28-06-2012, 12:51 PM
Tutorial
Tutorial.pdf (Size: 170.21 KB / Downloads: 118)
Fiber Basics
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.
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
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.
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.
Tutorial.pdf (Size: 170.21 KB / Downloads: 118)
Fiber Basics
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.
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
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.
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.