Absorption: That portion of fiber optic attenuation resulting in conversion of optical power to heat.

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

Attenuation: The reduction in optical power as it passes along a fiber, usually expressed in decibels (dB). (See optical loss.)

Attenuator: A device that reduces signal power in a fiber optic link by inducing loss.

Average power: The average over time of a modulated signal.

Back reflection, optical return loss: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4 percent of the incident light. Expressed in dB relative to incident power.

Backscattering: The scattering of light in a fiber back toward the source, used to make optical time domain reflectometer (OTDR) measurements.

Bandwidth: The range of signal frequencies or bit rate within which a fiber optic component, link, or network will operate.

Bending loss, microbending loss: Loss in fiber caused by stress on the fiber bent around a restrictive radius.

Buffer: A protective coating applied directly on the fiber.

Cable: One or more fibers enclosed in protective coverings and strength members.

Cable plant, Fiber optic: The combination of fiber optic cable sections, connectors, and splices forming the optical path between two terminal devices.

Chromatic dispersion: The temporal spreading of a pulse in an optical waveguide caused by the wavelength dependence of the velocities of light.

Cladding: The lower refractive index optical coating over the core of the fiber that “traps” light into the core.

Connector: A device that provides for a demountable connection between two fibers or a fiber and an active device and provides protection for the fiber.

Core: The center of the optical fiber through which light is transmitted.

Coupler: An optical device that splits or combines light from more than one fiber.

Cutback method: A technique for measuring the loss of bare fiber by measuring the optical power transmitted through a long length, then cutting back to the source and measuring the initial coupled power.

Cutoff wavelength: The wavelength beyond which single-mode fiber only supports one mode of propagation.

dB: Optical power referenced to an arbitrary power level.

dBm: Optical power referenced to 1 milliwatt.

Decibel (dB): A unit of measurement for optical power that indicates relative power on a logarithmic scale, sometimes called dBr. dB=10 log (power ratio).

Detector: A photodiode that converts optical signals to electrical signals.

Dispersion: The temporal spreading of a pulse in an optical waveguide. May be caused by modal or chromatic effects.

Edge-emitting diode (E-LED): An LED that emits from the edge of the semiconductor chip, producing higher power and narrower spectral width.

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

Equilibrium modal distribution (EMD): Steady state modal distribution in multimode fiber, achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.

Enterprise System Connection ESCON: IBM standard for connecting peripherals to a computer over fiber optics.

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

Fiber distributed data interface (FDDI): 100 Mbps ring architecture data network.

Ferrule: A precision tube that holds a fiber for alignment for interconnection or termination. A ferrule may be part of a connector or mechanical splice.

Fiber amplifier: A fiber optic device that amplifies signals without converting them to electrical signals, used as a repeater in long-distance networks.

Fiber tracer: An instrument that couples visible light into the fiber to allow visual checking of continuity and tracing for correct connections.

Fiber identifier: A device that clamps onto a fiber and couples light from the fiber by bending, to identify the fiber and detect high-speed traffic of an operating link or a 2 kHz tone injected by a test source.

Fiber optics: Light transmission through flexible transmissive fibers for communications or lighting.

FO: Abbreviation for “fiber optics.”

Fresnel reflection: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4 percent of the incident light. (See back reflection, optical return loss.)

Fusion splicer: An instrument that splices fibers by fusing or welding them, typically by electrical arc.

Graded index (GI): A type of multimode fiber, which used a graded profile of refractive index in the core material to correct for dispersion.

Index of refraction: A measure of the speed of light in a material.

Index matching fluid: A liquid used of refractive index similar to glass used to match the materials at the ends of two fibers to reduce loss and back reflection.

Index profile: The refractive index of a fiber as a function of cross section.

Injection laser diode ( ILD): A semiconductor device that emits high-powered, coherent light when stimulated by an electrical current. Used in transmitters for single-mode fiber links.

Insertion loss: The loss caused by the insertion of a component, such as a splice or connector in an optical fiber.

Jacket: The protective outer coating of the cable.

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

Launch cable: A known good fiber optic jumper cable attached to a source and calibrated for output power used for loss testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.

Light-emitting diode (LED): A semiconductor device that emits light when stimulated by an electrical current. Used in transmitters for multimode fiber links.

Link, fiber optic: A combination of transmitter, receiver, and fiber optic cable connecting them capable of transmitting data. May be analog or digital.

Long wavelength: A commonly used term for light in the 1300 and 1550nm ranges.

Loss, optical: The amount of optical power lost as light is transmitted through fiber, splices, couplers, etc.

Loss budget: The amount of power lost in the link. Often used in terms of the maximum amount of loss that can be tolerated by a given link.

Margin: The additional amount of loss that can be tolerated in a link.

Mechanical splice: A semi-permanent connection between two fibers made with an alignment device and index matching fluid or adhesive.

Micron (mm): A unit of measure, 10-6m, used to measure wavelength of light.

Microscope, fiber optic inspection: A microscope used to inspect the end surface of a connector for flaws or contamination and a cleaved fiber for the quality of the end.

Modal dispersion: The temporal spreading of a pulse in an optical waveguide caused by modal effects.

Mode field diameter: A measure of the core size in single-mode fiber.

Mode filter: A device that removes optical power in higher-order modes in fiber.

Mode scrambler: A device that mixes optical power in fiber to achieve equal power distribution in all modes.

Mode stripper: A device that removes light in the cladding of an optical fiber.

Mode: A single electromagnetic field pattern that travels in fiber.

Multimode fiber: A fiber with core diameter much larger than the wavelength of light transmitted that allows many modes of light to propagate. Commonly used with LED sources for lower-speed, short-distance links.

Nanometer (nm): A unit of length, 9-10m, used to measure the wavelength of light.

Network: A system of cables, hardware, and equipment used for communications.

Numerical aperture (NA): A measure of the light acceptance angle of the fiber.

Optical amplifier: A device that amplifies light without converting it to an electrical signal.

Optical fiber: An optical waveguide, comprised of a light-carrying core and cladding that traps light in the core.

Optical loss test set (OLTS): A measurement instrument for optical loss that includes both a meter and source.

Optical power: The amount of radiant energy per unit time, expressed in linear units of Watts or on a logarithmic scale, in dBm (where 0 dB=1mW) or dBm (where 0 dBm=1 mW).

Optical return loss, back reflection: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Typically 4 percent of the incident light. Expressed in dB relative to incident power.

Optical switch: A device that routes an optical signal from one or more input ports to one or more output ports.

Optical time domain reflectometer (OTDR): An instrument that uses backscattered light to find faults in optical fiber and infer loss.

Overfilled launch: A condition for launching light into the fiber where the incoming light has a spot size and NA larger than accepted by the fiber, filling all modes in the fiber.

Photodiode: A semiconductor that converts light to an electrical signal, which is used in fiber optic receivers.

Pigtail: A short length of fiber attached to a fiber optic component such as a laser or coupler.

Plastic optical fiber (POF): An optical fiber made of plastic.

Plastic-clad silica (PCS) fiber: A fiber made with a glass core and plastic cladding.

Power budget: The difference (in dB) between the transmitted optical power (in dBm) and the receiver sensitivity (in dBm).

Power meter, fiber optic: An instrument that measures optical power emanating from the end of a fiber.

Preform: The large diameter glass rod from which fiber is drawn.

Receive cable: A fiber optic jumper cable known to be good that is attached to a power meter used for loss testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.

Receiver: A device containing a photodiode and signal conditioning circuitry that converts light to an
electrical signal in fiber optic links.

Refractive index: A property of optical materials that relates to the velocity of light in the material.

Repeater, regenerator: A device that receives a fiber optic signal and regenerates it for retransmission, used in very long fiber optic links.

Scattering: The change of direction of light after striking small particles that causes loss in optical fibers.
Short wavelength: A commonly used term for light in the 665, 790, and 850nm ranges.

Single-mode fiber: A fiber with a small core, only a few times the wavelength of light transmitted that only allows one mode of light to propagate. Commonly used with laser sources for high-speed, long-distance links.

Source: A laser diode or LED used to inject an optical signal into fiber.

Splice (fusion or mechanical): A device that provides for a connection between two fibers, typically intended to be permanent.

Steady state modal distribution: Equilibrium modal distribution (EMD) in multimode fiber, achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.

Step index fiber: A multimode fiber where the core is all the same index of refraction.

Surface-emitter light-emitting diode (LED): An LED that emits light perpendicular to the semiconductor chip. Most LEDs used in datacommunications are surface emitters.

Talkset, fiber optic: A communications device that allows conversation over unused fibers.

Termination: Preparation of the end of a fiber to allow connection to another fiber or an active device. It is also called “connectorization.”

Test cable: A short single-fiber jumper cable with connectors on both ends used for testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.

Test kit: A kit of fiber optic instruments, typically including a power meter, source, and test accessories used for measuring loss and power.

Test source: A laser diode or light-emitting diode (LED) used to inject an optical signal into fiber for testing loss of the fiber or other components.

Total internal reflection: Confinement of light into the core of a fiber by the reflection off the core-cladding boundary.
Transmitter: A device that includes an LED or laser source and signal conditioning electronics that is used to inject a signal into fiber.

Visual fault locator: A device that couples visible light into the fiber to allow visual tracing and testing of continuity. Some use lasers and are bright enough to allow finding breaks in fiber through the cable jacket.

Watts: A linear measure of optical power, usually expressed in milliwatts (mW), microwatts (mW), or nanowatts (nW).

Wavelength: A measure of the color of light, usually expressed in nanometers (nm), or microns (mm).

Wavelength division multiplexing (WDM): A technique of sending signals of several different wavelengths of light into the fiber simultaneously.

Working margin: The difference (in dB) between the power and the loss budgets (i.e., the excess power margin).

HAYES is president of Fotec in Medford, Mass. He can be reached at jeh@fotec.com.