A graded-index or gradient-index fiber is an
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
whose
core
Core or cores may refer to:
Science and technology
* Core (anatomy), everything except the appendages
* Core (manufacturing), used in casting and molding
* Core (optical fiber), the signal-carrying portion of an optical fiber
* Core, the centra ...
has a
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
that decreases with increasing radial distance from the
optical axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight.
The optical axis is an imaginary line that defines the path along which light propaga ...
of the fiber.
Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow
sinusoidal
A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in m ...
paths down the fiber. The most common refractive index profile for a graded-index fiber is very nearly parabolic. The
parabolic profile results in continual refocusing of the rays in the core, and minimizes
modal dispersion
Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenom ...
.
Multi-mode optical fiber
Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Multi-mode links can be used for data rates up to 100 Gbit/s. Multi-mode fiber has a fairly large ...
can be built with either graded index or
step index. The advantage of the multi-mode graded index compared to the multi-mode step index is the considerable decrease in modal dispersion. Modal dispersion can be further decreased by selecting a smaller core size (less than 5–10 μm) and forming a
single-mode step index fiber.
This type of fiber is normalized by the International Telecommunication Union ITU-T at recommendation G.651.1.
Pulse dispersion
Pulse dispersion in a graded index optical fiber is given by
where
is the difference in refractive indices of core and cladding,
is the refractive index of the cladding,
is the length of the fiber taken for observing the pulse dispersion,
is the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
, and
is the constant of graded index profile.
See also
*
Power-law index profile
For optical fibers, a power-law index profile is an index of refraction profile characterized by
: n(r) =
\begin
n_1 \sqrt & r \le \alpha\\
n_1 \sqrt & r \ge \alpha
\end
where
\Delta = ,
and n(r) is the nominal refractive index a ...
*
Gradient index optics
Gradient-index (GRIN) optics is the branch of optics covering optical effects produced by a gradient of the refractive index of a material. Such gradual variation can be used to produce lenses with flat surfaces, or lenses that do not have the ab ...
References
*
Optical fiber
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