The usage of optical fiber and WDM technology helps us not only increasing the transmission

speed but also decreasing the overall cost of the communication system. However, some technical

factors still limited the bit rate and the spanning distance of the optical communication. With the

improvement of fiber manufacturing and the invention of EDFA (erbium-doped fiber amplifier),

the war against attenuation has been won, while dispersion and nonlinearity are still taken into

main consideration in today’s high speed optical communication systems. This tutorial is going to

talk about the dispersion and dispersion compensation in fiber optic transmission of WDM system.

Dispersion in Optical Fibers

About dispersion, the most familiar example is probably a rainbow, in which dispersion causes the

spatial separation of a white light into components of different wavelengths (different colors). In

fiber optic transmission, dispersion is the spreading of light pulse as its travels down the length of

an optical fiber. Dispersion is a consequence of the physical properties of the transmission

medium , it can be reduced by fiber design and is closely related to the optical source selection.

Why is Dispersion a Problem in Optical Transmission?

Dispersion compensation is an important issue for fiber-optic links. It limits the bandwidth or

information carrying capacity of a fiber. The following picture will tell us why is dispersion a

problem in optical fiber transmission.

Dispersion and Bit Rate

As bit rates are increasing, dispersion is becoming a critical aspect of most systems. Their

relationship can be concluded as "the higher the dispersion the lower the bit rate". As we can see

from the picture below, the higher dispersion is, the longer the bit interval must be used, while a

longer bit interval means fewer bits can be transmitted per unit of time, ie. a lower bit rate.

Types of Dispersion in Optical Fibers

In general, there are two different types of dispersion in optical fibers – intramodal and intermodal

dispersion. Intramodal dispersion (chromatic dispersion) occurs in all types of fibers while

intermodal dispersion (modal dispersion) occurs only in multimode fibers. Each type of dispersion

mechanism leads to pulse spreading.

Intramodal Dispersion (Chromatic Dispersion)

Intramodal dispersion, or chromatic dispersion (CD) is caused by the fact that singlemode glass

fibers transmit light of different wavelengths at different speeds. There are two factors that cause

chromatic dispersion: material dispersion and waveguide dispersion. In general, both types of

dispersion may be present, although they are not strictly additive. Their combination leads to

signal degradation in optical fibers for telecommunications, because the varying delay in arrival

time between different components of a signal "smears out" the signal in time.

Chromatic Dispersion (CD)

Material Dispersion – Material dispersion comes from a frequency-dependent response of a

material to waves. For example, material dispersion leads to undesired chromatic aberration in a

lens or the separation of colors in a prism. Material dispersion is not helpful in optical

communications that it limits how much data can be sent, as the pulses will overlap and

information will be lost.

Waveguide Dispersion – Waveguide dispersion is caused by the wavelength dependence of the

group velocity due to specific fiber geometry. Waveguide dispersion is important in waveguides

with small effective mode areas. But for fibers with large mode areas, waveguide dispersion is

normally negligible, and material dispersion is dominant.

Intrermodal Dispersion (Modal Dispersion)

In a multimode fiber, different modes travel at different velocities, if a pulse is constituted from

different modes then intermodal dispersion occurs. We call this type of dispersion as intermodal

dispersion, or modal dispersion. Modal dispersion limits the bandwidth of multimode fibers. The

more modes are, the greater the modal dispersion is. Typical bandwidth of a step index fiber may

be as low as 10 MHz over 1 km. Modal dispersion may be considerably reduced, but never

completely eliminated, by the use of a core having a graded refractive index profile. However,

multimode graded-index fibers having bandwidths exceeding 3.5 GHz·km at 850 nm are now

commonly manufactured for use in 10 Gbit/s data links.

Polarization mode dispersion (PMD) – Polarization mode dispersion (PMD) is a special form of

modal dispersion. This phenomenon describes a situation in which the electromagnetic wave

components that make up an optical signal travel at differen speeds within the fiber. This causes a

multipath interference at the receiver. PMD is difficult to predict and may possible vary with

temperature and environment, the twisting of the cable as it was pulled, and even between

production runs from the same manufacturer.

In an ideal optical fiber, the core has a perfectly circular cross-section. In this case, the

fundamental mode has two orthogonal polarizations (orientations of the electric field) that travel at

the same speed. The signal that is transmitted over the fiber is randomly polarized, i.e. a random

superposition of these two polarizations, but that would not matter in an ideal fiber because the

two polarizations would propagate identically (are degenerate). However, there are random

imperfections that break the circular symmetry in a realistic fiber where two different polarizations

of light in a waveguide, which normally travel at the same speed, travel at different speeds due to

random imperfections and asymmetries, causing random spreading of optical pulses.

The symmetry-breaking random imperfections fall into several categories. First, there is geometric

asymmetry, e.g. slightly elliptical cores. Second, the birefringences of the fiber, in which the

refractive index itself depends on the polarization. Birefringence can be influenced by two factors,

material birefringence and waveguide birefringence, similar to CD, but more complex. Waveguide

birefringence is caused by geometrical variations in the fiber such as concentricity or ellipticity.

Material birefringence is mainly caused by stress on the fiber. As the complexity of PMD, it

usually needs to be compensated by the compensation device which uses a polarization controller

to compensate for PMD in fibers.

Polarization Mode Dispersion (PMD)

Dispersion Compensation

Dispersion effects, as one of the main factors (other two factors are fiber attenuation and

nonlinearity) which significantly limite the bit rate and the spanning distance of the optical

communication, have to be taken into consideration. Dispersion compensation is often used in a

more general sense of dispersion management, meaning the control the overall dispersion of some

system. Nowadays, there are several important fiber technologies used to provide dispersion

compensation, such as dispersion shifted fiber, non-zero dispersion shifted fiber, despersion

compensating fiber and dispersion compensation module etc.

Dispersion Shifted Fiber (DSF)

The standard single-mode fiber deployed today is manufactured to optimize transmission at 1310

nm by effectively eliminating dispersion at that wavelength. The dispersion in the 1550 nm

window far exceeds that for 1310 nm on standard fiber and hence is a limiting factor in single

channel or DWDM systems operating in that window. Dispersion shifted fiber (DSF) 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. It is best suited for application involving single channel transmission at 1550 nm taking

advantage of allowing a communication system to possess both low dispersion and low

attenuation. However, when used in wavelength division multiplexing systems, SDFs can suffer

from four-wave mixing which causes intermodulation of the independent signals. As a result non-

zero dispersion shifted fiber is often used.

Non-zero Dispersion Shifted Fiber (NZDSF)

Non-zero dispersion shifted fiber is a type of single-mode fiber which was designed to overcome

the problems of dispersion-shifted fiber. Because of the small non-zero amount of dispersion that

occurs in the 1550 nm window, four-wave mixing and other non-linear effects are minimized so

that it can support WDM system.

Dispersion Compensating Fiber (DCF)

Dispersion compensating fiber (DCF) is a new specialty fiber that has a very high negative value

of dispersion. It can actually reverse the effects of chromatic sidpersion suffered by 1550 nm

signals that traverse standard single-mode fiber. It is used as a sort of inline pre- or post-

equalization in form of a fiber spool of a particular length placed at one end of a link. However,

DCF is not an optimal dispersion compensating solution for DWDM systems because it is effcient

for only single wavelength. In addition, because DCF's high non-linear penalties, its optical power

tolerance is limited.

Dispersion Compensation Module (DCM)

Dispersion compensation module (DCM), as its name suggests, is a device which used for

compensating the chromatic dispersion of a long span of transmission fiber. The Dispersion

compensation modules (DCMs) are building blocks of the CWDM&DWDM optical transport

system and serve at optical communication nodes to provide negative chromatic dispersion known

as pulse spread phenomenon that limits the high-bit rateand maximal transmission distance of data

along optical fibers. Typically, such a module provides a fixed amount of dispersion (e.g. normal

dispersion in the 1.6-μm spectral region), although tunable dispersion modules are also available.

A module can easily be inserted into a fiber-optic link because it has fiber connectors for the input

and output. The insertion losses may be compensated with a fiber amplifier, e.g. an EDFA in a 1.5-

μm telecom system. A dispersion compensation module is often placed between two fiber

amplifiers. Due to its high performance, DCMs are now widely used in WDM systems.

In addition, there are some technologies which are able to provide channelized dispersion

management. For example, to correct dispersion at different wavelength (different channel in

DWDM system) and provide continuous flexibility to accommodate network change and upgrades

(eg. migrating from 8 to 16 channels).

Dispersion compensation module (DCM), as its name suggests, is a device which used for

compensating the chromatic dispersion of a long span of transmission fiber. The Dispersion

compensation modules (DCMs) are building blocks of the CWDM&DWDM optical transport

system and serve at optical communication nodes to provide negative chromatic dispersion known

as pulse spread phenomenon that limits the high-bit rateand maximal transmission distance of data

along optical fibers. Typically, such a module provides a fixed amount of dispersion (e.g. normal

dispersion in the 1.6-μm spectral region), although tunable dispersion modules are also available.

A module can easily be inserted into a fiber-optic link because it has fiber connectors for the input

and output. The insertion losses may be compensated with a fiber amplifier, e.g. an EDFA in a 1.5-

μm telecom system. A dispersion compensation module is often placed between two fiber

amplifiers. Due to its high performance, DCMs are now widely used in WDM systems.

In addition, there are some technologies which are able to provide channelized dispersion

management. For example, to correct dispersion at different wavelength (different channel in

DWDM system) and provide continuous flexibility to accommodate network change and upgrades

(eg. migrating from 8 to 16 channels).