Polarization Maintaining Fibers and Dispersion Compensating Fibers

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Polarization Maintaining Fibers and Dispersion Compensating Fibers

Polarization Maintaining Fibers and Dispersion Compensating Fibers

Dispersion Compensation Fiber
For the trunk line system using single-mode fiber, most of them are composed of fibers with zero dispersion in the 1.3pm band. However, the 1.55pm with the smallest loss, due to the practical application of EDFA, it will be very beneficial if the 1.55pm wavelength can be operated on the 1.3pm zero-dispersion optical fiber. Because, in the 1.3Pm zero-dispersion fiber, the dispersion in the 1.55Pm band is about 16ps/km/nm. If an optical fiber with the opposite sign of the dispersion is inserted into the optical fiber line, the dispersion of the entire optical line can be made zero. The fiber used for this purpose is called Dispersion Compensation Fiber (DCF: DisPersion Compe-nsation Fiber). 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 lines.
Polarization Maintaining Fiber
The light wave propagating in the fiber has the properties 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. Usually, because the structure of the optical fiber section is circularly symmetric, the propagation constants of the two polarization modes are equal, and the two polarized light beams do not interfere with each other, but in fact, the optical fiber is not completely circularly symmetric. The binding factors between the polarization modes are irregularly distributed on the optical axis. The dispersion caused by this change in polarized light is called polarization mode dispersion (PMD). For cable TV that mainly distributes images, the impact is not too great, but for some future ultra-broadband services that have special requirements, such as:
①When 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 and other requirements are related to polarization;
③ When making polarization maintaining optical couplers and polarizers or depolarizers, etc.;
④ Fabrication of optical fiber sensors using optical interference, etc.
In the case where the polarized wave is required to remain constant, the fiber that has been modified to keep the polarization state unchanged is called a polarization maintaining fiber (PMF: Polarization Maintaining fiber), or a fixed polarization fiber.