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Technology Products & Services        EZZ e-newsletter 09052016

The basics of an OTDR The event dead zone is the minimum distance after a Fresnel reflection where an OTDR can detect another event but the loss cannot be measured (Figure 5). The OTDR merges the consecutive events and returns a global reflection and loss for all merged events. To establish specifications, the most common industry method is to measure the distance at -1.5 dB from each side of the reflective peak. The attenuation dead zone is the minimum distance after a Fresnel reflection where an OTDR can accurately measure the loss of a consecutive event. The minimum required distance is measured from the beginning of a reflective event until the reflection is back to 0.5 dB over the fiber’s backscattering level (Figure 6). The dynamic range is the maximum length of fiber that the longest pulse can reach. For example, a SM OTDR with a dynamic range of 35 dB has a usable dynamic range of approximately 30 dB. Assuming typical fiber attenuation of 0.20 dB/km at 1550 nm and splices every 2 km (loss of 0.1 dB per splice), a unit such as this one will be able to accurately certify distances of up to 120 km. The pulse width is actually the time during which the laser is on. The shorter the pulse, the less energy it carries and the shorter the distance it travels due to the loss along the link, and vice versa (Figure 7). The ability for an OTDR to pinpoint the right distance of an event relies on a combination of different parameters—among them are the sampling resolution and the sampling points. Sampling resolution is defined as “the minimum distance between two consecutive sampling points acquired by the instrument”. This parameter is crucial, as it defines the ultimate distance accuracy and fault-finding capability of the OTDR. Depending on the selected pulse width and distance range, this value could vary from 4 cm up to a few meters (Figure 8).

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