Fiber Optic :: Glossary :: A

Absorption - Loss of power in a fiber optic cable resulting from conversion of optical power into heat. This is principally caused by impurities, such as transition metals and hydroxyl ions. It is also caused by exposure to nuclear radiation.

Acceptance angle - The half angle of the cone which incident light is totally, internally, reflected by the fiber core. It is the angle over which the core of an optical fiber accepts incoming light, usually measured from the fiber axis. It is equal to the arcsine (NA) where NA is the numerical aperture.

Active area - The area of a detector with greatest response.

AM - Amplitude Modulation.

Amplitude Modulation - A transmission technique in which the amplitude of a carrier is varied in sympathy with the information being communicated.

Analog - A format that uses continuous physical parameters to transmit information. Examples of parameters are voltage amplitude and carrier frequency.

Angle of deviation - In ray optic theory it is the net resultant angular deflection experienced by a light ray after one or more reflections or refraction's. The term is used in reference to prisms with air interfaces. The angle of deviation is the angle between the original incident ray and the emergent ray.

Angle of incidence - In ray optic theory it is the angle between an incident ray and the normal to a reflecting or refracting surface.

Angular misalignment loss - The optical power loss caused by angular deviation from the optimum alignment of source to optical fiber.

Angular tilt - The angle formed by the axes of 2 fibers to be joined. Angular tilt causes an extrinsic loss that depends upon the joining hardware and method.

APD - Avalanche photodiode.

APF - All Plastic Fiber.

Aramid yarn - Strength element used in Siecor cable to provide support and additional protection of the fiber optic cable bundles. Kevlar is a particular brand of Aramid yarn.

ATM - Asynchronous Transfer Mode. This is a new emerging data standard (protocol) that uses many of the same data rates as Fiber Channel and SONET.

Attenuation - In fiber optic cable, attenuation results from absorption, scattering and other radiation losses. It is usually expressed as decibels per kilometer (dB/km) without the negative sign. Calculations and equations involving loss show and use the negative sign.

Attenuation-limited operation - The condition in a fiber optic cable based communications link when operation is limited by the power of the receive signal, rather than by bandwidth or by distortion.

Attenuator - A passive optical component that intentionally reduces the optical power propagating in a fiber optic cable.

Avalanche Photodiode (APD) - A photodiode that exhibits internal amplification of photocurrent. It accomplishes this by avalanche multiplication of carriers in the junction region. As the reverse-bias voltage approaches the breakdown voltage, electron-hole pairs created by absorbed photons acquire sufficient energy to create additional electron-hole pairs when they collide with ions. A multiplication or signal gain is thereby achieved.

Average power - The average level of power in a signal that varies with time.

Axial ray - A light ray that travels along the axis of a fiber optic cable.