Fiber Optics Essay Research Paper Both corporations

Fiber Optics Essay, Research Paper

Both corporations and individuals today are demanding high bit rates for various applications. Corporations demand this large bit rate for supercomputer interconnection, remote site backup for large computer centers, digital video production and distribution, and many other various tasks. This bit rate demand is seen most prominently in the home users need for large amounts of bandwidth to support the multimedia rich web sites found today on the Internet. These individuals want to be able to point, click, and receive an instant response from anywhere to anyone on the Internet. They want to be able send and receive with little latency, almost instantaneous. This need for an any-to-any communication is seen by many to be the key to optimizing the use of communication technology. As the Internet continues to evolve and grow, so will the demand for higher bit rates. If the estimates based on Figure 1 are correct, then the part of the communication infrastructure apportioned to the Internet must grow by about 10^9 to keep up with demand. (Green 1)

When all is said and done, there is only one physical transmission medium that is capable of meeting these demands: optical fiber. Fiber optic cable is being installed at a rate of 4,000 miles per day now so that in the United States alone, there exists over 10 million miles. Along with the large amount of fiber is the added feature of its gigantic capacity. Each fiber has a usable bandwidth of 25,000 GHz, about 1,000 times the usable radio spectrum, and this capacity is underused by a factor of 10,000 with the current technology of time division multiplexing. (Green 2) However, rapidly developing technologies will soon lead us to open the full potential of fiber optics.

Fiber optics is a technology that uses glass threads, or fibers, to transmit data. A fiber optic cable consists of a bundle of fibers, each of which is capable of transmitting messages modulated onto light waves. Some of the advantages of fiber include its high bandwidth characteristics, the ability to carry many signals, it is light weight, it is less prone to corrosion than is copper, it is immune to interference, and once installed, it is practically future proof . Fiber optic cable consists of three components, as show in Figure 2. At the center is the core, a very narrow strand of high quality glass. Around the core is the cladding, also high quality glass with an index of refraction slightly different from, and usually within 1%-2% of, the core. The third component is the buffer or jacket, usually structured from plastic or coverall fibers. There sometimes can be both a primary and a secondary buffer. The central premise behind fiber optics is perfect internal reflection. When the light rays reach the outside of the central glass core, they hit the cladding. Because of the slightly different indices of refraction, there is total internal reflection or no light escapes. Because of this, there is very little attenuation and in turn fiber can be used to transmit data over long distances. The light is transmitted onto the fiber by a light emitting diode (LED) or a laser transmitter in one of two ways: single-mode or multi-mode.

The core diameter in multi-mode fiber, ranging from 50 micrometers to 1,000 micrometers, is large compared to a wavelength of light, about 1 micrometer. This means that light waves can propagate down the fiber in many different ray paths, or modes. There are two basic types of multi-mode fiber. One is step index fiber and the other is graded index fiber. In step index fiber, the index of refraction is the same throughout the length of the fiber, resulting in propagation as shown in Figure 3. Because of the many modes, different rays travel different distances and take different amounts of time to propagate the length of the fiber. Because of this, when a pulse of light is injected into a fiber, the various rays of that pulse will reach the destination at different times. Thus, the output pulse will have a longer duration than the input pulse. This occurrence is known as modal dispersion and it limits the number of pulses per second that can be put on a fiber and still be recognized as different pulses. This limits the bandwidth of multi-mode fiber, limiting it typically to 20 to 30 MHz per kilometer. (The Glass Story 2) Taking advantage of the fact that light travels faster in a low index of refraction material than a high one, in graded index fiber the index of refraction is gradually changed from maximum at the center to minimum at the edges. The modes that travel near the edges of the core travel faster for a longer distance while the low-order modes, or modes traveling in the center, are slower for a shorter distance, decreasing the amount of modal dispersion, as shown in Figure 4. Therefore the ability to transmit pulses closer together without interfering with each other exists in multi-mode graded index fiber, supporting higher bandwidth, typically from 200 MHz per kilometer up to 1 GHz per kilometer.

The core diameter of single-mode fiber measures about 9 micrometers and is much closer to the diameter of a wavelength. This limits light transmission to a single ray or mode, hence the name. There are two slightly different types of single mode fiber in use today and both are completely interchangeable and compatible. The two types are matched clad and depressed clad with the