7.4 Fiber Optics

An optical transmission system has three key components: the light source, the transmission medium, and the detector. Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. The transmission medium is an ultra-thin fiber of glass or plastic. The detector generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end. Higher bandwidth links can be achieved using optical fibers. One of the best substances used to make optical fibers is ultrapure fused silica. These fibers are more expensive than regular glass fibers. Plastic fibers are normally used for short-distance links where higher losses are tolerable.

Optical fiber links are used in all types of networks, LAN and WAN. The frequency range of fiber optics is approximately 180 THz to 330 THz. There are two types of fiber optics cables:

Multimode fiber - Light rays can only enter the core if their angle is inside the numerical aperture of the fiber. 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.

Fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from host A to host B. The second fiber carries data from host B to host A. The fibers are similar to two one-way streets going in opposite directions. This provides a full-duplex communication link. 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. There are no crosstalk issues with fiber. It is very common to see multiple fiber pairs encased in the same cable. One cable can contain 2 to 48 or more separate fibers. Fiber can carry many more bits per second and carry them farther than UTP 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 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. 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 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 50µm core and a 125µm diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber.

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.

Infrared Light Emitting Diodes (LEDs) types of light source usually used with multimode fiber. LEDs are cheap 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 2 km.

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 µm 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 µm 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. 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.

Single-mode fiber is capable of higher bandwidth and greater cable run distances than multimode fiber. Single-mode fiber can carry LAN data up to 3 km. Although this distance is considered a standard, newer technologies have increased this distance. Multimode is only capable of carrying up to 2 km. 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. Multimode and single-mode fibers are shown in Figure 20.

Warming: The laser light used with single-mode has a longer wavelength than can be seen. The laser can seriously damage eyes. Do not look at the near end of a fiber that is connected to a device at the far end. Do not 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!

Multimode and Single-mode Fiber
Figure 20: Multimode and Single-mode Fiber
Types of Cables and Connecting a Networking Devices The Physical Layer Wireless Links and Transmission