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TitleOptical Fiber Project
TagsOptical Fiber Attenuation Infrared Optics Telecommunication
File Size516.6 KB
Total Pages26
Table of Contents
                            Chapter 5
5.0  Optical fiber

	A bundle of optical fibers                 A TOSLINK fiber optic audio cable being illuminated at one end
	5.1 Optical fiber History

	Index of refraction
	Light scattering
	Termination and splicing
	Free-space coupling
	Fiber fuse
Fiber optic communication
Document Text Contents
Page 1

Chapter 5

5.0 Optical fiber

An optical fiber (or fibre) is a glass or plastic fiber that carries light along its
length. Fiber optics is the overlap of applied science and engineering concerned
with the design and application of optical fibers. Optical fibers are widely used in
fiber-optic communications, which permits transmission over longer distances
and at higher bandwidths (data rates) than other forms of communications.
Fibers are used instead of metal wires because signals travel along them with
less loss, and they are also immune to electromagnetic interference. Fibers are
also used for illumination, and are wrapped in bundles so they can be used to
carry images, thus allowing viewing in tight spaces. Specially designed fibers are
used for a variety of other applications, including sensors and fiber lasers.

Light is kept in the core of the optical fiber by total internal reflection. This causes
the fiber to act as a waveguide. Fibers which support many propagation paths or
transverse modes are called multi-mode fibers (MMF), while those which can
only support a single mode are called single-mode fibers (SMF). Multi-mode
fibers generally have a larger core diameter, and are used for short-distance
communication links and for applications where high power must be transmitted.
Single-mode fibers are used for most communication links longer than
550 meters (1,800 ft).

Joining lengths of optical fiber is more complex than joining electrical wire or
cable. The ends of the fibers must be carefully cleaved, and then spliced together
either mechanically or by fusing them together with an electric arc. Special
connectors are used to make removable connections.

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Tetrahedral structural unit of silica (SiO2)

The amorphous structure of glassy silica (SiO2). No long-range order is present,
however there is local ordering with respect to the tetrahedral arrangement of
oxygen (O) atoms around the silicon (Si) atoms.

Silica exhibits fairly good optical transmission over a wide range of wavelengths.
In the near-infrared (near IR) portion of the spectrum, particularly around 1.5 μm,
silica can have extremely low absorption and scattering losses of the order of 0.2
dB/km. A high transparency in the 1.4-μm region is achieved by maintaining a
low concentration of hydroxyl groups (OH). Alternatively, a high OH
concentration is better for transmission in the ultraviolet (UV) region.

Silica can be drawn into fibers at reasonably high temperatures, and has a fairly
broad glass transformation range. One other advantage is that fusion splicing
and cleaving of silica fibers is relatively effective. Silica fiber also has high
mechanical strength against both pulling and even bending, provided that the
fiber is not too thick and that the surfaces have been well prepared during
processing. Even simple cleaving (breaking) of the ends of the fiber can provide
nicely flat surfaces with acceptable optical quality. Silica is also relatively
chemically inert. In particular, it is not hygroscopic (does not absorb water).

Silica glass can be doped with various materials. One purpose of doping is to
raise the refractive index (e.g. with Germanium dioxide (GeO2) or Aluminum

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oxide (Al2O3)) or to lower it (e.g. with fluorine or Boron trioxide (B2O3)). Doping is
also possible with laser-active ions (for example, rare earth-doped fibers) in order
to obtain active fibers to be used, for example, in fiber amplifiers or laser
applications. Both the fiber core and cladding are typically doped, so that the
entire assembly (core and cladding) is effectively the same compound (e.g. an
aluminosilicate, germanosilicate, phosphosilicate or borosilicate glass).

Particularly for active fibers, pure silica is usually not a very suitable host glass,
because it exhibits a low solubility for rare earth ions. This can lead to quenching
effects due to clustering of doping ions. Aluminum silicates are much more
effective in this respect.

Silica fiber also exhibits a high threshold for optical damage. This property
ensures a low tendency for laser-induced breakdown. This is important for fiber
amplifiers when utilized for the amplification of short pulses.

Because of these properties silica fibers are the material of choice in many
optical applications, such as communications (except for very short distances
with plastic optical fiber), fiber lasers, fiber amplifiers, and fiber-optic sensors.
The large efforts which have been put forth in the development of various types
of silica fibers have further increased the performance of such fibers over other


Fluoride glass is a class of non-oxide optical quality glasses composed of
fluorides of various metals. Due to their low viscosity, it is very difficult to
completely avoid crystallization while processing it through the glass transition (or
drawing the fiber from the melt). Thus, although heavy metal fluoride glasses
(HMFG) exhibit very low optical attenuation, they are not only difficult to
manufacture, but are quite fragile, and have poor resistance to moisture and
other environmental attacks. Their best attribute is that they lack the absorption
band associated with the hydroxyl (OH) group (3200–3600 cm−1), which is
present in nearly all oxide-based glasses.

An example of a heavy metal fluoride glass is the ZBLAN glass group, composed
of zirconium, barium, lanthanum, aluminum, and sodium fluorides. Their main
technological application is as optical waveguides in both planar and fiber form.
They are advantageous especially in the mid-infrared (2000–5000 nm) range.

HMFG's were initially slated for optical fiber applications, because the intrinsic
losses of a mid-IR fiber could in principle be lower than those of silica fibers,
which are transparent only up to about 2 μm. However, such low losses were
never realized in practice, and the fragility and high cost of fluoride fibers made
them less than ideal as primary candidates. Later, the utility of fluoride fibers for
various other applications was discovered. These include mid-IR spectroscopy,
fiber optic sensors, thermometry, and imaging [disambiguation needed]. Also, fluoride fibers

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Other uses of optical fibers

A Frisbee illuminated by fiber optics

Fibers are widely used in illumination applications. They are used as light guides
in medical and other applications where bright light needs to be shone on a
target without a clear line-of-sight path. In some buildings, optical fibers are used
to route sunlight from the roof to other parts of the building (see non-imaging
optics). Optical fiber illumination is also used for decorative applications,
including signs, art, and artificial Christmas trees. Swarovski boutiques use
optical fibers to illuminate their crystal showcases from many different angles
while only employing one light source. Optical fiber is an intrinsic part of the light-
transmitting concrete building product, LiTraCon.

Optical fiber is also used in imaging optics. A coherent bundle of fibers is used,
sometimes along with lenses, for a long, thin imaging device called an
endoscope, which is used to view objects through a small hole. Medical
endoscopes are used for minimally invasive exploratory or surgical procedures
(endoscopy). Industrial endoscopes (see fiberscope or bore scope) are used for
inspecting anything hard to reach, such as jet engine interiors.

In spectroscopy, optical fiber bundles are used to transmit light from a
spectrometer to a substance which cannot be placed inside the spectrometer
itself, in order to analyze its composition. A spectrometer analyzes substances by
bouncing light off of and through them. By using fibers, a spectrometer can be
used to study objects that are too large to fit inside, or gasses, or reactions which
occur in pressure vessels.[17][18][19]

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An optical fiber doped with certain rare earth elements such as erbium can be
used as the gain medium of a laser or optical amplifier. Rare-earth doped optical
fibers can be used to provide signal amplification by splicing a short section of
doped fiber into a regular (undoped) optical fiber line. The doped fiber is optically
pumped with a second laser wavelength that is coupled into the line in addition to
the signal wave. Both wavelengths of light are transmitted through the doped
fiber, which transfers energy from the second pump wavelength to the signal
wave. The process that causes the amplification is stimulated emission.

Optical fibers doped with a wavelength shifter are used to collect scintillation light
in physics experiments.

Optical fiber can be used to supply a low level of power (around one watt) to
electronics situated in a difficult electrical environment. Examples of this are
electronics in high-powered antenna elements and measurement devices used in
high voltage transmission equipment.

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