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The basics of an OTDR
An OTDR combines a laser source and a detector to provide an inside view of the fiber link (Figure 1). The laser source sends a signal into the fiber where the detector receives the light reflected from the different elements of the link. This produces a trace on a graph made in accordance with the signal received (Figure 4), and a post-analysis event table that contains complete info on each network component is then generated.
As the pulse travels along the fiber, a small portion of the pulse’s energy returns back to the detector due to the reflection of the connections and the fiber itself. When the pulse has entirely returned to the detector, another pulse is sent until the acquisition time is complete. After 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, optical return loss (ORL) and fiber attenuation.
As examined, the OTDR provides a view of the link by reading the level of light that returns from the pulse that was sent. There are two types of light levels: a constant low level created by the fiber called Rayleigh backscattering (Figure 2) and a high-reflection peak at the connection points (mechanical splice, bulkhead and opened connection) called Fresnel reflection (Figure 3). Rayleigh backscattering is used to calculate the level of attenuation in the fiber as a function of distance (in dB/km). Fresnel reflections lead to an important OTDR specification known as Event and Attenuation dead zones, both are expressed in meters. 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. |
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