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Is your fibre clean or not

What does “clean” mean?
Automated assessment of fibre end faces in fibre optic connections guarantees consistent quality and functionality.

In the world of data transmission via fibre optic cable, it is widely known that defects such as scratches or chipping and, above all, contamination on the fibre end faces of connectors are the principal cause of faults and the deterioration of transmission quality on the transmission paths. From a metrological point of view, impairments of this kind lead to increased reflections, which manifest themselves in reduced return loss at connector transitions and an increase in insertion loss along the entire path. Scratching or even destruction of the fibre end faces of other connectors during patching procedures are possible mechanical consequences.

In light of ever-growing bandwidth requirements and increasing use of fibre optic cables in networks, contaminated and damaged connectors are impairing network performance more and more often and can even lead to the failure of whole transmission paths.

Therefore, it is imperative that fibre end faces are always inspected for cleanliness before establishing a connection. This applies to all phases in a system’s life cycle, starting at assembly or installation, continuing through regular operation and regular maintenance work, up to fault tracing when malfunctions occur.

But when is a fibre end face deemed to be “clean” and ready for operation? As this concerns areas in which faces with diameters within a narrow micrometer range are pertinent, a simple visual inspection will by no means be sufficient. Every technician who works with fibre optic cables should at least carry with them a simple manual hand-held microscope, which is designed specifically for inspecting fibre end faces. Of course, this is in addition to appropriate cleaning equipment for removing any contamination that may have been detected before connecting for the first time (see fig. 1).

Standardized cleanliness

A standard devised by the International Electrotechnical Commission (IEC) provides a definition of “clean” and operational readiness. The standard’s designation is IEC 61300-3-35. This standard defines general requirements regarding the quality of fibre end faces in order to guarantee optimum insertion loss and return loss. It contains pass/fail criteria for testing and analyzing the end faces of optical connectors. As part of this, separate requirements are specified for various types of connections, e.g. SM-PC, SM-UPC, SM-APC and MM and multifibre connectors. Compliance with the required limit values guarantees a consistent level of performance from the optical connector.

Because the suitability of technicians is variable and cannot be verified and due to inconsistent light conditions and display quality, inspection and analysis with a purely manual fibre microscope is, however, not a reliable or reproducible method for guaranteeing compliance with the IEC standard. In addition, no test report is generated during manual inspection and therefore, it is not possible to document the quality of the fibre end faces directly on site.

As compliance with the IEC standard is the only way to fulfill the performance potential of modern fibre optic cable networks with their numerous connectors, it is suggested that the process of fibre end face testing be automated.

Automated assessment guarantees quality

A fibre microscope is used for this, which takes the pass/fail criteria of the IEC standard as a basis for an assessment, in conjunction with a piece of analysis software.
The automation of this inspection with a system of this kind removes the uncertainties associated with manual testing, produces a documented quality certificate at the place where the fibre end face is installed and ensures that the process is replicable and reliable. These benefits make automatic inspection of fibre end faces the most effective way of guaranteeing and verifying compliance with the IEC standard during the whole life cycle of the fibre optic cable and of fulfilling the performance potential of next-generation networks.

The fibre end faces to be assessed are divided into various zones, radially around the centre of the connector. 4 different zones are identified around the centre of the connector (see fig. 2). The various fault criteria for damage and contamination are specified for each individual zone according to number, size and position in relation to the fibre core.

Of course, it only makes sense to inspect the fibre end faces as part of a larger procedure, which involves alternating cleaning and inspection measures. The IEC standard also features an appropriate flowchart for this, so as to clearly define good and bad connectors. Consistent compliance with this workflow ensures that the inspection is carried out correctly each time and that the fibre end faces are clean before the connection is established. This prevents contaminated or damaged fibreglass from being connected to the network, along with the problems that presents.

Figure 2: Assessment zones for multi-mode and single-mode fibre end faces

Video microscopes for practical application

This process usually falls due for the first time in the life cycle of a fibre optic system during assembly or installation of the cables if the purpose is to determine the optical properties such as attenuation or reflection behaviour in order to document either proper assembly or correct installation.

Essentially, a distinction is made between two levels when measuring fibre optic cables. Level 1 describes only attenuation measurements which are usually carried out with standalone attenuation measuring stations or auxiliary modules on certification devices for copper cabling. This involves comparing readings for transmission to fixed limit values which are either calculated from the permissible properties of the associated individual components or devised from the requirements of an application standard.

Level 2 supplements the attenuation descriptors regarding reflectance curves for visualising occurrences on the fibre optic cables and at the same time, it also requires that only the fibre end faces of the relevant cables be documented.

For both types of measurement and before a measurement cable, delay line or follow-up line is connected, inspection and cleaning cycles should always be carried out in accordance with the procedure described below (see fig. 3), so that smooth operation of a system can be guaranteed.

Figure 3: Flowchart for inspection/cleaning of fibre end faces

Softing manufactures devices for both measurement levels onto each of which a video microscope can be connected via the USB port and for which the assessment in accordance with IEC 61300-3-35 can be carried out internally. In each case, the results are presented in the form of a graph and can be distributed as a separate document or together with the relevant associated measurement results or archived for the purposes of verification at a later date.

For measuring in accordance with Level 1, the WireXpert 4500, the flagship certifier, is fitted with fibre optic cable measurement adapters rather than copper cabling measurement adapters. As well as the classic single and multi-mode adaptors for the optical windows of 850/1300 nm and 1310/1550 nm, it also has modules capable of assessing multifibre connectors, so-called MPO connectors, in a measurement cycle using up to 12 multimode fibres. These multifiber systems are also already included in the IEC 61300-3-35 standard. Only the external zones C and D are not included in it.

The FiberXpert device is available for measurements in accordance with Level 2. A classic OTDR (optical reflectometer) in two designs, one a purely multimode device for the optical window of 850/1300 nm and the other a quad device which can be used for both multimode and single-mode systems and covers all four of the most common optical windows (see fig. 4)

Figure 4: Softing FiberXpert OTDR with connected video microscope for inspection of fibre end faces (example image)

Conclusion

All of the benefits of automatized assessment listed above ensure that automatic fibre end face testing is currently the most effective method of certifying compliance with the IEC standard throughout a fibre optic system’s life cycle and of ensuring that the potential of next-generation networks is fulfilled.