FA1000-62-85A1-0024A Datasheet, PDF (5/18 Pages) Glenair, Inc. – The Glenair Eye-Beam™ Fiber Optic Revolution

QwikConnect
Fiber In Focus: Singlemode and Multimode
Fiber Optic Systems
Fiber optics transmits data as light pulses down
extremely thin strands of glass or plastic fiber.
Singlemode and multimode are the two types of fiber
used in optical fiber systems. Rays of light passing
through a fiber do not travel randomly. Rather, they are
channeled into modes—the thousands of possible paths
a light ray may take as it travels down the fiber. A fiber
can support as few as one mode and as many as tens of
thousands. The number of modes in a fiber is significant
because it helps determine the fiber’s
bandwidth.
Singlemode fibers are manufactured with the
smallest core size (approximately 8 - 10 um in diameter),
eliminating modal dispersion by forcing the light
pulses to follow a single, direct path. The bandwidth of
a singlemode fiber so far surpasses the capabilities of
multimode fiber that its information-carrying capacity
is essentially infinite. Singlemode fiber is thus the
preferred medium for long distance and high bandwidth
applications.
Both multi- and singlemode fibers
have an outside diameter of 125 microns
- a little thicker than a typical human hair.
Light rays travel through the core of the
fiber. Multimode fiber has a much larger
core than singlemode fiber (typically
62.5 microns for multimode compared
to 9 microns for singlemode), allowing
hundreds of rays of light to propagate
through the fiber simultaneously.
Singlemode fiber’s smaller core allows
only one mode of light through.
Paradoxically, the higher the number of
modes, the lower the bandwidth of the
cable. The reason is dispersion.
A singlemode fiber has a much thinner core than a multimode fiber. Light pulses
follow a single direct path. Bandwidth essentially approaches infinity—limited
practically to about 100,000 gigahertz.
Dispersion
“Modal” dispersion is caused by the
different path lengths followed by light
rays as they bounce down the fiber (some
rays follow a more direct route down
the middle of the fiber, and arrive at
their destination well before those rays
which bounce back and forth against the
sides). “Material” dispersion occurs when
different wavelengths of light travel at
different speeds. By reducing the number
of possible modes, you reduce modal
dispersion. By limiting the number of
wavelengths of light, you reduce material
dispersion, but both of these reductions
in dispersion also reduce the information-
carrying bandwidth of your fiber optic
system.
Multimode fiber, with a wider core than singlemode, allows numerous light
beams to travel in different paths through the fiber­—but since the beams arrive at
different times, the aggregate “pulse” of the modes is dispersed.
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Refractive ₊
Index
-
SLOWER
FASTER
FASTER
Dispersion
Graded-Index Multimode fiber’s core has a refractive index that decreases from the
center to the edge. Light rays moving down the center axis move slower than those
at the edge, which take a curved path instead of zigzagging as they do in step-
index fiber. The modes at the edges arrive closer together with the modes in the
middle which allows for less dispersion in the pulse.
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Step-index multimode Plastic Optical Fiber
is used today in automobile on-board
information and entertainment systems
Multimode fiber is generally chosen
for applications where bandwidth
requirements fall below 600 MHz and
is also ideally suited for short distance
applications such as interconnect
assemblies used within a single premise
or contained space. Because of its larger
size, multimode fiber is easier to polish
and clean than singlemode, a critical
concern in interconnect applications
which expose the polished ends of the fibers to debris
during connector mating and unmating.
Two types of multimode fiber
Step-index multimode fiber
This was the first fiber design, engineered with a
relatively large core—up to 100 microns in diameter.
Some of the light rays take a straight path through the
fiber, while others bounce off the cladding and zigzag
through. This causes the different groupings of light rays,
called modes, to arrive separately at a receiving point.
The pulse, which is an aggregate of different modes,
begins to spread out and lose definition, or overlap. This
can be prevented by leaving space between pulses, but
this spacing limits bandwidth.
Today, step-index multimode is typically used in
Plastic Optical Fiber. This is a large-core fiber (1mm) used
for low-speed, short distance transmission applications
like home or industrial networks, home appliances, or
video surveillance systems. It has also gained a foothold
in automobile on-board information and entertainment
fiber systems like MOST and Flexray. This low-cost
technology has the potential to carry broadband access
to increasing numbers of businesses and homes, but
is not suitable for longer distances or the higher speed
data transfer needs of some applications.
Graded-index multimode fiber
This type of multimode fiber uses a core in which
the refractive index gradually decreases from the center
of the fiber out toward the cladding. Light rays moving
straight down the center axis advance more slowly than
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rays near the outside edge. Also, rather than taking a
zigzag path, light takes a helical curved path through
the graded index fiber which shortens its travel distance.
The faster light rays at the edge of the fiber arrive
closer together with the slower straight rays from the
center, allowing for a digital pulse with less dispersion.
Bandwidth is hundreds of times greater than step index
fiber. Graded index multimode fiber offers the easier
use and durability of a larger-core fiber without the
dispersion disadvantage of step-index fiber.
Today, fiber optic cable is an integral part of
communication technology. From high-reliability
graded-index multimode fiber used in aerospace or
military applications, to integrated plastic optical
fiber systems for automobiles, to high bandwidth
singlemode fiber cables that run across oceans, fiber
optic technology continues to develop and serve global
information and communication needs.
CS Long Lines, a ship designed to lay the Trans-Atlantic fiber optic
cable for AT&T. This ship was used to conduct the first deep-sea trials
of fiber optic cable in 1982. AT&T laid and opened the first fiber-optic
cable across the Atlantic in 1988.
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