What is Fiber Dispersion? How to Dispersion Compensation?

NADDOD Abel Jan 12, 2023

What is Fiber Dispersion?

Fiber dispersion shows a state of propagation of the input signal inside the fiber and refers to the signal distortion caused by different frequency components or different mode components of the optical signal propagating at different speeds. It mainly includes inter-mode dispersion, chromatic dispersion, and polarization mode dispersion.

Inter-mode Dispersion
Inter-mode dispersion is a signal distortion mechanism that occurs in multimode fibers and other waveguides. In multimode fibers, light rays that enter the fiber at different incidence angles are defined as a path or a mode. Since each mode has a different transmission path, its transmission speed (i.e., group speed) varies, so the signal transmission between the modes creates a time difference to reach the fiber terminal. Typically, some light passes directly through the core (axial mode), while other light is reflected back and forth between the cladding/core boundaries and travels forward along the waveguide zigzag, as shown in the figure below for a step refractive index multimode fiber. The fact is that once the light is refracted, intermodal dispersion/modal dispersion follows. In particular, intermodal dispersion is positively correlated with the transmission path, i.e., higher-order modes (where the light travels a longer distance at a greater angle) cause higher intermodal dispersion than lower-order modes (where the light travels a shorter distance at a smaller angle).

Multimode fibers can accommodate up to 17 light propagation modes simultaneously and have much higher intermodal dispersion than single-mode fibers. This is due to the fact that single-mode fibers have a single propagation mode, i.e., light travels along the core (axial mode) without reflecting to the cladding boundary, and therefore no intermodal dispersion occurs.

However, with gradual refractive index multimode fibers, the situation is different. Although the light also propagates in a different mode, the intermodal dispersion can be greatly reduced by choosing a suitable refractive index distribution because the refractive index of the core is not uniform, the light path is no longer straight but curved, and the speed of light propagation changes.

Chromaticity Dispersion
Chromatic dispersion refers to the spreading of light pulses caused by different wavelength components in the fiber at different group speeds, including material dispersion and waveguide dispersion.

Material dispersion is caused by the wavelength dependence of the refractive index on the core material, while waveguide dispersion is caused by the dependence of the mode propagation constants on the fiber parameters (core radius, refractive index difference between core and cladding) and the signal wavelength. At some specific frequencies, the material dispersion and waveguide dispersion can cancel each other to obtain a wavelength with approximately 0 chromatic dispersion.

In fact, chromatic dispersion is not always unfavorable. Light propagates at different speeds in different wavelengths or materials, causing the optical pulse to be broadened or compressed in the fiber, which makes it possible to customize the refractive index profile to produce fibers for different applications. g.652 fiber is one example.

Polarization Mode Dispersion
Polarization mode dispersion (PMD) reflects the polarization dependence of the propagation characteristics of light waves in optical fibers. Ideally, the two polarization modes should have the same propagation characteristics, but in general, there are slight differences between the different polarization modes. This is due to factors such as temperature and pressure changes or perturbations during propagation, which cause the two polarization modes to have different transmission speeds and generate time delays, resulting in polarization mode dispersion.

Polarization mode dispersion has little effect on networks with link speeds below 2.5 Gbps, even if their transmission distance is greater than 1000 km. However, as the transmission speed increases, especially when the rate exceeds 10 Gbps, the impact of polarization mode dispersion increases dramatically and becomes a fiber parameter that cannot be ignored. In addition to polarization mode dispersion, which is mainly generated during the glass manufacturing process, factors such as fiber cabling, installation and usage environment can all have an impact on it.

How to Compensate for Dispersion?

Although fiber dispersion does not attenuate the signal, it shortens the distance the signal travels within the fiber and causes signal distortion. For example, an optical pulse of 1 nanosecond at the transmitter side will spread to 10 nanoseconds at the receiver side, causing the signal to not be received and decoded properly. Therefore, it is critical to reduce fiber dispersion or perform dispersion compensation in long-haul transmission systems such as dense wavelength division multiplexing (DWDM). Three common dispersion compensation strategies and methods are described below.

Dispersion Compensated Fiber
With dispersion compensated fiber (DCF) technology, optical fibers with negative dispersion can be added to conventional fibers. Compared with dispersion-compensated fibers, the dispersion value of conventional fibers is very large and positive, which reduces or even eliminates the light distribution in these fibers. By adding a negative dispersion compensating fiber, the total dispersion of the entire fiber line is approximately zero, thus achieving high speed, high capacity and long distance communication. There are three main compensation mechanisms for dispersion compensated fibers, including front compensation, back compensation and symmetric compensation. Dispersion compensated fibers are widely used to upgrade fiber links that have been installed at 1310 nm to operate at 1550 nm.

Fiber Bragg Gratings
A fiber Bragg grating (FBG) is a reflection device consisting of an optical fiber that modulates its core refractive index over a range of distances. The use of this device can significantly reduce dispersion effects in long distance transmission systems such as 100 km. When the light beam passes through the fiber grating, the wavelengths that meet its modulation conditions are reflected, and the remaining wavelengths continue to be transmitted along the fiber through the fiber grating. The use of fiber Bragg gratings for dispersion compensation is extremely advantageous because fiber gratings can be integrated with other passive fiber devices with low insertion loss and low cost. In addition, fiber gratings can be used not only as filters for dispersion compensation, but also as sensors, wavelength stabilizers for pump lasers, and for narrowband wavelength division multiplexing add/drop filters.

Electronic Dispersion Compensation
Electronic dispersion compensation (EDC) is a method of dispersion compensation in optical communication links using electronic filtering (also called equalization), i.e., filtering in the communication channel to compensate for the signal attenuation caused by the transmission medium. Electronic dispersion compensation is usually implemented by a transverse filter whose output is a weighted sum of a series of delayed inputs, and which automatically adjusts the filter weights according to the characteristics of the received signal, i.e., adaptive. Electronic dispersion compensation can be used in single-mode fiber systems and multimode fiber systems, in addition to being combined with other features on 10Gbit/s receiver ICs. It can significantly reduce the transmitter cost in single-mode fiber systems and also increase the transmission distance of multimode fiber systems with a small loss of receiver cost.


Although fiber dispersion can affect signal propagation and even cause signal distortion in several ways, it is not entirely detrimental to signal transmission over fiber links in general. In fact, when WDM is used, a certain amount of fiber dispersion can be used to mitigate nonlinear effects. And when the fiber dispersion is too large, the above-mentioned dispersion compensation fiber, fiber Bragg grating, electronic dispersion compensation and other methods can be chosen for dispersion compensation.