Hello everyone!
Today I continue to explain how to adjust the OTDR parameters to make the measurement more accurate. This is the last article about this theme.
Pulse width
This is not the first parameter for adjusting but it is one of the most important parameters. The width of the impulse affects several things. When we choose a smaller pulse width (e.g. from 3 ns to 10 ns), we have a small “dead zone”, we can see close events (e.g. splice and connector pair at close range), we have a good resolution but we have a small range. If we choose a large pulse width (e.g. 500 ns), we have a large “dead zone”, we cannot see close events, we have a poor resolution but we have a large range.
Figure 1. Pulse with
(https://www.thefoa.org/tech/ref/testing/OTDR/OTDR.html)
Experience is required to work with OTDR, such as pulse width adjustment. The recommendation is as follows: for optical links less than 1 km, we should use a pulse width of 3 ns to 10 ns. For medium distances, the pulse width should be up to 20 ns to 100 ns. And for very long cables, a pulse width of 500 ns to several microseconds should be used.
Range
The range is the total length of the optical path to be tested. If we misjudge the length of the optical link, a measurement error occurs. It is therefore recommended to adjust the twice longer range.
Duration
This parameter refers to the duration of the measurement. If the duration time is short, we will get poor resolution, we will not see all the events clearly on the OTDR display. If we put a longer measurement time, we will get a good resolution and we will see all the events. The only disadvantage is waiting for the measurement process to complete.
Index of refraction
Index of refraction or shortly IOR is the index of refraction of the optical fiber core. The speed of light propagation through the core/fiber depends on this parameter. OTDR measures time and converts it into distance. This parameter can lead to a measurement error. That is why it is useful to set this parameter. Generally, it can do without this parameter, but in order to increase reliability, it needs to be entered.
I finished the article “OTDR analysis”. In one of the following articles, I will describe OTDR instruments specialized in PON measurements.
Thank you!
