What Is Optical Fiber Dispersion?

Optical fiber dispersion describes the process of how an input signal broadens/spreads out as it propagates/travels down the fiber. Normally, dispersion in fiber optic cable includes modal dispersion, chromatic dispersion and polarization mode dispersion.

Modal Dispersion

Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because of different propagation velocity for all modes. As we know, light rays entering the fiber at different angles of incidence will go through different paths/modes. Some of these light rays will travel straight through the center of the fiber (axial mode) while others will repeatedly bounce off the cladding/core boundary to zigzag their way along the waveguide, as illustrated below with a step-index multimode fiber. Whenever there is a bounce off, modal dispersion (or intermodal dispersion) happens. The longer the path is, the higher the model dispersion will be. For example, the high-order modes (light entering at sharp angles) have more model dispersion than low-order modes (light entering at smaller angles).

Multimode fiber can support up to 17 modes of light at a time, suffering much modal dispersion. Whereas, if the fiber is a single mode fiber, there will be no modal dispersion since there is only one mode and the light enters along the fiber axis (enters in axial mode) without bouncing off the cladding boundary.

However, things are different if one uses a graded-index multimode fiber. Although the light rays travel in different modes as well, the modal dispersion will be greatly decreased because of the various light propagation speeds.

Chromatic Dispersion

Chromatic dispersion is a phenomenon of signal spreading over time resulting from the different speeds of light rays. The chromatic dispersion is the combination of the material and waveguide dispersion effects.

Material dispersion is caused by the wavelength dependence of the refractive index on the fiber core material. Waveguide dispersion occurs due to dependence of the mode propagation constant on the fiber parameters (core radius, and difference between refractive indexes in fiber core and fiber cladding) and signal wavelength. At some particular frequency, these two effects can cancel each other out giving a wavelength with approximately 0 chromatic dispersion.

What’s more, chromatic dispersion isn’t always a bad thing. Light travels at various speeds at different wavelengths or materials. These varying speeds cause pulses to either spread out or compress as they travel down the fiber, making it possible to customize the index of refraction profile to produce fibers for different applications. For example, the G.652 fibers are designed in this way.

Polarization Mode Dispersion

Polarization mode dispersion (PMD) represents the polarization dependence of the propagation characteristics of light waves in optical fibers. In optical fibers, there is usually some slight difference in the propagation characteristics of light waves with different polarization states. When the light is defined as an energy wave or energy region, it possesses 2 mutually perpendicular axes, namely the electromotive force and magnetomotive force. The moment the energy inside these two axes transfers at different speeds in a fiber, PMD occurs.

PMD has small effects for networks whose link speeds are lower than 2.5 Gbps even if the transmission distance is longer than 1000 km. However, as speeds increase, it becomes a more important parameter especially when the speeds are over 10 Gbps. In addition to the major inherent PMD caused by the glass manufacturing process, the PMD can be affected or caused by the fiber cabling, installation and the operating environment of the cable as well.