Wheeler 515 Tutorial paper
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Wheeler 515 Tutorial paper

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Silica Telecommunication Optical Fiber FabricationOPTI 515R(Tutorial Paper)October 29, 2009Sean Wheeler1. IntroductionThis paper provides an overview of the process for fabricating optical fibers from preform generation to fiber drawing. Though there are numerous variations for a number of the steps involved that are more or less application driven, this paper focuses on the process of fabricating silica glass fiber as used in modern wide area optical fiber telecommunications networks. Considerations made with respect to process and materials selection trade-offs and best practices are described as they pertain to this particular application of optical fibers. 2. Types of Fibers2.1 Single Mode/MultimodeA single mode fiber is one that only supports one guided wave mode. In general, no fiber supports only one mode, since the mode depends not only on the materials present in and geometry of the fiber, but also the wavelength of light passing within it. A parameter called the normalized frequency V defined in step index fibers as:V = 2πa(NA)/λshows the relationship. Here, a is the fiber core diameter, NA the numerical aperture of the fiber and λ is the wavelength of light as it is measured external to the fiber. When V is less than or equal to approximately 2.405 the fiber is said to be single mode. For a fixed geometry and numerical aperture, the wavelength that satisfies V = 2.405 is called the cutoff wavelength. All wavelengths introduced ...

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Silica Telecommunication Optical Fiber Fabrication
OPTI 515R
(Tutorial Paper)
October 29, 2009
Sean Wheeler
1. Introduction
This paper provides an overview of the process for fabricating optical fibers from preform
generation to fiber drawing. Though there are numerous variations for a number of the steps involved
that are more or less application driven, this paper focuses on the process of fabricating silica glass
fiber as used in modern wide area optical fiber telecommunications networks. Considerations made
with respect to process and materials selection trade-offs and best practices are described as they
pertain to this particular application of optical fibers.
2. Types of Fibers
2.1 Single Mode/Multimode
A single mode fiber is one that only supports one guided wave mode. In general, no fiber
supports
only
one mode, since the mode depends not only on the materials present in and geometry of
the fiber, but also the wavelength of light passing within it. A parameter called the normalized
frequency
V
defined in step index fibers as:
V = 2πa(NA)/λ
shows the relationship. Here,
a
is the fiber core diameter,
NA
the numerical aperture of the fiber and
λ
is the wavelength of light as it is measured external to the fiber. When
V
is less than or equal to
approximately 2.405 the fiber is said to be single mode. For a fixed geometry and numerical aperture,
the wavelength that satisfies
V
= 2.405 is called the cutoff wavelength. All wavelengths introduced
into the fiber shorter than the cutoff wavelength excite multiple guided modes. Fibers operating in
single mode are less susceptible to certain kinds of dispersion and so are better suited to long distance
applications. It's possible to reduce the effect of modal dispersion in multi-mode fibers by fabricating a
core index profile that is graded (called graded index fiber), but over the long haul it is usually still
better to use single mode.
2.2 Dispersion shifted fibers
Silica fibers have a minimum in dispersion near 1.3 um, but telecommunication fibers are often
made to have attenuation minimized at 1.5 um. It is possible to shift the dispersion minimum (that is,
the zero dispersion point) to coincide with the attenuation minimum by careful engineering of core-clad
index profile[1]
.
Unfortunately, if the fiber is carrying multiple channels (as is the case for wavelength
division multiplexing systems) this can lead to intermodulation and give rise to spurious emissions
within the fiber [2]. In order to avoid this, the index profile is made to shift the dispersion point either
above or below the operational range so that the dispersion is small but nonzero. Fibers that do this are
called nonzero dispersion shifted.
3. Preform Generation Techniques
A preform is an assemblage of glass that shares the same profile as the fiber, but is much larger.
While fibers are typically on the order of 100 um in diameter, preforms are usually several centimeters.
Most fibers are drawn from preforms that are generated through a number of techniques.
3.1 MCVD
Modified chemical vapor deposition (MCVD) was originally developed for fabricating silica
fibers for telecommunications in the 1970s[3]. MCVD offers a high level of control over the eventual
optical properties of the fiber. This is particularly important for producing the desired index profile in
the core and optimizing performance over the desired spectral range (approximately 1.3 um – 1.6 um).
The MCVD process begins by cleaning and etching a hollow fused silica tube to remove surface
contaminants. An historic set of dimensions for the tube is 19 mm inner diameter, 25 mm outer
diameter and upwards of 1 m in length[3]. The tube is then placed on a specialized lathe and spun at a
modest rate about its long axis while heated externally by a torch or furnace to temperatures in excess
of 1800 °C . This heat source is slowly moved in a continuous fashion along the length of the tube.
For fibers used in telecommunications, O
2
gas is bubbled through solutions of SiCl
4
and GeCl
4
. Once
the chemicals reach the heat of the burner, they oxidize and precipitate a sooty substance that is
deposited on the inside of the tube. The soot is then sintered into SiO
2
and GeO
2
(glass) as the torch
slowly passes. This process is repeated numerous times (perhaps 30 or more). Once the desired
amount of material has accumulated, the torch's temperature is increased to 2000 °C, and the tube
collapses creating the preform. Careful control of rotation speed, temperature, and internal pressure of
the tube as well as torch translation speed allow for good preservation of the structure of the core and
cladding during collapse, minimizing the occurrences of bubbles and other defects[3].
Controlling the chemical gas flow is crucial in infusing the fiber with the desired optical
properties. GeCl
4
, which turns into GeO
2
, is used to increase the index of the silica and is ultimately
responsible for allowing guided modes within the fibers. Silica fibers doped with GeO
2
can be made to
have minimum in attenuation losses (less than 1 dB/km) near 1.5 um [3]. Conversely, various fluorine
Illustration 1: Schematic MCVD system*
compounds may be introduced to decrease the index [4]. Different chemical mixtures can be used in
successive layers of sintered glass to allow for very tightly controlled index profiles within the preform.
In general, many chemicals can be used in MCVD to obtain various properties for the fiber, but
some chemicals originally put into use for fiber fabrication should be avoided for our application.
P
2
O
5
can be used to dope fibers to increase the index of refraction and also because it tends to reduce
losses due to Rayleigh scattering, but unless OH levels in the glass are very low, absorption bands
between P and OH show up in the 1.5 um-1.7 um region [3].
3.2 PMCVD
Plasma enhanced MCVD is nearly identical to MCVD except an RF oxygen plasma (3-5 MHz
at around 10,000 °C) is used to increase the efficiency of soot deposition. This is accomplished due to
the high temperature gradient of the plasma which drives a powerful thermophoretic force. Both the
coil and the torch are moved along the length of the tube, but independently so that deposition and
solidification are separated and more finely controlled. Deposition rates with PMCVD are achieved in
excess of 3.5 g/min as to where MCVD peters out around 1 g/min. Another advantage of PMCVD is
more of the preform length is usable for drawing further enhancing overall efficiency. Other plasma
techniques use microwave frequency plasmas and attempt to use the plasma for deposition as well as
solidification but the lower pressures involved tend to limit deposition rate to around 1 g/min [3].
Illustration 2: OH absorption peaks in silica
fibers*
4. Drawing and Coating
The drawing and coating process has an affect on the performance properties of fibers.
Transmission losses, bandwidth, and fiber strength are all influenced by it [3]. Once the preform is
complete it is placed on a drawing tower. The lower end of the preform is heated to melting and
gravity begins its work of drawing the glass downward. As it stretches, the melted glass thins. The
thickness of the fiber is dependent upon the rate of draw. Typically, the faster the fiber is drawn the
narrower the diameter. The profile of the preform is preserved in the fiber. As material leaves the
preform a mechanical feeder ensures a continuous flow of material. Any lag or overpressure would
cause inconsistencies in the flow and therefore diameter of the fiber. Good control of the convection
from the furnace is also important in this regard [3].
Once the fiber leaves the furnace beneath the preform, its thickness is measured by a laser
micrometer. Fiber thickness can be measured to within 0.1 um or less [3]. This is part of a feedback
system that controls the draw rate. If the fiber is measured to be too thin or too thick it slows down or
speeds up the draw rate accordingly. Next the fiber is coated with a UV sensitive epoxy or polymer
followed by UV curing. There may be more than one coating phase depending on the fiber properties
or intended working environment. Improper coating and curing can lead to microbending losses in the
fiber. Fibers with germanium doped cores are somewhat vulnerable to UV exposure and may create
Illustration 3: Fiber
drawing tower schematic*
color centers within the fiber active area leading to degraded performance [3]. Once cured, fibers are
measured again with a laser micrometer to ensure uniform coating has taken place. The coating is
essential in protecting the fiber from abrasive damage or environmental contaminants that can affect
performance. Once cured, the fiber is spooled onto a low tension drum where it awaits QA/QC testing.
5. Conclusions
The process for fabricating optical fiber depends greatly on its ultimate purpose. What is
necessary or acceptable to use in one application may destroy a fiber's usefulness in another. In the
case of long distance telecommunications, parameters like dispersion, attenuation, and other sources of
loss are extremely important considerations. For successful commercialization and widespread use, it
has been important to develop techniques that allowed for a high level of control over fiber
characteristics while having high reproducibility at rapid production rates. MCVD and fiber drawing
offer this and more. Both processes are evolving to meet the needs of fiber optic applications.
References
[1]
Saleh, Bahaa E.A. and Malvin Carl Teich,
Fundamentals of Photonics 2
nd
Edition
, John Wiley
& Sons, Inc., 2007.
[2]
RP Photonics, “Encyclopedia of Laser Physics and Photonics: Four Wave Mixing”,
http://www.rp-photonics.com/four_wave_mixing.html
[3]
Nagel S. R., MacChesney J. B., Walker K. L., "An Overview of the Modified Chemical Vapor
Deposition (MCVD) Process and Performance",
IEEE Journal of Quantum Electronics
, Vol.
QE-18, No. 4, p. 459, April 1982.
[4]
K. Abe, “Fluorine doped silica for optical waveguides~’ in Proc. 2nd European Corrfi Opt.
Fiber Cornmun., Paris, France, 1976, pp. 59-61.
[5]
Mirabito, Michael M.A; and Morgenstern, Barbara L.,
The New Communications Technologies:
Applications, Policy, and Impact
, 5th. Edition. Focal Press, 2004.
*Image licensed through GFDL or has been released to public domain.