Fiberoptic Slab Manual

7/30/2019 Fiberoptic Slab Manual

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Manual for Fiber Optics Experiments

Project QCC TechASCEND

This material is based upon work supported by the National Science Foundation underGrant No. 0206101

Any opinions, findings, and conclusions or recommendations expressed in this material arethose of the author(s) and do not necessarily reflect the views of the National ScienceFoundation.


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Table of Contents

Page1. Introduction to Fiber Optic CablesStripping and Cleaving 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. Fiber-optic Star Coupler 37


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Introduction to Fiber Optics CablesStripping and Cleaving

Optical fiber has changed telecommunications all over the world. Because a single opticalfiber can carry really huge numbers of telephone conversations, long distance calls that usedto be very expensive have become cheap enough for many people to make them often. Howoptical fibers carry multiple conversations is fascinating.

How does the long-distance digital telephone system work?

When you talk into a telephone transmitter, the sound waves from your voice move adiaphragm on the transmitter. This causes electricity to flow. The electric signal varies withtime in a way that imitates the way the air pressure varied in the sound wave from yourvoice. In the modern telephone system, the (changing) strength of this electric signal ismeasured several thousand times each second. The measured strength is then made into acode. If you knew how to interpret this code, you could get the strength of the electricsignal as time went forward during the conversation that caused the signal. This code iswhat is sent through the telephone system. In the telephone system, the person at the otherend 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 thatreproduces the original electric signal from the transmitter. This electric signal goes to aspeaker, where it causes the air to vibrate just the way it did when the person on the otherend talked into the transmitter. The listener then hears a faithful reproduction of the originalconversation. Amazingly, all of this takes place so fast that the listener thinks that s/he ishearing the original talking as it is happening.

Where does fiber optics enter the long-distance telephone system?

In a fiber optic system, the code that is sent through the system is not electrical. Instead, theelectrical 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 itsdestination is it changed back into an electric code and from that into the electric signal thatoperates the speaker in the receiving phone.

Optical fiber

For much of modern telecommunication, the path over which the signals travel is opticalfiber. Optical fiber for most purposes is made of a very special kind of glass that is drawninto a very thin, long fiber. In some ways, this is similar to the fiberglass that is used forinsulation in homes. Unlike fiberglass, however, optical fiber is made of a much differentkind 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 cylinderthere is a core, and surrounding the core is a layer called the cladding. Both core andcladding are glass they are slightly different types, however. A cross section of the fiber isshown in Figure 1.


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Figure 1. Fiber cross section

In Figure 1, the diameter of the core is half the diameter of the cladding. This is typical ofone type of fiber. In a slightly different type of fiber, the core diameter is 0.4 time that of thecladding. Both of these are called multimode fiber. A third type of fiber is used for verylong distance telecommunication. Its core diameter is about 1/10 the diameter of itscladding. This type of fiber is called single mode fiber. There is one thing about Figure 1that is very misleadingits diameter. The outside diameter of all standard real optical fiberis 125 microns. A micron is 1/1,000,000 of a meter, or 1/1000 of a millimeter. This meansthat the outside diameter of standard optical fiber is only 1/8 of a millimeter. That is reallysmall. 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 easierto handle). It is quite amazing that the diameter of the light-carrying part of single modefiber is about 1/10 the diameter of a human hair.

We will examine two samples of fiber, one multimode and one singlemode, using amicroscope. 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 (orstrip) the coating. Although you can actually strip fiber with anXacto 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 shouldmake a small angle with the fiber, not over about 20 degrees.

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 tobreak the fiber. Since fiber is very thin, this takes lots of practice. Ask you instructor todemonstrate the technique.

Each group of participants will have a commercially available fiber optic stripper. As youwill discover, this makes the task of stripping much easier. You should strip between 1/4inch and 1/2 inch of the plastic coating each time you use the tool. Before you strip another1/4 - 1/2 inch, you need to brush off the blade with the brush provided. If you do not dothis, 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 (2.5-5cm) of the buffer.

cladding

core

Optical fiber Knife blade


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- 5 It is important to keep the work area clean and orderly. This is not just for appearance. Aclean, orderly work area improves safety. Optical fiber is made of glass. Since it is verythin, little pieces of it act like rather nasty splinters. If they get into your skin or eyes, theycan 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 nearlyperfectly smooth. You might think that the only way to achieve this would be to polish thefiber 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 simplerprocedure is equally good. This procedure is called controlledfracture. To cleave a fiberusing controlled fracture, we put a little stress on the fiber and make a very light scratch onit. 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, youwill crush the fiber, and the end will be jagged. To use a hand cleaver, bend the optical fiberaround 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 smallpieces of fiberthat are left on the cleaverin the red-covered fiber disposal units that shouldbe on your desk or on the tray of equipment you will receive. You should practice as manytimes as you can, examining the results with the microscopes that will be available for youruse. Your instructor may also show you how to use the semi-automatic cleavers that are inthe 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.)


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Fiber Optic Connectors

The fiber part of the fiber optic telecommunications system is made up of lengths ofoptical fiber that must be connected to each other to provide pathways for the optical signalsto 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 fibersare melted together), there are many situations in which you do not want a permanentconnection. For example, you might want to connect a television camera to one TV set atone 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 connectionto one set and installing another permanent connection to the other. When you want to havea 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(sometimescalled FC-PC), SC, and STtypes. One type of dual fiber connector is specified in the FDDI(Fiber Distributed Data Interface) documents. Each of these connector types must meetvery tight specifications. There are many other connector types, but these are among themost common.

What Makes a Connector Good?

To be useful, a connector has to have certain properties. Among them are thefollowing:

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 theconnector, the more likely it is that it will be expensive. People who design connectors mustmake 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 opticcable. We will use cable that is 3mm in outside diameter. We will use several differenttools to remove the various protective layers that surround the fiber in the cable. We willthen 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 takeanother.


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- 7 TERMINATION PROCEDURE--MOLEX 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 afiber optic cable and installing connectors on each end. A clean work area isless 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) Thenon-threadedend of the nut must face the

boot.

2- For jacketed cable, strip the outer jacketusing the Template (Figure 1) as a guideand use the jacket stripping tool. (This hasa red handle and a wire cutter. Use the1.3mm cavity.) Use the same tool withthe .4 mm cavity to remove the whitetubing. On the template, the section

marked Buffer is the white tubing.

3- Using the buffer stripping tool (No-NIK), precisely strip the buffered fiber tothe length on the Template. Use a brushto remove debris from the tool. A cloggedtool 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.

inch boxes


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- 8 4- Slowly insert the fiber into theconnector until it comes out of the otherend. A slight rotation of the connector willhelp. (Figure 2)

5- Pull the fiber back out.

6- Slide the protection sleeve over theferrule.

7- Carefully inject the epoxy into the backof the connector until it appears on the faceof the ferrule. Apply a drop of epoxy tothe threads of the connector beforescrewing on the rear nut. Re-insert the

fiber into the connector. (Figure 3)

8- Insert the connector into the plasticassembly fixture (which may be red or

black) and holding it firmly screw the rearnut onto the connector, capturing theKevlar strength members in the threads.Remove the assembly fixture. Be carefulnot to break the fiber. (Figure 4)

NOTE:-Clean any epoxy from the side of the

ferrule with lint free tissue and alcohol.

-Wash epoxy from your hands,

immediately.

9- Put the connector with the protectivesleeve into one of the oven holes for 5minutes at 100o. Let it cool then removethe protection sleeve.

10- Holding the cleaving tool (in plastic

tube) with the trigger upwards, slowlyinsert the ferrule with its protruding fiberinto the slot of the cleaving tool. Insert theconnector until the ferrule end face stopsagainst 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

Insert fiber into connectorThen pull back as shown

Face of

Ferrule

Apply epoxy here

Figure 2

Re-Insert fiber into connector

Figure 3

Assembly Fixture

Screw On Nut

Figure 4

Figure 5


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11- Insert the connector into the polishingtool. Using the glass plate mount a sheetof BLACK lapping film. Put a few dropsof water on the film. Perform 10 figure8 motions over the entire length of thelapping film. (Figure 6) Virtually no

pressure should be applied to the tool atthe beginning of this step.

Gently clean the contact with a lint-free

tissue and alcohol between eachpolishing step.

12- Mount a sheet of GREEN lappingfilm and perform at least 15 to 20 figure8 motions over the entire length of thelapping film.

13- Slide the boot up until it fits securelyover the rear nut.

14- Remove the connector from thepolishing tool and carefully clean theferrule with lint-free tissue and alcohol.Make sure that it is dry. Use the VideoFiber Microscope (or a 100X microscope)to inspect the quality of the polish. No

scratches or cracks should be visible.(Figure 7)

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.

Lapping Film Connector

Glass Plate Polishing Tool

Figure 6

Chip or Crack Scratch

GOOD BAD

Figure 7

Diagrams and some text used with permission ofMolexCorporation.


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Fiber Optic Test Set

The simplest test we can make on a fiber optical system is to see whether light is gettingthrough and how much is lost. The instruments used for this are called an optical source anda receiver. Together, they form a fiber optic test set.

Most fiber optic test sets work with several different wavelengths. The usual choices are850, 1310, and 1550 nanometers (nm). The receivers can be set for any one of these. Thesources you will use are dual wavelength sources. They can provide signals at 1310 nm or1550 nm. You might wonder why we need several wavelengths. The basic reason for thisis 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 reactsdifferently 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 ofthe source. Connect the other end to the input port on the receiver. Turn the source andreceiver on (by pushing the appropriately marked button). Press either theWAVELENGTH or the button (depending on your meter) until the desired wavelengthappears on the display. If your receiver has a zero button, you may zero the meter. To dothis, just press the ZERObutton. You next connect the cable (or other system) that you aretesting between the commercial cable and the receiver. To do this, you will need to use abulkhead connector. This is a metal piece that has two receptacles, one for the originalcable and the other for the system you are testing. Check to make sure that the wavelengthat which you are operating is the wavelength you want. Choose the units that you want touse. On some of the meters, your choice will be between dB, dB, and dBm. On others,

your choices will be dBm or dB. Choose dB if it is available. If you have zeroed yourmeter, the readings will be in dB unless you change to dBm. Finally, you read the result.Thats all there is to the operation.

There is, however, the matter of interpreting the reading. If you choose to get results in dBor in dBm, you will need to subtractthe reading with the cable you are testing from thereading 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 thisstep.

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 dBmOptical power is often expressed in units called dBm. These units tell us the optical powerin a convenient way.


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A microwattis 1 millionth of a watt. Microwatt is abbreviated as mW. An ordinary 100watt 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 = 0.5 microwatts (0.5 mW)3 dBm = 2 -6 = 0.25

6 dBm = 4 -9 = 0.125

9 dBm = 8 -10 = 0.1 10 = 10

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 formilli. Forexample, 3 dBm = 2 milliwatts instead of 2 microwatts. The prefix micro means 1millionth, 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 itrefers to light, however, it is related to a ratio of powers. If, for example, we say that thedifference between two light signals is 6 dB, it means that the power of the stronger signal is4 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 dBmand dBm express the ratio between the measured signal and a standard, either 1 microwattor 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 72microwatts. What is the dB difference between these two sources?


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Fusion Splicing

If you want to join two lengths of optical fiber together with the least possible loss of opticalpower, the method to choose is fusion splicing. In fusion splicing, the cores and claddingsof 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 smalldiameter, it requires a very precise instrument to join the cores in a way that lets the mostlight pass through the point of joining. (For single mode fiber, the diameter is about 1/100of 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 sosmall, it is difficult to line up two fibers so that their cores line up nearly perfectly. If thecores are not lined up just right, the light will not pass through. In addition, the heating must

be very exact. If you dont heat the two fiber ends enough, they will not melt together. Ifyou 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 byhand, using precision fiber positioners. These are thumbwheel-operated devices that movethe fibers by very small, very finely controllable amounts. When the operator rotates thethumbwheel through a fairly large angle, the fiber moves a very small distance. It takesconsiderable skill and practice to use these manual splicers.

Automated splicers

Nowadays, most companies use automated fusion splicers. When you put two properlystripped 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 thatafter you do the automated procedure.

The automated fusion splicer is designed to work with many different types andcombinations of fibers. In the procedure section, you will find out how to select the correctmode for the splices we will make.

Procedure

The procedure for making a fusion splice using the Ericsson model FSU975 fusionsplicer 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 foroptical fiber from Amherst Fiber Optics. (Using this tool is not essential, but it is


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convenient because this stripper strips just the right length for the cleaver we use.) In ourexperiment, 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 protectivestrands.) 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 pliers-like stripping tool, the end

of the clamping device should be flush against the stopping screw on the stripper, and thefiber should extend out over the end of the strippers clamping device. Guide the fiberthrough the hole and into the groove on the other side. The end of the fiber should restagainst the end of the groove. Grasp the clamping device firmly with your thumbexerting pressure on the actual clamp so that the fiber cannot slip within the clamp.Squeeze the yellow-handled 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 endfor 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 (onthe far right of the Ericsson model EFC11 fiber cleaver) is horizontal and the bladerelease lever (on the far left of the cleaver) is up. When you have made sure of this,carefully lift the clamp with the clamped fiberand put it on the cleaver. Put the fiberholder 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 handclamp (on the cleaver) down. Move the tensioning lever down. This puts a small amountof tension on the section of fiber between the clamps. Next, move the blade release leverdown. You will see the blade move in to touch the fiber. If all goes well, the fiber willcleave.

Splicing the fiber

Description of the splicer

As Figures 1 and 2 below show, a modern automated fusion splicer has many parts andcontrols. Figure 1 shows the top of the Ericsson fusion splicer. It is copied from themanual. Please disregard the page numbers given on the diagram. They refer to themanual.

Toward the top of the picture, you see the parts that hold the fibers to be joined bysplicing 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 thegrooves. The electrode block holds the electrodes. These electrodes produce the smallelectric arc that heats up the fibers so they will fuse. The safety shield covers theelectrodes and the fiber during the actual splicing to protect the eyes and bodies of the

people doing the splicing.


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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 beforeyou proceed with the experiment. The functions of the appropriate buttons are discussed

below as we come to them in the procedure.


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Now prepare the FSU 975 splicer to receive the cleaved fiber. The safety shield may beclosed. If it is, you will see DANGER HIGH VOLTAGE in white on its top. Open theshield by locating the button-like knob on the right side of the raised part in the center ofthe 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 pushedbackwards (toward the top of the splicer) so that they are horizontal. When the levers areproperly positioned, you will see white grooves into which the fiber will go. Carefullylift the fiber still clamped in its holder off the cleaver and onto one side of the Ericssonsplicer. Do not slide the fiber into the groove on the splicer. Instead, tilt the end of thefiber up slightly and tilt it into place without sliding. Remove the cut off end of the fiberfrom 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 thesplicer). Be sure to insert the fiber in such a way that the two clamps open in

the same direction. If you do, the V-grooves in the two clamping holders willbe on opposite sides.

Now that you have the clamped fibers on the splicer, it is time to turn on the splicer. To dothis, press the ON/OFF button in the upper left hand corner of the splicer. On the LCDmonitor screen (Refer again to Figure 1.), you will see two lines of print. The upper lineshould say FSU 975 AUTO MODE. The second line should start by sayingPARAMETER UPDATING. . . Once the updating is completed, the line should readINSERT FIBER and CLOSE SAFETY SHIELD. If you have positioned the fiberscorrectly, 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 button-like knob on the right side of thehousing toward the top of the splicer. (See Figure 3.) The DANGER HIGH VOLTAGElettering should be visible.

Figure 3

After you close the safety shield, the message line on the monitor will change. The firstmessage will tell you which splicing program was used last. The other messages will tell


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you about further stages in the splicing process. These include AUTOALIGNING/FUSION and PRESS FUSE BUTTON.

Figure 4

Before you do anything else about splicing the fibers, you should check the fibers on themonitor to make sure they are clean and cleaved well. (See Figure 4 for pictures of defectsin the cleaning or cleaving that require fixing.) If you see any defects, take the fibers off thesplicer and out of the clamps and re-prepare them. You can view the fiber from twodifferent 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. DoNOT 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 thiscombination. 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

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?

+

__

ENTER


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If the program displayed is P04, you should press the button and proceed to thesection headed Auto mode splicing below. If the program displayed is not P04, you needto change it.

Changing the program

To change the program, press the button. Next, press the button. At the top

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 button. The display should now read

NORMAL MM + MMNEW PROGRAM?

You should press the button. The display should now read

NORMAL MM + MMEDIT PARAMETERS?

Press the button again. The display screen should now show the two fibers and

FSU 975 AUTO MODENORMAL MM + MM.

Auto mode splicing

Now that the fibers are in place and the safety shield has been closed, the splicer will displaya series of messages on the bottom line that tell you it is ready. It should read

AUTO ALIGNING/FUSION PRESS FUSE BUTTON.

To start the automatic splicing process, press the button. The splicer willautomatically rough align the fibers, prefuse them, focus the view, align the fibers (a finealignment), and fuse them. A series of messages will appear on the display to let you knowwhere the splicer is in its sequence. A typical series of messages reads as follows:

ROUGH ALIGNMENT

PREFUSION WILL START

ROUGH ALIGNMENT

ALIGNING FIBERS

YES VIEW

ENTER

NO

NO

Fuse

NO


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SPLICING WILL START

SPLICING

If the splicer cannot carry out some part of the automatic sequence, it will print out a faultmessage on the bottom line of the display. These are usually easy to understand, and theway to fix them is usually pretty obvious. If you get a fault message and find it difficult tofigure 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 nextstep.

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 iscompleted, the splicer automatically estimates the splice loss. While it is doing this, thedisplay reads

FSU 975 AUTO MODE

CHECKING SPLICE.

When it has finished estimating the loss, the splicer displays it by printing

ESTIM. LOSS: #.## dBNEXT 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. Thescreen will read

ESTIM. LOSS: ##.# DbFOR RE-FUSING

PRESS FUSE BUTTON

To use this option, press the button again.

Do not re-fuse the splice more than once. If you do, you will probably weaken the splice

and/or increase the splice loss.

Visual splice evaluation

Fuse


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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

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 notlook 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 andaccessed with the 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 roundglass 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

pressing the button. Next, press the button. At the top of the monitor

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.

VIEW

VIEW

ENTER

YES ENTER


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Press the button. The display should now read

NORMAL SS + SSNEW PROGRAM?

You should press the button. The display should now read

NORMAL SS + SSEDIT PARAMETERS?

Press the button again. The display screen should now show the two fibers and

FSU 975 AUTO MODENORMAL 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 findthat 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 multimodecores are.

Manual splicing

Sometimes it is useful to align fibers manually, not using the automatic capability og thesplicer. 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.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 properlyplaced on the splicer, you are ready to begin. Be sure that you have the correct splicingprogram 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 Changingthe 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

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

NO

NO

Mode

ENTER

Fuse


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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 button. Make sure that the gap is also correct when it

is viewed from the other camera angle.

Now you should press the button. This will heat the fiber enough to clean off a smallamount of residue from the fiber. If the fiber is too dirty, however, the prefusion will notcure the problem.

After prefusion is finished, the splicer will tell you that it is waiting for you to complete finalinspection and alignment by displaying the message

READY FOR SPLICING

PRESS FUSE BUTTON

Before you press the button to splice the fibers, you should check the fibers. Use thebutton to switch between viewing angles. Look for dust or other problems

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 thefine alignment. The first step is to set the space between the fibers (the gap). Us theand buttons to bring the fibers as close together as possible without overlappingthem. The last movement you cause before you set the gap must be to move the fibers

closer together.

Briefly press the button. The gap should be as narrow as possible, but still wideenough 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 twofibers. Make sure that the two fibers line up straight across the screen.

Do not forget to fine-tune the alignment from both camera angles. Remember that you

change the angle by using the button.

After you finish the fine-tuning, but before fusion, the alignment should be as follows:The outer edges of the fibers line up, and the gap between their end-faces is centered on

the LCD monitor screen and is as small as possible while still allowing the fibers to moveup and down.

VIEW

Fuse

VIEW

GAP

VIEW

Fuse


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When you satisfied with the fiber alignment, press the button and start fusion. Thesplicer will follow the last splicing program you chose. As the splicer operates, the messagefield will read

Splicing. . .

Now you need to check the splice. Review the Automated splice evaluation and Visualsplice evaluation sections above for the procedure.

If you were going to use the spliced fibers in a real communications system, you would needto protect the splice with a heat shrinkable sleeve. We will not do this, but the process isvery simple.

Figures and some text excerpted with permission of the Ericsson Corporation.

Fuse


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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 thetelegraph in the early 1850s. Since it was very expensive to string even one cable over longdistances, this idea (which is called multiplexing) was very important. Modern telephonesystems 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 intelegraphy. The method we will use works on the same principle as a method in whichtelegraph messages were sent using sounds of different pitches. The pitch of a sound, trebleor bass, depends on its frequency. The way you can use different frequencies to sendseveral 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 oneend 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 differentfrequencies of sound to send different messages. Why not transmit all the messages at thesame frequency, and just transmit faster on that frequency? At least at the beginning oftelegraphy, the answer was simple. Human operators were transmitting and receiving thesignals, and they could only go so fast. Eventually, machines were used to do thetransmitting 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. Whendifferent frequencies of visible light reach our eyes, we see different colors. Thus, the colorof light is similar to the pitch of sound. Both depend on frequency. This can lead us to amethod for multiplexing optical signals. If we send messages in code using light, we cansend 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 theinformation carrying capacity of optical fiber. Why not use a single frequency of light andsend code at a very fast rate by turning the laser light on and off very rapidly? (That is, whynot 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 electronictransmitters and receivers, we cannot turn the light from the laser transmitters off and onnearly as fast as we would need to in order to send the maximum amount of information


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possible through an optical fiber. Furthermore, even if we could send the maximum amountof information, there are no receivers that could read the information that fast. By sendingthe information at many different frequencies, we can transmit much slower on each of thefrequencies 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. Inthat way, no transmitter has to operate at an excessive speed, and no receiver has to receiveat such a speed.

Real fiber optics communications systems do not use visible light. Infrared light worksbetter. But when you use these systems, you cannot see anything. You need detectors thatreact to the infrared light. For our experiment, we shall, therefore, use a system that usesvisible light and sends it through air, rather than through optical fiber.

The setup is shown in the diagram. Two lasers produce beams of different colors. Thebeams 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 wouldneed 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, wecombine the beams using a beamsplitter. This is a device that allows about half of the beamaimed at it to pass through and reflects the other half. If we put the laser beams at rightangles and the beamsplitter at 450 to both beams, the straight-through beam from one laserwill combine with the reflected beam from the other. In this way, we have combined thetwo messages so we can send them at the same time over the same path. In a more realisticsystem, we would send this combined message over an optical fiber so that atmospheric

Tun-

ableHeNe

Red HeNeDiffractiongrating Beamsplitter or

dichroic mirror


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conditions would not mess up the signals and so that other people could not easily read ourmessages without our realizing that they were. To combine signals with differentfrequencies for transmission through an optical fiber, you need a special device called awavelength division multiplexer. Our setup can be thought of as an over the air wavelengthdivision multiplexer.

When the combined signal reaches its destination, we must separate the two beams so thatwe can once again read the messages. There are several ways to do this. We have chosen touse a diffraction grating. This device reflects different colors in different directions. Afterthe combined beam reflects from the grating, the different colored beams come off atdifferent angles, so we can examine them individually. This process is calleddemultiplexing.

WARNING: Throughout this lab, never look directly at any of the

laser beams!!

Procedure

1. Put the beamsplitter mount at a convenient height and position. Mount thebeamsplitter on it.

2. Set up the multicolor helium neon laser so that its beam is horizontal and at theheight above the table at which you have the beamsplitter. The laser shouldalready be on. Keeping your eyes away from the opening, open the beamstopperon the front of the laser. Adjust the setting of the micrometer on the rear of thelaser to the reading necessary to get the color you need. Ask you instructor forthe 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 areat right angles to each other.

3. Use the laser power meter to measure the power in the laser beam before andafter 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 light-sensitive surface in the sensor. If themeasured power is less than 2 mW, lower the setting of the meter.

e. Record the maximum power reading for the transmitted beam and thereflected beam.

f. Compare the sum of the readings in part (e) to that in part (d). Whichone is bigger? Suggest a reason for your result.

4. Stop the beam of the red He Ne laser using the sliding beamstopper. Do notturnthe laser off.


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5. Set up the red He Ne laser at a right angle to the non-red one. Follow theprocedure of part (2), but move the laser rather than the beamsplitter.

6. Use the laser power meter to measure the power in this laser beam before andafter it is split. Follow the same procedure as in (3) a-f except start with themeter at its 2 mW setting.

7. Open the beamstopper on the non-red He Ne laser so its beam can emerge. Youshould have the reflected red beam and the transmitted beam of the second colormoving along about the same path. The next step is to make these beams movealong exactly the same path, as they would in a fiber optic system. Put a piece ofwhite paper or cardboard very near the beamsplitter on the side where the beamsshould merge. Move the red light laser until you get the beams as close togetheras possible. The beamsplitter mount can be turned. It can also be tilted using thetilting 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 asingle path. Remove the white object. Final adjustment can be done by lookingfrom 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.8. Use the laser power meter to measure the power in the combined beam. Startwith the meter at its 20 mW setting. Record your reading.

9. Mount the diffraction grating in the combined beam. Never touch the surfaceof 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 thegrating, one red and the other the second color you are using.

10. Measure the power in each of the separated beams.11. 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 tomodulate 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) orASCII (American Standard Code for Information Interchangethe code used incomputers) or make up your own. (Tables of Morse code and ASCII are at theend of this writeup.) For Morse code, use a long pulse of light for a dash (-) anda noticeably shorter pulse for a dot (.). Use a short light burst for a zero and anoticeably 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 theway ASCII code is transmitted in a real telecommunications system.

12. Two other members of each group (one for each separated beam color) shouldreceive the messages. They should record the dots and dashes for Morse code orthe ones and zeros for ASCII or for your own made-up code. Then they shoulddecode the messages and compare them to the originals. Submit the coded anddecoded 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 othersin your group or in other groups.


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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 000110001 100110010 200110011 300110100 400110101 500110110 6

00110111 700111000 800111001 900111010 : (colon)00111011 (semi-colon)00111100 < (less than)00111101 = (equal sign)00111110 > (greater than)00111111 ? (question mark)01000000 @ (AT symbol)01000001 A

01000010 B01000011 C01000100 D01000101 E01000110 F01000111 G01001000 H01001001 I

01001010 J01001011 K01001100 L

01001101 M01001110 N01001111 O01010000 P01010001 Q01010010 R01010011 S01010100 T01010101 U01010110 V01010111 W

01011000 X01011001 Y01011010 Z01011011 [ (left/opening bracket)01011100 \ (back slash)01011101 ] (right/closing bracket)01011110 ^ (caret/cirumflex)01011111 _ (underscore)01100000 `01100001 a01100010 b

01100011 c01100100 d01100101 e01100110 f01100111 g01101000 h01101001 i01101010 j01101011 k01101100 l01101101 m

01101110 n01101111 o01110000 p01110001 q01110010 r01110011 s01110100 t01110101 u


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01110110 v01110111 w01111000 x

01111001 y01111010 z

Morse Code AlphabetThe 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 ..--..


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The Optical Time Domain Reflectometer

Even though the highest quality modern optical fibers are extremely clear, they still scattersome of the light sent through and absorb some of it. This fact can be used to allow us tomeasure important facts about the fiber. When light passes from one substance intoanother substance, some of the light will be reflected. This comes about because indifferent substances light travels at different speeds. If an optical fiber has a smooth endthat makes a right angle with its length, about 4 % of the light coming through the fiberwill reflect at the end (where glass meets air). This can be used for necessarymeasurement.

The OTDR (Optical Time Domain Reflectometer)

The instrument we use is called an optical time domain reflectometer. For doing the kindsof 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 atabout 850 nanometers (nm), 1310 nm, and/or 1550 nm. Some instruments canoperate at more than one wavelength.

2. Detector. This produces an electrical output proportional to the strength of thelight that strikes it. The detector must have a very fast response time.

3. Signal analyzer. This measures the signal from the detector and sends themeasured value to an appropriate memory unit.

4. Microprocessor (or computer). This stores the measurement and, when instructedto do so, performs mathematical operations (such as averaging) on the measuredvalues. It can also locate and identify any connectors, fusion splices, and breaksin the fiber.

5. Display. This displays on a CRT or other screen the results of the measurementsand 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 reflectsback through the fiber to the detector. The time between when the signal left the pulsedlaser and when it reaches the detector is measured accurately. Since the speed of light inthe fiber is generally known (because the manufacturer sends its value), themicroprocessor can calculate how far the break is from the OTDR. But an OTDR cangive us much more information than that. When light travels through an optical fiber, asmall 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


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backscattered light as a function of the time it takes to reach the detector, the OTDR canproduce a graph of the strength of the signal go ing forwardas a function of distancealong the fiber. It can do that because the backscattered signal is a fixed fraction of theforward signal and because the measured time for backscattering is directly related to thedistance 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 findout how far they are from the OTDR.

Procedure1. Turn the power switch to ON. It is on the upper left corner of the front panel.2. If the screen is not lit to your liking, adjust the INTENSITY control.3. Power up the Data Logger by pressing its switch to the ON position,.4. 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 (orparameters) that you canadjust. 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, theappropriate parameter values are as follows:

WAVELENGTH To be chosen in consultation with the instructor.

FIBER RANGE 2.5

REFR INDEX 1.4600

AVERAGING TIME MAX

UNITS METERS

LABEL DISPLAY ON

FIBER LENGTH SHORT FIBER

The REFR INDEX value is particularly interesting. The abbreviation stands forrefractive index. The refractive index (for which the standard symbol is the letter n) isdefined by the following equation:

.v

c

fiberinlightofspeed

vacuumainlightofspeednrefractionofindex ===

____

_______

Since the speed of light in a vacuum is a known constant (which equals 300,000km/second), if we specify the value of the index of refraction n, we are really telling theOTDR the speed of light in the optical fiber we are using. The manufacturer of the fibersupplies this number with the fiber. Why does the OTDR need this information? TheOTDR 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 OTDRis a computer that calculates the distance from which the signal came based on the time ittook. But to do this calculation (which just uses distance = speed times time), thecomputer needs to know the speed of the light inside the fiber.

To change a parameter value, first use the and (up and down) arrowbuttons to the left of the SETUP MENU button. The parameter that can beadjusted will be highlighted by appearing in reverse video (black on green,instead of green on black). When the parameter you want to adjust is


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highlighted, you use the large knob on the lower middle right to make thechange. Turn the knob until a rectangle encloses the desired value. In thecase of the REFR INDEX parameter, the rectangle does not move. When itis 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 = parameterdisplay. The parameter will be one of the seven that can be adjusted withoutusing 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 ineither km/div or in m/div. A division is the distance from one of the short vertical lineson the bottom horizontal scale to the next.

9. Press the MARKER button. Turn the knob to move the marker all the way tothe left end of the signature. (This is the zero position, meaning zero distancefrom the OTDR.) Note that a small symbol (oricon) representing the markerappears in the lower right region of the screen after the words KNOB=. Theicon for each display button function is under the button.

10. Press the Km SCALE button. Turn the knob until the horizontal scale (at the

lower right of the display) changes to 32M/DIV.11. 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 to32 m/division you will be able to examine the beginning of the signature indetail. In particular, you should be able to see the signal that comes fromscattering from the polished end of the fiber in the connector attached to theOTDR.


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12. Press the Km SCALE button. Turn the knob until the horizontal scalechanges to 256 M/DIV. This is appropriate for cables between 1.28 km and2.56 km long. For other lengths, choose a scale that will show the entiresignature on the screen.

13. 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 settingcan be changed by first pressing the dB SCALE button and then turning theknob until the desired value appears at the lower left of the display.

14. You are now ready to obtain a fully averaged signature for your fiber. To getsuch a signature, we press the AVERAGE button. If the range is set for 2.5km and if the averaging time is set for MAX, the total averaging time will be16 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 thencontinues. If you press the FREE RUN button, averaging is cancelled. Theaveraging 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 (lessthan 1 dB) may be swallowed up in the noise of the FREE RUN trace.Most of them are clearly visible in an averaged trace.

15. When the signature stops improving due to averaging, you may push theAVERAGE button to cause the OTDR to pause. If there is enough time, youcan let the averaging process continue until the display indicates 100 % xMAX.

16. You are now ready to print out the signature on the Data Logger. You canprint a label on the OTDR. (You can make up your own label. It is a goodidea to include the date.) To put the label on the OTDR, press theADVANCED MENU button. Label is automatically highlighted. Press thedown arrow button in the MENUS area of the front panel untilCHARACTER POS (character position) is highlighted. You will see a smallrectangle to the right of the colon (:). You will enter a letter or symbol in thisrectangle. You can move the rectangle by turning the knob. If the rectangle isnext to the colon, the letter or symbol you put in the rectangle will bedisplayed at the far left of the top line of the graph. The farther to the right ofthe colon the rectangle appears, the farther to the right the letter or symbol inthe rectangle will be from the left edge of the graph.

Next, press the down arrow to highlight CHARACTER. As you turn theknob, 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 theknob 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 inthe label.

Continue until the entire label is displayed.


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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 ABORTRECORD PLOTS SAVES TO DISK OVERPLOTSTurn the knob to select PLOTS. Press the PLOTS/DISK button a secondtime to start the plotter. Wait until the plotter completes the plot beforecontinuing. 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 asection of the fiber that has no splices or connectors. The portion of the signaturefrom 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. Thisposition 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 solid-line marker to the right end of the section. It iscalled 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 theloss in dB per kilometer. If you need to, you can exchange the active marker with thereference 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 notremove the completed plot.

23. Press the SETUP MENU button.24. Press the up or down button to highlight the WAVELENGTH parameter. Ifit 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 bedisplayed.


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26. Perform steps 8 through 15 at the 1300 nm wavelength. If you wish, you maychange 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 theplots at the two different wavelengths?

Automatic Measurement of Splice Loss

The Photon Kinetics OTDR can automatically measure the loss in dB at a splice. Themethod used by the OTDRs computer to do this is indicated in the diagram:

The operator moves the marker to the approximate position of the splice. The machine thencomputes the precise position of the splice. Next, it examines the trace to the left and rightof the splice. It establishes where the normal backscatter (the backscattering that does notresult from the splice) ends to the left of the splice and restarts to the right of the splice. Itthen fits the best straight line to the segment on the left and the best straight line to thesegment on the right. These two segments should be parallel to each other, with thesegment to the right being lower than the segment to the left. The vertical distance betweenthe two parallel lines (in dB) is the splice loss. The OTDR computes all of thisautomatically.

Automatic Splice Loss Procedure

1. Select the splice that you wish to measure.2. Press the LOSS MEASUREMENT ON/OFF button so that it is ON (lit).

2

3

5


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3. Make sure that the knob is controlling the marker. If the display at the lowerright-hand corner does not verify this, press the MARKER button.

4. Turn the knob to position the marker near the splice.5. 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.

6. Press the km SCALE button. Turn the knob until the entire straight-line sectionof the backscatter signal on both sides of the splice (up to the next splice ordiscontinuity in both directions) is shown on the screen. It does not matter ifanother splice or discontinuity appears on the screen.

7. Press the dB SCALE button. Turn the knob to decrease the number of dB perdivision. This will enlarge the signature. Do not allow the straight-line portionof the backscatter signal on either side of the splice to be cut off at the top or

bottom of the screen.8. 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.9. Press the SELECT button until the LED labeled AUTO SPLICE is lit.

10. Press the COMPUTE button. Watch what happens as the OTDRs computergoes through its calculations. Sometimes the OTDRs computer cannotcalculate the loss for the splice nearest the marker. This can happen, forexample, 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 oftensolve this problem by using the horizontal MOVE button to allow you to

position the signature so that only one of the tow close-together splices is on thescreen. (This works because the automatic splice loss calculations only includeinformation 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. Follow steps 1 through 8 above in the Automatic Splice Loss Procedure section.2. 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 atthe KNOB = display on the screen, the MARKER is automatically selected.Turn the knob to move the center marker (which is solid) to the proper positionon the splice. For a reflective splice, this position is on the left side of the splice1.5 dB lower than the peak. For a non-reflective splice (such as fusion splice),the marker should be at the start of the fall-off in the backscatter signal.

3. Press the MARKER button. This causes the center marker to remain fixed andallows you to move the first marker on the left. Turn the knob to move the leftmarker as far left as you can without getting too near another discontinuity. (Bytoo near we mean in the region where the backscatter is not a straight line.)

4. Press the MARKER button again. This causes the far left marker to remainfixed and allows you to move the second marker on the left. Move this to a

position just before the beginning of the splice.


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5. Press the MARKER button. Move the fourth marker to the end of the splice.6. Press the MARKER button. Move the fifth marker as far to the right as possible

without getting too near another discontinuity. (See step 3.)7. 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 indetail. However, there are quite a few OTDRs available today. Like the Photon Kineticsmodel, some are laboratory models. These are usually fairly large and have many functionsavailable. They also tend to have powerful lasers that can be used to test fibers that are verylong. There are also several portable models available. These are designed for techniciansto use in the field. They are much smaller and lighter than the laboratory models. Althoughthey 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 lessexpensive than the laboratory models.


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Fiber-optic Star Coupler

Optical fiber and connectors with transmitters and receivers would be all that we wouldneed if we only wanted to connect one person with another. But if we want to connectmany 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 ofdoing this, but the simplest is to use a passive device, one that requires no power for itsoperation. Of course, such a device cannot increase the power of the signalit can onlydecrease it. Fiber-optic couplers are devices that split, combine or multiplex optical signalswithout added power. A (1 X N) (read one by N) splitterortree-coupler(Fig. 1), splits asingle input into N equal channels or combines signals from N channels into one. A starcouplerusually has equal numbers N of input and output ports (N X N). It may also have anunequal number of input and output ports (M X N).

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). Thoseinclude the following

1. Splitting Loss (Ls) :Is the loss (expressed in decibels) that occurs naturally due to thefact 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 fiber-optic connectors through which the signal must passand the loss within the coupler itself, which is called the excess loss ( LE). In a star couplerthe signal coming from an output port has to go through 2 optical connections, one to getinto the coupler and one as it comes out. The loss for each optical connector coupled pairtends to be about 1 dB.

couplerinput 1

N

output Starcoupler

a

b

c

1

2

3


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2. Excess Loss (LE) : This is the total power lost in the coupler and never recaptured atany 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 lossbetween 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. Back-reflection (B) : Referred to as backscattered power is a measure of theoptical power (PREFL) reflected back to an input port for an optical power (P IN)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 reflectivityconnectors at the coupler ports and the fiber jumpers (UPC or APC connectors). We willnot measure this loss.

5. Directivity (DIR) : Is a measure of how well the coupler sends thepower from an input port to the port(s) to which it

is intended (output only).


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Light is launched into an input port and the optical power coming out of the other inputport(s) is measured with all output ports optically terminated. Optical termination can bedone by immersing the fiber end in index matching fluid, coiling the fiber before theconnector, 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 insertionloss 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 theinsertion 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 inTable 1. For each pair of input/output optical powers measured, the corresponding physicalquantity to be calculated according to the definitions is shown as a matrix element. In thischart, L stands for insertion loss, B stands for back reflection loss (which we will notmeasure), and D stands for directivity loss. For example, if you connect port 1 to the opticalsource and measure the dB loss between that port and port a, the result goes in place of the Lin 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 portswhen the power source is connected to port a and the meter is connected to port b.


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(Receive (Launch Power)Power) a b 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 lossmeasurements on two coupler ports. This is a two-step procedure and includes sending a

pre-measured amount of optical power into each coupler port and then measurement of theoptical power coming out of all ports. Fiber-optic loss measurements can be performedusing the following two configurations shown in Figures 3 and 4.

Reference Reading A

Reference Reading BFigure 3. Fiber optic loss measurement. Configuration #1

First we measure the power entering the star coupler by connecting a power meter to asource with a fiber jumper cable, and record the input power. Next, connect the source to aninput port of the fiber coupler, measure and record the power emerging from each output

port.

Reference Reading A

Detector

Reference

cable ASource

Reference

cable B

Reference

cable C Detector

Device UnderTestSource

ConnectorAdapter

ConnectorAdapter

source detectorReference: cable A


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Reference Reading B

Figure 4. Fiber optic loss measurement. Configuration #2

In both configurations the actual loss reading is the difference between reading B andreading 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 theinput 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 metermodel M 200 is adequate for wavelengths that are between roughly 800 and 900 nm. This iscalled the first window. The FOTEC model S 310 source is useful for measuring the totalloss at about 850 nm. For the loss at 1300 nm or 1550 nm, different sources and meters arerequired. We shall limit ourselves to the first window of wavelengths, where the FOTECmodel 310 sources and model M 200 meters provide the necessary signal and detectioncapability.

Plug in the transformers for the sources into the appropriate sockets and connect the powercord to the FOTEC source. The detectors are battery powered. Connect the appropriateadapters to the source and the meter. If the cables from the star coupler have 906 SMAconnectors (with ends shaped like ) put (white plastic) cylinders over the tips. Makesure that the top of the cylinder is flush with the top of the tip. This changes the SMAconnector to a type 905 configuration (shaped like ). The white plastic adapters are inthe 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 thestar coupler is symmetric, you can choose either side to be the input side.) Next connect anoutput cable of the coupler to a FOTEC meter. If you have several FOTEC meters, connecteach 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

Device UnderTest

detectorsource


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output port by connecting them to the meter one after the other. (Of course, the reason wecan 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 Busing every coupler port as input port and record the results in Table 2. A convenient datacollection table version for the 3X3 multimode fiber star couplers available for laboratory

testing is shown in Appendix 3.


a

b

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:

10/10 mdBPower -= (in mW)


a

bc

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 insertionlosses 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.


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a

b

c1

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 excessloss. 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 L tot. Fromequation 1, we see that this equals to LE + 2LC. In the measurement procedure however, theloss portion 2LC is included in both the reference reading A as well as in reading B and theactual power reading is calculated by subtracting the two readings A and B. Therefore thetotal dB loss will be equal to LE .

Once you have calculated Ltot, you can use equation 2 together with measurements of theindividual 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 whatextent is this true?

Measurements with Two Sources

Get a second source and measure its output power with a direct cable to the meter. Connectthe output of this source to another input port while leaving the original source attached toits input terminal. Measure the power at each output port. Using the same reasoning as thatin equations 1 through 11, predict what your expected output power should be. Compareyour prediction(s) with the actual measurements.

Symmetry

Most star couplers are symmetrical in the sense that the ports on either end can be used asthe input ports with the ports on the other end serving as the output ports. Test this propertyexchanging the input and output ends and measuring the port losses and total loss as you didin the first part of this experiment.

Documents

Fiberoptic Slab Manual