Single Mode Fiber: the central glass core is very thin (the core diameter is usually 9 or 10 m) and can transmit only one Mode of light. Therefore, the inter-mode 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 spectrum width is narrow, the stability is better. Later, it was found that at 1.31 m wavelength, the material dispersion and waveguide dispersion of the single-mode fiber were positive and negative, and their sizes were exactly the same. This means that at 1.31 m wavelength, the total dispersion of the single-mode fiber is zero. From the loss characteristics of optical fiber, 1.31 meters is just a low loss window of optical fiber. In this way, the 1.31 m wavelength region becomes a very ideal working window for optical fiber communication, and it is also the main working band of the current practical optical fiber communication system. The main parameters of 1.31 m conventional single-mode optical fiber are determined by the ITU - T in the G652 recommendation, so the optical fiber is also known as G652 optical fiber.
In single-mode optical fiber, in-mode dispersion is the main constraint factor of bit rate. Due to its stability, if necessary, the dispersion can be compensated by adding a length of "dispersion compensated single-mode fiber". The zero-dispersion compensating fiber USES a length of fiber with a large negative dispersion coefficient to compensate the fiber with a high dispersion at 1550nm. By making the dispersion of the fiber near 1550nm small or zero, the fiber can achieve 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.
