Network Sales and Services Handbook (Cisco Press Networking Technology)
The light used in fiber optic transmission is not the same as the light found in a flashlight or light bulb; optical network light sources are more precise. The ITU has specified six transmission bands for fiber optic transmissions. These bands are measured and represented in terms of wavelength (l) sizes measured in nanometers (nm), one billionth of a meter, or microns (µm), one thousandth of a meter.
The six bands, or transmission windows, are these:
O-Band (1260 nm to 1310 nm)
E-Band (1360 nm to 1460 nm)
S-Band (1460 nm to 1530 nm)
C-Band (1530 nm to 1565 nm)
L-Band (1565 nm to 1625 nm)
U-Band (1625 nm to 1675 nm)
NOTE
The human hair is about 100 µm wide. |
The higher the transmission window, the lower the signal degradation (attenuation); the tradeoff is more expensive electronics.
The reflective path taken by light in a fiber is considered the mode. Each mode has its own pattern of electromagnetic fields as it propagates through the fiber. From a cross section of the fiber, these modes can be viewed as multiple headlights beaming at you. In multimode fiber, multiple modes are generated, causing pulse dispersion at the receiving end.
There are two modes used in fiber optic networks:
Multimode Fiber (MMF)
Single Mode Fiber (SMF)
MMF
MMF optic cable is made up of multiple strands of glass fibers and has a larger core than single-mode fiber. MMF optic cables have a combined diameter of 50 to 100 microns, with each cable carrying independent signals. MMF has greater bandwidth capabilities than single-mode fiber due to its larger core size, which also leads to ease of coupling and interconnecting. MMF is used for specific applications where the distance limitation of two kilometers is not an issue.
To increase the range, manufacturers developed Graded Index (GI) fiber, an improved multimode fiber that operates in the second transmission window (band) at around 1,300nm. Graded Index fiber nearly eliminates modal dispersion by gradually decreasing the Refractive Index out toward the cladding where the modes are longest. Waves on the longer modes travel faster than on the shorter modes; therefore, the entire pulse arrives at the receiver at about the same time.
On distances over 2 km, Graded Index fibers need high-powered lasers introducing the issue of modal noise. With modal noise, the fiber and connectors interact so that there are power fluctuations at the receivers. This increases the signal-to-noise ratio in a link limiting the length of the fiber.
SMF
SMF uses a single glass strand with a smaller core than that of multimode fiber. Because single-mode fiber uses a smaller core of 8 µm to 10 µm, single-mode fiber enables only one mode of light to travel over the fiber. With a single mode, many multimode problems, such as modal noise and modal dispersion, are no longer an issue. SMF can reach as far as 100 to 200 kilometers before a fiber optic repeater, or amplifier, is required.
SMF is more expensive than MMF. The minute size of these cores demands that components have much tighter tolerance, which increases costs. These costs, however, are easily outweighed by the increased bandwidth and distances of 80 km and longer.