VDV Works Virtual Hands-On Training
Fiber Optics Testing: Using OTDRs
 
Part of the VDV Academy Fiber Optic Training Programs

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 Dos & Don'ts

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Take Data

Viewing the trace

Measure Loss

Modify Test Parameters

Compare Traces

Troubleshooting

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More Useful References

 Understanding OTDRs

"Virtual Hands-On" Termination

"Virtual Hands-On" Testing


Troubleshooting With The OTDR

It's what the OTDR is best at. Use it to find breaks in the fiber, stress points, high loss connectors and splices and points of high reflectance.

The very best troubleshooting tool is good documentation! If you know the lengths of the cable segments and where connectors and splices are, you can simplify troubleshooting tremendously.

Bad Connectors Or Spices: If the dB loss of a connector is over the acceptable limit, the OTDR will show it. The horizontal zoom tool may be used to enlarge the trace and provide a clearer view of each connector.
We know this is a connector or perhaps a mechanical splice because of the large back reflection. A fusion splice rarely has a reflection unless it has an air bubble trapped in the fused area.

Locating Breaks
A break in a fiber can be caused by any number of reasons, usually traced to human error (that's why we call the typical OSP fault "backhoe fade"!)
The break will obviously be the end of the fiber, where it may have little or no reflection, depending on the shape of the fiber where it is broken. Since the OTDR length measurement is the fiber length not the cable length, the fault is probably about 1% closer than the fiber length. You can shoot in both directions and average the length to the fault to reduce uncertainty.



Cable Stress Points
Note the "rolloff" at the end of the cable. This point of higher attenuation before the end may indicate a severe stress problem on the cable due to tension or too tight a bend. The condition of the cable should be investigated.



Ghosts: Some events that appear in the trace to the right may appear as bad connectors, however they are "ghosts."

Ghosts are caused by high reflectance events - usually connectors - that cause multiple reflections as the reflected pulses bounce back and forth in the fiber. Short, highly reflective fibers can have two or three ghosts! Ghosts are especially prevalent in short multimode premises cables since multimode connectors are not optimized to reduce reflectance.

Ghosts can be spotted by several telltale signs. They are always at a multiple distance of a highly reflective event. Note the ghost above is at about 14 km, twice the distance of the highly reflective event at 7 km. This pulse is also in the noise after the end of the cable, but it may not appear so in shorter fibers.

Knowing the length of the fiber will prevent confusion from ghosts!

Remedy: Index matching fluid can be applied to shield the reflectance that occurs at the connectors. If proper index patching fluid is not available, a dab of mineral oil or petroleum jelly may be used instead. Be careful to clean the gel off of the connectors after the test process is complete, so it dust or debris won't stick to it.

Here is another example of a ghost, where the ghost is not in the middle of the noise. The end of the fiber (at the right of the red marker) is highly reflective and causes the ghost at the blue marker. The reflection is so large it saturates the OTDR receiver which has not yet recovered by the ghost, making it look like the end of the fiber is actually the ghost caused by a high loss connector!

Saturation: Reflective events with flat tops indicate the receiver is overloaded by the return pulse. 

Pulses like this cause long recovery times, longer than would be expected from the OTDR pulse width chosen. If the pulse is overloading the OTDR, it may affect the accuracy of measurments following it (like the fiber attenuation) and cannot be used for measuring back reflection. Sometimes using an attenuator in the launch cable or a longer launch cable can reduce the pulse level enough to prevent saturation.

BE WARY OF AUTOTEST!

All OTDRs have an autotest button that is very tempting. It analyzes the trace and chooses the proper setup, finds all the events and measures all the fiber attenuation. No operator input is required. Sounds tempting, doesn't it?

Well, here is an example of what can happen. This OTDR operator used a 200 foot (60 m) launch cable to test a 75 foot cable from one end (missing the connector on the far end) at both 850 and 1300 nm. The reflections show flat tops which indicate the OTDR receiver was saturated at both wavelengths, but at 1300 nm, the recovery of the OTDR was longer than the cable being tested, making it impossible to find reference points for loss measurements with markers. 

However, this OTDR autotest function says the cable passes at both wavelengths, even though the saturated 1300 nm trace shows a 0.39 dB GAIN at the joint (-0.39 dB means a gain.) The customer of the installer who submitted these traces for verification of the installation rejected them and required over 1,000 cables be retested at the installer's expense!

Our advice - never use autotest unless a knowledgeable operator has tested some representative fibers and verifies the OTDR autotest function is trustworthy.

bad example

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