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Number of decibels = 10 log10
800 1000 1200 1400 1600 1800
Figure 8.14 Attenuation as a function of wavelength.
OPTICAL FIBERS AND COMPONENTS
The lost power of an optical signal can be restored using an optical amplifier (see Section 8.3.3).
Dispersion is due to a number of reasons, such as modal dispersion, chromatic dispersion, and polarization mode dispersion.
Modal dispersion is associated with multi-mode fibers. As discussed in the previous section, when light is launched at the end of the fiber many fiber modes are created and propagated down the core of the fiber. Now, as shown in Figure 8.12(a), due to the different angles at which rays enter the core of a multi-mode fiber, some modes travel a longer distance to get to the end of the fiber than others. In view of this, the modes have different delays, which causes a spreading of the output pulse (see Figure 8.15(b)). Pulse spreading increases with the length of the fiber. In the case of graded-index multi-mode fiber (see Figure 8.15(c)), pulse spreading is minimum. This is because in graded-index fibers, the rays travel closer to the center of the core due to the parabolic refractive index. Consequently, the modes do not have a significant delay difference.
At high speeds, pulse spreading can cause pulses to run into one another, to the point where the data stream cannot be recovered. In the case of a single-mode fiber, pulse spreading is almost non-existent. This is because the core is small and only one ray is transmitted through.
Chromatic dispersion is due to the fact that the refractive index of silica, the material used to make the core of the fiber, is frequency dependent. In view of this, different frequencies travel at different speeds, and as a result they experience different delays, as in the case of the modes in a multi-mode fiber. These delays cause spreading in the duration of the output pulse. Chromatic dispersion is measured in ps /nm — km, where ps refers to the time spread of the pulse, nm is the spectral width of the pulse, and km corresponds to the length of the fiber. Chromatic dispersion is usually overshadowed by model dispersion in multi-mode fibers. This type of dispersion is called material dispersion.
Chromatic dispersion can be corrected using a dispersion compensating fiber. The length of this fiber is proportional to the dispersion of the transmission fiber. Approximately, a spool of 15 km of dispersion compensating fiber is placed for every 80 km of transmission fiber. Dispersion compensating fiber introduces attenuation of about 0.5 dB/km.
Another type of dispersion is the waveguide dispersion, which is important only in single-mode fibers. Single-mode fibers are designed so that material dispersion and waveguide dispersion cancel each other out.
The polarization mode dispersion (PMD) is due to the fact that the core of the fiber is not perfectly round. When light travels down a single-mode fiber it gets polarized and it
Time (a) Input pulse
(b) Output pulse (step-index)
(c) Output pulse (graded-index)
Figure 8.15 Pulse spreading.
travels along two polarization planes which are vertical to each other. In an ideal circularly symmetric fiber the light traveling on each polarized plane has the same speed with the light traveling on the other plane. However, when the core of the fiber is not round, the light traveling along one plane might travel either slower or faster than the light polarized along the other plane. This difference in speed will cause the pulse to break.
8.2.3 Types of Fibers
The multi-mode fiber has been used extensively in LANs and, more recently, in 1-Gigabit and 10-Gigabit Ethernet. A vast majority of the installed multi-mode fiber has a core diameter of 62.5 ^m and operates in the region of 850 nm and 1300 nm. It provides speeds up to 100 Mpbs. A small percentage of multi-mode fiber adheres to an earlier standard which has a core diameter of 50 ^m and operates in both regions of 850 nm and 1300 nm.
Single-mode fiber is used for long-distance telephony, CATV, and packet-switching networks. The following are various different types of single-mode fiber, classified according to their dispersion loss.
1. Standard single-mode fiber (SSMF): Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm.
2. Non-zero dispersion fiber (NZDF): This fiber has zero dispersion near 1450 nm.
3. Negative dispersion fiber (NDF): This type of fiber has a negative dispersion in the region 1300 to 1600 nm.