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In order to understand dispersion shifted fiber, we have to know what is chromatic dispersion. Let's look at this graph first. Most glass optical fibers are made of fused silica and this graph shows that the refractive index of silica changes as a function of wavelength, which means that different wavelengths travel at different speeds in silica.
Because of this speed difference, in a light pulse, some light colors travel faster, and some light colors travel slower, this makes the pulse to spread out in time, which is called pulse spreading.
Chromatic dispersion measures this pulse spreading at difference wavelengths, as you can see the second curve on this graph. Chromatic dispersion has a unit of ps/nm-km. So the number +20 at 1.6um means that a light pulse which has a wavelength range of 1600nm to 1601nm will be 20 ps wider in time after traveling 1km of fiber.
However, the total chromatic dispersion is actually the sum of two dispersions, the material dispersion as we saw in the previous graph, and the wavelength dispersion.
As we learned from the last graph, material dispersion is determined by the material, and it can not be modified by fiber designs unless you change the material.
However, the second dispersion, waveguide dispersion is caused by the distribution of light between core and cladding and it can be modified by using different fiber designs, such as changing the refractive index profiles and dimensions etc. This means we can use waveguide dispersion to offset material dispersion.
This graph shows the material dispersion, waveguide dispersion and the total chromatic dispersion of a standard step-index single mode fiber, which is also called ITU-T G.652 fiber. SMF-28e+ fiber from Corning is one such fiber.
After the offset, the total dispersion is 0 at 1310nm wavelength. This means a light pulse at 1310nm will have no pulse spreading problem.
Standard step-index single mode fibers has 0 dispersion at 1310nm, but 1310nm is not the lowest loss point. Instead, the lowest attenuation point is at 1550nm and 1550nm has been widely used for long distance telecom networks.
So in order to match this lowest attenuation point at 1550nm, engineers invented zero dispersion-shifted fibers in mid-1980s. They moved the waveguide dispersion even further down so the total chromatic dispersion is 0 at 1550nm.
This fiber was first simply called dispersion-shifted fiber, but after the invention of non-zero dispersion-shifted fiber, which I will discuss in the next slide, these fibers are now called zero dispersion-shifted fibers. Zero means their dispersion is zero in the middle of the erbium-doped fiber amplifier band.
Although this design worked well for single-channel systems, it proved unsuitable for WDM. When multiple optical channels pass through the same fiber at wavelengths where dispersion is very close to zero, they suffer from a type of crosstalk called four-wave mixing. The degradation is so severe that zero dispersion-shifted fiber cannot be used for dense-WDM systems.
Zero dispersion-shifted fibers were installed in some systems, but never came into wide use and are no longer manufactured.
And that is why engineers invented non-zero dispersion shifted fibers.
The way to avoid four-wave mixing is to move the zero-dispersion wavelength outside of the wavelength band used for erbium-doped fiber amplifiers. The name Non-Zero comes from the fact that their dispersion is shifted to a value that is low -- but not zero -- in the 1550nm band of erbium-fiber amplifiers.
This small dispersion is enough to keep signals at closely spaced wavelengths from staying in phase over long distances and causing serious crosstalk.
This small dispersion can be provided by moving the zero-dispersion wavelength either shorter or longer than the erbium-fiber 1550nm band, as shown in this figure.
For dense-WDM applications using erbium-doped fiber amplifiers, the current favorite is a zero-dispersion point at a wavelength of 1500nm or less.
Dispersion-shifted fibers do this by using different refractive index profile designs. So they can adjust the amount of waveguide dispersion differently. These pictures show refractive index profile example of one zero dispersion-shifted fiber and one non-zero dispersion-shifted fiber.
So there you have it. Please don't forget to visit www.fo4sale.com for more free fiber optic tutorials.