There are two factors limiting of light propagation in fiber. The first one is attenuation. The dispersion is the second main impairment. It’s about light pulses spreading as they travel along the fiber, leading to signal distortion. This limits the signal bandwidth and the reach of the fiber.
Two general types of dispersion affect optical systems:
1. Chromatic Dispersion (CD): different wavelengths propagate at different speeds
CD implies that different wavelengths (sometimes also named “colors”), injected into the same fiber, will arrive at their destination at slightly different times, even if they are part of the same single end-to-end signal. In the third window, the CD is about 17 picoseconds per nanometer per kilometer but varies even within the same “small” windows. For standard fibers, there is no CD at 1310 nm. The effect of CD increases by the square of the bit rate.
2. Polarization Mode Dispersion (PMD): different light polarizations travel at different speeds
PMD occurs because of the asymmetry of the fiber, typically caused by intrinsic geometric imperfections. Some other times, PMD is due to the wrapping material that stresses the fiber around the core. PMD originates from the manufacturing process and occasionally, after the process due to laying conditions that may cause mechanical stress and fiber bending or twisting.
Symbol is whatever is used to represent user data in an optical form. It could be as simple as a light pulse, or something more complex. Inter-symbol interference makes it difficult to distinguish consecutive symbols.
The inter-symbol interference is the consequence of light pulse dispersion; that is, each symbol interferes with contiguous symbols, with a consequent lower performance of the whole system.
In single-mode fibers, CD is originated by two main components: material dispersion and waveguide dispersion.
As previously explained, material dispersion occurs when wavelengths travel at different speeds. Thus, even if a laser emits a very narrow spectrum, it won't be one single wavelength but several of wavelengths. Each one of them will travel the medium with a different speed.
The latter occurs because of different refractive indexes of core and cladding:
* At short wavelengths, the light is confined as expected within the core; that is, the effective refractive index experienced by those wavelengths is close to the refractive index of the core material.
* At medium wavelengths, the light spreads slightly into the cladding. Since the refractive index of the cladding is lower than the core’s, the effective refractive index experienced by those wavelengths is lower.
* At long wavelengths, a greater portion of light spreads into the cladding. That is, the effective refractive index experienced by those wavelengths is even lower.
