OTDR instruction Settings

In order to accurately measure fiber length and decay when using OTDR, the relevant parameters must be set correctly before starting the test. The main parameters are: refractive index, pulse broadband and average time; At the same time, how to use the cursor to accurately pick points is also crucial.

First, refractive index setting

Whether the refractive index of optical fiber group is set accurately has great influence on the length measurement. The refractive index value is given by the optical fiber manufacturer. In addition, the distance algorithm of OTdrs from different manufacturers is slightly different. Generally speaking, the slender test distance error of OTDR is composed of the following three factors:

0.000025% ´ Test error caused by range ± OTDR range resolution ± refractive index of the fiber

Here is an example to illustrate the influence of optical fiber group refractive index on the length test: Suppose the measured optical fiber is disconnected 120km away from the test point. If XX Company's YY OTDR is used for the test, the range resolution is 8m if the sampling point is 32,000 points within this range. We set the error value of the refractive index of the optical fiber group to 0.001 (because the operator usually set the refractive index between 1.467 and 1.468) :

D = 0.000025% ´120,000m + 8m + 120,000m´ 0.001/1.467 = 100.8m

The error caused by refractive index is 81.8m, accounting for 81.15% of the total error.

From the above example we can understand how important refractive index setting is for fiber length measurement!!

Second, pulse width and average time Settings

Theoretically, for the same section of fiber, the greater the pulse width, the greater the distance measurement error. However, if the pulse width is very small, the boundary between the optical fiber end and the noise level cannot be accurately identified. The operator should choose the appropriate pulse width according to the actual situation. The principle is to set the pulse width as small as possible while ensuring that the end of the fiber can be identified.

In general, it is difficult to mechanically define the relationship between the test distance and the pulse width used, because each fiber has a different decay, it is difficult to use a standard scale to measure the pulse width used to test a certain distance of the fiber. However, two principles must be adhered to:

1, use the smallest possible pulse width to test the fiber, so that the accuracy of distance and decay can be guaranteed. Only if the pulse width is small enough to be able to see the general shape of the curve, can the curve be measured by averaging.

2, when the pulse width is determined, the selected average time should be long enough, generally between 15 seconds and 60 seconds. The longer the measured fiber is, the longer the average time is (and the pulse broadband is also larger).

Three, the correct use of the cursor to take points

Operators in the use of OTDR, because of the error brought by the point is inevitable. For the emission event, the point should be at the starting point of the steep rise of the curve. For non-reflective events, the point should be at the beginning of the steep drop of the curve. During the test, the curve at the fault point should be enlarged before determining the exact fault point location.

Although the event table of OTDR contains the corresponding distance value for each event, this distance value is not necessarily very reliable for technicians who undertake the task of emergency repair. Because the distance value in the event table is only accurate if all OTDR parameters are set correctly and the average time is long enough. Therefore, to accurately locate the fault point, you should use a manual method to determine the distance value: first move the cursor to the fault point, zoom in on the area, and then find the exact point.

Conclusion: Whether the above parameters are set correctly is very important for the test of OTDR. In addition, onsite personnel can improve the accuracy and speed of fault location based on long-term experience.