24-12-2012, 04:40 PM
Fiber Basics
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Optical fibers are circular dielectric
waveguides that can transport optical
energy and information. They have a
central core surrounded by a
concentric cladding with slightly
lower (by ≈ 1%) refractive index.
Fibers are typically made of silica
with index-modifying dopants such
as GeO2. A protective coating of one
or two layers of cushioning material
(such as acrylate) is used to reduce
crosstalk between adjacent fibers and
the loss-increasing microbending
that occurs when fibers are pressed
against rough surfaces.
For greater environmental protection,
fibers are commonly incorporated
into cables. Typical cables have a
polyethylene sheath that encases the
fiber within a strength member such
as steel or Kevlar strands.
The Fiber as a Dielectric
Waveguide: Fiber Modes
Since the core has a higher index of
refraction than the cladding, light will
be confined to the core if the angular
condition for total internal
reflectance is met. The fiber geometry
and composition determine the
discrete set of electromagnetic fields
which can propagate in the fiber.
These fields are the fiber’s modes.
There are two broad classifications of
modes: radiation modes and guided
modes. Radiation modes carry energy
out of the core; the energy is quickly
dissipated. Guided modes are
confined to the core, and propagate
energy along the fiber.
Bandwidth Limitations
Bandwidth of an optical fiber
determines the amount of
information that can be supported, in
other words, the data rate. The
mechanism that limits a fiber’s
bandwidth is known as dispersion.
Dispersion is the spreading of the
optical pulses as they travel down the
fiber. The result is that pulses then
begin to spread into one another and
the symbols become
indistinguishable. There are two main
categories of dispersion, intermodal
and intramodal.
Intermodal Dispersion
As its name implies, intermodal
dispersion is a phenomenon between
different modes in an optical fiber.
Therefore this category of dispersion
only applies to mulitmode fiber. Since
all the different propagating modes
have different group velocities, the
time it takes each mode to travel a
fixed distance is also different.
Therefore as an optical pulse travels
down a multimode fiber, the pulses
begin to spread, until they eventually
spread into one another. This effect
limits both the bandwidth of
multimode fiber as well as the
distance it can transport data.
Intramodal dispersion, sometimes
called material dispersion, is a
category of dispersion that occurs
within a single-mode. This dispersion
mechanism is a result of material
properties of optical fiber and applies
to both single-mode and multimode
fibers. There are two distinct types of
intramodal dispersion: chromatic
dispersion and polarization mode
dispersion.
Chromatic Dispersion. In silica, the
index of refraction is dependent upon
wavelength. Therefore different
wavelengths will travel down an
optical fiber at different velocities.
Attenuation
Light power propagating in a fiber
decays exponentially with length due
to absorption and scattering losses.
Attenuation is the single most
important factor determining the cost
of fiber optic telecommunication
systems as it determines spacing of
repeaters needed to maintain
acceptable signal levels.
In the near infrared and visible
regions, the small absorption losses of
pure silica are due to tails of
absorption bands in the far infrared
and ultraviolet. Impurities—notably
water in the form of hydroxyl ions—
are much more dominant causes of
absorption in commercial fibers.
Recent improvements in fiber purity
have reduced attenuation losses.
State-of-the-art systems can have
attenuation on the order of 0.1 dB/km.
Scattering can couple energy from
guided to radiation modes, causing
loss of energy from the fiber. There are
unavoidable Rayleigh scattering losses
from small scale index fluctuations
frozen into the fiber when it solidifies.
This produces attenuation
proportional to l/λ4. Irregularities in
core diameter and geometry or
changes in fiber axis direction also
cause scattering.
Numerical Aperture (NA)
The Numerical Aperture (NA) of a fiber
is defined as the sine of the largest
angle an incident ray can have for total
internal reflectance in the core. Rays
launched outside the angle specified
by a fiber’s NA will excite radiation
modes of the fiber. A higher core
index, with respect to the cladding,
means larger NA. The trade-offs
involved in increasing NA include
higher scattering loss from greater
concentrations of dopant. A fiber’s NA
can be determined by measuring the
divergence angle of the light cone it
emits when all its modes are excited.
Fiber Termination
End surface quality is one of the most
important factors affecting fiber
connector and splice losses. Quality
endfaces can be obtained by polishing
or by cleaving. Polishing is employed
in connector terminations when the
fiber is secured in a ferrule by epoxy.
The following describes the popular
connectors and their endface
preparation styles.
Fiber Optic Connector Types
A detailed component list is also available
for individual projects.
SMA—Due to its stainless steel
structure and low-precision, threaded
fiber locking mechanism, this
connector is used mainly in
applications requiring the coupling of
high-power laser beams into largecore,
multimode fibers. Typical
applications include laser beam
delivery systems in medical, bio-med,
and industrial applications. The
typical insertion loss of an SMA
connector is greater than 1 dB.