Fiber Optics Communication System -- A Brief
Fiber optic communication which was invented in the early 1970s
paved way for the telecommunication systems. The phenomenon
of transmitting images, data and voice through the passage of
light via thin transparent fibers is known as the fiber optics
Compared with electrical cables, fiber-optic cables are very
Fiber-optic cables are immune to problems that arise with
electrical cables, such as ground loops or electromagnetic
interference (EMI). Such issues are important, for example, for
data links in industrial environments.
Mostly due to their very high data transmission capacity, fiberoptic transmission systems can achieve a much lower cost than
systems based on coaxial copper cables, if high data rates are
needed. For low data rates, where their full transmission capacity
cannot be utilized, fiber-optic systems may have less of an
economic advantage, or may even be more expensive (not due to
the fibers, but the additional transceivers). The primary reason,
however, for the still widespread use of copper cables for the “last
mile” (the connection to the homes and offices) is simply that
copper cables are already laid out, whereas new digging
operations would be required to lay down additional fiber cables.
Fiber communications are already extensively used within
metropolitan areas (metro fiber links), and even fiber to the
home (FTTH) spreads more and more – particularly in Japan,
where private Internet users can already obtain affordable
Internet connections with data rates of 100 Mbit/s – well above
the performance of current ADSL systems, which use electrical
telephone lines. In other countries, one often tries to squeeze out
higher transmission capacities from existing copper cables, e.g.
with the technique of vectoring, in order to avoid the cost of laying
down fiber cables to the premises. This, however, is more and
more seen only as a temporary solution, which cannot satisfy
further growth of bandwidth demand.
Fiber-based systems have largely replaced radio transmitter
systems for long-haul optical data transmission. They are widely
used for telephony, but also for Internet traffic, long highspeedlocal area networks (LANs), cable TV (CATV), and
increasingly also for shorter distances within buildings. In most
cases, silica fibers are used, except for very short distances, where
plastic optical fibers can be advantageous.
Now let us try to explore what is Fiber Optics Technology
In earlier centuries, when someone wanted to convey some
important news to another person who was miles away, it was not
that easy. A lot of traditional methods were employed to deliver
the message. But, in this contemporary period, much
advancement like mobile communication, communication
through mail, satellite, and even more has made exchange of
information easier and quite simpler.
John Tyndall in 1854 experimentally proved that the light rays
could evenly pass through a curved stream of water. Thus, he tried
and succeeded in proving that the light signal can be bent.
Further advancement was made in 1880 by Alexander Graham
Bell. He invented his photophone that year which transmitted a
voice signal on a single beam of light. The basic mechanism
behind his invention was focusing solar energy over a mirror and
he then talked over a mechanism that made the mirror to vibrate.
A detector was placed at the receiving end that reproduced the
same voice received by the receiver. Any bad weather like a cloudy
day can easily interfere with the working of this invention.
John Logie Baird
It was in 1902 an English man, John Logie Baird, along with an
American, Clarence.W.Hansell, patented their unique innovation.
They worked on using sequences of rods that were transparent
that could easily transmit images for televisions.
It was a medical student Heinrich Lamm, who first assembled a
bundle of optic fibers in order to carry an image; it was in the year
1930. He successfully reported transmitting the image of a light
bulb. But then, the image he made was of a very low quality. His
patent for this innovation was denied.
Since there was no efficient communication, people were not able
to hear about the research work of others in nearby countries. In
1954, two scientists wrote papers on imaging bundles individually.
The two were a Dutch scientist, Abraham Van Heel and a British
scientist, Harold.H.Hopkins. The only difference between their
papers was that Hopkins reported on bundles of unclad fibers
whereas Abraham reported on a bundle of clad fibers. He tried
covering the fibers with a transparent lower refractive index
material. This helped a lot in reducing the interference between
the fibers by restricting the external distortion.
Abraham Van Heel and Harold.H.Hopkins
A theoretical description on single mode fibers was published in
1961 by Elias Snitzer who belonged to the American optical. He
tried proving that a small core fiber can carry only a single waveguide mode. His set up had a disadvantage that it had a light loss
of about one decibel per kilometer.
In the year of 1970, a good team of scientists started
experimenting with the usage of silica. They chose silica as it had
a high melting point with good purity and also a comparatively
lower refractive index. Patent number #3,711,262 marks the
invention of fiber optics wire by corning glass researchers, Robert
Maurer, Donald Keck and Peter Schultz. These optical wave guide
fibers were more advantageous. They carried 65,000 times more
information than the randomly employed copper wire. Even miles
apart, the information can be received with a very high rate of
efficiency. The information was decoded with a particular pattern
of light waves. This team concentrated more on solving the
Robert Maurer, Donald Keck and Peter Schultz
problems with fiber optics that were spotted by Dr.C.K.Kao.
The application of fiber optics extends to a wider range. In the
year of 1975, the government of the United States of America
planned of linking the computers that were present in the NORAD
headquarters using fiber optics techniques. They also executed the
plan hoping that will help in reducing the interference.
Under the downtown of Chicago, fiber optic communication was
laid for about 1.5 miles in the year of 1977. Here, every single
optical fiber carried the equivalent of about 672 voice channels.
At present, fiber optic communication is used across the world in
every nook and corner. As much as about eighty percentage of the
long distance traffic is being carried out with the optical fibers
that were designed by the three people, Maurer, Keck and Schultz.
The capacity of fibers for data transmission is huge: a single silica
fiber can carry hundreds of thousands of telephone channels,
utilizing only a small part of the theoretical capacity. In the last
30 years, the progress concerning transmission capacities of fiber
links has been significantly faster than e.g. the progress in the
speed or storage capacity of computers.
The losses for light propagating in fibers are amazingly small: ≈
0.2 dB/km for modern single-mode silica fibers, so that many
tens of kilometers can be bridged without amplifying the signals.
A large number of channels can be reamplified in a single fiber
amplifier, if required for very large transmission distances.
Due to the huge transmission rate achievable, the cost per
transported bit can be extremely low.
− Telecom Windows
Optical fiber communications typically operate in a wavelength
region corresponding to one of the following “telecom windows”:
The first window at 800–900 nm was originally used.
GaAs/AlGaAs-based laser diodes and light-emitting
diodes (LEDs) served as transmitters, and
silicon photodiodes were suitable for the receivers. However, the
fiber losses are relatively high in this region, and fiber
amplifiers are not well developed for this spectral region.
Therefore, the first telecom window is suitable only for shortdistance transmission.
The second telecom window utilizes wavelengths around 1.3 μm,
where the loss of silica fibers is much lower and the
fibers' chromatic dispersion is very weak, so that dispersive
broadening is minimized. This window was originally used for
long-haul transmission. However, fiber amplifiers for 1.3 μm
(based on, e.g. on praseodymium-doped glass) are not as good as
their 1.5-μm counterparts based on erbium. Also, low dispersion
is not necessarily ideal for long-haul transmission, as it can
increase the effect of optical nonlinearities.
The third telecom window, which is now very widely used, utilizes
wavelengths around 1.5 μm. The losses of silica fibers are lowest
in this region, and erbium-doped fiber amplifiers are available
which offer very high performance. Fiber dispersion is usually
anomalous but can be tailored with great flexibility (→ dispersionshifted fibers).
The second and third telecom windows are further subdivided
into the following wavelength bands:
O band original
E band extended
S band short wavelengths
C band conventional (“erbium window”) 1530–1565 nm
L band long wavelengths
U band ultralong wavelengths
The second and third telecom windows were originally separated
by a pronounced loss peak around 1.4 μm, but they can effectively
be joined with advanced fibers with low OH content which do not
exhibit this peak.
Part IV -- Fiber Optics System Design
The simplest type of fiber-optic communication system is a fiberoptic link providing a point-to-point connection with a single data
channel. Such a link essentially contains a transmitter for sending
the information optically, a transmission fiber for transmitting the
light over some distance, and a receiver. The transmission fiber
may be equipped with additional components such as fiber
amplifiers for regenerating the optical power ordispersion
compensators for counteracting the effects of chromatic
dispersion. The article on fiber-optic links gives more details.
A typical channel capacity for long-haul transmission is nowadays
2.5 or 10 Gbit/s; 40, 100 or even 160 Gbit/s may be used in the
future. More advanced systems increase the transmission capacity
by simultaneously using several, dozens or even hundreds of
different wavelength channels (coarse or dense wavelength
division multiplexing). The main challenges are to suppress
channel cross-talk via nonlinearities, to balance the channel
powers (e.g. with gain-flattened fiber amplifiers), and to simplify
the systems. Another approach is time division multiplexing,
where several input channels are combined by nesting in the time
domain, and solitons are often used to ensure that the
sent ultrashort pulses stay cleanly separated even at small pulseto-pulse spacings.
Another important development is that of systems which link
many different stations with a sophisticated fiber-optic network.
This approach can be very flexible and powerful, but also raises a
number of non-trivial technical issues, such as the need for
adding or dropping wavelength channels, ideally in a fully
reconfigurable manner, or to constantly readjust the connection
topology so as to obtain optimum performance, or to properly
handle faults so as to minimize their impact on the overall system
performance. As many different concepts (e.g. concerning
topologies, modulation formats, dispersion management,
nonlinear management, and software) and new types of devices
(senders, receivers, fibers, fiber components, electronic circuits)
are constantly being developed, it is not clear so far which kind of
system will dominate the future of optical fiber communications.
For a discussion of aspects such as bit error rates and power
penalties, see the article on optical data transmission.
− Transmission Capacity of Optical Fibers
Within the last 30 years, the transmission capacity of optical
fibers has been increased enormously. The rise in available
transmission bandwidthper fiber is even significantly faster than
e.g. the increase in storage capacity of electronic memory chips, or
in the increase in computation power of microprocessors.
The transmission capacity of a fiber depends on the fiber length.
The longer a fiber is, the more detrimental certain effects such
intermodal orchromatic dispersion are, and the lower is the
achievable transmission rate.
For short distances of a few hundred meters or less (e.g. within
storage area networks), it is often more convenient to
utilize multimode fibers, as these are cheaper to install (for
example, due to their large core areas, they are easier to splice).
Depending on the transmitter technology and fiber length, they
achieve data rates between a few hundred Mbit/s and ≈ 10 Gbit/s.
Single-mode fibers are typically used for longer distances of a few
kilometers or more. Current commercial telecom systems typically
transmit 2.5 or 10 Gbit/s per data channel over distances of ten
kilometers or more. Future systems may use higher data rates per
channel of 40 or even 160 Gbit/s, but currently the required total
capacity is usually obtained by transmitting many channels with
slightly different wavelengths through fibers; this is
called wavelength division multiplexing (WDM). Total data rates
can be several terabits per second, sufficient for transmitting
many millions of telephone channels simultaneously. Even this
capacity does not reach by far the physical limit of an optical fiber.
In addition, note that a fiber-optic cable can contain multiple
In conclusion, there should be no concern that technical
limitations to fiber-optic data transmission could become severe
in the foreseeable future. On the contrary, the fact that data
transmission capacities can evolve faster than e.g. data storage
and computational power, has inspired some people to predict
that any transmission limitations will soon become obsolete, and
large computation and storage facilities within high-capacity data
networks will be extensively used, in a similar way as it has
become common to use electrical power from many power
stations within a large power grid. Such developments may be
more severely limited by software and security issues than by the
limitations of data transmission.
− Key Components for Optical Fiber Communications
Optical fiber communication systems rely on a number of key
optical transmitters, based mostly on semiconductor
lasers (often VCSELs), fiber lasers, and optical modulators
optical receivers, mostly based on photodiodes (often avalanche
optical fibers with optimized properties concerning losses, guiding
properties, dispersion, and nonlinearities
semiconductor and fiber amplifiers (mostly erbium-doped fiber
amplifiers, sometimes Raman amplifiers) for maintaining
sufficient signal powers over long lengths of fibers, or as
preamplifiers before signal detection
optical filters (e.g. based on fiber Bragg gratings) and couplers
optical switches and multiplexers (e.g. based on arrayed
waveguide gratings); for example, optical add/drop multiplexers
(OADMs) allow wavelength channels to be added or dropped in
a WDM system
electrically controlled optical switches
devices for signal regeneration (electronic or optical
regenerators), clock recovery and the like
various kinds of electronics e.g. for signal processing and
computers and software to control the system operation
In many cases, optical and electronic components for fiber
communications are combined on photonic integrated circuits.
Further progress in this technological area will help optical fiber
communications to be extended to private households (→ fiber to
the home) and small offices.