Optical Fiber Reduces Dispersion

What is Chromatic Dispersion in Optical Fiber?

When you shine a white beam of light into a prism, you see that the light consists of rainbow bands, or spectra. This phenomenon is an example of dispersion. Red light with a wavelength of 700nm and violet light with a wavelength of 400nm are at opposite ends of the visible spectrum. But what causes the different wavelengths of light to separate from each other? It's glass!

Whether it's a prism made of glass or an optical fiber with a fused silica glass core, they both have the ability to bend different wavelengths of light into different angles because glass is a dispersive medium.

To characterize glass or other types of media through which light can pass, a parameter called the index of refraction (or also called the index of refraction) is used. This number refers to the speed at which light travels through the medium. The typical refractive index of single-mode fiber is about 1.46 1, which means that light travels 1.46 times faster in a vacuum than in fiber. However, this value varies slightly at different wavelengths. Generally in optics, the longer the wavelength, the lower the refractive index (see Figure 1 below).

fiber dispersion

The definition of the speed of light is: speed = speed of light / refractive index

As a result, the different colors of the spectrum travel at different speeds due to the difference in the index of refraction, in the image above red light travels faster than blue light due to the lower index of refraction. At a distance, red and blue are farther away, so the signal is wider:

fiber optic pulse

Over longer distances, this small "scattering" eventually adds up, making it impossible for the receiver to correctly distinguish between two consecutive signals when the pulses begin to overlap:

fiber optic signal

If left unchecked, this can cause serious problems for 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. 10G systems can run up to 100 km without issue, while 40G systems can run hundreds of km without dispersion compensation solutions.

It is worth noting that fiber specifications often specify group index values in terms of group velocity rather than phase velocity. As a result, the refractive index value increases with wavelength.

How Dispersion Compensation Optical Modules Reduce Dispersion

The dispersion compensation module (or DCM) is used to compensate the accumulated dispersion in the single-mode fiber, and the dispersion coefficient is used to characterize the dispersion value. Conventional SMF is about +16~17 ps/(nm*km) at 1550nm. For proper management, the DCM is built into the module using a special type of dispersion compensating fiber with negative dispersion coefficient values ranging from -30 to -300 ps/(nm*km).