Fiber Optics Lab Manual

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Fiber Optics Lab Manual Instructor’s Manual For Classroom and Hands-On Laboratory Sessions Contacts For Assistance You can contact the FOA for questions and advice by phone, fax or email: The Fiber Optic Association, Inc., 1119 S. Mission Ave 355, Fallbrook. CA 92028 Phone: 1-760-451-3655 Fax: 1-760-207-2421 Email: infofoa.org Availability of plastic optical fiber (POF) The plastic optical fiber used in some of these experiments is available for science distributors. It is a 1000micron (1mm) POF available from several suppliers. FOA has samples available at no cost for teachers at schools in the US. Contact us at the email above. This information is provided by The Fiber Optic Association, Inc. as a benefit to those interested in teaching, designing, manufacturing, selling, installing or using fiber optic communications systems or networks. It is intended to be used as a overview and/or basic guidelines and in no way should be considered to be complete or comprehensive. These guidelines are strictly the opinion of the FOA and the reader is expected to use them as a basis for learning, as a reference and for creating their own documentation, project specifications, etc. Those working with fiber optics in the classroom, laboratory or field should follow all safety rules carefully. The FOA assumes no liability for the use of any of this material. Fiber Optics Lab Manual PREFACE This series of fiber optics laboratory experiments was developed by Professor Elias Awad for the FOA under a NSF grant. It is intended to introduce students in technical high schools and colleges to the technology of fiber optics. No previous experience in fiber optics is required. Students are expected to read all sections of each laboratory write-up before starting with the “procedure” section of each experiment. In some cases, the teacher may wish to use this laboratory manual as a text. We included in each experiment enough theory, tutorial material and examples to fulfill the needs of some programs. Teachers may wish to hold the students responsible to do the exercises provided with the laboratory write-ups. Teachers offering a full semester of fiber optics will find it necessary to have a regular textbook for the lecture section of the course. The FOA Textbook, The Fiber Optic Technicians Manual, is one choice, but at a college level, a text with more theory, such as Fiber Optic Communications by Jim Downing or Jeff Hecht’s Understanding Fiber Optics Several laboratory write-ups suggested that the students do the experiment with more than one type of fiber. If this requirement is not fulfilled due to the lack of resources, the educational benefit of the experiment will not be lost. Elias A. Awad email: photonsflash.net Lab Exercise 1 1- TITLE: Termination of various lengths plastic optical fibers into a metallic, 1000µm, ST™ fiber optic connector for plastic fiber. 2- OBJECTIVE: This is an exercise. It is intended to develop your manual dexterity while teaching you the proper installation of an ST connector on the ends of a plastic optical fiber Students are expected to pay attention to the proper way to install the connector on the fiber. This must be done in a way that will ensure longevity and prevent premature failure of the fiber optic link. Students will use room temperature epoxy and mechanical crimping to secure the fiber firmly into the connector. Oven cured epoxy may not be used with plastic fiber. Each student (or group of students, depending on the available budget) will prepare 9 optical fibers of various lengths. Later in this manual, you will use these same fibers to perform other experiments. 3- PURPOSE: We introduced this exercise to teach you the importance of installing connectors on optical fibers. Without connectors, there will be no reliable way to align two fibers. Alignment is important to permit the transmission of an optical signal between two fibers or between a fiber and a transmit or receive instrument. Connectors are the weakest link for a signal in a fiber optic line. It is where maximum power losses may be found. You will learn that it is worth your while to understand and practice proper connector installation to avoid unnecessary service calls. Unnecessary service calls may annoy an engineer and will eat away at the profit margin of a contractor who is responsible for the performance of an installation. 4- TUTORIAL: x1 Typically, optical fiber is made of thin and solid strands of glass. In this case, we are using plastic optical fiber with 1000µm diameter. The hole in the ST connector is also 1000µm in diameter. Plastic optical fiber is not popular in long distance nor is it popular in high frequency applications. In long distance applications, it exhibits unacceptable losses, this is called attenuation. In high frequency applications, it exhibits greater pulse distortion, this is called modal distortion. At this writing, a type of plastic optical fiber called “graded index plastic optical fiber” is underdevelopment. It promises to perform satisfactorily at substantially higher frequencies but over a distance of only 100 meters or less. So as you can see, while it may be useable at higher frequencies, its application will continue to be limited to a short distance. i.e.: from the street to the home. Not only its higher losses, but also its large core diameter contribute to the poor performance of plastic optical fiber. In a large core fibers, a large number of modes is transmitted inside the core. Each of these modes will travel a different optical path. These optical paths are unequal. This is what gives rise to modal distortion. Again, none of these optical paths is equal in length. This causes different segments of a single optical signal (one flash of light, one bit) to exit from the output end of the fiber at different times, thus distorting the optical pulse. This will also cause an overlapping of adjacent optical pulses. one bit input; one flash of light output with a modal distortion two or more bits input output with a modal distortion and pulse overlap The number of modes for each data bit in a multimode fiber is given by the “M” number of the fiber. The “M” number is given by the “V” number of the fiber: x1 2 M = V / 2 True only for values of V ≥ 10 2 2 V = (2 π a)/ λ n - n 1 2 n and n are the indices of refraction of the core and cladding, respectively. a is the 1 2 core radius and λ is the free space wavelength. n 2 cladding a guided guided core n guided 1 cladding Acceptance Cone Critical Modes guided mode 5- THEORY: The theory of connector making is a combination of mechanical and optical skills. Mechanically, the connector must be easy to assemble and use. It must withstand repeated cycling and various environmental conditions. Today, connectors are being made from stainless or other metallic materials. Also, ceramic and composite materials are used often. Connectors for plastic optical fiber are also found as “all plastic” or “all metallic” connectors. A large variety of connectors are found and/or being developed for glass optical fiber. Some plastic optical fiber uses a plastic “snap in” connector. Also, the standard ST connector with a modified hole diameter is used to accommodate the plastic fiber. x1 Optical concerns in connector making are related to the surface preparation of the connector which may cause pulse distortion. These are higher level concerns that do not come into play in common typical applications. And most certainly, they do not come into play in plastic optical fiber. Remember, this exercise is intended to develop your manual dexterity and introduce you to the installation of connectors (in their simplest form) on an optical fiber. 6- MATERIAL: Each group of 2 or 3 students needs the following: 30 meter of plastic optical fiber 18 AT&T ST stainless optical fiber connectors, 1000µm hole 2 pads of alcohol wipes Epoxy, room temp cure or UV cure. (not oven cure) Single edge razor blade Suitable crimp tool OVERVIEW: Each student will prepare three or more different lengths of plastic optical fiber. Students will work in groups of 2s or 3s. Each fiber will have an AT&T ST style connector on each end. When completed, each group will have nine terminated fibers of different lengths as follow: 0.1m, 0.5m, 1m, 2m, 3m, 4m, 5m, 6m and 7m 7- PROCEDURE: Cut fibers longer than the desired lengths by about 5 cm. This will allow approximately 2.5 cm of fiber to come through the front ferrule of each connector on either side of the cable. Strip the outer jacket from either end of the fiber a distance of about 5 cm. Feed the fiber through the connector as a “ dry run” That is to see if the fiber fits and x1 all of the dimensions are as you want them. When all of the dimensions are as you want them, wipe the fiber clean with alcohol before you apply the epoxy (epoxy will not bond to “oily” glass or plastic). Mix the epoxy in accordance with the epoxy manufacturer’ specs. “Scoop” epoxy with the portion of the bare fiber and outside jacket that will be inside the connector. Take special care not to let epoxy contaminate the outside of the connector. If you get epoxy on the outside of the front ferrule, it will no longer fit into the receptacles of the instruments. If you get epoxy on the spring or spring housing of the connector. you will not be able to lock the connector into position. This is a detailed sketch of the ST connector and the expected position of the fiber in it. Below this sketch is a simplified one that shows the two connectors at either end of a cable. Key Core and Cladding Back Ferrule Front Ferrule Cable outer jacket Core and Cladding This style does not have a separate crimp sleeve This is the AT&T ST connector. This is the STRAIGHT BODY style. Plastic optical fiber is made of a plastic core surrounded by a plastic cladding. The two are housed inside an outer protective jacket. No other protective or strength member is present. Glass optical fiber have additional strength members built into the cable. This makes the termination of plastic optical fiber the easiest of all fibers. x1 The epoxy is used to “bond” the plastic fiber to the inside walls of the connector and the mechanical crimping is intended to increase that bonding where the cable’s outer jacket meets the back of the connector. Stainless Stainless Rubber Front Ferrule Back Ferrule Strain Relief Fiber with outside Jacket, The AT&T ST Bush into connector Connector as far as it can go. After epoxy and crimping, slide the Before polishing, cut Fiber without outside Jacket strain relief into position. HOLD the fiber flush with the THE CONNECTOR WHILE connector using the PUSHING THE STRAIN RELIEF razor Apply the epoxy, insert the fiber into the connector, crimp ASAP and let stand to cure in accordance with the epoxy specifications. About 15 min. or more. Slip the strain relief while holding the connector. I said while holding the connector. Cut the fiber flush at the tip of the front ferrule. KEEP THE FIBERS CLEAN, NO FIGURE PRINTS AND NO SCRATCHES. Wipe clean with alcohol, Place the dust cover on all connectors and keep all cables in a safe place for future experiments. 8- CALCULATION: In this exercise, there are no calculations to be performed. 9- RESULTS: Typically, the results of a laboratory exercise gives the student the opportunity to contrast the actual outcome to the anticipated outcome quantitatively. This does not apply in this exercise. x1 10- DISCUSSION: Student: Please write a short discussion. Describe, in your own words, if you had to, how you would re-write this lab. What you would change in it, and how, to make it easier and clearer for another student to do this lab. What technical and practical advise would you give to someone intending to do this exercise. 11- CONCLUSION: Students: Please write a conclusion about this exercise. Contrast what was intended (as you understood it) to what was accomplished. Explain the reason(s) for any differences between the two. In addition, offer recommendation(s) on how to make the outcome coincide with the objective. Do that so others doing this exercise will be able to make use of your recommendations to achieve better match between the objective and conclusion. x1 Lab Exercise 2 TITLE: Polish and visually inspect terminated plastic optical fibers “from the previous laboratory exercise.” OBJECTIVE: This is an exercise. It is intended to develop your manual dexterity while teaching you the proper procedure for polishing terminated plastic optical fiber. Students are expected to follow the instruction to the proper way to polish and protect the fiber and the connector in which it is housed. This must be done so as to maximize the performance of the fiber link at the connection point. Students will use 12µm Aluminum Oxide polishing film. All polishing will be dry polishing. No lubricating liquids will be used. Each student will polish one or several fibers of various lengths. This depends on the number of fibers and the number of students in each group. Slide as you polish. This will keep clean polishing film on the fiber at all times Polish using figure 8 motion as shown. Actually, the height and width of the figure 8 should fit into an imaginary circle as shown on the right above. KEEP IT CLEAN at all times, touch only with cleaning material x2 Do not transport contamination from one polishing film to the next. Always clean your polishing fixture with alcohol wipe. PURPOSE: We introduced this exercise to teach you the important points when polishing optical fibers. Student should remember that there are some differences in polishing plastic and glass optical fibers. In this exercise. you are polishing plastic optical fiber. look for additional details when polishing glass optical fiber in the future. Connectors are the weakest link in a fiber optic line. You will learn that it is worth your while to understand and practice proper connector polishing and protection techniques to avoid unnecessary service calls and to maximize the system’s performance. Unnecessary service calls will eat away at the profit margin of a contractor who is responsible for the performance of an installation. Poor system performance will eat away at the power budget of the system and may requires increased operating power. TUTORIAL: Typically, optical fiber is made of thin and solid strands of glass. In this case, we are using plastic optical fiber with 1000µm diameter. The hole in the ST connector is also 1000µm in diameter. Plastic optical fiber is not popular in long distance nor is it popular in high frequency applications. In long distance applications, it exhibits unacceptable losses, this is called attenuation. In high frequency applications, it exhibits greater pulse distortion, this is called modal distortion. At this writing, a type of plastic optical fiber called “graded index plastic optical fiber” is underdevelopment by a company just outside the Boston, Massachusetts area and it promises to perform satisfactorily at substantially higher frequencies up to a distance of 100 meters. So as you can see, while it maybe useable at higher frequencies, its applications will continue to be limited to a short distance. Not only its higher losses, but also its large core diameter contribute to the poor performance of plastic optical fiber. In a large core fibers, a large number of modes x2 is transmitted inside the core. Each of these modes is traveling a different optical path. None of these optical paths is equal in length. This causes different segments of the optical signal to exit from the fiber at different times, thus distorting the optical pulse. This will also cause an overlapping of adjacent optical pulses. Optical power maybe lost from the core of a fiber due to various reasons. One of these reasons is the radiation of higher order modes into the cladding. In the cladding, these modes will attenuate rapidly and be lost for ever. In multi mode fiber (such as we are using in this exercise) higher order modes are always present and are vulnerable to radiating out. Poorly polished connector may give rise to and promote the development of higher order modes and perhaps the conversion of these modes into a non-guided modes, they will become lost modes. In the figure below, you can see that the difference between the guided and unguided modes is the angle at which the light is incident. Again, a poorly polished connector may very well promote the conversion of guided modes into an unguided ones. Lost cladding guided guided core guided cladding Lost Core - Cladding Interface Critical Angle x2 When the CRITICAL ANGLE is decreased, light will enter the cladding and it will be lost When the CRITICAL ANGLE is increased, light will enter the core and it will be guided THEORY: The theory behind polishing glass or plastic optical fiber is very simple. Use fine grit polishing film to take out all of the surface scratches and irregularities. Leave the surface of the fiber and connector flat, smooth and in the same plan. Under such circumstances, when two fibers are brought close together, their surfaces will touch and make a full and smooth contact to allow the optical energy to cross from one to the next with only minimum loss. A large variety of connectors are found and/or being developed for glass optical fiber. Plastic optical fiber has a plastic “snap in” connector and the standard ST connector with an enlarged hole to accommodate the plastic fiber. Remember, this exercise is intended to develop your manual dexterity and introduce you to the polishing and inspection technique of a plastic optical fiber. MATERIAL: Each group of 2 or 3 students needs the following: 6 sheet Aluminum Oxide polishing film 12µm grit size 1 hard but smooth polishing surface such as glass for each student. 6 pads of alcohol wipes 1 suitable polishing fixture for each student 1 suitable inspection microscope with 60 or 100X magnification. x2 OVERVIEW: Each student will polish one or more connector depending on the total available Students will work in groups of 2s or 3s. Each end of each fiber will be polished and inspected several times. When completed, each group will have nine terminated and polished fibers of different lengths as follow: 0.1m, 0.5m, 1m, 2m, 3m, 4m, 5m, 6m and 7m When polishing one end of the fiber, make sure the opposite end protected so it will not drop on the floor, drag, collect dust or scratch. PROCEDURE: You had cut the fibers longer than the desired lengths by about 5 cm. This allowed approximately 2.5 cm of fiber to come through the front ferrule of each connector on either side of the cable. Now, cut that piece (the 2.5 cm) flush with the surface of the connector using a razor. You may have already done that in the previous lab. It is recommended at this point to inspect the cut and unpolished fiber with the microscope. USE ONLY ROOM LIGHT to inspect the fiber. Follow the microscope manufacturer’s instruction. This will allow you to see what the fiber looks like prior to polishing and give you an appreciation to the importance of polishing. Place the connector through the hole in the polishing fixture, be sure the tip of the front ferrule sticks out from the other side of the polishing fixture. The polishing fixture will secure the fiber and connector at 90° to the polishing surface. You must ensure that the polishing fixture stays flat on the polishing surface. Start polishing by making figure 8 motion on the polishing film. Note: the height of the figure 8 must equal its width Apply moderate pressure on the polishing film. Ensure that the tip of the connector, and thus the fiber, is in continuous contact with the polishing film at all times. Ensure that the polishing fixture does not lift off of the polishing film when making figure 8. x2 Do five or so laps of figure 8. Stop. Clean the tip of the fiber with alcohol. Inspect with a microscope. allow ROOM light to enter the fiber on one end while inspecting the other end with the microscope. Repeat the process, polish, stop, clean, inspect and polish again. Monitor and observe the progress of the surface being polished. . . . Polished plastic optical fiber will look like this. There will always be some minor scratches. The large cross section area of plastic fiber allows you to ignore these scratches. You may not ignore them in glass optical fiber Polished plastic optical fiber with no visible scratches. In most cases, this is hard to achieve and is not worth the effort for plastic fiber. x2 KEEP THE FIBERS CLEAN, NO FINGER PRINTS AND NO NEW SCRATCHES. Wipe clean with alcohol, Place the dust cover on all connectors and keep all cables in a safe place for future experiments. 8- CALCULATION: In this exercise, there are no calculations to be performed. 9- RESULTS: Typically, the results of a laboratory exercise gives the student the opportunity to contrast the actual outcome to the anticipated outcome quantitatively. This does not apply in this exercise. 10- DISCUSSION: Student: Please write a short discussion. Describe, in your own words, how you would re-write this lab, if you had to. What you would change in it and why. What advise would you give to someone intending to do what you had just finished. 11- CONCLUSION: Students: Please write a conclusion about this exercise. Contrast what was intended to what was accomplished. Explain the reason(s) for any differences between the two. In addition, offer recommendation(s) on how to make the outcome coincide with the objective. Do that so others doing this exercise will be able to make use of your recommendations and observations to achieve better match between the objective and conclusion. x2 Lab Exercise 3 TITLE: Power launching and the testing of optic power loss between two plastic optical fibers in ST connectors OBJECTIVE: This is a laboratory experiment. It is intended to develop your understanding of the testing methodology and the testing and calculation procedure relating to optical power loss between two terminated plastic optical fibers. Students are reminded that there are several methods used to test connector loss. In this lab, we are using the “launch cable method” without mode stripping, the simplest one of all. In future labs, we may revisit this subject and use a different testing procedure. Students will test the connector loss of only the shortest plastic fibers individually, develop understanding of the connector loss, testing procedure and calculate the loss value in different units. PURPOSE: Optical power loss in optical fibers is the reason we use regenerators or amplifiers in the line. Most of the power loss usually occurs at the point of connecting two fibers. It is important that the loss at this connection point be kept to a minimum.. testing connector loss is essential to the understanding of how to keep it to a minimum. Students will learn how to set up the testing procedure correctly, how to measure the loss, how to convert that loss between different units and how the loss maybe affected by outside variables. Once the loss and the reasons behind it are understood, the student will have learned how to prevent it in future terminations. TUTORIAL: x3 1- THE UNITS OF MEASUREMENTS: In optical fiber, we use the milliWatt scale to measure the transmitted optical power. At the receiver, we may find ourselves having to use the microWatt scale to evaluate the received optical power. The reason for selecting these units is very simple. They are just the right “size” for the technology today. We need not have one nor several Watts of power to get today’s optical communication systems to perform. Another unit of measure is the deciBel, also known as dB. The dB is used to measure the relative change of power between two points in a link. p 2 in watts 10 Log = is given the unit 10 P of deciBel, dB, which may 1 in Watts have a positive or a negative value Here, students are reminded that half the power is lost for every 3dB drop. Also, you are reminded that in the definition above, its value will be + if P P 2 1 and - if P P 2 1 P any system P 1 2 Since the typical optic power used in fiber optic communication systems is in the mW range, we need to modify the dB unit above only to inform the reader that we are using the mW and not any other range. If you fix the value of p to 1 mW, the 1 equation above will now be written as follows and the units will be referred to as dBm p 2 of any value 10 Log = is given the unit 10 1mW of deciBel or dB m m This is not a millideciBel, it is a deciBel referenced to 1 mW. Here, students are also reminded that half the power is lost for every 3dB drop. That is, if the power drops from 1 mW to a 0.5 mW, we say the drop is 3dB and that puts x3 us at the -3dBm point. And if the power drops by a half again, we say the total drop is 6dB and that puts us at the -6dBm point or 0.25 milliWatt. 1mW any system P 2 EXAMPLE: What would be the dBm reading of a 2mW optical signal SOLUTION: using the definition we write: p 2 of any value 10 Log = is given the unit 10 1mW of deciBel or dB m m Anytime you fix the denominator to we write: 1mW, your answer is in dBm 2 mW 10 Log = 3.01 dBm ≈ 3 dBm 10 1mW AN EXAMPLE for you: How many dBm is there in 1 mW of an optical signal. Do this example and pass it in to your teacher with your lab write-up 2-THE MISALIGNMENT ISSUE: One cause for optical power loss between two fibers is the misalignment of the two fibers. Misalignment can be caused by improper polishing which may have caused the surface of the fiber to be at an angle x3