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BER (Bit Error Rate): In digital applications, the ratio
of bits received in error to bits sent. BERs of one errored
bit per billion (1 × 10-9) sent are typical.
Buffer: Material used to protect optical fiber from
physical damage, providing mechanical isolation and/or
protection. Fabrication techniques include tight or loose
tube buffering, as well as multiple buffer layers.
Burrus LED: A surface-emitting LED with a hole
etched to accommodate a light-collecting fiber. Named
after its inventor, Charles A. Burrus of Bell Labs.
Chromatic Dispersion: Spreading of a light pulse
caused by the difference in refractive indices at different
wavelengths.
Cladding: The dielectric material surrounding the core
of an optical fiber.
Coarse Wavelength Division Multiplexing (CWDM):
CWDM is a cost-effective solution to dense wave-
length division modulation (DWDM) that was devel-
oped to have channel spacing by the International
Telecommunication Union (ITU) in 2002. This standard
allows for a 20 nm spacing of channels using wave-
lengths between 1270 nm and 1610 nm.
Coherent: Light source (laser) in which the amplitude
of all waves is exactly equivalent and rise and fall
together.
Core: The central region of an optical fiber through
which light is transmitted.
Coupler: An optical component used to split or com-
bine optical signals. Also known as a “Splitter,” “T-cou-
pler,” “2 × 2,” or “1 × 2” coupler.
Coupling Loss: The power loss suffered when coupling
light from one optical device to another.
Critical Angle: The smallest angle from the fiber axis
at which a ray may be totally reflected at the core-clad-
ding interface.
Cutoff Wavelength: The shortest wavelength at which
only the fundamental mode of an optical waveguide is
capable of propagation.
Dark Current: The external current that, under speci-
fied biasing conditions, flows in a photodetector when
there is no incident radiation.
Data Rate: The maximum number of bits of informa-
tion that can be transmitted per second, as in a data
transmission link. Typically expressed as megabits per
second (Mb/s).
Decibel (dB): The standard unit of level used to express
gain or loss of optical or electrical power.Dense Wavelength Division Multiplexing (DWDM):
An enhancement of WDM (see Wavelength Division
Multiplexing) that uses many wavelengths in the
1550 nm window (ranges 1530 nm to 1560 nm) for
transmitting multiple signals, and often uses fiber optic
amplification. Many narrowband transmitters send sig-
nals to a DWDM Optical Multiplexer (Mux), which
combines all of the signals onto a single fiber. At the
other end a DWDM Optical Demultiplexer (Demux)
separates the signals out to the many receivers.Detector: A transducer that provides an electrical out-
put signal in response to an incident optical signal. The
current is dependent on the amount of light received and
the type of device.Dispersion: Spread of the signal delay in an optical
waveguide. It consists of various components: modal
dispersion, material dispersion, and waveguide disper-
sion. As a result of its dispersion, an optical waveguide
acts as a low-pass filter for the transmitted signals.Ferrule: A component of a fiber optic connection that
holds a fiber in place and aids in its alignment.Fiber Data Distributed Interface (FDDI): An emerg-
ing standard developed by AT&T, Hewlett-Packard Co,
and Siemens Corp., using a 100 Mbps token ring net-
work that employs dual optical fibers.Fiber Optic: Any filament or fiber made of dielectric
materials, that guides light.Fiber Optic Link: A fiber optic cable with connectors
attached to a transmitter (source) and receiver (detec-
tor).Fresnel Reflection: The reflection of a portion of the
light incident on a planar surface between two homoge-
neous media having different refractive indices. Fresnel
reflection occurs at the air–glass interfaces at entrance
and exit ends of an optical fiber.Fundamental Mode: The lowest order mode of a
waveguide.Graded Index Fiber: An optical fiber with a variable
refractive index that is a function of the radial distance
from the fiber axis.Incoherent: An LED light source that emits incoherent
light as opposed to the laser which emits coherent light.
(See Coherent.)