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According to the transmission mode of light in the optical fiber, it can be divided into: single-mode optical fiber and multi-mode optical fiber. So, how do we distinguish between single-mode fiber and multimode fiber?
SM stands for single-mode, and MM stands for multi-mode. The single-mode is generally marked with 12DB1 or B4, and the multi-mode does not have such a mark as B1/B4.
The core diameter of the multimode fiber is 50~62.5 μm, the outer diameter of the cladding is 125 μm, the core diameter of the single mode fiber is 8.3 μm, and the outer diameter of the cladding is 125 μm. The working wavelength of the optical fiber is short wavelength 0.85μm, long wavelength 1.31μm and 1.55μm. Optical fiber loss generally decreases as the wavelength increases. 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. This is the lowest loss of optical fiber. The wavelength is 1.65 Losses above μm tend to increase. Due to the absorption of OHˉ, there are loss peaks in the ranges of 0.90~1.30μm and 1.34~1.52μm, and these two ranges have not been fully utilized. Since the 1980s, single-mode fiber tends to be used more, and the long wavelength 1.31 μm is used first.
Single-mode fiber: The central glass core is very thin (core diameter is generally 9 or 10μm), and only one mode of light can be transmitted, and the single-mode fiber propagates along a straight line without reflection, so its propagation distance Very far. Therefore, its intermodal dispersion is very small, suitable for long-distance communication, but there are still material dispersion and waveguide dispersion, so 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 the stability better. 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 their sizes are exactly equal. This means that at a wavelength of 1.31 μm, the total dispersion of the single-mode fiber is zero. From the point of view of the loss characteristics of the optical fiber, 1.31 μm is just a low loss window of the optical 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.
Multimode Fiber : The central glass core is relatively thick (50 or 62.5μm), which can carry the transmission of multiple optical signals. However, its intermodal dispersion is relatively large, which limits the frequency of digital signal transmission, and it will be more serious with the increase of distance. For example: 600MB/KM optical fiber has only 300MB bandwidth at 2KM. Therefore, the transmission distance of multimode fiber is relatively short, generally only a few kilometers.
