This document provides information on optical fibers, including their history, principles of operation, types, advantages over metallic wires, and a comparison of step index and graded index fibers. It notes that optical fibers were first described in 1842 and were used for data transmission starting in 1965. The key advantages of optical fibers are their low attenuation over long distances, small size, electromagnetic isolation, high bandwidth, and ability to carry large data rates. Optical fibers are classified as either step index or graded index based on their refractive index profile.

1. H.R.H. PRINCE OF WALES
INSTITUTE OF ENGG. &
TECHNOLOGY
JORHAT,ASSAM

2.  HISTORY
 OPTICAL FIBRE
 ADVANTAGES OF OPTICAL FIBRE OVER
METALLIC WIRES
 PRICIPLE OF OPERATION
 CLASSIFICATION OF OPTICAL FIBRES AND
THEIR ADVANTAGES
 COMPARISION OF STEP INDEX AND
GRADED INDEX FIBRES

3.  Daniel Colladon first described this
"light pipe" in an 1842 article titled On the
reflections of a ray of light inside a parabolic
liquid stream. This particular illustration
comes from a later article by Colladon, in
1884.
 The first working fiber-optical
data transmission system was
demonstrated by German physicist
Manfred Börner at Telefunken Research
Labs in 1965.NASA used fiber optics in
the television cameras that were sent to
the moon.

4. Daniel Colladon's Lightfountain or
Lightpipe , LaNature(magazine),1884

5.  An optical fiber is a flexible, transparent
fiber made by silica or plastic to a diameter
slightly thicker than that of a human hair.
Optical fibers are light equivalent of
microwave wavelengths with additional
advantage of a very wide bandwidth.
 Optical fibers typically include a core
surrounded by a transparent cladding material
with a lower index of refraction. Light is kept
in the core by the phenomenon of total
internal reflection which causes the fibre to act
as a waveguide.

6. an optical fiber

9. ADVANTAGES OF OPTICAL FIBRES OVER
METALLIC WIRES:-
An optical fibre has advantages over wire
or radio system and this is why
telecommunication industries have introduced
the fibre optic systems significantly. The
following are the main advantages of optical
fibres:-
i) Attenuation: Attenuation in a fibre is
markedly lower than that of co-axial cable or
twisted pair and is constant over a very wide
range. So, transmission within wide range of
distance is possible without repeaters.

10. ii) Smaller size and lighter weight: Optical fibers
are considerably thinner than co-axial cable or
bundled twisted pair cable. So, they occupy much
less space.
iii) Electromagnetic isolation: Electromagnetic
waves generated from electrical disturbances or
electrical noises do not interfere with light signals.
As a result, the system is not vulnerable to
interference, impulse noise, or cross talk.
iv) No physical electrical isolation is required
between the sender and receiver.
v) As fibers are very good dielectrics, isolation
coating is not required.

11. vi) The fiber is much more reliable, because it can
better withstand environmental conditions such as
pollution, radiation and salt produces no
corrosion. Moreover, it is normally affected by the
nuclear radiation. Its life is longer than in
comparison to copper wire.
vii) Almost there is no cross-talk in optical fibers
and hence transmission is more secure and private,
as it is very difficult to tap into a fiber.
viii) Greater Bandwidth: Bandwidth of the
optical fiber is higher than that of an equivalent
wire transmission line.

12. ix) Data rate is much higher in a fiber and hence
much more information can be carried by each
fiber than by equivalent copper cables.
example:- at the immensely high frequencies of
an optical fiber, data rates of 2Gbps over tens of
kilometer have been demonstrated . Compare this
to the practical maximum of hundreds of Mbps
over 1km for twisted pair.

13.  An optical fiber is a cylindrical dielectric
waveguide (non-conducting waveguide) that
transmits light along its axis, by the process of
total internal reflection. The fiber consists of a
core surrounded by a cladding layer, both of
which are made of dielectric materials. To
confine the optical signal in the core, the
refractive index of the core must be greater
than that of the cladding. The boundary
between the core and cladding may either be
abrupt, in step-index fiber, or gradual, in graded-
index fiber.

14.  TYPES OF FIBERS:-
i) Single mode
ii) Zero dispersion
iii) Dispersion modified
iv) Infrared
v) Rare earth ion doped
vi) Polarization maintaining
vii) Polarization maintaining and absorption
reducing fibres (PANDA).

15.  CLASSIFICATION OF FIBERS
The optical fiber is simply a cylindrical
waveguide system operating at optical
frequencies. It consists of a core at the centre
and a cladding or sheath out the core. The core
is generally a cylindrical dielectric glass with a
refractive index η1 and the cladding is the
second dielectric cover usually of glass with a
refractive index η2. Sometime the cladding is
surrounded by a buffer coating.

16. Variation in the composition of the core material
gives rise to two types optical fiber. They are:-
i)Step index fiber
ii)Graded index fiber
STEP INDEX FIBER:-
The basic structure of stepped index fiber is
shown in below. It has two portions. It’s structure
is something like to two concentric cylinders. Inner
cylinder is called the core. The outer cylinder may
be made of air.
In case of step index fiber, the refractive index
‘η1’ of the core material is uniform throughout and
undergoes an abrupt change in step at the core
cladding boundary.

17. Step index fibers have two types:-
i) Monomode
ii) Multimode

18.  Fibers that support many propagation paths
or transverse modes are called multi-mode
fibers, while those that support a single mode
are called single-mode fibers (SMF). Multi-
mode fibers generally have a wider core
diameter and are used for short-distance
communication links and for applications
where high power must be transmitted. Single-
mode fibers are used for most communication
links longer than 1,000 meters (3,300 ft).

19. In step index monomode fiber, the transit time
dispersion can be solved by making the core
very thin, so that the diameter of the core is of
the same order as the wavelength of the light
wave to be propagated. The chief
characteristics of step index monomode fiber
are:-
i) Very small core diameter.
ii) Low numerical aperture
iii) Low attenuation.
iv) Very high bandwidth.

20. There are disadvantages of monomode fiber.
Inspite of it’s so many good qualities, the use of
very thin cores creates mechanical difficulties
in the manufacture, handling and splicing the
fibers. Hence this type of fiber is very
expensive. This type of fiber is used as under-
sea cables where the expensive is justified by
the high return of earned income.
Advantages of multimode fibers:-
Multimode fibers posses several advantages
as compared to the single mode fibers. These
are:-

21. i)The larger radii of the multimode fibers make
it easier to launch the optical power in to the
fiber.
ii) The second advantage is that light can be
launched in a multimode fiber with the help of
LED source, whereas in single mode launched
by laser diode.

22. GRADED INDEX FIBRE:-
Graded index fibre have intermediate
bandwidth and capacity. It is a less expensive
method of overcoming transit time dispersion.
This fibre has a property of gradually
changing refractive index. The graded index
fibre together with its refractive index changing
profile is given in fig below.
From the fig below it is clear that light
waves with large angles of incidence travel
more path-lengths than those with smaller
angles. But we know that the decrease of
refractive index allows a higher velocity of
light energy propagation.

24. Thus, all waves will reach a given point along
the fibre, at virtually the same time. As a result,
the transit time dispersion is greatly reduced.
This type of light wave propagation is referred
to as graded index multimode propagation.
*Light velocity decreases with increasing
refractive index, for modes near the core centre
the light velocity is less than that near the core
boundary.
In case of a GIF, the refractive index of the
core is made to vary as a function of radial
distance from the centre of the fibre.
*The use of GIF is gradually diminishing.
*GIF may be single(mono) mode and
multimode.

25. COMPARISION OF STEP AND GRADED INDEX
FIBRES:-
1.Bandwidth:- A good quality Step Index
Fibre(SIF) may have a bandwidth of 50MHz km
where as the equivalent Graded Index Fibre(GIF)
can have 200,400 or 600MHz km bandwidth.
2.Attenuation:- GIF tends to be generally lower
attenuation than the SIF.
3.There is an inherent advantage in mode
dispersion for the graded index fibre, compared to
the SIF.
4.Numerical aperture(NA):- NA of GIF is
generally smaller than SIF.