People get funny with decibels (dB). To make 10 dB of attenuation sound worse than 0.1 dB of attenuation they put a negative sign in front of it. That gets confusing, because theoretically -0.174 dB of attenuation is the same as 0.174 dB of gain (if your fiber is designed to be non-linear it might give you gain). It also gets based on perspective, as in if attenuation is a good thing (something you are designing for) or a bad thing (something real you need to overcome).

All that said, if your spec is 0.100 dB of attenuation, it looks like you are failing by 0.074 dB. That isn't much; is your OTDR tester properly calibrated?

Z

thx zappedagain for your reply.
I confirm,our tester is properly calibrated.
warm regards

Generally, fiber optic cable has no gain, being purely passive. However, there are certain wavelengths that might excite non-linear behavior, such as 2nd harmonic generation.

What material is the fiber?
What wavelength is the OTDR using?

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thanks for reply.
1550 nm and 1330 nm for wavelength.

So, negative attenuation is essentially gain, but it's unlikely that you actually have gain, so there may be a coupling error somewhere in the path.

How does the system actually do the measurement, i.e., what is it comparing against to determine attenuation? I'm thinking that there's a measurement with and one without the cable, and the system takes the difference to be the attenuation.

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