What is Optical Fiber and Cisco Multimode

Fibers?
An optical fiber is a flexible filament of very clear glass capable of carrying
information in the form of light. Optical fibers are hair-thin structures created by
forming pre-forms, which are glass rods drawn into fine threads of glass protected by
a plastic coating. Fiber manufacturers use various vapor deposition processes to make
the pre-forms. The fibers drawn from these pre-forms are then typically packaged into
cable configurations, which are then placed into an operating environment for decades
of reliable performance.

Anatomy of an Optical Fiber
The two main elements of an optical fiber are its core and cladding. The "core", or the
axial part of the optical fiber made of silica glass, is the light transmission area of the
fiber. It may sometimes be treated with a "doping" element to change its refractive
index and therefore the velocity of light down the fiber.
The "cladding" is the layer completely surrounding the core. The difference in
refractive index between the core and cladding is less than 0.5 percent. The refractive
index of the core is higher than that of the cladding, so that light in the core strikes the
interface with the cladding at a bouncing angle, gets trapped in the core by total
internal reflection, and keeps traveling in the proper direction down the length of the
fiber to its destination.
Surrounding the cladding is usually another layer, called a "coating," which typically
consists of protective polymer layers applied during the fiber drawing process, before
the fiber contacts any surface. "Buffers" are further protective layers applied on top of
the coating.

Basic View of an Optical Fiber

Types of Fiber and Various Parameters
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Fibers come in several different configurations, each ideally suited to a different use
or application. Early fiber designs that are still used today include single-mode and
multimode fiber. Since Bell Laboratories invented the concept of application-specific
fibers in the mid-1990s, fiber designs for specific network applications have been
introduced. These new fiber designs - used primarily for the transmission of
communication signals - include Non-Zero Dispersion Fiber (NZDF), Zero Water
Peak Fiber (ZWPF), 10-Gbps laser optimized multimode fiber (OM3), and fibers
designed specifically for submarine applications. Specialty fiber designs, such as
dispersion compensating fibers and erbium doped fibers, perform functions that
complement the transmission fibers. The differences among the different transmission
fiber types result in variations in the range and the number of different wavelengths or
channels at which the light is transmitted or received, the distances those signals can
travel without being regenerated or amplified, and the speeds at which those signals
can travel.
A number of key parameters impact how optical fibers perform in transmission
systems. The specifications for each parameter will vary by fiber type, depending
upon the intended application. Two of the more important fiber parameters are
attenuation and dispersion. Attenuation is the reduction in optical power as it passes
from one point to another. In optical fibers, power loss results from absorption and
scattering and is generally expressed in decibels (dB) for a given length of fiber, or
per unit length (dB/km) at a specific transmission wavelength. High attenuation limits
the distance a signal can be sent through a network without adding costly electronics
to the system. Figure 2 illustrates the variation of attenuation with wavelength taken
over an ensemble of fiber optic cable material types. The three principal windows of
operation, propagation through a cable, are indicated. These correspond to wavelength
regions where attenuation is low and matched to the ability of a transmitter to
generate light efficiently and a receiver to carry out detection. Hence, the lasers
deployed in optical communications typically operate at or around 850 nanometers
(nm) (first window), 1310 nm (second window), and 1550 nm (third and fourth
windows).

Attenuation Versus Wavelength and Transmission Windows

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Dispersion is inversely related to bandwidth, which is the information-carrying
capacity of a fiber, and indicates the fiber's pulse-spreading limitations. Chromatic
dispersion in single-mode fiber links causes pulse spreading because of the various
colors of light traveling in the fiber at different speeds, causing a transmitted pulse to
spread as it travels down the fiber. Similarly, modal dispersion in multimode fiber
links causes pulse spreading because of the geometry of a multimode fiber core
allowing for multiple modes of the laser to separate at the fiber interface and
propagate simultaneously down the fiber. These modes travel with slight delays
relative to each other, causing the transmitted pulse to spread as it travels along the
fiber. When pulses spread too far, they overlap and the signal cannot be properly
detected at the receiving end of the network. Figure 3 depicts a generic view of pulse
spreading.

Pulse Spreading Caused by Dispersion

Types of Optical Connectors
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The connector is a mechanical device mounted on the end of a fiber optic cable, light
source, receiver, or housing. It allows it to be mated to a similar device. The
transmitter provides the information-bearing light to the fiber optic cable through a
connector. The receiver gets the information-bearing light from the fiber optic cable
through a connector. The connector must direct light and collect light. It must also be
easily attached and detached from equipment.
There are many different connector types. Table 2 illustrates some types of optical
connectors and lists some specifications. Each connector type has strong points. For
example, ST connectors are a good choice for easy field installations; the FC
connector has a floating ferrule that provides good mechanical isolation; the SC
connector offers excellent packing density, and its push-pull design resists fiber end
face contact damage during unmating and remating cycles.

Common Types of Fiber Optic Connector
Connect
or
FC

Appearan Insertion
ce
Loss

Repeatability

0.5--1.0 dB

0.2 dB

LC

0.15 db (SM)
0.10 dB (MM)

0.2 dB

MT
Array

0.3-1.0 dB

0.25 dB

SC

0.2-0.45 dB

ST

Applications

SM,
MM

High-density
interconnection

SM,
MM
Type. 0.4 dB Type. 0.4 dB SM,
(SM)
(SM)
MM
Type. 0.2 dB
(MM)

0.1 dB

Fiber
Type
SM,
MM
SM,
MM

Datacom, telecom
High-density
interconnection,
datacom, telecom

Datacom, telecom
Inter-/intra-building,
security, U.S. Navy

Type. 0.2 dB
(MM)

Multimode fiber, the first to be manufactured and commercialized, simply refers to
the fact that numerous modes or light rays are carried simultaneously through the
waveguide. Modes result from the fact that light will only propagate in the fiber core
at discrete angles within the cone of acceptance. This fiber type has a much larger
core diameter, compared to single-mode fiber, allowing for the larger number of
modes, and multimode fiber is easier to couple than single-mode optical fiber.
Multimode fiber may be categorized as step-index or graded-index fiber.
Step-index multimode was the first fiber design but is too slow for most uses, due to
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the dispersion caused by the different path lengths of the various modes. Step-index
fiber is barely used in current telecom and datacom applications.
Graded-index multimode fiber uses variations in the composition of the glass in the
core to compensate for the different path lengths of the modes. It offers hundreds of
times more bandwidth than step index fiber.

Types of Multimode Fiber and Associated Transceivers
Various Types of Multimode Fiber

Cable types are defined slightly differently by each standard body. In practical
situations, four main fiber types are commonly used:
 FDDI-grade is the legacy multimode fiber with 160 MHz*km bandwidth at 850
nm
 OM1 is another 62.5 micron fiber with little bit more bandwidth
 OM2 is the traditional 50 micron fiber
 OM3 is the laser-optimized fiber, ideally suited for VCSEL-based transmitters at
850 nm
Multimode Transceiver/Fiber Type Compatibility Matrix

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Interface Type

Wavelength (nm)

Fibers Supported

Reach (m)

MCP
Requirement

1000BASE-SX

850

FDDI-grade

220

No

OM1

275

No

1000BASE-LX

10GBASE-SR

10GBASE-LX4

10GBASE-LRM

1300

850

1300

1300

OM2

550

No

OM3

Not specified

FDDI-grade

550

Yes

OM1

550

Yes

OM2

550

Yes

OM3

Not specified

FDDI-grade

26

No

OM1

33

No

OM2

82

No

OM3

300

No

FDDI-grade

300

Yes

OM1

300

Yes

OM2

300

Yes

OM3

Not specified

FDDI-grade

220

Yes

OM1

220

Yes

OM2

220

Yes

OM3

220

No

The table as above summarizes various optical interfaces and their performance over
the different fiber types. The table is directly derived from the IEEE 802.3-2005
standard and specifies the maximum reach achievable over each fiber type and the
requirement for a mode conditioning patch cord (MCP).
These performance levels are guaranteed. If we go beyond the standard, longer
reaches may be achievable depending on the quality of each link. Fiber quality can
vary for a specific type due to the aging factor or to the random imperfections it was
built with. In order to really know if a link can work, the rule is to try and see if the
performance is satisfactory. The link should be either error-free for critical
applications, or the bit error should remain below 10-12 as per minimum standard
requirement.
As an example it may be possible to reach much longer distances than 550m with an
OM3 laser-optimized fiber and 1000BASE-SX interfaces. Also, it may be possible to
reach 2 km between two 1000BASE-LX devices over any fiber type with mode
conditioning patch cords properly installed at both ends.
In addition, future fibers are already under study. Manufacturers are generally
working on improving the bandwidth in the 850-nm window, where low-cost VCSELbased transceivers are an attractive alternative for new deployments. The OM3+ with
a modal bandwidth of 4700 MHz*km is already available, but the IEEE would need
to approve its standardization.
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