A. Multimode fiber : Multimode fiber can transmit several modes . At present, the commonly used multimode optical fiber mainly has two kinds: 50/125m and 62.5/125m. Multimode fiber -- fiber core is thick (50 or 62.5 micron), because the geometric size of the fiber (mainly core diameter d1) is much larger than the light wavelength (about 1 micron), there will be dozens or even hundreds of propagation modes in the fiber. At the same time, because of its large dispersion between modes, the transmission frequency is limited, and the increase with the distance will be more serious. According to the above characteristics, multimode optical fiber used for the transmission rate is relatively low, relatively short transmission distance of the network, such as local area network (LAN), usually in such networks with nodes, joint, detours, and the number of connectors, couplers, the unit length of optical fiber using the characteristics of more active devices, using a multimode fiber can reduce the network cost.
B.Single-mode fiber., while single-mode fiber can only transmit one mode for a given working wavelength.The core diameter of single-mode fiber is usually 9/125 micron . Single-mode fiber has a small core (about 9 microns) and can transmit only one mode of light. Therefore, the dispersion between modes is very small, suitable for remote communication, but there are still material dispersion and waveguide dispersion, so single-mode fiber has higher requirements for the spectral width and stability of the light source, that is, the spectral width should be narrow, and the stability should be good. Single-mode optical fiber is mostly used in the line with long transmission distance and relatively high transmission rate, such as long-distance trunk transmission, man construction, etc., the current FTTx and HFC networks are mainly single-mode optical fiber.