dispersion shifted fiber

Dispersion shifted fiber
When the working wavelength of a single mode fiber is 1.3Pm, the mode field diameter is about 9Pm, and its transmission loss is about 0.3dB/km. At this point, the zero dispersion wavelength happens to be at 1.3pm. In quartz optical fibers, the transmission loss in the 1.55pm segment is the smallest (approximately 0.2dB/km) from the perspective of raw materials. Due to the fact that the already practical erbium-doped fiber amplifier (EDFA) operates in the 1.55pm band, if zero dispersion can also be achieved in this band, it is more conducive to the application of long-distance transmission in the 1.55Pm band. So, by cleverly utilizing the combined cancellation characteristics of quartz material dispersion and core structure dispersion in fiber optic materials, the zero dispersion originally in the 1.3Pm range can be shifted to the 1.55pm range, which also constitutes zero dispersion. Therefore, it is named Dispersion Shifted Fiber (DSF). The method of increasing structural dispersion is mainly to improve the refractive index distribution performance of the fiber core. In long-distance transmission of optical communication, zero fiber dispersion is important, but not unique. Other properties include low loss, easy connection, and minimal changes in characteristics during cable formation or operation (including bending, stretching, and environmental changes). DSF considers these factors comprehensively in design.
Dispersion flattened fiber
Dispersion shifted fiber (DSF) is a single mode fiber designed with zero dispersion in the 1.55pm band. Dispersion flattened fiber (DFF), on the other hand, can achieve very low dispersion in a wider band from 1.3Pm to 1.55pm, almost reaching zero dispersion. Due to the need for DFF to reduce dispersion in the range of 1.3pm to 1.55pm. A complex design of the refractive index distribution of the fiber is required. However, this type of fiber is very suitable for wavelength division multiplexing (WDM) lines. Due to the complexity and high cost of DFF fiber technology. In the future, as production increases, prices will also decrease.
Dispersion compensating fiber
For trunk systems using single-mode optical fibers, most of them are composed of fibers with zero dispersion in the 1.3pm band. However, with the smallest loss of 1.55pm, due to the practicality of EDFA, it would be very beneficial to operate at a wavelength of 1.55pm even on a 1.3pm zero dispersion fiber. Because in a 1.3Pm zero dispersion fiber, the dispersion in the 1.55Pm band is about 16ps/km/nm. If a segment of fiber with the opposite dispersion symbol is inserted in this fiber optic circuit, the dispersion of the entire fiber optic circuit can be zero. The fiber used for this purpose is called a dispersion compensation fiber (DCF). Compared with the standard 1.3pm zero dispersion optical fiber, DCF has a finer core diameter and a larger refractive index difference. DCF is also an important component of WDM optical circuits.
Polarization maintaining fiber
The light wave propagating in the optical fiber has the nature 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, because the cross section structure of the fiber is circular symmetry, the propagation constants of the two polarization modes are equal, and the two beams of polarized light do not interfere with each other. But in fact, the fiber is not completely circular symmetry, for example, there will be a combination factor between the two polarization modes with 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 significant yet, but for some future ultra wideband services with special requirements, such as:
① When heterodyne detection is used in coherent communication and requires more stable polarization of light waves;
② When the input and output characteristics of optical machines are related to polarization;
③ When making polarization maintaining optical couplers and polarizers or depolarizers, etc;
④ Making fiber optic sensors that utilize light interference, etc,
Where polarization waves are required to remain constant, a fiber that has been improved to maintain its polarization state is called a polarization maintaining fiber (PMF), or a fixed polarization fiber.
Birefringent fiber
Birefringent fiber refers to a fiber that can transmit two inherent polarization modes that are orthogonal to each other in a single mode fiber. The phenomenon of refractive index variation with polarization direction is called birefringence. It is also known as PANDA fiber, which is a polarization maintaining and absorption reducing fiber. The glass part with large coefficient of thermal expansion and circular cross section is set at two transverse sides of the fiber core. During the high-temperature fiber drawing process, these parts contract, resulting in stretching in the y-direction of the fiber core and compressive stress in the x-direction. The photoelasticity effect of the fiber is caused, and the refractive index is different in the X direction and the y direction. According to this principle, achieve the effect of maintaining a constant polarization.