Definition of special optical fiber: optical fiber with special design or material.

Special optical fiber is generally considered to have at least one special property, which can be separated from ordinary standard optical fiber. However, there is no general definition of standard optical fiber. It can be simply considered that a standard optical fiber is a step index optical fiber, which is made of standard materials (ordinary glass fiber, quartz) and has ordinary parameter values (core size and numerical aperture). Therefore, special optical fibers include the following categories:

Some optical fibers use non-standard materials. For example, fluoride fiber can transmit light in a longer wavelength range. There is also phosphate glass fiber, which can be doped with high concentration of laser active rare earth ions. Radiation resistant optical fiber is made of materials that are not affected by radiation, and can be used in the space field. Some special materials can also be used as optical fiber coatings and optical fiber protective sleeves.

Active optical fibers doped with laser active ions (usually rare earth ions) are also called special optical fibers. Double clad fiber has additional waveguide structure to conduct pump light, so it is also very special.

Some optical fibers have very special parameters, such as large fiber core diameter or very large numerical aperture. In addition, there are large mode area optical fibers with very large area optical fiber modes. Dispersion shifted fiber has improved dispersion characteristics. In the dispersion decreasing fiber, the dispersion varies with the length.

In some optical fibers, the waveguide function cannot be realized simply by increasing the refractive index of the fiber core (see step index fiber), but by other methods. For example, there is also fiber crystal fiber, which contains air holes, so it is also called porous fiber.

The polarization maintaining fiber with different design can keep the light in linear polarization state in any long transmission distance. Single polarization fiber can only propagate light in a specific polarization direction.

The tapered fiber is made by stretching the fiber, and the fiber diameter becomes smaller, and the diameter changes with the length. It can be used in a mode field adapter.

Because of these different special properties, these optical fibers are not always called special optical fibers, but more specific expressions are preferred, especially in scientific publications.