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ABSTRACT Communication is an important part of our daily life. The communication process involves information generation, transmission, reception and interpretation. As needs for various types of communication such as voice, images, video and data communications increase demands for large transmission capacity also increase. This need for large capacity has driven the rapid development of light wave technology; a worldwide industry has developed. An optical or light wave communication system is a system that uses light waves as the carrier for transmission. An optical communication system mainly involves three parts. Transmitter, receiver and channel. In optical communication transmitters are light sources, receivers are light detectors and the channels are optical fibers. In optical communication the channel i.e, optical fibers play an important role because it carries the data from transmitter to the receiver. Hence, here we shall discuss mainly about optical fibers.

Optical Fibers in Communications Page 2 1. Introduction Optical fibers are arguably one of the world’s most influential scientific developments from the latter half of the 20th century. Normally we are unaware that we are using them, although many of us do frequently. The majority of telephone calls and internet traffic at some stage in their journey will be transmitted along an optical fiber. Why has the development of fibers been given so much attention by the scientific community when we have alternatives? The main reason is bandwidth – fibers can carry an extremely large amount of information. More indirectly, many of the systems that we either rely on or enjoy in everyday life such as banks, television and newspapers as (to name only a very limited selection) are themselves dependent on communication systems that are dependent on optical fibers.

Page 3 2. Fundamentals of Fibers The fundamental principle that makes optical fibers possible is total internal reflection. From Snell’s Law we find that refraction (as shown by the dashed line) can only occur when the angle theta1 is large enough. This implies that as the angle is reduced, there must be a point when the light ray is reflected, where theta1 = theta2. The angle where this happens is known as the critical angle and is:

 When an ray of light travels from a denser to a rarer medium such that the angle of incidence is greater than the critical angle, the ray reflects back into the same medium this phenomena is called TIR.  In the optical fiber the rays undergo repeated total number of reflections until it emerges out of the other end of the fiber, even if fiber is bend.

Page 4 Acceptance Angle: Acceptance Angle is the maximum angle with the axis of the optical fiber ,at which light may enter the fiber, in order to be propagated through it.

Numerical Aperture: Numerical aperture is considered as a light gathering capacity of an optical fiber. NA is defined as the Sine of Half of the angle Of fiber’s light acceptance cone.

Page 5 V Number / Normalized Frequency Definition: a normalized frequency parameter, which determines the number of modes of a fiber.

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If V is less than 2.405 then the fiber is single mode

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If V is greater than 2.405 then fiber is multimode.

V number is also related with the number of modes is the fiber as: N = V2/ 2 for step index fiber. Number of modes for graded index fiber is N = V2/ 4

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CLASSIFICATION OF OPTICAL FIBERS NOW WE ARE GOING TO DISCUSS ABOUT THE CLASSIFICATION OF OPTICAL FIBERS BASED ON THEIR:1. MATERIAL USED 2. NUMBER OF MODES AND 3. REFRACTIVE INDEX A. GLASS FIBERS: THEY HAVE A GLASS CORE AND GLASS CLADDING. THE GLASS USED IN THE FIBER IS ULTRA PURE, ULTRA TRANSPARENT SILICON DIOXIDE (SIO2) OR FUSED QUARTZ. IMPURITIES ARE PURPOSELY ADDED TO PURE GLASS TO ACHIEVE THE DESIRED REFRACTIVE INDEX. B. PLASTIC CLAD SILICA: THIS FIBER HAS A GLASS CORE AND PLASTIC CLADDING. THIS PERFORMANCE THOUGH NOT AS GOOD AS ALL GLASS FIBERS, IS QUITE RESPECTABLE. C. PLASTIC FIBERS: THEY HAVE A PLASTIC CORE AND PLASTIC CLADDING. THESE FIBERS ARE ATTRACTIVE IN APPLICATIONS WHERE HIGH BANDWIDTH AND LOW LOSS ARE NOT A CONCERN.

Page 7 Based On Number Of Modes Single Mode Fiber :  In single mode fiber only one mode can propagate through the fiber.  It has small core diameter (5um) and high cladding diameter (70um).  Difference between the refractive index of core and cladding is very small.  There is neither dispersion nor degradation therefore it is suitable for long distance communication.  The light is passed through the single mode fiber through laser diode.

Multi Mode Fiber :  It allows a large number of modes for light ray travelling through it.  The core diameter is 40um-100um and that of cladding is 70-125um.  The relative refractive index difference is also large than single mode fiber.  There is signal degradation due to multimode dispersion.  It is not suitable for long distance communication due to large dispersion and attenuation of signal.

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13. References 

Optical Fibers And Sources For Communications ---Adams and Henning,

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Principles Of Modern Optical Systems --- Andonovic and Uttamchandani

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An Introduction to Optical Waveguides ---Adams, M. J.