The core of a conventional
optical fiber is the part of the fiber that guides the light. It is a cylinder of
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
or
plastic
Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...
that runs along the fiber's length. 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. As a result, the fiber transmits all
rays that enter the fiber with a sufficiently small angle to the fiber's axis. The limiting angle is called the
acceptance angle, and the rays that are confined by the core/cladding boundary are called
guided rays.
The core is characterized by its
diameter
In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest Chord (geometry), chord of the circle. Both definitions a ...
or
cross-sectional area. 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. This allows for deviations from circularity due to manufacturing variation.
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 . For single-mode fiber, the mode field diameter is larger than the physical diameter of the core, because the light penetrates slightly into the cladding as an
evanescent wave.
The three most common core sizes are:
* 9
μm diameter (
single-mode)
* 50 μm diameter (
multi-mode)
* 62.5 μm diameter (multi-mode)
See also
*
Buffer (optical fiber)
References
Fiber optics
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