Optical Fiber Glossary of Terms | Corning

Technical Information

Glossary of Terms

Glossary of Terms

Corning is committed to providing education on the basic concepts of optical fiber through our Fiber 101 program. For additional information, please explore the links at the bottom of the page.

Basic Optical Fiber Concepts

Basic Optical Fiber Concepts


Attenuation is the reduction in signal power between two points in the system. It is defined as the ratio of the power at those two points (typically for optical fiber it is the input and output) and is expressed as the logarithm of that ratio in decibels (dB). For optical fiber, in trade and commerce, it is common to normalize the attenuation by the fiber length. This attenuation coefficient is expressed in dB/unit length, typically kilometers (km). 

Core and Cladding

The two basic parts of an optical fiber are its core and cladding. The core, or the axial part of the optical fiber, is the inner glass that carries information in the form of light signals. It is completely surrounded by the cladding layer, which ensures the light pulses remain within the core. Learn more about optical fiber's core and cladding.


Generally, light is sent through the fiber in the form of a pulse. As pulses travel down the fiber they spread out in time, which is known as dispersion. Dispersion is undesirable because it can cause bit errors when the signal reaches the receiver. 

Mode-Field Diameter

In a single-mode fiber, not all of the light is confined to the core of the fiber; sometimes the light is distributed through both the core and the cladding. The "mode field" is the distribution of light through the core and cladding of a particular fiber. Mode field diameter (MFD) defines the size of the power distribution. When coupling light into or out of a fiber, MFD is important in understanding light loss. 

Multimode vs. Single-mode

A mode is a defined path along which light travels. A light signal can propagate through the core of an optical fiber on a single path (single-mode fiber) or on many paths (multimode fiber). Single-mode fiber is used in metro, access, and long-haul applications, while multimode is used in enterprise and private networks.

Numerical Aperture

To couple into the fiber's core, incident light must fall within an acceptable angle. The numerical aperture (NA) is the sine of the half-angle over which the fiber can accept light rays (determined by the difference between the core and cladding refractive indices). It measures the range of acceptance of light into a fiber.

Polarization Mode Dispersion

Polarization mode dispersion (PMD) is where the two orthogonal polarization states of the mode separate, resulting in pulse spreading.

Total Internal Reflection

Total internal reflection is what causes light to be guided along the length of an optical fiber. It is the result of the refractive index of the cladding being less than the refractive index of the core. Light guiding to the core-cladding interface at an angle smaller than a critical angle (determined by the difference between the core and cladding refractive indices) will reflect and guide light longitudinally along the fiber. Learn more about total internal reflection.