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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

Absorption

In an optical fiber, loss of optical power results from the conversion of that power into heat. Intrinsic causes of absorption in a fiber involve the 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 is 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 is caused by the angular deviation from the optimum alignment of the 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 is applied to an optical surface to reduce reflectance and 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 the wavelength.

Attenuation

In an optical fiber, the diminution of average optical power. Note: In an 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.

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B

Backscattering

The portion of scattered light which returns in a direction generally reverses 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; is 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 an 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 a 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 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 the coaxial cable would reveal concentric circles, each with the same center point.

Coherent

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

Cladding Mode Stripper

A device that encourages the 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 that allows an optical fiber or cable to be repeatedly connected to or disconnected from a device such as a source or a 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 is defined by its high refractive index. The core is normally in the center of a fiber, bounded by a concentric layer of cladding with a 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 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.

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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 that 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 that 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" system, 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

An individual component has one operation or function in itself. Examples of such are: resistors, capacitors, and LEDs.

Dispersion

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

Distortion

The changing of a signal's 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 that 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 a 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 is 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 the multimode optical waveguide is 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 is caused by the 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.

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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 was 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 fiber optic connection that holds a fiber in place and aids in its alignment.

Fiber (optical)

Any filament, made of dielectric materials, 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

The material used to protect an optical fiber or cable from physical damage, providing mechanical isolation or protection. Fabrication techniques include a 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 the entrance and exit ends of an optical fiber. Resultant transmission losses (about four percent per interface) can be virtually eliminated by the use of anti-reflection coatings or index-matching materials.

FSMA (F-SMA) Connector

The SMA connector was the first widely used standard connector, developed in the 1970s by Amphenol using the design geometry of the SMA RF connector. It was designed for large-diameter multimode fiber applications, for which it is still widely used in industry and medicine. The regulatory standard separated it from the existing standard by providing an ‘F’ at the beginning to define ‘Fiber’ SMA. At this time most people have dropped the ‘F’ and just refer to it as SMA. The regulatory standard still separates from the original standard by referring to it at FSMA. (IEC 61754-22 International standard; Part 22 Type F-SMA connector). Also known as SMA-905 connector.

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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

HCS (Hard Clad Silica)

A polymer coated optical fiber system invented by OFS. A proprietary polymer is applied to the pristine surface of pure, fused silica as they draw the glass down to its final, step-index, multimode optical fiber form. They then add additional buffer layers to improve chemical and abrasion resistance. This final step increases the finished diameter to a dimension that is easier for field-technicians to handle.

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 means one thousand ( 1 x10³). Abbreviation k.

L

Lambertian Radiator

An optical source that 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 is caused by transverse or lateral deviation from the optimum alignment of the 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 that 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).

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M

Macrobending Loss

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

Material Dispersion

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

Mega

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

Meridional Ray

A ray 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 means one millionth (1 x 10^-6). Abbreviation, µ.

Microbending Loss

In an optical fiber, light loss is 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 is equal to 10^-6 meters. Abbreviation µm.

Milli

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

Modal Dispersion

In a multimode optical fiber, pulse distortion results from differential mode travel rates.

Modal Noise

The noise generated at the 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 satisfy 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 the 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.

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N

n

A symbol is used to represent the 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 the infrared region, are 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 an 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 the 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 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 has 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 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, results 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) is 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 its 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 the internal drift of holes and electrons. See also: Photon.

Pigtail

A short length of optical fiber permanently fixed to a component, is 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 a 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 an electric current in a load circuit.

Planck's Constant

A universal constant (h) 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 the 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 wavefront indicates 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; is 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 a 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 occur due to imperfections in the core and cladding. See also: Backscattering; Mode; Rayleigh scattering.

Sensitivity

1. Minimum optical power at the 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 is 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.

SMA Connector

SMA (Sub-Miniature version A) coaxial RF connectors were developed by Amphenol in the 1960s as a sub-miniature connector interface for RF cables with a screw-type coupling mechanism. Since then, many variants of the SMA series have evolved to meet the ever-changing requirements of the RF and microwave industry. At the same time, the fiber optic industry began to develop and Amphenol was a pioneer in developing the first fiber optic industry standard by modifying their existing SMA connectors to use fiber. The first fiber optic SMA connector is commonly known as a SMA 905 connector.

SMA 905 Connector

Also known as a FSMA Connector, it is one of the first fiber optic interconnect systems that has gained industry wide acceptance. This connector is still widely used today for industrial, military, and medical applications.

SMA 906 Connector

Is a fiber optic connector like the SMA 905 connector except that it has a stepped ferrule design. The stepped ferrule design allows for a full-length alignment sleeve to be used when mating these connectors. The SMA 906 connector typically has a lower insertion loss compared to an SMA 905 connector.

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. Maybe thermally fused or mechanically applied.

Splitter

A passive optical device that 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 is 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 a 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 is 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 is 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 the laser threshold. See also: Spontaneous emission.

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T

Total Internal Reflection

The total reflection 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 the 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 the air.

W

Wave Particle Duality Theory

A theory in modern physics that maintains 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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