Step-index Profile
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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 ...
, a step-index profile is 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 ...
profile characterized by a uniform refractive index within the
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 ...
and a sharp decrease in refractive index at the core-
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...
so that the cladding is of a lower refractive index. The step-index profile corresponds to a
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 ...
with the profile parameter approaching infinity. The step-index profile is used in most
single-mode fiber A transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the radiation's propagation direction. Transverse modes occur in radio waves and microwav ...
s and some
multimode 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 ...
s. A step-index fiber is characterized by the core and cladding refractive indices ''n1'' and ''n2'' and the core and cladding radii a and b. Examples of standard core and cladding diameters 2a/2b are 8/125, 50/125, 62.5/125, 85/125, or 100/140 (units of µm). The fractional refractive-index change \triangle \, = \frac \ll \ 1. The value of n1 is typically between 1.44 and 1.46, and \triangle is typically between 0.001 and 0.02. Step-index optical fiber is generally made by doping high-purity fused silica glass (SiO2) with different concentrations of materials like titanium, germanium, or boron.
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 ...
in a step index optical fiber is given by \text = \frac\,\! where :\triangle\,\! is the fractional index of refraction :n_1\,\! is the refractive index of core :\ell\,\! is the length of the optical fiber under observation :c is the speed of light.


See also

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Graded-index fiber A graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber. Because parts of the core closer to the fiber axis have a higher ref ...
*
Critical angle Critical angle may refer to: *Critical angle (optics), the angle of incidence above which total internal reflection occurs *Critical angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference lin ...
*
Numerical aperture In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the proper ...


References

Fiber optics {{optics-stub