What is Optical Fiber?
Optical fiber, in its many forms, cable configurations and connectorizations,
has become the standard building block for advanced communications
networks. Essentially, optical fiber functions as a "light
pipe," carrying light generated by lasers and other signal
transmission sources to its destination. Glass optical fiber provides
high-capacity, economical transmission of voice, data and video
signals from short to very long distances. It does this at very
high data rates and with very low signal loss.
Optical fibers are hair-thin structures created by forming preforms,
which are glass rods drawn into fine threads of glass protected
by a plastic coating. Fiber manufacturers use various processes
to make the preforms, such as Modified Chemical Vapor Deposition
(invented by Bell Laboratories), Vapor-phase Axial Deposition and
Outside Vapor Deposition. The fibers drawn from these preforms are
then typically packaged into cable configurations, which are then
placed into an operating environment for decades of reliable performance.
Basic Fiber Mechanics
Optical fiber uses a construction of concentric layers for optical
and mechanical advantages. The "core" is made of silica
glass, sometimes treated (or "doped") with another element
to change its refractive index (velocity of light down the fiber).
This core is completely surrounded by a "cladding," which
acts as a guide to the light waves, preventing light from leaking
out of the core. Cladding keeps light traveling in the proper direction
down the length of the fiber to its destination. Surrounding the
cladding is usually another layer, called a "coating,"
that 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(s).
Figure 1. The basic design of a single-mode optical fiber (a high index core
surrounded by a lower index cladding and covered with a protective coating)
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.
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 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. When
pulses spread too far, the signal cannot be properly detected at
the receiving end of the network.
Types of Fiber
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-1990's, 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 and fibers designed specifically for transoceanic
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/received, the distances
those signals can travel without being regenerated or amplified,
and the speeds at which those signals can travel.
Single-mode fibers (Fig. 2) have a small core size (< 10 Ám)
which permits only one mode or ray of light to be transmitted. Single-mode
fibers have low attenuation and zero dispersion at 1310 nm. This
fiber is a general-purpose fiber for systems of moderate distance,
transmission rates and channel count.
Figure 2. Single-mode fiber
Multimode fibers (Fig. 3) have larger cores that guide many modes
or rays simultaneously. When one pulse of a signal is generated
into a multimode fiber, the multiple modes enter the fiber core
from different angles and each mode propagates at a different speed.
This causes pulse broadening (modal dispersion), limiting the speed
at which subsequent pulses may be generated without overlapping.
Multimode fibers are generally used for short distance applications,
such as within buildings.
Figure 3. Graded-index multimode fiber
Non-Zero Dispersion fibers were designed specifically to address
the needs of long-haul optical networks - the ability to move a
large amount of information over a long distance cost effectively.
To accomplish this, the dispersion characteristics are modified
from conventional single-mode fiber such that it has low, but non-zero,
dispersion around 1550 nm, where attenuation is lowest (Fig. 4).
It also has low dispersion slope, which means it has similar transmission
characteristics across a wide range of wavelengths - perfect for
handling many transmission channels on one fiber.
Zero Water Peak fibers are single-mode fibers in which the water
absorption characteristics on conventional single-mode fibers, which
impact signal loss, have been adjusted to make more wavelengths
available for high quality transmission (Fig. 5). These fibers allow
50% more channels by opening up the wavelength range around 1400
nm, previously unavailable for transmission. This capability is
a perfect match for the requirements of metropolitan networks.
Figure 4. Dispersion
Figure 5. Attenuation