Some Fiber Properties
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Numerical aperture (NA)
of the fiber defines which light will be propagated and which will not. NA defines the light-gathering ability of the fiber. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated.
NA has an important consequence. A large NA makes it easier to inject more light into a fiber, while a small NA tends to give the fiber a higher bandwidth. A large NA allows greater modal dispersion by allowing more modes in which light can travel. A smaller NA reduces dispersion by limiting the number of modes.
Attenuation.
Attenuation is loss of power. During transit, light pulses lose some of their energy. Attenuation for a fiber is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from approximately 0.5 dB/km for singlemode fibers to 1000 dB/km for large-core plastic fibers.
Attenuation varies with the wavelength of light. There are three low-loss "windows" of interest: 850 nm, 1300 nm, and 1550 nm. The 850-nm window is perhaps the most widely used for networking because 850-nm devices are inexpensive. The 1300-nm (actually, 1310 is the preferred wavelength) window offers lower loss, but at a modest increase in cost for LEDs. The 1550-nm window today is mainly of interest to long-distance telecommunications and cable television applications for two reasons. First, it offers the lowest attenuation and hence the greatest transmission distances. Second, it is most compatible with fiber-based amplifiers and dense wavelength-division multiplexers , which are two technologies that significantly extend the capabilities of fiber. In general, the wavelengths work out like this:
Networks: 850 nm, multimode fiber 1310 nm, multimode and single-mode fibers Telecommunications: 1310 nm, multimode and single-mode fibers 1550 nm, single-mode fibers
Single-mode fibers are used almost exclusively in the 1310 and 1550 nm
Bandwidth.
Fiber bandwidth is given in MHz-km. A product of frequency and distance, bandwidth scales with distance: if you half the distance, you double the bandwidth. If you double the distance, you half the bandwidth. The bandwidth for 62.5/125-m fiber is 160 MHz at 850 nm and 500 MHz at 1300 nm. Single-mode fibers have bandwidths of hundreds of gigahertz: the practical information-carrying capacity of a single-mode fiber is determined more by the narrowness of the wavelengths launched into the fiber than by the fiber itself. Single-mode fibers carry 10 Gb/s signals today and will allow 40 Gb/s signals soon.
While the bandwidth of a single-mode fiber relies on the nature of the light it carries, its practical bandwidth can be multiplied by launching signals of different wavelengths into the fiber. Each wavelength will travel through the fiber separately and can be separated at the other end. You could, for example, launch a 10-Gb/s signal at 1545 nm, one at 1550 nm, one at 1555 nm, and one at 1560 nm to achieve an information carrying capacity of 40 Gb/s. This technique, called dense wavelength-division multiplexing, allows great leaps in fiber capacity. DWDM systems are capable of carrying 8, 16, 32 or more separate wavelengths.
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