Loading

Alison's New App is now available on iOS and Android! Download Now

Study Reminders
Support
Text Version

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

We now start by looking at how communication in general has evolved through the ages.
Take a guess on what was the first kind of communication people did. Was it cell phones?
Obviously not. What else? Before cell phones, what was the method of communication people
had used? Let us try to trace back. Landlines? Before landline telephones how did people
communicate? Telegraph cables? We are talking about the electromagnetic way of communication,
or, optical communication, in that sense.
It turns out that, communication as such started with optical communication – through fire and
beacons. People used to use beacons, mirrors, where they had persons as repeaters. Each
“repeater” would get some light and would reflect that with mirrors. You can do multi-point
communication in this manner. Apparently, that is how all communications started. If you look
at the evolution of the methods of communication, before the 1700s, people used polished bronze,
which would work like mirrors. They would use mirrors, which would be used to direct, re-route
information. People used fire beacons, smoke signals. In fact, people used different coloured
smoke to indicate different information. Of course, we know Graham Bell, who had the first
patent for the telephone. He first actually made a photo-phone, and not the telephone that we know
of today, where he used light and used mirrors to steer the light for communication.
But, the first formal way of communication was done in 1792. It was a French gentleman by
name Claude Chappe actually set up a link across entire France with semaphore signals,
which are the two flags as this figure is indicating, and you can see that the way the flags are held
represents a particular symbol. There was a general understanding of the way the flags were
held to what it represented. This is something like a preliminary version of Morse code, and
these are called as semaphore signals. He linked entire France with this, and by 1700s or
early 1800s, this signalling was existing through France in a full-fledged manner, with 200 relay
stations. So, there were people whose job was to learn these codes and to physically hold these
flags to represent information. We can say this is also an optical communication; it is just that it
is visual. But this is the first record of formal optical communication. Bit rate would be
determined by how fast you can move your flag. Usually, in one second, people could change the
signal once, and hence the bit rate was 1 bits-per-second.

NPTEL-Fiber Optic Communication Technology, Lecture 2 Page 2
But after that, electrical communications took over, because, a line-of-sight would be required
for this method. It would require a large number of relay stations, and so many people trained
and doing all this, and keeping a watch out for the signalling all the time. So, from semaphore
signalling, we moved on to electromagnetic/electrical way of communication and the first
method of electrical way of communication is the telegraph. So, Morse code, which is similar to a
semaphore, was developed, where there are the dots and dashes which represent different
alphabets or symbols. The typical rate was 10 bits per second.
The first Trans-Atlantic telegraph was set up in 1866. Incidentally, the very first time they tried
to establish the link, it was a big failure. It remained as an incomplete challenge for a long time.
On one side of the transatlantic side, there was Lord Kelvin, and on the other side of the Atlantic
Ocean, there was Wildman Whitehouse. Their way of communicating was only through this line.
Imagine, they are sitting on two opposite ends of the Atlantic Ocean, trying to establish a
telegraph line and there is no other way of communication. They are trying to establish a link
from two different continents across the sea, and they are using the telegraph line to try to see
whether they are able to communicate with each other. It turns out that a Lord Kelvin was using
very low voltages, and he was using a mirror galvanometer to detect the presence or absence of a
Morse code, whereas, Whitehouse was using very high voltages, which used to throw the
galvanometer out of scale. It was very difficult because they did not have a way of
communicating and knowing that both of them were operating at two entirely different voltages.
So, there is no way that Lord Kelvin could have communicated what he was doing. It would

NPTEL-Fiber Optic Communication Technology, Lecture 2 Page 3

have taken months to go by ship to convey that information. But then, 1866 was when the Trans-
Atlantic telegraph line was established.

Twisted pair came up in another 10 years, which was a telephone line, for analogue
communication. The speech was converted into an analogue signal with the help of a microphone,
which was transmitted through a copper cable which is the twisted pair. This was a standard way
of communicating for many, many years, but once the number of connecting subscribers
increased, the bandwidths were not sufficient. So, in order to fulfil the demand for increased
bandwidth and increased distances, the twisted pairs got replaced with coaxial cables. These
coaxial cables, even as early as 1940, could provide 3 MHz bandwidth, which is quite a large
number. To put it in perspective, the bandwidth of the current LTE signals is 20 MHz.
One TV channel is equivalent to multiple voice channels, so television channel requires a larger
bandwidth. Coaxial cables were used for all the communication systems, but the problem with
those was the frequency-dependent loss of the cable. The cable acts as a transmission line,
having its own characteristic inductance and capacitance. So, the frequency scaling and
bandwidth scaling was not possible. In order to increase bandwidth, people started using
microwave communication systems. The advantage of microwave communication systems is
that they are suitable for free-space communication links. So, as early as 1948, carrier
frequencies of 1 to 10 GHz were being used, with a speed of 100 Mbps. They had repeaters
which could work at different spatial locations, and transmit the information. Since it is free
space, it requires having a line of sight, and if the receiver is not in the line of sight, or not within
the range of the signal, the signal is lost. So, in order to transmit the signal to longer distances,
repeaters were used.
As early as the 1950s, communication system with electrical/electromagnetic communication was
very well established, but the limitation was bandwidth. How do you scale bandwidth? That is
when optical carriers started being considered. In case of microwave carriers, we talk about
several GHz of frequencies, and you know from your fundamental understanding that the
bandwidth or information capacity increases only when the carrier frequency increases. So,
optical carriers came out to be a very good choice because the carrier frequency is of the order of
THz, and hence could be modulated at very fast rates. So, about 1960 is when we started having
optical communication, and this was feasible because of the invention of lasers.
In terms of optical communication, how did things evolve? The word fibre optics was first
coined by an Indian by name Narinder Singh Kapany. He was a practising doctor in the UK and
he had used optical fibers for a different purpose, not for communication purpose. In fact,
Kapany had first used it for endoscopes, and he actually recorded improved imaging with an
optical fiber. The optical fiber could be inserted through the holes of the body and the imaging of
what is going on inside the body could be done, which is not in the line of sight. So, the concept
of endoscopes existed as early as that, but the optical fibers were not used for communication,
because the optical sources and detectors were not cheap and were unavailable. The attenuation

NPTEL-Fiber Optic Communication Technology, Lecture 2 Page 4
of the optical fiber for longer distances was very high. For short-distance applications like an
endoscope, even a lossy fibre would have worked fine.

The next breakthrough came in 1960, when multiple labs simultaneously demonstrated
the semiconductor laser. Beyond that, the next breakthrough came when people identified what
is that material which will give you low loss when you are trying to transmit light from one point
to the other. Kao and Hockham suggested that glass should be the ideal material. Corning Glass
first made a commercially deployable fiber which came in 1970. The first fiber that was
commercially deployed had a loss of 20 dB/km which means that the signal power decreased by a
factor of 100 after propagating through 1 km of fiber. The commercial link was established in
The US, in the northeast corridor, by 1983. The fabrication procedure needed to be improved so that
long lengths of fiber with uniform performance across the length could be manufactured, and
the process took many years before the first fiber system was deployed.
The next path-breaking technology in progress of optical communication is the invention of
optical amplifiers. In 1986, Desurvire and David Payne simultaneously demonstrated an Erbium-doped fiber amplifier, about which we will learn in the course, and with this amplifiers in place,
which could amplify multiple colours (wavelengths), there was no looking back for optical fiber
communication technology. So we had the fiber which was the very low loss, and to compensate for the
loss, we had the amplifier. Then 1996 was when the first commercial WDM (wavelength
division multiplexing) system was deployed. As early as 1996, we had data rates of 20 Gbps for
long distances. The commercial 10 Gbps standard came in 1996, and this standard remained as
such as late as 5 years ago. We could say that it is the widespread deployment of 10 Gbps
standard has enabled internet technology to reach into very remote areas.

NPTEL-Fiber Optic Communication Technology, Lecture 2 Page 5
Starting 2007 is when the standards changed, from 10 Gbps it became 100 Gbps, and we are
currently talking about 400 Gbps and 1 Tbps standard. While 1 Tbps is not yet a standard,
several service providers have started deploying 400 Gbps. But, that's where we are leading to.
So, by the end of the course, we would understand the technology that drives the 400 Gigabits
per second or the technology that would drive the next-generation optical communication
systems. So, that is about the key developments in optical communication.