It‘s glass! Whether it is a prism made of glass or an optical fiber with a fused silica glass core, since glass is a dispersion medium, they all have the ability to bend different wavelengths of light to different angles. To characterize glass or other types of media through which light can travel, a parameter is used, called the index of refraction (or also called the index of refraction). This number refers to the speed at which light travels through the medium. The typical refractive index of single-mode fiber is about 1.461, which means that light travels 1.46 times faster in a vacuum than in fiber. However, this value varies slightly at different wavelengths. Typically in optics, the longer the wavelength, the lower the refractive index.
As a result, where different colors of the spectrum travel at different speeds due to the difference in refractive index, in the image above, red light travels faster than blue due to the lower refractive index. At a distance, red and blue will be further apart, so the signal will be wider。
If not managed, this can cause serious trouble in network communication systems, especially in fast bit rate applications. 40G systems are more prone to dispersion than 10G systems because the signal pulses are denser at the source. A 10G system can run up to 100 kilometers without failure, while a 40G system can run for only a few kilometers without a dispersion compensation solution.
(It should be noted that the specifications for optical fibers will often state the group index value in terms of group velocity rather than phase velocity. As a result, the index value will increase with wavelength. This can be achieved, for example, by Corning® SMF-28® Ultra single mode fiber see where RFI @ 1310nm is 1.4676 and @ 1550nm is 1.4682)
How Dispersion Compensation Modules Reduce Dispersion
The dispersion compensation module (or DCM) is used to compensate the accumulated dispersion in single-mode fiber, and the dispersion coefficient is used to characterize the dispersion value. Regular SMF is about +16~17 ps/(nm*km) at 1550nm. To properly manage this problem, DCMs are built using a special type of dispersion compensating fiber inside the module that has a negative dispersion coefficient ranging from -30 to -300 ps/(nm*km).
For example, the cumulative dispersion for a 10km length of fiber will be +160~170 ps/nm, so to compensate for this amount of dispersion, DCM will be added in the link to reduce the total dispersion close to 0 ps with the specified and calculated fiber length /(nm*km). Shenzhen Xianyitong Technology provides high-quality dispersion compensators for G652 and G655 optical fibers, especially for long-distance signal transmission at rates above 10G.
