The single-mode and multi-mode in the optical module actually only refer to the type of optical fiber. LC/PC refers to the type of connector, single-mode and multi-mode will have a standard type on the module, single-mode SM, single-mode is used for long distances, and the fiber is yellow. Multimode MM, multimode is used for short distances, and the fiber is orange.
The wavelength window generally used for multi-mode is 850nm, while the single-mode is mainly based on the wavelengths near the 1310nm and 1550nm windows. Due to the serious dispersion between modes, multimode is only used for short-range, that is, SR. The single-mode tube module is mostly used for long-distance transmission such as LR, ER/ZR.
Multimode fiber is mostly used in networks with relatively low transmission rates and relatively short transmission distances, such as local area networks, etc. Such networks usually have many nodes, many joints, and many detours, and the amount of connectors and couplers is large. Unit The length of the fiber uses the characteristics of a large number of light sources, and the use of multimode fiber can effectively reduce the network cost. Single-mode fiber is mostly used in lines with long transmission distance and relatively high transmission rate, such as long-distance trunk transmission and metropolitan area network construction.
There are several ways to classify optical fibers, according to the transmission mode of light in the optical fiber: single-mode optical fiber and multi-mode optical fiber.
The core diameter of the multimode fiber is 50~62.5 μm, and the outer diameter of the cladding is 125 μm, and the core diameter of the single-mode fiber is 8.3 μm, and the outer diameter of the cladding is 125 μm. The working wavelengths of the optical fibers are 0.85 μm for short wavelengths, 1.31 μm and 1.55 μm for long wavelengths. The fiber loss generally decreases with the wavelength. The loss of 0.85μm is 2.5dB/km, the loss of 1.31μm is 0.35dB/km, and the loss of 1.55μm is 0.20dB/km, which is the minimum loss of the fiber, and the wavelength is 1.65 Losses above μm tend to increase. Due to the absorption effect of OHˉ, there are loss peaks in the range of 0.90~1.30μm and 1.34~1.52μm, and these two ranges are not fully utilized. Since the 1980s, single-mode fibers have tended to be used, and the long wavelength of 1.31 μm has been used first.
multimode fiber
Multimode fiber: The central glass core is thicker (50 or 62.5μm), which can transmit light in multiple modes. But its intermodal dispersion is large, which limits the frequency of transmitting digital signals, and it will be more serious with the increase of distance. For example: 600MB/KM fiber has only 300MB bandwidth at 2KM. Therefore, the distance of multimode fiber transmission is relatively short, generally only a few kilometers.
single mode fiber
Single-mode fiber (Single Mode Fiber): The central glass core is very thin (the core diameter is generally 9 or 10 μm), and only one mode of light can be transmitted. Therefore, its intermodal dispersion is very small, which is suitable for long-distance communication, but there are also material dispersion and waveguide dispersion, so the single-mode fiber has higher requirements on the spectral width and stability of the light source, that is, the spectral width should be narrow and stable. Be good. Later, it was found that at the wavelength of 1.31 μm, the material dispersion and the waveguide dispersion of the single-mode fiber are positive and negative, and the magnitudes are exactly the same. This means that at a wavelength of 1.31 μm, the total dispersion of a single-mode fiber is zero. From the loss characteristics of the fiber, 1.31μm is just a low-loss window of the fiber. In this way, the 1.31μm wavelength region has become an ideal working window for optical fiber communication, and it is also the main working band of practical optical fiber communication systems. The main parameters of 1.31μm conventional single-mode fiber are determined by the International Telecommunication Union ITU-T in the G652 recommendation, so this fiber is also called G652 fiber.
In single-mode fiber, intra-modal dispersion is the main limiting factor for bit rate. Since it is relatively stable, if necessary, the dispersion can be compensated by adding a length of "dispersion-compensating single-mode fiber". Zero dispersion compensation fiber is to use a section of fiber with a large negative dispersion coefficient to compensate for the fiber with higher dispersion at 1550nm. The dispersion of the optical fiber near 1550nm is small or zero, so that the optical fiber can have a higher transmission rate at 1550nm.
In single-mode fibers, another dispersion phenomenon is polarization mode dispersion (PMD), which cannot be compensated because PMD is unstable.
multimode fiber
In multimode fibers, modal dispersion and intramodal dispersion are the main factors affecting the bandwidth. The PCVD process can well control the refractive index profile and give an excellent refractive index profile. For graded multimode fiber (GIMM), it can limit modal dispersion and obtain high modal bandwidth.
When the bandwidth of the whole system reaches a certain level, it is also restricted by the intra-modal dispersion. Especially at 850 nm, the intra-modal dispersion of the multimode fiber is very large. The dispersion coefficient of multimode fiber at 850nm given by some international standards is -120ps/(nm·km), while the dispersion value of PCVD multimode fiber is between -95 and -110 ps/(nm·km).
