D/A or DAC

Abbreviation for digital-to-analog converter. A device used to convert digital signals to analog signals.

D/A or DAC


A format for component digital video tape recording working to the ITU-R 601, 4:2:2 standard using 8-bit sampling.



The VTR standard for digital composite (coded) NTSC or PAL signals that uses data conforming to SMPTE 244M.



A composite digital video recording format that uses data conforming to SMPTE 244M.



An uncompressed tape format for component digital video which has provisions for HDTV recording by use of 4:1 compression.


Dark Current

The induced current that exists in a reversed biased photodiode in the absence of incident optical power. It is better understood to be caused by the shunt resistance of the photodiode. A bias voltage across the diode (and the shunt resistance) causes current to flow in the absence of light.

Dark Current

Data Dependent Jitter

Also called data dependent distortion. Jitter related to the transmitted symbol sequence. DDJ is caused by the limited bandwidth characteristics, non-ideal individual pulse responses, and imperfections in the optical channel components.


Data Rate

The number of bits of information in a transmission system, expressed in bits per second (b/s or bps), and which may or may not be equal to the signal or baud rate.



Abbreviation for decibel relative to a carrier level.



Abbreviation for decibel relative to milliwatt.

dBµ: Abbreviation for decibel relative to microwatt.



Abbreviation for digital broadcast system. An alternative to cable and analog satellite reception that uses a fixed 18-inch dish focused on one or more geostationary satellites. DBS units receive multiple channels of multiplexed video and audio signals as well as programming information, and related data. Also known as digital satellite system.



See Duty Cycle Distortion Jitter.



Abbreviation for data circuit-terminating equipment. 1) In a data station, the equipment that performs functions such as signal conversion and coding, at the network end of the line between the data terminal equipment (DTE) and the line, and may be a separate or an integral part of the DTE or of intermediate equipment. 2) The interfacing equipment that may be required to couple the data terminal equipment (DTE) into a transmission circuit or channel and from a transmission circuit of a channel into the DTE.



See discrete-cosine transform.



See Data Dependent Jitter.


Decibel (dB)

A unit of measurement indicating relative power on a logarithmic scale. Often expressed in reference to a fixed value, such as dBm or dBµ.

dB = 10·Log10 (P1/P2)



A device used to convert data by reversing the effect of previous coding.



A module that separates two or more signals previously combined by compatible multiplexing equipment.


Dense Wavelength-division Multiplexing (DWDM)

The transmission of many of closely spaced wavelengths in the 1550 nm region over a single optical fiber. 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.


Destructive Interference

Any interference that decreases the desired signal. For example, two light waves that are equal in amplitude and frequency, and out of phase by 180°, will negate one another.

Destructive Interference


An opto-electric transducer used to convert optical power to electrical current. Usually referred to as a photodiode.


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.

Diameter-mismatch Loss

Dichroic Filter

An optical filter that transmits light according to wavelength. Dichroic filters reflect light that they do not transmit. Used in bulk optics wavelength-division multiplexers.

Dichroic Filter


Any substance in which an electric field may be maintained with zero or near-zero power dissipation. This term usually refers to non-metallic materials.



Any substance in which an electric field may be maintained with zero or near-zero power dissipation. This term usually refers to non-metallic materials.


Differential Gain (DG)

A type of distortion in a video signal that causes the brightness information to be incorrectly interpreted.


Differential Phase (DP)

A type of distortion in a video signal that causes the color information to be incorrectly interpreted.


Diffraction Grating

An array of fine, parallel, equally spaced reflecting or transmitting lines that mutually enhance the effects of diffraction to concentrate the diffracted light in a few directions determined by the spacing of the lines and by the wavelength of the light.

Diffraction Grating


A signal that consists of discrete states. A binary signal has only two states, 0 and 1. Antonym of analog.


Digital Compression

A technique for converting digital video to a lower data rate by eliminating redundant information.



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 (LEDs and laser diodes) and detectors (photodiodes).




Abbreviation for dual in-line package. An electronic package with a rectangular housing and a row of pins along each of two opposite sides.



A device that combines two or more types of signals into a single output. Usually incorporates a multiplexer at the transmit end and a demultiplexer at the receiver end.


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.

Directional Coupler


See near-end crosstalk.


Discrete-cosine Transform (DCT)

A widely used method of data compression of digital video pictures that resolves blocks of the picture (usually 8 x 8 pixels) into frequencies, amplitudes, and colors. JPEG and DV depend on DCT.



The temporal spreading of a light signal in an optical waveguide caused by light signals traveling at different speeds through a fiber either due to modal or chromatic effects.


Dispersion Management

A technique used in a fiber optic system design to cope with the dispersion introduced by the optical fiber. A dispersion slope compensator (illustrated) is one dispersion management technique.

Dispersion Management

Dispersion Penalty

The result of dispersion in which pulses and edges smear making it difficult for the receiver to distinguish between ones and zeros. This results in a loss of receiver sensitivity compared to a short fiber and measured in dB. The equations for calculating dispersion penalty are as follows:

ω = Laser spectral width (nm)
Dλ = Fiber dispersion (ps/nm/km)
τ = System dispersion (ps/km)
f = Bandwidth-distance product of the fiber (Hz • km)
L = Fiber length (km)
FF = Fiber bandwidth (Hz)
C = A constant equal to 0.5
FR = Receiver data rate (b/s)
dBL = Dispersion penalty (dB)

Dispersion Penalty

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 nonlinearity at the zero-dispersion wavelength.



nonlinearities in a unit that cause harmonics and beat products to be generated.


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.

Distributed Feedback Laser (DFB)

Distribution System

Part of a cable system consisting of trunk and feeder cables used to carry signals from headend to customer terminals.


Dominant Mode

The mode in an optical device spectrum with the most power.



An impurity added to an optical medium to change its optical properties. EDFAs use erbium as a dopant for optical fiber.



Thick liquid or paste used to prepare a surface or a varnish-like substance used for waterproofing or strengthening a material.


Double-window Fiber

1) Multimode fibers optimized for 850 nm and 1310 nm operation. 2) Single-mode fibers optimized for 1310 nm and 1550 nm operation.



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.



Abbreviation for data signaling rate. The aggregate rate at which data pass a point in the transmission path of a data transmission system expressed in bits per second (bps or b/s).



Abbreviation for dispersion supported transmission. In electrical TDM systems, a transmission system that would allow data rates at 40 Gb/s by incorporating devices such as SOAs.



A transmission rate in the North American digital telephone hierarchy. Also called T-carrier.



Abbreviation for data terminal equipment. 1) An end instrument that converts user information into signals for transmission or reconverts the received signals into user information. 2) The functional unit of a data station that serves as a data source or a data sink and provides for the data communication control function to be performed in accordance with link protocol.



Abbreviation for data terminal ready. In a communications network, a signal from a remote transmitter that the transmitter is clear to receive data.



Abbreviation for digital television. Any technology, using any of several digital encoding schemes, used in connection with the transmission and reception of television signals. Depending on the transmission medium, DTV often uses some type of digital compression to reduce the required digital data rate. Except for artifacts of the compression, DTV is more immune (than analog television) to degradation in transmission, resulting in a higher quality of both audio and video, to the limits of signal reception.


Dual Attachment Concentrator

A concentrator that offers two attachments to the FDDI network which are capable of accommodating a dual (counter-rotating) ring.


Dual Attachment Station

A station that offers two attachments to the FDDI network which are capable of accommodating a dual (counter-rotating) ring.


Dual Ring (FDDI Dual Ring)

A pair of counter-rotating logical rings.


Duplex Cable

A two-fiber cable suitable for duplex transmission.

Duplex Cable

Duplex Transmission

Transmission in both directions, either one direction at a time (half-duplex) or both directions simultaneously (full-duplex).

Duplex Transmission

Duty Cycle

In a digital transmission, the fraction of time a signal is at the high level.

Duty Cycle

Duty Cycle Distortion Jitter

Distortion usually caused by propagation delay differences between low-to-high and high-to-low transitions. DCD is manifested as a pulse width distortion of the nominal baud time.



Abbreviation for digital video broadcast-asynchronous serial interface. An interface used to transport MPEG-2 files. The interface consolidates multiple MPEG-2 data streams onto a single circuit and transmits them at a data rate of 270 Mb/s.



See dense wavelength-division multiplexing.


Ground Loop Noise

Noise that results when equipment is grounded at points having different potentials thereby creating an unintended current path. The dielectric properties of optical fiber provide electrical isolation that eliminates ground loops