Fiber Optic :: Glossary :: D

Dark current - The thermally induced current that exists in a photodiode in the absence of incident optical power.

Data link - Transmitter with Modulator, Transmission medium and Demodulator with Receiver that transmits data between 2 points. When the Transmission medium is a fiber optic cable the data link is a fiber optic data link.

Data rate - Also Data Transmission Rate. The number of bits of information sent per second in a data communications transmission system. It is generally expressed in Bits Per Second, BPS. This may or may not be equal to the Baud rate.

dB - Decibel, a measure of loss or equivalently attenuation. It is computed as a standard logarithmic unit for the ratio of 2 powers, voltages or currents. In fiber optics the ratio is power and defined by: dB = 10 Log10 (P₁/P₂).

dB loss budget - The amount of light available to overcome the attenuation in the fiber optic data link and still maintain BER (or equivalent) performance specifications.

dBm - Decibels below 1 mW.

dBm - Decibels below 1mW.

Demultiplex - Separation of channels which has been multiplexed in order to share a common transmission medium. With respect to a fiber optic cable medium it is the process of separating optical channels.

Detector - A device that generates an electrical signal when illuminated by light. The electrical current is dependent upon the amount of light received. Common detectors encountered in fiber optic data communications are photodiodes, photodarlingtons and phototransistors.

D4 - A connector type. It is very similar to the FC connector with its threaded coupling, tunable keying and PC end finish. The main difference is its 2.0-mm diameter ferrule. Designed originally by the Nippon Electric Corp.

Diameter-mismatch loss - The loss of power at a joint that occurs when the transmitting half has a diameter greater than the diameter of the receiving half. The loss occurs when coupling light from a source to a fiber optic cable, from a fiber optic cable to another fiber optic cable or from a fiber optic cable to a detector.

Diamond connector - A type of connector.

Dichroic filter - An optical filter that transmits light selectively according to wavelength.

Dielectric - Non-metallic and therefore non-conductive. Glass fiber optic cable is therefore considered dielectric. A dielectric cable contains no metallic components.

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

Digital - A data format that uses a discrete, countable and finite number of levels to transmit information. Binary is a special case of this corresponding to 2 levels.

DIN 47256 - A connector type.

Directivity - This is also referred to as near end crosstalk. It is the amount of power observed at a given input port with respect to an initial input power.

Dispersion - A general term for those phenomena that cause a broadening or spreading of light as it propagates down a fiber optic cable. This is the major cause of bandwidth limitations with fiber optic cable. There are 3 types of dispersion- modal, material and waveguide. Differential optical path lengths in multi-mode fiber optic cables cause modal dispersion. Material dispersion is caused by a differential delay of various wavelengths of light in a waveguide material. Waveguide dispersion is caused by light travelling in both the core and cladding materials in single-mode fiber optic cables.

Distortion-limited operation - Generally synonymous with bandwidth limited operation.

Dopan - Materials added to a core of a fiber optic cable in order to change its characteristics.

Drawing - The manufacturing process by which fiber optic cable is pulled from preforms.

Duplex cable - A 2 fiber cable suitable for duplex (2 way) transmission.

Duplex operation - Transmission on a data link in both directions. Half duplex refers to such transmission, but in a time-shared mode- only one direction can transmit at a time. With full duplex there can be transmission in both direction simultaneously.

Duty cycle - In digital transmission, the ratio of high levels to low levels or the ratio of on time - signal present - to total time - as averaged over many bit or Baud intervals.