These powerful OTDRs include intelligent Optical Link Mapper (iOLM) software that turns trace analysis into a single, one-touch process. They also feature a wide selection of test wavelengths, for first-class flexibility.
OTDR Basics
OTDR is an invaluable tool for determining the condition of fiber optic cables. It can verify splice loss, measure length and locate faults. It can also be used to create a “picture” of an installed cable before it is removed from service or replaced, which makes it ideal for troubleshooting and analysis.
During OTDR testing, the instrument sends a pulse of light through a fiber and measures the backscattered power level. This signal strength is then compared to the original signal in order to determine whether there are any losses. The difference in the backscattered power level of the two signals gives an indication of the position and degree of any loss.
The OTDR uses the effects of Rayleigh scattering and Fresnel reflection to detect backscattered signal levels. When a splice or connector is installed, the backscatter power level of the fiber changes due to the reduced transmitted light intensity from the connector or splice. This change in backscatter power is measured by the OTDR, which can then calculate the loss for that specific event.
One of the most important features an OTDR has is its dynamic range. This range is a function of the attenuation of the fiber in use and is typically 5 to 8 dB higher than the maximum loss that will be encountered during the test.
A high dynamic range is important to ensure accurate results. It is especially important for premises testing, which is often done in environments with less room for error.
Another essential feature of OTDR is its event dead zone. An event dead zone is the minimum distance between two consecutive events detected by the OTDR. This is important for detecting closely spaced events in the link, such as the patchcords that connect data centers.
The shortest possible event dead zone allows technicians to identify and trace a problem with more accuracy. Using an OTDR with a long event dead zone can lead to missing an event or having to search for it for long periods of time.
When selecting an OTDR, it is important to choose one with a short event dead zone so that you can be sure you are locating the smallest possible loss in your test. It is also essential to find an OTDR with a low noise floor and a wide dynamic range, as this will allow you to accurately read the trace.
Setting Up the OTDR
An OTDR is a powerful instrument that tests fiber optic cables. It sends pulses of laser light through the optical fiber and then collects the reflected light back to its receiver. The reflected light is then used to determine the condition of the fiber. The Palm OTDR uses Rayleigh scattering and Fresnel reflection to analyze the quality of the fiber.
OTDRs can be configured to detect all types of light loss events, including connector losses and splices. These loss measurements are essential for reliable operation of fiber optic networks.
A good OTDR can measure up to 250km of cable in a few seconds. It will then create a trace which shows the losses over time, allowing you to compare them against installation documentation and splice closures.
Most OTDRs also have an "auto test" function. This is a great timesaver when it is properly set up. However, the auto-test function is not foolproof and can give inaccurate results if you do not have the knowledge to use it correctly.
Before using the "auto-test" function, make sure you understand how the OTDR works and how to read an OTDR trace. If you do not, you can easily get stuck with unreliable tests and wasted money.
As a general rule, it is best to avoid using the auto-test function when you are not well-versed in how to use the OTDR and when you are not familiar with the characteristics of the cable plant being tested (length, number and location of splices and connectors). If you do try it, be sure to have an experienced operator look over your test before pressing the "auto-test" button.
One of the most common sources of error with OTDRs is the difference in backscatter between two different fibers or inline splices. This is caused by the different power levels of the light that gets scattered and reflected from the fibers. By measuring the losses in both directions and averaging, you can eliminate this source of error and get accurate results.
When you are ready to run a test, select a wavelength on the OTDR's display that matches the one on the network or cable plant being tested. If the OTDR cannot detect a wavelength that matches the one on the network or cable, you will receive a message stating that the selected pulse width is too wide. You should then disconnect the fiber from the SM Live port and start another acquisition. The Mini OTDR trace should then look normal again.
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Reading the Trace
When you first use an OTDR, the trace it produces is a lot to take in. The trace shows you how the test pulse travels down the fiber, allowing you to measure attenuation and reflectivity (scattering and absorption) and it also shows you how the signal changes from one point on the trace to the next.
When the test signal goes past a splice or connector it will show up in the trace as a drop. This is due to the loss of the splice or connector, but it will also show a peak at that location in the trace because of the reflection from the connection. The reflection can be an indication of a problem such as an erroneous gain or ghost.
OTDRs can also compare traces in the same window, which is useful for confirming data collection and contrasting different test methods on the same fiber. Using comparisons also helps you troubleshoot because it allows you to see what has changed since the last time the trace was taken.
Traces can also be averaged. This is important because it allows you to get better resolution by using longer averaging times. This is especially helpful when the 7 inch multifunction OTDR is capturing a large number of traces, which can be useful when you want to cover long distances or find events that were lost with shorter test pulse widths.
You can also set the OTDR to different wavelengths, lengths or pulse widths to see what setting gives you the best compromise between noise and resolution or to find events that were lost with wide pulse widths. This also helps you decide how many averages to collect.
The OTDR can also be calibrated for the cable it is testing to ensure that it is reading the correct length. This is typically done by looking at the cable jacket for the cable's length markings.
This can be a time consuming process, but it is an important step in making sure that your OTDR is working as it should. It is not foolproof, though. It is a good idea to have an experienced technician help you with the calibration.
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Troubleshooting
Using an OTDR can be a tricky business. A few mistakes can result in a less than gratifying experience for the operator and their crew. However, with the right equipment and a little know-how, OTDRs can deliver on the promise of the next generation network.
EXFO has got you covered with its line of genuine OTDRs. Our AXS-100 Access OTDR offers several configurations and a large variety of options for first-class flexibility. Optimized for testing passive optical networks (PON) within FTTx architectures, it combines the industry’s best OTDR technology with a few power meter related functions in a small and portable package. The unit also has the honor of being the best suited to its intended use as an FTTx tester. Those lucky enough to own one can rest assured they are in for a treat.
