For the trunk system with single-mode fiber, most of them are composed of fiber with zero dispersion at 1.3pm band. However, due to the practical application of EDFA, it will be very beneficial if the 1.55pm wavelength can be operated on the 1.3pm fiber with zero dispersion. Because, in the 1.3Pm zero dispersion fiber, the 1.55Pm band dispersion is about 16ps/km/nm. If an optical fiber opposite to the dispersion sign is inserted in the optical fiber line, the dispersion of the entire optical line can be zero. DisPersion compensated Fiber (DCF) used for this purpose is called dispersion compensated fiber (DCF). Compared with standard 1.3pm zero dispersion fiber, DCF has a smaller core diameter and a larger refractive index difference. DCF is also an important part of WDM optical circuit.

Polarization preserving fiber

The light wave propagating in fiber has the property of electromagnetic wave, so in addition to the basic single mode of light wave, there are essentially two orthogonal modes of electromagnetic field (TE, TM) distribution. Generally, due to the circularly symmetric structure of the fiber cross section, the propagation constants of the two polarization modes are equal, and the two polarized light beams do not interfere with each other. However, in fact, the fiber is not completely circularly symmetric, for example, it has a curved part, and the binding factors between the two polarization modes will appear, showing an irregular distribution on the optical axis. The dispersion caused by this change in polarized light is called polarization mode dispersion (PMD). For cable TV, which mainly distributes images, the impact is not too big, but for some future UWB services with special requirements, such as:

When heterodyne detection is used in coherent communication, the polarization of light wave is required to be more stable;

(2) when the input and output characteristics of optical machines are related to polarization;

(3) When making polarization keeping optical couplers and polarizers or depolarizers;

(4) the production of optical fiber sensors using optical interference,

When the Polarization wave is required to be constant, the optical fiber that has been improved so that the polarization state is unchanged is called the polarization preserving fiber (PMF: keeping-polarization fiber), or fixed-polarization fiber.