Lasers (VCSELs) are two types of light source usually used with multimode fiber. Use one or the other. LEDs are a little cheaper to build and require somewhat less safety concerns than lasers. However, LEDs cannot transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6,560 ft). Web Links Multimode Fiber http://searchnetworking.techtarget.com/ sDefinition/0,,sid7_ gci212613,00.html
Content 3.2 Optical Media 3.2.7 Single-mode fiber Single-mode fiber consists of the same parts as multimode. The outer jacket of single-mode fiber is usually yellow. The major difference between multimode and single-mode fiber is that single-mode allows only one mode of light to propagate through the smaller, fiber-optic core. The single-mode core is eight to ten microns in diameter. Nine-micron cores are the most common. A 9/125 marking on the jacket of the single-mode fiber indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter. An infrared laser is used as the light source in single-mode fiber. The ray of light it generates enters the core at a 90-degree angle. As a result, the data carrying light ray pulses in single-mode fiber are essentially transmitted in a straight line right down the middle of the core. This greatly increases both the speed and the distance that data can be transmitted. Because of its design, single-mode fiber is capable of higher rates of data transmission (bandwidth) and greater cable run distances than multimode fiber. Single-mode fiber can carry LAN data up to 3000 meters. Multimode is only capable of carrying up to 2000 meters. Lasers and single-mode fibers are more expensive than LEDs and multimode fiber. Because of these characteristics, single-mode fiber is often used for inter-building connectivity. Warning: The laser light used with single-mode has a longer wavelength than can be seen. The laser is so strong that it can seriously damage eyes. Never look at the near end of a fiber that is connected to a device at the far end. Never look into the transmit port on a NIC, switch, or router. Remember to keep protective covers over the ends of fiber and inserted into the fiber-optic ports of switches and routers. Be very careful. Figure compares the relative sizes of the core and cladding for both types of fiber optic in different sectional views. The much smaller and more refined fiber core in single-mode fiber is the reason single-mode has a higher bandwidth and cable run distance than multimode fiber. However, it entails more manufacturing costs. Web Links Single Mode Fiber http://searchnetworking.techtarget.com/ sDefinition/0,,sid7_ gci212992,00.htm
Content 3.2 Optical Media 3.2.8 Other optical components Most of the data sent over a LAN is in the form of electrical signals. However, optical fiber links use light to send data. Something is needed to convert the electricity to light and at the other end of the fiber convert the light back to electricity. This means that a transmitter and a receiver are required. The transmitter receives data to be transmitted from switches and routers. This data is in the form of electrical signals. The transmitter converts the electronic signals into their equivalent light pulses. There are two types of light sources used to encode and transmit the data through the cable: Each of these light sources can be lighted and darkened very quickly to send data (1s and 0s) at a high number of bits per second. At the other end of the optical fiber from the transmitter is the receiver. The receiver functions something like the photoelectric cell in a solar powered calculator. When light strikes the receiver, it produces electricity. The first job of the receiver is to detect a light pulse that arrives from the fiber. Then the receiver converts the light pulse back into the original electrical signal that first entered the transmitter at the far end of the fiber. Now the signal is again in the form of voltage changes. The signal is ready to be sent over copper wire into any receiving electronic device such as a computer, switch, or router. The semiconductor devices that are usually used as receivers with fiber-optic links are called p-intrinsic-n diodes (PIN photodiodes). PIN photodiodes are manufactured to be sensitive to 850, 1310, or 1550 nm of light that are generated by the transmitter at the far end of the fiber. When struck by a pulse of light at the proper wavelength, the PIN photodiode quickly produces an electric current of the proper voltage for the network. It instantly stops producing the voltage when no light strikes the PIN photodiode. This generates the voltage changes that represent the data 1s and 0s on a copper cable. Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC connector). On single-mode fiber, the Straight Tip (ST) connector is frequently used. In addition to the transmitters, receivers, connectors, and fibers that are always required on an optical network, repeaters and fiber patch panels are often seen. Repeaters are optical amplifiers that receive attenuating light pulses traveling long distances and restore them to their original shapes, strengths, and timings. The restored signals can then be sent on along the journey to the receiver at the far end of the fiber. Fiber patch panels similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allowing quick changes to the connection of devices like switches or routers with various available fiber runs, or cable links. Lab Activity Lab Exercise: Fiber-Optic Cable PurchaseThis lab will introduce the variety and prices of network cabling and components in the market. The student will gather pricing information for fiber patch cables and fiber bulk cable.
Content 3.2 Optical Media 3.2.9 Signals and noise in optical fibers Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end. A buffer and an outer jacket that stops light from entering or leaving the cable cover the cladding. Furthermore, the transmission of light on one fiber in a cable does not generate interference that disturbs transmission on any other fiber. This means that fiber does not have the problem with crosstalk that copper media does. In fact, the quality of fiber-optic links is so good that the recent standards for gigabit and ten gigabit Ethernet specify transmission distances that far exceed the traditional two-kilometer reach of the original Ethernet. Fiber-optic transmission allows the Ethernet protocol to be