Knowledge base · Measuring

Index of refraction: the one number the whole length rests on

An OTDR does not measure distance. It measures time and converts it into metres through the fiber's index of refraction. Get the fourth decimal of that number wrong and you are metres out over a kilometre.

Check the IOR in your own file

The interface calls it IOR; cable datasheets call it the index of refraction, or the group index.

Where it comes from

Three sources, and they do not always agree.

The instrument's settings

Whatever value the technician had set at the moment of the measurement is what got written into the file. Often it is the instrument's default, left over from the previous site.

The cable's datasheet

The real value for this fiber type at this wavelength. That is the correct one — and it differs between manufacturers, and between wavelengths of the very same fiber.

Common sense

If the optical length along the trace disagrees with the design and the closures sit "almost" where they should, the cause is usually here rather than in the cable. It takes a minute to check: put in the datasheet value and see whether things line up.

What recomputing does

It does not change the shape of the curve, only its distance axis: every point — the events, the fiber end, the cursors — moves proportionally. Levels in decibels stay as they were. The saved file keeps a record of which value was changed into which, so whoever opens it after you sees both the new number and the old one.

What we do not fill in for you

There is no default index of refraction in the service, just as there are no acceptance thresholds. A plausible number in a field would read as a recommendation, and the correct value is known by your cable's datasheet, not by us. The recomputation is done with your number, and by hand.