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Specialty optical fibers are usually understood as optical fibers which have at least one special property, distinguishing them from standard fibers. However, there is no universally accepted definition of the term standard fiber. One may regard a standard fiber as a simple step-index fiber, made with a standard material (in case of glass fibers: silica) and with common values of parameters like the core size and numerical aperture. Specialty fibers can then belong to various groups:
- Some fibers use non-standard materials. For example, there are fluoride fibers which can transmit in longer (infrared) wavelength regions (→ mid-infrared fibers). Also, there are phosphate glass fibers, which can be more highly doped with laser-active rare earth ions. Radiation-resistant fibers are made from materials which are less affected by radiation, and used for example in space applications. Some fibers are made from monocrystalline material; they are called single-crystal fibers. Special materials may also be used for fiber coatings and jackets.
- Active fibers, being doped with laser-active ions (generally with rare earth ions), are often generally regarded as specialty fibers. Double-clad fibers, having an additional larger waveguide structure for pump light, are even more special.
- Some fibers exhibit extreme or specially tailored parameters, such as an unusually large fiber core diameter or a very high numerical aperture. Also, there are large mode area fibers, exhibiting particularly large fiber modes. Dispersion-shifted fibers have tailored chromatic dispersion properties. In dispersion-decreasing fibers, the chromatic dispersion even varies over the length.
- There are fibers where the waveguide function is not obtained simply with a somewhat increased refractive index in the core (→ step-index fibers), but in some other way. For example, there are photonic crystal fibers, containing air holes and therefore also being called holey fibers.
- Polarization-maintaining fibers of different designs can be used to maintain a linear polarization state over arbitrarily long propagation distances. Single-polarization fibers guide only light with a certain polarization direction.
- Tapered fibers have been stretched to obtain a reduced fiber diameter, which usually changes along the length. They can be used, for example, for mode field adapters.
As there can be so different special properties, it is preferable to use a more specific term than specialty fibers, particularly in scientific publications.
See also: fibers, fiber optics, rare-earth-doped fibers, double-clad fibers, photonic crystal fibers, mid-infrared fibers, single-crystal fibers, polarization-maintaining fibers, single-polarization fibers, few-mode fibers, multi-core fibers
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