An optical fiber usually has some kind of fiber core. The area around the core is then called the fiber cladding.

Figure 1: Light can be launched into the core of a fiber, which is surrounded by the cladding.

For fibers which are not simple step-index fibers, but have a more complicated refractive index profile, one usually considers the cladding to be only the area where the refractive index starts to stay constant – up to the outer cladding radius, where one may have air or some polymer coating, for example.

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

Usually, the cladding is fabricated together with the fiber core by pulling from a fiber preform.

Cladding Diameter

The diameter of a fiber cladding is often chosen to be 125 μm, which is a standard value. Although it is often irrelevant concerning the guiding properties, it can nevertheless have a substantial practical importance:

When fibers are made thicker, they more easily break when one tries to bend them. However, increased cladding diameters are sometimes necessary, e.g. for high-power devices or for multi-core fibers with a larger number of cores.

Fusion splicing and mechanical splicing are usually difficult when the two fibers have different cladding diameters. There are special solutions, however, for such cases.

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