Thursday, February 20, 2020

To measure the reflectance, the OTDR measures

Reflectance

To measure the reflectance, the OTDR measures the amount of light that returns both from the backscatter in the fiber and from that which is reflected from a connector or splice. The calculation of the reflectance is a complex process that involves noise at the baseline of the OTDR, the level of backscatter and the power at the reflected peak. Like all backscatter measurements, the uncertainty of the measurement is quite high, but an OTDR has the advantage of showing where the reflective events are located so that they can be corrected if necessary.

Plot comparison
Comparing two paths in the same window is useful for confirming data collection and contrasting different test methods on the same fiber. The comparisons are also used to compare the graphical traces of the fiber during troubleshooting or restoration with paths obtained just after installation to see what has changed. All OTDRs offer this feature, by which you can copy one path and paste it to another to compare them.
OTDR measurement uncertainty

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The greatest source of measurement uncertainty that occurs when testing with an OTDR depends on the backscatter coefficient of the fibers being tested, the amount of light from the outgoing test pulse that is dispersed back to the OTDR. The backscattered light that is used for measurement is not a constant but depends on the attenuation of the fiber and the diameter of the fiber core.

If you look at two different fibers spliced ?? or connected together in an OTDR, you will notice that the difference in backscattering of each fiber is a major source of error. If both fibers are identical, as happens when splicing a broken fiber again, the backscatter will be the same on both sides of the joint, so the OTDR will measure the actual splice loss. However, if the fibers are different, the uneven backscatter coefficients will cause a different percentage of light to be sent back to the OTDR.
Fiber optic cable certification
If the first fiber has more dispersion (shown as attenuation) than the other after connection, the percentage of light from the OTDR test pulse will go down, so that the loss measured in the OTDR will include the actual loss plus a loss error caused by a lower backscatter level, which will cause the loss shown to be greater than it really is. If you look from the opposite side, from a low attenuation fiber to a high attenuation fiber, we will find that the backscatter goes up, making the measured loss less than it really is. In fact,

Although this source of error is always present, it can be virtually eliminated by taking the readings in both directions and averaging the measurements. In addition, many OTDRs have this function programmed in their measurement routines. This is the only way to test splices online to check for loss and obtain accurate results.

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