26-06-2012, 11:34 AM
Principles of Optical Fibers
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INTRODUCTION
The idea of using light to send messages has been developed since the eighth
century B.C., when the Greeks used fire signals for sending alarms or calls for help[1].
Thus before optical fibers was developed such techniques used for communication
needed to be prearranged between both the sender and receiver[2]. It was only in the mid
1960s did Charles K. Kao determined that glass had a loss of 20db/km, which spurred
researchers into exploring methods for making glass more pure[3]. This discovery sparked
a revolution in the telecommunication industry as a new industry of processing optical
fibers becomes commercially important.
LIGHT THEORY
When light is directed into an optical fiber the effectiveness of the wire depends
on its ability to guide the light ray far distances with little scattering or absorption of the
light as possible. Doing so means that the optical fiber must exhibit total internal
reflection within the wire. Thus when considering the propagation of light for an optical
fiber the refractive index of the dielectric medium needs to be accounted for. As light
rays become incident on an interface between two dielectrics with different index of
refractions, refraction occurs between the two mediums.
THE OPTICAL FIBER
The typical fibers today are made out of glass or plastic since it is possible to
make them thin and long. Also both glass and plastic are transparent at particular
wavelengths, which allow the fibber to guide light efficiently [7].
The fiber is constructed with a core with a high index surrounded by a layer of
cladding at a lower index. The core and the cladding can be made out of both plastic and
glass.
FIBER CONSTRUCTION (FLAME HYDROLYSIS)
There are many different variations of vapor phase deposition that have been used
to produce low loss optical fibers. In general such vapor phase techniques used today fall
into two categories: Flame hydrolysis and Chemical Vapor Deposition (CVD), however
this paper will only focus on Flame hydrolysis techniques such as Vapor Axial
Deposition (VAD) and Outside Vapor Phase Oxidation (OVPO).