Core (optical fiber)

[1] The core is surrounded by a medium with a lower index of refraction, typically a cladding of a different glass, or plastic.

Light travelling in the core reflects from the core-cladding boundary due to total internal reflection, as long as the angle between the light and the boundary is greater than the critical angle.

In most cases the core's cross-section should be circular, but the diameter is more rigorously defined as the average of the diameters of the smallest circle that can be circumscribed about the core-cladding boundary, and the largest circle that can be inscribed within the core-cladding boundary.

Another commonly quoted statistic for core size is the mode field diameter.

This is the diameter at which the intensity of light in the fiber falls to some specified fraction of maximum (usually 1/e2 ≈ 13.5%).