Wheeler 515 Tutorial paper

Sean Wheeler

Informations

• Publié par: Viwyung
• Nombre de lectures: 22
• Langue: English

Extrait

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.