Which reflectometer to choose for new fiber-optic lines?

Modernization of 100/400G networks and deployment of 5G networks at the proper quality level require testing of optical fiber communication lines (OFC). Choosing the right methods and additional instruments is critical when conducting bulk testing, as mistakes can be very costly.

If we talk any modern optical networks, fiber and coupling quality are critical. Modernization of existing networks involves checking their condition as well as laying new OFCs—testing the quality of connections. The higher the speed of fiber-optic lines, the more stringent the requirements for the quality of their diagnostics. And here the problem of choosing a testing methodology arises: for example, is "testing to the maximum" using bidirectional tests always the best option?

Bidirectional testing of OFCs with a reflectometer: the answer or new problems?

It is important to understand that fiber-optic communication is related to reflectometry. Without a high-quality optical reflectometer (OTDR), it is impossible to create a reliably operating fiber-optic link. The solution to the problem of testing comes down to the correct initial selection of equipment and determining the most suitable methodology.

There are two main methods of reflectometry: one-way (one compensation coil at one end of the line; launch cable box) or two-way (Bi-dir OTDR, with a compensation coil at the near end and the same at the far end). .

The direction of the emitted light can influence the test results. In any fiber, there is a difference in the return loss coefficients, and in one particular direction, the light loss may be greater.

Single-sided OTDR testing can miss many anomalies. Wire junctions with different return loss values can compensate for the signal loss in one orientation and attenuate it in the other. In addition, there are dead zones in which the OTDR does not record events.

The figure below shows an example of the difference in signal loss depending on the measurement direction. One side of the testing even shows a negative loss of -0.3 dB, which is, of course, impossible. In this case, an amplification effect is observed due to the difference in the backscatter coefficient (BSC) at the junction of the two cables.

Therefore, one-way OTDR testing is better suited for simple tests such as finding and locating fiber breaks and bends, assessing total signal attenuation in fibers, testing connectors, etc. In this case, there is no need to waste time on bidirectional testing. The main advantage of unidirectional fiber-optic testing is that it is enough to inspect and clean the fiber just once for any one line. This is important since contamination of the connectors can lead to failure of the fiber-optic line, which was working properly before testing. Sometimes customers insist on two-way "full testing," but this can be excessive and even harmful as it introduces an unnecessary risk of damage while cleaning the optical connectors.

Bidirectional fiber inspection without running around

One-sided tests are not accurate when laying new lines and monitoring the performance of FOCs. In such cases, industry standards require bidirectional testing with an optical time domain reflectometer (Bi-dir OTDR) measuring the signal at both ends of the line. This is necessary to identify anomalies that are not detected by conventional one-way reflectometry. In addition, there are unique cases where a line uses cables with different fiber diameters, where bidirectional effects can be important. Bi-dir OTDR testing makes it possible to average the measurement parameters and give a clear assessment of the quality of the fiber-optic link. Ultimately, it is possible to unambiguously determine whether the tested line supports high-speed data transfer. This saves time and money that the customer may lose if the cable system fails on launch.

Traditional bidirectional testing of fiber-optic links implies first measuring one side:

And then testing the optical line on the other side:

Then obtain the average parameters. This is an important job that requires manual data processing. It can be accelerated by using two similar instruments with two external cable routes simultaneously (the bidirectional reflectometer technique), but this requires additional equipment expenses.

Another effective technique is testing with an optical reflectometer with a closed circuit (loopback). This method involves using a reflectometer at one end of the line and a reference fiber loop at the other. This way, one can test two optical lines in both directions, alternately swapping cables. This is the most effective method in terms of cost and labor savings since it requires only one reflectometer, loop switching, and two compensation coils.

A major drawback of OTDR-loopback testing is the labor-intensive process of recording and merging test data. Often, manual work with data leads to errors, and repeated tests are needed. This problem has been solved in modern optical reflectometers, such as the VIAVI T-BERD/MTS-4000 V2.

When using such advanced devices, the task of specialists is reduced to the high-quality implementation of procedures for cleaning contamination of optical connectors and one-way checking of launch cables before starting bi-directional testing.

Thus, preference should be given to OTDR modules and platforms that offer the ability to choose any optical cable diagnostic technique with maximum automation. This choice should be made based on practical needs and not on the capabilities of existing equipment. Ultimately, this approach reduces the likelihood of OFC failures.