When classifying optical fibers, strictly speaking, they should be classified from the aspects of the material composition of the optical fiber, the manufacturing method of the optical fiber, the transmission point modulus of the optical fiber, the refractive index distribution and the operating wavelength of the optical fiber cross section. Now the most commonly used classification method in computer networks is to classify according to the difference of transmission point modulus. According to the different transmission point modulus, optical fiber can be divided into single-mode fiber and multi-mode fiber. The so-called "mode" refers to a beam of light that enters an optical fiber at a certain angular velocity. Single-mode fibers use solid-state lasers as light sources, and multi-mode fibers use light-emitting diodes as light sources. Single mode fiber (Single Mode Fiber): The center glass core is very thin (the core diameter is generally 9 or 10 μm) and can only transmit one mode of light. Therefore, the inter-mode dispersion is very small, suitable for long-distance communication (the propagation distance is generally: at the farthest distance of 120 kilometers from 100M, which is mainly due to the power problem of the laser 1000M does not exceed 70 kilometers), but there are also material dispersion and waveguide dispersion, In this way, the single-mode fiber has high requirements on the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability should be good. Later, it was found that at 1.31μm wavelength, the material dispersion of the single-mode optical fiber and the waveguide dispersion are positive and negative, and the magnitude is 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 point of view of the loss characteristics of optical fiber, 1.31μm is just a low loss window of optical fiber. In this way, the 1.31μm wavelength region has become an ideal working window for optical fiber communication, and 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.