| Decibel
(dB) |
The
standard unit used to express gain or loss of optical
power. A standard logarithmic unit for the ratio
of two powers, voltages or currents. In fiber optics,
the ratio is power. dB = 10olog10 (P1/P2) |
| dBc
|
Abbreviation
for decibel relative to a carrier level. |
| dBµ
|
Abbreviation
for decibels relative to microwatt. |
| dBm |
Abbreviation
for decibels relative to milliwatt. |
| Dense
Wavelength-division Multiplexing (DWDM) |
This
refers to the transmission of a multiplicity of
closely spaced wavelengths in the 1550 nm region.
Wavelength spacings are usually 100 GHz or 200 GHz
which corresponds to 0.8 nm or 1.6 nm. DWDM bands
include the C-Band, the S-Band, and the L-Band. |
| Detector |
An
opto-electric transducer used in fiber optics to
convert optical power to electrical current. Usually
referred to as a photodiode. |
| DFB |
See
distributed feedback laser. |
| Diameter-Mismatch
Loss |
The
loss of power at a joint that occurs when the transmitting
fiber has a diameter greater than the diameter of
the receiving fiber. The loss occurs when coupling
light from a source to fiber, from fiber to fiber,
or from fiber to detector. |
| Dielectric |
Nonmetallic.
All-dielectric designs are inherently nonconductive;
all-dielectric cables contain no metal and are lightning
resistant |
| Digital |
A signal
that consists of discrete states. A binary signal
has only two states, 0 and 1. |
| Digital
Compression |
An
engineering technique for converting a cable television
signal into a digital format (in which it can easily
be stored and manipulated) which may then be processed
so as to require a smaller portion of spectrum for
its transmission. It could allow many channels to
be carried in the capacity currently needed for
one signal. |
Diode
|
An
electronic device that lets current flow in only
one direction. Semiconductor diodes used in fiber
optics contain a junction between regions of different
doping. They include light emitters (LED's and laser
diodes) and detectors (photodiodes). |
| Diode
Laser |
Synonymous
with injection laser diode. |
| Directional
Coupler |
A coupling
device for separately sampling (through a known
coupling loss) either the forward (incident) or
the backward (reflected) wave in a transmission
line. |
| Dispersion |
The
cause of bandwidth limitations in a fiber. Dispersion
causes a broadening of input pulses along the length
of the fiber. Two major types are mode dispersion
caused by differential optical path lengths in a
multimode fiber and material dispersion caused by
a differential delay of various wavelengths of light
in a waveguide material |
| Dispersion-compensating
Fiber (DCF) |
A fiber
that has the opposite dispersion of the fiber being
used in a transmission system. It is used to nullify
the dispersion caused by that fiber. |
| Dispersion-compensating
Module (DCM) |
This
module has the opposite dispersion of the fiber
being used in a transmission system. It is used
to nullify the dispersion caused by that fiber.
It can be either a spool of a special fiber or a
grating based module. |
| Dispersion-shifted
Fiber (DSF) |
A type
of single-mode fiber designed to have zero dispersion
near 1550 nm. This fiber type works very poorly
for DWDM applications because of high fiber non-linearity
at the zero dispersion point. |
| Dispersion
Management |
A technique
used in the system design of a fiber optic transmission
to be able to cope with the dispersion introduced
by the optical fiber. |
| Dispersion
Penalty |
Dispersion
in an optical fiber causes pulses and edges to be
smeared. As the edges smear, the receiver has more
difficulty discriminating between ones and zeros.
This results in a loss of sensitivity compared to
a short fiber and referred to as the dispersion
penalty in dB. |
| Distortion
|
Nonlinearities
in a unit that cause harmonics and beat products
to be generated. |
| Distortion-limited
Operation |
Generally
synonymous with bandwidth-limited operation. |
| Distortion-limited
Operation |
Generally
synonymous with bandwidth-limited operation. |
| Distributed
Feedback Laser (DFB) |
An
injection laser diode which has a Bragg reflection
grating in the active region in order to suppress
multiple longitudinal modes and enhance a single
longitudinal mode. |
| Double-window
Fiber |
This
term is used two ways. For multimode fibers, the
term means that the fiber is optimized for 850 nm
and 1310 nm operation. For single-mode fibers, the
term means that the fiber is optimized for 1310
nm and 1550 nm operation. |
| DSL
|
Abbreviation
for digital subscriber line. In an integrated systems
digital network (ISDN), equipment that provides
full-duplex service on a single twisted metallic
pair at a rate sufficient to support ISDN basic
access and additional framing, timing recovery,
and operational functions. See also ISDN. |
| Dual
Ring (FDDI Dual Ring) |
A pair
of counter-rotating logical rings. |
| Duplex
Cable |
A two-fiber
cable suitable for duplex transmission. |
| Duplex
Transmission |
Transmission
in both directions, either one direction at a time
(half-duplex) or both directions simultaneously
(full-duplex). |
| Duty
Cycle |
In
a digital transmission, the fraction of time a signal
is at the high level |
| DWDM
|
See
dense wavelength-division multiplexing |
