D/A or DAC: Abbreviation for digital-to-analog converter. A device used to convert digital signals to analog signals.

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.

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.

dBc: Abbreviation for decibel relative to a carrier level.

dBm: Abbreviation for decibel relative to milliwatt.

DC: Abbreviation for direct current. An electric current flowing in one direction only and substantially constant in value.

DCE: 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.

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 dB¦Ì .

dB = 10•Log₁₀ (P₁/P₂)

Demultiplexer: 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.

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Detector: An opto-electric transducer used 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.

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.

Dielectric: 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.

Digital: 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.

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

Diode Laser: Synonymous with injection laser diode.

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

Diplexer: 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.

Directivity: See near-end crosstalk.

Dispersion: 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-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.

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 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:

Where:
ω = Laser spectral width (nm)
D_λ = Fiber dispersion (ps/nm/km)
τ = System dispersion (ps/km)
f = Bandwidth-distance product of the fiber (Hz ?km)
L = Fiber length (km)
F_F = Fiber bandwidth (Hz)
C = A constant equal to 0.5
F_R = Receiver data rate (b/s)
dB_L = Dispersion penalty (dB)

Distortion: 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.

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.

Dope: 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.

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.

DSR: 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).

DST: 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.

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

DTE: 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.

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

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 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.