Glossary Of Fiber Optic Terms

Fiber Optics

Glossary of Terms

A • B • C • D • E • F • G • H • I • J • L • M • N • O • P • R • S • T • U • V • W • Z

There are some very specialized technical terms used in describing fiber optic technology, methods, and the relevant tools of the trade. Here are some important terms and principles on the topic of fiber optics, the greater physics field of optics, and related chemistry.

• A •

Acceptance Angle: The angle at which the core of an optical fiber accepts light and allows it to remain within the core and therefore be propogated onward. At the outer limit of the acceptable angle range is the critical angle, the point at which the signal integrity is corrupted.
Angled Physical Contact (APC) Connector: A manual connector in fiber optics cables where the adjoining ends of each cable are polished to create a precise (8° to 12°) curvature which forces the fibers to touch, eliminating the air gap between cables. The incoming signal enters at a slight angle minimizing back reflection on the signal. Back reflection is least with repeated APC connector applications, as opposed to other connector choices.
Angular Misalignment Loss: Power loss caused by straying from proper angular alignment anywhere between points along the path, such as between waveguides, sources, and light sensors.
Arc Fusion Splicing: A fiber cutting method in which a precision machine aligns fiber ends until ideal transmission is detected across the gap. Then two electrodes are used to create an arc which heats the optical fibers, resulting in one continuous fused fiber. Splices created using this process alone often have under half the strength of the original fiber's integrity.
Attenuation (Loss): The rate at which the intensity and amplitude of a signal decreases. In optics this refers to the degredation of a beam of light over a certain distance. Glass fiber has a low attenuation, making it an ideal choice for telecommunication over long distances.

• B •

Back Relfection: A situation occurring when a beam of light encounters a boundary between to media in such a way that the light is returned in the direction from which it came.
Bendable Fibers: A sub-grouping of optical fiber which can be bent significantly further than standard cable without adverse signal degradation.
Borescope: An optical device comprised of a tube with an viewpiece on one end and a lens on the other end. Optical fibers are commonly used in this device for illumination and often times also as the actual sheath of the relay connecting the two ends of the instrument. Borescopes are used for inspection of delicate areas normally inaccesible to the human eye, such as within components of aircraft mechanics and turbines.
Broomsticking: The process by which a molten glass preform is gravity-drawn into a fiber form.
Buffer: A layer between the cladding and the jacket of a fiber optic cable used for insulation, identification, and protection of the enclosed fiber. Buffers come as either loose buffers or tight buffers.

• C •

Chalcogenide Glass: A glass comprised of one or more chalcogenide element (such as sulfur, selenium or tellurium) as a main ingredient. Chalcogenide glass has a low refractive index, making it one of the reasons why it is a common choice for manufacturing optic lenses and optical fibers. This material is also used in compact disc technology.
Chemical Vapor Deposition (CVD): A coating method for optical fibers whereby certain high-quality materials such as silicon are deposited upon a surface.
Cladding: The layer (sometimes layers) of material covering the core of the optical fiber and in turn covered by the buffer. This layer is of lower refractive index than the core in order to propogate the wave correctly.
Cleaving: A controlled break deliberately made in an optical fiber whereby a flat endface is created which is at a right angle to the length (longitude) of the fiber.
Coherence: A state by which a group of waves are all in phase and are propogating at the same frequency. Coherence is a necessary state for the photons that comprise a laser beam.
Collimation: In optics, the process wherby a signal beam is lined up to as close as possible to being parallel with the waveguide, and more specifically the fiber's core.
Core: The inner-most portion of fiber optic cable which carries the light signal, immediately surrounded by the cladding.
Coupling Loss: Loss that happens when signals are transferred from one device or medium to another deivce or medium.
Critical Angle: The angle of incidence (past the acceptance angle) above which a state of total internal reflection is reached.
Critical Bend Radius: The angle of incidence at which light refracts in a manner that allows it to travel along the interface between materials (total internal reflection), as in between the core and cladding of an optical fiber.
Cross-Talk (XT): A situation arising from one transmitted signal inadvertantly causing an unwanted effect on another signal. Minimizing or (optimally) preventing cross-talk is a very large consideration in maintaining signal integrity.

• D •

Decibel (dB): A unit of measurement commonly used in the field of optics to quantify the power of a light wave. Often used to represent signal loss (attenuation) over a line or through a connector/termination end.
Demodulator: A device that performs the reverse operation of modulation. One of the components used in "decoding" the information from the carrier signal wave.
Dielectric: A nonconductive nonmetal material - an insulator. The term is commonly used when describing the effect of electric fields on the material.
Dielectric Mirror: A mirror comprised of multiple thin layers of dielectric material(s), designed specifically for a certain relectivity of targeted wavelengths of light. These mirrors can be produced through several means, including physical vapor deposition and chemical vapor deposition.
Dielectrics: The study of dielectric substances, and how electric fields behave inside a material.
Diffraction: The bending effect which occurs when propagating waves encounter obstacles in their paths.
Dispersion: Any process whereby a signal wave propogating through a medium disperses, or fans out, due to the different components (frequencies) responding differently to the medium present. Variations of dispersion in optics are material dispersion, profile dispersion, and waveguide dispersion.
Dispersion-Shifted Fiber (DSF): A specific type of single-mode fiber designed with a core-clad refractive index that moves the zero-dispersion wavelength from the silica-glass standard of 1300 nm to the minimum-loss figure of 1550 nm.
Distributed Temperature Sensors (DTS): Any number of devices which measure temperatures along an optical fiber cable acting as the sensor. These can be found anywhere temperature monitoring is of great concern, as in storage tanks and vehicles, engineering plants, and furnaces.
Drawing Tower: A peice of machinery that fabricates optical fibers from a preform via pulling, with the preform being heated close to its melting point. Close controls are held to ensure proper fiber diameter.

• E •

Electric Arc Fusing: A method used to fuse (attach) two optical fibers by placing the cleaved ends of each optical fiber adjacent to each other and using a controlled electric arc to super-heat the interface and bond them together.
Electromagnetic Interference (EMI): Occurs when an outside source's electromagnetic radiation disturbs an electric circuit.
Electromagnetic Wave Equation: A mathematical equation that describes the movement of waves through a medium. It can be used to determine the speed of a propogating wave through the medium.
Endoscope: A borescope which is used specifically for medical means within the human body.
Erbium-Doped Fiber Amplifier (EDFA): The most common type of optical fiber amplifier, which can boost boost both Conventional (C-band) and Long (L-band) bands.
Evanescent Wave: A wave whose intensity decays exponentially with the distance from its source. Commonly found during total internal reflection.

• F •

Fermat's Principle: The optics principle of least time which ditates that light takes the path between two points with has the least optical path length. Further used to extrapolate Snell's law of refraction as well as the law of reflection.
Fiber Fuse: A situation where high optical power meets a damaged or imperfect portion of a fiber cable and destroys the core of the fiber. The resulting fuse is reflective and therefore propogates back towards its source.
Fiber Launch System: The coupling of a free-space laser into an optical fiber. This is the generator of the wave propogating through the fiber optic cable's core.
Fiber Optic Gyroscope (FOG): A gyroscope that detects rotation via two light beams passing in opposite directions in optical fiber. By noting the phase-shifting effects encountered when a beam travels against the rotation, the rotation can be measured.
Fiber Optics: The science governing, the study of, and the engineering involved in the design and use of optical fibers.
Fiberscope: A flexible fiber optic cable that has a lens on one end and an eyepiece on the other, used for inspect of small components usually in hard to access areas.
Fiber To The Home (FTTH) Installations: Fiber optic communication where the optical signal is delivered to the user's personal space without being stepped-down to a cable format prior.
Flame Hydrolysis: A reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water (H2O) in an oxyhydrogen flame.
Flare (Lens Flare): The "superimposing" effect of light passing through a lens in an undesirable method, resulting from such things as internal reflection and scattering. Common examples are sunset haze and glare of light through a window.
Fresnel Reflection: An alteration of the signal and therefore degredation that occurs when light moves between two different medium which have different refractive indexes. A portion of the signal can be reflected backwards at such interfaces. This is why index-matching is a great concern in maintaining signal integrity over any connector in a fiber optic system.

• G •

Gain Medium: Any material or system wherein input amplification takes place without resulting in feedback. Examples include lasers and optical amplifiers.
Gap Loss: Power loss in fiber optic systems resulting from a signal being transferred from one section of fiber across open space to another fiber. Scattering can also occur as a by-product of gap loss due to the light signal dispersing somewhat over the gap.
Geometric Optics: The study of and the science involved in optical systems, specifically the geometric properties of the light rays transmitted and received.
Germanium Dioxide (GeO₂): One common component in the creation of optical fibers and waveguides. This compound is also referred to as germanic acid, germania and germanium oxide.
Germanium Tetrachloride (GeCl₄): A chemical compound commonly used in the fabrication of the core of optical fibers due to its low optical dispersion and high index of refraction.
Gigabit (Gb): A unit denoting storage capacity, equal to one billion (1,000,000,000) bits.
Graded-Index Fiber: Sometimes referred to as gradient-index fiber, this is an optical fiber whose core has a refractive index which decreases with increased distance from the center of the core outwards towards the cladding. This is usually created by multiple layers of cladding surrounding the core. When compared to common multimode fiber, this type of optical fiber shows significantly lower modal dispersion.
Gradient Index Optics: The sub-field of optics dealing with optical effects yielded by a material's gradual variation of the refractive index. The most common phenomenom encountered in gradient index optics is the illusion of wet patches on a highway on a hot day.

• H •

Hydrogen Darkening: A chemical degrading of silica glass resulting from free hydrogen atoms bonding with it over time.
Hydrophone: An underwater microphone. Fiber optic cable is a good choice of transmission line for hydrophone use, due to its data retention characteristics and resistance to underwater pressures.

• I •

Index Of Refraction: Also referred to as refractive index. This measurement denotes a medium's effect in reducing the speed of light passing through it. This number is a coefficient multiplier in comparison to the speed of light travelling in air or vacuum. The physical property of light partially reflecting from surfaces which have a different refractive index than their immediate surroundings is at the heart of the science involved in optical fiber technology.
Index-Matching Gel: A gel commonly used on ends, splices, or connectors to reduce the effects of Fresnel reflection by closely matching the fiber's index of refraction.
Insertion Loss: In optical fibers, a decrease in a signal's power arising from a device being placed inline on the fiber.
Inside Vapor Deposition (IVD): A production technique for engieering optical fibers. Specific mixtures of germanium and silica are vaporized and then deposited as soot on the inside surface of a hollow glass cylinder. By mixing the compounds in different portions, different refractive index layers can be created. The resulting product is the preform which is then fabricated via further means into the optical fiber.
Interferometry: A technique of superimposing (interfering) waves to detect their differences.

• J •

Jacket: The layer of a fiber optic cable covering the buffer, usually made of plastic.
Jetting: A compressed air assisted method of pushing optical fiber cables into ducting.

• L •

Laser: Light Amplification by Stimulated Emission of Radiation. Lasers typically broadcast light in a narrow, single-colored beam with a clearly-defined wavelength. Lasers are the heart of the broadcast signal transmitted over optical fibers.
Laser Diode: A laser which is semiconductor-based and converts an electrical input into a light signal, used for transmissions through optical fibers. A laser diode emits light either from its edge or its surface (beneficial inside optical fiber's core) and creates a smaller beam than a laser-emitting diode (LED), therefore making it easier for use with the smaller core found in singlemode optical fibers.
Lens: A symmetrical optical device used to transmit and refract light by concentrating or diverging the beam.
Light-Emitting Diode (LED): A semiconductor diode that generates infrared or visible light when voltage is applied to it, commonly used an indicator or light source in fiber optic systems.
Light Guide: A flexible bundle of fibers, used as a waveguide to transmit light.
Loss (Signal Loss): The opposite of gain, signal loss is the reduction in signal strength due to any of a number of causes. Measured in decibels (dB).

• M •

Material Dispersion: Dispersion (spreading of the signal) caused by the inherent properties of a specific material. In pure silica optical fiber, material dispersion is minimized around wavelengths of 1.27 m. Practical fibers have a slightly longer target wavelength.
Maxwell's Equations: A set of mathematical equations used by James Clerk Maxwell which explain electric and magnetic fields as functions relating to position and time. These equations are Gauss' law, Gauss's law for magnetism, the Maxwell-Faraday equation, and Ampere's circuital law.
Mechanical Splice: A non-permanent coupling of optical fibers held in place by a self-contained connector. Less reliable than fusion slicing's permanent method.
Megawatt: An SI unit of power equal to 1 million watts or 1,000 kilowatts.
Micrometer (µm): A micron, one millionth of a meter. The common unit of measurement for wavelengths corresponding to electromagnetic waves.
Modal Dispersion: A distortion phenomenom which occurs in multimode fibers, limiting the bandwidth. The signal is spread in time because the propagation velocity of the signal is not constant for all modes. Modal dispersion may be reduced, but it can never be eliminated completely. Other common names for this occurance are multimode distortion, multimode dispersion, and modal distortion.
Modulation: The process by which a periodic waveform signal is varied (modulated), to carry an information signal (and therefore data). Varying any or all elements of the wave's amplitude, phase, or its frequency can sucessfully modulate a signal.
Modulator: A device used to perform signal modulation on a wave. Requires polarized light as an input.
Mono-Mode Fiber: Another name for Single-Mode Fiber.
Multimode Fibers (MMF): Optical fibers which support transverse modes or many propagation paths, and generally have a large-diameter core. Used for short-distance communication links or for situations where a high power signal must be transmitted.

• N •

Network: Telecommunications nodes and links organized in such a way that messages may be transmitted from one part of the overall structure and received at another. Telecommunication networks are composed of three primary components: the Control Plane (control signal carrier). the Data/User Plane (carries users' traffic), and the Management Plane (carries network management administration traffic). Fiber optic cable may be found in networking applications.
Non-Imaging Optics: The sub-field of optics dealing with optical systems transferring light between a source and a target, without attempting to form an image of the source.
Normal Mode: A pattern of motion in which all parts of the system move sinusoidally with the same frequency.
Numerical Aperture (NA): A number that denotes the range of angles over which an optical system can receive or transmit light. This number is the product of the refractive index of the medium multiplied by the sine of one half the angle of the cone of light that can pass through the lens. The bigger the numerical apertuture value is, the better the resolving power of the lens is.

• O •

Optical Amplifier: An optical device that boosts the power of a signal directly, without having to convert it first to an electrical signal. Amplifiers that fall into this category are laser amplifiers, doped fibre amplifiers (DFAs), semiconductor optical amplifiers (SOAs), Raman amplifiers, and optical parametric amplifiers.
Optical Fiber: A glass or plastic fiber designed specifically to guide light along its length. Optical fibers are immune to outside electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise.
Optical Fiber Connectors: A variety of mechanical termination ends for optical fibers which allow for easier and faster disconnection and connection than true splicing.
Optical Gain: A measurement of any recordable increase in power of a beam of light as it moves along in a gain medium.
Optical Gyroscope: Another name for a fiber optic gyroscope.
Optical Path Difference (OPD): The difference in optical path lengths from one path in comparison to another.
Optical Path Length (OPL): The product derived by multiplying the length the path light follows by the index of refraction of the medium it passes through. Governs interference and diffraction of light and is an important determinant of the phase of the light along the path.
Optical Pumping: A process commonly found in laser construction. Light is utilized to elevate (pump) electrons to a highter energy level from a lower one.
Optics: The field of science describing the properties and behavior of light as well as light's interaction with matter.
Optoelectronics: The field of science dealing with electronic devices that interact with light.
Oxyhydrogen Torch: A high temperature torch utilizing a mixture of hydrogen and oxygen gases. This torch is often used for splicing applications and fusing fiber optic cables.

• P •

Payout Spool: The amount of fiber optic cable wound onto a finished spool.
Photonic Crystal Fiber (PCF): A new grouping of optical fiber centered around properties inherent to photonic crystals.
Photonic Crystals: Structures designed to affect photon motion in a way similar to how semiconductor crystals affect electron motion.
Physical Contact (PC) Polish: A compound used to prepare the ends of two pieces of optical fibers to be joined.
Plastic-Clad Silica Fiber (PCS): Also referred to as polymer-clad silica fiber, this is optical fiber uses a silica-based core and cladding made of plastic. These fibers generally have lower performance, higher transmission losses, and lower bandwidths than fibers comprised of all-glass.
Plastic Optical Fiber (POF): An optical fiber made out of plastic, usually with acrylic for the core, and polymers as the cladding. The core of these fibers are commonly far larger than all-silica fibers.
Polarization-Maintaining Optical Fiber (PMF): An optical fiber in which polarization of the light entering into the fiber is maintained. These fibers are used for special applications, such as fiber optic sensing and often-times in telecommunications for connecting the source laser to a modulator (which requires polarized light as an input to operate). Has higher attenuation than singlemode fiber.
Polymer: A substance made of large molecular mass molecules in repeating structural units connected by chemical bonds. Plastics are common polymers.
Preform: The piece of glass from which the optical fiber waveguide is drawn. The shape of the preform differs depending upon the application the finished product is intended for.
Propogation: In optics, the motion of light waves along or through a waveguide or other medium.
Pulling: The traditional method for inserting fiber optic cables into cable ducts. As with other cable types, pulling denotes inserting by a winch line. This method has widely been replaced by jetting.

• R •

Ray: A geometric representation for light propogation. These rays are used to calculate, represent, and determine the behavior of optical systems.
Reflection Loss: Signal loss that occurs resulting in at least some of the energy being reflected back to the source of the signal.
Refraction: In waves (including light), the change in direction resulting from a change in its speed. A common example is the "distortion" illusion when light contacts water (i.e. a partial submerged object appears to be broken or bent at the interface of the surface).
Refractive Index: Also known as index of refraction. A coefficient measurement of how much the speed of light is reduced inside a medium. Obtained by dividing the phase velocity of the wave by the phase velocity of the medium itself.
Repeater: An electronic device that performs any of a number of amplification or modulation functions on an input signal prior to retransmission. Often used to offset the effects of attenuation when a signal is sent over a long distance.
Relative Density: Also known as specific density. A ratio a substance's density to a refernce material's density.
Resolving Power: Also referred to as spectral resolution. A measure of an imaging device's power to resolve features. Calculated by dividing a given wavelength by the smallest difference in wavelengths which can be distinguished.
Return Loss: The power reflected due to imperfections in an optical link. Calculated by dividing the power of the wave reflected upon encountering the imperfection by the power of the transmitted wave.

• S •

Scattering: In optics, diffuse reflections occuring when light is forced to change direction due to anomalies in the medium which it encounters.
Sheath: A common protective jacket covering optical fibers.
Signal-To-Noise Ratio (SNR or S/N): The ratio of a desired signal's power to the background noise's power which corrupts the signal. The lower the signal-to-noise-ratio, the better.
Signal Wave: The wave transmitted.
Silica Optical Fiber: A low-loss variation on optical fiber technology utilizing silica glass.
Silicon Tetrachloride (SiCl₄): A chemical compound used in the creation of silicon. Silicon is used extensively in semiconductors and as a main component in silica optical fibers.
Singlemode Fibers (SMF): Optical fibers designed to support only a single ray of light down its propogation path. SMF fibers do not exhibit the dispersion that multimode fibers do. These fibers can transfer a higher bandwidth than multi-mode fibers can, and are a lower cost item than MMF.
Sintering: A method of fusing the core and cladding of optical fibers together by using a sub-melting point heat and pressure.
Specific Gravity: The density of a substance divided by the density of water.
Speckle: A random occurance where a pattern is produced by the interference of many wavefronts with phase differences and/or intensity fluctuations.
Step-Index Fiber: Fibers which have a uniform refractive index within the core and a pronounced decrease in refractive index at the interface of the core and cladding.
Strain: In optical fibers, the degradation of the cable or individual fiber resulting from stress.
Strain Relief: Implements designed to minimize the occurance and effects of strain on fiber optic cables.

• T •

Telecommunication: The field of science concerned with the transmission and reception of information over large distances, specifically for communication purposes. Fiber optic cable is quickly becoming a larger component of the telecommunications field.
Tensile Strength: A measurement of how much stress is required to pull some material to the point which it breaks.
Thermophoresis: Also referred to as the Soret effect or thermodiffusion. This is the movement of molecules from a hot to cold region (or vice-versa) resulting from certain isotpes being subjected to controlled temperature changes. This effect is utilized in the manufacturing stage of optical fiber to create the desired refractive properies of the core and cladding.
Tolerances: Refers to the permitted limits of variation in a product's dimensions and other physical characteristics. By adhering to strict tolerances during manufacturing, consistently uniform (and therefore interchangable) products may be produced.
Total Internal Reflection: An occurance in the field of optics encountered when a ray of light meets the boundary of a medium at an angle larger than the critical angle, with respect to the normal to that medium's surface. If the other side of the boundary has a lower refractive index then no light can pass through, effectively reflecting the entire ray away. This is a crucial theory behind the science of fiber optics. Total internal reflection ensures that the transmitted signal may be reflected countless times along the length of the core without the signal being lost.
Transverse Mode (TM): In optics, a pattern of light propogation whose magnetic field vector is perpendicular to the propagation path of the light. The amount of modes in optical fibers differentiates single-mode fiber from multi-mode fiber. Allowable modes are calculated by using Maxwell's equations in relation to the conditions pertaining to the boundaries of the waveguide.

• U •

Ultra Physical Contact (UPC) Connector: A machine-polished end manual connector in single-mode fiber optics cables. Although there is somewhat more signal loss with UPC connectors as opposed to APC connectors, these connectors can be repaired by a skilled technician.

• V •

Vapor Axial Deposition (VAD): A production process utilizing oxyhydrogen flame to radially create optical fibers.
Voltage: A measurement of the electric potential energy contained at a given point within an electric or electronic circuit. Voltage is expressed in volts in the SI system (where 1 volt= 1 joule/coulomb).

• W •

Watt (W): The SI unit of power. The power in a circuit where a current of a single ampere flows across a potential difference of one volt is one watt. 1 W = 1 joule/second.
Waveguide Dispersion: Dispersion resulting from discrepencies in phase and group velocities due to a combination of core radius, numerical aperture, and the wavelength of the signal wave. In circular waveguides such as optical fiber, the figure is calculated by dividing the core radius by wavelength of the light wave passing through it. Of concern only in single-mode fibers where ideally waveguide dispersion and material dispersion cancel one another out.
Wavelength (Λ): The distance between consecutive peaks or crests of a wave (usually light, sound or water) propogating at a given frequency.
Wavelength-Division Multiplexing (WDM): A technology which allows for multiple optical signals to be carried over a single fiber by using different wavelengths of lasers to carry each different signal. This allows for increased performance via multiple signals, in both directions, over a single optical fiber.
Wavelength Shifter: A photofluorescent compound that receives high frequency photons and produces low frequency photons in return. Used to produced a controlled alteration of the signal it is placed into.
Waveguide: An optical structure made of dialectric material which utilizes total internal reflection to guide waves. The most common type of optical waveguide is fiber optic cable. Another variety is photonic-crystal fiber.

• Z •

Zero-Dispersion Wavelength: The wavelength(s) at which waveguide dispersion and material dispersion have a cancelling effect on one another. Found only in single-mode optical fibers.

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