Fiber Cladding

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

What is a Fiber Cladding?

An optical fiber usually has some kind of fiber core. The area around the core is then called the fiber 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 because it is large enough to make the fiber sufficiently stable but also small enough to allow substantial bending. 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.

Silica Fibers

In the case of silica fibers, the cladding often consists of pure silica, while the core is doped e.g. with germania 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.

Double-clad Fibers

There are so-called double-clad fibers, which in addition to the outer cladding have a pump cladding (or inner cladding), into which one may inject pump light e.g. for a fiber amplifier.

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 mostly consisting of air, apart from thin glass struds.

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. Often, that fraction is only a few percent, but in some cases — mostly with the fiber being operated close to a mode cut-off — it can be substantially larger.

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 power 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).

Frequently Asked Questions

Questions and Comments from Users