November FNT Instructor Tip

Remember to Check the Index of Refraction (IOR).

By: Jeff Dominique, Chief Instructor


IOR values may not be changeable when using auto test, so be aware of this and only use Real Time or Expert Mode when you really want to know the near exact distance to an event. The reason I say near is because the OTDR is measuring glass distance which is longer than what would be printed on the cable jacket due to the helix or spiraling effect that occurs within the cable structure.

There are two ways to find out what the true IOR is for a specific fiber under test. The first is to get it direct from the manufacturer using the cable code, fiber type and born-on-date. Most often, the manufacturer will affix a reel card to the outside flange of the reel, which includes test data and pertinent information such as the glass IOR.

Another way to find the true IOR is to locate an event location somewhere down stream and measure that distance. Make any necessary length corrections by multiplying the cables helix factor (found on manufacturers specs online) by the printed length on the cable jacket. While shooting the trace in Real Time, move the IOR number up or down until the location of the event matches the exact length of your calculation. By placing a cursor of the OTDR, where your calculated value is, this helps to really hone in on the true IOR.

FNT Instructors Recommend

When testing fiber optic cable with an OTDR, don’t forget to set the appropriate value of IOR (Index of Refraction) of the fiber under test. Doing so ensures that events will be positioned exactly where they occur, along the length of the fiber.

Note that a tenth of a point change in either direction from an accurate IOR entry can significantly shift event locations, sometimes by as much as hundreds of feet in longer circuit traces.

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