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Measurement error in OTDR

Posted on 2nd Nov 2019 @ 1:41 AM

Optical fibers are measured using an optical time-domain reflectometer (OTDR), sometimes the fiber link is short. A large attenuation value is often obtained when measuring short fiber links, so that the attenuation index cannot obtain a good evaluation. What method can be used to get a better test result?

The OTDR tester generally has a measurement blind zone. The length of this blind zone depends on the quality of the tester and the selected pulse width and reflection value. Generally, Anritsu 9083, 9090, JDSU, EXFO-150, -200 imported OTDR tester are about 20 meters at 5N seconds pulse width. The pulse width of 100ns is 70 meters, 200ns is 140 meters, and 1us is about 180 meters. Some no brand machines may not be able to measure the curve, and cannot find the nodes, the end points and other issues.

Due to the short fiber link (generally less than 1km), the OTDR measurement curve has volatility, and these small fluctuations affect the final numerical calculation.

Among the technical parameters describing the OTDR, there is an index about the linearity. The general OTDR is 0.05 dB/dB for this index. In fact, this index describes the fluctuation range of the OTDR curve. So for short fibers, the accuracy of OTDR measurements is challenging. This issue is also a common problem faced by OTDR at present, so what method is used to reduce this impact?

1, Add 2km test dummy fiber (OTDR Launch cable).

Add 2km OTDR Launch cable, the effect is reduced. However, there is a test errors for the increased length. So, this method as a test data to verify the quality of the project, it has considerable reference value.

2, Use a light source and an optical power measure the link loss, measure length with an OTDR.

Since the link loss is measured close to the standard loss measurement method using a light source and an optical power meter, the accuracy of the loss measurement is improved. For short fibers, use an optical power meter with a higher resolution accuracy level to reduce the error. Length measurements are obtained using an OTDR. The unit length attenuation value is obtained by dividing the loss value by the length value.

3, The OTDR measurement curve is for reference only, and the OTDR measurement focuses on eliminating link barriers.

In short fiber optic measurements, the OTDR measurement curve is best used as a reference only, as a basis for qualitative and not as a basis for quantitative. More tasks of the OTDR are used to deal with obstacles in the link, such as the quality of the connector connection, the quality of the fusion splice, and the micro-bend of the fiber.