Different Types of Single Mode and Multimode Duplex Fiber

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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 has 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.