OTDRs and Location Accuracy

One of the many reasons to use an OTDR for testing and troubleshooting is to get data about a severe bend or break in your cable plant. The OTDR can provide the location of a break so that you can send out a crew to repair it. But, how accurate is that location? Are you setting up the OTDR properly to make the right calculations for distance?
 
Here is the main challenge for your OTDR when reporting an event location - in many cable designs, the fiber length is different than the cable length. Yes, there is more fiber than there is cable sheath, especially in outdoor, loose tube cables. The OTDR is measuring the length of the fiber you are testing – not the physical cable.
 
Let’s back up…
An OTDR uses the fiber’s “index of refraction” (IOR) value to calculate distances. This is provided by the fiber manufacturers and input into the settings of your OTDR. With the right IOR value, you will get an accurate fiber length report and accurate distances to ‘events’ like connectors, breaks, etc. But, what if there is 3 or 4 or 5% more fiber than there is cable? Your OTDR will give you a break location that could be hundreds of feet further than it really is.
 
So, how do you compensate for this?
To plan for this, we have to go back to when you receive a cable – before actually installing it. Performing proper acceptance testing when you receive optical cable is critical – otherwise locating a break in the outside plant could be challenging.
 
Performing an OTDR test on each cable spool and reporting the cable markings on each length of cable are essential. First get the IOR value and properly set up your OTDR. The cable documentation should provide the fiber’s IOR value – or call your vendor to get the data.  Document the sequential markings on each end of the cable to get the actual length of the cable on the reel. Then, using the IOR provided, take an OTDR reading and document the length of the fiber.
 
Once you have both of these values, you can derive a ratio between fiber and cable length. This is known as the helix percentage. Some cable manufacturers will provide a helix percentage with the cable documentation. Some OTDRs have a place you can enter this value to automatically compensate for the difference in lengths and determine the proper location.
 
If you do not have helix compensation features in your OTDR, you can easily calculate your own adjusted index of refraction using simple math, as shown in this example:

A bulk spool of cable has cable markings that indicate the cable length is 2,500 meters. You enter the fiber’s IOR of 1.468 into your OTDR setup screen and run an OTDR test. The OTDR reports the fiber length as 2,600 meters.
Divide the fiber distance by the cable distance to determine the helix factor.
In this case, 2600 / 2500 = 1.04.
Multiply the helix factor by the fiber’s initial IOR to get your adjusted IOR: 1.04 x 1.468 = 1.5267.
Input the adjusted IOR in your OTDR settings to get a location that is much more accurate – although rarely perfect!

 
Note: If you are testing an optical cable at more than one wavelength, remember to calculate an adjusted IOR for each wavelength to properly document your cable spans.