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Fiber Optics Communications
Lecture 2
Introduction to Fiber Optic
Communication System
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Communications refers to information transmission and reception. The information
being transmitted could be of various forms, either analog (voice, video, text) or
digital form such as binary codes which when sampled properly carries essentially all
information of analog signal
Motivation of new communication systems is to increase data rate so that more
information could be sent
One of the earliest known optical transmission links was the use of fire signal by the
Greeks
In 1838 the telegraph was invented by Samuel Morse. This ushered in a new era in
communications i.e. electrical communications
Amount of information that can be transmitted is directly related to the frequency
range over which the carrier operates, increasing the carrier frequency theoretically
increases the available transmission bandwidth and therefore larger capacity
The trend was to employ higher frequencies which offer increases in bandwidth and
capacity. This lead to the birth of radio, tv, radar, and microwave links
Telecommunications usually refers to fiber optic communications, satellite
communication, or mobile communication.
Introduction to Fiber Optic
Communication System
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Two basic elements in any telecommunicating system
– Transmission Carrier: an electromagnetic wave with particular frequency in spectrum.
Information rides on carrier to be delivered
– Transmission media: material in which carrier signal propagates
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Examples
– AM or FM signal
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Transmitted information is voice. The carrier is radio waves (3 KHz to 300MHz). The signal can be
transmitted tens or hundreds of miles
– Microwave transmission
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Carrier in (300 MHz to 30 GHz). Transmission media is coaxial cables, waveguide or air
Fiber optic communication
– Carrier is light (near infrared just outside visible light). The actual wavelength used includes
850 nm, 1300 nm, and 1550 nm. The transmission medium is optical fiber
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The optical frequency n is related to wavelength l through
c = λn
Example, a 1552.5 nm wavelength light signal has a frequency of 193.1 THz (193.1
x 1012 Hz)
Introduction, contd
• From EM spectrum, the higher the frequency,
the shorter wavelength, and the higher is the
EM energy. Energy is related to wavelength by
• E = hn, where h is Planck’s constant and v is
the frequency in Hz, also
E[J]= hc/λ [m]
E[ev]= 1.24 /λ[mm]
EM spectrum
The Development of Fiber Optic
Communication System
• In ancient times, Greeks used reflected light from mirrors to communicate
between watch towers, ship captains used both mirrors and lanterns to
communicate with their fleets
• Lightwave communication started with the invention of photophone by
Alexander Graham Bell in 1880, which used Sun light as carrier and air as
transmission media. Sound information is transmitted this way up to 200
meters.
• The biggest obstacle for using optic fibers for communications was high
loss. Prior to 1970, the best silica (SiO2) fiber had attenuation coefficient a
=1 dB/m or 1000 dB/km. Let us see what this means:
• Using such fiber of 1 Km long to transmit light of wavelength lo=1 mm. In
order to receive a single photon at the end of fiber, find the energy to be
transmitted into the beginning of the fiber.
• Fiber loss is defined as Loss[dB]= 10 log10 PTX/PRX where PTX is transmitted
energy and PRX is received energy.
• At the receiver, a single photon’s energy is PRX[eV]= 1.24/1=1.24
[eV]=1.986x10-19 [J]
Development, contd
• The attenuation coefficient is defined as a[dB/km] = (10/L[km])
log10 PTX/PRX , where L is the length of the fiber. Therefore, the
required transmitted energy is
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PTX = 10 α/10PRX = 10 100 x 1.986 x10-19 ~ 2 x 10 81 [J]
• This is equivalent to all light energy of the sun’s radiation in 1047
years, if all radiation is of lo=1 mm.
• In 1970, Corning Glass Works did successfully grow fused silica fiber
with loss less than 20dB/km
• Bell labs made successfully the room temperature continuous wave
AlGaAs (Aluminum Gallium Arsenide) that operated at 850 nm.
Semiconductor laser light has high purity spectrum or very narrow
wavelength range and of high efficiency. It was lucky that 850nm
wavelength happens to correspond to one of the low loss window in
silica fiber
The Generation of Fiber Optic
Communication System
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From then onward, within 7 years of time, fiber optic communication started to be
commercialized. From 1977 to date, there has been 3 generations of fiber optic.
The first generation (1977-1997)
Point to point pipelines. Routing and switching are carried out by electric
components. The basic feature of 1st generation is that all aspects of regeneration
were accomplished using electrons. Optic signal transmitted is converted to
electric signal, cleared, resynchronized, and amplified and then transmitted back
to optical signal. The regeneration is both expansive and complex. Further
classification of 1st generation
– λ= 850nm. Multimode fiber. Light source AlGaAs semiconductor laser. Typical bit rate 44.736
Mbps
– λ= 1300nm. Multimode fiber. 1300 nm corresponds to second window of low loss in silica and
lowest in dispersion. Single mode fiber was developed in 1984. Single mode fiber has much
lower fiber dispersion and lower loss. Therefore, used in long distances and transoceanic
communication. Typical data rate is 1.7 Gbps with repeater spacing about 50 km
– λ = 1550nm single mode. 1550 nm corresponds to the lowest loss window for silica and can
operate at 10Gbps for repeater distance of 100 km
The second generation (1997-2000)
• 2nd generation is no longer point to point pipeline, it is then
optical network. Light is no longer dumb carrier of data.
Optic amplification was realized using optic amplifier e.g.
EDFA (Erbium Doped Fiber Amplifier) which revolutionized
fiber optic systems
-Eliminated optic-electric-optic conversion and replaced
regeneration by all optic in line amplifiers
-Pushed the application of wavelength division multiplexing
(WDM). Because EDFA can directly and simultaneously
amplify 40 nm bandwidth to 37 dB gain.
• EDFA and WDM is mainstream of today’s high speed fiber
optic. Thus, reduced cost, easier for maintaining and greatly
expanded capacity. Aggregate data rate distance product
up to 2.56 Tbps-Km is currently realizd.
The third generation (evolving)
• The new generation of optic communication
network is still evolving. Not only long haul
optic communication, the fiber to home
(FTTH) and local networks will be controlled
optically.
• Nowadays, long distance (> 100km) and
transoceanic communication are dominated
by fiber optics.