Fiber optic cables are the medium of choice in telecommunications
infrastructure, enabling the transmission of high-speed voice, video,
and data traffic in enterprise and service provider networks. Depending
on the type of application and the reach to be achieved, various types
of fiber may be considered and deployed, such as single mode duplex fiber and multimode duplex fiber optic cable.
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 fiber optic cable),
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.
There are two different
types of fiber optic cable: multimode and single-mode(MMF and SMF).
Both are used in a broad range of telecommunications and data networking
applications. These fiber types have dominated the commercial fiber
market since the 1970's. The distinguishing difference, and the basis
for the naming of the fibers, is in the number of modes allowed to
propagate in the core of a fiber. The "mode" is an allowable path for
the light to travel down a fiber. A multimode fiber allows many light
propagation paths, while a single-mode fiber allows only one light path.
In
multimode fiber, the time it takes for light to travel through a fiber
is different for each mode resulting in a spreading of the pulse at the
output of the fiber referred to as intermodal dispersion. The difference
in the time delay between the modes is called Differential Mode Delay
(DMD). Intermodal dispersion limits multimode fiber bandwidth. This is
significant because a fiber's bandwidth determines its information
carrying capacity, i.e., how far a transmission system can operate at a
specified bit error rate.
The optical fiber guides the light
launched into the fiber core (Figure 1). The cladding is a layer of
material that surrounds the core. The cladding is designed so that the
light launched into the core is contained in the core. When the light
launched into the core strikes the cladding, the light is reflected from
the core-to-cladding interface. The condition of total internal
reflection (when all of the light launched into the core remains in the
core) is a function of both the angle at which the light strikes the
core-to-cladding interface and the index of refraction of the materials.
The index of refraction (n) is a dimensionless number that
characterizes the speed of light in a specific media relative to the
speed of light in a vacuum. To confine light within the core of an
optical fiber, the index of refraction for the cladding (n1) must be
less than the index of refraction for the core (n2).
Fibers
are classified in part by their core and cladding dimensions. Single
mode duplex fiber have a much smaller core diameter than multimode
duplex fiber optic cable. However, the Mode Field Diameter (MFD) rather
than the core diameter is used in single-mode fiber specifications. The
MFD describes the distribution of the optical power in the fiber by
providing an "equivalent" diameter, sometimes referred to as the spot
size. The MFD is always larger than the core diameter with nominal
values ranging between 8-10 microns, while single-mode fiber core
diameters are approximately 8 microns or less. Unlike single-mode fiber,
multimode fiber is usually referred to by its core and cladding
diameters. For example, fiber with a core of 62.5 microns and a cladding
diameter of 125 microns is referred to as a 62.5/125 micron fiber.
Popular multimode product offerings have core diameters of 50 microns or
62.5 microns with a cladding diameter of 125 microns. Single-mode
fibers also have 125 micron cladding diameters.
A single-mode
fiber, having a single propagation mode and therefore no intermodal
dispersion, has higher bandwidth than multimode fiber. This allows for
higher data rates over much longer distances than achievable with
multimode fiber. Consequently, long haul telecommunications applications
only use single-mode fiber, and it is deployed in nearly all
metropolitan and regional configurations. Long distance carriers, local
Bells, and government agencies transmit traffic over single-mode fiber
laid beneath city streets, under rural cornfields, and strung from
telephone poles. Although single mode duplex fiber has higher bandwidth,
multimode fiber supports high data rates at short distances. The
smaller core diameter of single mode duplex fiber also increases the
difficulty in coupling sufficient optical power into the fiber. Relaxed
tolerances on optical coupling requirements afforded by multimode fiber
enable the use of transmitter packaging tolerances that are less
precise, thereby allowing lower cost transceivers or lasers. As a
result, multimode duplex fiber optic cable has dominated in shorter distance and cost sensitive LAN applications.
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