OTDR measurements in the last mile

Troubleshooting GPON and XGS-PON

Across the UK, PON technology is conquering the last mile. Numerous new fibre networks are being laid and households and businesses connected at a record pace.

The first priority when working on optical fibres is cleanliness. The largest proportion of problems with fibre optic connections is attributable to contamination. Technicians are equipped with inspection and cleaning tools and, over the last few years, with optical power meters too, but when these confirm a poor level it would be ideal if the technician could carry out troubleshooting using an OTDR.

An OTDR is the only tool capable of identifying the nature of the anomaly and where this anomaly is to be found.

Are all OTDR equal? – What's special about the last mile?

The largest proportion of fibre connections rolled out in the last mile are passive optical networks, or PONs. Currently, millions of customers in Europe are already connected via GPON  (ITU-T G.984.3); at the same time, many countries are beginning to roll out XGS-PON (ITU-T G.9807.1, symmetrical 10 gigabit PON) over the same fibre links; in many places, a hybrid or mixed operation is emerging. PONs are always singlemode fibres with a relatively short line length, theoretically up to maximum distance of 20km, in practice often much shorter. An OTDR should take this into account by enabling small pulse widths and dead zones. The criterion is the wavelengths that the OTDR should be able to cover. GPON uses 1310nm (up) and 1490nm (down), XGS-PON uses 1270nm (up) and 1577nm (down).

An OTDR suitable for PON troubleshooting should therefore support at least two wavelengths: one of the above mentioned commonly used wavelengths (e.g. 1310nm) if the network or the section to be measured is out of service; and a second wavelength that allows interference free in service measurements, which is particularly important if the technician does not have access to both ends of the fibre. The latter should have the greatest possible separation from the live wavelengths; a maintenance wavelength of 1650nm is a good choice here. The technician then has one wavelength in the second optical window and one in the third window. Optical windows are wavelength ranges that are particularly suitable for data transmission, as certain ranges are better suited and some less so due to the composition of the fibre.

An OTDR for the access segment simply and rapidly identifies problems from the individual subscriber to the splitter and beyond. If the problem is behind the splitter, i.e. further towards the OLT, the problem is likely to affect the entire PON branch and thus multiple subscribers at the same time.

What does an OTDR actually do?

An OTDR measures the line and event attenuation and can indicate not only the line length but splices and connector information too. (see Fig. 1). If the OTDR has two wavelengths, it's even possible to detect and pinpoint a bending radius violation (macrobend).

Example events mapped on an OTDR trace
Fig 1. - Example events mapped on an OTDR trace


An OTDR generates a pulse of light using a laser diode that is coupled into an optical fibre. Via the same coupler, it then analyses the pulse components reflected back by Rayleigh scattering (attenuation) and Fresnel reflection (localisation).

Electromagnetic waves, including light, are scattered in all directions by tiny molecules (Rayleigh scattering), because the material of a glass fibre is not ideal but rather contains impurities. This, among other factors, is due to the manufacturing process and the quality of the materials used. These impurities in turn cause unwanted fluctuations in the refractive index (IoR) of the fibre.

The intensity of light scattered back to the OTDR is measured (backscatter measurement) resulting in the optical return loss (ORL), which allows the calculation of fibre length, the attenuation per kilometre as well as the insertion loss (IL) and reflectance at events. We can say that the greater the backscatter, the greater the attenuation on the line.

Fresnel reflection plays a role in localising events. In fibre optic technology, the "total" reflection of electromagnetic waves at material transitions due to a refractive index transition occurs primarily at connectors (glass-air-glass). Fibre breaks, the end of an open fibre, contamination or scratches on fibre end surfaces also lead to these effects, known as Fresnel losses. This phenomenon can also reveal a poorly executed splice. The light of a specific wavelength is incident on such an event and is reflected much more strongly and directionally in relation to the Rayleigh scattering.

The insertion of a precisely defined pulse, e.g. with a duration of exactly 10ns, makes it possible to detect the reflection of the pulse at an event and thus to measure its transit time (outward and return path). With this measured time and the knowledge of the refractive index of the fibre in question, the distance of the event from the OTDR instrument can be determined extremely precisely.

What should we look for when choosing an OTDR instrument?

In addition to the variable pulse width and the insertion level, the available dynamic range represents a pivotal quality feature. The pulse width and transmission level together determine how much energy can be put into the pulse and thus onto the line. The dynamic range describes how much the transmitted pulses can be attenuated by distance and events and still be recognised after scattering and reflection. It can also be described as the sensitivity of the OTDR.

The instrument permits users to define a preferred averaging time to increase the dynamic range somewhat through repeated measurement. When the period over which the measuring instrument takes multiple measurements and averages the results, e.g. 60 seconds, is known, this results in an improved signal-to-noise ratio and thus, ultimately, better results.

However, for an OTDR used primarily on PONs, a dynamic range of 20dB is entirely sufficient. In combination with a high insertion level and a pulse width of 100ns it is possible – depending on the quality of the fibre and the number of events – to draw conclusions about distances of several kilometres. Certainly, units with much greater dynamic ranges and higher averaging times are also available, but for short distances, these represent a substantial, unnecessary additional expense. When working with the OTDR ideal insertion conditions are significantly more important. If these are poor, e.g. due to a defective connector, scratches or contamination, this reduces the intensity of the pulse and thus the dynamic range and, as a consequence, the accuracy of the measurement. That is why the inspection and cleaning mentioned at the outset are of great importance.

Pulse width and dead zones

The most important criterion is the localisation resolution, i.e. how accurately can the OTDR provide line lengths and distances to events. In this context, we can say that the narrower the pulse, the better the resolution: in theory, accuracies of up to 1m can be achieved at 10ns. Of course, deviations and uncertainties still arise due to the length of the line section, the measurement itself, the temperature and other influences, but these are no longer so significant.

Therefore, it is important to ensure that the pulse width is adjustable within certain parameters. At first glance, one might be inclined to always choose the highest value, but beware: the wider the pulse, the more likely it is to conceal closely spaced events. As a rule of thumb, one can say that two events can be distinguished if they are at least half a pulse width apart. So if you expect "many" events when measuring short lines, it is advisable to choose a shorter pulse. It should also be possible to configure the instrument for the expected fibre optic length, as this determines the transit time of the pulse on the line and thus has a direct influence on the dead zone.

There are two types of dead zone. Firstly, there is the event dead zone, which specifies the distance at which two events can be distinguished from each other and the attenuation dead zone, which determines the minimum distance between two events such that the attenuation can still be determined precisely - both together comprise the so-called total dead zone. This is usually specified for very short pulse widths (e.g. 10ns) and should be in the range of a few metres.

The accuracy of the attenuation measurement, the linearity, whether of that of the distance or the event, is another important selection criterion; a deviation of ±0.05dB is entirely sufficient for use in the PON branch.

Overall, however, the accuracy is determined by the number of recorded data points, i.e. in how many individual values the measuring instrument can store for the measurement. The more data points available, the higher the resolution and accuracy in the end. For the application described above, 100,000 data points are sufficient. 300,000 data points is ideal, especially for longer distances.

As with TDR measurements on copper lines, it is also important to know the details of the fibre to be measured when performing OTDR measurements. The instrument should thus enable you to enter important fibre parameters in order to obtain the most accurate results. These include first and foremost the index of refraction, (IoR), the Rayleigh backscattering coefficient (BC) and the attenuation coefficient (ACI).

How can the OTDR instrument support the user?

Instruments with an auto mode perform many tasks fully automatically and are recommended especially for beginners. Tests with different wavelengths and pulse widths are carried out automatically and the results are conveniently displayed in an event table with the help of symbols, length and distance information. Ideally, this even includes pass/fail evaluation, and a good OTDR tester should permit definition of the corresponding limit values for the splice, macro bend and connector events. Some instruments even come with a predefined assessment according to ITU-T G.671 or TIA 568.3-D, which of course makes servicing a little easier, but the values cannot then be customised to your own quality requirements – which may be higher.

However, since every novice will have gained sufficient experience by some point, it makes a lot of sense to ensure that the instrument also comes with a manual mode with dedicated OTDR graphs and a real-time mode that enables users to reliably detect rapidly changing events as well. With auto modes, this is more or less left to chance.

Launch and tail fibre cables and the benefits of port savers

Regardless of whether you are a novice or an expert, whether you are using auto mode or real-time measurement, never troubleshoot without using launch and tail fibre cables and make sure that you are offered them with when you purchase an OTDR. Often it is precisely the connector of the fibre to be measured that causes the greatest problems; wear and contamination are the most common and severe faults here. If one were to plug the fibre to be measured directly into the OTDR instrument, it would not be possible to assess the influence of the connector on the overall line, even with narrow pulses, since the event dead zone is greater than zero, but the connector is virtually at zero. By inserting a launch fibre of e.g. 1000m, we can ensure an accurate evaluation of this event.

A similar effect occurs at the end of fibre. If the last connector of the fibre to be measured is open, there is a complete reflection at the end, making it impossible to draw conclusions about the quality of this downstream connector. Whether it is precisely this last connector that the technician was called out to fix cannot be determined. Only the use of a tail fibre makes this last connector a fully fledged connector with measurable attenuation.

Launch and tail fibres should always be of the same fibre type and longer than the own attenuation dead zone. Be sure to use high quality Launch and tail fibres; the connectors in an OTDR measurement must always be of the highest quality and they should not wear out quickly, if at all possible. A port saver should therefore be included in the scope of delivery. It can be connected between the OTDR tester and the leading fibre to protect both the actual connector of the measuring instrument and the launch fibre and can be easily and cheaply replaced after a few 100 mating cycles.

Measurement logging

When performing contract work, e.g. for a network operator or for your own documentation, it is also important to be able to export all data obtained from the OTDR for later analysis. The device should be able to export all data points to a Standard OTDR Record (SOR) file. Free SOR viewers then enable an in-depth analysis down to individual data points. When choosing optical test and measurement equipment, carefully consider the sector in which you wish to use your instrument, so that you have exactly the right solution at hand when problems arise. The use of a multifunction tester can save a lot of time, as it enables a prompt initial assessment: Especially in the case of "minor" problems (contamination, contact problems, etc.), it is then not necessary to wait for the expert!

Article by Dennis Zoppke, Product Manager at Intec GmbH