How to Use OTDR Testing to Troubleshoot Fiber Optic Cable Installations?
OTDR testing is a useful tool for troubleshooting fiber optic cable installations and certifying new ones. It can verify splice loss, measure length and find faults such as breaks.
An OTDR uses a high power laser light pulse to measure backscattered light in the fiber and reflected light from connectors and splices. It then displays the return signal as a trace or signature from the measurement of the fiber.
Attenuation Coefficient
The attenuation coefficient is an important tool in OTDR testing because it explains the amount of energy emitted, reflected, transmitted or received in a given time frame. This information is very useful for nuclear diagnostics, radiation protection, gamma ray physics, and other fields that require accurate attenuation data.
The OTDR uses an algorithm that compares the amount of light reflected by one test pulse to the amount of light returned by another pulse. The difference is called backscatter and is a function of the amount of attenuation in the fiber, the diameter of the core, and the glass that makes up the inside of the fiber.
When the OTDR sees a spike in backscatter that is greater than the amount of splice loss, it interprets this as a gainer. This false reading can be a major source of confusion for new OTDR users, so it’s best to avoid these areas while testing.
A gainer is typically caused when two fibers are spliced together with different backscatter levels. When this occurs, the OTDR reads the level at the end of the first fiber as a higher level than the end of the second and plots that data point higher up on the trace.
This is a very common cause of false results in splice testing. The reason this happens is because the second fiber has a higher backscattering coefficient than the first. When the two fibers are spliced, more of the first fiber gets scattered back into the fusion splice, making it appear to have a higher backscatter level than the second fiber.
This is why you should always use a launch cable, or a "pulse suppressor," when testing a splice or connector with an OTDR. This allows the OTDR to settle down after it sends a test pulse into the fiber, so that it can determine the amount of loss that has been absorbed by the splice or connector.
Splice Loss
The OTDR uses mathematical derivation to calculate loss from distance and time between two markers, one on each end of the fiber or connector under test. The OTDR does this by extrapolating the fiber traces from both ends of the event (connector or splice) using the least squares approximation method. The OTDR also subtracts the loss from the length of fiber between the markers to determine the actual attenuation of the connector or splice.
Splice Loss varies widely between different types of spliced fibers because of the many different factors that can affect loss. In general, splice loss depends on intrinsic and extrinsic parameters, such as the mode field diameter (MFD), core ellipticity and geometry parameters of the spliced fibers and also on the type of splicing process used.
For example, splice loss between fibers with different MFDs can be as high as 0.04 dB. Similarly, splice loss between fibers of different core ellipticities can be as high as 0.06 dB.
A higher splice loss can occur in the case of a mechanical splice when two fibers are mismatched at the splice point. This is due to the difference in backscattering coefficients of the two fibers. If the first fiber has a lower backscattering coefficient than the second fiber, less light gets scattered back from the splice.
However, if the first fiber has a higher backscattering coefficient than the second, more light will be scattered back from the splice. This causes the Palm OTDR to read the backscatter level at the splice point as higher than what it should be, which can lead to misleading results.
This is called a gain splice. Usually the Mini OTDR will show this as a negative loss on the splice loss screen. In order to get an accurate loss budget, it is important to include these gains in the average of the splice losses.
For these reasons, the splice loss average is a very important factor in constructing a system. The goal is to have a splice loss average that is equal to or better than the total loss budget. If the average is lower than the budget, then adjustments must be made in the splice process to achieve the desired result.
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Connector Loss
OTDR is an invaluable tool for testing fiber optic systems, from construction and installation to maintenance and fault locating. However, it is not without its limitations. Some OTDR test results may not be meaningful or accurate when testing very long fibers, for example.
The first step in using otdr testing is to set up the reference cables and ensure they are in good condition, clean and free of damage. Ideally, the reference cables should be 1-3 meters long and compatible with connector mating adapters for easy connection to the OTDR.
Once the reference cables are properly prepared, it is time to get down to business. The OTDR can launch one or more test pulses into the fiber under test, and it uses a technology rather like a radar set to detect echoes that return to the OTDR along the fiber. The OTDR can then measure the backscatter power of this light, and use it to estimate any losses on the fiber.
If there are any additional loss events in the network, they can also be detected by the OTDR and used to deduce their degree. This can include things such as a connector that has been broken, or a splice where two different connections have been made.
When OTDR traces are generated, they usually follow a pattern that is characterized by a curve that slops downwards and then peaks before dropping down dramatically. This decline is interrupted by sharp shifts that are usually caused by loss events such as connectors, splices or breaks.
As these losses take their toll on the reflected and backscattered light, the 7 inch multifunction OTDR shows trace results by plotting reflected and backscattered light against distance along the fiber as shown in figure 4. The Y-axis represents the power level and the X-axis shows the distance.
The OTDR trace provides a snapshot of the amount of loss in a fiber, which helps to identify the loss event and aids with installation troubleshooting. It can also be a source of valuable data for tracing and restoration later on.
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Ghosts
Ghosts are a type of false reflection that can occur in OTDR trace when the OTDR's light pulse passes through a fiber. These events are confusing to the technician because they appear as a real reflective event, but do not show loss and can be confused with actual faults in the fiber cable.
These ghosts are usually caused by high reflectance events, such as connectors, that cause multiple reflections in the fiber. The OTDR will record multiple traces as each of these reflections is bounced back and forth in the fiber.
When this happens on short cables, it can be very confusing and often leads to a false result when you test the cable. The best way to avoid ghosting is to only use the shortest test pulses possible and use averaging as much as possible. This will significantly reduce the number of ghosts and their effects on the OTDR trace.
Another problem with ghosting is that it can obscure the appearance of a true fault in the fibre, such as a bad splice or connector. A visual fault locator will also help here, as it injects a bright red laser light into the fiber to find faults in the trace that may not be visible to the OTDR's camera or detector.
Some of these faults are very easy to find, but others can be a little more tricky. For example, if there is a lot of attenuation at the end of the cable before the end connector and the OTDR does not show an end event, it may be a sign that the end connector has a severe stress problem due to tension or a very tight bend.
To avoid this kind of problem, it is a good idea to always compare the OTDR's trace with the documentation from the cable plant, which should include the cable length. This will ensure that the tester will not be fooled by ghosts and will only look at the point where the OTDR has detected a ghost, and not anywhere else.
It's also a good idea to check the distance scale in the event table to make sure that the OTDR has a valid measurement range. If the trace is very long, this might indicate that the OTDR is not properly set up for the length of the cable.