100 repeaters
are necessary.
– Two fiber pairs, each operating at 295.6 Mbps and with
special coding methods, provide a total capacity of
40,000 voice channels.
• More advanced systems use lower loss fibers and optical amplifiers to
reduce (or eliminate) the need for repeaters
• Low weight give them advantage for submerged cable application
because of relative
ease
and
transporting and laying the fibers
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Systems
– Hi OVIS also stands for Highly Interactive Optical Visual
Information system. The system consists of a center,
sub center, and home terminals linked by optic
transmission lines.
– The lines connect computers and video equipment.
Each home terminal has a TV set, camera, microphone,
and keyboard. Two way interactive communication is
obtained.
– Services: Video request service, Home study course,
Information about local events, medical facilities, train
timetables etc. Fiber Optic Communication
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Systems
Applications of Fiber Optic
Applications of Fiber Optic
Communications
Communications
• Metallic communications links
– Metallic communications links installed along electrified
railway tracks suffer from electromagnetic interference
from the electricity powering the vehicles.
– Because of fiber rejection of EMI, signals traveling
through fibers laid along the track do not degrade.
• Optic communications are compatible with electrified railways.
Wire systems are not.
• Similarly, fibers can be placed near high voltage power lines
without adverse effects, whereas wire systems would be noisy.
• Fibers can even pass unaffected through area where electrical
power is generated or through power substations. Optic cables
can be suspended directly from power line towers, or poles, if
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clearance spaceFiber
permits
and if the load can be tolerated
Systems
• Cable Television System (CATV)
– Cable television systems collect and distribute a large number of
color channels.
– The distances covered range from a few tens of meters to several
kilometers.
– CATV systems obtain their signals from various sources.
– These sources are satellite earth stations, microwave links,
antennas picking up broadcasts from nearby transmitters, and local
studios where programming originates.
– All these sources can be connected to the central distribution
location (the CATV head end) by fibers.
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Applications of Fiber Optic
Applications of Fiber Optic
Communications
Communications
• Fiber System for Transmission of Digital Data
– Fiber system are particularly suited for transmission of
digital data such as that generated by computers.
– Interconnections can be made between the central
processing unit (CPU) and peripherals, between CPUs
and memory, and between CPUs.
– Example: the connection of several hundred cathode
ray-tube (CRT) terminals, located throughout a highrise, to a processor
located
on one of the floors.
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• Military Applications
– Military applications include communications ,command
and control links on ships and aircraft, data links for
satellite earth stations, and transmission lines for
tactical command-post communications.
– The important fiber characteristics are low weight ,small
size ,EMI rejection, and no signal radiation.
– On aircraft and ships the reduced shock, fire, and spark
hazards are significant
assets.
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