Cladding Fibers

An optical fiber usually has some sort of fiber core, and the area around the core is called the fiber cladding.

For fibers with a more complex refractive index distribution that is not a simple step index, it is generally thought that the cladding is simply in the area where the refractive index remains constant all the way to the outer radius of the cladding, where there is some polymer coating in the air。

For single-mode fibers, the cladding typically covers a much larger area than the core, but for some multimode fibers, the opposite may be true.

Typically, the cladding is manufactured with the fiber core by pulling it out of the fiber preform.

Cladding diameter

The diameter of the fiber cladding is usually 125 μm, which is a standard value. Although it has nothing to do with bootstrap properties, it still has important practical implications:

• When the fibers are made thicker, they can break easily when you try to bend them. However, there are times when it is necessary to increase the cladding diameter, such as for high-power devices or multi-core fibers with a higher number of cores.
• Many fiber tools, such as fiber strippers and fusion splicers, are optimized for standard cladding diameters of 125 microns and therefore are not suitable for non-standard fiber diameters.
• When two fibers have different cladding diameters, fusion splicing and mechanical splicing are often difficult. However, there are special solutions for this situation.
• The cladding diameter is kept constant during the fiber pull-out from the preform, which continuously measures the diameter of the pull-out fiber and adjusts and pulls out the speed appropriately.

The numerical aperture of the cladding

The numerical aperture of an optical fiber is generally considered to be the value applied to the core or cladding interface. However, taking into account the interface between the cladding and the coating on the outside of the cladding, it is possible to specify the numerical aperture of the cladding.

Silica fibers

In some cases of quartz fibers, the general cladding consists of pure quartz, while the core is doped with germanium oxide to achieve an increased refractive index. In other cases, a large-core multimode fiber may have an undoped core and a cladding with a reduced refractive index, where the refractive index is slightly lower, such as fluorine-doped or boron-doped.

Double-clad fiber

There are so-called double-clad fibers, which, in addition to the outer layer, have a pump cladding (or inner cladding) into which the pump light can be injected and can be used for fiber amplifiers.

Some photonic crystal fibers have an air cladding, which is a fiber cladding that is mainly surrounded by a barrier composed of dominant air.

Light in the core and cladding

The fiber core can direct the light It mainly travels in the core, but a part of the optical power can travel in the area around the core and extends into the cladding. Of course, it's only a few percentage points, but in some cases – as the fiber approaches the mode cutoff frequency – it can be much larger. In addition, there is a cladding mode that covers most of the melt.

In most cases, an attempt is made to emit light into the fiber's guide mode rather than into the cladding pattern. However, due to imperfect emission, due to excessive bending of the fiber (→ bending loss), and due to the non-uniformity of the core (especially for low NA fibers), a large amount of power can go into cladding mode.