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In the case of an OTDR, the consecutive event is detected, but the loss cannot be measured. Still using the car example mentioned above, when your eyes are blinded by another car, after a few seconds you could notice an object on the road without being able to properly identify it. In other words, it is the minimum length of fiber needed between two reflective events. The event dead zone is the minimum distance after a Fresnel reflection where an OTDR can detect another event. Also, using different methods to calculate the distance could also return a shorter dead zone than what it really is. For instance, a -55 dB reflection for singlemode fiber provides more optimistic specifications of a shorter dead zone than using -45 dB, simply because -55 dB is a lower reflection and the detector recovers faster. For this reason, it is important to read the specification sheet footnotes since manufacturers use different testing conditions to measure the dead zones-pay particular attention to the pulse width and the reflection value. Most manufacturers specify dead zones at the shortest available pulse width and on a -45 dB reflection for singlemode fibers and -35 dB for multimode fibers. In the OTDR world, time is converted into distance therefore, more reflection causes the detector to take more time to recover, resulting in a longer dead zone. A dead zone is defined as the length of time during which the detector is temporary blinded by a high amount of reflected light, until it recovers and can read light again-think of when you drive a car at night and you cross another car in the opposite direction your eyes are blinded for a short period of time. Both originate from Fresnel reflections and are expressed in distance (meters) that vary according to the power of those reflections. There exist two types of dead zones: event and attenuation. What Are Dead Zones?įresnel reflections lead to an important OTDR specification known as “dead zones”. Examples of such reflections are connectors, mechanical splices, bulkheads, fiber breaks or opened connectors. Fresnel reflection is identifiable by the spikes in an OTDR trace. When the light hits an abrupt change in index of refraction (e.g., from glass to air) a higher amount of light is reflected back, creating Fresnel reflection, which can be thousands of times bigger than the Rayleigh backscattering. The second type of reflection used by an OTDR-Fresnel reflection-detects physical events along the link. Higher wavelengths are less attenuated than shorter ones and, therefore, require less power to travel over the same distance in a standard fiber. When hit, some particles redirect the light in different directions, creating both signal attenuation and backscattering. This phenomenon comes from the natural reflection and absorption of impurities inside optical fiber. Rayleigh backscattering is used to calculate the level of attenuation in the fiber as a function of distance (expressed in dB/km), which is shown by a straight slope in an OTDR trace. Note that there are two types of light levels: a constant low level created by the fiber called “Rayleigh backscattering” and a high-reflection peak at the connection points called “Fresnel reflection”. Learn all about OTDR testing and EXFO's OTDR products Reflection Is KeyĪs previously examined, the OTDR provides a view of the link by reading the level of light that returns from the pulse which was sent. The main advantage of using an OTDR is the single-ended test-requiring only one operator and instrument to qualify the link or find a fault in a network. Therefore, many acquisitions will be performed and averaged in a second to provide a clear picture of the link’s components.Īfter the acquisition has been completed, signal processing is performed to calculate the distance, loss and reflection of each event, in addition to calculating the total link length, total link loss, ORL and fiber attenuation. When the pulse has entirely returned to the detector, another pulse is sent-until the acquisition time is complete. As the pulse travels along the fiber, a small portion of the pulse’s energy returns to the detector due to the reflection of the connections and the fiber itself.
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A clock then precisely calculates the time of flight of the pulse, and time is converted into distance-knowing the properties of this fiber. The signal sent is a short pulse that carries a certain amount of energy. This produces a trace on a graph made in accordance with the signal received, and a post-analysis event table that contains complete information on each network component is then generated. The laser source sends a signal into the fiber where the detector receives the light reflected from the different elements of the link. Return to glossary The Fundamentals of an OTDR The BasicsĪn OTDR combines a laser source and a detector to provide an inside view of the fiber link.
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