Fig 1 Rayleigh scattering in optical fiber

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1 The optical time domain reflectometry Experiment 2: The optical time domain reflectometry The purpose of this laboratory exercise is to learn to know the fundamentals of an optical time domain reflectometry and a fusion splicer. Both instruments are frequently used in field work by professionals and also very useful in laboratory work. After this laboratory exercise the reader should know how to splice fibers with the fusion splicer and how to perform basic measurements with the OTDR. Background/Theory Introduction to scattering and attenuation While light propagates in an optical fiber the power of light is decreased bit by bit. Attenuation in optical fibers can be divided to in two categories: intrinsic and extrinsic attenuation. Intrinsic attenuation is a result of the material characteristics of fiber and it is composed of absorption and Rayleigh-scattering. Instead, extrinsic attenuation is a result of the external factors of fiber, bends for example. Though intrinsic and extrinsic attenuation are notable in long fibers more attenuation is often produced by connections between fibers. In all connections made by optical connectors light is partly reflected back or leaked out of the fiber. Nevertheless, when long fibers and fusion spliced connections are used it is intrinsic attenuation that limits how far light can travel through fiber before it is too weak to detect. Most of the light which is sent to the fiber can be detect at the other end, but a part of it is always absorbed or scattered. Absorption and scattering are caused by imperfections of fiber, small grains of dirt, for instance. Scattering means that light is not absorbed but it is just sent in different angle after it hits small particles in optical fiber (Fig. 1). Some of the light is scattered to the direction it came from. This is called backscattering. Backscattering forms the basis to the use of the optical time domain reflectometry, which is introduced in next chapter. Fig 1 Rayleigh scattering in optical fiber Recently, because of the development of the manufacturing methods of optical fibers, damage of attenuation produced by scattering and absorption has been decreased significantly. Nevertheless, scattering and absorption limits the use of different wavelengths in optical technology. Rayleigh-

2 scattering reduces the use of wavelengths smaller than 800 nm. It is also hard to make good use of wavelengths larger than 1700 nm because of infrared-absorption. Introduction to the Optical Time Domain Reflectometry The optical time domain reflectometry is used to examine the fiber, faults and connections for example. The attenuation profile of fiber can also be found out easily. The OTDR is very suitable for those kinds of operations because the whole fiber can be examined in one measurement and measurements can be done where fiber is mounted. Optical time domain reflectometry can also probe more than one fiber or other passive components at the same time. In measurements we can find location and attenuation of different components or possible faults of fibers between them. Optical time domain reflectometry is based on scattering and reflections. OTDR sends an optical pulse to the fiber and measures the received backscattering. The signal which is received consists naturally only of scattering and reflections of pulse which was sent. By interpreting signal as a function of time OTDR can draw an attenuation of a measured component, fiber for instance, as a function of distance. Theory of the OTDR Optical time domain reflectometry measures backscattering as a function of time and graph is then drawn as a function of distance (Fig. 2). The graph represents the power of signal which the detector of the OTDR receives. The graph of fiber probed by OTDR consists of two spikes with gradually decreasing line between them. The line between spikes is decreasing because the received signal is decreased as a function of distance in accordance with attenuation coefficient of fiber. At the both ends of fiber reflection is rather large (Fresnel reflection) which creates spikes to the graph. Length of the fiber can therefore be measured from the width of the graph. Fig 2 OTDR signal as a function of distance In the graph of OTDR single components and other sources of attenuation, faults for instance, are shown as a drop in the power of received signal (Fig. 3). Size of a drop depends on an amount of power that is lost due to the component. The lost power represents of course the attenuation of 2

3 component. Components and faults in fiber are either reflective or nonreflective. Reflective components create a spike to the graph of OTDR the same way as the both ends of fiber do. With nonreflective components there are no spikes because no excess light is reflected back. In most cases reflective attenuation is caused by connectors or other passive components and nonreflective attenuation is usually caused by fusion splice or similar fault in fiber. Fig 3 Attenuation of different faults Calculation of attenuation caused by components or faults is done by measuring and comparing the power level of signal before and after the drop of power. Instead, attenuation coefficient of fiber can be measured by examining the part of the graph straight. Fig 4 Measuring attenuation of fault By calculating the slope of straight part, x 1 - x 2 (Fig. 4) for instance, we get the attenuation (db/km) of measured fiber. Attenuation coefficient α can therefore be calculated simply by formula (1). 3

4 y2 y3 α = ( db / km) (1) x x 2 1 Attenuation of components, or suchlike faults, can be calculated from the graph of OTDR by examining the power levels nearby the drop. The difference between points y 1 -y 2 (Fig. 4) tells us an attenuation shown in graph. The size of the second attenuation could be calculated in the same way. Introduction to fusion splicing While working with optical fibers there often occurs a need for longer fibers or we have to fix broken ones. We also need connectors to the ends of fiber to be able to use fibers with other optical instruments. For these kinds of problems we need a fusion splicer. The idea of fusion splicing is to connect two fibers without connectors. Nevertheless, splicing is not a simple procedure and it needs very specialized instruments. In fusion splicing the ends of two fibers are spliced together. Because the core of fibers is very small (~9 µm) fibers have to be aligned precisely at the right angle and distance. If alignment is not done correctly light leaks or reflects out of splice and the attenuation of the splice increases significantly. Attenuation can be due to three kinds of faults (Fig. 5). In fusion splicing the fusion splicer takes care of the alignment of fibers. There are two cameras which scan the splice from two different angles. According to the data got from the cameras, splicer can align fibers correctly. When fibers are aligned the ends of the fibers are melted together by arc aimed to the splice. While melting splicer also pushes fibers a bit together. Fig 5 Possible faults of splices If the ends of the fibers are not clean enough the attenuation of splice increases remarkably (Fig. 5) because small bubbles or grain of dirt may be left in the splice. So, to get the lowest possible attenuation for the splice we have to finish the ends of fibers as carefully as we can. When splicing is done strength of splice is tested by fusion splicer. Fusion splice is quite durable to longitudinal stress but if splice is bent it breaks easily. To prevent breaking heat-shrinkable jacket is threaded to the fiber. Jacket is melted in the oven of fusion splicer after splicing. 4

5 List of equipment The Optical Time Domain Reflectometry The buttons of the front panel of optical time domain reflectometry are shown in figure 7. While performing measurements we mostly need the buttons located on the right side of the screen. Buttons on the left side of the front panel have to be adjusted only at the beginning of the measurements. To get a good resolution to measurements we should keep the fiber range as small as it can be (50 km) because the fibers we are using are quite short. The wavelength which is used at measurements can be adjusted by Wave Length button. Buttons on the right side of the front panel can be used to adjust the view of screen and perform the calculations while measuring fibers. By Vertical Position and Center Distance buttons we can move the graph on the screen and by db Scale and Distance Scale buttons we can adjust the scale of the graph. All calculations are done by Loss Measurements buttons and Mode buttons are used to change the way of measuring. Avg button is used to view the average of the measurement data. After pressing Avg button OTDR starts to average the signal which is received and when it is pressed again the calculations are stopped. We are encouraged to use average before all calculations because the graph of OTDR is more defined with averaging. Pulse Width button is used to adjust the width of the pulse which OTDR sends to the fiber. Smaller width of the pulse gives better measurement results. Fig 6 Optical Time Domain Reflectometry Fiber which is measured is connected to lower right corner of the front panel with FC/PC connectors. On/Off button of OTDR is located in the back panel. The Fusion splicer The front panel of the fusion splicer consists of three parts (Fig 6). At the top of the front panel there is oven, which is used to heat the shrinkable jacket. In the middle of the front panel is a cover. Under the cover there are clips for fibers. At the bottom of the front panel there are buttons and a folding screen that are used to control and observe splicing. 5

6 Fig 7 Fusion splicer We can turn the fusion splicer on by pressing 1 -button. While starting the splicer performs initial tests and finally returns to main menu. Buttons at the left side of front panel are used to operate in different menus of the fusion splicer and buttons on the right are used to start splicing and heating. Reset-button can always be used to return to main menu. We can find instruction of the use of buttons at the bottom of the folding screen. Fibers that are going to be spliced are placed under the cover (Fig 6). To prevent fibers to move while splicing there are two clips for both fibers. One must remember not to break or dirt the end of fiber while placing it to the splicer. The ends of the fibers should be placed between electrodes and because there is not much space fibers must be placed very carefully. When fiber is placed in the oven after splicing, splice must be placed in the middle of the heatshrinkable jacket. We also have to take care of keeping fiber straight inside the jacket. The bends that are left in the jacket may increase the attenuation. To keep the fiber straight it must be pulled a bit while closing the second clip and the cover of oven. Variable attenuator, fibers and connectors Because even the smallest impurity will greatly affect the attenuation of optical connectors and fibers we have to be very careful when operating with optical instruments. Connectors have to be cleaned always before connecting. In this exercise we are using two long fibers and one a bit shorter. One variable attenuator is also needed. Variable attenuator uses batteries and On/Off button is located in the front panel. 6

7 Measurement goals Aims of this laboratory exercise are to first splice two fibers to longer one and then characterize the fiber with OTDR. The measurements are also done to variable attenuator which is connected to spliced fiber. We are about to measure the length and the attenuation of the fibers as well as the attenuation of the separate component, variable attenuator. Fig 8 Setup of part 2 The first part of this laboratory work consists of splicing fibers and in the second part (Fig 8) the fiber and the splice are measured with OTDR. Spliced fiber is connected to OTDR with FC/PC connectors. In the last part of measurements variable attenuator and third fiber are connected to fiber which was measured in part 2 (Fig 9). Fig 9 Setup of part 3 Measurement 1: Splicing 1. Turn on fusion splicer. It takes a while to start, so you can straight away begin to prepare fibers. 2. Before splicing you have to remove the cladding of the fiber. There is a special pair of tongs for that purpose. Place the fiber in tongs and pull the cladding of the fiber away. You should peel at least a few centimeters because you still have to finish the end of the fiber with a staple cutter. Remark that the fiber which has no coating is very thin and sharp. Beware that you don t hurt anyone with the sharp end of the fiber and take care that the clippings of the fiber end in garbage can. 3. When the cladding is removed fiber has to be cleaned. Before cleaning you still have to thread a heat-shrinkable jacket in the fiber. Water the cleaning cloth in a cleaning solution and wipe the end of the fiber a few times. Fiber has to be as clean as possible because even the smallest amount of dirt increases the attenuation of splice significantly. 4. To get better splices, the end of the fiber has to be finished with a staple cutter before splicing. In the cutter there are numbers from one to three that guide you to use the cutter. Ask assistant for help if you need it. Notice that after finishing the end of the fiber it easily breaks again if you 7

8 handle it carelessly. If the end of the fiber is not prepared well enough the fusion splicer may advice you to finish it again. 5. After finishing the fibers place them in the fusion splicer and begin the splicing. Ask an assistant to advice you to place the fibers in the splicer properly. Make sure that you have threaded a heat-shrinkable jacket in the fiber before splicing. When you have placed both fibers in fusion splicer follow the instruction given in screen and perform the splicing. After splicing find an attenuation of the splice from the screen and write it down in your measurement report. You still have to heat the shrinkable jacket to protect the splice. When heating is done you can move to part two of this exercise. Measurement 2: Measuring fibers 1. We begin the part two by measuring the length of the spliced fiber. Connect fiber to the OTDR with FC/PC connectors and find the graph from the screen of the OTDR. Every time you connect fibers remember to clean the connectors well. 2. Locate the both ends of the fiber and try to find the splice from the graph of the OTDR. You can adjust db Scale and Distance Scale to get better view. To find the value of certain spot at the screen use a Marker-function. After finding the splice and the both ends write down required values to the measurement report. It is easier to get good measurement results if you use the Average-function of the OTDR. A Delta-function may also help you to define the length of the fiber. 3. To specify the attenuation of the fiber there is Delta-function in the OTDR. By the Deltafunction you can calculate the slope of the graph and so find the coefficient of attenuation. For calculating the slope two points - x 1 and x 2 - are needed (Fig 4). First move a marker to point x 1 and press the Delta-button and then do the same with the point x 2. Then you can find the value of the coefficient of attenuation from the screen (db/km). Write it down to the measurement report and answer questions concerning the part 2 of the measurements. Measurement 3: Measuring variable attenuator 1. Connect the variable attenuator and the third fiber to the setup used in the part 2 (Fig 9). Always remember to clean the connectors carefully when you are using optical fibers or components. Use FC/PC connectors. Set the wave length of the OTDR to 1300 nm. 2. Set value 0 db to the variable connector and adjust the screen so that you can locate the variable connector and the third fiber from the graph of the OTDR. Experiment how different values of attenuation affect the graph. At the end set the variable attenuator back to value 0 db and answer questions 3.1 and 3.2 in the measurement report. 3. Finally, we are going to measure the real attenuation of the variable attenuator with values 0, 2 and 4 db. For measuring the attenuation of the component there is a Splice Loss-function in the OTDR. You can use either manual or automatic calculation and you can switch between those two options by pressing Splice Loss-button 5 seconds. Calculations are done as explained in chapter 2.2. Ask an assistant for advice if necessary. After measuring attenuations answer the question in measurement report and return it to your assistant. 8

9 MEASUREMENT REPORT Date: Group number: Student IDs:,,, Part 1 Splicing 1.1. Attenuation of splice: 1.2. Is that good value for an optical splice? Attenuation of connection done by connectors is usually 0,5-1 db and attenuation coefficient of optical fiber 0,3 db/km for example. Part 2 Measuring fibers 2.1. Length (km): Length of the spliced fiber: Location of splice: 2.2. Attenuation coefficient (db/km): Attenuation: Wave length: 1300 nm 1550 nm 2.3. Why is the attenuation of the fiber different with different wavelengths? 2.4. It may be difficult to find splice from the graph, but could the connection done by connectors be located? Part 3 Measuring variable attenuator 3.1. Is attenuator a reflective or a nonreflective component? 3.2. Does the variable attenuator create attenuation with value 0 db? Why? 9

10 3.3. Attenuation of the variable attenuator: Shown value 0 db: 2 db: 4 db: Real attenuation (db) 3.4. Is there something wrong in the scale of variable attenuator? 10