Hello, everyone!
Today, I will continue to explain OTDR and OTDR trace analysis. This is the second article about this theme.
There are two types of events:
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Reflective events. These types of events are connector pair, mechanical splice, etc. - due to "mismatch" the signal is reflected. And
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Non-reflective events. They are fusion splices, macro bends, etc.
I will detail explain these events in the next article. Let's start from the beginning.
Fiber length
We can measure the length of the optical trace, or more precisely the optical fiber. The length is usually given in m or km. It is a horizontal scale on the display. There may be confusion because the optical fiber is longer than the optical trace. The fiber is longer than the optical trace for about 1-2%.
Total attenuation of the optical trace/fiber
OTDR can calculate the total attenuation of the optical trace or optical fiber. The unit is dB. Total attenuation includes attenuation of fibers (in dB/km), attenuation of splices, connector pairs, macro bends, passive optical splitters, WDMs, etc.
Typical attenuation on the optical trace or optical fiber:
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The attenuation of optical fibers depends on the wavelength. The attenuation of SM fiber at 1310nm is about 0.3-0.35 dB/km, at 1550nm it is about 0.2-0.25 dB/km. The attenuation of MM fiber at 850nm is about 3 dB/km and at 1300nm is about 1 dB/km.
The attenuation of fusion splice is about 0.1 dB (MM splices – 0.1-0.5 dB, SM splices – 0.05 dB),
The attenuation of the connector pair is about 0.5 dB (MM conectors – 0.2-0.5 dB, SM connectors (which are factory made) – 0.1-0.2 dB, and SM connectors (wich are field terminated) – 0.5-1 dB),
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Optical splitter attenuation: 1:2 – about 3.5 dB, 1:4 – about 7 dB, 1:8 – about 11 dB, etc
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WDM attenuation is about 0.5 - 0.8 dB, etc.
If the splicing is done very well, the OTDR may not see the fusion splice. Due to the appearance of strong reflections, "ghosts" can appear. Different types of fibers lead to a phenomenon called “gainer”. Macro bends are seen on the optical trace as fusion splices. Strong reflections affect the resolution of the OTDR instrument. We cannot see close to the events. Then, problems with measuring on short links, such as FTTH, etc. All these phenomena can lead to confusion. Therefore, I repeat, it is important to have good documentation and know the operation of the instrument.
I will continue on the next article >> OTDR trace analysis (3)
Thank you!
