Limits The Bandwidth Of Multi Mode Fiber example essay topic

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 ligh 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 former having the cladding's index of refraction the same as the core and the latter have the cladding's index of refraction slightly lower than that of the core. Regardless of type, using single-mode fiber eliminates the dispersion problems of multi-mode fiber, enables transmission over much longer distances, and produces higher bit rates than that of multi-mode fiber. This goal of achieving higher bit rates is a necessity based on the constantly increasing demand for bandwidth in the telecommunications industry today. Our infrastructure today is not meeting the bandwidth demands caused by increased information flow of businesses and home users.

A quick and cost effective solution must be implemented if this trend is to continue. There are several alternatives to the problem of producing bandwidth to meet the demand. An obvious solution would be to simply add more fiber optic cable. This is being done at a steady rate, but because fiber installation costs are often the greatest expense of building a network, it may not always be economically feasible.

Another factor making this alternative less attractive is that the fiber that is currently installed is not nearly being used to its full capacity. Using common roadway traffic as an analogy, adding more fiber is like adding more roadways. It takes a long time to do, is disruptive to the current system, and is expensive. (ADC 1) Another simple solution is to light up dark fiber. This phrase refers to using fiber that is already installed but not currently being used. Frequently, when fiber is installed, more than is needed is put in to curb the effects of the growth of a system.

The extra fiber is called dark fiber because it is not being used so there is no light on it. However, because this boom in bandwidth demand has been going on for some years now, most of the dark fiber has already been lit up. Once again using the traffic analogy, lighting up dark fiber would be like opening up unused roads. This is a good solution if there are any unused roads available. The use of faster transceivers with the standard time division multiplexing can double or quadruple bandwidth but there is a fundamental limit because if fiber dispersion and time division multiplexing is usually restricted to short-range systems. (ADC 1) Also, the addition of faster mux and demux equipment is a huge cost.

Another solution is an alternative to the time division multiplexing scheme that is so common in telecommunications today. Wavelength division multiplexing is based on using different wavelengths of light on one fiber. Broadband WDM doubles the capacity of a system by using two-channel wavelength division multiplexing. These systems operate at the 1310 nanometer and 1550 nanometer wavelengths. (ADC 2) The channel spacing of 240 nanometers is typical on these systems, which have been in use for about a decade.

In narrowband WDM the channel spacing is decreased down to between 12 and 24 nanometers and as many as four channels are put on the fiber using a wavelength range of 1530 nanometers to 1565 nanometers. (ADC 2) Broadband and narrowband WDM have the capacity to quadruple bandwidth, however with the demand as such a high growing rate, this will is only a temporary solution. So how can we most efficiently utilize the fiber currently installed in the ground The answer lies in dense wavelength division multiplexing, or DWDM. As with broadband and narrowband WDM, dense WDM increases the capacity of a fiber by putting multiple wavelengths on it. However, because of the extremely small channel spacing of 1.6 nanometers or less, there be as many as eight, sixteen, thirty-two, or more wavelengths injected into the fiber.

(ADC 2) More and more can be added to quickly and significantly increase the bandwidth on that fiber. Using the traffic analogy once again, DWDM is like putting thirty-two lanes on a road, but requires everyone to drive a much narrower car. Dense wavelength division multiplexing requires equipment that will be able to multiplex, or combine, multiple wavelengths from different fibers onto one fiber and to demultiplex, or separate, the wavelengths from that single fiber back onto multiple fibers at the destination. This equipment must be precise because multiplexing and demultiplexing must be done with a low degree of loss and a high degree of accuracy to ensure that there is little or no channel cross talk. There are three essentially different methods used in the dense wavelength division technology, but the fact that remains constant is that they all multiplex at the source and demultiplex at the destination. These three technologies are interference filters, planar wave-guides, and fiber couplers and gratings.

(ADC 2) Interference filters are expensive to produce and are prone to the effects of aging and the environment. Put in a mechanical optical assembly are multilayer dielectric interference filters combined with micro-optics. (ADC 3) Using rod lenses with graded index technology, the light on each fiber is aligned and refocused back onto the fiber passing through a series of filters. Each filter is designed to only transmit one wavelength and reflect all others. This is how the multiplexing and demultiplexing processes are achieved. The lenses, filters, and fiber all have to be aligned in an extremely precise manner for the system to function properly.

This predicament results in the high price of these components as well as limiting the number of channels that can be multiplexed. Another technology used in dense wavelength division multiplexing involves the use of planar wave-guides. A wave-guide is a rectangular, circular, or elliptical tube used to control the path of a wave. The combined wavelengths pass through multiple wave-guides causing a slight phase shift between each of the channels, resulting in an interference pattern that separates the wavelengths. (ADC 3) Using planar wave guides often results in a high loss rate, and another disadvantage of this technology is that it is very temperature sensitive, requiring expensive active thermal control. Using Fused B iconic Taper couplers and Fiber Bragg Gratings can create a mux / demux.

With this method, the signal always remains on the fiber, eliminating the precise alignment problems of interference filters and wave-guides. The result is a simple and effective low-loss multiplexing and demultiplexing system. A Bragg grating is an intermittent change in the refractive index of a material. (ADC 4) At each change in the refractive index, a reflection occurs. The reflections will add up beneficially so that wavelength is completely reflected and all other wavelengths are transmitted. (ADC 4) A coupler simply splits incoming wavelengths into two fibers.

In a four-channel situation, this would put two channels on each fiber. A Bragg grating can then be used on each fiber to reflect one wavelength and transmit the other, resulting in a multiplexed signal being de multiplexed. The opposite can be achieved through the same technology. For a service provider in today's world, the ability to rapidly expand to meet the demand of customers and efficiently manage the bandwidth available within fiber optic cable is the key to success. Dense wavelength division multiplexing technology is the obvious choice for achieving this goal.

Optical fiber is the present and the future of computer networking. Through the use of technologies such as multiplexing, the current fiber optic infrastructure is seen as future-proof. This infrastructure is presently the backbone of our networks, however, as the economic barrier continues to be broken down, fiber will begin to be brought to the desktop and the home. Fiber to the desktop, or F TTD, is the only medium that will be able to meet the high bandwidth demands, and it is the future of computer networking.

Bibliography

ADC Telecommunications, Inc. Dense Wavelength Division Multiplexing (DWDM) An Overview Applications and Technologies. Internet. web October 2000.
Green, P.E. Jr. The Fiber-Optic Challenge of Information Infrastructures. Internet. IBM T.J. Watson Research Center. 16 November 2000.
Optical Cable Corporation. The Glass Story. Internet. web 11 November 2000.
Cisco World: Technology Overview, Calculating Fiber Loss and Distances. Internet. Publications and Communications, Inc. (PCI). 30 November 2000.