OTDR traces are valuable tools for troubleshooting network problems, especially when the traces are taken after installation to see how the network has changed over time. But how do you read an OTDR trace?
How do I read an OTDR trace?
OTDRs are an essential tool for testing fiber optic cables, enabling contractors to ensure that cable installation has been done properly. They are also used to find locations of breaks, splices and connectors, if necessary, in order to troubleshoot and repair problems.
An OTDR trace shows a number of things, including the amount of light loss and attenuation, or "reflectance," on a given section of fiber optic cable. It does this by sending a pulse of light down the fiber and measuring how much light is scattered back towards the OTDR. This "backscatter" is then measured along with reflected light from connectors or cleaved ends, so an OTDR can determine how much loss or attenuation there is on any section of fiber.
To make a measurement, the OTDR sends a pulse of light down the fiber at different wavelengths and then measures how much of that light is scattered and how much is reflected back to the OTDR. It then displays this data on the screen as a "trace," which can be used to identify connector losses, splices, breaks and other events in the fiber.
Depending on the OTDR you are using, you may be able to adjust your test settings so that multiple samples of the same section of fiber are taken and averaged into one trace. In some cases, you will even be able to use a trace to see the overall loss and attenuation of a whole cable plant.
This ability to do multiple tests and average them together is what gives the OTDR its unique ability to find events lost at certain wavelengths, while not being influenced by noise. This allows you to find splices that are faulty, for example, by measuring how much backscatter is in the fiber.
In a long outside-plant fiber run, where there are many splices and connectors, it is important to understand how to read an OTDR trace. There are a few things you should be aware of:
First, Ghosts are produced when there is a large reflection in a short section of fiber, and the light bounces back and forth. Ghosts cause repeats of a trace, so they can be easily spotted.
What is an OTDR trace?
OTDR is a test tool that uses a pulse of laser light to measure the optical power loss and reflectance along the length of a fiber link. As the laser pulse passes through the fiber, its power diminishes due to scattering from the fiber, or at events such as connections, breaks, cracks, splices, sharp bends and the end of the fiber.
During this time, the light signal passes through a lens called a Fresnel and produces two types of signals that are visible on the OTDR trace: backscatter and reflection. The backscatter is caused by the light hitting slight impurities in the fiber, which scatter the light and then return it up the fiber link to the OTDR.
The reflection is produced when the light hits a polished connector or mechanical splice at the end of the fiber, or by a fault in the fiber. These events are called “events” in OTDR jargon and are typically shown as reflective peaks on the OTDR trace.
Event dead zones are important for a good Palm OTDR trace. These are the minimum distances that a Fresnel reflection will cause an OTDR detector to be temporarily blinded so that it can no longer read the light. This is similar to driving a car at night and being suddenly blinded by a bright object.
A shorter event dead zone will allow the OTDR to detect more closely spaced events in a given test. This is especially important for premises networks, where patchcords are short and often connect a large number of connectors.
Another factor that can affect OTDR traces is the instrument’s resolution and sampling points. These parameters define the ultimate distance accuracy and fault-finding capability of the instrument. A narrower pulse width can see more detail on the OTDR trace but also imposes a limit on how far it can see along the cable plant.
OTDR traces can be very useful for identifying problems in fiber cabling infrastructure. However, the OTDR must be properly set up and used with the right measurement parameters to produce accurate, clear and reliable results. A knowledgeable OTDR technician can help determine the best measurement settings and interpret the OTDR traces to identify issues in the network.
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What is an OTDR event?
The OTDR trace is the result of a series of optical pulses injected into a fiber, then portions of this light are reflected and refracted back down the fiber to the OTDR’s photo detector. This information enables the OTDR to calculate distance and loss, identify connectors and faults, and characterize the link’s capabilities from start to finish.
Using the correct OTDR parameters is key to getting accurate results, especially for tests that need to be run in a specific environment or require a certain level of accuracy. These parameters adjust the pulse width, averaging time, dead zones, and the distance range for your given fiber run to offer the best possible results.
Sampling resolution is an important parameter, as it relates to the ability of the OTDR to accurately determine the distance between two consecutive sampling points. Depending on the selected pulse width and distance range, this value could vary from 4 cm up to several meters.
Another important measurement is instrument resolution, which measures how close two fiber events can be spaced and still be recognized as separate events by the Mini OTDR. The duration of the measurement pulse and data sampling interval are both factors that impact this value, but the shorter the pulse duration, the better the resolution.
Event dead zones are a critical part of the OTDR specification, as they determine the minimum length of fiber necessary for an OTDR to detect a second event after a Fresnel reflection. Having the shortest event dead zone allows an OTDR to recognize closely spaced events, such as a splice or connector.
A gainer is a change in backscatter that is caused by a connection between two fibers with different backscatter coefficients, numerical apertures, core diameters, or mode field diameters. These differences in backscatter can cause the OTDR to measure more loss than it really is. This is a common confusion for new OTDR users, so check the fiber types on either side of a connector or splice to make sure they are of matching size and quality.
OTDR equipment and OTDR-like equipment are typically used indoors in controlled environments, such as central offices or equipment huts. In rare cases, they are used outdoors in manholes, aerial platforms, or open trenches.
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What is an OTDR event table?
An OTDR, or Optical Time Domain Reflectometer, is a test instrument that integrates a laser source with a detector to provide an inside view of a fiber link. The signal is reflected by the different elements of a link and then graphed, creating a post-analysis event table that contains complete information about each network component.
OTDRs are equipped with a processor that analyzes the trace to improve the signal-to-noise ratio. The processor collects tens of thousands of data points that make up a fiber trace or waveform. The trace is displayed in a window on the screen. Each point on the screen represents an average of a dozen sampling points.
To display the trace, the 7 inch multifunction OTDR processor must first calculate each point's distance and loss. Depending on the type of event, each point can be represented by a spike or a drop in backscatter level. A spike indicates a reflective event, such as connectors, mechanical splices or breaks on the fiber, while a drop in backscatter level recognizes nonreflective events, which can be fusion splices, opened connectors or bends.
Each event or loss on the OTDR trace is grouped together into an event table (Figure 4). The event table shows the type of event, distance, loss, reflectance and attenuation.
The event table also identifies two types of reflection: Rayleigh and Fresnel, which are based on physical events on the fiber link. The first kind, Rayleigh backscattering, is derived from the backscattering of light when it encounters an obstruction such as a bulkhead or connector. The second, Fresnel reflection, is generated when light encounters a sudden shift in refraction index.
Both types of reflection can be recognized on the OTDR trace, but a higher peak in the OTDR trace indicates Fresnel, which is a much more potent type of backscattering than Rayleigh. These high spikes are caused by a rapid shift in refraction index, such as bending of the fiber or opening of a connector.
An OTDR has four test modes: real-time, average, dead zone and auto tests. Each test mode is operated by selecting the corresponding menu item in the operating interface.
