Backbone system
The transmission network that carries high speed transmission of telecommunications between regions.

Back reflection
Occurs when light propagating through the fiber is mirrored back into the fiber at the end face, instead of continuing through to the receiving source.

Backscattering
The scattering of light in a direction opposite the original one.

Balanced coupler
A coupler whose output has balanced splits: for instance 1 by two 50/50, or 1 by 4: 25/25/25/25.

Bandwidth (1)
The range of frequencies, usually expressed in hertz and defined by the number of hertz carried, or the upper and lower limiting frequencies. By definition, analog is wide bandwidth. Sometimes a device or system, such as transmitters, fibre, connectors, detectors, etc. are described by their bandwidth as a range of frequencies that can be conveyed with losses less than 3 dB from maximum response. CATV systems are bandwidth rich, often maintaining a bandwidth of 300 to 750 MHz.

Bandwidth (2)
The wavelength range around the central wavelength (CWL) of an optical transmission window, usually expressed in nm.

Bandwidth distance product
The information-carrying capacity of a transmission medium is normally referred to in units of MHz-km. This is called the bandwidth-distance product or more commonly bandwidth.

Beamsplitter
A device used to divide an optical beam into two or more separate beams. Sometimes used instead of coupler
or splitter.

Bend loss
Loss or attenuation due to the escape of light due to the excessive bending of a fibre. This loss is usally due to internal light paths exeeding the critical angle for Total Reflection. The generally accepted lower limit for bending radius in standard singlemode fibre (9/125) is about 25 mm. Bending losses can be caused by a wide variety of sources. The bending loss depends from fibre type and wavelength.

Bend radius
Radius of curvature that a fibre can follow without degradation or increased attenuation.

Biconic
Fibre optic connector developed by AT&T, now largely out of favor.

Bi-directional couplers
Couplers that operate in the same way in both directions.

Bi-directional transmission
Signals are passed and carried in both directions along a single fibre or device.

Blown fibre
A method for installing fibre in which fibres are through a vessel, for instance a tube.

BPON
Broadband passive optical network

Broadband
Transmission facilities capable of handling a wide range of frequencies or wavelength simultaneously.

Buffer
The coating material that covers and protects the fibre. Typically a plastic material, it protects the fibre from external stresses or abrasion.
Buffer tubes
Extruded cylindrical tubes covering optical fibers(s) used for protection and isolation. (See Loose Tube.)

Bundle
Many individual fibers contained within a single jacket or buffer tube. Also a group of buffered fibers distinguished in some fashion from another group in the same cable core.