TYPES OF FIBER OPTICS.pdf

Ankit

1. PRESENTED BY -ANKIT KUMAR
BRANCH : -ECE
UNI. ROLL -12500319018

2. Introduction
Fibers are classified according to the number of modes that they can propagate. Single mode fibers
can propagate only the fundamental mode. Multimode fibers can propagate hundreds of modes.
However, the classification of an optical fiber depends on more than the number of modes that a fiber
can propagate. An optical fiber's refractive index profile and core size further distinguish single mode
and multimode fibers. The refractive index profile describes the value of refractive index as a function
of radial distance at any fiber diameter.
OBJECTIVE
After reading this unit, you should be able to understand:
Multimode and single mode step-index and graded-index fibers.
Identify the two basic types of single mode step-index fibers.
Outdoor cable and Indoor cable applications.

3. FIBER TYPES –SINGLE MODE AND MULTI-MODE
The refractive Index profile describes the relation between the indices of the core and cladding. Two
main relationships exist:
(I) Step Index
(II) Graded Index
The step index FIBER has a core with uniform index throughout. The profile shows a sharp step at the
junction of the core and cladding. In contrast, the graded index has a non-uniform core. The Index is
highest at the center and gradually decreases until it matches with that of the cladding. There is no
sharp break in indices between the core and the cladding. By this classification there are three types
of FIBERs :
(I) Multimode Step Index FIBER (Step Index FIBER)
(II) Multimode graded Index FIBER (Graded Index FIBER)
(III) Single-Mode StepIndex fibre (Single Mode Fibre)

4. STEP-INDEX MULTIMODE FIBER
Step Index multimode Fiber has a large core, up to 100 microns in diameter. As a result, some of the
light rays that make up the digital pulse may travel a direct route, whereas others zigzag as they
bounce off the cladding. These alternative pathways cause the different groupings of light rays,
referred to as modes, to arrive separately at a receiving point. The pulse, an aggregate of different
modes, begins to spread out, losing its well-defined shape. The need to leave spacing between pulses
to prevent overlapping limits bandwidth that is, the amount of information that can be sent.
Consequently, this type of fiber is best suited for transmission over short distances, in an endoscope,
for instance.
Fig : 1 STEP-INDEX MULTIMODE FIBER

5. GRADED-INDEX MULTIMODE FIBER
It contains a core in which the refractive index diminishes gradually from the center axis
out toward the cladding. The higher refractive index at the center makes the light rays
moving down the axis advance more slowly than those near the cladding.
Fig : 2GRADED-INDEX MULTIMODE FIBER
Also, rather than zigzagging off the cladding, light in the core curves helically because
of the graded index, reducing its travel distance. The shortened path and the higher
speed allow light at the periphery to arrive at a receiver at about the same time as the
slow but straight rays in the core axis. The result: a digital pulse suffers less dispersion.

6. SINGLE-MODE FIBER
It has a narrow core (nine microns or less), and the index of refraction between the core
and the cladding changes less than it does for multimode fibers. Light thus travels
parallel to the axis, creating little pulse dispersion. Telephone and cable television
networks install millions of kilometers of this fiber every year.
Fig : 3 SINGLE-MODE FIBER

7. References and Suggested Further Readings
ITU-T manual on OF installation.
EI of BSNL.
EI on underground OF cable laying works by BBNL.
Fiber Optics Technician's Manual.
Understanding optical communication by Dutton.
Planning Fiber Optic Networks by Bob Chomycz.
www.timbercon.com
http://www.ofsoptics.com
http://www.thefoa.org/
http://www.corning.com
http://www.fiber-optics.info
http://www.rp-photonics.com
http://www.occfiber.com
and other websites