Page 34 - Optical Communications Essentials

Optical Communication Systems Overview

24 Chapter Two

Figure 2.2. Illustration of the modified chemical vapor
deposition (MCVD) process.

fiber from mechanical and environmental effects. (Chapter 4 gives the details of
how a fiber guides light.)
The first step in making a fiber is to form a clear glass rod or tube called a
preform. Currently a preform is made by one of several vapor-phase oxidation
processes. In each of these processes, highly pure vapors of metal halides (e.g.,
SiCl 4 and GeCl 4 ) react with oxygen to form a white powder of SiO 2 particles. The
particles are then collected on the surface of a bulk glass (such as the outside of a
rod or the inside of a tube) and are sintered (transformed to a homogeneous glass
mass by heating without melting) to form a clear glass rod or tube. Figure 2.2
shows one such process, which is known as the modified chemical vapor depo-
sition (MCVD) process. Here as the SiO 2 particles are deposited, the tube is
rotated and a torch travels back and forth along the tube to sinter the particles.
Chapter 20 describes this and other fiber fabrication processes.
Depending on how long a fiber is desired, the preform might be a meter long
and several centimeters in diameter. The preform has two distinct regions that
correspond to the core and cladding of the eventual fiber. As illustrated in Fig. 2.3,
fibers are made by precision feeding the preform into a circular furnace. This
process softens the end of the preform to the point where it can be drawn into
a long, very thin filament which becomes the optical fiber.
Prior to 1970 most researchers had tried to purify compound glasses used for
standard optics, which are easy to melt and draw into fibers. A different approach
was taken at the Corning Glass Works where Robert Maurer, Donald Keck, and
Peter Schultz started with fused silica. This material can be made extremely
pure, but has a high melting point. The Corning team made cylindrical preforms
by depositing purified materials from the vapor phase. In September 1970, they
announced the fabrication of single-mode fibers with an attenuation of 17dB/km
at the 633-nm helium-neon line (a loss factor of 50 over 1km). This dramatic
breakthrough was the first among the many developments that opened the door
to fiber optic communications. The ensuing years saw further reductions in
optical fiber attenuation. By the middle of 1972 Maurer, Keck, and Schultz had
made multimode germania-doped fibers with a 4-dB/km loss and much greater
strength than the earlier brittle titania-doped fibers.

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