25-06-2012, 12:44 PM
Attenuation in silica-based optical fibers
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Introduction
Figure 1 –The reduction in the attenuation of optical fibers from 1965 to present times.
Data from Kapron [1970], Miya [1979], Kanamori [1986], Nagel [1989], Kato [1999], Tsukitani [2002],
Nagayama [2002].
Silica based optical fibers constitute the backbone of optical communication systems
and carry much of the information that is being transmitted around the world. One of
the properties of optical fibers that make them a preferred medium for data
transmission is their high transparency, making the attenuation, or loss of light small.
A typical attenuation coefficient of optical fibers for transmission is 0.2 dB/km.
Optical fibers are not only used as a transmission medium, but also for components
within an optical communications system. Typically fibers used for components have
attenuation coefficients several times larger than the transmission fibers. Even though
the fiber loss is compensated by the introduction of amplifiers to a system, a low fiber
attenuation is always desired since by reducing the attenuation, the number of
amplifiers in a system can be reduced, thereby reducing the cost and complexity of the
system and also improving the signal to noise ratio.
A common reaction to the title of this work “Attenuation in silica-based optical
fibers” is: Wasn’t that done many years ago? And true, a lot has happened since the
results on the first optical fibers with attenuation coefficients in the order of 1000
dB/km were reported (Figure 1). In 1970 results were reported on fibers having an
attenuation coefficient of approximately 20 dB/km at 632 nm [Kapron et.al 1970].
Optical fibers
Refractive index profiles and cross sections of optical fibers with single-clad and triple-clad
index profiles.
Introduction
The fibers discussed in this work are all silica-based and manufactured using the
Modified Chemical Vapor Deposition (MCVD) process. The light guiding properties
of an optical fiber are determined by the refractive index profile (n®), which for the
fibers presented here is either a single-clad or a triple clad refractive index profile
(Figure 2).
The fiber with a single-clad index profile has a core with a refractive index (n) that is
higher than that of the surrounding cladding (nsilica). The fiber with triple-clad index
profile has a core with a high refractive index surrounded by a trench with a lower
refractive index followed by a ring having a higher index than the cladding. The triple
clad refractive index profile is by some authors described as a dual concentric core
refractive index profile.
Refractive indexes given in this work will be given as refractive index differences
(Δn) with reference to the refractive index of pure silica (nsilica):
Manufacturing of optical fibers.
In the MCVD process, high-purity material is deposited inside a horizontally mounted
rotating silica tube [MacChesney 1974]. The material deposited inside the tube can be
either pure or doped silica. Typical dopants include Fluor (F) that decreases the
refractive index of the silica, Germanium (Ge) that increases the refractive index of
the silica and Phosphor (P) that increases the refractive index and lowers the viscosity
of the glass. The sources of Si, Ge and P are typically the chlorides SiCl4, GeCl4,
POCl3, which at room temperature are liquids having a high vapor pressure.
The material
The optical fibers discussed in this work are made of silica-based glass.
Even though glass has been known through centuries, there exists no universally
accepted definition of a glass. One possible definition is that given by Shelby [2005]
that a glass is: An amorphous solid lacking long-range periodic atomic structure and
exhibiting a region of glass transformation behavior.
If a crystal is being heated above its melting temperature an abrupt change in
enthalpy will be observed. When the melt is cooled below Tm the material will
normally rearrange to obtain crystalline order and the enthalpy will drop to the value
for the crystal. If however crystallization does not occur, a supercooled liquid is
obtained. Upon further cooling, the viscosity of this liquid will increase (fluidity will
decrease) and the structural rearrangements will slow down. At some point, the
viscosity of the liquid will be so high that the atoms can no longer be completely
rearranged to obtain the equilibrium structure, and the enthalpy will thus begin to
deviate from the equilibrium.
Attenuation in fibers with high Δncore
Attenuation of optical fibers as a function of index difference between the core and the
cladding (Δncore). The dots represent measured attenuation; the solid lines the contributions to the
attenuation given by Equation 10 and Equation 14.
Introduction
One of the central subjects of this thesis has been the understanding of the attenuation
of fibers having a high index difference between the core and the cladding (Δncore).
OFS has chosen that some of the results obtained must be kept confidential.
Consequently some of the equations presented in this chapter will only be fully
disclosed and/or discussed in the confidential part of this report. Furthermore, the
relations between layer indexes and layer widths in the refractive index profiles
shown are altered to secure that they are not reproducible. The alterations of index
profiles do not affect the conclusions.