Glossary of fiber optic network terms Suggest a term We’re always adding new fiber optic network terms to our list. If you can’t find what you’re looking for, get in touch! We’ll respond directly, and consider adding the term to our list. µm A micron; a millionth of a meter. Common unit of measurement for fiber-optic diameters. 1-by-N coupler The first number designates the input fiber and N designates the amount of output fibers (e.g. 1X2, 1X8, etc.) Absorption The portion of optical attenuation in optical fiber resulting from the conversion of optical power to heat. Caused by impurities in the fiber such as hydroxyl ions. AC Abbreviation for alternating current. An electric current that reverses its direction at regularly recurring intervals. Acceptance Angle The half-angle of the cone (a) within which incident light is totally internally reflected by the fiber core. It is equal to sin-1(NA) Active device A device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Examples include controlled power supplies, transistors, LEDs, amplifiers, and transmitters. American National Standards Institute (ANSI) The American National Standards Institute (ANSI) is the official American standards body through which standards are published, and various other standards-setting committees are accredited. Amplifier A device, inserted within a transmission path that boosts the strength of an electronic or optical signal. Amplifiers may be placed just after the transmitter (power booster), at a distance between the transmitter and the receiver (in-line amplifier), or just before the receiver (preamplifier). Analog A continuously variable signal. Opposite of digital. Angled physical contact (APC) A polishing technique for fibers/ferrules that minimizes reflective light. A style of fiber-optic connector with a 5 -15 degree able on the connector tip for the minimum possible back reflection. Usually used in AM or DWDM single-mode transmission systems using laser light sources. Typically 65-70 dB. Application-specific optical fiber (ASOF) Fibers built for specific applications such as erbium fibers used in EDFA optical amplifiers. Other types include high N.A. fibers used for manufacturing filters and gratings, etc. Arrayed waveguide grating (AWG) A device that allows multiple wavelengths to be combined and separated in a DWDM system. An array of planar waveguides diffracts light at angles that depend on the wavelength. The central element is an array of narrow, curved waveguides that run parallel to each other between a pair of mixing regions. The waveguides differ in length by an increment that is much larger than the wavelength. The input signal enters the first mixing region and is coupled into the waveguides to pass into the second mixing region. Attenuation The loss of optical power, whether caused intrinsically (absorption, scattering and microbends), or by extrinsic components such as connectors, splices, splitters and other optical components. External stresses such as microbends and macrobends result in fiber attenuation. The term can also be used synonymously with attenuation coefficient, which is expressed in dB/km. Attenuator Either fixed or variable components that are manufactured for testing electro-optical components or to incorporate a specific amount of loss into an operational optical network. Attenuators also provide a “safety margin” in planned networks to allow for electronics degradation over time, or physical changes to the optical component portion of the network. Available as fixed with specific amounts of attenuation or a variable optical attenuator (VOA) where the attenuation value can be changed for testing optical components or systems. Attenuator, optical Passive components that produce a controlled signal attenuation in an optical fiber transmission line. Automatic test equipment (ATE) Test equipment that is computer programmed to perform a number of test measurements on a device without the need for changing the test setup. Especially useful in testing components and assemblies. Avalanche photodiode (APD) A photodiode designed to take advantage of avalanche multiplication of photocurrent. It converts one photon to multiple electrons. Back reflection A percent of the transmitted signal reflected back towards the source from a fiber-optic interface. Referenced in dB. Backscatter coefficient The ratio of the optical pulse power (not energy) at the OTDR output to the backscatter power at the near end of the fiber (z=0). This ratio is inversely proportional to the pulsewidth, because the optical pulse power is independent. It is expressed in dB. Backscattering The scattering of light into a direction generally reverse to the original one. Band A range of optical spectrum allocated based on optical amplifiers. Six bands are specified: O (original), E (enhanced), S (short), C (conventional), L (long), and U (ultra). These cover the optical spectrum from 1260 nm to 1675 nm. Bandwidth The difference between the highest and the lowest frequencies of a transmission channel or path. Identifies the amount of data that can be sent through a given channel. The greater the bandwidth, the greater the information-carrying capacity. Beam displacer A design that uses a birefringent crystal. They are used in optical couplers, optical switches, and optical isolators. Beamsplitter A passive-optical coupler that uses simple optical lenses with a reflective coating tailored to control the split ratio of the device. Bend Radius The smallest radius an optical fiber or fiber cable can bend before excessive attenuation of breakage occurs. BER The fraction of bits transmitted that are received incorrectly. The number of coding violations detected in a unit of time. Bi-directional Operating in both directions. Bi-directional transceiver A device that sends information in one direction and receives information from the opposite direction. Binary Base two numbers with only two values, 0 or 1. Bit error rate (BER) A measurement of transmission accuracy. It is a ratio of bits received in error versus bits sent. Bit error rate tester (BERT) Test equipment that measures the bit error rate (BET) of digital transmission systems. Bragg Grating A technique for building optical filtering functions directly into a piece of optical fiber based on interferometric techniques. Bypass switch A high-speed switch to transfer an optical signal to an alternate fiber. Carrier-to-noise ratio (CNR) A quantification of system noise. The ratio, expressed in decibels, of the level of the carrier to that of the noise in the receiver bandwidth before any nonlinear process such as amplitude limiting and detection takes place. C-band The C-band is the “conventional” DWDM transmission band, occupying the 1530 to 1565-nm wavelength range. All DWDM systems deployed prior to the year 2000 operated in the C-band. Center Wavelength In a laser, the nominal value central operating wavelength. It is the wavelength defined by a peak mode measurement where the effective optical power resides. Channel A communications path or the signal sent over that path. Through multiplexing several channels, voice channels can be transmitted over an optical channel. Chirp In laser diodes, the shift of the lasers center wavelength during single pulse durations. Chromatic dispersion (CD) The variation in the velocity of light (group velocity) as a function of wavelength. It causes pulses of a modulated laser source to broaden when traveling within the fiber, up to a point where pulses overlap and bit error rate increases. CD is a limiting factor in high-speed transmission and must be properly compensated, which implies proper testing. Circulator, optical The optical circulator allows for optical add/drop multiplexing in conjunction of integrating a fiber Bragg grating (FBG) and is commonly used in long haul, metropolitan area networks and networks. In a circulator, the internal passive components are arranged so that light passes from port 1 to 2, from port 2 to 3 and from port 3 to 4 while preventing it from traveling in the opposite direction. Because each level of the circulator is identical, the steps can be repeated as many times as necessary. Cladding Material that surrounds the core of an optical fiber. Its lower index of refraction compared to that of the core causes the transmitted light to travel down the core. CLEC A company that provides alternative services to customers that were historically served by local telephone companies. These services are normally data and video transport that the existing telephone network cannot handle. Coarse wavelength division multiplexing (CWDM) Applies to greater separation of wavelengths than DWDM. In the case of single-mode applications CWDM defines 20-nm separation from 1270nm to 1610nm, with 1470nm to 1610nm the most commonly used wavelengths. With multimode fibers, the wavelengths are 778, 800, 825 and 850 nm. Coating The material surrounding the cladding of a fiber. Generally a soft plastic material that protects the fiber from damage. Coder A device, also called an encoder, that converts data by the use of a code, frequently consisting of binary numbers in such a manner that reconversion to the original form is possible. Concatenation The process of connecting pieces of fiber together. Coupler An optical device used to interconnect three or more optical fibers. Also called a splitter. Can be equal splits or a percentage, e.g., 10/90%. These have a number of characteristics that determine their function and application, including the number of input and output ports, signal attenuation, wavelength selectivity, et al. Coupling loss The total optical power loss within a junction, expressed in decibels, attributed to the termination of the optical conductor. Coupling ratio Coupling ratio is a measure of how a device distributes light from its inputs to its outputs. It is expressed as either a percentage or in dB. Critical angle The maximum angle at which light can be propagated within a fiber. Sine critical angle equals the ratio of the numerical aperture to the index of refraction of the fiber core. CSRZ-DQPSK Carrier suppressed return-to-zero differential quadrature phase shift keying. These technologies make it possible to generate DWDM signals with bit rates of 100 Gbps and beyond per channel and transmit them over long distances. Cutoff Wavelength In single-mode fiber, the wavelength below which the fiber ceases to be single-mode. Data Rate The number of bits of information in a transmission system. Expressed in bits per second and which may or may not be equal to the signal rate. dB/km A logarithmic unit describing the ratio of loss of power per kilometer distance. dBm Decibels relative to one milliwatt. A positive number indicates the power is above one milliwatt; a negative number indicates the power is below. This unit has become common in fiber-optic communication systems because the power of light sources used with optical fibers is on the order of one milliwatt. Decibel (dB) A logarithmic unit describing the ratio of two powers. Used to measure loss (or attenuation) of quality, reflectance, and amplification of optical signals. The ratio of two power levels, P1 and P2, expressed by –10 log10(P1/P2). P1 and P2, expressed by -10 log10(P1/P2). Demultiplexer A device that separates the two or more signals that have been combined into a common signal. An optical demultiplexer filters signals at different wavelengths. Dense wavelength division multiplexing (DWDM) Combining four or more wavelengths into an optical window (e.g., 1550 nm). DWDM devices have a channel spacing less than or equal to 1000 GHz and can cover one or more spectral bands. Detector A device that converts optical energy to electrical energy, such as PIN or APD photodiodes. Differential group delay (DGD) A delay caused by different arrival times of optical signals, which causes dispersion. In multimode fibers it is the delay difference of the various modes, whereas in single-mode fibers it is the delay caused by chromatic, waveguide, and polarization mode dispersion. Differential mode delay (DMD) A distortion of the signal-bearing light-pulse over lengths of multimode optical fiber. The light paths traveling the full width of the core will take longer to get to the receiver than those closer to the core. Diffraction grating An array of fine, parallel, equally-spaced reflecting or transmitting lines that mutually enhance the effects of diffraction to concentrate the diffracted light in a few directions determined by the spacing of the lines and by the wavelength of the light. Digital A data format that uses discrete varying signals to contain information. Diplexer Used primarily in OLT and ONT of an FTTx network to combine or separate the 1310-nm detection and 1490-nm transmission of voice and data streams and includes an internal WDM module for wavelength separation. Directional coupler A fiber-optic coupler that preferentially transmits light in one direction. Directionality The suppression backreflections, generally measured in dB. If a 0 dBm signal passes through a coupler with 50 dB directionality, only -50 dBm will pass in the wrong direction. Dispersion The cause of bandwidth limitations in a fiber. Dispersion causes a broadening of input pulses along the length of the fiber. Two major types are (a) modal dispersion caused by differential optical path lengths in a multimode fiber; and (b) material dispersion caused by a differential delay of wavelengths in a waveguide material. Dispersion compensator A optical passive device designed to provide a solution to dispersion problems in high speed and long distance installations. They can be fixed using an FBG, or tunable for ROADM applications. Dispersion-compensating fiber A specialized optical fiber with a negative dispersion, which counters the affects of standard positive dispersion (G.652 and G.655) single-mode fibers. Dispersion-compensating module A module that incorporates a chirped FBG and an optical circulator, which act as an individual wavelength or channel filter. Dispersion-shifted fiber (DSF) A single-mode fiber manufactured with its best dispersion characteristics at 1550 nm instead of 1300 nm to take advantage of lower attenuation at 1550-nm wavelength. Distortion A change in the shape of a signal’s waveform. Distributed feedback (DFB) A type of laser using an internal grating to reduce line width of the laser that may be used for analog applications, e.g., AM/FM/DWDM applications. Duplex transmission Transmission in both directions, either one direction at a time (half duplex) or both directions simultaneously (full duplex). Dynamic range For an optical instrument, defined as the ratio (in dB) of the smallest signal that can be observed at a specified wavelength separation in the presence of a strong, nearly-saturating signal. E-band The “extended” DWDM transmission band that uses the 1360 to 1460 nm wavelength range. Electromagnetic interference (EMI) The frequency spectrum of electromagnetic radiation that extends from subsonic frequency to X-rays. This term should not be used instead of the term RFI. Electro-optical switch A solid-state optical switch with no moving parts and a very fast response time. End finish Surface condition at the optical conductor face. End separation loss The optical power loss caused by distance between the end of a fiber and a source, detector, or another fiber. Erbium-doped fiber amplifier (EDFA) An optical amplifier that uses active erbium doped fiber and a pump source (laser) to boost or amplify the optical signal. Etalon Used in wavelength lockers, the etalon is a Fabry-Perot filter paired with a beamsplitter cube. Ethernet A data communications protocol originally developed for premises and local access networks (IEEE 802.3) operating at speeds from 10 Mbps to 10 Gbps. It was originally developed for peer-to-peer communications using shared media over relatively short distances. Ethernet features variable length packets that allow data to be sent with less overhead. Excess Loss The amount of light lost in a coupler, beyond that inherent in the splitting to multiple output fibers. Extinction Ratio The ratio of the low or OFF optical power level to the high or ON optical power level. Extrinsic Loss Loss caused by imperfect alignment of fibers in a connector or splice such as lateral offset, angular misalignment, end separation, and end finish. Generally synonymous with insertion loss. Eye Pattern A diagram that shows the proper function of a digital system. The openness of the eye relates to the Bit Error Rate (BER) that can be achieved. Fabry-Perot (FP) A standard laser diode consisting of a semiconductor cleaved on each end forming a resonant chamber to create the lasing effect. Used in digital applications. Fabry-Perot interferometer An optical cavity similar to a laser but without the laser gain medium. It consists of two partially-transparent mirror aligned to be parallel so that they bounce light back and forth. Interference effects select wavelengths that resonate within the cavity. Fall Time Also called turn-off time. The time required for the trailing edge of a pulse to fall from 90% to 10% of its amplitude; the time required for a component to produce such a result. Faraday rotator Materials that rotate the plane of polarization differently depending on which direction light is passing through them. Forward Error Correcting (FEC) A communication technique used to compensate for a noisy transmission channel. Extra information is sent along with the primary data payload to correct for errors that occur in transmission. Four Wave Mixing A nonlinearity common in DWDM systems where myultiple wavelengths mix together to form new wavelengths called interfering products. Interfering products that fall on the original signal wavelength become mixed with the signal, mudding the signal and causing attenuation. Interfering products on either side of the original wavelength can be filtered out. FWM is most prevalent near the zero-dispersion wavelength and at close wavelength spacings. Fiber A single optical transmission element characterized by a core, a cladding, and a coating. Fiber amplifier An all-optical amplifier using erbium or other doped fibers and pump lasers to increase signal output power without electronic conversion. Fiber Bragg grating (FBG) A piece of photorefractive fiber that is exposed to high-intensity ultraviolet interference patterns that will cause it to reflect a specific wavelength while being transparent to all other wavelengths. Used in WDM systems. Fiber-optic test procedure (FOTP) Specific substandards within the TIA/EIA 455 standard, used predominantly for test and measurement. Fiber-optics Light transmission through optical fibers for communications purposes. Fiber sensor A sensing device in which the active sensing element is an optical element attached directly to an optical fiber. The measured quantity changes the optical properties of the fiber so that it can be detected and measured. Fiber to the building/business (FTTB) A topological reference to a PON network that supports multiple subscribers in a single structure, i.e., a business or a building. Fiber to the curb/customer (FTTC) Distribution of communication services by providing fiber-optic links to a central point in each neighborhood and continuing to the homes by either twisted pair or coax. Fiber to the desk (FTTD) Transmission system using fiber-optics as the medium throughout, from transmitter to desktop. Fiber to the home (FTTH) The distribution of communications services by providing fiber-optic links all the way to each house. Fibre Channel (FC) A high-speed interconnection standard for connecting supercomputers with peripheral devices up to 10 km away at transmission rates over 1 Gbps. Filter See optical filter. Four wave mixing (FWM) A collective name for a group of non-linear processes where up to three different incident waves interact in the medium, leading to a fourth resulting wave. Frequency The number of cycles per unit of time, denoted by Hertz; 1 Hertz = 1 cycle per second. Fresnel reflection Reflection of a portion of incident light at a planar interface between two homogeneous media having different refractive indices. Full spectrum WDM (FSWDM) A technology platform based on the use of spectrally enriched optical pulses for signal transmission at speeds of 10 Gbps and higher. Full width half maximum (FWHM) Used to measure the spectral width of light sources. Measure the spectral width at -3 dB (half power from peak) and at the full width of the source’s power peak. Fusion Splicer An instrument that permanently bonds two fibers together by heating and fusing them. Gigabit (Gb) One billion bits. Gigahertz (GHz) A unit of frequency equal to one billion Hertz. Graded-index fiber A type of fiber where the refractive index of the fiber core decreases radically towards the outside of the fiber. GRIN lens A graded-index lens, a lens where the refractive index varies along its length. Hybrid Fiber Coax (HFC) A transmission system or cable construction that incorporates both fiber-optic transmission components and copper coax transmission components. HFC Network A telecommunication technology in which optical fiber and coaxial cable are used in different sections of the network to carry broadband contrent. Index of refraction (I) The ratio of the speed of light in a vacuum to the speed of light in a material. When light strikes the surface of a transparent material, some light is reflected while some is bent (refracted) as it enters. The I is used to calibrate OTDRs for measuring fiber length. Infrared Light wavelengths extending from 770 nm on. Insertion loss Total optical power loss caused by the insertion of an optical component such as a connector, splice or coupler into a previously continuous path. Measured in dB. Institute of Electrical and Electronics Engineers (IEEE) A committee that represents the United States on the ISO in the areas of electrical or electronic standards. Interference filter Pass a single wavelength while reflecting others. These include line filters, band filters, and cut-off filters. Interferometer A measurement instrument that projects interference bands across the face of fiber-optic connector. The bands are used to determine the centering, angle of apex offset and radius of curvature of the fiber-optic connector. Interleaver An optical passive multiplexer that combines sets of even and odd channels with tight channel spacings. International Standards Organization (ISO) An international body funded by the United Nations, that provides consistent worldwide standards. U.S. membership is provided by ANSI. International Telecommunications Union (ITU) The international body for communications standards. The Telecommunications group within ITU is designated as ITU-T. Formerly known as CCITT. Isolator A fiber-optic component that either allows only unidirectional passing of light or that passes only some wavelengths of light. Jitter Small and rapid variations of timing of a waveform due to noise, changes in component characteristics, supply voltages, imperfect synchronizing circuits, ets. Kilometer (km) 1,000 meters or 3,281 feet (0.621 miles); the standard length of measurement for fiber and fiber-optic cable. Laser Light Amplification by Stimulated Emission of Radiation; a coherent source of light with a narrow spectral width. DFB, Fabry-Perot and VCSEL are the three types of lasers used in fiber-optic communication systems. Laser diode A semiconductor diode that emits light in a narrow spectrum; typically over 90% of the light output power concentrated within one angstrom. Laser-optimized multimode fiber Laser-optimized multimode fibers have higher bandwidths than legacy multimode fibers, which are designed for transmission using LED sources. Specified in the TIA-492AAAC-A standard. Laser optimized MMF can be 50/125 or 62.5/125 and are recommended for use with fiber-optic Gigabit transmission systems. L-band The “long” DWDM transmission band, occupying the 1565 to 1625 nm wavelength range. Light The region of the electromagnetic spectrum that can be perceived by human vision, designated by the visible spectrum and nominally covering the wavelength range of 400-770 nm. In optical communications, it includes the much broader portion of the electromagnetic spectrum that can be handled by the basic optical techniques used for the visible spectrum. This region is not clearly defined but may be considered to extend from the near-ultraviolet region of approximately 300 nm, through the visible region, and into the mid-infrared region to 30,000 nm. Light-emitting diode (LED) A semiconductor device that emits incoherent light formed by the P-N junction. Link An optical cable with connectors attached to a transmitter and receiver. Liquid crystal switch Used in fiber-optic switches. Triggered by electrical voltage. Local area network (LAN) An interconnected system of separate stations, usually computers, in one relatively small geographical location such as a building or complex. Long wavelength A commonly-used term for light in the 1300/1310 and 1550 nm ranges. Loss The portion of energy applied to a system that is dissipated and performs no useful work. Also called attenuation. Loss budget The tolerable difference between the light impulse where it originates and the light impulse where it arrives at the receiving end. If too much light power has been lost along the way through deficiencies in the cable or connectors, the signal cannot be read and interpreted. Loss windows Fiber-optics transmission is typically at the 850 and 1300 nm regions for multimode fibers and the 1310, 1550 and 1625 nm regions for single-mode fibers. The history of the usage comes from the availability of sources and detectors and their operating characteristics over an optical fiber due to the absorption effects at different wavelengths. Mach-Zender interferometer Used in the manufacturing of optical interleavers. Macrobending In a fiber, all macroscopic deviations of the fibers axis from a straight line that will case light to leak out of the fiber causing signal attenuation. Margin The additional amount of loss that can be tolerated in a link. Material dispersion Caused by variances in the speed of light at different wavelengths. Material dispersion and waveguide dispersion make up chromatic dispersion. Mean time between failure (MTBF) Developed by the military to estimate maintenance or replacement times for various pieces of high-end equipment, MTBF is based upon statistical evidence derived from in-use testing under extreme conditions (simulated or actual environment). Testing is performed by the equipment’s manufacturer or an independent test facility. Mega (M) A prefix meaning one million. Also represented by the letter M. Megahertz (MHz) Unit of frequency equal to one million Hertz. Metropolitan area network (MAN) An interconnected data transmission system connecting users and LANs in a localized geographical area such as a city. Usually a self-healing ring topology. Microbending Small imperfections in the core/cladding boundary. The larger the core, the less effect imperfections will have. Also defined as pinching effects. Microbending loss In an optical fiber, loss caused by sharp curvatures involving local axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Such bends may result from fiber coating, cabling, packaging, installation, et cetera. Micron (µm) Millionth of a meter = 10-6/meter Modal dispersion The difference in the arrival times of higher order and lower order modes. Modal dispersion limits effective transmission distance on multimode fibers. Mode A light path. An electromagnetic field pattern that satisfies Maxwell’s equations. Mode coupling In an optical fiber, the exchange of power among modes. The exchange of power may reach statistical equilibrium length. Mode field diameter (MFD) The portion of a single-mode fiber that actually transmits the light energy. Generally 20% larger than the physical core. Modulated laser A laser module that allows users to control output power by varying a control voltage. The laser is activated as needed. Beam modulation may be used to synchronize a laser with an analytical instrument or camera. Laser diode modules use either analog or TTL modulation. Mean Time Between Failure (MTBF) Time after which 50% of the units of interest will have failed. Multimode fiber (MMF) An optical waveguide that allows more than one mode to be guided. Graded index and most step index fibers are multimode. 50/125 62.5/125 and 100/140 are the most common styles. Multiplex Combining two or more signals in a single fiber. Multiplexer A device that combines two or more separate signals for transmission through a single fiber. Optical multiplexers combine signals at different wavelengths. Electronic multiplexers combine signals electronically before converting them into optical form. Nanometer (nm) One billionth of a meter = 10-9/meter. Most common unit of measurement for light. Nanosecond (ns) One billionth of a second, expressed as 10-9/sec National Institute of Standards and Technology (NIST) The NIST works with industry and government to advance measurement science and develop standards in support of industry, commerce, scientific institutions, and all branches of government. Formed in 1901 as the National Bureau of Standards, part of the U.S. Department of Commerce. Nonzero dispersion-shifted (NZDS) A single-mode fiber designed for use with DWDM and optical amplifier applications. Specified by ITU-T G.655. Numerical aperture (NA) A measure of the angular acceptance for a fiber. It is approximately the sine of the half-angle of the acceptance cone. The numerical aperture of an optical fiber defines a characteristic of the fiber in terms of its acceptance of impinging light. The “degree of openness,” “light gathering ability,” and “acceptance cone” are all terms describing this characteristic. O-band The “original” DWDM transmission band, occupying the 1260 to 1360 nm wavelength range. OM1 Specified by IEC 11801, this specifies the attenuation and bandwidth of the “legacy” 62.5/125 graded index multimode cabled fiber. OM2 Specified by IEC 11801, this specifies the attenuation and bandwidth of the “legacy” 50/125 graded index multimode cabled fiber. OM3 Specified by IEC 11801, this specifies the attenuation and bandwidth of the “laser optimized ” 50/125 graded index multimode cabled fiber with a bandwidth of 2 Ghz (minEMB). OM4 Specified by IEC 11801, this specifies the attenuation and bandwidth of the “laser optimozed ” 50/125 graded index multimode cabled fiber. with a bandwidth of 4.7 Ghz (minEMB). Optical add/drop multiplexing (OADM) Used with DWDM systems allowing a wavelength to be added or dropped optically instead of electronically. Optical amplifier A device that amplifies light without converting it to electrical signal. Optical Bandpass The range of optical wavelengths which can be transmitted through a component. Optical cross-connect Switches that direct optical signals among multiple pathways. M possible inputs must be directed to any of N possible outputs. Optical dispersion compensator (ODC) A optical passive device designed to provide a solution to dispersion problems in high speed and long distance installations. They can be fixed using an FBG, or tunable for ROADM applications. Optical Fall Time The time interval for the falling edge of an optical pulse to transition from 90% to 10% of the pulse amplitude. Optical fiber An optical waveguide comprised of a light-carrying core and cladding, which traps light in the core. Optical filters Passive components used to modify the optical radiation passing through it, usually by altering the spectral distribution. Employed to reject or absorb optical radiation in particular ranges of wavelength while transmitting it in other ranges. Tunable optical filters have the ability to track the signal wavelength variation over its operating wavelength range while non-tunable models have fixed values. Optical isolator (OI) A non-reciprocal device intended to suppress backward reflections along an optical fiber transmission line while having minimum insertion loss in the forward direction. Optical line terminal (OLT) In FTTx, the OLT is the main services interface between the customer and the service provider. Located in the central office, this equipment provides voice, video, and data downstream to the customer, while using a voice and/or data interface upstream from the customer to request services. The OLT handles downstream transmission at either/or 1490 nm and 1550 nm and upstream traffic from the ONT at 1310 nm through an optical diplexer or triplexer. Optical loss The amount of optical power lost as light is transmitted through fiber, splices, couplers, etc. Also known as attenuation; measured in dB. Optical network terminal (ONT) The ONT, used in FTTx applications, provides the subscriber interface (upstream requests for voice video and data) over the fiber-optic network to the optical line terminal (OLT). Conversely, the ONT distributes to the subscriber the services requested through the ONT from the service providers. The ONT demultiplexes downstream 1490nm and 1550 nm transmission while multiplexing upstream 1310 nm transmission through an optical diplexer or triplexer. Optical power The amount of radiant energy per unit time, expressed in linear units (watts) or on a logarithmic scale (dB). Optical return loss (ORL) The amount of light reflected from optical fibers and optical components. The fiber, connectors, or splices in an optical system can cause the reflection. Optical Signal to Noise Ratio The ratio of the total signal to the noise which shows how much higher the signal level is than the level of the noise. A measure of signal quality. Optical spectrum analyzer (OSA) The OSA provides a visual display of optical power versus wavelength by scanning the spectrum of the input signals. It can plot power levels on all WDM channels for performance assessments while showing indications of nonlinear events and perturbations that can occur in a DWDM network. The instrument is widely used at all levels from testing components to entire fiber systems. Originally designed as a high performance instrument for lab use, but as WDM spread through communication networks, OSAs were adapted for field use. They have become important instruments for verifying proper WDM function and for troubleshooting. Optical switch A passive component possessing two or more ports that selectively transmits, redirects, or blocks optical power in an optical fiber transmission line. Optical to electrical to optical (OEO) Used as specifying a network switch that receives an optical signal, then demultiplexes, switches, multiplexes and re-transmits the signal optically. Optical transport network (OTN) Designed for high speed services including 10 and 40 Gigabit systems including ROADM networks. Specified by the ITU-T G.709 standard. Optomechanical switch Bipolar switch based on moving parts used to switch optical signals between fibers. Currently available versions include switches based on moving fibers, or using moving mirrors. OS1 Specified by IEC 11801, this specifies the attenuation and bandwidth of the ITU-T G.652 A and B single-mode fibers. OS2 Specified by IEC 11801, this specifies the attenuation and bandwidth of the ITU-T G.652 C and D single-mode fibers. Passband The region of usable frequency in electronics or wavelength in optics. Passive A component that requires no electrical power to operate, i.e., optical splitters or fixed optical attenuators. Passive-optical network (PON) A point-to-multipoint system, specified by the ITU and IEEE, that is made up of fiber-optic cabling, passive splitters and couplers that distribute an optical signal from the service provider to homes or buildings. Photodetector An electro-optic device that transforms light energy into electrical energy. Photodiode A semiconductor that converts light into an electrical signal, used in fiber-optic receivers. Picosecond One millionth of one millionth, or one trillionth (10-12). PIN diode Positive intrinsic negative diode, a type of photodiode used to convert optical signals in a receiver. Planar lightwave circuit A PLC formed on fused silica that includes an optical waveguide. This technology can integrate complex passive-optical devices into small areas and is used to multiplex and demultiplex WDM systems, optical switches, etc. Planar lightwave circuit (PLC) A technology that enables photons to pass through a wafer structure in much the same way they do through fiber. It has become an accepted technology for WDM systems, largely in the form of multiplexer/demultiplexer modules based on arrayed-waveguide gratings. Polarization A term used to describe the orientation of the electric and magnetic field vectors of a propagating electromagnetic wave. Polarized dispersion loss (PDL) The difference in dB between the maximum and minimum values of loss (attenuation) due to the variation of the polarization states of light propagating through a device. The ITU defines PDL as polarization dependent loss, the maximum variation of insertion loss due to a variation of the state of polarization (SOP) over all SOPs. Polarized mode dispersion (PMD) Typical single-mode fibers support two perpendicular polarizations of the original transmitted signal, which may travel at different speeds and arrive at different times. The average difference in arrival times of the two polarization modes, normalized with length, is referred to as PMD. Power The rate at which energy is absorbed, received, transmitted, transferred, et cetera, per unit time, measured in dBm or watts. Pulse broadening An increase in pulse duration. Pulse width The time the source is on, measured at full width half maximum (FWHM). Time duration of the laser. Quadrature Amplitude Modulation A coding technique that uses many discrete digital levels to transmit data with minimum bandwidth. QAM256 uses 256 discrete levels to transmit digitized video. Quality of service (QoS) A measure of the telephone service quality provided to a subscriber. Raman Amplifier An optical amplifier based on Raman scattering which generates many different wavelengths of light from a nominally single-wavelength source by means of lasing or action or by the beating together of two frequencies. The optical signal can be amplified by collecting the Raman scattered light. Random Jitter Random jitter is due to thermal noise and may be modeled as Gaussian process. The peak to peak value of RJ is of a probabilistic nature and thus any specific value requires an associated probability. Receive (Rx) Refers to the detection of the light source, LED or laser. Receiver An electronic unit that converts an optical signal to an electrical signal. Reconfigurable optical add/drop multiplexer (ROADM) Unlike original OADMs, a reconfigurable OADM can be managed via a network connection without need for a truck roll. They function as optical switches, allowing for remote service changes, and provide an express wavelength path and power monitoring. Reflectance The percentage of light reflected from an event within the fiber system Reflection The abrupt change in direction of a light beam at an interface between two dissimilar media so that the light beam returns into the medium where it originated, i.e., a mirror. Refraction The bending of a beam of light in transmission between two dissimilar materials or in a graded index fiber where the refractive index is a continuous function of position. Refractive index The ratio of light velocity in a vacuum to its velocity in the transmitting medium. Regenerator A repeater designed for digital transmission in which digital signals are amplified, reshaped, retimed and retransmitted. Repeater/ regenerator A repeater is a device inserted at intervals along a circuit that detects a weak signal, amplifies it, cleans it up, and retransmits it in optical form. A regenerator is a receiver and transmitter combination used to reconstruct signals for digital transmission. Ring Network A network Topology in which terminals are connected in a point to point serial fashion in an unbroken circular configuration. Rise Time The time taken to make a transition from one state to another, usually measured between the 10% and 90% completion points of the transition. Shorter or faster rise times require more bandwidth in a transmission channel. S-band The “short” DWDM transmission band, occupying the 1460 to 1530 nm wavelength range. Short wavelength A commonly-used term for light in the 665, 790, and 850 nm ranges. Signal-to-noise ratio (SNR) The ratio of the power of the signal versus the power of the background noise, usually measured in decibels. A term used to describe the quality of an electronic transmission system. Single-mode An optical fiber in which only the lowest order bound mode (which may consist of a pair of orthogonally polarized fields) can propagate at the wavelength of interest. Single-mode fiber (SMF) A step-index waveguide in which only one mode will propagate above the cutoff wavelength at a single wavelength. SONET Also know as Synchronous Optical Networking. A standardized protocol used by the telecom industry in North America to transfer multiple digital bit streams synchronously within a single framing protocol. See also Synchronous digital hierarchy (SDH). Source Usually an LED or laser used to convert an electrical information-carrying signal into a corresponding optical signal for transmission by an optical waveguide. Spectral width A measure of the wavelength extent of a spectrum that is used to determine the optical width of light sources such as LEDs and lasers in optical systems. Speed of light 2.998 x 108 meters per second. Splitter See coupler. Star coupler A coupler in which many fibers are brought together to a single optical element in which their signals are mixed. The mixed signals are then transmitted back through all the fibers. The name comes from the geometrical arrangement; all fibers come together at a single point. Steady state modal distribution Equilibrium modal distribution (EMD) in multimode fiber, achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance. Step-index fiber A type of fiber where the refractive index of the core is uniformly higher than that of the surrounding cladding. Storage area network (SAN) A network which links host computers to storage servers and systems. The network protocols can include FC-AL, SSA, ATM and Fast Ethernet. The storage technology can be a collection of servers on a network, or a more complex and expensive host storage server such as a mid-range or mainframe computer. Switch A device for re-routing signals from one optical fiber into others. Synchronous digital hierarchy (SDH) A worldwide, high-speed synchronous protocol standard transmitting at up to 10 Gbps. See also Synchronous optical network (SONET), the protocol standard used in North America. Synchronous optical network (SONET) A standardized protocol used by the telecom industry in North America to transfer multiple digital bit streams synchronously within a single framing protocol. See also Synchronous digital hierarchy (SDH), the protocol standard used in Europe. Synchronous transmission A transmission method in which data characters are synchronized by timing signals generated at the sending and receiving stations (as opposed to start/stop communications). Both stations operate continuously at the same frequency and are maintained in a desired phase relationship. Several data codes may be used for the transmission as long as the code utilizes the required line control characters. Also called “bi-sync” or “binary synchronous.” Tee coupler A fiber-optic coupler in which three fiber ends are joined together, and a signal transmitted from one fiber is split between the other two. A conceptual drawing looks like the letter T, which accounts for the name. Telcordia Technologies (Bellcore) Formerly known as Telcordia-Bell Communications Research, it is the unofficial standards development body providing technical specifications for the regional Bell operating companies. Telecommunications Industry Association (TIA) Combines with the Electronic Industries Alliance (EIA) to provide test and measurement methods, establish procedure standards and publish “RS” standards such as RS-232. Terminator An optical plug with the fiber dead-ended so that there is no return loss. This is done by breaking the end of the fiber and index matching it inside the connector. As they have to be measured for ORL, it is best to have several types with the measurement value required. Thermo-electric switch A variation on the electro-optic switch, the thermo-optic switch uses temperature instead of an applied voltage to change the refractive index of the active waveguide arms. Slower than their electro-optic counterparts, with typical response times in the milliseconds, but less costly. Thermoplastic A material that will soften, flow, or distort appreciably when subject to sufficient heat and pressure, i.e., polyvinyl chloride (PVC) or polyethylene (PE). Total internal reflection 100% reflection and 0% transmission of light at the interface of two optical media. Transducer A device that converts energy from one form to another such as optical energy to electrical energy. Transmit (Tx) The light source, i.e. LED or laser. Transmitter An electronic unit that converts an electrical signal to an optical signal. Transponder The part of a networking device that receives and transmits a signal Triplexer Commonly known as a passive WDM, which is included in the transceiver package that combines and separates the three common wavelengths of 1490 nm, 1550 nm downstream and 1310 nm upstream as needed in FTTx networks. Utilizing both lasers and detectors, this provides optical-to-digital and digital-to-optical conversion in the active, three-wavelength transceiver used in the OLT and ONT. Also used to describe the active transceiver as a whole. Tunable dispersion compensator Correctly adapt or compensate for the dynamic change in fiber distance in reconfigurable fiber-optic systems such as long haul and metropolitan area networks. FBG-type dispersion compensators can also be made tunable. Changing the temperature gradient along the length of the grating changes the group delay response of the filter, thus changing the dispersion. Tunable gratings typically have dispersion versus temperature tables so that the user can set the appropriate amount of dispersion by adjusting the temperature to a specific value. U-band The “ultra long” DWDM transmission band, occupying the 1625 to 1675 nm wavelength range. Ultra physical contact (UPC) Refers to endface polish of a connector that is performed on a polishing machine. Typically 60 dB return loss. Variable optical attenuator (VOA) A fiber system attenuator with adjustable attenuation; often used to test system performance by increasing attenuation until the system fails. Vertical cavity surface-emitting laser (VCSEL) A high-speed, low-cost laser that is used for applications such as Gigabit Ethernet and Fiber Channel where the modulation rate of LEDs is insufficient. Visible light Electromagnetic wavelengths, ranging from 380 to 770 nm, that are visible to the human eye. Waveguide dispersion The distortion of mode due to the geometric properties of the waveguide. Wavelength The optical term for frequency. Fiber-optics generally uses the 850 nm, 1300/1310 nm, 1550 nm and 1625 nm wavelengths for transmission purpose due to the marriage of performance with light sources, optical fibers, and optical detector technologies. Wavelength blocker Used in ROADM systems the wavelength blocker rejects dropped wavelengths so they are not expressed through the node. Wavelength division multiplexing (WDM) Combination of two or more optical signals for transmission over a common optical path, usually a single fiber. WDM devices have a channel wavelength spacing greater than or equal to 50 nm. They typically separate a channel in one conventional transmission window (e.g., 1310 nm) from another (e.g., 1550 nm). Wavelength locker Used to prevent optical drifting of sources. DWDM systems can have multiple wavelengths as closely spaced as 0.4 nanometers, or 50 gigahertz. Such narrow spacing requires a precision monitoring circuit to provide error correction in case the laser wavelength begins to drift. The wavelength locker employs a Fabry-Perot filter, or “Etalon” which is paired with a beamsplitter cube, while a tap coupler samples a small percentage of the modulated laser output and passes it through the beamsplitter. The light then passes through the etalon and into both an etalon photodiode and a reference photodiode. As the laser’s wavelength changes, an error signal between the reference and etalon photodiode is generated. This error signal then drives a Peltier cooler, which controls the laser temperature, thus locking the wavelength to the desired value. Wavelength meter Wavelength meters provide digital displays of both wavelength and optical power level and are used in both factory and field testing applications. They are more accurate than OSAs for testing wavelengths. Wavelength selectable switch (WSS) A type of ROADM used in DWDM networks to allow a network operator to change the direction of an added or dropped wavelength through the use of mirrors mounted on micro-electrical-mechanical positioners. Wavelength selective coupler Wavelength-selective couplers distribute signals according to their wavelength and are mainly used to route WDM signals to the appropriate ports, or to separate wavelengths traveling on a single fiber for different purposes. They also block wavelengths from reaching the wrong destination such as in EDFA amplifiers and WDM systems. WDM coupler A passive device that is designed to either a) optimally combine light of multiple predetermined wavelengths into a single core; or b) optimally sort and segment those wavelengths and couple them separately into output fiber cores. Wide area network (WAN) A telecommunications or computer network that extends over a wide geographic area (as opposed to local area networks or LANs) and allows business, education or government users to share data regardless of their location. Window See band. Y coupler One of four coupler configurations, defined by the number of input and output ports, that dictate the function of the device. Y (and T) couplers, also known as “taps”, are 3-port devices that split one input to a pair of outputs. The signal may be split evenly between the outputs or split in some other ratio. They are often directional devices. Zero-dispersion Wavelength In a single-mode optical fiber, the wavelength at which material dispersion and waveguide dispersion cancel one another. The wavelength of maximum bandwidth in the fiber. Also called zero-dispersion point.