Fiber Optic Cables and Their Parts - Essay Example

Fiber optic cables I think we should start our topic by just asking a simple but important question what is a fiber optic cableA fiber optic cable is a cylindrical pipe. It may be up of plastic or glass a combination of glass and plastic. Fabrication of it is in such a way that this pipe can guide light from one end of it to the other. The idea of having light guided through bent glass is not new or high tech. Concept of total internal reflection of light through bent glass plays an important role for working fiber optic cable. The phenomenon of guiding light through bent glass has been early given by Leonardo DaVinci in one of his notebooks. But, he has not been able to verify this assertion. What is known for certain is that total internal reflection of light in a beam of water, basically guided light, and was given by the physicist John Tyndall in either 1854 or 1870, depending upon which reference you consult. Tyndall showed that light could be bent around a corner while it traveled through a jet of pouring water. Using light for communications came after this. . In 1934 the first patent on guided optical communications over glass was obtained by AT &T. unfortunately, no materials were available at that time to fabricate a glass (or other type of transparent material) fiber optic cable with sufficiently low attenuation to make guided optical communications possible. Between 1968 and 1970, experts who were working at a number of different academic, industrial and government laboratories dropped the attenuation of glass fiber optic cable from over 1000 dB/km to less than 20 dB/km. Corning patented its fabrication process for the cable. In the late 1980's and 1990's this progress increased with the even lower cost plastic fiber optic cable and Plastic Clad Silica (PCS). In last few years a number of oceanic fiber optic cables have been fabricated. One cable was fabricated in 1990. That was relatively non-controversial. However, as additional cables were introduced, the coastal fishing industry became increasingly concerned about the loss of fishing ground resulting from cable placement, and their liability should they come into contact with a cable. Another cable was fabricated in 1998. To represent their interests a number of coastal fishers formed a committee. After discussion on numerous issues, the fishers and the cable owner reached on a conclusion that has served as the basis for later agreements' between these two groups in Oregon. Two concentric layers termed the core and the cladding are the basic composition of a fiber optic cable. These layers are shown in the following figure. Fiber Optic Cable, 3 dimensional view and its basic cross section Both core and cladding have different refraction indexes with the core having R1 and the cladding R2. Light is piped through the core. In Fiber optic cable an additional coating termed as jacket is also provided around the cladding. Core, cladding and jacket are all shown in the three dimensional view on the left side of above Figure. The jacket is usually made up of one or more layers of polymer. This jacket protects the core and cladding from shocks that might affect their optical or physical properties. It acts like a shock absorber. The jacket also provides protection from abrasions, solvents, Small oil micro-deposits, dust and other contaminants. Eventually the jacket does not have any optical properties so it can not affect the propagation of light within the fiber optic cable. The illustration on the left side of above Figure is very simple. In reality, there may be a strength member added to the fiber optic cable so that it can be pulled during installation. This would be added just inside the jacket. Buffer should also be introduced between the strength member and the cladding. Parts of a Typical-Optic Cable This protects the core and cladding from damage and allows the fiber optic cable to be bundled with other fiber optic cables. We have to properly clean the end of a fiber optic cable and the inner surface of an optical module lens because these constitute optical surfaces and maintained to ensure optimum reliability and system performance. There should be no dust particles and Small oil micro-deposits on fiber optic cable optical surfaces because that may cause a loss of light or degraded signal power which may cause a number of sporadic problems in the optical connection. The remedy of this is clean dry air it insures that the aerosol is free of dust, water, and oil. Filtered compressed air or canned compressed air, which is available at any laboratory supplier or photo/camera shop, should be used. Above figure shows Small oil micro-deposits and dust that can collect on fiber cable connector tips and canals. Contaminants into the optical surfaces may be burn due to Laser power density and that may cause the fiber to produce inaccurate results finally rendering it unusable. Now the question is how the light is guided down the fiber optic cable in the core This occurs because the core and cladding have different indices of refraction. Index of the core, R1, always should be greater than the index of the cladding, R2. Next figure shows how this is employed to effect the propagation of light down the fiber optic cable and restrict it to the core. In our illustration a light ray is injected into the fiber optic cable on the right. As the phenomenon of total internal reflection states, if the light ray is injected and strikes the core-to-cladding interface at critical angle, an angle greater than an entity, then light ray is reflected back into the core. For total internal reflection the angle of incidence is always equal to the angle of reflection. The light ray will then continue this bouncing path throughout the length of the fiber optic cable. But if the light ray strikes at an angle less than the critical angle on core-to-cladding interface then it passes into the cladding where with propagation distance it is attenuated very rapidly. And Light can be guided down the fiber optic cable if it enters at less than the critical angle. This angle for required phenomenon is fixed by the indices of refraction of the core and cladding and is given by the formula: Ac = arc cosine (R2 /R1) The critical angle is measured from the cylindrical axis of the core. For illustration, if R1 = 1.446 and R2= 1.430 then a quick computation by above formula will show that the critical angle is 8.53 degrees, a quite small angle. Propagation of a light ray down a fiber optic cable It must be noted that a light ray enters the core from the air outside, to the left of above Figure. We have to take the refractive index of the air into account in order to assure that a light ray in the core will be at an angle less than the critical angle. This can be done quite simply. The following basic rule then applies. Let us assume that a light ray enters the core from the air at an angle less than an entity called the external acceptance angle - Aext it will be guided down the core and given by formula. Aext = arc sin [(R1/ R0) sin (Ac)] Here R0 is the index of refraction of air. This angle is also measured from the cylindrical axis of the core. In the example above a calculation shows it to be 12.4 degrees - again a quite small angle. True to the idea of "one-stop shopping", Weidmller can also give fiber optic connecting cables. Besides Fiber Optic patch cables, based on multimode zipcord cables with 50 and 62.5 micro-meter fiber diameters, a system cable for tough field installations rounds off our range. All the Fiber Optic cables from Weidmller can be supplied with SC duplex or ST connectors. FO patch cable - SC or ST plug both ends - Multimode glass fiber with 50 or 62.5 micrometer core Diameter FO patch cable with SC plug-in connector FO patch cable with ST plug-in connector FO patch cable - Adapter cable, SC to ST plug - Multimode glass fiber with 50 or 62.5 micro-meter core diameter FO adapter cable FO system cable - SC or ST plug as selected - Multimode glass fiber with 50 or 62.5 micro-meter core diameter SC plug ST plug To simplify the installation and maintenance of Prism optical network products such as nodes and amplifiers, Fiber Optic Service Cables are designed. One end of each jacketed service cable is terminated by a functional unit with 2 mm color-coded sub-units that are connector zed for the particular Prism module. And service cables, at opposite ends, are 900m pigtails suitable for splicing in splice surroundings. Materials Selection for fiber optic cables 1. According to the project contamination control requirements plan, we shall be selected all the material. And material should be suitable for both the termination process and the environment in which the finished product will be used. 2. All materials used in vacuum or low-pressure compartments shall not release greater than 1.0 percent total mass loss (TML) or 0.1 percent collectable volatile condensable material (CVCM). 3. Outer parts of cable should be corrosion resistant. The use of cadmium plating is restricted. 4. Materials shall have no negative effects on the health of person during handling and when used for the intended purpose. 5. Fire-rated fiber optic cables are available for use for specific applications when required. 6. When we use fiber optic cables for flight applications Vinyl dust caps shall not be used. Environmental Conditions:- 1. Controlled Environment- for proper working of fiber optic work area shall have a controlled environment which limits the entry of contamination. Procedures shall require environmental parameters be recorded and documented. Dust, dirt, fiber fragments, or strength member pieces shall be cleared to preclude contamination of the hardware. The humidity and temperature of this area shall be monitored, documented, and maintained within the limits defined as the comfort zone in Figure. Comfort Zone--Temperature versus Humidity Requirements 2. Special Environmental Requirements- more stringent control of environmental conditions is required to begin the process of Parts or equipment. 3. Ventilation System- ventilation system for removing air contaminants is necessary in areas used for cleaning parts and areas where toxic or volatile Vapors are generated. The Ventilation system should be compatible with the recommendations and guidelines of the Occupational Safety and Health Administration (OSHA) requirements. 4. Field Operations Requirements- In field operations, for the situation when the required controlled conditions cannot be achieved effectively, special precautions shall be included to lower the effects of the uncontrolled environment on the operation being performed on the hardware. In the appropriate document, these precautions shall be identified clearly. 5. Lighting- On the surface where fiber optic fabrication operations are being performed, inspected, or tested, the Light intensity shall be a minimum of 1077 lumens per square meter. Extra lighting may be used to achieve the required light intensity. Fiber optic data link performance is a subject that will be discussed in full at the end of this chapter. As far as the performance of a fiber optic data link is concert the network users are interested in the effect that the fiber optic cable has on overall link performance. Consideration of performance comes to answering following three questions: 1) Through the external acceptance angle, how much light can be coupled into the core 2) In propagating down the core, how much attenuation will a light ray experience 3) How much time dispersion will light rays representing the same input pulse experience in propagating down the core The more light that can be coupled into the core the more light will reach the Receiver and the lower the BER. If the attenuation in propagating down the core is minimum then maximum light reaches the Receiver and the lower the BER. The less time dispersion found in propagating down the core the faster will be the signaling rate and the higher will be the end-to-end data rate from Source-to-User. For answering these questions we have to consider many factors. The main factors are the size of the fiber, the composition of the fiber and the mode of propagation. When we talk about size, fiber optic cables have exceedingly small diameters. Figure illustrates the cross sections area of the core and cladding diameters of four generally used fiber optic cables. The diameter sizes shown are in microns, 10-6 m. Fiber optic cable sizes are usually expressed by first giving the core size and the cladding size. Consequently, 50/125 indicates a core diameter of 50 microns and a cladding diameter of 125 microns; 100/140 indicates a core diameter of 100 microns and a cladding diameter of 140 microns. The larger the core the more light can be coupled into it from external acceptance angle cone. However, larger diameter cores may actually allow too much light in and too much light may cause Receiver saturation problems. The left most cable shown in Figure 2-4, the 125/8 cable, is often found when a fiber optic data link operates with single-mode propagation. The cable that is second from the right in Figure 2-4, the 62.5/125 cable, is often found in a fiber optic data link that operates with multi-mode propagation. Typical core and cladding diameters -Sizes are in microns When we talk about composition or material makeup fiber optic cables are of three types: glass, plastic and Plastic Clad Silica (PCS). These three types of material according to the requirement and differ with respect to attenuation and cost. Attenuation and cost will first be mentioned for selection of material. Physical effects absorption and scattering are the principal factors for causing attenuation. Absorption removes signal energy in the interaction between the propagating light (photons) and molecules in the core. Due to scattering light redirects to out of the core to the cladding. For a fiber optic cable attenuation is dealt with quantitatively it is referenced for operation at a particular optical wavelength, a window, where it is decreased. Now we talk about mode of propagation; fiber optic cable can be one of two types, multi-mode or single-mode. These modes provide different performance with respect to both attenuation and time dispersion. The single-mode fiber optic cable provides the better performance but on the higher cost. To understand the difference in these types an explanation must be given of what is meant by mode of propagation. As we know, Light has a dual nature and can be viewed as either a wave phenomenon or a particle phenomenon (photons), according to the Huygens theory. For the present purposes we will consider it as a wave. When this wave is guided down a fiber optic cable it consist certain modes. These are variations in the intensity of the light, both over the cable cross section and down the cable length. Actually, all these modes are numbered from lowest to highest. In a very simple manner we can say each of these modes can be thought of as a ray of light. Though, it should be cleared, that the term ray of light is a hold over from classical physics and does not really describe the quantization physics and true nature of light. Figure describes the variation of attenuation versus wavelength taken over an ensemble of fiber optic cable material types. For propagation through a cable there are three principal windows of operation are indicated in the figure. These windows are according to the wavelength regions where attenuation is low and matched to the ability of a Transmitter to generate light efficiently and a Receiver to carry out detection. The 'OH' symbols in the figure indicates that at these particular wavelengths the presence of Hydroxyl radicals in the cable material cause a bump up in attenuation. Due to the presence of water these radicals comes in to consideration. During either a chemical reaction in the manufacturing process or as humidity in the environment radicals enter the fiber optic cable material. Attenuation vs. Wavelength Applications of Fiber optic cables:- 1. local area network 2. Telecommunication networks 3. Wide area network 4.CATV distribution networks 5. Point to point systems Safety Requirements:- All necessary precautions should be taken to protect personnel from injury while working with fiber optic cable assemblies. 1. Protection from bare fibers. A. persons who may come in contact with bare fibers shall wear wrap-around safety goggles for eye protection. B. Slivers of bare fiber shall be wrapped in a heavy tape (e.g., duct tape) and placed in a specially marked container for later disposal. 2. Protection from eye exposure to light sources shall be in accordance to the American National Standard for the Safe Use of Lasers. Some light sources used in testing and operating fiber optic cable assemblies may cause permanent eye damage. Storage and Handling:- 1. For the development and implementation of requirements and procedures necessary to prevent damage and to control conditions that could degrade the reliability of parts and deliverable items, the supplier is responsible. Containers shall be compatible with the stored materials. 2. Connector interfaces or stripped fibers should not be handled with bare hands due to risk of contamination to fiber interface. 3. Hands and tools should be cleaned properly before processing with optical parts. References:- 1. "Fibre optic cables", www.weidmuller.com/.../pdfs/datasheets/5652480000_Ethernet-FO_Cables.pdf 2. Jeff Kroft, "Fiber Optic Cables: The Oregon Experience or Things Look Different Here", www.econ.state.or.us/telecom/cablespeech.pdf 3. "Fibre optic cables", www.conductix.com/public/docs/CX/Cable_Machinery_Fiber_Optic_Cables_200 4. "Fiber Optic Service Cables", www.cisco.com/application/pdf/en/us/guest/products/ps8970/c1650/cdccont_0900aecd806c5bf4.pdf 5. "PREVENTIVE MAINTENANCE OF FIBER OPTIC CABLES AND OPTICS" ftp://ftp.hp.com/pub/networking/software/ProCurve-Fiber-Optics-Cleaning-Guide.pdf Read More