Glossary Archive | Alker Fibre Optic Specialists






























A material such as silicon carbide, aluminum oxide, silica, cerium oxide, emery or rouge that is used to figure, shape, or finish optical elements. Abrasives differ from polishing materials mainly in particle size.


Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibers is explained by three factors: imperfections in the atomic structure of the fiber material, the intrinsic or basic fiber material properties and the extrinsic (presence of impurities) fiber material properties.

Acceptance Angle

The angle over which the core of an optical fiber accepts incoming light; usually measured from the fiber axis. Related to numerical aperture, NA.

Acceptance Cone

The cone within which optical power may be coupled into the bound modes of an optical fiber. The acceptance cone is derived by rotating the acceptance angle about the fiber axis.


An adapter is a mechanical device designed to align fibre optic connectors. It contains the split sleeve, also known as the interconnect sleeve, that holds the two ferrules together. Adapters can help mate or connect a variety of fibre optic cables together.

See also Coupler Adapter sleeve

Adapter Sleeve

A mechanical fixture within an adapter body that aligns and holds two terminated fibre connectors. The adapter sleeve material is normally: phosphor bronze, zirconia, ceramic or polymer.

See also Adapter

Add-drop multiplexer (ADM)

A device that drops and/or add one or more optical channels to a signal.

Aligned Bundle

An assembly of fibers in which the coordinates of each fiber are the same at the two ends of the bundle. Also called coherent bundle.

All Silica Fibre

Also known as all glass fibre. A fibre with both a silica core and a silica cladding, regardless of the presence of a polymer overcoat or buffer.


The maximum value of the electromagnetic wave, measured from the mean to the extreme: the height of the wave.

Angle of Deviation

The net angular deflection experienced by a light ray after one or more refractions or reflections. The total deviation ε of a light ray can be computed, given the incidence angle α. The same calculation allows for determination of the emergence angle δ. The angle of deviation is the angle between the original incident ray and the emergent ray. Since the larger angle of deviation corresponds to a larger refractive index, violet light would emerge with the largest angle of deviation and red light would emerge with the smallest angle. The difference between the angles of deviation of any two rays is called the angle of dispersion.

Angle of Incidence

The angle between an incident ray and the normal to a reflecting or refracting surface.

Angle of Refraction

The angle formed between a refracted ray and the normal to the surface. This angle lies in a common plane with the angle of incidence.

Angled Physical Contact (APC)

A style of fiber optic connector with a 5°-15° angle on the connector tip for the minimum possible backreflection. The APC, angled physical contact, connector is best for high bandwidth applications and long haul links since it offers the lowest return loss (RL) of connectors currently available. In an APC connector, the endface of a termination is polished precisely at an 8-degree angle to the fiber cladding so that most RL is reflected into the cladding where it cannot interfere with the transmitted signal or damage the laser source. As a result, APC connectors offer a superior RL performance of -65 dB.

Angular Misalignment

Loss at a connector due to fiber end face angles being misaligned.


Area of a fiber core which will accept light entry, also known as light acceptance. Angle of a light ray which can be used for injecting light into a fiber core or waveguide.

AR Coating

Antireflection coating. A thin, dielectric or metallic, film applied to an optical surface to reduce its reflectance and thereby increase its transmittance.


Aramid fibers are a class of heat-resistant and strong synthetic fibers. Provides support, protection and tensile strength. Also referred to as Kevlar® a brand of aramid yarn.


A component intended to protect the critical internal components, buffer tubes or fibers, from damage related to external mechanical attack or abrasion.
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Reduction of signal magnitude, or loss. The reduction in optical power as it passes along a fibre. The decrease in signal strength along a fibre optic waveguide caused by absorption and scattering. Attenuation is usually expressed in dB/km.
See also Absorption Attenuator

Attenuation Coefficient

Characteristic of the attenuation of an optical fiber per unit length, in dB/km.

Attenuation Constant

The real part of the axial propagation constant for a particular mode. The attenuation coefficient for the mode power is twice the attenuation constant.

Attenuation Limited Operation

The condition in a fiber optic link when operation is limited by the power of the received signal (rather than by bandwidth or distortion).

Attenuation Limited Power

The limitation on performance imposed by the Amplitude of a received signal rather than distortion.

Attenuation Meter

A device used to measure power loss in fiber optic connectors, cables, or systems.


A passive device for reducing the amplitude of a signal without appreciably distorting the waveform.
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Axial Propagation Constant

The propagation constant evaluated along the axis of a fiber in the direction of transmission.


The center of an optical fiber.

Back Reflection Optical Return Loss

Optical return loss (or back reflection) of a connection is the amount of light that is reflected back up the fiber toward the source by light reflections off the interface of the polished end surface of the mated connectors and air. Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Expressed in dB relative to incident power.

Back Scattering

Scattering of light in the direction opposite to that in which it was originally traveling.
The return of a portion of scattered light to the input end of a fiber.
The scattering of light in the direction opposite to its original propagation.


The frequency range over which the connector or device can operate without degradation of performance.

Beam Attenuator

A device designed to decrease flux density or power per unit area of a light beam through absorption and scattering of the beam.

Beam Splitter

A device that divides incident light into two separate beams. An optical device, such as a partially reflecting mirror, that splits a beam of light into two or more beams. Used in fiber optics for directional couplers.

Bend Loss

A form of increased attenuation in a fiber that results from bending a fiber around a restrictive curvature (a macrobend) or from minute distortions in the fiber (microbends).

Bend Radius

The radius of curvature that an optical fiber can bend without sustaining damage. The smaller the bend radius, the greater the material flexibility (as the radius of curvature decreases, the curvature increases). A minimum bend radius is the radius below which an optical fiber or fiber optic cable should not be bent. The manufacturer should specify the minimum radius to which the cable may safely be bent during installation, and for the long term, the former is somewhat shorter than the latter. The minimum bend radius is in general also a function of tensile stresses during installation, while being bent around a sheave while the fiber or cable is under tension.
POF is a very flexible medium and is not readily suscpetible to damage from bending.
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Bending of Light

The action occurring when light passes through an optical Interface at other than normal incidence, i.e., refraction.

Biconic Connector

A type of fiber optic connector consisting of two cone shaped ferrules aligned by a mating sleeve.
AT&T named this fiber optic connector the “Biconic” since it had a conical molded-plastic ferrule that fitted into a bi conical mating adapter when connected to another connector.


The ability for light to transmit in both directions through a fiber. Fibers are generally used in pairs, with one fiber carrying a signal one way, and the other fiber carrying it the other way. However, bidirectional communications is possible over one fiber strand by using two different wavelengths and appropriate coupling/splitting devices. Bidirectional couplers split or combine light the same way when it passes through them in either direction.

Bifurcated Fibre

A branched fibre optic light guide.
Bifurcated fibres have two fibres in the common end and break out into two legs at the other end. The fibre type used in each leg can be the same or different, allowing optimization of fibre core diameter or wavelength range based on the application.


The separation of a light beam, as it penetrates a doubly refracting object, into two diverging beams, commonly known as ordinary and extraordinary beams.


A form placed onto the fiber optical component used in connectors to seal and/or strain relieve a cable assembly.

Breakout Cable

A type of fiber optic cable containing several fibers, each with its own jacket and all of them surrounded by one common outer jacket.
Breakout cables are also called fanout cables.
alker offer internal and external fibre optic breakout cables for different environments.


A protective coating applied directly on the fiber. In a Fiber optic communication cable, one type of component used to encapsulate one or more optical fibers for the purpose of providing such functions as mechanical isolation, protection from physical damage and fiber identification. The buffer may take the form of a miniature conduit, contained within the cable and called a loose buffer, or loose buffer tube, in which one or more fibers may be enclosed, often with a lubricating gel. A tight buffer consists of a polymer coating in intimate contact with the primary coating applied to the fiber during manufacture.


A rigid or flexible group of fibers assembled in a unit. Coherent fiber bundles have fibers arranged in the same way on each end and can transmit images.

Butt Splice

A joining of two fibers without optical connectors arranged end-to-end by means of a coupling.


Wavelengths of about 1530 to 1565 nm, where erbium-doped fiber amplifiers have their strongest gain. Normally erbium-fiber amplifiers operate in either C- or L-band.


One or more optical fibers enclosed, with strength members, in a protective covering.

Cable Jacket

The outermost protective covering applied over the internal cable elements (core, cladding, buffer, and strength member).

Center Wavelength

It is the wavelength defined by a peak mode measurement where the effective optical power resides.


A layer of glass or other material surrounding the core of a fibre, forming the conduit that carries the light through the fibre. It has a refractive index slightly lower than the core.
The low-refractive-index material that surrounds the core of an optical fiber to contain core light while protecting against surface contaminant scattering.
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.

Cladding Diameter

The average of the diameters of the smallest circle that can be circumscribed about the cladding, and the largest circle that can be inscribed within the cladding.

Cladding Mode

An undesired mode that is confined to the cladding of an optical fiber by virtue of the fact that the cladding has a higher refractive index than the surrounding medium, i.e., air or primary polymer overcoat. Modern fibers have a primary polymer overcoat with a refractive index that is slightly higher, rather than lower, than that of the cladding, in order to strip off cladding modes after only a few centimeters of propagation.

Cladding Mode Stripper

A mechanism or device, especially a coating with a refractive index equal to or slightly greater that that of an optical fiber’s cladding, that removes modes propagating through the cladding by allowing them to radiate out of the fiber.


The process of scoring and breaking the optical fiber end in order to terminate a connector. Also known as cleaving. In an optical fiber, a deliberate, controlled break, intended to create a perfectly flat endface, perpendicular to the longitudinal axis of the fiber. A cleave is made by first introducing a microscopic fracture into the fiber with a special tool, called a cleaving tool, which has a sharp blade of hard material, such as diamond, sapphire, or tungsten carbide. If proper tension is applied to the fiber as the nick is made, or immediately afterward (this may be done by the cleaving tool in some designs, or manually in other designs), the fracture will propagate in a controlled fashion, creating the desired endface. A good cleave is required for a successful splice of an optical fiber, whether by fusion or mechanical means. Also, some types of fiber optic connectors do not employ abrasives and polishers, instead, they use some type of cleaving technique to trim the fiber to its proper length, and produce a smooth, flat perpendicular endface.


An outer plastic layer applied over the cladding of a fiber for mechanical protection. The material surrounding the cladding of a fiber. Generally a soft plastic material that protects the fiber from damage.


A term describing light as waves which are in phase in both time and space. Monochromaticity and low divergence are two properties of coherent light.

Coherence Length

The length over which energy in two separate waves remains constant. With respect to a laser, the greatest distance between two arms of an interferometric system for which suficient interferometric effects can be obtained.

Coherent Bundle of Fibres

Fibres packaged together in a bundle so they retain a fixed arrangement at the two ends and can transmit an image.


The process of aligning the optical axes of optical systems to the reference mechanical axes or surfaces of an instrument. The adjustment of two or more optical axes with respect to each other.


An optical instrument consisting of a well corrected objective lens with an illuminated slit or reticle at its focal plane. Collimators are used in lens testing to determine focal lengths, and in other applications where a distant object at a known location is required.

Composite Cable

A cable containing both fiber and copper conductors. Also known as hybrid cable.


A central core surrounded by one or more layers of materials, all sharing a common central axis.


The measurement of how well centred the core is within the cladding.


A device mounted on the end of a fiber optic cable, light source, receiver, or housing that mates to a similar device to couple light into and out of optical fibers.
A connector joins two fiber ends, or one fiber end and a light source or detector.
A mechanical or optical device that provides a demountable connection between two fibers or a fiber and a source or detector.
Hardware installed on fiber cable ends to provide cable attachment to a transmitter, receiver or other cable.
Usually a device that can be connected and disconnected repeatedly.
Numerous connector styles have been developed, each meant to offer better performance, easier and/or termination and lower cost.
Different connector types have different characteristics, different advantages and disadvantages, and different performance parameters. But all connectors have the same four basic components: ferrule, connector body, cable and coupling device. Each connector type has strong points, for example: ST connectors are a good choice for easy field installations; the FC connector has a floating ferrule that provides good mechanical isolation; the SC Connector offers excellent packing density and its push-pull design resists fiber end face contact damage during unmating and remating cycles.
Since fiber optic technology was introduced in the late 70s, numerous connector styles have been developed. Each new design was meant to offer better performance (less light loss and back reflection), easier and/or termination and lower cost.

Connector Induced Fibre Loss

Insertion loss, expressed in dB, due to impurities or structural changes to the fibre optic cable by termination or handling with the connector.

Connector Insertion Loss

Total optical power loss caused by insertion of an optical component such as a connector, splice, or couple into a previously continuous path. The loss in the power of a signal that results from inserting a passive component into a previously continuous path. Examples of such passive devices are connectors, inline star couplers and splices.

Connector Receptacle

The fixed or stationary half of a connection that is mounted on a panel/bulkhead. Receptacles mate with plugs.

Connector Return Loss

The amount of power reflected from the connector to connector interface.
Return loss values are expressed as dB.
Connector Type Typical Reflectance
Flat with air gap -20 dB
Physical Contact (PC) -30 tp -40 dB
Ultra PC -40 to -50 dB
APC (angled) -60 dB or higher

Connector Variation

The maximum value in dB of the difference in insertion loss between mating optical connectors.
Also known as optical connector variation.

Constructive Interference

Any interference that increases amplitude of the resultant signal. For example, when the wave forms are in phase, they can create a resultant wave equal to the sum of multiple light waves.


The transparent glass or plastic section with a high refractive index. The center of the optical fiber through which light is transmitted. The core is the part of the fiber that carries the light signal that has information impressed upon it. The fiber optic core is made of very pure glass and its diameter is measured in microns. The central region about the longitudinal axis of an optical fiber, which region supports guiding of the optical signal. For the fiber to guide the optical signal, the refractive Index of the core must be slightly higher than that of the cladding. In different types of fibers, the core and core cladding boundary function slightly differently in guiding the signal. Especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding.

Core Area

The part of the cross sectional area of an optical fiber within which the refractive index is everywhere greater than that of the innermost homogeneous cladding, by a specified fraction of the difference between the maximum refractive index of the core and the refractive index of the innermost cladding.

Core Diameter

In the cross section of a realizable optical fiber, ideally circular, but assumed to a first approximation to be elliptical, the average of the diameters of the smallest circle that can be circumscribed about the core cladding boundary, and the largest circle that can be inscribed within the core cladding boundary.

Core Eccentricity

A measure of the displacement of the center of the core relative to the cladding center.

Coupled Modes

A mode that shares energy among one or more other modes, all of which propagate together. The distribution of energy among the coupled modes changes with propagation distance.


The method for connecting two connectors that cannot intermate.
If the two ends of a coupler are different then it is usually referred to as an adapter.
Another meaning of coupler: an optical fibre device with one or more input fibre and one or several output fibres. Light from an input fibre can appear at one or more outputs, with the power distribution potentially depending on the wavelength and polarization. Such couplers can be fabricated in different ways, for example by thermally fusing fibre so that their cores get into intimate contact. If all involved fibres are singlemode (support only a single mode per polarization direction for a given wavelength), there are certain physical restrictions on the performance of the coupler. In particular, it is not possible to combine two or more inputs of the same optical frequency into one single-polarization output without significant excess losses. However, such a restriction does not occur for different input wavelengths: there are couplers which can combine two inputs at different wavelengths into one output without exhibiting significant losses. Such couplers are used e.g. in fibre amplifiers to combine the signal input and the pump wave. Other wavelength-sensitive couplers are used as multiplexers in wavelength division multiplexing (WDM) telecom systems to combine several input channels with different wavelengths, or to separate channels.

See also Adapter


Transfer of light into or out of an optical fiber. (Note that coupling does not require a coupler). An optical device that splits or combines light from more than one fiber. A device that connectors three or more fiber ends, dividing one input between two or more outputs or combining two or more inputs into one output.

Coupling Efficiency

The fraction of available output from a radiant source that is coupled and transmitted by an optical fiber.

Coupling Loss

The power loss suffered when coupling light from one optical device to another. There are intrinsic losses (non-ideal fiber parameters) and extrinsic losses (mechanical effects).

Crimp Die

Portion of the crimp tool that shapes the crimp.

Crimp Sleeve

A crimped metal cylinder that holds the connector to the cable through the cable’s strength member.
For some type od connectors the fiber optic jacket has to be securely fastened to the connector body. A band or ring is crimped around the jacket to secure it to the connector body.
Other times a piece of shrink tubing is used to secure the jacket to the connector body.

Crimp Tool

Mechanical device that holds the crimp die and is used to perform the crimping function.


The physical compression (uniformed deformation) of a metal band or ring around the connector body.

Critical Absorption Wavelength

That Wavelength at which the Absorption of a given element or system begins to demonstrate an inconstant value.

Critical Angle

The least angle of incidence at which total internal reflection takes place. The angle of incidence in a denser medium at the interface between the denser and less dense medium, at which the light is refracted along the interface. When the critical angle is exceeded, the light is totally reflected back into the denser medium. The critical angle varies with the indices of refraction of the two media with the relationship: sin Ic = n´/n where Ic is the critical angle; n´ the refractive index of the less dense medium; and n the refractive index of the denser medium.

Cutback Method

A destructive technique for determining certain optical fiber transmission characteristics, such as attenuation and bandwidth, by performing the desired measurements on a long length of the fiber under test, cutting the fiber under test at a point near the launching end, repeating the measurements on the short length of fiber, and subtracting the results obtained on the short length to determine the results for the residual long length.

Cutoff Mode

The highest order mode that will propagate in a given waveguide at a given frequency.

Cutoff Wavelength

The wavelength beyond which singlemode fiber only supports one mode of propagation. The longest wavelength at which a single mode fiber can transmit two modes.

Dark Fibre

Unused fibre. Carrying no light. Dark fibre is fibre optic cable that has not been used yet and is not connected to any device. Optical fibre installed without transmitter and receiver, usually to provide expansion capacity.

Dense Wavelength Division Multiplexing (DWDM)

Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. A WDM system uses a multiplexer at the transmitter to join the several signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer. DWDM is also sometimes called wave division multiplexing (WDM).

Destructive Interference

Any interference that decreases the desired signal.

Diameter Mismatch Loss

The loss of power at a joint that occurs when the transmitting fiber has a diameter greater than the diameter of the receiving fiber. The loss occurs when coupling light from a source to fiber, from fiber to fiber, or from fiber to detector.


A material such as a glass fiber, which is not metallic and is not conductive.

Dielectric Coatings

Dielectric coatings, also called thin film coatings or interference coatings, consist of thin layers of transparent dielectric materials, which are deposited on a substrate. Their function is essentially to modify the reflective properties of the surface by exploiting the interference of reflections from multiple optical interfaces. They can be used for highly reflecting laser mirrors or partially transmissive output couplers, for dichroic mirrors, for anti-reflection coatings, for various kinds of optical filters, beam splitters, heat reflectors, solar cell covers, and thin film polarizers.

Dielectric Filter

An optical fiber that selectively transmits one wavelength and reflects others based on interference effects inside the structure. Also called interference filter.


As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated. These secondary wavefronts will interfere with the primary wavefronts as well as with each other to form various diffraction patterns.

Diffraction Angle

The angle that lies between the direction of an incident light beam and any resulting diffracted beam.

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.

DIN Connector

The DIN connector have a single unit body with spring loaded free floating zirconia ferrule. Offers superior performance in a compact and DIN 47256 compatible design.

Directional Coupler

A coupler in which light is transmitted differently when it goes in different directions.


The stretching of light pulses as they travel in an optical fiber, which increases their duration. The temporal spreading of a light signal in an optical waveguide caused by light signals traveling at different speeds through a fiber either due to modal or chromatic effects.

Dispersion Compensation

Offsetting the dispersion of one fiber by using different fibers or other components that have dispersion of the opposite sign. Usually done for chromatic dispersion.

Dispersion Slope

The change in dispersion with wavelength.

Distribution Cable

Distribution cable is the most popular indoor cable, as it is small in size and light in weight, contain several tight-buffered fibres bundled under the same jacket with Kevlar strength members and sometimes fibre glass rod reinforcement to stiffen the cable and prevent kinking. The fibres are double buffered and can be directly terminated, but because their fibres are not individually reinforced, these cables need to be broken out with a breakout box or terminated inside a patch panel or junction box to protect individual fibres.[1]

Dominant Mode

The mode in an optical device spectrum with the most power.


An impurity added to an optical medium to change its optical properties.

Double Clad Fibre

Optical fibre with a structure consisting of three layers of optical material instead of the usual two. The inner most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices.

Dual Window Fibre

For multimode fibres, the term means that the fibre is optimized for 850nm and 1310nm operations. For single-mode fibres means that the fibre is optimized for 1310nm and 1500nm operation.


In cables, one that contains two fibres. For connectors, one that connects two pairs of fibres.
See also Simplex

Duplex Transmission

Transmission in both directions, either one direction at a time (half duplex) or both directions simultaneously (full duplex).

E2000 Connector

The E2000 connector hold a single fiber in a ceramic ferrule. E2000’s have a moulded plastic body similar to the LC’s but are smaller. The E2000 also exhibits pull latching mechanism and integrates a protective cap over the ferrule, which acts as a dust shield and shield users from laser emission. The protective cap is loaded with an integrated spring to ensure proper closing of the cap. Like other small for factor connectors, the E-2000 connector is suited for high density application.


The displacement of the optical axis from the mechanical axis. A measure of the center of a conductor’s location with respect to the circular cross-section of the insulation surrounding it, expressed as a percentage of center displacement of one circle within the other.

Effective Area

All nonlinear effects are dependent upon the intensity of the electromagnetic field in the medium. However, it is the total optical power entering and leaving the fibre that is usually measured. Some method is required for converting between the two when comparing theoretical and experimental results.
The field in a single mode fibre is not evenly distributed or even fully contained within the core. It is larger at the fibre axis than near the core cladding interface and extends into the cladding to a degree depending on the actual refractive index profile. The effective area parameter has been defined for the purposes of calculating nonlinear effects. The larger the effective area or (MFD), the lower the power density and so the lower impact of non linear effects, and the smaller the effective area or (MFD), the higher the power density which leads to higher impact of the non linear effects.
The effective area is sometimes included in the description of the fiber name, such as Corning`s LEAF (Large Effective Area Fiber).

Effective Mode Volume

For an optical fiber, the square of the product of the diameter of the near field pattern and the sine of the radiation angle of the far field pattern. The diameter of the near field radiation pattern is defined as the full width at half maximum and the radiation angle at half maximum intensity. Effective mode volume is proportional to the breadth of the relative distribution of power amongst the modes in a multimode fiber. It is not a spatial volume but rather an optical volume equal to the product of area and solid angle.

Effective Numerical Aperture

The real Numerical Aperture (NA) of a fiber when the computed NA is not valid because of change in the glass indices during drawing and fusion.

Effective Refractive Index

The phase delay per unit length in a waveguide relative to the phase delay in vacuum.

Electrical to Optical Converter (E/O)

A device that converts electrical signals to optical signals, such as a laser diode.


Describes the fact that the core or cladding may be elliptical rather than circular.

End Face

Term often used to describe the end of a ferrule. The end face is finished or polished to have a smooth end, which can minimize connector loss or back reflection. Typical polish types are PC, UPC, and APC.

End Finish

The quality of the end surface of a fiber prepared for splicing or terminated in a connector.

End Separation Loss

The optical power loss caused by distance between the end of a fiber and a source, detector, or another fiber.

Epoxy Connector

A type of fiber optic connector that is terminated onto a cable with use of epoxy to perform a chemical bond.

Epoxy Resin

A thermosetting resin used to secure the fiber with the connector ferrule. Plastic materials that become hard, infusible solids upon the addition of a hardening agent. Epoxy resins have excellent adhesive action, high chemical, solvent, and thermal resistance, and low shrinkage on curing.

Equilibrium Modal Distribution (EMD)

EMD of light travelling in an optical waveguide or fiber, is the distribution of light that is no longer changing with fibre length or with input modal excitation. Achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.
The condition in a multimode fiber wherein after propagation has taken place for a certain distance, called the equilibrium length, the relative power distribution among modes becomes statistically constant and remains so for the course of further propagation down the fiber.


Erbium is a chemical element in the lanthanide series, with symbol Er and atomic number 68. Optical fibers doped with erbium, can amplify light in the 1550 nm region when pumped at around 980 or 1480 nm by an external light source. The 1550 nm wavelength is important for optical communications because standard singlemode optical fibers have minimal loss at this particular wavelength. This process results in an unusually mechanically simple laser optical amplifier. Fiber amplifiers are used to support dense wave length division multiplexing (DWDM) and to expand to the other wavelength bands supported by fiber.

Erbium Doped Fibre Amplifier (EDFA)

EDFA is an optical device that is used to boost the intensity of optical signals being carried through a fibre optic communications system. An optical fibre is doped with the rare earth element erbium so that the glass fibre can absorb light at one frequency and emit light at another frequency.

Evanescent Wave

Guided light waves that extend beyond the boundary of a fiber core into the cladding. Evanescent waves can transfer energy between waveguides. Light guided in the inner part of an optical fiber’s cladding rather than in the core, i.e. the portion of the light wave in the core that penetrates into the cladding.

Excess loss

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

Extrinsic Fibre Loss

Optical fibre loss caused by the imperfect alignment of fibres in a connector or splice. Contributors include angular misalignment, axial misalignment, end separation, and end finish.

Fan Out Cable

Is an optical fiber cable containing several jacketed simplex optical fibers packaged together inside an outer jacket. Multi fiber cable constructed in the tight buffered design. Designed for ease on connectorization and rugged applications for intra or interbuilding requirements. Same as a breakout cable.

Fast Axis

In a birefringent material, the index of refraction varies with the direction of vibration of a lightwave. That direction having a low refractive index is the fast axis; at right angles to it is the slow axis, with a high index of refraction.

FC Connector

FC stands for Fixed Connection. It is fixed by way of a threaded barrel housing. FC connectors are typical in test environments and for singlemode applications. Were designed for use in high-vibration environments, offer extremely precise positioning of the fibre optic cable with respect to the transmitter’s optical source emitter and the receiver’s optical detector. Once installed the position is maintained with absolute accuracy. They feature a position locatable notch and a threaded receptacle. FC connectors are generally constructed with a metal housing and are nickel-plated. They have ceramic ferrules and are rated for 500 mating cycles. The insertion loss for matched FC connectors is 0.25 dB. FC is the fibre optic connector standard for Nippon Telephone & Telegraph (NTT) installations, developed with Nippon Electric Co. (NEC).

The 2.14 mm FC APC keyway format is typically called “wide key”, “NTT” or “type N”.The narrower 2 mm keyway format is called “Reduced” or “type R”. Some manufacturers mark type R keys with a single scribe mark on the key and type N keys with a double scribe mark.
See also Connector


Feedthroughs are designed to allow optical fibers to penetrate compartments at different pressure levels with terminations on both sides. They allow optical fibre to pass through the wall of a pressure or hermetically sealed vessel permitting the use of test and monitoring equipment, probes, laser delivery systems, optical sensors and more while maintaining the integrity of the vessel.


A precision tube which holds a fiber for alignment for interconnection or termination. Ferrule may be part of a connector or mechanical splice.
The ferrule is a rigid tube used to confine and support the stripped end of a fiber as found in connectors.
The ferrule, usually made of metal or ceramic, is the central part of the male connector. It is designed to both align and protect the fiber core during connection. The ferrule tip is polished to ensure a smooth finish on the fiber end. Polish can also minimize connector loss or backreflection, depending on the angle used. There are four types of polish: PC, super PC (SPC), ultra PC (UPC) and angled PC (APC).


An optical instrument consisting of an objective lens, a coherent (usually flexible) fiber bundle and an eyepiece to examine the output of the fiber bundle.


A single, separate optical transmission element, characterized by a core and a cladding. Any filament or fibre, made of dielectric materials, that guides light, whether or not it is used to transmit signals.

Fibre Amplifier

An optical fibre doped to amplify light from an external source. The most important type is the erbium-doped fibre amplifier.

Fibre Axis

The mechanical centerline through the core of an optical fibre. The longitudinal centre of symmetry of an optical fibre.

Fibre Bandwidth

The lowest frequency at which the magnitude of the fibre transfer function decreases to a specified fraction of the zero frequency value. Often, the specified value is one half the optical power at zero frequency.

Fibre Buffer

The material used to protect an optical fibre or cable from physical damage, providing mechanical isolation or protection. Fabrication techniques include both tight jacket or loose tube buffering as well as multiple buffer layers.

Fibre Bundle

To achieve high throughput with flexible glass or plastical optical fibres, multiple fibres are often arranged in a bundle. This is also desirable when light sources are not point-like and thus cannot be focused to small diameters. However light losses are inevitable for two reasons:
1. For a given fibre, the maximum usable area is defined by the core and cladding diameters. These diameters determine the fibre packing factor (FPF).
2. When fibres are arranged in a bundle, not all of the bundle cross-sectional area is filled with fibres, even when tightly packed arrangements are used. The particular packing arrangement defines the bundle packing fraction (BPF).

Fibre Cleaving

The controlled fracture of an optical fibre along a crystalline plane resulting in a smooth surface.

Fibre Core

The inner portion of the fibre that transmits light. The index of refraction is higher than that of the cladding that surrounds it.
The core diameter can range from very small, on the order of the wavelength of light, to several millimetres or more. Flexible glass and silica (quartz) fibres have core diameters which range from ∼50 μm to about 1000 μm. Light launched into a fibre will, after a given length, reach the core material boundary and pass to another medium (glass, air, etc). Depending on the incident angle, some of the energy will be refracted outward (leaky modes) and some will reflect back into the core material (guided modes).

Fibre Curl

A property of optical fibres that results from thermal stresses during manufacturing and is defined as the amount of curvature over any specified length of the fibre. The curl of the fibre is frozen at the time of manufacturing and is not known to vary throughout its operating life. It does, however, vary along its length and so it is useful and considered good practice to measure the curl even within a cable manufacturing environment to ensure the consistency of the product.

Fibre Loss

The deterioration of the light signal in transmission through a fibre optic cable.
The various losses in the fibre are due to absorption, scattering, dispersion and bending.
Fibre bending produces at least two loss mechanisms. In multimode fibres, the number of propagating modes is reduced as a function of the fibre bend radius. An additional problem when fibres are bent is electromagnetic radiation loss due to differences in propagation (wavefront) velocity. All these losses contribute to increased attenuation. For multimode fibres, this effect is relatively small when compared to the reduction of propagating modes. This type of attenuation can seriously affect single mode fibres when they are bent.
Absorption losses are largely due to impurities in glass material from residual often unwanted atoms and hydrogen/oxygen molecules. Unwanted impurities include metal ions such as Cr3+, Fe2+ and Cu2+, absorbing between 500 nm and 1000 nm.
Dispersion Loss: as an optical signal travels along the fibre, it becomes increasingly distorted. This distortion is a sequence of intermodal and intramodal dispersion. Intermodal dispersion is a distortion mechanism occurring in multimode fibres in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Intramodal dispersion is the pulse spreading that occurs within a single mode. Material dispersion or spectral dispersion or chromatic dispersion results because of variation due to the refractive index of the core as a function of wavelength, because of which pulse spreading occurs even when different wavelengths follow the same path. Waveguide Dispersion is whenever any optical signal is passed through the optical fibre, practically 80% of the optical power is confined to the core and the rest 20% optical power into the cladding.
Scattering losses occur due to microscopic variations in the material density, compositional fluctuations, structural inhomogeneities and manufacturing defects. One significant scattering mechanism at low wavelengths is Rayleigh scattering. Spatially there are high-density gradients which alter refraction index and thereby cause scattering. The effect evidences itself in, among other things, strong reverse scattering. Another scattering mechanism is Mie scattering, which mainly results in forwarding scattering. This mechanism comes from material inhomogeneities at longer wavelengths. Waveguide Scattering Losses is a result of variation in the core diameter, imperfections of the core-cladding interface, change in RI of either core or cladding. Finally, Stimulated Raman Scattering and Stimulated Brillouin Scattering is non-linear radiation-induced effects, which take place when particular intensity thresholds are exceeded. In practice, such non-linear effects only take place when high-intensity laser light is transmitted.

Fibre Optic Assembly

A length of fibre optic cable that has been terminated with a connector, pigtail or another component. There are many types of fibre optic cable connectors used in cable Assemblies: ST Connectors: slotted bayonet type connector with long ferrule. Common connector for multimode fibres. FC connectors: screw on type connector. Popular with single mode fibres. SC Connectors: push/pull connector that can also be used with duplex fibre construction. LC connectors: Much like the ST connector but with a ferrule that is half the size. MT-RJ connectors: connector configured for duplex fibres with both fibres in one ferrule. MU connectors: Much like the SC connector but with a ferrule about half the size.

Fibre Optic Cable

A package for an optical fibre or fibres that may include cladding, buffering, strength members and an outer jacket.
Fibre optic cable functions as a light guide, guiding the light introduced at one end of the cable through to the other end.
Singlemode cable is a single strand of glass fibre with a diameter of 8.3 to 10 microns, has one mode in which light will propagate, 1310 or 1550nm. Singlemode fibre carries higher bandwidth than multimode fibre but requires a light source with a narrow spectral width. The small core and single light wave virtually eliminate any distortion that could result from overlapping light pulses providing the least signal attenuation and the highest transmission speeds of any fibre cable type.
Typical multimode fibre core diameters are 50, 62.5, and 100 micrometres. Multimode fibre gives high bandwidth at high speeds over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable’s core typically 850 or 1300nm. However, in long cable runs multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission.
The cladding is usually coated with a tough resin buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fibre but do not contribute to its optical waveguide properties. Rigid fibre assemblies sometimes put light absorbing glass between the fibres, to prevent light that leaks out of one fibre from entering another. This reduces cross-talk between the fibres or reduces flare in fibre bundle imaging applications. For indoor applications, the jacketed fibre is generally enclosed, with a bundle of flexible fibrous polymer strength members like Aramid (e.g. Twaron or Kevlar), in a lightweight plastic cover to form a simple cable.
Each end of the cable may be terminated with a specialized optical fibre connector to allow it to be easily connected and disconnected from transmitting and receiving equipment. For use in more strenuous environments, a much more robust cable construction is required. In loose-tube construction, the fibre is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fibre itself. This protects the fibre from tension during laying and due to temperature changes. Alternatively, the fibre may be embedded in a heavy polymer jacket, commonly called tight buffer construction. These fibre units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching. The cable may also be armoured to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armoured in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power signals that are carried to power amplifiers or repeaters in the cable. Modern fibre cables can contain up to a thousand fibres in a single cable, so the performance of optical networks easily accommodates even today’s demands for bandwidth on a point to point basis.
POF is a plastic-based cable which promises performance similar to glass cable on very short runs but at a lower cost. Plastic core cable is used for short distance applications.

Fibre Optics (FO)

A general term describing a lightwave or optical communications system. In such a system, electrical information is converted to light energy, transmitted to another location through optical fibres and is there converted back into electrical information.
The branch of optical technology concerned with the transmission of radiant power through fibres made of transparent materials such as glass, Fused Silica or plastic. Common abbreviation is FO.

FibreBragg Grating (FBG)

A Fibre Bragg Grating is a periodic or aperiodic perturbation of the effective refractive index in the core of an optical fibre. The perturbation is approximately periodic over a certain length of e.g. a few millimetres or centimetres, and the period is of the order of hundreds of nanometers. This leads to the reflection of light propagating along the fibre in a narrow range of wavelengths, for which a Bragg condition is satisfied.
A fibre Bragg grating reflects the selected wavelength and transmits others. A fibre Bragg grating can, therefore, be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.


The process whereby the shape of an optical surface is altered by polishing.


A mechanical shoulder on the outside housing, enabling attachment to a panel; can also allow for seals with gaskets or o-rings.

Flat Ferrule

The Ferrule of the connector is flat, thus allowing the whole ferrule face to make contact in a connection.


That quality of a cable or cable component which allows for bending under the influence of outside force, as opposed to limpness, which is bending that occurs due to the cable’s own weight.

Flooding Compound

A substance surrounding the buffer tubes of a fiber optic cable, to prevent water intrusion into the interstices in the event of a breach of the jacket.

Free Space Optics (FSO)

Also called free space photonics. The transmission of modulated visible or infrared (IR) beams through the atmosphere via lasers, LEDs, or IR-emitting diodes (IREDs) to obtain broadband communications. The energy beam is collimated and sent through clear air or space from the source to the destination, rather than guided through an optical fiber. If the energy source does not produce a sufficiently parallel beam to travel the required distance, collimation can be done with lenses. At the source, the visible or IR energy is modulated with the data to be transmitted. At the destination, the beam is intercepted by a photodetector, the data is extracted from the visible or IR beam (demodulated), and the resulting signal is amplified and sent to the hardware. FSO systems can function over distances of several kilometers. As long as there is a clear line of sight between the source and the destination, communication is theoretically possible, but there are limitations. Rain, dust, snow, fog, or smog can block the transmission path and shut down the network.


The number of times an electromagnetic wave oscillates in a second, or the number of wave peaks that pass a point in second; measured in hertz.

Fresnel Reflection Loss

A loss that takes place at discontinuities of refractive index, especially at an air-glass interface such as a fiber endface. At those interfaces, a fraction of the optical signal is reflected back toward the source.

Front Mounted

A connector is front mounted when it is attached to the outside or mating side of a panel. A front mounted connector can only be installed or removed from the outside of the equipment.

Full Duplex

The full duplex system uses two fibers to communicate. This allows one fiber to transmit from point A to point B while the other fiber transmits from B to A. Therefore, both ends of a full-duplex system have both transmitters and receivers. Manufacturers include identification methods for fibers used in a full-duplex system, such as color coding or a ridge marking.

Fundamental Mode

The lowest order mode of a waveguide.

Fused Coupler

A fused coupler basically consists of two, parallel optical fibers that have been twisted, stretched and fused together so that their cores are very close to each other. The length of this coupling region determines the coupling ratio from one fiber to the other. During the manufacturing process, light is launched into an input fiber and the output power from each output fibers is carefully monitored. When the desired coupling ratio is achieved, the fully automated manufacturing process is stopped. The resulting coupler is essentially one fiber with two cores that are very near to one another.
Fused couplers are used to split optical signals between two fibers, or to combine optical signals from two fibers into one fiber. They are constructed by fusing and tapering two fibers together. This method provides a simple, rugged, and compact method of splitting and combining optical signals.

Fusion Splice

Fusion splice is the process of fusing or welding two fibres together by the application of localized heat sufficient to fuse or melt the ends forming a continuous single fibre. The splicing process begins by preparing each fibre end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fibre. The fibre is then cleaved. The quality of each fibre end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fibre end faces. The basic fusion splicing apparatus consists of two fixtures on which the fibres are mounted and two electrodes. Inspection microscope assists in the placement of the prepared fibre ends into a fusion splicing apparatus. The fibres are placed into the apparatus, aligned, and then fused together. The small size of the fusion splice and the development of automated fusion splicing machines have made electric arc fusion one of the most popular splicing techniques in commercial applications.
Multimode fibres can be harder to fusion splice as the larger core with many layers of glass that produces the graded-index profile are sometimes harder to match up, especially with fibres of different types or manufacturers.

See also Mechanical Splice

Fusion Splicer

An instrument that permanently bonds two fibers together by heating and fusing them.

Gap Loss

The power loss that occurs when an optical signal is transferred from one fiber to another that is axially aligned with it, but longitudinally separated from it. The gap allows light from the transmitting fiber to spread out as it leaves the fiber endface. When it strikes the receiving fiber, some of the light will enter the cladding, where it is quickly lost. As a result of signal strength and cohesion being lost (due to the scattering of the light), a fiber optic signal suffering from gap loss is degraded in both quality and throughput. The three basic types of gap loss are angular misalignment loss, lateral offset loss, and longitudinal displacement loss. The losses tend to be proportional to the ratio of the core radius to the size of the gap or displacement.
Gap loss can be reduced by filling the gap with a gel that matches the index of refraction of the fiber as closely as possible.

Gap Loss Attenuator

An optical attenuator that exploits the principle of gap loss to reduce the optical power level when inserted in-line in the fiber path. e.g., to prevent saturation of the receiver.

Graded Index Fibre

A type of multimode fibre which used a graded profile of refractive index in the core material to correct for dispersion. An optical fibre with a core having a refractive index that decreases with increasing radial distance from the fibre axis. The most common refractive index profile for a graded index fibre is very nearly parabolic. The parabolic profile results in continual refocusing of the rays in the core and compensates for multimode distortion. Has greater bandwidth than step index fibre, but less bandwidth than single mode fibre.

Graded Index Profile

Any refractive index profile that varies with radius in the core.

Group Index

For a given mode propagating in a medium of refractive index, the group index, is the velocity of light in a vacuum, divided by the group velocity of the mode.

Hard Clad Silica Fibre

A type of optical fibre in which a silica glass core (diameter: typically 200 µm) is surrounded by a hard polymer or similar material much stronger than the customary cladding material (diameter: 230 µm typically).
The core and cladding can be separated from each other. The hard cladding layer imparts unique properties on these fibres, serving to provide total internal reflection, enhancing the fundamental strength of the fibre, and providing a crimp-worthy thin coating for simple and reliable connectorization in the field.
HCS fibres have large silica cores and medium bandwidth, supporting transmission rates of less than 100 Mb/s. HCS fibres are suitable for distances at such data rates of several tens of meters to several hundred meters.

Homogeneous Cladding

Cladding in which the nominal refractive index is constant throughout. An optical fiber may have several homogeneous claddings, each having a different refractive index.

Hybrid adapter

Hybrid adapters are used to join dissimilar connectors together. The adapter’s key element is the split sleeve, normally made from zirconia, having a specific resistance force to insertion and withdrawal of a ferrule that ensures axial alignment of the cores.

See also Adapter Adapter sleeve

Hybrid Cable

A fiber optic cable containing two or more different types of fiber, such as multimode and singlemode. Also a cable containing both optical fiber and copper wire. Also known as composite cable.

Hydroxyl Ion Absorption

The absorption of electromagnetic waves, including the near infrared, due to the presence of trapped hydroxyl ions remaining from water as a contaminant. The hydroxyl (OH-) ION can penetrate glass during or after product fabrication, resulting in significant attenuation of discrete optical wavelengths used for communications via optical fibers.

Incident Angle

The angle between an incident ray and a line perpendicular to an optical surface.

Incoherent Bundle

A Bundle of filaments of optical glass or other transparent materials that transmit only light, not optical images. The arrangement of the individual fibers in the bundle is not sufficiently regular to transmit optical images.

Index Dip

An undesired decrease in the refractive index at the center of the core.

Index Matching Material

A substance, usually a liquid, cement (adhesive), or gel with refractive index similar to glass used to match the materials at the ends of two fibers to reduce loss and back reflection. Index Matching Material has an index of refraction that nearly equals that of the fibers core. Used to reduce Fresnel Reflection loss at fiber ends. Also known as index matching gel or index matching material, it is used to enhance the transmission of light across the joint.

Index of Refraction

The speed of light in a vacuum divided by the speed of light in a material, abbreviated n, which measures how materials refract light.

Index Profile

The refractive index of a fiber as a function of cross section.

Infrared (IR)

Light with wavelengths longer than 700nm and shorter than about 1mm, invisible to the human eye, which we can feel as heat. Glass optical fibers transmit infrared signals at 700 to about 1650nm in the infrared.

Insert Retention

The axial load in either direction that an insert must withstand without being dislocated from its normal position in the connector shell.

Insertion Loss

It is expressed in decibels as the ratio of power received at the load before insertion to the power received at the load after insertion. The total optical power loss caused by the insertion of a component such as a splice or connector in an optical fiber system: insertion loss from absorption. Insertion loss from misalignment either from mismatched core diameters or from lateral misalignment. Insertion loss from an air gap: this occurs from no physical contact. Insertion loss from contamination; contamination is the most common cause of insertion loss, such as dirt, scratches, or chips.

Intensity Modulation (IM)

In optical communications, a form of modulation in which the optical power output of a source varies in accordance with some characteristic of the modulating signal.


The way that waves add together, depending on their phase. Constructive interference occurs when the waves are in phase and their amplitudes add. Destructive interference occurs when the waves are 180 degree out of phase and their amplitudes cancel.

Interference Filter

An optical filter that selectively transmits one wavelength and reflects others based on interference effects inside the structure. Also called dielectric filter.


An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces.
A beam of light or another type of electromagnetic radiation is splitted into two equal halves using what’s called a beam-splitter, a piece of glass whose surface is very thinly coated with silver, half the light passes straight through and half of it reflects back—so the beam-splitter is like a cross between an ordinary piece of glass and a mirror. One of the beams known as the reference beam shines onto a mirror and from there to a screen or camera. The other beam shines at or through something you want to measure, onto a second mirror, back through the beam splitter, and onto the same screen or camera. This second beam travels an extra distance or in some other different way to the first beam, so it gets slightly out of phase.
When the two light beams meet up at the screen or camera, they overlap and interfere, and the phase difference between them creates a pattern of light and dark areas (a set of interference fringes). The light areas are places where the two beams have added together (constructively) and become brighter; the dark areas are places where the beams have subtracted from one another (destructively). The exact pattern of interference depends on the different way or the extra distance that one of the beams has traveled.
Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers.


The protective outer coating of the cable.

Jumper Cable

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


A very strong, very light, synthetic compound developed by DuPont which is used to strengthen optical cables.


The slot or groove in which a key slides.


Wavelengths of about 1570 to 1625nm where some erbium-doped fiber amplifiers operate.

Lateral Displacement Loss

Or Lateral Offset Loss. The loss of power that results from lateral displacement of optimum alignment between two fibers or between a fiber and an active device.

LC Connector

Lucent Technologies first developed the LC connector for TelCo environment uses; LC stands for Lucent Connectors. The LC connector is a small form factor fibre optic connector. The LC connector resembles a small SC connector. The LC connector has been standardized as FOCIS 10 (Fiber Optic Connector Intermateability Standards) in EIA/TIA-604-10. The LC connector uses a 1.25 mm ferrule, half the dimension of the ST ferrule or a regular ceramic ferrule connector.
Features of LC Connectors: half the dimension of regular connectors, push& pull mechanism, detachable clips for simplex as well as duplex connectors.
See also Connector

Leaky Mode

A mode having a field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance. Such a mode gradually “leaks” out of the waveguide as it travels down it, producing attenuation even if the waveguide is perfect in every respect.

Loose Buffer

A protective tube loosely surrounding a cabled fibre, often filled with gel. A type of fibre optic cable construction where the fibre is contained within a loose tube in the cable jacket.
See also Tight Buffer


Attenuation of optical signal, normally measured in decibels. The amount of a signal’s power that is lost in connectors, splices, or fiber defects.

Loss Budget

An accounting of overall attenuation in a system.
The distribution of total loss among the components of a system. It is calculated by adding the average losses of all the components used in the cable plant to get the total estimated end-to-end loss.


All macroscopic deviations of the fibre’s axis from a straight line that cause signal attenuation.
Macrobends are bends that are large enough to be seen by the human eye. Macro bend loss refers to losses induced in bends around mandrels (or corners in installations), the bends necessary to fit fibres inside splice enclosures or patch panels.
Bend-insensitive fibres are intended to reduce macro-bending loss so cables could be run around corners better and fibres would not have as much loss when stuffed into small spaces.

Mandrel Wrapping

Mandrel wrapping is a technique used to preferentially attenuate high order mode power of a propagating optical signal.
It consists of the wrapping of several turns of optical fibre around a mandrel approximately 1 cm in diameter in order to filter out excess high-order modes that are launched into the optical fibre.
If the fibre is propagating substantial energy in affected modes, the modal distribution will be changed.

For multimode fibre optics, a technique used to modify the modal distribution of a propagating optical signal.


To join two connectors halves in a normal engaging mode.

Material Dispersion

Material dispersion is a phenomenon in which different optical wavelengths propagate at different velocities, depending on the refractive index of the material used in the fibre core.
The refractive index of the glass varies according to the wavelength of the optical signal, different wavelengths travel at different speeds in the medium.
The effect of material dispersion is that the various wavelengths comprising the pulse travel at different velocities through the medium.
So, the pulse can spread over a distance simply due to the interaction of various wavelengths with the matter in the crystalline core, which causes some portions of a pulse to arrive earlier than other portions.
As the wavelength increases (and frequency decreases), material dispersion decreases.

Mechanical Splice

A splice in which fibres are joined mechanically but not fused together. An optical fibre splice accomplished by fixtures or materials, rather than by thermal fusion.
When the optical fibres are secured by mechanical means, the gap between the optical fibre endfaces usually is filled with a refractive index matching gel to reduce reflections.

See also Fusion Splice


Mechanical stress on a fibre that introduces local discontinuities, which results in light leaking from the core to the cladding by a process called mode coupling.
The introduction of minute curves with small radii in the optical axis of the guide or the core-cladding interface surface at which incidence angles of light rays propagating in the guide are, or tend to become, less than the critical angle so that some of the radiant power in the ray escapes from the waveguide core into the cladding and out of the guide.

Minimum Bend Radius

The smallest radius an optical fiber or fiber cable can bend before increased attenuation or breakage occurs.

Misalignment Loss

The loss of power resulting from angular misalignment, lateral displacement and fibre end separation.

Multi Mode Fibre

Transmits or emits multiple modes of light. An optical waveguide with a much larger core (50µm +) than the singlemode waveguide core (2µm to 9µm) and which permits approximately 1,000 modes to propagate through the core compared to only one mode through a singlemode fibre.
An optical fibre that has a core large enough to propagate more than one mode of light. The typical diameter is 62.5 micrometres or 50 micrometres.
See also: Optical Fibre Single Mode Fibre

Numerical Aperture (NA)

The sine of half the angle over which a fibre can accept light multiplied by the refractive index of the medium containing the light. For air, the index is almost equal to 1.
NA also describes the angular spread of light from a central axis, as in exiting a fibre, emitting from a source, or entering a detector.

Optical Bandpass

The range of optical wavelengths which can be transmitted through a component. Pertaining to a device that will allow or support a fixed band.

Optical Fibre

An optical fibre (or optical fiber) is a flexible, transparent fibre made by drawing glass (silica) or plastic. Optical fibres typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by the phenomenon of total internal reflection which causes the fibre to act as a waveguide.
Fibres that support many propagation paths or transverse modes are called multi-mode fibres (MMF), while those that support a single mode are called single-mode fibres (SMF). Multi-mode fibres generally have a wider core diameter and are used for short-distance communication links and for applications where high power must be transmitted.
An important aspect of a fibre optic communication is that the losses brought about by joining two different cables are kept to a minimum.
For applications that demand a permanent connection, a mechanical splice which holds the ends of the fibres together mechanically or a fusion splice that uses heat to fuse the ends of the fibres together could be used. Temporary or semi-permanent connections are made by means of specialized optical fibre connectors.

Optical Return Loss (ORL)

The ratio (expressed in dB) of optical power reflected by a component or an assembly to the optical incident power.
ORL is the ratio of the light reflected back from a device under test to the light launched into that device, usually expressed in dB.

Panda Fibre

Panda is a common style of polarisation-maintaining fibre, using round and symmetrical stress rods on either side of the core to induce polarisation.
See also Polarisation Maintaining Fibre

Patch Panel

A panel on which fibre optic connectors and fibre optic couplers are mounted in an organized array for easy access. Both sides are usually accessible with sufficient spacing between components to facilitate handling individual components. Also board panel, console or patch bay.

Polarisation Maintaining Fibre

An optical fibre in which the polarisation planes of lightwaves launched into the fibre are maintained during propagation with little or no cross-coupling of optical power between the polarisation modes. Also PM fibre or PM optical fibre. Used in special applications such us sensing and interferometers.
See also Panda Fibre


The act of smoothing ends of fibres generally using abrasives. Optically smooth surfaces allow maximum transmission of light between fibres at connections and minimize coupling loss.

Profile Dispersion

Dispersion attributed to the variation of refractive index contrast with wavelength.
The dispersion that occurs because the refractive index contrast, the profile dispersion parameter and the profile parameter of the waveguide are different for each of the different wavelengths in the spectral width of the lightwave.

Quick Disconnect

A type of fibre optic connector shell that permits rapid locking and unlocking of two connector halves.
See also Connector

Refractive Index

Of a medium: the ratio of the velocity of propagation of an electromagnetic wave in vacuum to the velocity of propagation of the wave in the medium. Common symbol n.

SC connector

SC connector is an abbreviation for subscription channel connector. SC is a connector with a 2.5 mm ferrule and a push-pull latching mechanism. It features high packing density, low loss, low back reflection, and low cost. It was the connector standardized in TIA-568-A. It is also available in a duplex configuration.
See also Connector


A single fibre cable.
See also Duplex

Single Mode Fibre

A small core optical fibre through which only one mode will propagate.
A fibre having a small core diameter (approximately 9 microns) with a cladding having a refractive index very close to that of the core will transmit light in a single-mode entering at a narrow-angle and transmitting over very wide bandwidths.

See also: Optical Fibre Multi Mode Fibre

SMA Connector

The SMA connector was developed by Amphenol from its line of microwave connectors known as “subminiature A”. The connector has a 3 mm stainless steel ferrule and a connector gap that is threaded on the inside. They are mostly used in military applications.
In addition to their compact size, the SMA connector has exceptional mechanical durability. The ferrule is traditionally made of steel, although ceramic versions are available. SMA connectors types: the SMA-905 has a straight ferrule, the SMA-906 has a stepped ferrule with lower insertion loss. SMA 905 is also available as a keyed connector.
See also Connector

ST Connector

ST stands for Straight Tip. ST is a fibre optic connector originally developed by AT&amp.
Quick-release bayonet style connector, cylindrical with twist lock coupling, 2.5mm keyed ferrule.
They are easily inserted and removed due to their design. The typical Insertion Loss for matched ST connectors is 0.25 dB.
See also Connector

Tight Buffer

A protective coating extruded tightly over fibre for mechanical and environmental protection. A thick coating of a plastic-type material is applied directly to the outside of the fibre itself. The coating material is either nylon or PVC. This results in a smaller overall diameter of the entire cable and one that is more resistant to crushing or overall impact forces. The buffering offers excellent physical and flexing properties and higher micro-bending sensitivity. Tight buffer cable is normally lighter in weight and more flexible than loose-tube cable.
See also Loose Buffer

Ultra Physical Contact (UPC)

A term describing a specific fibre optic contact polishing protocol or process yielding improved return loss.
UPC connectors are polished with no angle.
See also Connector

Variable Optical Attenuator (VOA)

An attenuator in which the attenuation can be varied.
In fibre optic systems a device that is inserted into a fibre optic link and attenuates the intensity of lightwaves over a continuous range of attenuation.
See also Attenuation Attenuator

Zero Dispersion Wavelength

Zero Dispersion wavelength is the wavelength at which the net chromatic dispersion of an optical fibre is nominally zero. In a single-mode fibre, the wavelength at which waveguide dispersion cancels out material dispersion.


Zipcord is a type of cable made by two or more cables held together by an insulating jacket that can be easily separated simply by pulling apart. In fibre optics, zipcord format is most often seen in duplex fibre optics cables.

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