What is the purpose of setting the index of refraction parameter correctly on an OTDR?

The purpose of setting the index of refraction (IOR) parameter correctly on an OTDR (Optical Time Domain Reflectometer) is to ensure accurate distance measurements along the fiber optic cable. The index of refraction is a property of a material that describes how much light slows down when traveling through it. Light travels slower in glass than in a vacuum, and the index of refraction quantifies this difference. The OTDR calculates the distance to events (like connectors, splices, or breaks) by measuring the time it takes for a pulse of light to travel down the fiber and for the reflected or backscattered light to return. To accurately convert this time into a distance, the OTDR needs to know the speed of light within the fiber. This speed is determined by the index of refraction of the fiber core. If the index of refraction is set incorrectly, the OTDR will miscalculate the distance to events. For example, if the IOR is set too low, the OTDR will underestimate the distance to events, and they will appear closer than they actually are. Conversely, if the IOR is set too high, the OTDR will overestimate the distance. Fiber manufacturers specify the index of refraction for their fibers, and this value should be entered into the OTDR before making measurements. Small errors in the IOR setting can lead to significant distance errors over long fiber spans. For example, an error of 0.001 in the IOR setting can result in a distance error of several meters over a 10-kilometer cable. Therefore, it is crucial to set the IOR correctly to ensure accurate distance measurements and reliable troubleshooting of fiber optic networks.