Signal Dispersion in Fiber Optic Communication

For modern glass fiber optics, the most serious problem is not signal attenuation, but dispersion, which means that the signal gradually spreads and overlaps after a certain distance of transmission in the fiber, making it difficult for the receiving end to distinguish whether the signal is high or low. There are many causes of dispersion in optical fibers. Taking modal dispersion as an example, the non-uniform axial speed of the transverse mode of the signal leads to dispersion, which also limits the application of multimode fibers. In single-mode fibers, the dispersion between modes can be suppressed very low.

However, there is also a dispersion problem in single-mode optical fibers, commonly known as group velocity dispersion. The reason is that the refractive index of glass is slightly different for incident light waves of different wavelengths, and the light waves emitted by the light source cannot be without spectral distribution. This also results in different refractive behaviors of light waves inside the fiber due to slight differences in wavelength. Another common type of dispersion in single-mode fibers is called polarization mode dispersion, which is caused by the fact that although single-mode fibers can only accommodate one transverse mode light wave at a time, this transverse mode light wave can have polarization in two directions. Any structural defects and deformations in the fiber may cause the two polarized directions of light waves to have different propagation speeds, which is also known as fiber birefringence. This phenomenon can be suppressed through polarization maintaining optical fibers.