Most likely, it is because of a heavily dirty patch cord on the side closer to operator, but there may be a defect in the optical socket, cable damage near the cross, or bending of the fiber.If you made measurements and obtained the attenuation for the entire line of 0.66 dB and the length of your line is 3 km, then the kilometric attention will be 0.66 3 0.22 dB km.The X-axis shows the distance, the Y-axis is the signal power level.
In general, the graph is descending, because everything in the fiber optic line introduces attenuation into the sent signal: all kinds of connections, defects, and the fiber itself also has constant attenuation. The fiber itself, due to Rayleigh scattering, reflects back a little and analyzing the power of the reverse reflection and the time, at which this instantaneous power came, the OTDR puts dots on the axial plane, connecting them into a graph. If somewhere there is a non-reflective fault (splice, kink), then the reflectometer level of the reflected signal before the fault will be higher than after it and a step is formed on the graph. If there is a reflective fault (mechanical connection, break, fiber end), then the OTDR trace indicates in this place a powerful reflection, much higher than the light coming from Rayleigh scattering, and we see a peak on the graph. Since the first impulse is very rough and noisy, for a quality OTDR trace, a lot of impulses (thousands and tens of thousands) are sent to the line repeatedly, and the resulting OTDR trace is their average. The more impulses, the more accurate and smooth the OTDR trace, but then you need to wait longer for the end of measurement. The length of the duct starts from the very beginning of the reading scale, that is, the dead zone is already part of the duct we are measuring. It prevents us from seeing what happens at the very beginning of the duct, and it is sad (we cannot see directly whether the cross connection is good and whether the pigtail is well spliced with the cable). Yet we cannot see, say, the splice of a pigtail with a fiber of a cable in a cross, we can analyze it only from indirect data. ![]() You need to set up a shorter impulse and measure the fiber again. Therefore, the dead zone and all other events are stretched, accuracy disappears, and small details are lost. Closer to the right, there is a phantom peak at twice the distance from the end of the duct, see below for it. Moreover, more: the OTDR trace, as you can see, is cut off in amplitude, peaks are cut off from above. This is already a feature of an inexpensive OTDR, but this effect usually does not interfere with seeing events on the duct. If the dead zone is not only wide, but also goes into the duct smoothly (in the form of a hyperbola parabola), and even unevenly with noise, this is a sure sign that something is wrong at the very beginning of the duct. Either one of the ports (on the OTDR or on the cross) is dirty, or the socket on the cross or on the OTDR itself is broken. In the latter case, with repeated disconnection connection, the result will vary greatly until complete attenuation and no sign or a signal), or the patch cord pigtail is bad, or splice inside the cross is bad. Alternatively, the rarest and most unpleasant option, right next to the cross (tens of meters) the cable is damaged. Sometimes, when measuring the signal at the cable hunk (or when it is necessary to measure a line not terminated with a cross connect with a cable end simply hanging), if there is no device to quick enter fibers, it is necessary to splice each fiber to the pig-tail connected to the OTDR, and after the measurement, you need to break the splice, splice another fiber, measure again, break it again, and so on. In manual mode, looking at the fusion splicer screen, you can bring the fibers together very accurately; still, a barely noticeable axial displacement of the fibers already strongly affects the light passage through the core.
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