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How to understand the OTDR trace?

How to understand the OTDR traces? : An OTDR acts like a radar. It sends a pulse down the fiber and looks for the return signal from the measurement of the fiber to create a display called "trace" or "signature". Two factors contribute to the return signal from the OTDR test pulse: reflection and backscatter. This article is not based on how the OTDR works, but it is based on how we understand the OTDR trace.

When we connect OTDR to a fiber for the test, then which types of the peaks are seen. We will discuss same in this article. The reason for getting peaks in the OTDR is also that reflection signals are peaks in the OTDR trace produced by the polished fiber at a connector or cleave where the light backs up the fiber.
OTDR trace

Meaning of peaks in OTDR trace

We receive different types of peaks during the OTDR trace, now we will try to find out what those peaks mean and why. Usually the following types of peaks are found in the OTDR trace-
1. Connector Pair
2. Fusion Splice
3. Fiber Bend
4. Fiber End
5. Dead Zone
6. Reflectance and Optical Return Loss

Now we will try to know about above mentioned points in brief-

Connector Pair

Optical fiber cable experts believe that a high return loss is a good thing and usually results in low insertion. In optical networks, OTDR displays the connecting points at the point where we use optical connectors via a peak, fiber optic pair or connector greatly affects the performance of an established fiber optic link. High connector loss, low return loss, or high reflectivity will impair an application.

Connector pair create losses so optical loss of connectors also called attenuation, any type of optical losses simply  reduction of optical power induced by transmission through a medium such as a pair of fiber optic connectors. 

When we calculate the optical link budget, we consider the connector loss is 0.1% but most fiber connectors typically have a range of 0.3 to 0.5dB for standard loss and 0.15 to 0.2 for low loss.

Fusion Splice

Splice loss in OTDR trace

Splice losses usually depend on the drum length of the optical fiber cable laid in the optical network. If laid the drum of 2Km-4Km or 6Km the splice losses shows respectively in the OTDR trace. When you take the OTDR trace, you receive the trace like above. The above trace is not quite straight, some peaks are seen in the middle. These peaks are due to fusion splice loss.

Splice loss is usually treated as follows-
➤losses range between 0.15 to 0.25dB/km for 1550nm systems. 
➤0.25 to 0.35 dB/km for 1310 nm single-mode. 
➤0.5 to 1.5 dB/km for 1300 nm multimode.
➤2 to 3.5 dB/km for 850 nm systems.

Fiber Bend

Analysis of OTDR trace

The bending looks almost identical to the splice, but with a smaller difference. The splice will produce approximately the same attenuation in both wavelengths. In the case of fiber bending, the measurement at 1310 nm will not show at all, or it will be poorly visible, and will produce several decibels at 1550 nm.

How can you understand that the fiber has to bend, not a bad splice. If the bend loss is visible at a place where there is no splice closure, then we should understand that, it is a worrying sign that something is wrong with the cable. It is necessary to look and see, perhaps, the cable is torn off by fasteners and simply damaged.

Another reason created by splicer for the cable bending is when the cassette in the splice closure is designed for a 40 mm FO Splice protection kit, and splicer use 60 mm splice protection kit was crammed into it. 

Overall Bend loss depends on how the FOC is bend, if there is too much bend then that point will show too much bend loss. Bend loss 0.5 to 0.75dB can be considered.

Fiber End

See the picture above to see how the end of the fiber peak appears in the OTDR trace. The end of the fiber can be recorded in the OTDR as both a large peak and a small peaks, sometimes no peak can occur and the line immediately closes in noise.

When fiber is end, we do not pay attention to how small or big the peak is. We should understand one more thing here if the far end of the fiber is connected to the telecom equipment and the peak is still very high, it may be necessary to clean mechanical cross connections at that end of the fiber.

Keep in your mind another very important point that Ghosts peaks can also be introduced on the OTDR trace if we incorrectly set the distance range.

Dead Zone

Dead Zone in OTDR trace
OTDR photodiodes require time to recover from the saturated state; During this time, it will not correctly detect the backscatter signal. The total optical length which is not fully characterized during the recovery period is termed the dead zone.

To get complete information about Dead Zone, open the link given below-

Reflectance and Optical Return Loss

Reflectance and Return loss
Look carefully at the image above, which clearly shows reflectance in the OTDR trace. The reflectance of an event represents in the OTDR trace and actually it is the ratio of the reflected power to the incident power at that discrete location in a fiber span.

We can express the the Optical Return Loss as the total optical power returning
to the source from the complete fiber span. To get complete information about Optical Return Loss, open the link given below- 

Last Word

Dear friends, I have tried my best to provide enough information about the OTDR trace. How did you feel about this article "How to understand the OTDR trace?" Do let me know by commenting.

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