optical fiber

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Major milestones in electrical communication
Communication Systems – 20th Century
Problems of Electrical Communication Systems
History of Optical Communication
Figure of merit for Communication Systems –
the B.L product
• Optical Communication systems – free-space
and optical fiber
• Optical Fiber Communication (OFC) –
advantages
• Major developments in OFC since 1980

1.Major milestones in Electrical
Communication

2.Communication Systems of the 20th
Century

• 1838 – Samuel F.B. Morse invented Telegraphy
• 1866 – first transatlantic telegraph cable
• 1876 – Alexander Graham Bell invented
Telephone
• 1905 – Triode based Electronic amplifier
• 1940 – first coaxial-cable system (3 MHz –
3,000 voice channels or ONE
television channel)
• 1948 – first microwave system (4 GHz)
• 1975 – the most advanced coaxial system with a
bit rate of 274 Mb/s

• Wire – Telegraphy (2 wires for telegraph
transmission – simplex & duplex)
• Wire – Telephony (2 wires for telephone
transmission of 1 channel)
• Carrier telephony (long-distance telephony
for multiple channels – 4,8,16)
• Coaxial cable systems (for 32 channel
PCM systems – 32x64kb/s = 2.048 Mb/s)

3.Problems of Electrical
Communication systems

4.History of Optical Communication
Systems

• Affected by EMI
• Low bandwidth (4 kHz – telephone,
100-500 MHz per km – coaxial cable )
• High attenuation (20 dB/km – typically)
• High system cost (due to too many
repeaters for a given Bandwidth/ data rate)
• Prone to tapping
• Bulky

• Optical communication is older than
electrical communication !
• 8th century B.C. - Greeks used fire signals
for sending alarms, calls for help, etc
• 1792 – Claude Chappe’s Optical telegraph
• 1880 – Alexander Graham Bell used light
beam for transmission of speech
(Photophone)
• 1960 – invention of Ruby LASER

History of Opt.Commn…..contd
• 1966 – light confinement using sequence of gas
lenses
• 1966 – suggestion to use optical fiber (Kao &
Hockham)
• 1970 – Corning Glass optical fiber with 20 dB/km
near 1 µm
• 1970 - Semiconductor Laser with CW operation
at room temp.
• 1980 onwards – wide spread use of Optical
Fiber Communication

5. Figure of merit for Communication
Systems – the B.L product
• A commonly used figure of merit for communication
systems is the
bit rate-distance product, BL
where B is the bit rate, and L is the repeater spacing.
• 1970 – Communication systems had a maximum value
of BL product = 100 Mb/s-km only, due to fundamental
limitations.
• It was realized that BL product could only be increased
through the use of optical waves as carrier.

6.What is Optical Fiber
Communication (Fiber Optics) all
about?
• Optical transmission of electrical signals
using an electrical-to-optical converter
(E/O converter), an optical fiber, and
optical-to-electrical converter (O/E
converter).
• E/O converters: LEDs, Laser Diodes
• O/E converters: Photodetectors
Increase in bit rate-distance product (BL) during the period 1850-2000.
(source: Chapter 1 - GP Agrawal, Fiber-Optic Communication Systems, 3rd
edition, John Wiley & Sons., Inc., New York, 2002)

Advantages of Optical Fiber
Communication (Fiber Optics)
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Very high bandwidth (10 - 100 GHz, typ.)
Very low attenuation (lowest 0.16 db/km)
Immune to EMI
Data security (almost impossible to tap
information)
• Lower system cost (fewer repeaters due to
low attenuation of fibers)
• Very low Bit Error Rate ( 10^-10 typically)

Basics of Optical Fiber
Communication
An Optical Fiber Communication System
consists of
• Transmitter (Optical source + driver circuit)
• Optical Fiber
• Receiver (Photodetector + receiver circuit)
• Based on the communication system
requirements, the appropriate source,
fiber, photodetector combination is
chosen.

7. Optical Fiber

Optical Fiber Types

Optical Fiber Dimensions

Cross section and refractive-index profile for step-index and gradedindex fibers
(source: Chapter 1 - GP Agrawal, Fiber-Optic Communication Systems, 3rd
edition, John Wiley & Sons., Inc., New York, 2002)

Attenuation Characteristics –
Single Mode Fiber

Optical Fiber Communication Transmission windows

Improvements in Optical fiber attenuation and popular
transmission windows for Optical fiber communication
(Source: Chapter 1, Gerd Keiser, Optical Fiber Communications, 3rd
edition, McGraw-Hill International Editions, Singapore, 2000)

Optical Fiber Cables

Loose Tube Fiber Cable

• For Outdoor applications optical fibers
need to be armored.
• Unlike copper cables optical fibers do not
have high tensile strength.
• Optical fibers are put inside loose tubes or
V-grooves.
• For Indoor applications tight buffered
cables with strengthening materials such
as Kevlar are often employed.
Optical Fiber Cable for Outdoor Applications

Tight Buffered Cable
8. Transmitter

Optical Fiber Cable for Indoor Applications

Transmitter……contd.
• LEDs - used for low to medium bit rate applications (less
than 100 Mbits/sec) and lower optical link lengths.
- are cheap and rugged
- can be switched on and off (for digital modulation)
using simple logic drivers.
• Laser diodes are used for high bit rate and longer optical
link applications.
- are very sensitive to temperature changes
- require sophisticated circuits for their field use.
- Most commonly used circuits monitor the average
optical power and adjust the drive current
automatically to maintain the required optical
power.

• Optical transmitter is an electrical-to-optical
converter.
• Sources – LED or Laser Diode (LD)
• Principle – Varying the optical power of the
source by varying the current
• LED – for short range and low data rate
applications
• LD – for long range and high data rate
applications
• Analog or Digital modulation of source current

Transmitter……contd.
• Generally laser diodes come with fiber pigtails,
which are aligned in factory for optimum power
coupling.
• An optical transmitter consists of an optical
source (LED or LD) and a drive circuit which
drives the required amount of current through
the LED or LD.
• LED transmitters typically have output powers of
10 – 50 µW at the end of a fiber MMF pigtail.
• LDs typically give anywhere from 1mW – 20mW
on a SMF pigtail.

Optical Spectrum of LEDs & Laser
Diodes

9. Receiver
• An optical receiver is an optical-to-electrical converter +
amplifier and decision circuits.
• Photodetectors are used for O/E conversion.
• Two types - PIN and Avalanche Photodetector (APD)
• Principle – generation of photo current using the light
from the fiber falling on the depletion region of a photo
detector
• PIN – used for modest applications, no internal gain
mechanism, cheap and rugged
• APD – used for applications requiring high sensitivity;
provide internal optical gain of several tens. They require
high bias voltages (>200V). Quite expensive.

Receiver………..contd.
• The photodetector (PIN or APD) followed by a
low noise amplifier.
• The optical power detected is typically 1µW or
less.
• front end amplifier must be a low noise
amplifier.
• The bandwidth required at the receiver is
generally very high (several hundreds of
MHz).
• Design of a fiber optic receiver circuit is quite
a challenge.

10. Optical Fiber Connectors

Receiver………..contd.
– Most of the noise in the low noise amplifier is introduced
by the first device.
– For high frequency applications a matching MESFET
device is chosen as the front end amplifier device.
– For simple, low bit rate applications a simple current-tovoltage converter (using an opamp) is good enough.
– Low noise preamplifer circuit will be followed by a Post
amplifier (to raise the electrical signal to the required
levels)
– For digital applications a high-speed comparator
employed to finally convert the signal to the required
logic levels.

11. Permanent Joints - Splicing
• Fusion Splicing is the most common method used for
joining fibers.
• Fibers for indoor use with primary and secondary buffer
coatings generally come in lengths of about 2km.
• Outdoor fiber cables are quite bulky and come in much
smaller lengths (100m to 500m).
• With modern day fusion splicing machines splice losses
are typically of the order of 0.01 dB per splice.
• These machines automatically align the two pieces of
fibers for maximum power before they are joined.
• Sophisticated splicing machines match the refractiveindex profiles of the fibers as well.

12.Major developments in OFC since 1980

Increase in the capacity of optical fiber systems realized after 1980.
The change in the slope after 1992 is due to the advent of WDM
technology
(source: Chapter 1 - GP Agrawal, Fiber-Optic Communication Systems, 3rd
edition, John Wiley & Sons., Inc., New York, 2002)

Major developments since 1980…..contd.

Increase in the BL product since 1975 through several generations of
optical fiber systems
(source: Chapter 1 - GP Agrawal, Fiber-Optic Communication Systems, 3rd
edition, John Wiley & Sons., Inc., New York, 2002)

Major developments since 1980…..contd.

Fourth Generation systems

• First generation systems – 1975 to 1980 –
850nm systems, and multimode fibers, data
rates below 100 Mb/s,
• Second generation systems – early 1980s –
1300nm systems, single mode fibers with 0.5
dB/km loss, data rates up to 1.7 Gb/s, repeater
spacing of 50km
• Third generation systems – mid 80s - 1550nm,
0.2 dB/km loss, dispersion-shifted fibers with
minimum dispersion at 1550nm, data rates
4Gb/s, repeater spacing of 100km

• Drawback of the 3rd generation systems – signal
regenerated using electronic repeaters, spaced
typically 60-70km.
• Demonstration of Fiber amplifiers - 1989
• 4th generation systems – 1990 - make use of
Optical amplification (for increased repeater
spacing) and Wavelength-division multiplexing
(WDM) for increased data rate.
• Resulted in a data rate of 10 Tb/s by 2001.

4th Generation systems…contd.
• In most systems fiber losses are
compensated periodically using erbiumdoped fiber amplifiers spaced at 60-70km.
• 1991 – demonstration of a data
transmission using re-circulating-loop
configuration
- 21,000 km at 2.5Gb/s
- 11,300 km at a bit rate of 5 Gb/s

Under sea Cable (submarine cable)
Communication
• One of the most challenging means of
communication - used since 1858
• The cable of 1858 worked only for a few weeks
• 1866 – the first transatlantic telegraph cable
(North America to Europe)
• Telegraph operator could send about 17 words
per minute, at a cost of $5 per word.
• 1956 – the first transatlantic telephone cable
(TAT-1) – 48 telephone circuits between
Newfoundland and Scotland.
• Was based on analog systems

Undersea cables….contd.
• By 1983 – TAT cable capacity increased
to 4200 voice circuits using Frequency
Division Multiplexing (FDM)
• From 1956 to 1983 the capacity of the
TAT increased at an annual rate of 20%
• 1988-89 – the first undersea fiber optic
communication system with a capacity of
280 Mb/s on each of the three fiber pairs.

Power on repeatered cables
• repeaters need to be powered.
• The standard approach is to send a constant current
of about 1A from one end of the cable to the other,
along a copper sheath which lies outside the fibres
and inside the armour (if present).
• Each km of cable offers a resistance of some 0.7
ohm. Voltage drop across each repeater is typically
40V (on four fibre-pair cable)
• a requirement of close to 10 KV across a typical 7500
km transatlantic crossing with 100 repeaters.

4th Generation Optical Fiber
Submarine Systems
• 1996 - the first cable (TAT-12/13) using fully
optical amplification via erbium-doped fiber
amplifiers (EDFAs) came into service.
• Because of the optical amplifiers the need for
the two signal conversions is avoided.
• This change from regeneration to optical
amplification considerably reduced the number
of active components which had to be qualified
for 25 years of undersea service
• Significantly improved the intrinsic reliability of
the cable systems (though that is so high that it
is difficult to measure).

Optical Fiber Undersea cable
communication…contd.
• First system used hybrid optical systems –
repeaters converted the incoming signals
from optical to electrical, regenerated the
data with high-speed ICs, and
retransmitted the data with a local
semiconductor laser.

Wavelength division multiplexing
(WDM)
• Transmitting signals at more than one
wavelength on each fiber pair, thus
increasing bandwidth.
• STM-16 (2.5 Gbps) is the transmission speed
in the SDH hierarchy which is being most
widely used today
• Modern submarine cable systems can
transmit STM-16 signals at four or eight
different wavelengths, to give a total capacity
of 10 or 20 Gbps per fiber pair.

WDM Cable Network between Germany and
Singapore (SEA-ME-WE-3)