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.
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.
- Many fiber tools such as fiber strippers and fusion splicers are optimized e.g. for the 125-μm standard cladding diameter and may then not work well for non-standard fiber diameters.
- Fusion splicing and mechanical splicing are usually difficult when the two fibers have different cladding diameters. There are special solutions, however, for such cases.
During fiber pulling from a preform, the cladding diameter is held constant by a feedback system, which continuously measures the diameter of the drawn fiber and adjusts the pulling speed appropriately.
Numerical Aperture of the Cladding
The numerical aperture of a fiber is usually considered as a value applying to the core/cladding interface. However, one may specify the numerical aperture of the cladding, considering the interface between the cladding and a coating outside the cladding, for example.
In case of silica fibers, the cladding often consists of pure silica, while the core is doped e.g. with germania in order to achieve a somewhat increased refractive index. In other cases, e.g. often in large-core multimode fibers, one may have an undoped core and an index-depressed cladding, where the refractive index is somewhat reduced e.g. with fluorine or boron doping.
Air Cladding of Photonic Crystal Fibers
Some photonic crystal fibers have an air cladding. This is a fiber cladding which is surrounded by a barrier which consists mostly of air.
Light in Core and Cladding
A fiber core can guide light such that it mostly propagates in the core, but some smaller or larger fraction of the optical power may propagate in the region just around the core, i.e., extending somewhat into the cladding. Besides, there are also cladding modes which can cover much of the cladding area.
In many cases, one tries to launch light only into the guided modes of the fiber and not into any cladding modes. However, substantial powers may get into cladding modes due to imperfect launching, or later on due to excessive bending of the fiber (→ bend losses) or because of inhomogeneities of the fiber core (particularly for low-NA fibers).