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Glossary of Terms

This glossary contains definitions and descriptions which are intended to help everyone understand the meanings of the words as they are used on our site. Many words have several shades of meaning, or even totally different meanings, when used within other contexts.

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

A

Absorption - In an optical fiber, loss of optical power resulting from conversion of that power into heat. Intrinsic causes of absorption in a fiber involve tails of the ultraviolet and infrared absorption bands. Extrinsic components causing loss include impurities, e.g., the OH- ion and transition metal ions, and defects, e.g., results of thermal history and exposure to nuclear radiation. See also: Attenuation.

Acceptance Angle - The angle within which a fiber will accept light for transmission along its core. This angle is measured from the centerline of the core.

Acousto-Optic Modulator - A device that varies the amplitude and phase of a light beam, e.g., from a laser, by sound waves.

Active Port Diameter - On a light source or detector, the diameter of the area in which light can be delivered to or received from an optical fiber.

Analog - An electrical signal which varies continuously. Sound waves and ocean surf repeatedly striking the beach are examples of analog signals. Analog signals have a frequency range (bandwidth) measured in Hertz (Hz). See also: Digital.

Angle of Incidence - The angle formed between an incoming ray of light striking a surface and a perpendicular line drawn to that surface at the point of incidence (the point at which the ray strikes the surface.)

Angle of Reflection - The angle formed between an outgoing ray of light after striking a surface, and reflecting from it, and a perpendicular line drawn to that surface at the point of incidence.

Angstrom (Å) - A unit of optical wavelength (obsolete). 1 Å= 10-10 meters. Note: The angstrom has historically been used in the field of optics, but it is not an SI (International System) unit.

Angular Alignment - The alignment of two optical fibers with respect to the angle formed by their axes.

Angular Misalignment Loss - The optical power loss caused by the angular deviation from the optimum alignment of source to optical fiber, fiber-to-fiber, or fiber-to-detector. See also: Extrinsic joint loss; Lateral offset loss.

Anode - The element of an electron tube or semiconductor device towards which the primary electron stream flows. It can be thought of as the positive potential terminal.

Anti-Reflection (AR) Coating - A thin layer of material applied to an optical surface to reduce reflectance and to increase transmittance. The ideal value of the refractive index of a single-layered film is the square root of the product of the refractive indices on either side of the surface to which it is applied, the ideal optical thickness being one quarter of wavelength.

Attenuation - In an optical fiber, the diminution of average optical power. Note: In optical fiber, attenuation results from absorption, scattering and other radiation losses. Attenuation is generally expressed in dB without a negative sign. Calculations and equations involving loss show and use the negative sign. Attenuation is often used as a synonym for attenuation coefficient, expressed in dB/km. This assumes the attenuation coefficient is invariant with length.

Attenuation Coefficient - A factor expressing optical power loss per unit of length, expressed in dB/km. The sum of scattering and absorption coefficients.

Attenuation-Limited Operation - The condition prevailing when the received signal amplitude (rather than distortion) limits performance. See also: Bandwidth-limited operation; Distortion-limited operation.

Avalanche Photodiode (APD) - A photodiode that has gain in its output power compared to the optical power that it receives through avalanche multiplication of photocurrent. Note: As the reverse-bias voltage approaches the breakdown voltage, hole-electron pairs created by absorbed photons acquire sufficient energy to create additional hole-electron pairs when they collide with ions; thus, a multiplication (signal gain) is achieved. See also: Photon; PIN photodiode.

Axis - A straight line, real or imaginary, passing through a body and indicating its center; a line so positioned that various portions of an object are located symmetrically in relation to the line. Plural = Axes.

Axial Ray - A light ray that travels along the optical fiber's axis. See also: Meridional ray; Skew ray.

B

Backscattering - The portion of scattered light which returns in a direction generally reverse to the direction of light travel. See also: Rayleigh scattering; Reflectance; Reflection.

Bandwidth - The range of frequencies handled by a device or system. See also: Fiber bandwidth.

Bandwidth-Limited Operation - The condition prevailing when the system bandwidth, rather than the amplitude (or power) of the signal, limits performance. This condition is reached when material and modal dispersion distort the shape of the waveform beyond specified limits. See also: Attenuation-limited operation; Distortion-limited operation; Material dispersion; Modal dispersion.

Beam Diameter - The distance between two diametrically opposed points at which the irradiance is a specified fraction of the beam's total irradiance; most commonly applied to beams that are circular or nearly circular in cross section. Synonym: Beamwidth. See also: Beam divergence.

Beam Divergence - The increase in beam diameter with increase of distance from the source.

Beamwidth - See Beam diameter.

Beamsplitter - A device for dividing an optical beam into two or more separate beams; often a partially reflecting mirror. See also: Coupler; Splitter.

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

Bit Error Rate (BER) - In digital applications, the ratio of bits received in error to bits sent. BERs of 10-9 (one error bit in billion sent) are typical.

Boltzman Constant - A constant equal to 1.38 X 10-23.

Buffer - See Fiber buffer.

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C

Cable - An optical fiber, multiple fibers, or fiber bundle which may include a cable jacket and strength members, fabricated to meet optical mechanical, and environmental specifications. See also: Fiber buffer; Fiber bundle.

Cathode - The element of an electron tube or semiconductor device from which the primary electron stream flows. It can be thought of as the negative potential.

Characteristic Angle - The angle at which a given mode travels down an optical fiber.

Chromatic Dispersion - The change in refractive index versus wavelength which causes a difference in the travel speed of light in a fiber.

Cladding - A low-refractive-index, glass or plastic that surrounds the core of a fiber. Optical cladding promotes total internal reflection for the travel of light in a fiber.

Cladding Mode - A mode that is confined by virtue of a lower refractive index medium surrounding the cladding. See also: Mode.

Coaxial - Having the same centerline. A cross-section of coaxial cable would reveal concentric circles, each with the same center point.

Coherent - Signals, at whatever frequency, that are at the same frequency and in phase with each other, as within a laser beam.

Cladding Mode Stripper - A device that encourages conversion of cladding modes to radiation modes. As a result of its use, cladding rays are stripped from the fiber. A cladding mode stripper often uses a material having a refractive index equal to or greater than that of the waveguiding cladding to induce this conversion. See also: Cladding; Cladding mode.

Collimation - The process by which a divergent or convergent beam of radiation is converted into a beam with the minimum divergence possible for the system (ideally a parallel bundle of rays). See also: Beam divergence.

Combiner - A passive device in which optical power from several input fibers is collected at a common point. See also: Coupler.

Connector - A junction which allows an optical fiber or cable to be repeatedly connected to or disconnected from a device such as a source or detector.

Concatenation - The process of connecting pieces of fiber to a link, either by splicing or connectors.

Core - The light-conducting portion of a fiber, defined by its high refractive index. The core is normally in the center of a fiber, bounded by a concentric layer of cladding with lower refractive index.

Coupler - A device whose purpose is to distribute optical power among two or more ports, or to concentrate optical power from two or more fibers into a single port. Couplers may be active or passive. See also: Combiner; Splitter; Star Coupler.

Coupling Efficiency - The fraction of available output from a radiant source which is received and transmitted by an optical fiber, The coupling efficiency for a Lambertian radiator is usually equal to the sin2 Q maximum for the optical fiber being used. See also: Lambertian radiator.

Coupling Loss - The power loss suffered when transferring light from one optical device to another. See also: Angular misalignment loss; Extrinsic joint loss; Insertion loss; Intrinsic joint loss; Lateral offset loss.

Critical Angle - The smallest angle at which a meridional ray may be totally reflected within a fiber at the core-cladding interface. When light travels in a homogeneous medium of relatively high refractive index (n1) onto an interface with a homogeneous material of lower index (n2), the critical angle is defined by sin-1 (n1/n2). See also: Acceptance angle; Angle of incidence; ; Reflection; Refractive index (of a medium); Total internal reflection.

Cutback Technique - A technique for measuring fiber attenuation or distortion by performing two transmission measurements. One is at the full length of the fiber and the other is with a portion cut back from the original length.

D

Dark Current - The external current that, under specified biasing conditions, flows in a photodetector when there is no incident radiation. The average or DC value of this current is identified by the symbol, Id.

Data Rate - The maximum number of bits of information which can be transmitted per second, as in a data transmission link. Typically expressed as megabits per second (Mbps).

Decibel (dB) - A logarithmic unit of measure, based upon the ratio of the output power to the input power. Ten times the base-ten logarithm of the ratio of the two power levels.
equation for decibel

Demodulator - A device which removes an information signal from a carrier wave.The information signal is a temporary add-on, such as a long-distance telephone conversation. The signal may be added to laser light in an optic fiber, transmitted to its destination, and then removed and translated back into sound when it reaches its intended listener.

Depletion Region - The portion of a semiconductor device that has no charge carriers.

Detector - A transducer that provides an electrical output signal in response to an incident optical signal. The current or voltage is dependent on the amount of light received and the type of detector. See also: Receiver.

Diffuse Reflection - Reflection from a surface that makes it appear matte or dull. Opposite of the specular reflection occurring from a mirror.

Digital - Refers to the transmission of a signal, using the binary form. The binary system can be thought of as an "off/on" systems, in which an "off" is represented by a zero (0) and an "on" by the numeral 1. An example of a digital appliance is a toaster. It is either on or off. See also: Analog.

Diode - An electronic device that lets current flow in only one direction.

Diode Laser - A solid-state semiconductor device consisting of at least one P-N junction, capable of emitting coherent radiation under specific conditions.

Discrete - A individual component has one operation or function in itself. Examples of such are: resistors, capacitors, LEDs.

Dispersion - Distortion of an electromagnetic signal caused by different propagation characteristics (speed) of different wavelengths and the differing path lengths of modes in a fiber. See also: Material dispersion; Modal dispersion.

Distortion - The changing of a signals wave shape. Examples of distortion include dispersion and clipping in an amplifier circuit.

Distortion-Limited Operation - The condition prevailing when distortion of a received signal, rather than its amplitude (or power), limits performance. The condition reached when a system distorts the shape of the waveform beyond specified limits. In a fiber-optic system, it usually results from material and modal dispersion. See also: Attenuation-limited operation; Bandwidth-limited operation; Material dispersion; Modal dispersion.

Duplex - A fiber-optic cable that contains two optical fibers.

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E

Electromagnetic Waves - One form that radiant energy can assume as it travels through space. Electromagnetic waves include radio waves, gamma rays, X-rays, infrared, ultraviolet and visible light. All electromagnetic waves have an electrical component and a magnetic component. As the waves move forward, they also travel up and down in "mountains and valleys" (crests and troughs) which are perpendicular to the forward direction of travel. See also: Wave-Particle Duality Theory.

Electron - A particle which orbits the nucleus of an atom. Electrons contain a tiny negative electrical charge of 1.6 x 10-19 coulombs.

Electro-optic Effect - Describes the change of a material's refractive index or the change of birefringence under the influence of an electric field, e.g., lithium-niobate.

End Finish - Quality of the surface at an optic fiber's end, commonly described as mirror, mist, hackle, chipped, cracked, or specified by final grit size used in polishing (1 µm, 0.3 µm, etc.)

Endoscope - A fiber-optic bundle that delivers light and views inside the human body.

End Separation Loss - Optical power loss caused by a longitudinal distance between the end of a fiber and a source, detector, or fiber. See also: Extrinsic joint loss.

Equilibrium Length - For a specific excitation condition, the length of multimode optical waveguide necessary to attain stable distribution of power among propagating (light travel) modes.

Equilibrium Mode Distribution (EMD) - The condition in a multimode optical fiber in which the relative power distribution among propagating modes is independent of length. Synonym: Steady-state condition. See also: Equilibrium length; Mode; Mode coupling.

Extraordinary Ray - A ray that has a non-isotropic speed in a doubly refracting crystal. It does not necessarily obey Snell's law upon refraction at the crystal interface. See also: Birefringence.

Extrinsic Joint Loss - Loss caused by imperfect alignment of fibers in a connector or splice. Contributors include angular misalignment, lateral offset, end separation and end finish. Generally synonymous with insertion loss. See also: Angular misalignment loss; End separation loss; Intrinsic joint loss; Lateral offset loss.

F

Fall Time - The time it takes an output to fall from a high level to minimum value, typically measured as the time to fall from 90% to 10% of minimum output.

Faraday Effect - The effect, discovered by James Faraday in 1945, whereby non-optically active materials rotate the polarization plane of polarized light passed through them when placed in a strong magnetic field.

Ferrule - A component of a fiberoptic connection that holds a fiber in place and aids in its alignment.

Fiber (optical) - Any filament, made of dielectric materials, that guides light, whether or not it is used to transmit signals. Synonym: Optical waveguide. See also: Fiber bundle.

Fiber Bandwidth - The frequency at which the magnitude of the fiber 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.

Fiber Buffer - Material used to protect an optical fiber or cable from physical damage, providing mechanical isolation or protection. Fabrication techniques include tight jacket or loose tube, buffering, as well as multiple buffer layers. See also: Fiber Bundle.

Fiber Bundle - An assembly of unbuffered optical fibers. Usually used as a single transmission channel, as opposed to multiple cables, which contain optically and mechanically isolated fibers, each of which provides a separate channel.

Fiberoptic Link - Any optical transmission channel designed to connect two end terminals or to be connected in series with other channels.

Fresnel Reflection - Reflection of a portion of the light incident on a planar interface between two homogeneous media with different refractive indices. A Fresnel reflection occurs at the air-glass interfaces at entrance and exit ends of an optical fiber. Resultant transmission losses (about four percent per interface) can be virtually eliminated by use of anti-reflection coatings or index matching materials.

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G

Graded-Index - An optical fiber core whose refractive index is changed in a systematic way from center to edges to decrease modal dispersion.

H

Hertz - (Hz) - A unit of frequency equivalent to one cycle per second.

I

Inclusion - The presence, within a body of glass, of an alien or extraneous material.

Incoherent - A term denoting the lack of a fixed phase relationship between two or more waves. In fiber optics it applies to LEDs which emit multiple or a band of frequencies.

Index-Matching Material - A material, often a liquid or cement, whose refractive index is nearly equal to the core index, used to reduce Fresnel reflections from an optical fiber's end face. See Also: Fresnel reflection; Refractive index.

Index of Refraction - The ratio of the velocity of light in a vacuum to the velocity of light in a given medium.

Index Profile - In a graded-index optical fiber, the refractive index as a function of radius.

Infrared (IR) - The span of electromagnetic wavelengths above the visible part of the spectrum (about 0.75 µm) and below microwaves (about 30 µm).

Infrared Emitting Diode (IRED) - A semiconductor diode that is very similar to LEDs which emits infrared radiation instead of visible. The manufacturing methods are also similar, but they are composed of different materials.

Injection Laser Diode - A solid state semiconductor device consisting of at least one p-n junction capable of emitting coherent, stimulated radiation under specified conditions.

Insertion Loss - Total optical power loss caused by insertion of an optical component such as a connector, splice, or coupler into a previously continuous path.

Interference - 1) The additive process whereby the amplitudes of two or more waves are systematically attenuated and reinforced. 2) The process whereby a given wave is split into two or more waves by, for instance, reflection and refraction of beam splitters, and then possibly brought back together to form a single wave.

Integrated Detector/Preamplifier - A single chip containing a detector and an amplifier which converts optical signals to usable electrical output.

Intrinsic Joint Loss - Loss caused by fiber-parameter (e.g., core dimensions, profile parameter) mismatches when two nonidentical fibers are joined. See also: Extrinsic joint loss; Lateral offset loss.

J

Jacket - A layer of material surrounding a fiber but not bonded to it.

Joules - A standard unit of measure for energy. Named for British physicist James P. Joule.

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K

Kilo - A prefix in the SI system meaning one thousand ( 1 x103). Abbreviation k.

L

Lambertian Radiator - An optical source which has radiance uniform in all directions, proportional to the cosine of the angle from the perpendicular.

Laser - A device that produces monochromatic, coherent light through stimulated emission. Most lasers used in fiber optic communications are solid-state semiconductor devices. See also: Injection laser diodes; Stimulated emission.

Lasing Threshold - The lowest excitation level at which a laser's output is dominated by stimulated emission rather than spontaneous. See also: Laser; Spontaneous emission; Stimulated emission.

Lateral Offset Loss - An optical power loss caused by transverse or lateral deviation from optimum alignment of source to optical fiber, fiber-to-fiber, or fiber-to-detector.

Launch Angle - The angle between an incoming light ray and the optical axis of an optical fiber or bundle.

Leaky Ray - In an optical waveguide, a ray for which geometric optics would predict total internal reflection at the core boundary, but suffer less by virtue of the curved boundary.

LED - Light Emitting Diode.

Light - 1) In a strict sense the visible spectrum, nominally covering the wavelength of 400 nm to 750 nm. 2) In the laser and optical communication fields, the much broader portion of the electromagnetic spectrum that can be handled by the basic optical techniques used for the visible spectrum extending from the near-ultraviolet region of approximately 0.3 µm, through the visible region, and into the far-infrared region to 30 µm. See also: Infrared (IR).

Light Emitting Diode (LED) - A semiconductor device which emits incoherent light from a p-n junction when biased with an electrical current. Light may exit from the junction stripe-edge or from its surface (depending on the device's structure).

M

Macrobending Loss - Light loss due to rays exiting the waveguide because the incident angle is less than critical angle due to bends greater than fiber diameter. Does not cause radiative losses.

Material Dispersion - Light impulse broadening caused by various wavelengths of light traveling at different velocities through a fiber. Material dispersion increases with increasing spectral width of the source.

Mega - A prefix in the SI system meaning one million (1 x 106). Abbreviation, M.

Meridional Ray - A ray that passes through the optical axis of an optical fiber (in contrast with a skew ray, which does not). See also: Axial ray; Numerical aperture; Skew ray.

Mesail Power - The mathematical average between high and low levels of power of a modulated signal, independent of duty cycle.
Equation for mesail power

Micro - A prefix in the SI system meaning one millionth (1 x 10-6). Abbreviation, .

Microbending Loss - In an optical fiber, light loss caused by sharp curvatures involving local axial displacements of a few micrometers and spatial wavelengths of a few millimeters. Such bonds may result from fiber coating, cabling, packaging, installation, etc. Note: Microbending can cause significant radiation losses and mode coupling. See also: Macrobending.

Micrometer - A unit of length in the SI system equal to 10-6 meters. Abbreviation m.

Milli - A prefix in the SI system meaning one thousandth (1 x10-3). Abbreviation, m.

Modal Dispersion - In a multimode optical fiber, pulse distortion resulting from differential mode travel rates.

Modal Noise - The noise generated at exit aperture of a waveguide when using a coherent light source. The effect is caused by interference between modes in the waveguide. See also: Mode; Interference.

Mode - In any cavity or transmission line, one of the electromagnetic field distributions that satisfies Maxwell's equations and the boundary conditions. The field pattern of a mode depends on the wavelength, refractive index, and cavity or waveguide geometry.

Mode Coupling - In an optical fiber, the exchange of power among modes. The exchange of power may reach statistical equilibrium after passage over a finite distance that is designated the equilibrium length. See also: Equilibrium length; Mode; Mode scrambler.

Mode Filter - A device for inducing mode coupling in an optical fiber to establish equilibrium.

Mode Scrambler - A device for inducing or promoting mode coupling in an optical fiber.

Modulate - To modify a single-frequency "carrier frequency" by adding or superimposing on that frequency a signal containing information. This is the way in which information (music and words) is added to radio waves, and it is the way that long-distance telephone conversations are added to laser light being transmitted through optic fibers. Modulation converts words into impulses of light that can be "translated" back into words when the signal reaches its destination.

Multifiber Cable - An optical cable that contains two or more fibers, each of which provides a separate information channel. See also: Fiber bundle; Optical cable assembly.

Multimode Fiber - A fiber that supports passage of more than one mode. The number of modes in a fiber is defined by boundary conditions and Maxwell's equations.

Multimode Distortion - See: Modal dispersion.

N

n - A symbol used to represent refractive index.

Nanometer - One millionth of a millimeter. A common unit of measure for the wavelengths of high-frequency energy such as light. Abbreviated nm.

Near-Infrared - The shortest wavelengths of infrared region, just a little longer wavelengths than the visible.

Noise Currents - Any noise voltage or current that prevents precise measurements. Dark current and thermal noise (from amplifiers and resistors) contribute to noise in fiber optic systems.

Noise Equivalent Power (NEP) - The rms value of optical power required to produce an rms signal-to-noise ratio of 1; and indication of noise level which defines the minimum detectable signal level.

Normal - Also referred to as line normal. An imaginary line that forms a right angle with a surface or with other lines. The word "normal" is often used rather than "perpendicular" when measuring/describing incident, reflected and refractive angles.

Numerical Aperture (NA) - The numerical aperture of an optical fiber defines the characteristic of the fiber in terms of its acceptance of impinging light. The larger the numerical aperture the larger the "openness" to which a fiber will accept light.

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O

Optical Cable Assembly - Generally, an optical cable that has been terminated with connectors on both ends and is ready for installation.

Optical Filter - An element that selectively transmits certain optical wavelengths and blocks a range of wavelengths.

Optical Spectrum - The electromagnetic spectrum in the wavelength region from 10 nm (ultraviolet) to 1000 µm (far-infrared).

Optical Time Domain Reflectometry (OTDR) - A method for characterizing a fiber wherein an optical pulse is transmitted through the fiber and the resulting backscatter and reflections are measured as a function of time. Useful in estimating attenuation coefficient as a function of distance and identifying defects and other localized losses. See also: Backscattering; Rayleigh scattering; Scattering.

Optical Waveguide - Any structure having the ability to guide the flow of radiant energy along a path parallel to its axis and, at the same time, to contain the energy within or adjacent to its surface.

Optoelectronics - The field of electronics which deals with lasers, photodetectors, light-emitting-diodes and other electronic devices which respond to, or produce optical radiation.

Ordinary Ray - A ray that has isotopic speed in a doubly refracting crystal. It obeys Snell's law upon refraction at the crystal surface. See also: Birefringence.

Output Power - Radiant power, expressed in watts.

P

Peak Wavelength - The wavelength at which the optical power of a source is at maximum.

Photon - A quantum of electromagnetic energy. The energy of a photon is equal to h * v where h is Planck's constant and v is the optical frequency.

Phase - A term used to define the relationship between two identical electromagnetic waves that are shifted a portion of a wavelength apart. Phase, frequency and amplitude all categorize different aspects of electromagnetic waves.

Photoconductivity - The conductivity increase exhibited by some nonmetallic materials, resulting from free carriers generated when photon energy is absorbed in electronic transitions.

Photocurrent - The current that flows through a photosensitive device (such as a photodiode) as the result of exposure to radiant power. See also: Dark current; Photodiode; Radiant Power.

Photodarlington - A light detector in which a phototransistor is combined in a device with a second transistor to amplify its output current.

Photodetector - A detector that responds to incident light upon it surface. See also: Photodarlington, Phototransistor, PIN Diode.

Photodiode - A two-electrode, radiation sensitive junction formed by a p-n junction in a semiconductor. The junction is reverse biased; current flows when photons create holes and electrons, caused by radiant optical power.

Photon - The fixed quantum elemental unit of light energy. Photons, when viewed as particles of light, are one of two ways we can explain the properties and behavior of light. See also: Wave-Particle Duality Theory.

Phototransistor - A transistor that detects light and amplifies the resulting electrical signal. Light falling on the base-emitter junction generates a current, which is amplified internally.

Photovoltaic effect - Production of a voltage difference across a p-n junction resulting from the absorption of photon energy. The voltage difference is caused by internal drift of holes and electrons. See also: Photon.

Pigtail - A short length of optical fiber permanently fixed to a component, used to connect power between a component and a transmission fiber.

Pin Diode - A semiconductor detector with an intrinsic region separating the p- and n-doped regions. This design gives fast, linear response and is widely used in fiber-optic receivers.

PIN Photodiode - A diode with a large intrinsic region sandwiched between p- and n-doped semiconducting regions. Photons absorbed in this region create electron-hole pairs that are separated by an electric field, thus generating electric current in a load circuit.

Planck's Constant - A universal constant (h) which gives the ratio of a quantum of radiant energy (E) to the frequency (v) of its source. It is expressed as E = hv. Named after German Physicist Max K. E. Planck. Its value is 6,625 x 10-34 joule-second.

Plastic-Clad-Silica (PCS) fiber - A fiber with a glass core and plastic cladding.

Polarization - The description of the electric field vector motion in an electromagnetic wave. Different types of polarization describe different types of sources.

Propagating - Transmitting energy or moving the energy along a path.

Pulse dispersion - The widening of an optical pulse as it transverses the length of a fiber. This property limits the useful bandwidth of the fiber and is usually expressed in terms of nanoseconds of widening per kilometer. The principal mechanisms are material dispersion and multimode distortion effect.

Q

Quantum efficiency - The conversion of photons/second to electrons/second for detectors and electrons/second to photons/second for sources.

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R

Radiant power - The time rate of flow of radiant energy, expressed in watts.

Ray - A geometric representation of a light path through an optical medium. A line perpendicular to the wave front, indicating the direction of radiant energy flow. See also: Mode.

Rayleigh scattering - Scattering by refractive index fluctuations (inhomogeneities in material density or composition) that are small with respect to wavelength. The scattered field is inversely proportional to the fourth power of the wavelength. See also: Scattering.

Receiver - A device that detects an optical signal and converts it into an electrical form usable by other devices. See also: Transmitter.

Reference surface - The surface of an optical fiber used to contact transverse alignment elements of a connector or other component.

Reflectance - The ratio of reflected power to incident power. Note: In optics, frequently expressed as optical density or as a percentage; in communication applications, generally expressed in dB.

Reflection - The return of radiation by a surface, without change in wavelength.

Refraction - The bending of a beam of light at an interface between two dissimilar materials or in a medium whose refractive index is a continuous function or position (graded-index medium).

Refractive Index - The ratio of the speed of light in a vacuum to the speed of light in a material. Abbreviated n.

Refractive-Index profile - The description of refractive index as a function of radius in a fiber. See also: Graded-index; Step index.

Repeater - In an optical fiber communication system, an optoelectronic device or module that receives an optical signal, converts it to electrical form, amplifies it (or in the case of a digital signal, reshapes, re-times, or otherwise reconstructs it) and re-transmits it in optical form.

Responsivity - The ratio of an optical detector's electrical output to its optical input, with the precise definition depending on the detector type; generally expressed in amperes per watt or volts per watt of incident optical power.

Rise Time - The time it takes an output to rise from a low level to peak value. Typically measured as the time to rise from 10% to 90% of maximum value.

S

Scattering - The changes in direction of light confined within an optical fiber occurring due to imperfections in the core and cladding. See also: Backscattering; Mode; Rayleigh scattering.

Sensitivity - 1. Minimum optical power at receiver input, which is required for proper system operation. 2. Sometimes incorrectly used as a synonym for responsivity.

Shot Noise - Noise generated by the statistical process of single electrons crossing a p-n junction. The mean square shot noise current is directly related to a diode's average current.
Equation for Shot Noise

Signal-to-Noise Ratio (SNR) - The ratio of signal level to noise level, related to bit error rate performance. See also: Bit error rate.

Single-Mode Fiber - A fiber that allows only one path for light because of the fiber's very small core diameter; less than 10 microns.

Skew Ray - A ray that does not intersect the optical axis of a fiber (in contrast with a meridional ray). See also: Axial ray; Meridional ray.

Source - A device that, when properly driven (with electrical energy), will produce information-carrying optical signals.

Spectral Width - A measure of the wavelength range of a source's output spectrum. One method of specifying spectral line widths is the full width at half maximum (FWHM), specifically the difference between the wavelengths at which the magnitude is one-half of its maximum.

Spectrum - A band of continuous frequencies or wavelengths; also the visible separation of the colors contained in white light.

Splice - A permanent junction between optical fibers. May be thermally fused or mechanically applied.

Splitter - A passive optical device which divides optical power among several output fibers from a common input. See also: Combiner; Coupler; Star coupler.

Star Coupler - A passive device in which power from one or more input optical fibers is distributed among a number of output fibers. See also: Combiner; Coupler; Splitter.

Spontaneous Emission - Radiation emitted when the internal energy of a quantum mechanical system drops from an excited level to a lower level without regard to the simultaneous presence of similar radiation. Examples of spontaneous emission include: 1) radiation from an LED; and 2) radiation from an injection laser below the lasing threshold. See also: Injection laser diode; Light emitting diode; Stimulated emission.

Step index - A fiber core which is of uniform-refractive index, surrounded by a cladding of lower refractive index.

Step-Index Fiber - A fiber in which the refractive index changes abruptly at the boundary between core and cladding.

Steradian (sr) - The unit of solid angular measure, being the subtended surface angle of a sphere divided by the square of the sphere's radius. There are 4 𝞹 steradians in a sphere. The solid angle subtended by a cone of half-angle θ is 2 𝞹 (1-cos θ).

Stimulated Emission - Radiation emitted when the internal energy of a quantum mechanical system drops from an excited level to a lower level when induced by the presence of radiant energy at the same frequency. An example is the radiation from an injection laser diode above laser threshold. See also: Spontaneous emission.

T

Total Internal Reflection - The total refection of light back into a material when it strikes a boundary at an angle exceeding the critical angle. This is what keeps light confined internally to an optical fiber. See Also: Critical Angle; Step index.

Transducer - A device designed to convert one form of energy into another. For example, a speaker converts audio-frequency electrical energy into audible sound; a phonograph cartridge converts mechanical movement of the needle into an electrical signal.

Transmission Loss - Total loss encountered in transmission through a system. See also: Attenuation; Reflection.

Transmitter - A device (or transducer) that converts an electrical signal into an optical signal for transmission on a fiber cable. See also: Receiver.

U

Ultraviolet - An invisible portion of the optical spectrum whose wavelengths begin immediately beyond the violet end of the visible spectrum. Ultraviolet wavelengths range from approximately 20 to 380 nm. Also the most damaging of the sun's rays to the skin and eyes.

V

Velocity of Light - The speed of light in a vacuum, in round numbers, is 300,000 kilometers per second, or 186,000 miles per second. It is less than .01 % slower in air.

W

Wave Particle Duality Theory - A theory in modern physics which maintains that the properties of light can best be explained by sometimes treating light as particles (a stream of photons), and sometimes as electromagnetic waves.

Wavelength - The distance an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Wavelengths of light are measured in nanometers or micrometers.

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