Definition: the region in an optical fiber which guides light
The core of a fiber is the region in which the light is guided, i.e., it is primarily responsible for the waveguiding effect. (The articles on fibers and waveguides explain more about the guiding properties.) In case of active fibers, it also provides amplification of light.
The article focuses on cores for glass fibers, although there are also plastic optical fibers.
Refractive Index Control
Figure 1: A single-mode fiber has a core which is very small compared with the cladding, whereas a multimode fiber can have a large core.
The waveguiding properties are determined by the refractive index profile, i.e., the increase in refractive index in the core relative to that of the cladding. For step-index profiles, the numerical aperture and the V number are frequently used parameters. Many fibers, however, are based on non-step-index designs, where those parameters are not well defined and thus of limited utility. For example, there are graded-index fibers with an approximately parabolic index profile, and single-mode fibers with a pronounced fabrication-related index dip at the center.
Usually, the core is a region of slightly increased refractive index, obtained not by using an entirely different glass, but by doping the glass with some index-raising material. In the case of silica fibers, typical index-raising dopants are
germania (GeO2 → germanosilicate)
alumina (Al2O3 → aluminosilicate)
phosphorus pentoxide (P2O5 → phosphosilicate)
boron oxide (B2O3 → borosilicate)
Alternatively or in addition, the index of the fiber cladding (or only of a limited trench around the core) may be lowered e.g. by fluorine doping. Index-lowering agents can also be used in the core if other required dopants (particularly for active fibers, see below) would make the refractive index difference too high. Alternatively, one may then produce a ring with slightly increased refractive index around the core.
Particularly for large mode area single-mode fibers with low numerical aperture, the exact refractive index profile can be important. For example, the frequently encountered effect that the center of the core exhibits a dip in the refractive index can be detrimental.
In conventional all-glass fibers, one usually has only a quite limited refractive index contrast, and the guided modes can be calculated using the scalar approximation, which very much simplifies the mathematical analysis.
In photonic crystal fibers, however, the waveguide function can be realized in different ways:
In some cases, a pattern of air holes around the core can be interpreted as creating a region with a somewhat lower average refractive index, which leads to a similar guiding mechanism as in conventional all-class fibers – although some properties can still be quite remarkable, e.g. endlessly single-mode guidance.
In other cases, waveguiding is based on photonic bandgaps effects, i.e., on an entirely different principle.
Typically, the waveguiding in photonic crystal fibers is not achieved with index-raising or index-lowering dopants, but essentially by a proper placement of tiny air holes.
Hollow-core Fibers
Photonic crystal fibers can even have a hollow core, i.e., an air hole with a diameter of typically a few microns. Waveguiding is then achieved with a kind of photonic bandgap. See the article on hollow-core fibers for details.
Cores for Active Fibers
Additional laser-active dopants are required for active fibers, i.e., for fibers which can be used for fiber amplifiers or lasers. In almost all cases, the fiber core contains rare earth ions such as Er3+ (erbium), Yb3+ (ytterbium) or Nd3+ (neodymium) – and normally some additional dopants, for example for improving the solubility of the rare earth ions, which is quite low for pure fused silica. One often uses aluminosilicate or phosphosilicate glasses.
The maximum phonon energy of the core glass material determines the potential for multi-phonon transitions and thus can have a strong influence on the lifetime of energy levels.
The glass composition also influences the possibility of efficient energy transfer between different ions, e.g. in erbium-ytterbium-doped fibers.
For details, see the article on rare-earth-doped fibers.
Birefringent Fibers
Although the fiber core is rotationally symmetrical for most fibers, there are methods to break this symmetry e.g. by using an elliptical core and/or by introducing asymmetric structures around the core. This can lead to strong birefringence (→ polarization-maintaining fibers) and even to polarization-dependent guidance (→ single-polarization fibers).
Various Relevant Fiber Core Properties
In addition to the waveguiding, the details of the core also influence other properties of a fiber:
The uniformity and material purity of the core affects the fiber losses through scattering and absorption. That holds particularly for cores with high numerical aperture (strong index contrast).
The tendency of a fiber for photodarkening effects as well as the resistance against radiation damage (radiation-induced aging) is also strongly dependent on the chemical composition. Certain dopants (e.g. germania) should be avoided in radiation-resistant fibers.
The photosensitivity of the fiber core determines the potential for fabricating fiber Bragg gratings by irradiation with ultraviolet light.
Non-centered Fiber Cores
In most cases, the fiber core is located at the center of the fiber's cross-section, because this facilitates splicing, launching light, the use of fiber connectors and other fiber joints, etc. However, for double-clad fibers it can be advantageous to use an off-centered core, because this can substantially improve the pump absorption. In rare cases, helical core fibers are used where the core winds around the fiber axis.
There are also multi-core fibers, containing multiple cores, where obviously at most of those can be centered on the fiber cross section.
