This document discusses different types of optical fibers. It describes single mode fibers as having a small diameter that supports only one propagation mode, while multimode fibers have a larger core diameter supporting multiple modes. Index profiles can be step index, where the core and cladding have uniform but different refractive indices, or graded index, where the core index decreases from the center outward. Single mode fibers typically have a step index profile, while multimode fibers can be either step or graded index. The document provides illustrations and explanations of step index and graded index fiber structures and their light propagation characteristics.

1. STEP INDEX
&
GRADED INDEX

2. WHAT ARE
OPTICAL FIBRES ?

3. Optical fibre (or "fibre
optic") refers to the
medium and the
technology associated with
the transmission of
information as light pulses
along a glass or plastic
strand or fibre.

6. TYPES OF OPTICAL FIBRE
Optical fibres are generally of
two types based upon the size
of the core and cladding:
• Single mode fibre(SMF)
• Multimode fibre(MMF)

7. 1. SINGLE MODE FIBRE
A Single Mode Fibre has a smaller diameter
and can support only one mode of propagation.
2. MULTIMODE FIBRE
A Multimode Fibre has a larger core diameter
and supports a number of modes.

8. INDEX PROFILE
Single mode fibres and Multimode fibres
can be distinguished on the basis of
index profile.
An index profile is a plot of refractive
index drawn on horizontal axis versus
the distance from the core axis drawn on
the vertical axis.

9. TYPES OF INDEXPROFILES
There are two types of index profiles:
1.STEP INDEX PROFILE(SI)
2.GRADED INDEX PROFILE(GRIN)
The index profile of a MMF can either be
step index or graded index type whereas
the index profile of SMF is usually a step
index type.

10. STEP-INDEX PROFILE
In this type of optical fibre the fibre core has a
uniform refractive index nf and cladding also has
uniform refractive index nc such that nf > nc . The
figure below represents the step-index optical fibre
where a and b are the radii of the core and cladding,
respectively. nf (fibre
core)nc
(cladding)
Air Air
n
b
a
O

11. The following figure shows the path of rays:
2 1
2
The rays entering at different angles, travel
different paths and emerge out the fibre at
different time along different directions. Two rays
are shown in figure. Clearly, an input pulse
widens as it travels along the fibre.
1

13. nf (fibre
core)nc
(cladding)
Air Air
b
n
a
O
Graded-index profile
In graded-index optical fibre the core has non-
uniform refractive index which increases from axis
towards the cladding. The cladding has a uniform
refractive index.

14. The path of rays is shown in the following
figure:
In this type, the entering ray is continuously bent
and travels a periodic path along the axis. The rays
entering at different angles follow different paths
with same period in space and time. Thus there is a
periodic self-focusing of the rays. In this type of
fibre the pulse-dispersion is less than that in step-
index fibre.
nc
n
f

16. SINGLE MODE STEP INDEXFIBRE
A single mode step index fibre has a very fine
thin core of uniform refractive index of a higher
value which is surrounded by a cladding of
lower refractive index. The refractive index
changes abruptly at the core-cladding
boundary, because of which it is known as step
index fibre.n1
n2 n2
n1
n2
n1
(a) (b) (c)
(a) Refractive index profile (b) Cross-section (c) Side view
showing mode of

17. The fibre is surrounded by an opaque
protective sheath. A typical SMF has a
core diameter of 4 µm, which is of the
order of a few wavelength of light.
Light travels in SMF along a single path
that is along the axis. Obviously, it is
zero order mode that is supported by a
SMF. A SMF is characterised by a very
small value of ∆. It is of the order of
0.002.
CONTINUED…

18. MULTIMODE STEP INDEXFIBRE
It is similar to the single mode step index
fibre but it has larger core diameter of
100µm which is very large as compared
to the wavelength of the light being
transmitted. Light follows the zigzag
path inside the fibre. Many such zigzag
paths of propagation are permitted in a
MMF. A typical structure and index
profile of a step index MMF are as shown:

19. n1
n2 n2
n1
n2
n1
(a) (b) (c)
(a) Refractive index profile (b) Cross-section (c) Side view
showing mode of
propagation
The NA of a MMF is larger as the core diameter of the f
is larger. It is of the order of 0.3 .
θ
CONTINUED…

20. n1n2
r
2a
n2
n(r)n
Cladding
2a
Cor
e
(a) (b) (c)
MULTIMODE GRADED INDEXFIBRE
In this type of fibre, the refractive index of
the core varies with distance form the fibre
axis. It has a high value at the centre and
falls off with increasing radial distance from
the axis. A typical structure and its index
profile are shown in figures:
(a) Refractive index profile (b) Cross-section (c) Side view
showing typical ray paths along a GRIN fibre.

21. Such a profile causes a periodic focussing of
the light propagating through the fibre. In
case of GRIN fibres, the acceptance angle and
numerical aperture decreases with radial
distance from the axis. For the fibres of the
parabolic index-profile, the numerical
aperture is given by:
NA = n1(2∆)1/2 √1-(r/a)2
CONTINUED…

22. THANK YOU
By
RITESH GOYAL
16103027