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This
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article
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was
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written
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by
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Joni
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Nurmi
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,Zhen
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Shi
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and
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Sun
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Hai
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!history-of-fiber-optic.jpg|width=300!
\[1\] Source of the figure: [http://fiberoptic101.blogspot.fi/2010/11/history-of-fiber-optic.html] |
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Principle of operation
Fiber optical cables are highly transparent, cylindrical conduits for light. These cables are formed of two materials with different refractive indices. This creates a optimal waveguide for transmitting light. When light enters the fiber made of material with higher refractive index than the cladding surrounding it, it stays inside the material due to total internal reflection and is thus transmitted forward.
Index of refraction
Index of refraction is a measurement of speed of light in material. By definition, vacuum has refractive index of 1(c in vacuum = 1). The higher the index, the slower the light will travel in the material. Typical value for fiber core is 1,62 and for the cladding 1,52.
Total internal refraction:
When light travelling inside material hits a boundary, it is partially refracted and partially reflected. However if the angle is greater than the critical angle of the materials them it is reflected back in its entirety. For this to occur the material in which the light is travelling in must have higher refractive index than the material outside.
Multi-mode fiber
In optical fiber technology, multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths.
Multi-Mode cable has a little bit bigger diameter, with a common diameters in the 50-to-100 micron range for the light carry component (in the US the most common size is 62.5um). Most applications in which Multi-mode fiber is used, 2 fibers are used (WDM is not normally used on multi-mode fiber). POF is a newer plastic-based cable which promises performance similar to glass cable on very short runs, but at a lower cost.
Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet 914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.
Single-mode fiber
In [optical fiber|http://searchtelecom.techtarget.com/definition/optical-fiber]
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technology, single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission.
Single Mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to 10 microns that has one mode of transmission. Single Mode Fiber with a relatively narrow diameter, through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width.
Single Modem fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fiber)
Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.
Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1300 to 1320nm.
Comparison of single-mode fiber and muti-mode fiber
Constructions are different
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\[2\] Source of the figure: [http://www.rp-photonics.com/multimode_fibers.html] |
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Transmissions are different
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!Fiber single mode.GIF!
!fiber multimode fiber.GIF!
\[3\] Source of the figure: [http://www.arcelect.com/fibercable.htm] |
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Special-purpose
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fiber:
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Polarization
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-
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maintaining
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fibers
What is polarization-maintaining
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fibers?
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Polarization-maintaining
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fibers
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is
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a
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special
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purpose
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single
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mode
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fiber,
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it
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preserves
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and
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transmits
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the
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polarization
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state
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of
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light
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that
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is
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launched
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into
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it.
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Why do we need the polarization-maintaining
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fibers?
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The
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reason
...
why
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we
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need
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to
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have
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polarization-maintaining
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fibers
...
is
...
that
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regular
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single
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mode
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fiber
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cannot
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maintain
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the
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polarization
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state
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of
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input
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laser
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beam.
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Because
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in
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a
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regular
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single
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mode
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fiber,
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the
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input
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polarized
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laser
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beam
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(consist
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of
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two
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perpendicular
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polarized
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modes
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-
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horizontal
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and
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vertical)
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have
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the
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same
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propagation
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constant
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(propagation
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constant
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is
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a
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measure
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of
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the
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change
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undergone
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by
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the
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amplitude
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of
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the
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wave
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as
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it
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propagates
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in
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a
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given
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direction),
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in
...
other
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words
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they
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both
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travel
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at
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the
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same
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velocity
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(not
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good).
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It
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would
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cause
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optical
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energy
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from
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one
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of
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these
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modes
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(horizontal
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or
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vertical
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mode)
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to
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transfer
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the
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other
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or
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“cross-couple”.
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That
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is
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the
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reason
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why
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single
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mode
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fiber
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cannot
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maintain
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the
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polarization
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state
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of
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input
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laser
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beam.
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How does polarization-maintaining
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fibers
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work?
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Then
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the
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polarization-maintaining
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fibers
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was
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invented
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to
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overcome
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this
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problem.
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The
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polarization-maintaining
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fibers
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are
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designed
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in
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such
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a
...
way
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that
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two
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perpendicular
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polarized
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modes
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are
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forced
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to
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travel
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at
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different
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propagation
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constants,
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which
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means
...
they
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travel
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at
...
different
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velocity.
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This
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difference
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in
...
velocities
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makes
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it
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very
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hard
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for
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optical
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energy
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to
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transfer
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between
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each
...
other.
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As
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the
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result
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that
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polarization
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state
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of
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input
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laser
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beam
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is
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preserved.
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This
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special
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purpose
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fiber
...
is
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commonly
...
used
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in
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fiber
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optic
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sensing,
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interferometry
...
and
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slab dielectric waveguides.
Cross Section of Polarization Maintaining Fibers
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Figure. 4. Cross Section of Polarization Maintaining Fibers \[4\] |
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The above three pictures illustrated the three different types of polarization maintaining fibers, which are Panda, Elliptical-clad,
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Bow-tie.
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There
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fibers
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contain
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a
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unique
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feature;
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they
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have
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stress
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rods
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in
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fibers
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(a
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feature
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not
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seen
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in
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other
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fiber
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types).
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The
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stress
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rods
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are
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two
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circles
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in
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the
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Panda
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PM
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fiber,
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a
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elliptical
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clad
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in
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elliptical-clad
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PM
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fiber
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and
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two
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bow-ties
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in
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the
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Bow-Tie
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type
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PM
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fiber.
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The
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most
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important
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property
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of
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stress
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rods
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is
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that
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it
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creates
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stress
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in
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the
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core
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of
...
the
...
fiber
...
such
...
that
...
the
...
transmission
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of
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only
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one
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polarization
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plane
...
of
...
light
...
is
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favored.
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YouTube video - A Brief Introduction of the Polarization-maintaining
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Fibers
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Video.1. A Brief Introduction of the Polarization-maintaining Fibers \[5\] |
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References:
Principle of operation - Special purpose fiber - http://en.wikipedia.org/wiki/Optical_fiber
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\[2\]Multimode fibers - [http://www.rp-photonics.com/multimode_fibers.html] |
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\[3\]The basic of fiber optic cable (single mode and multi mode) - [http://www.arcelect.com/fibercable.htm]/ |
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\[4\] What Are Polarization Maintaining Fibers? - [http://www.fiberoptics4sale.com/wordpress/what-are-polarization-maintaining-fibers/|http://www.fiberoptics4sale.com/wordpress/what-are-polarization-maintaining-fibers/] |
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\[5\] What is Polarization Maintaining Fiber? \- [http://www.youtube.com/watch?v=7rrb-_Iin-g|http://www.youtube.com/watch?v=7rrb-_Iin-g] |