Each fiber is generally made up of a very thin glass core covered by an outside glass layer, which has a lower refraction coefficient than that of the core. Fibers can be divided into categories: multimode fibers and single mode fibers. In a multimode fiber, as its name there are multiple different optical signal modes transmitted, which can bring in multimode dispersion, thereby limiting transmission capacity of a multimode fiber. In contrast, in a single-mode fiber, there is only a single optical signal mode internally reflected in the fiber core. There is no multimode dispersion. Rather, for a single-mode fiber, its dispersion limitation is material dispersion, which is also called chromatic dispersion. Such a dispersion changes with different central wavelength. At a central wavelength of around 1330nm, the smallest chromatic dispersion can be generally found for most common single-mode fiber.

In addition to chromatic dispersion, for a single-mode fiber, we also consider power loss ratio because a single-mode fiber is generally used in long-haul transmission systems. Single-mode fibers demonstrate different loss ratios at different central wavelength. Typically, they demonstrate the smallest power loss ratio at wavelength around 1550nm, which is typically 0.2dB/km.

We can see that the minimum chromatic wavelength and the minimum power loss wavelength are different. For fiber communications, a small chromatic dispersion is desired for a higher transmission capacity, but meanwhile a low power loss ratio is desirable as well for a long-distance transmission. Thus, there is often a tradeoff between power loss and chromatic dispersion. Nonetheless, due to the emergence of wavelength division multiplexing (WDM) techniques, the wavelength window centered at 1550nm is widely used for multiple wavelength transmission in a single fiber in today’s optical transmission system. This of course is also due to the central wavelength of an optical amplifier such as EDFA, which falls into the 1550nm region.

To enable today’s fibers to have the lowest chromatic dispersion at the central wavelength window of 1550nm, more advanced fiber fabrication techniques have been invented to produce some fibers such as Truewave fibers, which is characterized to show the flat chromatic dispersion coefficient crossing both 1330nm and 1550nm windows.

In addition to the fibers for long-distance transmission, other special fibers that are mainly used for the fabrication of optical component by taking advantage of high nonlinearity of these fibers. Specifically, these fibers include holy fibers, crystal fibers, Erbium doped fibers, etc.