Dark fibre
Unused fibre; a fibre carrying no light.

dB
Decibel: A measurement of gain or loss in optical power

dBm
Decibels below 1mW.

Decibel
The standard unit used to express the ratio of two power levels. It is used in communications to express either a gain or loss in power between the input and output devices.

a) 3dB loss is approximately 50% decrease in power
b) 2dB loss is approximately 37% decrease in power.
c) 1dB loss is approximately 20% decrease in power.
d) 0.5dB loss is approximately 11% decrease in power.

Demultiplexing, optical
The splitting of two or more wavelength from a common optical fibre onto a minimum of two different optical fibres.

DEMUX (Demultiplexer)
A device that separates a signal of different (multiplexed) wavelength into its original seperated components.

Depressed cladding
Cladding with two sections with different refractive index. The inner section has a uniform refractive index that is lower than the also uniform refractive index of the outer section.

DF
The Radia fiber optic connector.

Diameter mismatch loss
The loss of power that occurs when one fibre transmits to another and the transmitting fibre has a diameter greater than the receiving fibre. It can occur at any type of coupling, fibre to fibre, fibre to device, fibre to detector, source to fibre etc.. The amount of mismatch loss depends from wavelength and fibre parameters.

Dichromic filter or mirror
One that selectively transmits or reflects light according to selected wavelengths.

Directivity (D)
Proportion of the transmitted signal which is reflected to the parallel fibre on the same side of an optical coupler or splitter.

Dispersion
A measure of the frequency / wavelength dependence in transmittence according to several parameters. Also called pulse spreading since dispersion causes a broadening of the input pulses along the length of the fibre. Two major types of dispersion are modal and material. Can be caused by multiple frequencies / wavelengths components having different propagation characteristics, or the relationship between the refractive index and wavelength or frequency.

Dispersion flattened fibre
A single mode fibre that has a low chromatic dispersion throughout the range from 1300 to 1600 nanometers.

Dispersion shifted fibre
A single mode fibre that has zero dispersion wavelength at 1550 nanometers.

Dispersion unshifted fibre
A single mode fibre that has zero dispersion wavelength at 1300 nanometers. Often called conventional or unshifted fibre.

DNP
Dry non-polish connector used for POF: Plastic Optical Fibre.

DSLAM
Digital Subscriber Line Access Multiplexer

Dopant
A material, such as germanium or boron, added to silica to change its index of refraction.

Dual-window fibre
Graded-index fibre in which the refractive index profile is designed to give low modal dispersion at both 850 and 1300 nm.

Duplex cable
Two, side-by-side simplex cable sub-units surrounded by a single polymer jacket.

DWDM
Dense Wave Division Multiplexing is an optical multiplexing technique used to increase the carrying capacity of a fibre network.