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Why is the Fiber attenuation measurement not accurate when testing short fiber links?

As with all OTDR traces there are small variations in the signal level caused by noise. If we zoom in on a section of a trace we can clearly see this variation in signal (dB):

In the example above we observe a difference in signal of 0.108dB between marker A and marker B.

Let’s assume that the fiber under test has an attenuation of 1db/km at 1300nm. By doing the calculation we can see the impact the variation in signal could have on a short link (100m) compared to a longer link (10km):

Short Distance (100m):

Attenuation = Loss(dB) / Length (km)

Loss = 1 x 0.1 = 0.1db, Length = 0.1km

Attenuation = 1db/km

Now let’s add the 0.108dB difference we observe between markers A and B:

(0.1+0.108)/0.1 = 2.08dB/km (This represents a 108% increase in attenuation over a 100m link)

Long Distance (10km):

Attenuation = Loss(dB) / Length (km)

Loss = 1 x 10 = 10db, Length = 10km

Attenuation = 1db/km

Now adding the same 0.108dB difference observed between markers A and B:

(10+0.108)/10 = 1.0108dB/km (The same variation only causes a 1.08% increase in the attenuation measurement over a 10km fiber section)

In FastReporter, if you set a threshold value for the fiber section attenuation then you have the option of applying a minimum section length. The thresholds will not be applied to all fiber sections shorter in length than the user specified minimum section length.