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Lesson 1A Pile of Sand Becomes a Strand of Glass |
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For those of you who
may aspire to become a Physicist/Scientist some day, this is a fun and interesting
lesson. Pay particular attention to the Lab
Experiment(s). A Tale of Two Ovens Two people arise in the morning, driving to work at different buildings that contain ovens. Both begin with a very clean environment and wear sanitized, white garments. Each has a supply of ingredients and will carefully follow recipes and manage oven temperatures to create a product to be offered for sale. However, the products they bring to market could not be more different, as one is a baker who will produce bread and pastries that are quickly devoured by hungry consumers and the other is a physicist that will produce a strand of optical fiber for communications networks.
While the comparison above is an oversimplification of the process, it is notable that the modern reality of fiber optic communications resulted from many years of trial and error, culminating in methods for achieving large quantities of high-quality optical fibers with uniform performance characteristics. For example, early optical fibers could only carry signals a few meters (6 to 9 feet), yet today capable of passing signals thousands of meters (tens of thousands of feet). In the world of fiber optics, distances and physical measurements are usually expressed in metric values, so be prepared to do some simple math in your head. For example, 1 meter equals 3.28 feet, so a 1,000 meter section of cable is 3,280 feet in length. The formulas and techniques for manufacture of optical fibers are closely held secrets, but can be generally described as you will see below. While a successful chef does not release their prize recipe, they do allow a visit to the kitchen so you can see the work in progress. So it is with the manufacture of communication-grade optical fibers, as the exact recipe and processes are carefully guarded by manufacturers. It's All About The Sand Next time you are standing on a beach, try to imagine that the sand between your toes could someday be used to carry a movie to your home, relay a text message between cell towers, or connect you with a distant caller. Actually, the sand at most beaches is not of sufficient quality to become optical fibers (with exception of sand on Siesta and Crescent Beaches in Sarasota, FL but don't take our word for it, go see for yourself as these are the two finest beaches in America). No, the sand from Siesta Beach is not used for manufacturing fibers, but has been judged best in the world by international tourism councils.
The sand used for manufacture of optical communications fibers is silica-based and highly purified for this purpose. Much of the sand in the world is quartz-based, and much silica sand (Silicon Dioxide) is derived from broken down quartz crystals. Various chemicals can also be mixed with the sand to lower the melting point, resulting in much lower temperatures for ovens and furnaces. The process begins by converting silica sand into molten state, forming a highly purified silica tube which is placed in a special lathe. Think of it as a large test tube with no closed ends. The lathe constantly turns while flames provide an even and controlled heating of the tube as chemical vapors are blown through the hollow center. As chemical vapors react to heat, they form a "soot" evenly along the inside of the silica tube, a process known as "Chemical Vapor Deposition".
Among the chemicals blown through the tube are Silicon Chloride, Germanium Chloride, Oxygen and other chemicals as dictated by the "recipe" required to produce a specific transmission profile. Once the center of the tube is filled to the desired thickness with chemical deposits, the process is halted, resulting in what is called a "soot preform" which is then treated to subsequent heat and pressure processes to remove moisture and impurities, and collapse the entire preform into a solid glass rod. At this point, the glass in the center of the rod formed from the chemical deposits has a higher "index of propagation" than the outer portion of the glass. Think of this as a pinhole in the middle of a lens on a pair of sunglasses; light passes through the entire lens, but the pinhole exhibits a different degree of ability to pass light than the surrounding lens material. Once the preform meets the design criteria for the type of fiber to be sold, it is critical that no subsequent manufacturing processes modify or change these properties. Once the "soot preform" has been collapsed and solidified, it is called a "blank preform".
The blank preform is loaded into a tall furnace called a "drawing tower", and the tip of the preform subjected to extremely hot temperatures so the glass begins to melt. As the tip of the preform melts, a small "gob" of glass accumulates at the tip and when the weight is sufficient, breaks free from the preform, plummeting down the drawing tower toward the bottom trailing a string of fiber in its wake "drawing" fiber from the tip of the preform.
How About Some Fresh Pizza Have you ever grabbed a slice from fresh pizza where the cheese just kept stringing along as you pulled, or served a scoop of cheesy casserole leaving a string of cheese from bowl to plate? In both cases it seems the longer you pull on the string of cheese, the longer the string becomes, and you have to physically break it with a fork or knife. That is a very good way to describe what is happening in the drawing tower. Consider this: if the cheese in the pizza or casserole was too hot, it would be too liquified to string, and if too cold would become a solid. The trick is to keep the cheese the exact temperature that allows a constant string to be drawn from the source. As long as a cheesy dish is at the optimum temperature, the cheese will yield a nearly unlimited string. We will test this theory in our Lesson 1 Lab Experiments. Likewise, the tip of the fiber preform must be held to an exact temperature that yields a constant string of fiber to be "drawn" from the melting glass.
Once the initial fiber is drawn, automated take-up reels can provide the exact amount of tension required to draw the fiber into a uniform diameter of 125 microns (125 µm), slightly larger than a human hair. We will discover how this compares with your hair in the Lab Experiments for this lesson. Micron is the term for micro-meter (µm), which is 1,000,000th of a meter or 0.0049212598425197 Inch. In the photo below, the black spot on the fiber (at left) is a dust particle.
Lasers near the preform tip provide continuous measurements of the fiber size so that the process can be adjusted as required to product thousands of meters of uniform glass strand. Once the fibers are manufactured, they can be encased in a jacket or bundled with other fibers to become a cable. These processes will be examined in Lesson 3- How Fibers are Spliced and Connected to Equipment. Two Types of Fiber The manufacturing process can yield many specific types of fiber strands depending on specific user transmission requirements, however, two main classes of fibers are in use worldwide: · Single-Mode Fiber · Multi-Mode Fiber
The outside diameter for both fiber types is the same- 125 µm, but the difference relates to the size of the very center portion of the fiber. During the Vapor Deposition Process described earlier, the collapsed fiber blank exhibits two different Indexes of Propagation; one for the very center of the glass which is called the core, and a differing index for the surrounding glass called the cladding. Index of Propagation is an expression for the ability of glass to pass light. The point at which two differing indexes meet produces a mirroring effect, creating a mirrored tunnel in the center of the fiber. Therefore, the very center of the drawn fiber is one level of indexing, completely surrounded by glass of a differing indexing. We will explain the need for and difference between Single Mode and Multi-Mode fibers in Lesson 5-Bumper Cars - How Pulses of Light Race to the Finish Line. We're Not Done Yet Once the fiber is drawn, a final process encloses the glass with two "coating" of polymer-based coverings, the first a softer material to protect the fiber from abrasion, and the final coating a harder, yet flexible substance designed to withstand handling by human hands during splicing operations. The final coatings are also color-coded so that technicians can properly identify the fibers for use. Even the softest of skin would easily abrade an unprotected glass fiber, causing enough scratches to insure a breakage at the first bend. Therefore, much care is taken when the coatings are removed for splicing operations.
This completes Lesson 1 A Pile of Sand Becomes a Strand of Glass OK, now crank up your investigative skills and conduct the Lab Experiment (s) for this lesson by clicking the button at the top right of this page, then proceed to the Lesson Test by clicking the button also at the top right of this page. Don't forget you can order a sample fiber from a real telephone cable for use in some experiments by clicking ORDER FIBER or the button on top right of the Home page. Prices from $1.00 per fiber. Great way to earn extra credits in your next Technical Presentation, Science Fair or Merit Badge! |
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