1. science
2. physics
3. optics

­ 1 ­ Manual for Fiber Optics Experiments Project QCC TechASCEND This material is based upon work supported by the National Science Foundation under Grant No. 0206101 Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
­ 2 ­ Table of Contents 1. Introduction to Fiber Optic Cables—Stripping and Cleaving Page 3 2. Fiber Optic Connectors 6 3. Fiber Optic Test Set 10 4. Fusion Splicing 12 5. Wavelength Division Multiplexing 23 6. The Optical Time Division Reflectometer 29 7. Fiber­optic Star Coupler 37
­ 3 ­ Introduction to Fiber Optics Cables—Stripping and Cleaving Optical fiber has changed telecommunications all over the world. Because a single optical fiber can carry really huge numbers of telephone conversations, long distance calls that used to be very expensive have become cheap enough for many people to make them often. How optical fibers carry multiple conversations is fascinating. How does the long­distance digital telephone system work? When you talk into a telephone transmitter, the sound waves from your voice move a diaphragm on the transmitter. This causes electricity to flow. The electric signal varies with time in a way that imitates the way the air pressure varied in the sound wave from your voice. In the modern telephone system, the (changing) strength of this electric signal is measured several thousand times each second. The measured strength is then made into a code. If you knew how to interpret this code, you could get the strength of the electric signal as time went forward during the conversation that caused the signal. This code is what is sent through the telephone system. In the telephone system, the person at the other end of the conversation does not need to know the code. Instead, the machinery at the receiving end of the system reads the code and converts it back into an electric signal that reproduces the original electric signal from the transmitter. This electric signal goes to a speaker, where it causes the air to vibrate just the way it did when the person on the other end talked into the transmitter. The listener then hears a faithful reproduction of the original conversation. Amazingly, all of this takes place so fast that the listener thinks that s/he is hearing the original talking as it is happening. Where does fiber optics enter the long­distance telephone system? In a fiber optic system, the code that is sent through the system is not electrical. Instead, the electrical code is changed into a code based on laser light. It is this light that travels through a fiber optic cable nearly to its final destination. Only when the signal gets quite near its destination is it changed back into an electric code and from that into the electric signal that operates the speaker in the receiving phone. Optical fiber For much of modern telecommunication, the path over which the signals travel is optical fiber. Optical fiber for most purposes is made of a very special kind of glass that is drawn into a very thin, long fiber. In some ways, this is similar to the fiberglass that is used for insulation in homes. Unlike fiberglass, however, optical fiber is made of a much different kind of glass and comes in lengths that may be many kilometers long. Standard optical fiber is shaped like a very long thin cylinder. In the center of the cylinder there is a core, and surrounding the core is a layer called the cladding. Both core and cladding are glass; they are slightly different types, however. A cross section of the fiber is shown in Figure 1.
­ 4 ­ cladding core Figure 1. Fiber cross section In Figure 1, the diameter of the core is half the diameter of the cladding. This is typical of one type of fiber. In a slightly different type of fiber, the core diameter is 0.4 time that of the cladding. Both of these are called multimode fiber. A third type of fiber is used for very long distance telecommunication. Its core diameter is about 1/10 the diameter of its cladding. This type of fiber is called single mode fiber. There is one thing about Figure 1 that is very misleading—its diameter. The outside diameter of all standard real optical fiber is 125 microns. A micron is 1/1,000,000 of a meter, or 1/1000 of a millimeter. This means that the outside diameter of standard optical fiber is only 1/8 of a millimeter. That is really small. In fact, it is about the same as the diameter of a single hair of a typical human being. Furthermore, the light used in the telecommunication system travels in the core. The cladding is necessary to keep the light in the core (and to make the fiber stronger and easier to handle). It is quite amazing that the diameter of the light­carrying part of single mode fiber is about 1/10 the diameter of a human hair. We will examine two samples of fiber, one multimode and one singlemode, using a microscope. Notice the difference between the two types of fiber. Stripping and cleaving optical fiber We will also learn some skills necessary for working with optical fiber. Fiber comes with a thin plastic coating (called the buffer) to protect it. Before we can join two fibers together, we must first remove (or strip) the coating. Although you can actually strip fiber with an Xacto knife, it is much easier to do it with a fiber stripping tool. To strip with a knife takes practice. Use a very sharp knife, such as an Xacto knofe. The plane of the blade should make a small angle with the fiber, not over about 20 degrees. Optical fiber Knife blade
Figure 2. Proper positioning of knife for stripping optical fiber. You have to press hard enough to remove the plastic coating (buffer), but not hard enough to break the fiber. Since fiber is very thin, this takes lots of practice. Ask you instructor to demonstrate the technique. Each group of participants will have a commercially available fiber optic stripper. As you will discover, this makes the task of stripping much easier. You should strip between 1/4 inch and 1/2 inch of the plastic coating each time you use the tool. Before you strip another 1/4 ­ 1/2 inch, you need to brush off the blade with the brush provided. If you do not do this, the coating will accumulate on the blade of the tool, and it will stop stripping the fiber. For most purposes, you will need to strip between 1 and 2 inches (2.5­5cm) of the buffer. ­ 5 ­ It is important to keep the work area clean and orderly. This is not just for appearance. A clean, orderly work area improves safety. Optical fiber is made of glass. Since it is very thin, little pieces of it act like rather nasty splinters. If they get into your skin or eyes, they can be painful and dangerous. In many cases, the next step in preparing a fiber for several processes is to cleave the fiber. The purpose of cleaving is to prepare the end of the fiber so that it makes a very nearly perfect right angle with the cylindrical body of the fiber and that this end face is nearly perfectly smooth. You might think that the only way to achieve this would be to polish the fiber end while holding the fiber at a right angle to the polishing surface. Although this polishing method works (and is sometime necessary), for most applications a much simpler procedure is equally good. This procedure is called controlled fracture. To cleave a fiber using controlled fracture, we put a little stress on the fiber and make a very light scratch on it. When this is done correctly, the fiber will split, leaving ends that are very smooth and are perpendicular to the length of the fiber. Unfortunately, if you don't do this correctly, you will crush the fiber, and the end will be jagged. To use a hand cleaver, bend the optical fiber around your index finger. (This puts the needed stress on the fiber.) Scratch the fiber lightly by gently touching it with a diamond, sapphire, or tungsten carbide tipped hand cleaver. Your instructor will demonstrate the technique. Please be sure to dispose of the small pieces of fiber that are left on the cleaver in the red­covered fiber disposal units that should be on your desk or on the tray of equipment you will receive. You should practice as many times as you can, examining the results with the microscopes that will be available for your use. Your instructor may also show you how to use the semi­automatic cleavers that are in the laboratory. These devices produce an excellent cleave more than 95% of the time. Unfortunately, they are quite expensive. (They cost nearly $1500 each, as of 2001.)
­ 6 ­ Fiber Optic Connectors The fiber part of the fiber optic telecommunications system is made up of lengths of optical fiber that must be connected to each other to provide pathways for the optical signals to get from where they start to where you want them to go. Although the connections with the least loss of the signal are permanent fusion splices (in which the ends of two glass fibers are melted together), there are many situations in which you do not want a permanent connection. For example, you might want to connect a television camera to one TV set at one time and another TV set at another time. It would be very inconvenient to have a permanent connection that you could not change without physically breaking the connection to one set and installing another permanent connection to the other. When you want to have a connection that can be changed conveniently, you use fiber optic connectors. Common Connector Types There are quite a few types of connectors on the market at this time. You might still find SMA connectors on older equipment. They come in two styles (called 905 and 906), but are very rarely used nowadays. Current single fiber connectors include FC (sometimes called FC­PC), SC, and ST types. One type of dual fiber connector is specified in the FDDI (Fiber Distributed Data Interface) documents. Each of these connector types must meet very tight specifications. There are many other connector types, but these are among the most common. What Makes a Connector Good? To be useful, a connector has to have certain properties. Among them are the following:
1. A connector has to allow as much light to get through as possible. 2. A connector has to be rugged. 3. A connector should allow the same amount of light through each time it is disconnected and reconnected. 4. A connector should be easy and simple to install. 5. A connector should be inexpensive. It is not easy to meet these requirements. For example, the better the quality of the connector, the more likely it is that it will be expensive. People who design connectors must make compromises to get the best combination of properties for the least money. Our experiment. We are going to install a pair of ST connectors onto a length of jacketed fiber optic cable. We will use cable that is 3mm in outside diameter. We will use several different tools to remove the various protective layers that surround the fiber in the cable. We will then attach the connector ends, polish the fiber, and test the completed connectorized cable. The connectorization and polishing will take several sessions. Testing the cable will take another.
­ 7 ­ TERMINATION PROCEDURE­­MOLEX ST Connector # 86010 Ferrule/Connector Head Twist on Nut Boot TEMPLATE Figure 1. Template for stripping cable It is very important to WORK CLEAN if you wish to succeed in preparing a fiber optic cable and installing connectors on each end. A clean work area is less confusing and much SAFER. Please dispose of all fiber scraps as well as all pieces of cable jacket, buffer material and Kevlar as soon as possible. 1­ Slide the boot and Kevlar retention nut onto the cable as shown. (Figure 1a) The non­threaded end of the nut must face the boot. 2­ For jacketed cable, strip the outer jacket using the Template (Figure 1) as a guide and use the jacket stripping tool. (This has a red handle and a wire cutter. Use the 1.3mm cavity.) Use the same tool with the .4 mm cavity to remove the white tubing. On the template, the section marked “Buffer” is the white tubing. 3­ Using the buffer stripping tool (No­ NIK), precisely strip the buffered fiber to the length on the Template. Use a brush to remove debris from the tool. A clogged tool may break the fiber. Leave the yellow Kevlar strands as shown. Figure 1a Figure 1b Jacketed fiber before (a) and after (b) stripping NOTE: ­While holding firmly onto the buffer, it is recommended that you strip small lengths, approximately 1/8 inch of buffer at a time. ­Verify all strip lengths with the template. Snip off any excess fiber with the scissors. ­Carefully dispose of the excess cleaved fiber to avoid any possible injury. ¼ inch boxes
­ 8 ­ 4­ Slowly insert the fiber into the connector until it comes out of the other end. A slight rotation of the connector will help. (Figure 2) Insert fiber into connector Then pull back as shown 5­ Pull the fiber back out. Face of 6­ Slide the protection sleeve over the ferrule. 7­ Carefully inject the epoxy into the back of the connector until it appears on the face of the ferrule. Apply a drop of epoxy to the threads of the connector before screwing on the rear nut. Re­insert the fiber into the connector. (Figure 3) 8­ Insert the connector into the plastic assembly fixture (which may be red or black) and holding it firmly screw the rear nut onto the connector, capturing the Kevlar strength members in the threads. Remove the assembly fixture. Be careful not to break the fiber. (Figure 4) Ferrule Apply epoxy here Figure 2 Re­Insert fiber into connector Figure 3 Assembly Fixture NOTE: ­Clean any epoxy from the side of the ferrule with lint free tissue and alcohol. Screw On Nut ­Wash epoxy immediately. from your hands, 9­ Put the connector with the protective sleeve into one of the oven holes for 5 minutes at 100 o . Let it cool; then remove the protection sleeve. Figure 4 10­ Holding the cleaving tool (in plastic tube) with the trigger upwards, slowly insert the ferrule with its protruding fiber into the slot of the cleaving tool. Insert the connector until the ferrule end face stops against the stop screw.(Figure 5) ­Release the connector and gently press the trigger to cleave the fiber. ­Remove the connector from the tool. ­Safely dispose of excess cleaved fiber Figure 5
­ 9 ­ 11­ Insert the connector into the polishing tool. Using the glass plate mount a sheet of BLACK lapping film. Put a few drops of water on the film. Perform 10 figure “8” motions over the entire length of the lapping film. (Figure 6) Virtually no pressure should be applied to the tool at the beginning of this step. Lapping Film Connector Gently clean the contact with a lint­free tissue and alcohol between each polishing step. 12­ Mount a sheet of GREEN lapping film and perform at least 15 to 20 figure “8” motions over the entire length of the lapping film. Glass Plate 13­ Slide the boot up until it fits securely over the rear nut. Figure 6 14­ Remove the connector from the polishing tool and carefully clean the ferrule with lint­free tissue and alcohol. Make sure that it is dry. Use the Video Fiber Microscope (or a 100X microscope) to inspect the quality of the polish. No scratches or cracks should be visible. (Figure 7) Polishing Tool Chip or Crack Scratch NOTE: DO NOT put a wet connector into the Video Fiber Microscope. NOTE: If you use a 100X microscope, store it in a closed position when not in use. This will insure longer battery life. GOOD BAD Figure 7 Diagrams and some text used with permission of Molex Corporation.
­ 10 ­ Fiber Optic Test Set The simplest test we can make on a fiber optical system is to see whether light is getting through and how much is lost. The instruments used for this are called an optical source and a receiver. Together, they form a fiber optic test set. Most fiber optic test sets work with several different wavelengths. The usual choices are 850, 1310, and 1550 nanometers (nm). The receivers can be set for any one of these. The sources you will use are dual wavelength sources. They can provide signals at 1310 nm or 1550 nm. You might wonder why we need several wavelengths. The basic reason for this is that light of different wavelengths shows different losses in the same cable. In addition, you must “tell” the receiver the wavelength you are using because the detector reacts differently to different wavelengths. To use a test set, you connect the system being tested between the source and the receiver. To do this is not very complicated. You first make sure that the correct adapter is connected to each instrument. Next, connect a commercial cable to the chosen wavelength output of the source. Connect the other end to the input port on the receiver. Turn the source and receiver on (by pushing the appropriately marked button). Press either the WAVELENGTH or the λ button (depending on your meter) until the desired wavelength appears on the display. If your receiver has a zero button, you may zero the meter. To do this, just press the ZERO button. You next connect the cable (or other system) that you are testing between the commercial cable and the receiver. To do this, you will need to use a bulkhead connector. This is a metal piece that has two receptacles, one for the original cable and the other for the system you are testing. Check to make sure that the wavelength at which you are operating is the wavelength you want. Choose the units that you want to use. On some of the meters, your choice will be between dB, dBμ, and dBm. On others, your choices will be dBm or dB. Choose dBμ if it is available. If you have zeroed your meter, the readings will be in dB unless you change to dBm. Finally, you read the result. That’s all there is to the operation. There is, however, the matter of interpreting the reading. If you choose to get results in dBμ or in dBm, you will need to subtract the reading with the cable you are testing from the reading with just the commercial cable. This difference will tell you the dB loss in the cable being tested. If you have zeroed the meter with the commercial cable, you can skip this step. It will be helpful if you understand the use of dBm, dBm, and dB in the study of light. The section that follows should be useful. Decibels, dBm, and dBm Optical power is often expressed in units called dBm. These units tell us the optical power in a convenient way.
­ 11 ­ A microwatt is 1 millionth of a watt. Microwatt is abbreviated as mW. An ordinary 100 watt light bulb produces about 2 million microwatts of light power. A table of some equivalents may be handy. 0 dBm = 1 microwatt (1mW) 3 dBm = 2 “ 6 dBm = 4 “ 9 dBm = 8 “ 10 “ = 10 “ ­3 dBm = 0.5 microwatts (0.5 mW) ­6 “ = 0.25 “ ­9 “ = 0.125 “ ­10 “ = 0.1 “ If you add 10 to the number of dBms, you multiply the power by 10. So, for example, since 6 dBm is the same as 4 microwatts, 16 dBm is the same as 40 microwatts. In a similar way, if you subtract 10 from a number of dBms, you divide the power by 10. (The actual definition of dBm is given by the following equation: Power (dBm) = 10 log10 (Power in microwatts).) A similar unit is called dBm. The equivalents shown above for dBm are correct for dBm except that wherever there is a “m” you substitute an “m.” “m” is the symbol for milli. For example, 3 dBm = 2 milliwatts instead of 2 microwatts. The prefix micro means 1 millionth, while the prefix milli means 1 thousandth. You may also sometimes see plain dB without a m or an m after it. This stands for decibels. When it refers to sound, it has a special meaning that we will not discuss here. When it refers to light, however, it is related to a ratio of powers. If, for example, we say that the difference between two light signals is 6 dB, it means that the power of the stronger signal is 4 times as strong as that of the weaker signal. Similarly, if the difference is 9 dB, the ratio is 8:1. As you can see, these ratios are the same as those for dBm and dBm, except that dBm and dBm express the ratio between the measured signal and a standard, either 1 microwatt or 1 milliwatt. Discussion Questions 1. What is the power of a 23dBm optical source? 2. What is the power of a –16dBm optical source? 3. A certain source produces 18 microwatts of power. Another source produces 72 microwatts. What is the dB difference between these two sources?
­ 12 ­ Fusion Splicing If you want to join two lengths of optical fiber together with the least possible loss of optical power, the method to choose is fusion splicing. In fusion splicing, the cores and claddings of the two fibers are actually melted together. (The core is the central part of the fiber. The light signal travels almost completely in the core.) Because the core has a very small diameter, it requires a very precise instrument to join the cores in a way that lets the most light pass through the point of joining. (For single mode fiber, the diameter is about 1/100 of a millimeter. For multimode fiber, the diameter is 1/16 of a millimeter.) For comparison, a human hair is about 1/8 of a millimeter in diameter. Because the diameter of the fiber is so small, it is difficult to line up two fibers so that their cores line up nearly perfectly. If the cores are not lined up just right, the light will not pass through. In addition, the heating must be very exact. If you don’t heat the two fiber ends enough, they will not melt together. If you heat them too much, they will droop and will not line up properly. Fusion Splicers Manual splicers It is possible to buy manual fusion splicers. In these, the operator has to line up the fibers by hand, using precision fiber positioners. These are thumbwheel­operated devices that move the fibers by very small, very finely controllable amounts. When the operator rotates the thumbwheel through a fairly large angle, the fiber moves a very small distance. It takes considerable skill and practice to use these manual splicers. Automated splicers Nowadays, most companies use automated fusion splicers. When you put two properly stripped and cleaved fibers on these devices, they automatically line up the fibers, fuse them, and measure the dB loss at the splice. You will use an automated splicer in this experiment. However, it is possible to position the fibers manually with this device, and you will do that after you do the automated procedure. The automated fusion splicer is designed to work with many different types and combinations of fibers. In the procedure section, you will find out how to select the correct mode for the splices we will make. Procedure The procedure for making a fusion splice using the Ericsson model FSU975 fusion splicer is straightforward, but it does involve a number of steps. Stripping the Fiber To avoid having to measure the length of the stripped fiber, we use a special stripper for optical fiber from Amherst Fiber Optics. (Using this tool is not essential, but it is
­ 13 ­ convenient because this stripper strips just the right length for the cleaver we use.) In our experiment, we will use unjacketed optical fiber. (If we were using optical fiber cable, we would need to remove an appropriate length of the jacketing and the Kevlar protective strands.) Put the coated fiber into one of the pair of clamps provided with the stripper. The grooved end of the clamping device should face the pliers­like stripping tool, the end of the clamping device should be flush against the stopping screw on the stripper, and the fiber should extend out over the end of the stripper’s clamping device. Guide the fiber through the hole and into the groove on the other side. The end of the fiber should rest against the end of the groove. Grasp the clamping device firmly with your thumb exerting pressure on the actual clamp so that the fiber cannot slip within the clamp. Squeeze the yellow­handled stripper, and pull the clamping device (with the fiber in it) straight back along the track away from the stopping screw. This will strip the fiber end for a length just right for the cleaver. Thoroughly clean the bare fiber using an alcohol­ soaked wipe. Cleaving the fiber Before you put the stripped fiber on the cleaver, make sure that the tensioning lever (on the far right of the Ericsson model EFC11 fiber cleaver) is horizontal and the blade release lever (on the far left of the cleaver) is up. When you have made sure of this, carefully lift the clamp with the clamped fiber and put it on the cleaver. Put the fiber holder with the clamped fiber over its position on the cleaver and lower it into place. There is a screw that stops the holder at the correct position. Next, lock the right hand clamp (on the cleaver) down. Move the tensioning lever down. This puts a small amount of tension on the section of fiber between the clamps. Next, move the blade release lever down. You will see the blade move in to touch the fiber. If all goes well, the fiber will cleave. Splicing the fiber Description of the splicer As Figures 1 and 2 below show, a modern automated fusion splicer has many parts and controls. Figure 1 shows the top of the Ericsson fusion splicer. It is copied from the manual. Please disregard the page numbers given on the diagram. They refer to the manual. Toward the top of the picture, you see the parts that hold the fibers to be joined by splicing and the region where the splicing actually takes place. The left and right V­ grooves are used to align the fibers. The left and right fiber clamps hold the fibers in the grooves. The electrode block holds the electrodes. These electrodes produce the small electric arc that heats up the fibers so they will fuse. The safety shield covers the electrodes and the fiber during the actual splicing to protect the eyes and bodies of the people doing the splicing.
­ 14 ­ Figure 1 Figure 1 shows the top face of the fiber with each of the important features labeled. Figure 2 Figure 2 shows the buttons in greater detail. You should examine these figures before you proceed with the experiment. The functions of the appropriate buttons are discussed below as we come to them in the procedure.
­ 15 ­ Now prepare the FSU 975 splicer to receive the cleaved fiber. The safety shield may be closed. If it is, you will see “DANGER HIGH VOLTAGE” in white on its top. Open the shield by locating the button­like knob on the right side of the raised part in the center of the top of the splicer and pushing it towards you. There are two fiber clamps that are covered by the shield, each with a lever to open and close it. The levers should be pushed backwards (toward the top of the splicer) so that they are horizontal. When the levers are properly positioned, you will see white grooves into which the fiber will go. Carefully lift the fiber still clamped in its holder off the cleaver and onto one side of the Ericsson splicer. Do not slide the fiber into the groove on the splicer. Instead, tilt the end of the fiber up slightly and tilt it into place without sliding. Remove the cut off end of the fiber from the cleaver to clean it out for the next cleave. Repeat the above procedure for the other length of fiber that you want to splice (except that you put the fiber in its holder on the other side of the splicer). Be sure to insert the fiber in such a way that the two clamps open in the same direction. If you do, the V­grooves in the two clamping holders will be on opposite sides. Now that you have the clamped fibers on the splicer, it is time to turn on the splicer. To do this, press the ON/OFF button in the upper left hand corner of the splicer. On the LCD monitor screen (Refer again to Figure 1.), you will see two lines of print. The upper line should say “FSU 975 AUTO MODE.” The second line should start by saying “PARAMETER UPDATING. . .” Once the updating is completed, the line should read “INSERT FIBER” and “CLOSE SAFETY SHIELD.” If you have positioned the fibers correctly, you should see a (greatly magnified) image of them on the screen. If you do not, you will need to lift the clamps and try to reposition the fibers. Close the safety shield. To do this, slide the button­like knob on the right side of the housing toward the top of the splicer. (See Figure 3.) The “DANGER HIGH VOLTAGE’ lettering should be visible. Figure 3 After you close the safety shield, the message line on the monitor will change. The first message will tell you which splicing program was used last. The other messages will tell
­ 16 ­ you about further stages in the splicing process. These include “AUTO ALIGNING/FUSION” and “PRESS FUSE BUTTON.” Figure 4 Before you do anything else about splicing the fibers, you should check the fibers on the monitor to make sure they are clean and cleaved well. (See Figure 4 for pictures of defects in the cleaning or cleaving that require fixing.) If you see any defects, take the fibers off the splicer and out of the clamps and re­prepare them. You can view the fiber from two different directions by pressing the VIEW button (located to the right of the monitor screen). Be sure to check both views. You can adjust the focus of the picture on the monitor by pressing the + button or __ the button. The message field (bottom line) on the splicer now says “PRESS FUSE BUTTON.” Do NOT press this button yet!! Selecting the splicing mode and program First, you need to make sure that the correct splicing mode and program are selected. The splicer defaults to auto mode when you just turn it on. For this part of our experiment, that is fine. But we need to make sure that we are in the correct program for the fibers we will fuse. We will be fusing two multimode (MM) fibers. Program 04 is the correct one for this combination. To verify that the splicer is set for this mode (or to change the programmed mode if necessary), first press the button. When you do this, both lines on the ENTER
monitor will change. The top line will display the current splicing program. The bottom line will ask you if you want to change it by displaying “NEW PROGRAM?” ­ 17 ­ NO
If the program displayed is P04, you should press the button and proceed to the section headed “Auto mode splicing” below. If the program displayed is not P04, you need to change it. Changing the program To change the program, press the button. Next, press the button. At the top YES VIEW of the monitor screen, you will see a list of programs. Using the up arrow and down arrow buttons in the right side group of buttons, scroll to the line that reads NORMAL MM + MMP04. Press the ENTER button. The display should now read NORMAL MM + MM NEW PROGRAM? You should press the NO button. The display should now read NORMAL MM + MM EDIT PARAMETERS? NO Press the button again. The display screen should now show the two fibers and FSU 975 AUTO MODE NORMAL MM + MM. Auto mode splicing Now that the fibers are in place and the safety shield has been closed, the splicer will display a series of messages on the bottom line that tell you it is ready. It should read AUTO ALIGNING/FUSION PRESS FUSE BUTTON. Fuse To start the automatic splicing process, press the button. The splicer will automatically rough align the fibers, prefuse them, focus the view, align the fibers (a fine alignment), and fuse them. A series of messages will appear on the display to let you know where the splicer is in its sequence. A typical series of messages reads as follows: ROUGH ALIGNMENT PREFUSION WILL START ROUGH ALIGNMENT ALIGNING FIBERS ­ 18 ­ SPLICING WILL START SPLICING If the splicer cannot carry out some part of the automatic sequence, it will print out a fault message on the bottom line of the display. These are usually easy to understand, and the way to fix them is usually pretty obvious. If you get a fault message and find it difficult to figure out what to do, ask the instructor for help. If all has gone well, you will not get a fault message. In that case, you are ready for the next step. Automated splice evaluation You will need to wait for a little bit while the splicer completes the splicing. (While this is going on, the screen will continue to read, “SPLICING.”) After the splicing sequence is completed, the splicer automatically estimates the splice loss. While it is doing this, the display reads FSU 975 AUTO MODE CHECKING SPLICE. When it has finished estimating the loss, the splicer displays it by printing ESTIM. LOSS: #.## dB NEXT PICTURE, VIEW, where “#.##” stands for the number of decibels of loss across the splice. The splicer will offer you the chance to fuse the fiber again, in case the loss is too high. The screen will read ESTIM. LOSS: ##.# Db FOR RE­FUSING PRESS FUSE BUTTON Fuse
To use this option, press the button again. Do not re­fuse the splice more than once. If you do, you will probably weaken the splice and/or increase the splice loss. Visual splice evaluation ­ 19 ­ The estimated loss does not give all the information about the splice. You should also examine the splice using the greatly magnified image on the display. You can view the splice from two different directions. One is shown throughout most of the splicing steps. You can look from the other direction by pressing the VIEW button. Figure 5 shows an example of a successful splice. The important thing to note is that the core and the outer edges form straight lines. If your splice does not look like the one in Figure 5, you will need to repeat the splicing process. NOTE: The white line you see in the hot images (stored when the fibers are splicing and accessed with the VIEW button) is the core. In the live images, however, the white line is NOT the core. It is just light shining through the fiber being focused by the round glass fiber. Figure 5 Remove the spliced fiber. Fusing single mode fiber Multimode fibers are used for communication over relatively short distances. To communicate over long distances (tens of kilometers), one has to use single mode fiber. For our next experiment, we will fuse two single mode fibers. With the automated splicers we are using, the procedure for fusing single mode fibers is very similar to that for fusing multimode fibers. All we need to do is change the program. The steps are as follows: Note that when you open the safety shield to remove a splice, the splicer automatically turns off. Get two lengths of single mode fiber from the spool. Prepare them by stripping and cleaving them just as you did for the multimode fiber. Put the fibers in their holders onto the splicer. Then you are ready to splice. First, turn the splicer back on. To do this, press the ON/OFF button in the upper left hand corner of the splicer. Next, press the button. To change the program, start by ENTER pressing the button. Next, press the button. At the top of the monitor YES ENTER
screen, you will see a list of programs. Use the up arrow and down arrow buttons in the right side group of buttons to scroll to the line that reads NORMAL SS + SS P01. ­ 20 ­ Press the ENTER button. The display should now read NORMAL SS + SS NEW PROGRAM? You should press the NO button. The display should now read NORMAL SS + SS EDIT PARAMETERS? NO Press the button again. The display screen should now show the two fibers and FSU 975 AUTO MODE NORMAL SS + SS. You should now follow the instructions given (several pages) above starting with the section headed “Auto mode splicing.” If you do several single mode splices, you will probably find that your estimated losses will be somewhat higher than you got for multimode splicing. This is not surprising, since single mode cores are much smaller in diameter than multimode cores are. Manual splicing Sometimes it is useful to align fibers manually, not using the automatic capability og the splicer. To do this, you must change the splicer to Manual mode. All you need to do is press the button. The mode field will then read “FSU 975 MANUAL MODE. Mode When you want to change back to Automode, you just push the Mode button again. Prepare the fibers you want to splice just as you did before. After the two fibers are properly placed on the splicer, you are ready to begin. Be sure that you have the correct splicing program for the type of fiber you are splicing (P 01 for single mode or P 04 for multimode). If necessary, change the program following the instructions given above under “Changing the program.” The splicer will now display a series of messages telling you that it is ready. First, you should see READY FOR PREFUSING PRESS FUSE BUTTON Fuse
Do not press the button yet! The next step is to roughly align the fibers. By pressing the and buttons alternately for both the right and left fibers, you can line up the outer edges of the fibers. When you think the fibers are lined up, start centering the gap and adjusting its size. To do this, bring the fibers toward the center of the monitor by using the and buttons for each of ­ 21 ­ the fibers. Your goal is to get a gap centered on the display screen and half the width (diameter) of a fiber. Next, press the VIEW button. Make sure that the gap is also correct when it is viewed from the other camera angle. Fuse
Now you should press the button. This will heat the fiber enough to clean off a small amount of residue from the fiber. If the fiber is too “dirty,” however, the prefusion will not cure the problem. After prefusion is finished, the splicer will tell you that it is waiting for you to complete final inspection and alignment by displaying the message READY FOR SPLICING PRESS FUSE BUTTON Before you press the Fuse button to splice the fibers, you should check the fibers. Use the button to switch between viewing angles. Look for dust or other problems VIEW with the fiber. If you see dust or any other problem, stop the splicing procedure and re­ prepare the fibers. After you have made sure that the fibers are clean and free of other problems, you can do the fine alignment. The first step is to set the space between the fibers (the “gap”). Us the and buttons to bring the fibers as close together as possible without overlapping them. The last movement you cause before you set the gap must be to move the fibers closer together. Briefly press the GAP button. The gap should be as narrow as possible, but still wide enough to let the fibers move freely up and down. After you have set the gap, do not press the and buttons. Use the and buttons to fine tune the alignment of the outer edges of the two fibers. Make sure that the two fibers line up straight across the screen. Do not forget to fine­tune the alignment from both camera angles. Remember that you change the angle by using the VIEW button. After you finish the fine­tuning, but before fusion, the alignment should be as follows: The outer edges of the fibers line up, and the gap between their end­faces is centered on the LCD monitor screen and is as small as possible while still allowing the fibers to move up and down. ­ 22 ­ When you satisfied with the fiber alignment, press the Fuse button and start fusion. The splicer will follow the last splicing program you chose. As the splicer operates, the message field will read Splicing. . . Now you need to check the splice. Review the “Automated splice evaluation” and “Visual splice evaluation” sections above for the procedure. If you were going to use the spliced fibers in a real communications system, you would need to protect the splice with a heat shrinkable sleeve. We will not do this, but the process is very simple. Figures and some text excerpted with permission of the Ericsson Corporation. ­ 23 ­ Wavelength Division Multiplexing Introduction A little (fairly ancient) history. The idea of sending more than one message at a time over a single carrier dates back to the telegraph in the early 1850’s. Since it was very expensive to string even one cable over long distances, this idea (which is called multiplexing) was very important. Modern telephone systems transmit thousands of messages at the “same” time over a single cable. There are several methods that are used to send multiple messages. All of them started in telegraphy. The method we will use works on the same principle as a method in which telegraph messages were sent using sounds of different pitches. The pitch of a sound, treble or bass, depends on its frequency. The way you can use different frequencies to send several messages together is to send the telegraph (Morse) code for one message at 240 Hz, one on 360 Hz, another on 480 Hz, and so forth. These signals can be mixed together at one end of the cable and separated electronically at the other end. Why bother? You might wonder why the telegraphers needed to go to the trouble of using different frequencies of sound to send different messages. Why not transmit all the messages at the same frequency, and just transmit faster on that frequency? At least at the beginning of telegraphy, the answer was simple. Human operators were transmitting and receiving the signals, and they could only go so fast. Eventually, machines were used to do the transmitting and receiving, but even they had a maximum rate that was considerably lower than the ability of the telegraph wire to carry information. Fiber optic communication systems. Instead of sound, fiber optic communications systems use light to carry messages. When different frequencies of visible light reach our eyes, we see different colors. Thus, the color of light is similar to the pitch of sound. Both depend on frequency. This can lead us to a method for multiplexing optical signals. If we send messages in code using light, we can send several messages at the same time by sending them in different colors. Why bother? You might wonder why it is necessary to use different frequencies of light to maximize the information carrying capacity of optical fiber. Why not use a single frequency of light and send code at a very fast rate by turning the laser light on and off very rapidly? (That is, why not use a large number of very short light pulses made up of light of a single frequency?) The answer is very similar to that for telegraphy. Even with the best modern electronic transmitters and receivers, we cannot turn the light from the laser transmitters off and on nearly as fast as we would need to in order to send the maximum amount of information
­ 24 ­ possible through an optical fiber. Furthermore, even if we could send the maximum amount of information, there are no receivers that could “read” the information that fast. By sending the information at many different frequencies, we can transmit much slower on each of the frequencies and still send the same amount of information. We combine the different­ frequency signals and send them over a single fiber. At the other end of the fiber, we separate the different frequencies and send each of the frequencies to a different receiver. In that way, no transmitter has to operate at an excessive speed, and no receiver has to receive at such a speed. Real fiber optics communications systems do not use visible light. Infrared light works better. But when you use these systems, you cannot see anything. You need detectors that react to the infrared light. For our experiment, we shall, therefore, use a system that uses visible light and sends it through air, rather than through optical fiber. Tun­ able HeNe Diffraction grating Red HeNe Beamsplitter or dichroic mirror
The setup is shown in the diagram. Two lasers produce beams of different colors. The beams can be individually cut off or allowed to pass by a beam modulator in each path. This allows us to send two coded messages, one for each beam. The beam modulators can be very simple. A piece of cardboard will do. For very fast beam modulation, you would need fancier equipment. We will use cardboard. After the beams are modulated (that is, turned on and off rapidly), they must be combined. (This process is called multiplexing, just as it was with sound.) In our apparatus, we combine the beams using a beamsplitter. This is a device that allows about half of the beam aimed at it to pass through and reflects the other half. If we put the laser beams at right angles and the beamsplitter at 45 0 to both beams, the straight­through beam from one laser will combine with the reflected beam from the other. In this way, we have combined the two messages so we can send them at the same time over the same path. In a more realistic system, we would send this combined message over an optical fiber so that atmospheric ­ 25 ­ conditions would not mess up the signals and so that other people could not easily read our messages without our realizing that they were. To combine signals with different frequencies for transmission through an optical fiber, you need a special device called a wavelength division multiplexer. Our setup can be thought of as an over the air wavelength division multiplexer. When the combined signal reaches its destination, we must separate the two beams so that we can once again read the messages. There are several ways to do this. We have chosen to use a diffraction grating. This device reflects different colors in different directions. After the combined beam reflects from the grating, the different colored beams come off at different angles, so we can examine them individually. This process is called demultiplexing. WARNING: Throughout this lab, never look directly at any of the laser beams!! Procedure 1. 2. 3. 4. Put the beamsplitter mount at a convenient height and position. Mount the beamsplitter on it. Set up the multicolor helium neon laser so that its beam is horizontal and at the height above the table at which you have the beamsplitter. The laser should already be on. Keeping your eyes away from the opening, open the beamstopper on the front of the laser. Adjust the setting of the micrometer on the rear of the laser to the reading necessary to get the color you need. Ask you instructor for the appropriate setting. Adjust the position until the beam hits the beamsplitter. Locate the part of the beam that passes through the beamsplitter and the part that is reflected by the beamsplitter. Turn the beamsplitter until these two beams are at right angles to each other. Use the laser power meter to measure the power in the laser beam before and after it is split: With the meter off, a. Set the meter for 20 milliwatts (mW). b. Turn the meter on. c. Remove the sensor head from its bracket, and place it in the beam before the beamsplitter. d. Record the maximum reading you get by moving the sensor head so all of the beam falls on the light­sensitive surface in the sensor. If the measured power is less than 2 mW, lower the setting of the meter. e. Record the maximum power reading for the transmitted beam and the reflected beam. f. Compare the sum of the readings in part (e) to that in part (d). Which one is bigger? Suggest a reason for your result. Stop the beam of the red He Ne laser using the sliding beamstopper. Do not turn the laser off.
­ 26 ­ 5. 6. 7. 8. 9. 10. 11. 12. Set up the red He Ne laser at a right angle to the non­red one. Follow the procedure of part (2), but move the laser rather than the beamsplitter. Use the laser power meter to measure the power in this laser beam before and after it is split. Follow the same procedure as in (3) a­f except start with the meter at its 2 mW setting. Open the beamstopper on the non­red He Ne laser so its beam can emerge. You should have the reflected red beam and the transmitted beam of the second color moving along about the same path. The next step is to make these beams move along exactly the same path, as they would in a fiber optic system. Put a piece of white paper or cardboard very near the beamsplitter on the side where the beams should merge. Move the red light laser until you get the beams as close together as possible. The beamsplitter mount can be turned. It can also be tilted using the tilting screws with knurled tops. With patience, and by adjusting both the beamsplitter and the red laser, you can get both beams to travel together along a single path. Remove the white object. Final adjustment can be done by looking from an angle at the point where the combined beam hits a wall. You will probably need to make fine adjustments to both the laser and the beamsplitter. Use the laser power meter to measure the power in the combined beam. Start with the meter at its 20 mW setting. Record your reading. Mount the diffraction grating in the combined beam. Never touch the surface of the diffraction grating!! Handle it by its edges. A mount for the grating will be provided. Ask your instructor how to use it. Two beams will come from the grating, one red and the other the second color you are using. Measure the power in each of the separated beams. You can now send and receive two messages at the same time. Two members of each group send the messages. Each should use a piece of cardboard to ‘modulate’ one of the two beams by either stopping the beam or allowing it to pass. Each sender should write a short message and translate it into code. Choose any code you wish. You can use Morse code (the telegraphers’ code) or ASCII (American Standard Code for Information Interchange—the code used in computers) or make up your own. (Tables of Morse code and ASCII are at the end of this writeup.) For Morse code, use a long pulse of light for a dash (­) and a noticeably shorter pulse for a dot (.). Use a short light burst for a zero and a noticeably longer burst for a one in ASCII code. If you make up your own code, you are on your own. If you have a good sense of time, you can use a timed burst of light for a one and a timed dark period for a zero. This is closer to the way ASCII code is transmitted in a real telecommunications system. Two other members of each group (one for each separated beam color) should receive the messages. They should record the dots and dashes for Morse code or the ones and zeros for ASCII or for your own made­up code. Then they should decode the messages and compare them to the originals. Submit the coded and decoded messages, noting any errors. If you wish, you can time your messages. It might be interesting to compare your speed and accuracy with those of others in your group or in other groups.
­ 27 ­ Selected ASCII codes 00100000 SP (Space) 00100001 ! (exclamation mark) 00100010 " (double quote) 00100011 # (number sign) 00100100 $ (dollar sign) 00100101 % (percent) 00100110 & (ampersand) 00100111 ' (single quote) 00101000 ( (left/opening parenthesis) 00101001 ) (right/closing parenthesis) 00101010 * (asterisk) 00101011 + (plus) 00101100 , (comma) 00101101 ­ (minus or dash) 00101110 . (dot) 00101111 / (forward slash) 00110000 0 00110001 1 00110010 2 00110011 3 00110100 4 00110101 5 00110110 6 00110111 7 00111000 8 00111001 9 00111010 : (colon) 00111011 ; (semi­colon) 00111100 < (less than) 00111101 = (equal sign) 00111110 > (greater than) 00111111 ? (question mark) 01000000 @ (AT symbol) 01000001 A 01000010 B 01000011 C 01000100 D 01000101 E 01000110 F 01000111 G 01001000 H 01001001 I 01001010 J 01001011 K 01001100 L 01001101 M 01001110 N 01001111 O 01010000 P 01010001 Q 01010010 R 01010011 S 01010100 T 01010101 U 01010110 V 01010111 W 01011000 X 01011001 Y 01011010 Z 01011011 [ (left/opening bracket) 01011100 \ (back slash) 01011101 ] (right/closing bracket) 01011110 ^ (caret/cirumflex) 01011111 _ (underscore) 01100000 ` 01100001 a 01100010 b 01100011 c 01100100 d 01100101 e 01100110 f 01100111 g 01101000 h 01101001 i 01101010 j 01101011 k 01101100 l 01101101 m 01101110 n 01101111 o 01110000 p 01110001 q 01110010 r 01110011 s 01110100 t 01110101 u
­ 28 ­ 01110110 v 01110111 w 01111000 x 01111001 y 01111010 z Morse Code Alphabet The International Morse code characters are: A .­ B ­... C ­.­. D ­.. E . F ..­. G ­­. H .... I .. J .­­­ K ­.­ L .­.. M ­­ N ­. O ­­­ P .­­. Q ­­.­ R .­. S ... T ­ U ..­ V ...­ W .­­ X ­..­ Y ­.­­ Z ­­.. 0 ­­­­­ 1 .­­­­ 2 ..­­­ 3 ...­­ 4 ....­ 5 ..... 6 ­.... 7 ­­... 8 ­­­.. 9 ­­­­. Full stop .­.­.­ Comma ­­..­­ Question mark ..­­..
­ 29 ­ The Optical Time Domain Reflectometer Even though the highest quality modern optical fibers are extremely clear, they still scatter some of the light sent through and absorb some of it. This fact can be used to allow us to measure important facts about the fiber. When light passes from one substance into another substance, some of the light will be reflected. This comes about because in different substances light travels at different speeds. If an optical fiber has a smooth end that makes a right angle with its length, about 4 % of the light coming through the fiber will reflect at the end (where glass meets air). This can be used for necessary measurement. The OTDR (Optical Time Domain Reflectometer) The instrument we use is called an optical time domain reflectometer. For doing the kinds of measurements needed for testing optical fiber networks, it is the best instrument to use. The basic parts are as follows: 1. Pulsed laser. This produces short, very intense pulses of light in the near infrared. Typical pulses last between 10 and 100 nanoseconds. (A nanosecond is one billionth of a second, or 10 ­9 seconds.) Commercial instruments usually operate at about 850 nanometers (nm), 1310 nm, and/or 1550 nm. Some instruments can operate at more than one wavelength. 2. Detector. This produces an electrical output proportional to the strength of the light that strikes it. The detector must have a very fast response time. 3. Signal analyzer. This measures the signal from the detector and sends the measured value to an appropriate memory unit. 4. Microprocessor (or computer). This stores the measurement and, when instructed to do so, performs mathematical operations (such as averaging) on the measured values. It can also locate and identify any connectors, fusion splices, and breaks in the fiber. 5. Display. This displays on a CRT or other screen the results of the measurements and any indicated calculations. The display can show a graph of intensity of backscattered light reaching the detector as a function of distance along the fiber. It can also display results of more complex interpretations of the data performed by the microprocessor. 6. Printer and/or removable storage device. These make permanent records of the measured quantities. An OTDR sends a series of light pulses into the fiber we are examining. When the light pulse reaches a break in the fiber, about 4 percent of the light reaching the break reflects back through the fiber to the detector. The time between when the signal left the pulsed laser and when it reaches the detector is measured accurately. Since the speed of light in the fiber is generally known (because the manufacturer sends its value), the microprocessor can calculate how far the break is from the OTDR. But an OTDR can give us much more information than that. When light travels through an optical fiber, a small fraction of it gets scattered backwards as the light signal goes forward in the fiber. This backscattered light returns to the detector in the OTDR. By measuring the
­ 30 ­ backscattered light as a function of the time it takes to reach the detector, the OTDR can produce a graph of the strength of the signal going forward as a function of distance along the fiber. It can do that because the backscattered signal is a fixed fraction of the forward signal and because the measured time for backscattering is directly related to the distance along the fiber. Splices and connectors cause different patterns of reflected and scattered light, so an experienced user of an OTDR can identify these features and find out how far they are from the OTDR. Procedure 1. 2. 3. 4. Turn the power switch to ON. It is on the upper left corner of the front panel. If the screen is not lit to your liking, adjust the INTENSITY control. Power up the Data Logger by pressing its switch to the ON position,. Press the SETUP MENU button. On the CRT screen you will see a display with the title SETUP at the top. On the left are seven quantities (or parameters) that you can adjust. The values that are currently set are enclosed in rectangles. 5. Check with your instructor which settings are appropriate for the cable you are going to examine. If you are using the spool of fiber labeled with a 2.2 km sticker, the appropriate parameter values are as follows: WAVELENGTH FIBER RANGE REFR INDEX AVERAGING TIME UNITS LABEL DISPLAY FIBER LENGTH To be chosen in consultation with the instructor. 2.5 1.4600 MAX METERS ON SHORT FIBER The REFR INDEX value is particularly interesting. The abbreviation stands for refractive index. The refractive index (for which the standard symbol is the letter n) is defined by the following equation: . index _ of _ refraction = n =
speed _ of _ light _ in _ a _ vacuum c =
speed _ of _ light _ in _ fiber v Since the speed of light in a vacuum is a known constant (which equals 300,000 km/second), if we specify the value of the index of refraction n, we are really telling the OTDR the speed of light in the optical fiber we are using. The manufacturer of the fiber supplies this number with the fiber. Why does the OTDR need this information? The OTDR actually measures the time that passes between when it send out the original laser pulse and when the signal arrives back from each section of the fiber. Inside the OTDR is a computer that calculates the distance from which the signal came based on the time it took. But to do this calculation (which just uses distance = speed times time), the computer needs to know the speed of the light inside the fiber. To change a parameter value, first use the and (up and down) arrow buttons to the left of the SETUP MENU button. The parameter that can be adjusted will be highlighted by appearing in reverse video (black on green, instead of green on black). When the parameter you want to adjust is
­ 31 ­ highlighted, you use the large knob on the lower middle right to make the change. Turn the knob until a rectangle encloses the desired value. In the case of the REFR INDEX parameter, the rectangle does not move. When it is highlighted, turning the knob will change the number in the rectangle. Note that on the lower right side of the screen there is a KNOB = parameter display. The parameter will be one of the seven that can be adjusted without using the up/down arrow buttons. 6. Use an optical wipe to clean the ends of the optical fiber cable connector. If possible, also clean the sleeve on the OTDR into which the connector fits. 7. Insert the cable connector into the sleeve on the OTDR. 8. Press the FREE RUN button. You should get a signature similar to the following: The signature of a fiber means the trace you get on the OTDR from that fiber. In the lower right hand side of the screen, you will see the length per division. It will be in either km/div or in m/div. A “division” is the distance from one of the short vertical lines on the bottom horizontal scale to the next. 9. 10. 11. Press the MARKER button. Turn the knob to move the marker all the way to the left end of the signature. (This is the zero position, meaning zero distance from the OTDR.) Note that a small symbol (or icon) representing the marker appears in the lower right region of the screen after the words “KNOB=.” The icon for each display button function is under the button. Press the Km SCALE button. Turn the knob until the horizontal scale (at the lower right of the display) changes to 32M/DIV. Press the PULSE button and use the knob to change the pulse width to 1.25 m. By putting the marker at the extreme left and changing the horizontal scale to 32 m/division you will be able to examine the beginning of the signature in detail. In particular, you should be able to see the signal that comes from scattering from the polished end of the fiber in the connector attached to the OTDR.
­ 32 ­ 12. 13. 14. 15. 16. Press the Km SCALE button. Turn the knob until the horizontal scale changes to 256 M/DIV. This is appropriate for cables between 1.28 km and 2.56 km long. For other lengths, choose a scale that will show the entire signature on the screen. Check the decibels per division (DB/DIV) at the upper right of the display. 4 DB/DIV generally seems appropriate for lengths of 1 or 2 km. This setting can be changed by first pressing the dB SCALE button and then turning the knob until the desired value appears at the lower left of the display. You are now ready to obtain a fully averaged signature for your fiber. To get such a signature, we press the AVERAGE button. If the range is set for 2.5 km and if the averaging time is set for MAX, the total averaging time will be 16 minutes. You will be able to see the signature as it is constantly averaged. If you press the AVERAGE button while the averaging process is going on, the process pauses until you press the AVERAGE button again. It then continues. If you press the FREE RUN button, averaging is cancelled. The averaging process makes the signature trace much smoother. It allows you to see features that you could not see in the FREE RUN trace. Small details (less than 1 dB) may be “swallowed up” in the noise of the FREE RUN trace. Most of them are clearly visible in an averaged trace. When the signature stops improving due to averaging, you may push the AVERAGE button to cause the OTDR to pause. If there is enough time, you can let the averaging process continue until the display indicates 100 % x MAX. You are now ready to print out the signature on the Data Logger. You can print a label on the OTDR. (You can make up your own label. It is a good idea to include the date.) To put the label on the OTDR, press the ADVANCED MENU button. “Label” is automatically highlighted. Press the down arrow button in the MENUS area of the front panel until CHARACTER POS (character position) is highlighted. You will see a small rectangle to the right of the colon (:). You will enter a letter or symbol in this rectangle. You can move the rectangle by turning the knob. If the rectangle is next to the colon, the letter or symbol you put in the rectangle will be displayed at the far left of the top line of the graph. The farther to the right of the colon the rectangle appears, the farther to the right the letter or symbol in the rectangle will be from the left edge of the graph. Next, press the down arrow to highlight CHARACTER. As you turn the knob, letters and symbols appear in the rectangle one at a time. If you want a particular character at the position indicated by the rectangle, stop turning the knob when that character appears in the rectangle. To move the rectangle to the next position, press the down arrow button. This will shift the rectangle to the right. Turn the knob to the next character in the label. Continue until the entire label is displayed.
­ 33 ­ If you need to correct a character do the following: a. Press the up arrow button to highlight the CHARACTER POS function. b. Turn the knob to move the rectangle to the position at which you wish to make the correction. c. Press the down arrow button and turn the knob to choose the correct character d. To return to the signature, press the AVERAGE button. To produce the labeled printout, press the PLOT/DISK button once. At the bottom of the screen, you will see the following: PUSH PLOT/DISK TO CONFIRM; ANY OTHER BUTTON TO ABORT RECORD; PLOTS SAVES TO DISK OVERPLOTS Turn the knob to select PLOTS. Press the PLOTS/DISK button a second time to start the plotter. Wait until the plotter completes the plot before continuing. Do not remove the plot from the plotter. 17. Next, we measure the length of the fiber. To do this, a. Press the MARKER button. Then turn the knob until the marker is roughly 1.5 dB below the final peak of the signature on the leading edge of this reflection. You can use the markings on the side of the display window to locate the 1.5 dB point. b. Read the distance from the MARKER = readout, which is in the upper right of the screen. c. Record your reading. This is the length of the fiber. 18. Our next procedure is to measure the attenuation (loss of power) per unit length of a section of the fiber that has no splices or connectors. The portion of the signature from such a section will be a sloping straight line. To start, press the MARKER button. Turn the knob to put the marker at the left of the section of fiber. This position does not have to be very precise. You just need to be sure that you have the left marker on a smooth straight section with no spikes or sudden shifts. 19 Press the LOSS MEASURE ON/OFF button. 20. Press the SELECT button until the LED labeled DELTA is lit. When this occurs, the vertical part of the marker on the display will change from a solid line to a dotted line. On the right of the display, you will see DIST =, LOSS =, AND DELTA = readings. 21. Turn the knob to move the solid­line marker to the right end of the section. It is called the reference marker. The solid line is called the active marker. The DIST = readout shows the loss in dB between the markers. The DELTA = readout show the loss in dB per kilometer. If you need to, you can exchange the active marker with the reference marker by pressing the MARKER button. 22. We now record this information. Press the PLOT/DISK button once. Turn the knob to select PLOTS. Press the PLOTS/DISK button a second time to start the plotter. Do not remove the completed plot. 23. Press the SETUP MENU button. 24. Press the up or down button to highlight the WAVELENGTH parameter. If it is available, turn the knob until the 1300 NM value is enclosed by a rectangle. 25. Press the SETUP MENU button again. This should cause the fiber signature to be displayed.
­ 34 ­ 26. Perform steps 8 through 15 at the 1300 nm wavelength. If you wish, you may change the label for the second plot. 27. Perform steps 17 through 21. 28. Press the PLOTS/DISK button once. Turn the knob to select OVERPLOTS. 29. Press the PLOTS/DISK button to start overplotting. This will plot the 1300 nm data in a different color on the same plot as we had for the other wavelength data. 30. Tear off the graphs from the datalogger. Do you notice any difference between the plots at the two different wavelengths? Automatic Measurement of Splice Loss The Photon Kinetics OTDR can automatically measure the loss in dB at a splice. The method used by the OTDR’s computer to do this is indicated in the diagram: 3 1 2 4 5
The operator moves the marker to the approximate position of the splice. The machine then computes the precise position of the splice. Next, it examines the trace to the left and right of the splice. It establishes where the normal backscatter (the backscattering that does not result from the splice) ends to the left of the splice and restarts to the right of the splice. It then fits the best straight line to the segment on the left and the best straight line to the segment on the right. These two segments should be parallel to each other, with the segment to the right being lower than the segment to the left. The vertical distance between the two parallel lines (in dB) is the splice loss. The OTDR computes all of this automatically. Automatic Splice Loss Procedure 1. 2. Select the splice that you wish to measure. Press the LOSS MEASUREMENT ON/OFF button so that it is ON (lit). ­ 35 ­ 3. 4. 5. 6. 7. 8. 9. 10. Make sure that the knob is controlling the marker. If the display at the lower right­hand corner does not verify this, press the MARKER button. Turn the knob to position the marker near the splice. Press the horizontal MOVE button. Turn the knob until the splice you want to measure is within one division of the center of the display window. Press the km SCALE button. Turn the knob until the entire straight­line section of the backscatter signal on both sides of the splice (up to the next splice or discontinuity in both directions) is shown on the screen. It does not matter if another splice or discontinuity appears on the screen. Press the dB SCALE button. Turn the knob to decrease the number of dB per division. This will enlarge the signature. Do not allow the straight­line portion of the backscatter signal on either side of the splice to be cut off at the top or bottom of the screen. Press the MARKER button. Turn the knob to move the marker within one pulse width of the splice. The pulse width appears on the screen labeled PULSE. Press the SELECT button until the LED labeled AUTO SPLICE is lit. Press the COMPUTE button. Watch what happens as the OTDR’s computer goes through its calculations. Sometimes the OTDR’s computer cannot calculate the loss for the splice nearest the marker. This can happen, for example, when another splice or discontinuity is too close for the automatic program to be sure where one splice starts and the other ends. You can often solve this problem by using the horizontal MOVE button to allow you to position the signature so that only one of the tow close­together splices is on the screen. (This works because the automatic splice loss calculations only include information displayed on the screen.) Sometimes, however, you will get the message “UNRECOGNIZABLE FEATURE, MANUAL LOCATE.” In such cases, you will need to use a different procedure. Manual Splice Loss Procedure 1. 2. 3. 4. Follow steps 1 through 8 above in the Automatic Splice Loss Procedure section. Press the SELECT button until the MAN SPLICE LED (manual) is lit. You will see five vertical line markers on the screen. As you will see when you look at the KNOB = display on the screen, the MARKER is automatically selected. Turn the knob to move the center marker (which is solid) to the proper position on the splice. For a reflective splice, this position is on the left side of the splice 1.5 dB lower than the peak. For a non­reflective splice (such as fusion splice), the marker should be at the start of the fall­off in the backscatter signal. Press the MARKER button. This causes the center marker to remain fixed and allows you to move the first marker on the left. Turn the knob to move the left marker as far left as you can without getting too near another discontinuity. (By “too near” we mean in the region where the backscatter is not a straight line.) Press the MARKER button again. This causes the far left marker to remain fixed and allows you to move the second marker on the left. Move this to a position just before the beginning of the splice.
­ 36 ­ 5. 6. 7. Press the MARKER button. Move the fourth marker to the end of the splice. Press the MARKER button. Move the fifth marker as far to the right as possible without getting too near another discontinuity. (See step 3.) Press the COMPUTE button. After you complete step 7, straight lines will be placed by the computer, joining the markers. The splice loss measurement will appear in the MANUAL SPLICE LOSS readout. Some text excerpted from the Photon Kinetics manual. Used with permission of Photon Kinetics. In this writeup, we have discussed the Photon Kinetics optical time domain reflectometer in detail. However, there are quite a few OTDRs available today. Like the Photon Kinetics model, some are laboratory models. These are usually fairly large and have many functions available. They also tend to have powerful lasers that can be used to test fibers that are very long. There are also several portable models available. These are designed for technicians to use in the field. They are much smaller and lighter than the laboratory models. Although they are not as flexible, they are much easier to deal with, and they still give us what we need to troubleshoot a cable that is not performing well. In addition, they are much less expensive than the laboratory models.
­ 37 ­ Fiber­optic Star Coupler Optical fiber and connectors with transmitters and receivers would be all that we would need if we only wanted to connect one person with another. But if we want to connect many people with one another or send one television signal from one transmitter to many receivers, we need to be able to distribute the optical signal. There are several ways of doing this, but the simplest is to use a passive device, one that requires no power for its operation. Of course, such a device cannot increase the power of the signal—it can only decrease it. Fiber­optic couplers are devices that split, combine or multiplex optical signals without added power. A (1 X N) (read “one by N”) splitter or tree­coupler (Fig. 1), splits a single input into N equal channels or combines signals from N channels into one. A star coupler usually has equal numbers N of input and output ports (N X N). It may also have an unequal number of input and output ports (M X N). a 1 1 input Star output coupler b 2 coupler N c 3
Figure 1 : 1XN splitter / combiner Figure 2 : 3X3 star coupler The purpose of this lab is to measure loss parameters of a 3X3 star coupler (Fig. 2). Those include the following 1. Splitting Loss (Ls) :Is the loss (expressed in decibels) that occurs naturally due to the fact that the device divides its input power among the N output ports of the coupler. An ideal star coupler (one in which there is no loss of power at all) with 2N ports (where N = 2,3,4,...) divides the incoming power PIN into N equal output powers POUT, so POUT/PIN = 1/N, For a 6 port star coupler, (also written as 3 x 3, and read “3 by 3”), we have: POUT/PIN = 1/3 Expressing this in dB, we get Ls = 4.77 dB (This comes from, Ls = ­10 log(1/3) = +10 log 3 = 4.77) Real star couplers are not ideal. They have additional losses. These are the connector loss (LC) due to coupling of a pair of fiber­optic connectors through which the signal must pass and the loss within the coupler itself, which is called the excess loss ( LE). In a star coupler the signal coming from an output port has to go through 2 optical connections, one to get into the coupler and one as it comes out. The loss for each optical connector coupled pair tends to be about 1 dB. ­ 38 ­ 2. Excess Loss (LE) : This is the total power lost in the coupler and never recaptured at any of the output ports. In a 3X3 star coupler the excess loss for an input port “a” is calculated by putting a known optical power Pa in that input port and measuring the total output power (P1 + P2 + P3) from all output ports 1,2,3 respectively. The loss can be expressed as a ratio Excess Loss ratio = (P1 + P2 + P3) / Pa Or in decibel form LE = ­10 log[(P1 + P2 + P3) / Pa] in dB For a star coupler with 8 or less ports, the excess loss tends to be about 1 dB. 3. Insertion Loss (LINS) : This is the power drop across the coupler from an input port to a certain output port. It includes the splitting loss as well as any excess loss between the particular input and output ports. In a 3X3 coupler for example the insertion loss between input port “a” and output port “1” is Insertion Power Loss Ratio = P1 / Pa Or LINS = ­10 log(P1 / Pa ) in dB For an ideal coupler the excess loss is zero and the insertion loss is equal to the splitting loss. 4. Back­reflection (B) : Referred to as backscattered power is a measure of the optical power (PREFL) reflected back to an input port for an optical power (PIN) incident in that input port. The reflection loss for any coupler port is defined as Reflection loss ratio = PREFL / PIN or LREFL = 10 log(PREFL / PIN) in dB The reflection loss measurement requires a special test set (return loss test set), optical termination of all the other ports except the measured port and very low reflectivity connectors at the coupler ports and the fiber jumpers (UPC or APC connectors). We will not measure this loss. 5. Directivity (DIR) : Is a measure of how well the coupler sends the power from an input port to the port(s) to which it is intended (output only).
­ 39 ­ Light is launched into an input port and the optical power coming out of the other input port(s) is measured with all output ports optically terminated. Optical termination can be done by immersing the fiber end in index matching fluid, coiling the fiber before the connector, or using optical terminators to cut down any connector reflection. The directivities of a 3X3 coupler for an input port “a” can be expressed as DIRa,b = ­10 log(Pb / Pa) and DIRa,c = ­10 log(Pc / Pa) in dB 6. Uniformity : It is a measure of the maximum difference in insertion loss between output channels for a given input port. Ua = ( LINS)a,max ­ ( LINS)a,min Measurement method: One common way to represent coupler loss data is a chart called a Loss Matrix, showing the insertion loss (L) for every combination of input and output ports as well as directivities (D) and back reflectivities (B). An example of such a matrix for a 3X3 star coupler is shown in Table 1. For each pair of input/output optical powers measured, the corresponding physical quantity to be calculated according to the definitions is shown as a matrix element. In this chart, L stands for insertion loss, B stands for back reflection loss (which we will not measure), and D stands for directivity loss. For example, if you connect port 1 to the optical source and measure the dB loss between that port and port a, the result goes in place of the L in the first row of the chart where column 1 on the launch and row a on the receive cross. Similarly, replace the D where column a and row b cross with the loss between these ports when the power source is connected to port a and the meter is connected to port b.
­ 40 ­ (Receive Power) a b (Launch Power) c 1 2 3 a B D D L L L b D B D L L L c D D B L L L 1 L L L B D D 2 L L L D B D 3 L L L D D B Table 1. Loss Matrix elements for a 3X3 star coupler You can measure all matrix elements except backreflection by performing power loss measurements on two coupler ports. This is a two­step procedure and includes sending a pre­measured amount of optical power into each coupler port and then measurement of the optical power coming out of all ports. Fiber­optic loss measurements can be performed using the following two configurations shown in Figures 3 and 4. Source Source Figure 3. Reference Reference Reference cable A cable B cable C Connector Connector Adapter Adapter Reference Reading A Device Under Test Detector Detector Reference Reading B Fiber optic loss measurement. Configuration #1 First we measure the power entering the star coupler by connecting a power meter to a source with a fiber jumper cable, and record the input power. Next, connect the source to an input port of the fiber coupler, measure and record the power emerging from each output port. Reference: cable A
source detector Reference Reading A ­ 41 ­ source
Device Under Test detector Reference Reading B Figure 4. Fiber optic loss measurement. Configuration #2 In both configurations the actual loss reading is the difference between reading B and reading A. Configuration #1 assumes good quality fiber optic connectors and is the preferred one. Configuration #2 is simpler and is used if the connector quality is poor. Measuring dB loss and Individual Port Loss for a Star Coupler From theory, we see that measurement of coupler loss is straightforward. We measure the input power and the output power, calculate the required ratio and use it according to the definitions. A power meter with an SMA adapter will work. The FOTEC power meter model M 200 is adequate for wavelengths that are between roughly 800 and 900 nm. This is called the first window. The FOTEC model S 310 source is useful for measuring the total loss at about 850 nm. For the loss at 1300 nm or 1550 nm, different sources and meters are required. We shall limit ourselves to the “first window” of wavelengths, where the FOTEC model 310 sources and model M 200 meters provide the necessary signal and detection capability. Plug in the transformers for the sources into the appropriate sockets and connect the power cord to the FOTEC source. The detectors are battery powered. Connect the appropriate adapters to the source and the meter. If the cables from the star coupler have 906 SMA connectors (with ends shaped like ) put (white plastic) cylinders over the tips. Make sure that the top of the cylinder is flush with the top of the tip. This changes the SMA connector to a type 905 configuration (shaped like ). The white plastic adapters are in the FOTEC cases. If cables with other connectors are available, you can use them also. Follow configuration #2 for loss measurements. Reference Reading A Connect a cable from the source directly to a meter with a fiber jumper and measure the input power (launched power) in mW. Switch the meter to its dBm and record the same power in dBm. Disconnect the direct cable. Reading B Connect one of the input fiber cables of the star coupler to the FOTEC source. (Since the star coupler is symmetric, you can choose either side to be the input side.) Next connect an output cable of the coupler to a FOTEC meter. If you have several FOTEC meters, connect each one to a different output fiber cable. Measure the power output to each of the output ports. If you do not have as many meters as output ports, measure the power from each ­ 42 ­ output port by connecting them to the meter one after the other. (Of course, the reason we can do this successfully is that our source is very stable.) Record the power from each output port in dBm and put in the proper matrix box. Repeat the procedure for “Reading B” using every coupler port as input port and record the results in Table 2. A convenient data collection table version for the 3X3 multimode fiber star couplers available for laboratory testing is shown in Appendix 3. (Receive Power) a a b c 1 2 3 b (Launch Power) c 1 2 3 Table 2. Measured received power in dBm
Convert the powers from dBm to mW in the above matrix for excess loss calculations and record results in Table 3. To do this, you can either switch the meter to its mW setting or use a calculator. To convert dBm to mwatts, use the following formula: Power = 10 - dB m / 10 (in mW) (Receive Power) a a b c 1 2 3 b (Launch Power) c 1 2 3 Table 3. Measured received power in mW
Calculate the insertion losses and directivities in dB by subtracting the received powers (in dBm) from the launched power (also in dBm) and prepare the loss matrix showing insertion losses and directivities in dB (leave back reflection boxes empty). If you need help, ask you instructor how to do the calculation. Record results in Table 4.
­ 43 ­ (Receive Power) a a b c 1 2 3 b (Launch Power) c 1 2 3 Table 4. Measurement results (in dB) as Loss Matrix elements.
Compare Table 4 to Table 1 to identify the Loss Matrix parameters. Use Table 3 results to calculate Excess Loss values. Look above for the formula for excess loss. Calculate the excess loss ratio and use a calculator to calculate the excess loss in dB. Use your collected data in equation 10 of appendix 1 to find the total dB loss Ltot. From equation 1, we see that this equals to LE + 2LC. In the measurement procedure however, the loss portion 2LC is included in both the reference reading A as well as in reading B and the actual power reading is calculated by subtracting the two readings A and B. Therefore the total dB loss will be equal to LE . Once you have calculated Ltot, you can use equation 2 together with measurements of the individual output port powers to find the quantity 10 Log(l/N) = L ­ LE Since this quantity depends only on N, it should be the same for each output port. To what extent is this true? Measurements with Two Sources Get a second source and measure its output power with a direct cable to the meter. Connect the output of this source to another input port while leaving the original source attached to its input terminal. Measure the power at each output port. Using the same reasoning as that in equations 1 through 11, predict what your expected output power should be. Compare your prediction(s) with the actual measurements. Symmetry Most star couplers are symmetrical in the sense that the ports on either end can be used as the input ports with the ports on the other end serving as the output ports. Test this property exchanging the input and output ends and measuring the port losses and total loss as you did in the first part of this experiment. Do you get the same results when you reverse the ports
­ 44 ­ to which you attach the source and the meter? Why might the results be different? Discuss this with the rest of your group and ask your instructor for some ideas.
­ 45 ­ APPENDIX 1 Loss Calculations (If you want to know where the equations come from) If we assume that each connector produces the same loss and that the splitting ratio is uniform among all ports, we can write 1. POUT/PIN = 1/N and the total loss L for each output port as: 2. L = ­10 log (1/N) + LE + 2LC. If there were no excess loss and no connector loss, the sum of the output powers would have to equal the input power. Instead, we lose power at the input connector, within the star itself, and at each of the output connectors. Assume that the fraction of the incoming power transmitted by the input connector and in each of the output connectors is the same. Call it f. Then the power transmitted past the input connector is: 3. PIC = f Pi Let fE be the fraction of this power which emerges from the star coupler through all its output ports. Thus the power emerging from the star (POS) is given by: POS = fE PIC = fE f Pi In a carefully manufactured star, this power is divided evenly among the N output ports, so each output port emits optical power 4. Pop = 1/N fE f Pi Because of the loss in each output connector, the actual power emitted from each port is 5. PAO = f 1/N fE f Pi From the N output ports, the actual total power emitted is 6. PA Total = N f 1/N fE f Pi = fEf 2 Pi Then 7. PA total/ Pi = fEf 2 Now the excess loss LE is
­ 46 ­ 8. LE = ­10 log Pos/fPi = ­10 log fE And 9. LC = ­10 log PIC/Pi = ­10 log f Also 10. Ltot = ­10 log PA total/Pi = ­10 log fEf 2 = ­10(logfE + 2 logf) Comparing, we see that 11. Ltot = LE + 2LC Note that Ltot of equation 11 does not equal N times the dB loss L given by equation 2.
­ 47 ­ APPENDIX 2 A convenient data collection table for the 3X3 multimode fiber star couplers available for laboratory testing. Index 1 corresponds to side 1 of the coupler, index 2 to side 2 and indexes O, B, T correspond to the three colors of the fibers of each side: Orange, Blue and Transparent respectively. For example, if you connect the source to the orange cable on side 1 of the coupler (You choose which side to call 1), and connect the meter to the blue cable on side 2, your result goes in the rectangle with an X in it. Receive Port O1 (Launch port) O1 B1 B1 T1 O2 B2 T2
X T1 O2 B2 T2