Optical Networking Glossary

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A Absorption
The taking-in of energy, which can cause an ion to move to a higher energy state or can cause a material to heat up, for example. An optical signal losing its energy to atoms in a fiber is said to be absorbed.

Acceptance angle
A measure of the different angles through which light can be taken into an optical fiber.

Acceptance testing
The testing of an optical network system to make sure that it performs as required.

Acousto-optic effect
A change in a material’s refractive index caused by acoustic pulses or waves.

Active device
A device that requires a source of energy to function, unlike passive devices.

Active layer/Laser cavity
The layer in a laser that is designed to confine the light and allow it to build up before being emitted.

All-wavelength fiber
Optical fiber that removes the attenuation peak at 1400nm to increase the range of possible wavelengths that can be transmitted at low loss.

Amplified spontaneous emission (ASE)
Buildup of unwanted spontaneous emission in an optical fiber amplifier. ASE is an undesirable effect and adds “noise” to the amplifier system.

Amplifier/Repeater/Optical amplifier
Device that boosts the power of an optical signal. An optical amplifier does this without any conversion of the light into an electrical signal.

Angstrom
A length of 10 to the power of minus 10 meters, i.e. 0.0000000001m.

Arrayed waveguide gratings (AWGs)
Integrated optical circuits formed by a series of curved silica waveguides that can combine (multiplex) or separate (demultiplex) different wavelength signals in an optical network.

ASCII
American Standard Code for Information Interchange – a protocol for basic text.Asynchronous transfer mode (ATM)
A data-link layer protocol enabling integration of voice and data with the ability to provide QOS guarantees.Asynchronous transmission
A system in which data is transmitted at random times, as opposed to a synchronous system.

ATM adaptation layers (AALs)
Convert information into ATM cells.

Atom
Formed by a nucleus of positively charged protons and neutral neutrons, with negatively charged electrons orbiting the nucleus.

Attenuation/Loss
In an optical fiber, the absorption of light by molecules in the fiber — causing some of the intensity of the light to be lost from the signal. Usually measured in dB/km.

Available bit rate (ABR)
A QOS level guaranteeing a minimum rate of data transfer with the possibility of higher rates if available.

Avalanche photodiode (APD)
A semiconductor photodiode that can generate as many as 100 excited electrons for every incident photon.

B Backward pumping/Counter-pumping
Inserting the pump laser light at the end of the fiber in a Raman amplified system.

10Base-T
An Ethernet format that has a 10-Mbit/s transfer rate over unshielded twisted pair (UTP) cable with a length limit of around 100m.

100Base-T
An Ethernet format that has a 100-Mbit/s transfer rate over unshielded twisted pair (UTP) cable with a length limit of 100m (Fast Ethernet).

Best effort
The transmission of information through a network by using any remaining bandwidth not used elsewhere. No guarantees are provided in terms of a minimum level of service.

Binary
A method to represent numbers by sequences of 1s and 0s.

Bit (Binary digIT)
The smallest piece of information, transmitted as a flash of laser light (represented by a 1 digit) or the lack of a flash of laser light (represented by a 0 digit). Sequences of bits (1s and 0s) represent all the information transmitted in a digital system.

Bit error rate (BER)
A measure of the accuracy of information transmitted in a system, calculated as the number of incorrect bits divided by the total number of bits transmitted.

Bit rate
The speed of information transfer — the number of bits that can be transmitted each second (the number of on-off laser flashes possible per second).

Bit-rate transparency
A feature of a device enabling it to handle optical signals regardless of their bit-rates.

Border gateway protocol (BGP)
A protocol that establishes routes for IP routers.

Bragg wavelength
The wavelength of maximum reflection in a fiber Bragg grating. The Bragg wavelength depends on the amount of refractive index change in the fiber core and the spacing of these periodic changes.

Byte
Eight bits.

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C Cable
One or more optical fibers safely packaged up to be laid in the ground or under the ocean.

Carrier-sense multiple access (CSMA)
The method used by Ethernet to access a cable by sensing if anyone else is transmitting on it before sending data from a device.

C-band (Conventional band)
In erbium doped fiber amplifiers, the region of amplification approximately between wavelengths of 1530nm and 1580nm.

Cell
An envelope for data, as in an ATM cell.

Channel
A signal in an optical network. Different wavelengths can be referred to as different channels.

Chirping
Rapid changing of a laser’s peak emission wavelength.

Chromatic dispersion
The spreading out of light pulses in time, due to different wavelengths traveling at different speeds. A combination of material dispersion and waveguide dispersion.

Circuit switching
A protocol that enables communication between devices by fixing in place a connection (a "circuit") between them. Continuous information streams, such as phone calls, are best suited to circuits.

Cladding
The section of an optical fiber surrounding the core. The cladding’s lower refractive index serves to guide light within the core.

Cladding mode
Unwanted light waves that travel through an optical fiber’s cladding rather than its core.

Clock
A timing signal in an optical network, required so that devices know exactly at what point to look for bits of information. The clock signal is matched to the bit rate of the system.

Clock recovery/Timing recovery
The extraction of clock information from an optical signal, used to know exactly when to sample the signal to look for a 1 or a 0.

Coarse wavelength-division multiplexing (CWDM)
A version of WDM that employs multiple, widely spaced wavelengths, lying anywhere in the transmitting region of an optical fiber (1260 to 1620 nanometers). Coherent
A set of light waves that are in-sync together (known as being “in phase”).

Co-counter pumping
Inserting pump laser light at both the beginning and the end of the fiber in a Raman amplified system.

Connectionless
A protocol that does not require a connection to be in place in order to send information between two locations.

Connection-oriented
A protocol that must establish a connection between two devices before it can begin to send information between them.

Constant bit rate (CBR)
A QOS level giving a fixed and guaranteed amount of network capacity to a circuit, useful for time-sensitive information such as voice.

Conventional fiber
Optical fiber allowing only a single mode to travel through it, due to having a very small core diameter, relative to its cladding. Provides lowest loss in the 1550nm region and has zero dispersion at 1300nm. Has a core with a higher refractive index than the cladding.

Co-pumping/Forward pumping
Inserting the pump laser light at the beginning of the fiber in a Raman amplified system.

Core
The central part of an optical fiber, usually made from silica and surrounded by the lower refractive index cladding. Light travels along the core of an optical fiber.

Counter-pumping/Backward pumping
Inserting the pump laser light at the end of the fiber in a Raman amplified system.

Coupler
Device that combines or splits light from optical devices.

Coupling efficiency
The efficiency with which light can be transferred between two devices that are joined (coupled) together.

Cross-phase modulation
A nonlinear Kerr effect, whereby several different wavelengths in a WDM system can cause each other to spread out.

Crosstalk
One optical signal being adversely affected by the presence of another.

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D dB
Decibel — usually used as a figure of loss or gain in an optical network. Calculated as one tenth of the logarithm of the output power divided by the input power.

dBm
A unit of power, popular in optical networks, showing the power with respect to 1 milliwatt (0.001W). Uses the dB equation, with the input power set to 1 milliwatt. Zero dBm is a power of 1 milliwatt.

Demultiplexing
The separating of different wavelengths in a wavelength-division multiplexing system. The opposite of multiplexing.

Dense wavelength-division multiplexing (DWDM)
A variety of WDM that uses multiple wavelengths (or channels) in the 1550-nanometer region of the infrared spectrum. The wavelengths are closely spaced — usually evenly, on a grid.Detector
A device that detects laser light and converts it back into electrical form.

Diffraction
The deflecting of a light wave when it travels through an object such as a grating.

Digital wrapper
An encapsulation method for traffic of all protocols, allowing an optical network to be protocol transparent, its functions to be managed in an efficient manner, and its data to be transmitted more reliably.

Diode laser
Tiny salt-grain-sized semiconductor devices giving out intense light at one specific color when an electric current is passed through them.

Discrete amplification
Amplification that takes place at one particular point in a system.

Dispersion
Different wavelengths of light travel at slightly different speeds in optical fiber, and this causes optical pulses to spread out as they travel through a system.

Dispersion-compensating fiber
Optical fiber with highly negative dispersion at 1550nm to compensate for a buildup of positive dispersion in that region in conventional fiber.Dispersion-shifted fiber (DSF)
Optical fiber that has a point of minimum dispersion moved towards its point of lowest attenuation — around 1550nm.

Dispersion slope
Reflects the fact that different wavelengths experience different amounts of dispersion in an optical fiber.

Distributed amplification
Amplification that takes place over the length of a system, rather than at one discrete point.

Distributed feedback (DFB) laser
Lasers giving out a very sharply defined color of light. Similar to Fabry-Perot design but with the addition of a corrugated structure above the active layer. This feeds back one specific wavelength into the cavity, which is then amplified and emitted.

Doping
The adding of an impurity (dopant) to a material. Used to change the optical properties of optical fiber in erbium doped fiber amplifiers.

E Edge emitter
A regular design of laser (e.g. Fabry-Perot or distributed feedback) which gives out light from its edges. Opposite to a surface emitter.

Electrical excitation
In a laser, the achievement of population inversion through an electric current exciting electrons into higher energy states.

Electro-optic effect
Phenomenon relating to light interacting with an electric field.

Electron
A negatively charged particle present in all atoms.

Energy-wavelength relationship
The energy of a wave is a constant number divided by its wavelength. So a large wavelength has a low energy, and a short wavelength has a high energy.

Energy gap
The amount of energy required to move an ion/electron between specific excited or ground states.

Erbium
A rare earth element used to dope fiber in erbium doped fiber amplifiers.

Erbium doped fiber amplifier (EDFA)
Optical amplifiers made of short lengths of optical fiber doped with the element “erbium.” A pump laser excites erbium ions in the fiber, which can then give their energy to optical signals passing through.

Ethernet
A data-link layer protocol commonly used to transfer Internet Protocol packets over cables in local area networks.

Excited states/Excited levels
The energy states to which ions and electrons can be moved from the ground state if they are given some energy.

Eye diagram/Eye pattern
1 and 0 signals in an optical system are displayed on an oscilloscope to show how clearly they are defined. An “open” eye shows a good quality signal with clear differences between 1s and 0s. A “closed” eye means that some 0s could be confused for 1s and vice versa, and therefore is a sign of a poor transmission system.

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F Fabry-Perot laser
The most basic design of laser, consisting of two specially designed slabs of semiconductor material on top of each other, with another material between them forming what is known as the “active layer” or “laser cavity.” Electric current flows through the device from the top slab through to the bottom, and the emission of light occurs in the active layer.

Fast Ethernet
An Ethernet format that has a 100-Mbit/s transfer rate over unshielded twisted pair (UTP) cable with a length limit of 100m (100Base-T).Feedback
The return of some output back into the input. The returning of light back into the laser cavity in a distributed feedback laser.

Fiber/Optical fiber
Very thin strands of pure silica glass through which laser light travels in an optical network. Consists of a core surrounded by a less refractive index cladding.

Fiber Bragg gratings (FBGs)
Small sections of optical fiber that act like selective mirrors — only reflecting back specific wavelengths. This reflection is caused by a periodic change of refractive index in the fiber core, which reflects mainly at the Bragg wavelength.

Fiber grating laser
A cheaper alternative to DFB lasers; formed from conventional Fabry-Perot lasers with a fiber Bragg grating in the output that filters out unwanted wavelengths.

Fiber pigtail/Pigtail
A short section of optical fiber, constantly connected to a device and transmitting light from that device.

First window
The 850nm region of wavelengths used in optical networks.

FIT rate
A measure of reliability for a device — the number of failures in one billion device hours.

Forward equivalence class
Identifies flows and classifies them in terms of any QOS requirements in an MPLS network.

Forward error correction (FEC)
Mathematical algorithms performed on data to be transmitted in a network create FEC information that can be used at the receiving end to detect and correct any errors in the transmitted data.

Forward pumping/Co-pumping
Inserting the pump laser light at the beginning of the fiber in a Raman amplified system.

Four-wave mixing (FWM)
A nonlinear Kerr effect in which two or more signal wavelengths can interact to create a new wavelength.

Frame
An envelope for data, as in an Ethernet frame.

Frequency
The number of cycles per unit time, measured in Hertz (Hz).

Fusion splice/Splice
The connecting of two optical fibers by effectively melting their ends to join them together.

G Gain
The ratio of output optical power to input power, usually given in units of dB. Usually represents an increase in an optical signal; a negative gain means a loss of power.

Gain flattening
The art of getting equal amounts of amplification over a range of wavelengths in an optical amplifier.

Gbit/s — gigabits per second
1 gigabit per second is 1 billion bits per second.

Gigabit Ethernet
An Ethernet format that provides 1-Gbit/s access rates on optical fiber over distances of up to several kilometers.

Graded-index fiber
Optical fiber with a core whose refractive index changes with the distance from its axis.

Grating
A device designed to allow specific wavelengths to be reflected, while others pass through it.

Ground state
The usual location of ions and electrons, which can absorb energy and then move up to excited states.

H Humicubation
Lying on the ground, especially as a sign of penitence or humiliation.

I Ion
An electrically charged atom.

Infrared
The region of electromagnetic waves between around 700nm and 0.1mm in wavelength. Includes all the wavelengths that are transmitted in optical networks. (Visible light ranges from around 400nm to 700nm.)

Insertion loss
The loss introduced into a system by the connection of a device. Usually given in dB.

Integrated optical circuits
Similar in principle to electrical integrated circuits, but with the combining of many tiny versions of current optical components onto single silicon wafers.

Intensity
Strictly speaking, intensity is the power per unit area. Usually used just to mean the amount of light — the power.

Internet Protocol (IP)
A network-layer protocol that puts Internet data into packets and helps route them through a local area network.

IP address
An address used in an IP network, given in a so-called "dotted decimal" notation (eg., "129.67.219.9").

IP router
A device for directing IP packets towards their final destination.

IPv6
A new version of IP (version 6) that includes 16-byte addresses as one of its improvements.

Isolator
A device designed to allow light through it in only one direction.

ITU-Grid
Each different wavelength in a wavelength-division multiplexed system will be separated by a multiple of 0.8nm. This is sometimes referred to as “100GHz spacing”, which is the frequency separation, or as the “ITU-Grid” — named after the standards body that set the figure.

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J Jitter
Rapid changes in an optical signal — usually referring to changes in the time interval between optical pulses.

K Kerr effect
The refractive index change in an optical fiber core, due to the intensity of light traveling through it.

L Label Distribution Protocol (LDP)
A protocol ensuring that MPLS nodes throughout a network know which labels correspond to which routes.

Label edge router (LER)
A device that handles the entrance and exit of all information in an MPLS network.

Label switch router (LSR)
The nodes in between the source and destination LERs of an MPLS network that switch the labeled packets of information rapidly through the network.

Label switched path (LSP)
The paths over which labeled packets are switched by LSRs in an MPLS network.

LAN
Local area network.

Large effective area fiber
A fiber that allows light to be transmitted in a larger total area of the fiber in order to reduce the concentration of high powers that may cause adverse nonlinear effects.

Laser/Laser diode/Laser chip
Light Amplification by Stimulated Emission of Radiation. Tiny salt-grain sized semiconductor devices giving out intense light at one specific color when an electric current is passed through them. Used to transmit information in an optical network.

Laser cavity/Active layer
The layer in a laser that is designed to confine the light and allow it to build up before being emitted.

L-band (Long band)
In erbium doped fiber amplifiers, the region of amplification approximately between wavelengths of 1580nm and 1610nm.

Line amplification
Amplifying optical signals in the middle of an optical network link.

Linewidth/Spectral width
Any optical signal is spread at least a small amount onto neighboring wavelengths. Spectral width is the range of wavelengths over which one particular signal is transmitted.

Loss
The ratio of output optical power to input power, usually given in units of dB. Usually represents a decrease in an optical signal, a negative loss means a gain of power.

Loss/Attenuation
In an optical fiber, the absorption of light by molecules in the fiber — causing some of the intensity of the light to be lost from the signal. Usually measured in dB/km.

Loss budget/Power budget
The amount of optical power launched into a system that will be lost through various mechanisms, e.g., insertion losses and fiber attenuation. Usually given in dB.

M Macrobends
Large radius bends of an optical fiber, such as around the corner of a piece of equipment (maybe less than 10cm in radius). Macrobends can cause some loss of the signal within the fiber.

Magneto-optic effect
Phenomenon caused by light waves interacting with magnetic fields.

MAN
Metropolitan area network.

Material dispersion
Contributes to the overall chromatic dispersion of a system, causing different wavelengths to travel at different speeds due to the variation in refractive indexes of the fiber core corresponding to changing wavelengths.

Mbit/s — megabits per second
1 megabit per second is 1 million bits per second.

Medium access control (MAC) address
Every device connected to a LAN has an associated MAC address that is hard-wired to it.

Microbends
Tiny bends of an optical fiber, e.g., over a tiny sharp ridge on the ground. Microbends can cause some loss of the signal within the fiber.

Micro-electro-mechanical systems (MEMS) mirrors
Mirrors, no larger in diameter than a human hair, which can be arranged on special pivots so that they can be moved in 3 dimensions. Several hundred such mirrors can be placed together on mirror arrays no more than a few centimeters square in size, to form an optical crossconnect.

Micrometer/Micron
A length of 0.000001 meters.

Mie scattering
Light scattering in an optical fiber because its core is not always a perfect cylinder.

Mode
One of the various light waves that can be transmitted in an optical fiber. Each optical signal generates many different modes, but in singlemode fiber the aim is to only have one of them transmitted.

Modulation
The creation of patterns of 1s and 0s to represent useful information to be transmitted though an optical network. Performed by a modulator, which will usually control the output of a laser to generate the required sequences of 1s and 0s.

Modulation bandwidth
The frequency associated with a modulator at which the difference in light power between the 1s and 0s has dropped to half of the value it was at the very low frequencies.

Monochromatic
Of one “color” or one “wavelength.”

MPlambdaS
A futuristic application of MPLS in which wavelengths are switched through a network.

Multimode fiber
An optical fiber in which more than one mode can be transmitted through the core.

Multiplexing
The combining of different wavelengths in a wavelength-division multiplexing system. The opposite of demultiplexing.

Multiprotocol label switching (MPLS)
A protocol for the swift routing of data streams, bringing improved performance and QOS options to Internet Protocol traffic.

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N Nanometer (nm)
A length of 0.000000001 (one billionth) meters.

Node
A point in an optical network where optical signals can be processed and switched among various links.

Noise
Distortion of an optical signal that can affect the accuracy of transmitted information.

Noise figure (NF)
A measure of the amount of noise in a system.

Nonlinear effects
High optical powers causing various effects on signals in optical fibers, usually detrimental. The output does not scale linearly with the input, so if the input signal doubles in power then the output is less than double.

Non-return-to-zero (NRZ)
A method of transmitting 1s (flashes of laser light) and 0s (no light) whereby if several 1s are to be transmitted in succession, the laser light does not turn off in between 1s. It would just remain on throughout and not return to zero light. The opposite of a return-to-zero (RZ) system.

Non-zero dispersion-shifted fiber (NZ-DSF)
Optical fiber giving a small amount of dispersion in the 1550nm region that can reduce the effects of four-wave mixing.

OOpen shortest path first (OSPF)
A protocol that establishes routes for IP routers.

Open systems interconnection (OSI) model
The seven standardized layers of protocols that combine to transport data around a network.

Optical add-drop multiplexer (OADM)
A device that can take in (add) or take out (drop) individual wavelengths from an optical fiber completely in the optical domain (i.e., without any conversion of the light into electricity).

Optical amplifier
Device that boosts the power of an optical signal without any conversion of the light into an electrical signal.

Optical channel
An individual wavelength in a system.

Optical crossconnect (OXC)/Optical switch
Device that can move optical signals between different optical fibers, without the need for conversion to electrical signals.

Optical excitation
In a laser, the achievement of population inversion through light from another laser exciting electrons into higher energy states.

Optical fiber/Fiber
Very thin strands of pure silica glass through which laser light travels in an optical network. Consists of a core surrounded by a less refractive cladding.

Optical network
Optical fibers connecting locations many miles apart, carrying information in the form of on-off flashes of laser light.

Optical packet switch/router
A device of the future that can route individual packets through a network without needing to convert the signal into electrical format first, as in conventional routers.

Optical pulse/Pulse
A short burst of light energy.

Optical signal/Signal
Light energy in a system, carrying information.

Optical switch/Optical crossconnect (OXC)
Device that can move optical signals between different optical fibers, without the need for conversion to electrical signals.

Optical time-division multiplexing (OTDM)
The interleaving of optical signals from different sources to make a higher composite bit-rate, e.g., combining four 10-Gbit/s signals to give one 40-Gbit/s signal.

Optical time-domain reflectometer (OTDR)
Device used to inspect optical fiber links by sending optical pulses down them and monitoring the light reflected back to the device. Can calculate overall fiber attenuation and highlight points of high loss in the fiber, or even fiber breaks.

Optoelectronic devices
Devices converting electrical energy to optical energy or vice-versa.

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P Packet
An envelope for data, as in an IP packet.

Packet switching
A protocol that enables the sending of data in packets, to be routed through the network based upon address information contained within their headers. Sporadic data exchanges, such as Web traffic, are best served by packets.

Passive device
A device that does not require a source of energy to function, unlike active devices.

Passive optical network (PON)
Enables a single fiber access line to support a cluster of buildings through use of a passive splitter close to the cluster, so that light from a single fiber can be split into different wavelengths and steered to individual buildings via short lengths of fiber.

Path overhead (POH)
An overhead in an SDH frame whose function is to monitor the quality of the link and indicate the type of data payload the frame is carrying.

Permanent virtual circuit (PVC)
A connection that is constantly in place.

Photodetector/Photodiode
A device that converts optical signals (photons) into electrical signals (electrons).

Photon
A single “particle” of light.

Photonics
The field dealing with photons, including device and system design for optical networks.

Pigtail/Fiber pigtail
A short section of optical fiber, constantly connected to a device and transmitting light from that device.

P-i-n photodiode
A semiconductor photodiode that excites one electron per incident photon.

Polarization
The direction of the electric field in an electromagnetic wave.

Polarization-maintaining fiber
Optical fiber in which the different polarizations of light do not interfere with each other.

Polarization mode dispersion (PMD)
Dispersion of an optical signal due to the different polarizations of light traveling at different speeds through optical fiber, caused by imperfections in fiber symmetry and fluctuating fiber stresses.

Population inversion
In a semiconductor laser, the situation where the majority of electrons are in the excited state.

Power
The rate of transfer of energy per unit time in a system.

Power budget/Loss budget
The amount of optical power launched into a system that will be lost through various mechanisms, e.g., insertion losses and fiber attenuation. Usually given in dB.

Power amplification
Amplifying optical signals just after they have left the laser.

Praseodymium
A rare earth element that can be used in the same way as erbium in erbium-doped fiber amplifiers (EDFAs). Praseodymium provides gain in the 1300nm region.

Pre-amplification
Amplification of optical signals at the end of the system just before they reach the detectors.

Pre-amplifier
Adds an EDFA before a p-i-n photodetector to give better detection performance.

Pre-emphasis
Altering the initial powers of different wavelengths in a system, so that they all arrive at the end of the system with equal powers. This can counteract the effects of stages in a system that may incur different amounts of loss or gain at different wavelengths.

Protection switching
Automatic switching between fiber links, should one become unavailable.

Pulse/Optical pulse
A short burst of light energy.

Pump laser
A laser used to excite ions in a material, usually used in optical amplifiers.

Q Quality of service (QOS)
The level of service provided to a customer through a network connection.

R Raman amplification
Optical amplification process throughout the actual transmission fiber in an optical network, caused by a carefully selected pump-laser wavelength scattering from atoms in the fiber and changing its wavelength to that of the optical signal. Performs stimulated Raman scattering.

Rayleigh scattering
Light scattering in an optical fiber due to slight changes in the core’s refractive index.

Reamplification/1R regeneration
Boosting the power of an optical or electrical signal.

Refraction
The change in direction of light due to its passing between two different materials.

Refractive index
A property of a material that determines how fast light travels through it.

Receivers
Devices placed at the end of an optical network with detectors to convert the laser light back into electrical form.

Reed-Solomon
Mathematical algorithms that can be used for forward error correction.

3R regeneration
The process of amplifying (correcting loss), reshaping (correcting noise and dispersion), retiming (synchronizing with the network clock), and retransmitting an optical signal.

Repeater/Amplifier/Optical amplifier
Device that boosts the power of an optical signal. An optical amplifier does this without any conversion of the light into an electrical signal.

Reshaping
Removing noise from a digital signal and shaping it into clear 1s and 0s.

Retiming
Adjusting the precise locations of 1s and 0s in a detected signal in order to match them to the bit-rate of a system.

Return-to-zero (RZ)
A method of transmitting 1s (flashes of laser light) and 0s (no light) whereby if several 1s are to be transmitted in succession, the laser light turns off in between 1s. It therefore flashes each time it wants to give out a 1, returning to zero light in between 1s. The opposite of a non-return-to-zero (RZ) system.

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S Second window
The 1300nm region of wavelengths used in optical networks.

Self phase modulation
A nonlinear Kerr effect, whereby a wavelength can spread out onto adjacent wavelengths by itself.

Section overhead (SOH)
The first 9 columns of an STM1 frame in SDH. Provides a comprehensive range of facilities such as error monitoring, network management, and protection switching.

Semiconductor
A material that can behave as an insulator (not conducting electricity) or as a conductor (conducting electricity).

Semiconductor laser
Tiny salt-grain sized semiconductor devices giving out intense light at one specific color when an electric current is passed through them.

Semiconductor optical amplifiers (SOAs)
Similar to regular lasers, but with non-reflecting ends and broad wavelength emission. An incoming optical signal stimulates emission of light at its own wavelength, therefore amplifying it.

Signal/Optical signal
Light energy in a system, carrying information.

Signal-to-noise ratio (SNR)
Ratio of the amplitude of the optical signal to the amplitude of the noise.

Silica
Very pure glass from which optical fibers are manufactured, a combination of silicon and oxygen – Si0₂.

Singlemode fiber
A mode is one of the various light waves that can be transmitted in an optical fiber. Each optical signal generates many different modes, but in singlemode fiber the aim is to only have one of them transmitted. This is achieved through having a core of a very small diameter (usually around 10 micrometers). A.k.a. conventional, standard, or step-index fiber.

Snell’s law
A law of physics that allows light to be guided within the core of an optical fiber due to the lower refractive index of the surrounding cladding layer.

Soliton pulse
A specially designed optical pulse that takes advantage of nonlinear effects to reverse the effects of dispersion, and travel through a system while maintaining its shape and integrity.

Spectral width/Linewidth
Any optical signal is spread at least a small amount onto neighboring wavelengths. Spectral width is the range of wavelengths over which one particular signal is transmitted.

Splice/Fusion splice
The connecting of two optical fibers by effectively melting their ends to join them together.

Spontaneous emission of light
Light given out by ions or electrons naturally falling from excited states to ground states.

Stimulated Brillouin scattering
A nonlinear effect whereby light in fiber can create acoustic waves, which then scatter light to different wavelengths.

Stimulated emission of light
Light given out by ions or electrons falling from excited states to ground states, caused by other light in the system.

Stimulated Raman scattering
A nonlinear phenomenon whereby an optical signal scatters from atoms in an optical fiber, loses some energy to those atoms, and therefore increases in wavelength. Used in Raman amplification.

Submarine systems
Optical fiber links connecting countries through the world’s oceans.

Supervisory signals
Signals used for the control of various devices within an optical network, e.g., amplifiers.

Surface emitter
A design of laser that gives out light from its surface, usually with a vertical cavity (as in the vertical cavity surface emitting laser). Opposite of an edge emitter.

Switched virtual circuit (SVC)
Connection that can be dynamically created and destroyed as required by the network.

Synchronous digital hierarchy (SDH)
Physical-layer protocol for packaging data to send through an optical network. U.S. version is called Sonet.

Synchronous optical network (Sonet)
Version of SDH primarily used in U.S., Canada, and Japan.

Synchronous transmission
A system in which data is transmitted at regular time intervals, as opposed to an asynchronous system.

Synchronous transport module (STM)
The SDH frame.

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T Tag switching
See: Multiprotocol label switching (MPLS).

Tapered fiber
An optical fiber with a shaped end to allow more light to be coupled into its core.

TCP/IP
The protocol suite installed on desktop computers to enable interfacing with networks.

Third window
The 1550nm region of wavelengths used in optical networks. This is the point of minimum attenuation in conventional optical fiber.

Threshold current
The specific value of electrical current applied to a laser at which the stimulated emission of light really takes over from the spontaneous emission.

Thulium
A rare earth element that can be used in the same way as erbium in erbium-doped fiber amplifiers (EDFAs). Thulium provides gain in the 1400nm region.

Time-division multiplexing (TDM)
A constant stream of data traveling through the network, created by multiplexing several low bit-rate streams into one high bit-rate stream.

Time to live (TTL)
A field in an IP packet header that will be decreased by one every time the packet passes through an IP router in the network, until it reaches zero and the packet is then discarded.

Timing recovery/Clock recovery
The extraction of clock information from an optical signal, useful to know exactly when to sample the signal to look for a 1 or a 0.

Time-division multiplexing (TDM)
The interleaving of signals from different sources to make a higher composite bit-rate.

Trans-Atlantic Telephone cable (TAT)
A series of submarine systems placed across the Atlantic Ocean connecting the U.S. with Europe. The most recent cable is TAT-14.

Transmission
The sending of optical signals through optical fibers, and associated effects and phenomena.

Transmission control protocol (TCP)
A protocol used to track IP packets and make sure that they arrive where they should.

Transmitters
Devices placed at the beginning of an optical network to transmit light representing information. Usually consisting of lasers and modulators.

Transponder
A device that takes an optical signal, performs electrical 3R regeneration, and then retransmits the signal in optical form. The output can be of a different wavelength than the input if so required.

Tunable lasers
Lasers that can be adjusted to emit one of several different wavelengths, usually on the ITU-Grid.

U Unshielded twisted pair (UTP)
A cable used in Ethernet.

Unspecified bit rate (UBR)
A QOS level providing whatever spare capacity the network has at any one time.

VVariable bit rate (VBR)
A QOS level having the ability to grow and shrink in size with data needs, but with the possibility for some loss of cells if requirements cannot be met due to higher priority traffic from other sources.

Vertical cavity surface emitting lasers (VCSELs)
Lasers with a vertical cavity that emit light from their surface, in contrast with regular “edge emitters.”

Virtual channel (VC)
A connection established for information exchange in an ATM network. Many virtual channels are contained in a virtual path.

Virtual channel identifier (VCI)
An element in the ATM header specifying which channel is being used.

Virtual container (VC)
The data-carrying part of an SDH frame. In the case of an STM1 it's known as a VC4.

Virtual path
In ATM, a route between two points containing many (up to several thousand) virtual channels that are initiated for each specific information exchange.

Virtual path identifier (VPI)
An element in the ATM header specifying which path is being used.

Virtual private network (VPN)
A network over which a group of locations have exclusive connections with each other, although many other customers use the same physical infrastructure.

WWAN
Wide-area network.

Waveguide dispersion
Contributes to the overall chromatic dispersion of a system. Caused by a portion of the light traveling in the cladding of the fiber, which has a different refractive index to the core and therefore propagates light at a different speed.

Waveguide
A material that can guide optical energy (light).

Wavelength
All electromagnetic radiation (radio waves, microwaves, ultraviolet light, visible light, etc.) is transmitted in waves, and the wavelength is the distance between successive crests of the waves. In optical networks, you can think of different wavelengths as being different colors of light.

Wavelength-division multiplexing (WDM)
Transmitting many different colors (wavelengths) of laser light down the same optical fiber at the same time, in order to increase the amount of information that can be transferred.

X Xanthism
Abnormal or excessive yellowness.

Y Yapok
A semi-aquatic South American opossum.

Z Zero-dispersion slope
The rate of change of dispersion with respect to wavelength at the point of zero dispersion in an optical fiber.

Zero-dispersion wavelength
The wavelength of transmission in an optical fiber at which no overall dispersion occurs.

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