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OTDR Waveform Interpretation What’s on Screen OTDRs provide extensive information on the condition of your fiber optic lines. They may tell you there is something at 11.457 km with a loss of 2.346 dB and a reflectance of 46.7 dB. But it really doesn’t tell you what it is! If you are unfamiliar with OTDR waveforms and particularly if you don’t have any cable maps or original test results for comparison it can be difficult to correctly interpret the OTDR. Even the automatic measurement software found in modern OTDRs doesn’t identify what the item is, it just gives its characteristics. This application note shows the most common types of shapes, you will see on an OTDR waveform. By combining your knowledge of the system under test, and sometimes a little extra testing, you can identify what the waveform event actually represents on the fiber. Once you have identified the event you can compare it against your system specifications to see if it is good or go fix the problem if it is not acceptable! Tests shown in this application note are available from Tektronix for reference and training. The tests can be viewed using a TFP2A or TFS3031 OTDR, or using the Tektronix OTDR software program, FMTAP. The file name is shown for each picture so you can reference the events on your OTDR or PC. Pictures for this application note were taken with FMTAP.

The Fiber Optic Cable The first cable feature to look for with the OTDR is the fiber optic cable itself. With the OTDR you can see if there is a cable present by looking for the very small amount of light reflected from the glass itself. This light is called Backscatter: the light reflected or scattered back towards the OTDR. The OTDR trace shows this as a flat line or gradually dropping slope (see Figure 1, file pc-end2.cff).

Figure 1, Fiber Backscatter When you see the backscatter type of line it means there is cable, glass, present. If you don’t there is no cable or the OTDR signal has been stopped before reaching the point of interest. The backscatter is important in measurements. Where it stops, that is the distance to the event at that location. Vertical changes in the backscatter is loss or attenuation.

p

Backscatter(fiber present)

No backscatter(no fiber)

Copyright © 1998 p

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The End of the Fiber Now that you can see the fiber you want to know where it ends! In acceptance testing you need to confirm the fiber length. In restoration you want to find the distance to the break. Most OTDRs will mark the events on the fiber. The last marker or the last item in the event table is the end of the cable. Interpretation difficulties occur in determining if the end shown is the system end or a fault close to the far end. The shape of reflection at the end event gives you information about what is happening at that point. There are three types of end reflections. 1. Highly Reflective End When light traveling down glass fiber encounters the end of the fiber, light is reflected from the glass-air interface. This reflection is called a Fresnel (pronounced fra-nel) reflection. When the fiber end is flat, as with a precision cleave or a PC connector, there is a large reflection returned down the fiber (see Figure 2, file pc-end2.cff).

Figure 2, Flat Polish End Reflection When you see a large Fresnel reflection, typically greater than 30 dB reflectance, along with an end to the backscatter this

indicates a flat precision (polished or cleaved) end. The OTDR is able to test all the way to that connector. This does not mean that the connector itself is good, just that the OTDR can get to it. The only way to completely test a connector is to test through it. To check the quality of the far end connector you have to make and OTDR test from that end, or attach a long jumper cable to the far end connector. 2. Low Reflection End Some systems use angled end connectors (APC connectors). The end of the fiber is cleaved or polished at an angle to reduce return reflections. You need to know if your system used flat (standard or PC) or angled (APC) connectors to know what type of end reflection to expect. The angled connectors will have a reduced Fresnel reflection at the open end (see Figure 3, file APC-end2.cff).

Figure 3, APC end An APC end reflection is typically less that 45 dB reflectance. In some cases you may not see a Fresnel reflection at all. As with the flat polish connector seeing to the far end tells you the fiber reaches the connector but doesn’t tell you if that connector is good.

Fresnelreflection

End offiber

Small Fresnelreflection

End offiber

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3. Non-Reflective End A Fresnel end reflection is caused by a precision end, either cleaved or polished. A broken fiber usually does not have a Fresnel reflection because the end is shattered (it is glass). There is no precision surface to reflect large amounts of light. After the broken end the backscatter slope from the cable drops to the noise level at the bottom of the screen (see Figure 4, file brk-end2.cff).

Figure 4, Non-reflective End When you have no end reflection usually this indicates that the light is lost at that point due to a break or bend in the cable. Some very low-reflection APC connectors may look similar to this case. One non-reflective case is actually the end of the OTDR test, not the end of the fiber. If you see the fiber slope continue all the way to the noise level at the bottom of the screen the OTDR has run out of range before it found the end of the fiber (see Figure 5, file rng-end1.cff). Check your OTDR instructions on how to increase the range of its test or check for events that are causing much of the signal to be lost. This may occur on very long fiber or where a connector/splice has excessive loss.

Figure 5, End of Range Mid-Cable Reflective Events Mechanical connectors in the system (patch panel connectors or mechanical splices) create a Fresnel reflection similar to a precision end. The reflection is caused by the gap between fibers inside connector. This differs from an end reflection in that you can see the backscatter line from the fiber on both sides of the reflection (see Figure 6 top waveform, file rfl-mid1.cff).

Figure 6, Connector Reflections

APC connectors and gel-filled mechanical splices will have a smaller reflection than PC or flat polish connectors (see Figure 6, bottom waveform, file apc-mid1.cff). The APC connector angle reduces the amount of Fresnel reflection. The matching gel in mechanical splices serves a similar, Fresnel reflection reduction function.

No endreflection

End offiber

APCreflection

PCreflection

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The OTDR provides loss and reflectance measurements for the connector. The measurements can be compared to system specifications. Zero or very low loss reflections may be an echo of an event rather than a real event. Waveform echoes occur when light re-reflects off mechanical connectors (one of the connectors could be the OTDR connector). Echoes create a reflection that repeats the distance between the connectors. For example, if you have two connectors 1 km apart you may see an echo 2km from the first event. Some OTDRs will warn you if a reflection is a possible echo but most will not. Mid-Cable Non-Reflective Events Connectors and mechanical splices have Fresnel reflections because of the gap between the two sections of glass. This large reflection makes the event easy to find. Fusion splices and bends do not have a Fresnel reflection. In these events the glass is not interrupted but there is loss of light. A bend or fusion splice appears the same on the OTDR trace. The slope of the fiber backscatter drops at the location (see Figure 7, file bend1310.cff). The amount of vertical drop is the splice or bend loss. Check the loss against your system specifications to see if it is good or bad. The automatic measurement given by the OTDR is often the most accurate way of measuring the loss.

Figure 7, Splice or Bend Determining if this is a splice or a bend may be critical in fixing a bad (high-loss) point. The loss may actually be a bend near the splice (e.g., going into the splice case) rather than the splice. You need to know what you are looking for or you could waste time trying to fix a good splice! On singlemode fiber there is a simple test to differentiate a bend from a splice. One characteristic of a bend is that its loss is higher at 1550nm than at 1310nm. A fusion splice will have similar loss at both wavelengths. It is common to have a bend near a splice so the measurement is a mix of the splice loss and bend loss. For example, the loss of the event seen in Figure 7 was 1.21 dB at 1310nm. However, the same point has 4.75 dB loss at 1550nm! Both tests are shown in Figure 8 (Top waveform 1310nm file bend1310.cff, bottom waveform 1550nm file bend1550.cff). The problem at this point is a bend, not the splice. The bend is at, or close to the splice. OTDRs will group events close together so the bend may be several meters away from the splice and the OTDR is only able to show one loss event.

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Figure 8, Bend at Two Wavelengths Testing the event at 1310 and 1550nm wavelengths is an excellent method of determining the type of fault you are looking at. Bends may also be present in reflective events and will show similar wavelength dependant loss variation. Sorry, this method does not work with the 850/1300nm wavelengths used in multimode systems. Gainers and Intrinsic Loss In all events mentioned above the loss of the event was the difference in the fiber backscatter before and after the event. However in some events, changes in the fiber characteristics (changes in scattering coefficient, core size mismatch, etc.) can fool the OTDR loss measurement. These changes in fiber characteristics (intrinsic effects) can even make the loss appear to be a gain! The extreme case of these intrinsic fiber effects is a “gainer”. A gainer is where the waveform goes up instead of down as in a normal loss (see Figure 9, file gain-1.cff). Since the loss is assumed to be less light, the up-side-down loss will measure as a negative loss, or a gain. This can happen on both reflective and non-reflective events -- anywhere you join two different fibers.

Figure 9, - Gainer Gainers are a function of different (not bad, just different) sections of fiber. At the opposite end of the fiber section from the gainer you will get a larger than normal loss. The amount that the loss measurement is pushed up at the gain, it is pushed down at the opposite end of the fiber section. You may see (in more extreme cases) a whole section of fiber that seems higher than adjacent sections (see Figure 10, file gain-1.cff).

Figure 10, Raised Fiber Section Intrinsic measurement offsets can cause problems in determining if the splice loss meets your system specifications. The gainer measurement can’t be correct -- you don’t get real gains in joined sections of glass. The extra large loss at the far end of the section is only partially splice loss. You might try to fix a splice where the issue is the cable not the splice. Re-splicing will not fix the large loss measurement in these cases.

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As with bends, intrinsic events take a little extra testing to resolve. When you see a gainer you know you have intrinsic effects but you don’t know how much it is affecting the loss measurements. To resolve the issue, find the true splice loss, you need to test the splice from both ends and average the measurement. The amount the measurement is pushed up from one is the amount is will be pushed down when testing from the opposite direction. By averaging the two measurements you get the correct splice loss, without the intrinsic caused offsets. For example, you may have a splice that measures as a 0.20 dB gain (-0.20 dB loss) from one direction. Testing from the opposite direction the splice measures a 0.30 dB loss. Averaging the result: The averaged measurement solves two problems: 1. The 0.20 gain was obviously not an accurate splice loss measurement.

2. The 0.30 dB also was not an accurate measurement, though we don’t have the gainer clue to realize this if you were only looking at the loss of that event. For example, if your maximum allowable splice loss was 0.2 dB you would have tried to fix a splice that was already good. Many OTDR documentation software programs will do the bi-directional or two-way average for you. They will match up the splices and calculate the average splice loss. Figure 11 shows a bi-directional waveform comparison done with the FMTAP software program. Waveforms shows are A-demo01.cff and B-demo01.cff that come with the FMTAP software program.

Figure 11, Bi-directional Waveforms Figure 12 shows the bi-directional event table from the A and B-demo files.

-0.20 + 0.302

= 0.10 2

= 0.05 dB

A:A-demo01.cff B:B-demo01.cff Evt Distance Splice Evt Distance Splice Average Loss Loss Sp Loss # (km) (dB) # (km) (dB) (dB) 1 0.000 0.00 5 53.486 7.93 3.96 2 24.478 0.29 4 28.920 0.33 0.31 *3 49.063 0.04 3 4.423 0.23 0.13 4 51.285 0.29 2 2.215 -0.08 0.10 5 53.481 7.62 1 0.000 0.00 3.81

Figure 12, FMTAP Bi-directional Event Table