Dispersion
Is the spreading of a pulse of light as it travels down the length of an optical fiber. Dispersion limits the bandwidth or information carrying capacity of a fiber. The bit rate must be low enough to ensure that pulses do not overlap. A lower bit rate means that the pulses are farther apart and, therefore, that greater dispersion can be tolerated. There are five types of dispersion:
- Modal dispersion
- Material dispersion
- Waveguide dispersion
- Chromatic dispersion
- Polarization mode dispersion
Modal Dispersion
Modal dispersion occurs only in multimode fibers. It is the result of light rays following different paths through the fiber core and consequently arrives at the fiber end at different times. The input light pulse is made up of a group of modes. As the modes propagate along the fiber, light energy distributed among the modes is delayed by different amounts. The pulse spreads because each mode propagates along the fiber at different speeds. Since modes travel in different directions, some modes travel longer distances. Modal dispersion occurs because each mode travels a different distance over the same time span, as shown in figure 2-25. The modes of a light pulse that enter the fiber at one time exit the fiber a different times. This condition causes the light pulse to spread. As the length of the fiber increases, modal dispersion increases.
Material Dispersion
Material dispersion occurs because different wavelengths (colors) also travel at different velocities through a fiber, even in the same mode. Remember, n = c/v where “c” is the speed of light in a vacuum and “v” is the speed of the same wavelength in a material. Here the index of refraction will change according to the wavelength. Material dispersion occurs because the spreading of a light pulse is dependent on the wavelengths’ interaction with the refractive index of the fiber core. Different wavelengths travel at different speeds in the fiber material. Different wavelengths of a light pulse that enter a fiber at one time exit the fiber at different times. Material dispersion is a function of the source spectral width. The spectral width specifies the range of wavelengths that can propagate in the fiber. Material dispersion is less at longer wavelengths.
The amount of dispersion depends on two factors:
- The range of wavelengths injected into the fiber. A source does not emit asingle wavelength; it emits several. The range of wavelengths, expressed in nanometers, is the spectral width of the source. An LED can have a spectral width in the range of 35nm to well over 100nm. A Laser diodes spectral width is .1nm to 3nm.
- Longer “reddish” wavelengths travel faster than shorter “bluish” wavelengths.An 860nm wavelength travels faster than an 840nm wavelength. At 1550nm, the situation is reversed. The shorter wavelength travels faster than longer ones: a 1560nm wavelength travels slower than a 1540nm wavelength. At some point a crossover must occur where the bluish and reddish wavelengths travel at the same speed. This point is called the zero dispersion point occurs at 1300nm.
Material dispersion is of greater concern in single-mode systems. A standard single-mode fiber has the lowest material dispersion at 1300nm and the lowest loss at 1550nm. Or, it has the highest information-carrying capacity at 1300nm and the longer transmission distance at 1550nm. Dispersion is about five times higher at 1550nm than at 1300nm, while attenuation is about 0.2 dB lower.
A dispersion-shifted fiber attempts to give the designer the best of both worlds, low loss and high bandwidth at the same optical wavelength. The zero-dispersion wavelength is shifted from the 1300nm region to 1550nm.
Zero dispersion-shifted (DS) fibers have the zero dispersion point shifted to 1550nm to coincide with the low attenuation operating point. Material dispersion is reduced to zero. DS fibers work well when a single channel data stream is transmitted through the fiber. The newer systems send more than one channel through the fiber. They may send channels or streams of data at 1546, 1548, 1550, and 1552nm. Here an effect called four-wave mixing robs the signals of power and increase noise in the system. Four-wave mixing occurs in fibers that have the zero dispersion point at or near the wavelengths being transmitted. This mixing can seriously limit the use of multiple wavelengths in DWDM applications and this will lower transmission speeds.
Adding a small amount of dispersion can suppress four-wave mixing. Nonzero-dispersion-shifted (NZ-DS) fibers overcome this problem by shifting the zero dispersion point not to 1550nm, but to a point nearby. NZ-DS fibers, because of their ability to handle high data rates and multiple wavelengths, are widely used in communications applications, surpassing DS fibers.
Waveguide Dispersion
Waveguide dispersion occurs because the mode propagation constant (β) is a function of the size of the fiber’s core relative to the wavelength of operation. Waveguide dispersion is most significant in a single-mode fiber. The energy level travels at slightly different velocities in the core and cladding because of the slightly different refractive indices of the materials. Altering the internal structure of the fiber allows waveguide dispersion to be substantially changed, thus changing the specified overall dispersion of the fiber. About 80% of the light is propagated down the core with the remaining 20% traveling down the cladding.
To understand the physical origin of waveguide dispersion, we need to know that the light energy of a mode propagates partly in the core and partly in the cladding and that the effective index of a mode lies between the refractive indices of the cladding and the core. The actual value of the effective index between these two limits depends on the proportion of power that is contained in the cladding and the core. If most of the power is contained in the core, the effective index is closer to the core refractive index. If most of the power propagates in the cladding, the effective index is closer to the cladding refractive index.
The power distribution of a mode between the core and the cladding of a fiber is itself a function of the wavelength. More accurately, the longer the wavelength, the more power in the cladding. Thus, even in the absence of material dispersion, the refractive indices of the core and the cladding are independent of wavelength. If the wavelength changes, the power distribution changes.
Chromatic Dispersion (CD)
Chromatic Dispersion (CD) is the term given to the phenomenon by which different spectral components of a light pulse travel at different speeds. CD arises for two reasons. The first reason is that the refractive index of silica is frequency dependent. Thus different frequency components travel at different speeds in silica. This component of CD is called material Dispersion. The second reason is that although material dispersion is the principle component of chromatic dispersion for most fibers, there is a second component called Waveguide Dispersion.
Polarization Mode Dispersion (PMD)
Polarization mode dispersion (PMD is a minor type of dispersion that only becomes significant in a system that has already minimized other forms of dispersion and that is operating at gigabit data rates. Polarization mode dispersion arises from the fact that even a single mode can have two polarization states. These polarizations travel at slightly different speeds, thus spreading the signal. For a 100-km transmission distance, PMD limits the signal frequency to 40 GHz. The magnitude of PMD in a fiber is expressed as this difference, which is known as the differential group delay (DGD) and called Δτ (“delta Tau”).
Each type of dispersion mechanism leads to pulse spreading. As a pulse spreads, energy is overlapped. This condition is shown in figure 2-26. The spreading of the optical pulse as it travels along the fiber limits the information capacity of the fiber.
In multimode fibers, waveguide dispersion and material dispersion are basically separate properties. Multimode waveguide dispersion is generally small compared to material dispersion. Waveguide dispersion is usually neglected. However, in single mode fibers, material and waveguide dispersion are interrelated. The total dispersion present in single mode fibers may be minimized by trading material and waveguide properties depending on the wavelength of operation.
Modal dispersion is the dominant source of dispersion in multimode fibers. Modal dispersion does not exist in single mode fibers. Single mode fibers propagate only the fundamental mode. Therefore, single mode fibers exhibit the lowest amount of total dispersion. Single mode fibers also exhibit the highest possible bandwidth.