How Can the Dispersion Be Compensated in Optical Communication?

In optical fiber communication, there are three factors that can degrade the optical signal during transmission. They are attenuation, dispersion and nonlinear effects. Among these problems, attenuation has been solved with the improvement of fiber manufacturing and the invention of erbium-doped fiber amplifier (EDFA). But dispersion affects the system most and is tougher to overcome compared with the other two factors. How can the dispersion be compensated in optical communication?

What Is Dispersion?

In fiber optical transmission, dispersion is defined as the pulse spreading in optical fiber. In order to explain this phenomenon in a plain way, the most familiar example used is the rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths (different colors). Dispersion increases along the fiber length. It is a consequence of the physical properties of the medium within which the light is propagating.

Why Is Dispersion a Problem in Optical Transmission?

Dispersion is regarded as a problem in fiber optic links because it limits the potential bandwidth and transmission performance of a fiber. The overall effect caused by dispersion on the performance of a fiber optical system is called Intersymbol Interference (ISI). ISI occurs when the pulse spreading causes the output pulses of a system to overlap, rendering them undetectable, which is known as a form of distortion of signal.

Nowadays, in high bit rate long-haul transmission systems, dispersion becomes an even more critical aspect, because of the higher the dispersion, the lower the bit rate. We can see the effects caused by dispersion to bit rate according to the following figure. The higher the dispersion is, the longer bit interval must be used, which means fewer bits can be transmitted per unit of time, i.e. lower bit rate.

Two Types of Dispersion in Optical Fibers

Dispersion is generally divided into two categories: chromatic dispersion and modal dispersion, the former of which occurs in all types of fibers while the latter of which occurs only in multimode fibers.

Chromatic Dispersion

Chromatic dispersion (CD) is due to the fact that different wavelengths are propagated at different speeds in the fiber because the index of refraction of glass fiber is a wavelength-dependent quantity. CD can be divided into two parts: material dispersion and waveguide dispersion.


Material dispersion is due to the frequency dependency of a material to waves, i.e. The refractive index of the core varies as a function of the wavelength.

Waveguide dispersion is due to the physical structure of the waveguide. Waveguide dispersion has little effect in simple step-index fibers (large mode areas), but it will be significant in fibers with complex index profiles (small mode areas).

Modal Dispersion

Modal dispersion occurs in multimode fiber when the pulse spreading consists of different modes, caused by the time delay between lower-order modes and higher-order modes. Modal dispersion limits the bandwidth of multimode fibers.

Polarization mode dispersion (PMD) in one special form of modal dispersion. It occurs due to random imperfection and asymmetries that cause two different polarization modes, which normally travel at the same speed, to travel at different speeds. It will lead to rotation of polarization orientation along the fiber.


Different Dispersion Compensation Techniques

Dispersion compensation or dispersion management is the process of designing the fibers and compensating elements in the transmission path to control the total dispersion of a system to a small number. The most commonly employed techniques are as follows:

Dispersion-Shifted Fiber (DSF)

Dispersion-shifted fiber (DSF) is used to compensate dispersion at 1550nm wavelength (zero-dispersion wavelength). It is a type of single-mode optical fiber with a core-clad index profile tailored to shift the zero-dispersion wavelength from the natural 1300 nm in silica-glass fibers to the minimum-loss window at 1550 nm. But when it is used in wavelength division multiplexing (WDM) systems, DSFs have other effects such as high four wave mixing (FWM) and cross phase mixing (CPM). Thus non-zero dispersion shifted fiber is used.

Non-zero Dispersion Shifted Fiber (NZDSF)

Non-zero dispersion shifted fiber is designed to overcome the problems of DSF. Because of the small non-zero amount of dispersion that occurs in the 1550nm window, the FWM and CPM can be minimized so that it can be used in WDM systems.

Dispersion Compensation Fiber (DCF)

Dispersion compensation fiber is a special type of fiber that has large negative dispersion value equal to the transmitting fiber. Typically DCF dispersion can be in the range of -80 ps/(nm∙km). It can actually reverse the effects of chromatic dispersion suffered by the 1550nm signals that traverse standard single-mode fiber. It can be used as pre-compensation, post-compensation or in-line compensation fiber. One drawback of DCF is that it has high insertion loss. And because the length of DCF is determined by the length of the transport length, it leads to bulky terminal components together with high insertion loss. In addition, since DCF is only efficient for single wavelength, it is not a preferable choice for dispersion compensation in DWDM systems.

Fiber Bragg Grating (FBG)

Fiber Bragg grating (FBG) is used for chromatic dispersion compensation by recompression of the dispersed optical signals. It presents low insertion loss. Dispersion compensation using FBG is based on the wavelength-specific periodic variation by the way of chirped FBG. Together with a standard optical circulator, a highly effective dispersion-compensating module (DCM) can be achieved. Two types of FBG-based dispersion compensators are available in the market: multichannel (or channelized) and continuous. The latter type provides channel-spacing-specifc compensation. The former type provides continuous compensation through out the C band and L band, which is suitable for high bit rate systems.


In order to keep the bandwidth and transport performance at desired level, it is important to solve the dispersion problem in fiber optical communication systems. Thanks to the development of dispersion compensation technology, we are now able to employ different approaches to avoid the deterioration in performance of communication systems. And with the advance of dispersion management, more suitable dispersion modules will be introduced in the future.

Technologies Used in Multiplexing

Sending email is a commonplace occurrence in our daily life. When you send an email to a friend in another city, it will firstly join up with other messages being transmitted in your city, and then get dropped off at the correct destination in the correct city. How do all of these messages get to join together and be transmitted without getting mixed up? This process is achieved through the use of multiplmexing technology, which is a method that combines multiple analog message signals or digital data streams into one signal over a shared medium. Actually, multiplexing is widely used in many telecommunications applications. This article will introduce multiplexing technology from the aspect of common technologies used in multiplexing.

Optical multiplexing filter is an essential component in multiplexing technology, which is a physical device that combines each wavelength with other wavelengths (as shown in the following figure). Many technologies are applied in multiplexing, including thin-film filter (TFF), fiber bragg grating (FBG), arrayed waveguide grating (AWG) and interleaver, periodic filter, and frequency slicer.



Optical TFF typically consists of multiple alternating layers of high- and low-refractive-index material deposited on a glass or polymer substrate. This substrate is made to let only photons of a specific wavelength pass through, while all others are reflected.


A bragg grating is made of a small section of fiber that has been modified by exposure to ultraviolet radiation to create periodic variations in the refractive index of the fiber. And the process of creating periodic variations will generate wavelength-specific dielectric mirrors. Thus, the FBG can reflect particular wavelengths of light and transmit all others.


AWG devices can multiplex a large number of wavelengths into a single optical fiber. These devices are designed on the fundamental principle of optics that light waves of different wavelengths interfere linearly with each other. That’ to say, if each channel in an optical communication network makes use of light of a slightly different wavelength, then the light from a large number of these channels can be carried by a single optical fiber.

Interleaver, Periodic filter, and Frequency Slicer

Interleaver, periodic filter and frequency slicer are often used together to perform the function of multiplexing. The following figure shows how interleaver, periodic filter and frequency slicer work together to make a multiplexer device. Periodic filter is in stage 1, which is an AWG. Stage 2 represents the frequency slicer which is another AWG. The interleaver is at the output part, which is provided by six bragg gratings. Six wavelengths (λ) are received at stage 1 which breaks the wavelengths down into odd and even wavelengths. Then the odd and even wavelengths go to stage 2 respectively. Finally, they are delivered by the interleaver in the form of six discrete, interference-free optical channels.

interleaver, periodic filter and frequency slicer

All in all, the usual goal of multiplexing is to enable signals to be transmitted more efficiently over a given communication channel rather than save bandwidth. Nowadays, the most popular multiplexing technology is wavelength division multiplex (WDM), which can be divided into coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). It is hoped that multiplexing technology would offer significant gains in bandwidth efficiency.