Fiber Optic Glossary: A

Absorption: That portion of optical attenuation in optical fiber resulting from the conversion of optical power to heat. Caused by impurities in the fiber such as hydroxyl ions.
A/B Switch: A device that accepts inputs (optical or electrical) from a primary path and a secondary path to provide automatic or manual switching in the event that the primary path signal is broken or otherwise disrupted. In optical A/B switches, optical signal power thresholds dictate whether the primary path is functioning and signals a switch to the secondary path until optical power is restored to the primary path.
AC: Abbreviation for alternating current. An electric current that reverses its direction at regularly recurring intervals. Generally a sinusoidal waveform.
Acceptance Angle: The half-angle of the cone (a) within which incident light is totally internally reflected inside the fiber's core. It is equal to sin^-1(NA).
Active Device: A device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Examples include power supplies, transistors, LEDs, Lasers, Detectors, amplifiers, and transmitters.
A/D or ADC: Abbreviation for analog-to-digital converter. A device used to convert analog signals to digital signals.
Add/Drop Multiplexing: a multiplexing function offered in connection with SONET that allows lower speed signals to be added or dropped from a high-speed optical carrier in a wire center. The connection to the add/drop multiplexer is via a channel to a central office port at a specific digital speed (DS3, DS1, etc.)
ADM: Abbreviation for add-drop multiplexer. A device which adds or drops signals from a communications network. Common in CWDM and DWDM systems where a given wavelength is dropped and/or added at a node.
ADSL: Abbreviation for asynchronous digital subscriber line. See DSL.
Aerial Plant: Cable that is suspended in the air on telephone or electric utility poles.
AGC: Abbreviation for automatic gain control. A process or means by which gain is automatically adjusted in a specified manner as a function of input level or another specified parameter.
AM: Abbreviation for amplitude modulation. A transmission technique in which the amplitude of the carrier varies in accordance with the signal.
Amplified Spontaneous Emission (ASE): A background noise mechanism common to all types of erbium-doped fiber amplifiers (EDFAs). It contributes to the noise figure of the EDFA which causes a loss of signal-to-noise ratio (SNR).
Amplifier: A device, inserted within a transmission path, that boosts the strength of an electronic or optical signal. Amplifiers may be placed just after the transmitter (power booster), at a distance between the transmitter and the receiver (in-line amplifier), or just before the receiver (preamplifier).
Analog: A continuously variable signal. Opposite of digital.
Angular Misalignment: Loss at a connector due to fiber end face angles being misaligned.
ANSI: Abbreviation for American National Standards Institute. An organization that administers and coordinates the U.S. voluntary standardization and conformity assessment system.	http://www.ansi.org/
APC: Abbreviation for angled physical contact. A style of fiber optic connector with an angle on the connector tip (typically 8°) for the minimum possible backreflection. Any backreflection that does occur exceeds the acceptance angle and that light escapes into the cladding.
APD: See avalanche photodiode.
APL: Abbreviation for average picture level. A video quality parameter.
AR Coating: Antireflection coating. A thin, single or multi-layer dielectric or metallic film applied to an optical surface to reduce its reflectance and thereby increase its transmittance.
Armor: A protective layer, usually metal, wrapped around a cable.
ASCII: Abbreviation for American Standard Code for Information Interchange. An encoding scheme used to interface between data processing systems, data communication systems, and associated equipment.	(Click to Enlarge)
ASIC: Abbreviation for Application-Specific Integrated Circuit. A custom-designed integrated circuit.
ASTM: Abbreviation for American Society for Testing and Materials. An organization that provides a forum for the development and publication of voluntary consensus standards for materials, products, systems, and services that serve as a basis for manufacturing, procurement, and regulatory activities.	http://www.astm.org/
Asynchronous: Data that is transmitted without an associated clock signal. The time spacing between data characters or blocks may be of arbitrary duration. Opposite of synchronous.
Asynchronous Transfer Mode (ATM): A transmission standard widely used by the telecom industry. A digital transmission switching format with cells containing 5 bytes of header information followed by 48 data bytes. Part of the B-ISDN standard.
ATE: Abbreviation for Automatic Test Equipment. Test equipment computer programmed to perform a number of test measurements on a device without the need for changing the test setup. Especially useful in testing components and PCB assemblies.
ATSC: Abbreviation for Advanced Television Systems Committee. Formed to establish technical standards for advanced television systems, including digital high definition television (HDTV).	http://www.atsc.org
Attenuation: The decrease in optical level along a fiber optic waveguide caused by absorption and scattering. Attenuation is usually expressed in dB/km.
Attenuation-Limited Operation: The condition in a fiber optic link when operation is limited by the power of the received signal (rather than by bandwidth or distortion).
Attenuator: 1) In electrical systems, a usually passive network for reducing the amplitude of a signal without appreciably distorting the waveform. 2) In optical systems, a passive device for reducing the amplitude of a signal without appreciably distorting the waveform.
Avalanche Photodiode (APD): A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region. APD's usually require relatively high bias voltages and are very temperature sensitive. They offer much better sensitivity than PIN photodiodes. They are limited to digital applications.
Average Power: The average level of power in a signal that varies with time.
AWG (Arrayed Waveguide Grating): A device, built with silicon planar lightwave circuits (PLC), that allows multiple wavelengths to be combined and separated in a dense wavelength-division multiplexing (DWDM) system.
Axial Propagation Constant: For an optical fiber, the propagation constant evaluated along the axis of a fiber in the direction of transmission. The real part of the axial propagation constant is the attenuation constant. The imaginary part is the phase constant.
Axis: The center of an optical fiber.