Fibre Optics Glossary

Adaptor
To ensure proper alignment of two fibre optic connectors an 'adaptor' is used. This contains the split sleeve (interconnect sleeve) that holds the two connector prongs (ferrules) together.

Absorption
As light travels down an optical fibre it loses energy, either by 'absorption' of photons by impurities in the glass or by scattering. These are the prime causes of signal loss (attenuation).

Acceptance angle
The maximum angle to the axis (centre line) of an optical fibre at which a light ray will enter the core and be propagated along the fibre.

Add/drop multiplexer
One means of sending multiple streams of information down an optical fibre is to assign them different wavelengths. This is called WDM (wavelength-division multiplexing). An 'add/drop multiplexor' can add or remove information streams from the light travelling through a fibre.

Aramid yarn
An ingredient in optical fiber cable that provides support, protection and tensile strength. Also referred to as KEVLAR, which is a brand of aramid yarn.

ATM (Asynchronous Transfer Mode)
A mechanism (or protocol) for sending information throughout a network that relies on the fast transmission and switching of small packets of data.

Attenuation
The loss of signal strength (optical power) during transmission between two points. It expresses the inefficiency of an optical system, measured in decibels per kilometer (dB/km) at specific wavelengths of light.

Axis
The center of an optical fibre.

Bandwidth
The information-carrying capacity of an optical fibre (measured in MHz/km).

Buffer
A hard plastic coating on the outside of the fibre that protects the glass from moisture or physical damage.

Cable assembly
An optical fibre cable with connectors installed on one or both ends. See "pigtail" and "patch cord".

C band
See optical bands.

Channel spacing
The amount of bandwidth allotted to each channel.

Cladding
The material surrounding the core of an optical fibre. The cladding has a lower refractive index (faster speed) in order to keep the light in the core. The cladding and core make up an optical waveguide.

Cleave
The process of scoring and breaking the optical fibre end in order to terminate a connector.

Coating
A protective layer applied over the fibre cladding during the drawing process to protect it from the environment.

Connector
A mechanical device used on a fibre to provide a means for aligning, attaching and decoupling the fiber to a transmitter, receiver or other fibre. Commonly used connectors include 568SC (Duplex SC), ST, FDDI, FC, D4 and Biconic.

Core
The central region of an optical fibre through which light is transmitted. It has a higher refractive index (slower speed) than the surrounding cladding.

Coupler
A device that combines two or more fibre inputs into one fibre output or divides one fibre input into two or more fibre outputs.

Cutoff wavelength
The shortest wavelength at which a singlemode fibre transmits only one mode. At shorter wavelengths, it transmits two or more modes.

Dark fibre
Unused fibre lines that were laid to accommodate expanding future requirements.

dB (decibel)
A unit which expresses the (logarithmic) ratio of two power levels.

dBm (decibels milliwatt)
The logarithmic ratio of a given power level compared to one milliwatt (mW). e.g. 0dBm = 1mW, 10dBm = 10mW, 20dBm = 100mW, 30dBm = 1000mW, etc.


dBµ (decibels microwatt)
Similar to dBm but where 0dBµ = 1µW (microwatt).

Diffraction
The bending of light rays as they pass around corners or through holes smaller than their own wavelengths.

Dispersion
The spreading or broadening of light pulses as they travel through a fibre.

EDFA (erbium-doped fiber amplifier)
An optical amplifier that boosts all channels in the optical signal at the same time.

EMI (Electromagnetic Interference)
The interference in signal transmission or reception resulting from electrical or magnetic fields. Optical fibres are not susceptible to EMI which is a key advantage over copper cables.

Enclosure
A cabinet used to organize and enclose cable terminations and splices for use within main equipment rooms, entrance facilities, main or intermediate cross-connects and telecommunications closets.

End finish
The quality of the end surface of a fibre prepared for splicing or terminated in a connector.

Epoxy
A thermosetting resin used to secure the fibre with the connector ferrule.

Excess loss
The amount of light lost in a coupler, beyond that inherent in the splitting to multiple output fibres.

Ferrule
The rigid prong in a fibre-optic plug that aligns the fibre with the socket.

Fibre
A thin filament of glass or plastic consisting of a core (inner region) and a cladding (outer region) and a protective coating.

Fibre amplifier
An all optical amplifier that increases signal output power without converting the light signal to electricity and back again.

Fibre tracer
An instrument that couples visible light into the Fibre to allow visual checking of continuity and tracing for correct connections.

Fusion splice
The joining of two fibre ends by applying enough heat to fuse or melt the ends together to form a continuous single fibre.

Graded-index fibre
A type of multimode fibre which used a graded profile of refractive index in the core material to correct for dispersion.

Index of refraction (refractive index)
A measure of the speed of light in a material.

Index matching fluid
A liquid used of refractive index similar to glass used to match the materials at the ends of two fibres to reduce loss and back reflection.

Infrared
A range of light with wavelengths from approximately 700 to 1000nm. Fibre-optic systems transmit between 700 and 1700nm.

Injection loss, insertion loss
The amount of light that leaks out or is otherwise lost after being inserted into a fiber either from a light source or another fiber.

Jacket
The protective outer coating of the cable.

Jumper cable
A short single fibre cable with connectors on both ends used for interconnecting other cables or testing.

L band
See Optical bands

Laser
A device that generates a coherent beam of light i.e. one in which all the light has the same phase and a single (or nearly single) wavelength.

Laser diode
A laser made of semiconductor materials widely used to transmit light into optical fibres.

LED (light emitting diode)
A device that produces light with a wide range of wavelengths. LEDs are typically used with lower-bandwidth multimode fibre.

Loose tube
The protective tube surrounding one or more fibers. This is usually found in cables used for outdoor installations.

Macrobending
The loss due to large scale bending (extrinsic loss). Bending causes imperfect guiding of light which will exceed the critical angle of reflection. Macrobending loss can be reversed once the bend is corrected.

Mechanical splice
A semi-permanent connection between two fibres made with an alignment device and index matching fluid or adhesive.


MEMS (microelectromechanical systems)
Tiny components etched from a semiconductor material that can move under the control of electronic signals. MEMS devices include movable mirrors that can switch or redirect the path of light.

Microbending
The loss of light due to small distortions in the fiber, not usually visible to the naked eye.

Micron (µm)
One micrometer or one millionth of a meter. Used to express the geometric dimension of fibres.

Modal dispersion
The spreading of light pulses along the length of the fibre caused by the different optical paths taken by light rays in a multimode fiber.

Mode
A reflective path that a light ray takes in a fibre. For singlemode fibres, there's only one mode - straight through the centre of the fibre.

Mode field diameter
A measure of the core size in singlemode fibre.

Multimode
An optical fibre in which light travels in multiple modes. Commonly used with LED sources for lower speed and short distance links than singlemode fibre.

Multiplex
Combining two or more signals into a single bit stream that can be later individually recovered.

Nanometer (nM)
One thousandth of a micrometer.

Network
A system of cables, hardware and equipment used for communications.

Numerical aperture (NA)
A measure of the light acceptance angle of the Fibre.

OEO (Optical Electrical Optical)
Refers to devices that convert light back to electricity for manipulation and then back out to light. Contrast with OOO.

Optical amplifier
A device that boosts signals in an optical fiber.

Optical bands
The spectrum for transmission in singlemode optical fibres has been broken into the following wavelength ranges, or bands:



Optical channel
A signal transmitted at one wavelength in a fibre-optic system.

Optical return loss (back reflection)
Light reflected from the cleaved or polished end of a fibre caused by the difference of refractive indices of air and glass. Typically 4% of the incident light.

Optical switch
A device that routes an optical signal from one or more input ports to one or more output ports.

OTDR (Optical Time Domain Reflectometer)
An instrument that measures optical transmission characteristics by sending a short pulse of light down a fibre and observing backscattered light. Used to measure fibre attenuation and evaluate optical transmission at splices and connectors.

Passive optical network
A fibre-optic system with no active components between its distribution point and remote receiver nodes.

Photodiode
A device that receives optical power and changes it to electrical power. See photoelectric.

PC (Physical Contacting)
Refers to the type of fibre-optic connector that makes actual contact of two terminated fibre ends, keeping signal losses to a minimum.

Patch cord
A specific length of optical fibre cable with terminated connectors on each end. Used for connecting patch panels or optoelectronic devices.

Photon
A particle of light.

Photonic
Having to do with light or photons.

Pigtail
A short length of fibre in which one end is attached to a component and the other is free to be spliced to another fibre.

Polarization
The alignment of the perpendicular electrical and magnetic fields that make up a light wave.

Polishing paper
Also known as lapping film, it is a paper with a fine grit used to remove any imperfections in the fibre end surface that may exist after cleaving. Fibre ends terminated within a connector are polished flush with the end of the ferrule.

Polishing puck
A device used to hold the connector during the polishing of the fibre.

Raman amplifier
A device that boosts the signal in an optical fibre by transferring energy from a powerful pump beam to a weaker signal beam.

Receiver (RX)
An optoelectronic device that converts optical signals into electrical signals.

Reflection
The process that occurs when a light ray traveling in one material hits a different material and reflects back into the original material without loss of light.
 
Refraction
The bending of light rays as they pass through a transmission medium of one refractive index into a medium with a different refractive index.

Refractive index
The ratio of the velocity of light in a vacuum to the velocity of light in a specific material. Using 1.0 as the base reference, the higher the number, the slower light travels.

Repeater
A transceiver that converts optical signals to electronic and back out to optical.

Riser
A pathway for indoor cables that pass between floors.

S band
See optical bands.

Scattering
A property of glass that causes light to deflect from the fibre and contribute to losses (intrinsic attenuation).

SDH (Synchronous Digital Hierarchy)
A scale of standard data rates for fibre-optic systems defined by the ITU.

Singlemode
An optical fibre in which the signal travels in one mode (path). It typically has an 8-10 µm core within a 125 µm cladding.

Soliton
A laser pulse that retains its shape in a fibre over long distances.

Splice
A method for joining two optical fibre ends. Fusion splicing and mechanical splicing are the two types.

Splice closure
A container used to hold and protect splice trays.

Splice tray
A container used to hold, organize and protect spliced fibres.

Split sleeve
The part of a fibre-optic adapter that aligns the ferrules of two terminated connectors.

Splitter
A device that takes the light from one fibre and injects it into the cores of several other fibers.

Step-index fibre
A fibre in which the core and cladding each have a uniform, but different, refractive index.

Threshold current
The minimum current required to cause a diode laser to generate a beam of light.

Tight buffer
A protective coating (typically 900 µm) that is extruded directly over the primary coating of fibers. Provides high tensile strength, durability, ease of handling and termination.

Transceiver
A transmitter and receiver combined in one device.

Transmitter (TX)
An optoelectronic device that converts an electrical signal to an optical signal. It is usually an LED or laser diode.

Transparent network
A fibre-optic network that is entirely light based with optical switches and other optical-only devices.

Tunable laser
A laser that can change its frequency over a given range.

VOA (Variable Optical Attenuator)
A device that can be adjusted to block different fractions of light passing through it.

Waveguide
A structure that guides electromagnetic waves. An optical fibre is an optical waveguide.

Waveguide array
A device that separates wavelengths by passing them through an array of curved waveguides running between a pair of mixing regions.

Wavelength
The length of a wave measured from any point on one wave to the corresponding point on the next. The wavelengths of light used in optical fibres are measured in nanometers. Common wavelengths are 850, 1300 and 1350 nm.