The carrier of optical fiber transmission is light. Although the frequency band is extremely wide, it can not be fully utilized. This is due to the dispersion (inter-mode dispersion, material dispersion and waveguide dispersion) of light transmission in optical fiber. They affect fiber bandwidth to varying degrees
Inter-mode dispersion is due to the different total reflection angles of different modes of light on the core-pack interface, and the length of the zigzag path is different. Therefore, after a beam of light pulse bursts into the optical fiber, the time of each mode contained in it to reach the end point through a certain distance transmission will have a sequence, thus causing pulse broadening. It can extend a narrow pulse to about 20 nanoseconds/km, and the corresponding bandwidth of the optical fiber is about 20 MHz · km
Material dispersion is an internal mode dispersion. The light transmitted by the optical fiber, even the laser, also contains light components of different wavelengths with a certain spectral width. For example, the laser spectrum width emitted by GaAlAs semiconductor laser is about 2 nm. The transmission speed of light in the medium is related to the refractive index n, while the refractive index of quartz medium changes with the wavelength. Therefore, when a beam of light pulse rushes into the optical fiber, even if it is the same mode, the transmission group speed will be different due to different light wavelengths, resulting in the pulse broadening after reaching the end point, which is called material dispersion. Near 1.3 μ m, the refractive index changes very little with the wavelength, so the material dispersion is very small (for example, 3 picoseconds/km · nm). Eliminating intermodal dispersion can greatly improve the bandwidth of optical fiber. Pure quartz has zero dispersion at 1.27 micron wavelength
Waveguide dispersion is also a kind of intra-mode dispersion, which is caused by the group velocity difference caused by the change of mode propagation constant with wavelength. The waveguide dispersion is smaller. At the wavelength of 1.3 μ m, the material dispersion decreases significantly, so that the two are roughly the same, and may cancel each other. The types of optical fiber are divided into quartz optical fiber, multi-component glass optical fiber, plastic clad optical fiber and plastic optical fiber according to the materials used. Among them, quartz optical fiber uses high-purity SiO2 glass as optical fiber material, which has the advantages of low attenuation and wide frequency band, and plays a major role in research and application. For example, according to the refractive index of fiber core, there are mainly mutation type fiber and gradient type fiber. According to the mode of transmission, there are multimode fiber and single-mode fiber
