3.2.6 This page will introduce multimode fiber.
The part of an optical fiber through which light rays travel is called the core of the fiber. Light rays can only enter the core if their angle is inside the numerical aperture of the fiber. Likewise, once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called "multimode" fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber.
Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from device A to device B. The second fiber carries data from device B to device A. The fibers are similar to two one-way streets going in opposite directions. This provides a full-duplex communication link. Copper twisted-pair uses a wire pair to transmit and a wire pair to receive. Fiber-optic circuits use one fiber strand to transmit and one to receive. Typically, these two fiber cables will be in a single outer jacket until they reach the point at which connectors are attached.
Until the connectors are attached, there is no need for shielding, because no light escapes when it is inside a fiber. This means there are no crosstalk issues with fiber. It is very common to see multiple fiber pairs encased in the same cable. This allows a single cable to be run between data closets, floors, or buildings. One cable can contain 2 to 48 or more separate fibers. With copper, one UTP cable would have to be pulled for each circuit. Fiber can carry many more bits per second and carry them farther than copper can.
Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket.
The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. A core is typically glass made from a combination of silicon dioxide (silica) and other elements. Multimode uses a type of glass, called graded index glass for its core. This glass has a lower index of refraction towards the outer edge of the core. Therefore, the outer area of the core is less optically dense than the center and light can go faster in the outer part of the core. This design is used because a light ray following a mode that goes straight down the center of the core does not have as far to travel as a ray following a mode that bounces around in the fiber. All rays should arrive at the end of the fiber together. Then the receiver at the end of the fiber receives a strong flash of light rather than a long, dim pulse.
Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection. Standard multimode fiber-optic cable is the most common type of fiber-optic cable used in LANs. A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter (1ยต).
Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage. There are two basic cable designs. They are the loose-tube and the tight-buffered cable designs. Most of the fiber used in LANs is tight-buffered multimode cable. Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding. The most practical difference between the two designs is the applications for which they are used. Loose-tube cable is primarily used for outside-building installations, while tight-buffered cable is used inside buildings.
The strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests.
The final element is the outer jacket. The outer jacket surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode fiber is usually orange, but occasionally another color.
Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting 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).
The next page describes single-mode fiber.
3.2.7 This page will introduce 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. Although this distance is considered a standard, newer technologies have increased this distance and will be discussed in a later module. 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.
Warming: 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.
The next page introduces some components that are used with optical fiber.
3.2.8 This page explains how optical devices are used to transmit data.
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:
• A light emitting diode (LED) producing infrared light with wavelengths of either 850 nm or 1310 nm. These are used with multimode fiber in LANs. Lenses are used to focus the infrared light on the end of the fiber.
• Light amplification by stimulated emission radiation (LASER) a light source producing a thin beam of intense infrared light usually with wavelengths of 1310nm or 1550 nm. Lasers are used with single-mode fiber over the longer distances involved in WANs or campus backbones. Extra care should be exercised to prevent eye injury.
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). 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.
The next page will discuss data loss in optical fiber.
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