Series 101: Multimode vs. Singlemode

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Series 101 Multimode vs. Singlemode Author
Mark Mullins
www.flukenetworks.com 2006-2017 Fluke
Corporation
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Series 101 Multimode vs. Singlemode
Fiber optic cabling comes in two types
multimode and singlemode. Most of you likely know
that multimode cabling distances are shorter than
singlemode, and singlemode is therefore deployed
for outside plant long-haul fiber applications,
while multimode is the primary choice for data
centers and premise applications.
Singlemode fiber is however growing in
popularity in shorter-reach applications due to
its inherently higher bandwidth capabilities, and
more technicians are faced with installing both
singlemode and multimode fiber plants. It
occurred to us that not everyone understands the
technical differences between the two fiber
types. Lets take a closer look.
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Series 101 Multimode vs. Singlemode
Whats in a Mode? In fiber optic data
transmission, the term mode is used to describe
the means that a light signal travels through the
glass core of the fibera mode is simply the path
that the light follows. So in a singlemode fiber,
the light travels down one path, while in a
multimode fiber, the light travels in many paths.
Think of it this way. When you go down a water
slide, you bounce off the walls of the slide.
Each angle that you travel is a mode. Light
within a multimode glass core works the same way.
The light hits the glass at an angle and reflects
back again, moving the light along the length of
the core. Why doesnt the light just shine out of
the core? First of all, the light hits the glass
at a shallow angle, which causes the glass to act
like a mirror. Secondly, there is a layer around
the core called the cladding. To keep the light
inside the core, the cladding has a different
refractive index profile, which is the technical
parameter that determines how much light is bent,
or refracted, when entering a material.
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Series 101 Multimode vs. Singlemode
Bandwidth Limiting Delay Why does singlemode
fiber support significantly greater bandwidth and
distances? Sending light in a single mode
eliminates something called differential mode
delay (DMD), which is the primary
bandwidth-limiting factor of multimode
fiber. When multiple modes travel through a
multimode fiber, some travel down the center,
while others travel along pathways closer to the
core-cladding interface. Those travelling on the
outside edge are referred to as higher order
modes, while those near the center of the core
are lower order modes. Higher and lower order
modes travel at different velocities, and DMD is
the difference in travel time. The smaller the
DMD, the less the light pulses spread out over
time and the higher the bandwidth. When the time
difference between pulses is greater, the
receiver may not be able to properly distinguish
the pulses. DMD is directly related to
distanceit increases as the fiber length
increases. This is why multimode fiber has much
shorter distance requirements than singlemodeup
to about 500 meters for multimode compared to 10
kilometers for singlemode.
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Series 101 Multimode vs. Singlemode
DMD can also be caused by imperfections in the
fiber, and fiber manufacturers have learned to
limit DMD by carefully optimizing the fiber
refractive index profile. Rather than the modes
refracting only where the core meets the
cladding, multimode fiber uses a graded fiber
index profile that changes continuously from the
center of the core to the core-cladding boundary.
This creates a parabolic (i.e. symmetrical curve)
pathway that causes the lower order modes
travelling the shorter distance close to the core
of the fiber to travel slower, and the higher
order modes travelling longer distances near the
edge of the core to travel faster. This minimizes
time delays between pulses, thereby decreasing
DMD and allowing for higher bandwidth.
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Series 101 Multimode vs. Singlemode
More Obvious Differences Some of the more
obvious differences between multimode and
singlemode include cable color, light sources and
testing. Singlemode is almost always yellow,
while multimode is typically aqua. Different
types of multimode may also be different
colourswhile OM3 is almost always aqua, OM4
multimode is sometimes a pink colour called Erika
Violet to help distinguish it from OM3, and the
latest generation of multimode fiber, OM5, is
lime green. Another key difference is the light
source and associated cost of the transmission
equipment. Singlemode fiber requires a laser
light source with a narrow spectral width, which
results in more costly transceivers. While the
singlemode cable itself is less expensive than
multimode cable, singlemode transceivers cost
anywhere from 1.5 to 4 times that of multimode
transceivers. And when it comes to testing
multimode vs. singlemode, its important to
understand that the two fiber types cannot be
mixed and launch cords must match the fiber type
being tested. Testing multimode fiber also
requires Encircled Flux (EF) testing, which
characterizes how the light is launched into the
cabling under test. EF testing restricts the
number of modes launched to reduce variability
and achieve precise, repeatable test results.
Click here for more information on Encircled Flux
testing, or download the white paper if you
really want to delve in.
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