TRANSMISSION MEDIUM USED
1. FIBER OPTICS
Optical fiber is the medium in which communication signals are transmitted from one location to another in the form of light guided through thin fibers of glass or plastic. These signals are digital pulses or continuously modulated analog streams of light representing information. These can be voice information, data information, computer information, video information, or any other type of information.
These same types of information can be sent on metallic wires such as twisted pair and coax and through the air on microwave frequencies. The reason to use optical fiber is because it offers advantages not available in any metallic conductor or microwaves.
The main advantage of optical fiber is that it can transport more information longer distances in less time than any other communications medium. In addition, it is unaffected by the interference of electromagnetic radiation, making it possible to transmit information and data with less noise and less error. There are also many other applications for optical fiber that are simply not possible with metallic conductors. These include sensors/scientific applications, medical/surgical applications, industrial applications, subject illumination, and image transport.
Most optical fibers are made of glass, although some are made of plastic. For mechanical protection, optical fiber is housed inside cables. There are many types and configurations of cables, each for a specific application: indoor, outdoor, in the ground, underwater, deep ocean, overhead, and others.
An optical fiber data link is made up of three elements (Figure 2-1):
1. A light source at one end (laser or light-emitting diode [LED]), including a connector or other alignment mechanism to connect to the fiber. The light source will receive its signal from the support electronics to convert the electrical information to optical information.
2. The fiber (and its cable, connectors, or splices) from point to point. The fiber transports this light to its destination.
3. The light detector on the other end with a connector interface to the fiber. The detector converts the incoming light back to an electrical signal, producing a copy of the original electrical input. The support electronics will process that signal to perform its intended communications function.
OPTICAL FIBER
Optical fiber (Figure 2-3) is comprised of a light-carrying core surrounded by a cladding that traps the light in the core by the principle of total internal reflection. By making the core of the fiber of a material with a higher refractive index, we can cause the light in the core to be totally reflected at the boundary of the cladding for all light that strikes at greater than a critical angle. The critical angle is determined by the difference in the composition of the materials used in the core and cladding. Most optical fibers are made of glass, although some are made of plastic. The core and cladding are usually fused silica glass covered by a plastic coating, called the buffer, that protects the glass fiber from physical damage and moisture. Some all-plastic fibers are used for specific applications. Glass optical fibers are the most common type used in communication applications. Glass optical fibers can be singlemode or multimode. Most of today’s telecom and community antenna television (CATV) systems use singlemode fibers, whereas local area networks (LANs) use multimode graded-index fibers.
Singlemode fibers are smaller in core diameter than multimode fibers and offer much greater bandwidth, but the larger core size of multimode fiber makes coupling to low cost sources such as LEDs much easier. Multimode fibers may be of the step-index or graded-index design. Plastic optical fibers are large core step-index multimode fibers, although graded-index plastic fiber is under development. Because plastic fibers have a large diameter and can be cut with simple tools, they are easy to work with and can use low-cost connectors. Plastic fiber is not used for long distance because it has high attenuation and lower bandwidth than glass fibers. However, plastic optical fiber may be useful in the short runs from the street to the home or office and within the home or office.
There are two basic types of optical fiber—multimode and singlemode (Figure 2-4). Multimode fiber means that light can travel many different paths (called modes) through the core of the fiber, entering and leaving the fiber at various angles. The highest angle that light is accepted into the core of the fiber defines
the numerical aperture (NA). Two types of multimode fiber exist, distinguished by the index profile of their cores and how light travels in them (Table 2-1). Step-index multimode fiber has a core composed completely of one type of glass. Light travels in straight lines in the fiber, reflecting off the core/cladding interface. The NA is determined by the difference in the indices of refraction of the core and cladding and can be calculated by Snell’s law. Since each mode or angle of light travels a different path, a pulse of light is dispersed while traveling through the fiber, limiting the bandwidth of step-index fiber. In graded-index multimode fiber, the core is composed of many different layers of glass, chosen with indices of refraction to produce an index profile approximating a parabola, where from the center of the core the index of refraction gets lower toward the cladding. Since light travels faster in the lower index of refraction glass, the light will travel faster as it approaches the outside of the core. Likewise, the light traveling closest to the core center will travel the slowest. A properly constructed index profile will compensate for the different path lengths of each mode, increasing the bandwidth capacity of the fiber by as much as 100 times over that of step-index fiber.
Singlemode fiber just shrinks the core size to a dimension about six times the wavelength of light traveling in the fiber and it has a smaller difference in the refractive index of the core and cladding, causing all the light to travel in only one mode. Thus modal dispersion disappears and the bandwidth of the fiber increases tremendously over graded-index fiber.
Advantages of Fiber Optics
· Wider bandwidth and greater information capacity
Optical fibers have greater information capacity than metallic cables because of the inherently wider bandwidths available with optical frequencies (up to several thousand gigahertz).
· Immunity to crosstalk
Optical fiber cables are immune to crosstalk because glass and plastic fibers are nonconductors of electrical current. Therefore, fiber cables are surrounded by a changing magnetic field, which is the primary cause of crosstalk between metallic conductors located physically close to other.
· Immunity to static interference
Because optical fiber cables are nonconductors of electrical current, they are immune to static noise due to electromagnetic interference (EMI) caused by lightning, electric motors, relays, fluorescent lights and other electrical noise sources (most of which are man –made). For the same reason, fiber cables do not radiate electromagnetic energy.
· Environmental immunity
Optical fiber cables are more resistant to environmental extremes (including weather variations) than metallic cables. Optical cables also operate over a wider temperature range and are less affected by corrosive liquids and gases.
· Safety and convenience
Optical fiber cables are safer and easier to install and maintain than metallic cables. Because glass and plastic fibers are nonconductors, there are no electrical currents or voltages associated with them. Optical fibers can be used around volatile liquids and gases without worrying about their causing explosions or fires. Optical fibers are also smaller and much more lightweight and compact than metallic cables. Consequently, they are more flexible, easier to work with, require less storage space, cheaper to transport, and easier to install and maintain.
· Lower transmission loss
Optical fibers have considerably less signal loss than their metallic counterparts. Optical fibers are currently being manufactured with as little as few-tenths-of-a-decibel loss per kilometer. Consequently, optical regenerators and amplifiers can be spaced considerably father than with metallic transmission lines.
· Security
Optical fiber cables are more secure than metallic cables. It is virtually impossible to tap into a fiber cable without the user’s knowledge, and optical cables cannot be detected with metal detectors unless they are reinforced with steel for strength.
· Durability and reliability
Optical fiber cables last longer and are more reliable than metallic facilities because fiber cables have a higher tolerance to changes in environment conditions and are immune to corrosive materials.
· Economics
The cost of optical fiber cables is approximately the same as metallic cables. Fiber cables have less loss and require fewer repeaters, which equates to lower installation and overall system costs and improved reliability.
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