It is important to know how to use an OTDR and set up the instrument properly before you start testing. Failure to do so can result in measurement errors, splice problems and even damage to the OTDR itself.
Set Up the OTDR
An OTDR is a tool that can help you measure and troubleshoot fiber in your network. It combines a laser source and a detector to provide an inside view of the link. Using this technology, you can identify if there are any issues with the fiber and determine if there is a problem with the connectors or splices.
Before you can use the OTDR, it must be set up correctly. This includes choosing the proper measurement parameters and interpreting the traces that the OTDR generates.
The first thing you need to do is select the OTDR's test mode and set the test parameters. You should choose a test mode that best suits your needs. Most OTDRs offer single-mode and multi-mode modes, with each offering different levels of resolution and a range of other options for testing.
Next, you need to set the OTDR's test range. This will determine the maximum distance that the OTDR can display on the trace, and should be at least twice as long as the cable you are testing.
Another important setting is the test pulse. This will determine how closely the OTDR can see events in your network. Normally the OTDR can only see features that are 5 to 500 meters long (17 to 1700 feet) in the fiber, since the pulse is too long to see anything closer together.
This can be a problem in LANs, as the pulse cannot see patchcords and other close-spaced features that are typically in the cable plant. However, if you are using the OTDR to trace a long segment of a fiber and are only interested in finding a few closely-spaced events, this can be a useful feature.
Connect the OTDR to the Cable
An optical time domain reflectometer (OTDR) is a very useful tool in testing fiber optic cable. It can be used in a variety of situations. It is especially useful in long outside plant cables with many splices to verify that the cable was not damaged during installation or to determine if each splice has been properly made.
OTDRs use high power laser sources to create backscatter levels in the fiber that can be measured by the OTDR. The OTDR then uses that information to produce a trace that shows the level of attenuation in the fiber as a function of distance. This information is calibrated in dB/km to provide accurate test results.
The OTDR has a wide range of pulse widths that allow it to cover much more dynamic range than conventional fiber testers. However, this can cause problems when testing long lengths of fiber. For this reason, OTDRs typically have an option to control the test pulse power.
Most OTDRs also have a feature to compare two traces in one window. This is a great way to see how different test methods work on the same fiber. It is also helpful for comparing a trace that was taken just after installation to another to see what has changed since then.
Some OTDRs also have a visual fault locator built in to help with troubleshooting if a fiber trace is taking longer than expected or doesn’t look right. This is an extremely valuable feature that can save you a lot of time and money.
When a fiber is spliced, the two sides of the joint are often not identical and this can be a source of error when measuring splice loss. Looking at a splice that has a lower loss on one side and a higher loss on the other, the OTDR will show a "gainer". This can be confusing for new Mini OTDR users.
Turn on the OTDR
A fiber OTDR (optical time domain reflectometer) is an excellent tool for troubleshooting and fault locating in the construction of new fiber links. It works like radar, sending a pulse down the fiber and looking for a return signal. Depending on the model, this can give you the power loss at any point of the link, or an overall profile of the light loss across the entire link.
OTDRs use the effects of Rayleigh scattering and Fresnel reflection to make measurements. The light from the pulse travels through the fiber, gets scattered in all directions and then returns to the Palm OTDR.
Backscatter can vary considerably from one section to another, making it difficult to compare traces taken on the same length of fiber. Often, this backscatter variation is called ghosting and occurs when the OTDR isn't properly cleaned or wiped off after each test.
However, you can overcome these differences by comparing the losses from both ends of the same section with the OTDR and averaging them together. The errors cancel out, and the average value is closer to the true value of the splice or connector loss.
You can also use the OTDR's comparison window to copy and paste a trace from one end of the fiber to the other. This is useful for comparing different test methods on the same fiber, or when testing a trace from just after installation to see if there have been any changes.
Another thing to keep in mind when using an OTDR is the "dynamic range." This is the maximum optical loss an OTDR can measure from the end of the link down to a certain level of noise. This depends on the total amount of energy in the pulse and the sensitivity of the OTDR's detector.
Connect the Cable to the OTDR
OTDRs are a very powerful tool for testing fiber optic cable plants and identifying light loss points that can cause problems with the network. They do this by sending a high power pulse of laser light into the fiber where it is reflected back along the fiber and detected by the detector. This signal is processed to produce a trace on a graph that indicates where each light loss point is located and provides valuable information for the trained user.
However, OTDRs can be inaccurate for a variety of reasons. They may not have enough resolution to cover a length of cable plant, or they might be susceptible to test pulse overload. Also, some OTDRs are sensitive to reflectivity, which can cause nonlinearities in the trace.
To avoid these problems, OTDRs must be set up for a long enough distance to allow the instrument's receiver to recover from the test pulse. The pulse width determines how close two connections or splices can be resolved, so it is important to use the shortest possible pulse that will allow the OTDR to make a series of averages for the entire length of the cable plant.
OTDRs can also measure the overall loss of a cable plant using one or more reference cables. The type of reference cable used will vary depending on the type of cables in the cable plant being measured. For example, a patchcord test might require only one reference cable, while a fiber optic cable plant might need both a launch and receive reference cable to include tests for all connectors on both ends of the fiber plant being tested.
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Take a Trace
OTDRs are a great tool for detecting the location of loss in a fiber optic cable link. They provide a picture of the fiber with all the different elements reflected and can also pinpoint specific fiber locations on the network for further inspection or analysis.
During an OTDR trace, a laser source sends a short pulse into the fiber. The detector then receives the light reflected back from the different parts of the fiber link and produces a trace on a graph. This trace can be displayed in GTViewer and used as a visual representation of the location of the loss.
The OTDR is capable of taking multiple test samples and averaging the results to get a better idea of how much loss has occurred. This can help in determining the amount of fiber lost and the extent of the event.
A typical 7 inch multifunction OTDR pulse width ranges from 10 to 30 nanoseconds (ns). This allows a technician to see more detail on the trace and identify events that are close together.
It also provides the ability to compare traces at different wavelengths--850 nm on multimode fiber and 1300 nm on single mode--since fiber is more sensitive to stress at longer wavelengths. This can be very helpful in locating stresses caused by improper installation.
Another way an OTDR can help detect the exact location of a loss event is by showing the backscattered light from the event. This is called Rayleigh backscattering.
However, this can be a misleading indication of the actual loss of the fiber. It can also mask other events such as connectors or splices that reduce the amount of light returned to the OTDR.
