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Cladding in
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
s is one or more layers of materials of lower
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, o ...
, in intimate contact with a core material of higher refractive index. The cladding causes light to be confined to the core of the fiber by
total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflecte ...
at the boundary between the two.The Basics of Fiber Optic Cable
/ref> Light propagation within the cladding is typically suppressed for most fibers. However, some fibers can support ''cladding modes'' in which light propagates through the cladding as well as the core. Depending upon the quantity of modes that are supported, they are referred to as multi-mode fibers and single-mode fibers. Improving transmission through fibers by applying a cladding was discovered in 1953 by Dutch scientist Bram van Heel.


History

The fact that transmission through fibers could be improved by applying a cladding was discovered in 1953 by Dutch scientist Bram van Heel, who used it to demonstrate image transmission through a bundle of optical fibers. Early cladding materials included
oils An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturat ...
, waxes, and
polymers A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...
. Lawrence E. Curtiss at the
University of Michigan , mottoeng = "Arts, Knowledge, Truth" , former_names = Catholepistemiad, or University of Michigania (1817–1821) , budget = $10.3 billion (2021) , endowment = $17 billion (2021)As o ...
developed the first glass cladding in 1956, by inserting a glass rod into a tube of glass with a lower refractive index, fusing the two together, and drawing the composite structure into an optical fiber.


Modes

A cladding mode is a mode that is confined to the cladding of an
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
by virtue of the fact that the cladding has a higher
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, o ...
than the surrounding medium, which is either air or the primary polymer overcoat. These modes are generally undesired. Modern fibers have a primary polymer overcoat with a refractive index that is slightly higher than that of the cladding, so that light propagating in the cladding is rapidly attenuated and disappears after only a few centimeters of propagation. An exception to this is double-clad fiber, which is designed to support a mode in its inner cladding, as well as one in its core.


Advantages

In the production of glass fibers, there will inevitably be surface irregularities (ex. pore and cracks) that will scatter light when struck and lessen the total travel distance of the light. The inclusion of a glass cladding greatly reduces the attenuation caused by these surface irregularities. This is due to the light scattering less at the glass/glass interface than it would have at the glass/air interface for a fiber without cladding. The two primary factors that allow for this are the smaller change in index of refraction seen between two surfaces of glass, as well as surface irregularities on the cladding not interfering with the light beams. The inclusion of glass cladding is also an improvement over just applying a polymer coating, as glass will typically be stronger, more homogenous, and cleaner. Additionally, the inclusion of a cladding layer also allows for the usage of smaller glass fiber cores. With most glass fibers have a cladding that raises the total outer diameter to 125 microns.


Effect on numerical aperture

The numerical aperture of a multimode optical fiber is a function of the indices of refraction of the cladding and the core: : \rm = \sqrt The numerical aperture allows for the calculation of the acceptance angle of incidence at the fiber interface. Which will give the maximum angle at which the incidence light can enter the core and maintain total internal reflection: \rm = \sin(\theta_A) By combining both of these equations it can be seen in the diagram above how \theta_A is a function of n_1 and n_2, where n_1 is the index of refraction of the core and n_2 n_2 is the index of refraction of the cladding.


Recent developments

Fiber optic cores and cladding are typically made from highly purified silica glass due to its excellent transmission of light. Certain impurities can be added to impart various properties, such as increasing transmission distance or improving fiber flexibility. There has been significant work done in improving these properties within the last several years. ClearCurve is a fiber optic cable created by Corning in which alterations to the cladding allow for a fiber to be made hundreds of times more flexible than traditional fibers.


References

Fiber optics {{Optics-stub