You can’t function properly with dust in your eyes, and fibre optic channels face a similar problem.

When fibre optic cabling was used in the 1990s, the world was an easier place.

A high optical budget allowed easy handling during installation, testing and operation, and most links had a high tolerance.

With each new generation of application the performance requirements have increased.

Given the latest definition of 40Gbps and 100Gbps Ethernet fibre Standards, optical requirements are so tight that end users, installers and consultants have to realise that habits and things we used to know no longer apply.

Since the introduction of 10Gbps over Ethernet, the maximum power loss budget for multi-mode links has dropped from 12.5dB to 2.6dB.

Fibre handling, installation methods and equipment have only gradually improved, but it is obvious that a change is now required for 10/40/100Gbps.

Critically the demand for 1.5dB maximum power loss with OM4 fibres for 40Gbps and 100Gbps applications requires:

• accurate handling and cleaning of end faces;
• precise testing with calibrated test equipment;
• detailed analysis of test results against loss budgets;
• vigilant procedures while implementing fibre optic links;
• meticulous cleaning and inspection tools; and
• careful consideration of connector counts.

With a budget of 1.5dB for 40/100Gbps, any dust on the end face, or even a patch cord being dropped and not recleaned, may result in a non-functional channel.

We have known for many years that cleaning and inspection of end faces is compulsory before connectors are mated – every time. This even applies to brand new cables and particularly to test cords.

Most faults in the field stem from contaminated end faces, inadequate fibre test equipment or failure to analyse test results to eliminate things like gains while still on site.

Therefore cleaning equipment is a must for the initial installer and subsequent network operator. Such equipment is available in many forms for connectors, couplers/adaptors and even MPO cassettes.

Cleanliness of all connector interfaces and special test cords and/or reference connectors is the important factor in obtaining correct and reproducible test results.

With small core diameters of 9µm or 50µm, the tiniest particles on the connector surface can disturb and affect the result.

The often practised ‘blow the dust off the connector’ can lead to a surface appearance as shown in the accompanying illustration, so special lint-free cloths should be used for cleaning. Alcohol wipes and air cans are not recommended as a first choice.

The importance of having a clean end face is also shown. Two connectors have been mated once, twice and five times, and each time more dust is collected.

Even more important, the dust particles are moving towards the centre core. This has a detrimental effect for 1Gbps and 10Gbps channel testing and network operation.

As ISO/IEC 11801 has defined channel budgets of 2.6 for 10Gbps and down to 1.5dB for 100Gbps, any small end face imperfections and testing failures will be critical.

Dust particles might have a strong influence on the connector attenuation and return loss, as they disturb the physical contact between fibre cores. Always check that end faces are clean before mating connectors to each other, and before every repatching operation, then inspect again to see if the end faces are still clean.

When testing, always remove dust and dirt, and inspect again to ensure that the end face is actually clean.

Check (look) – clean – check (look)

Always make sure the transmitter is turned off or there is no network connection before inspection. Pay attention and be extra careful when using a direct-view microscope.

A microscope is a valuable tool for optical inspection, otherwise the user has no idea about the effectiveness of the cleaning progress or about the condition of the mating surface.

Design implications

With the latest designs in data centres, the number of connections is usually higher than for the typical two-connector link approach.

Beside the fact that the latest low-loss connectivity is needed, faulty or dirty end faces have a huge effect on performance because the losses are increased with each additional connector:

Example

A 100m link with two connections (0.5dB each) may result in about 1.35dB total loss at 850nm wavelength.

The same link with four connections would have a loss of 2.35dB. Just one dirty connector could exceed the channel budget of 2.55 dB and result in an operational malfunction.

Conclusion

This article summarises the necessity for accurate and precise fibre optic cleaning, testing and operations. New, tight limits are required for fibre to suit the higher bandwidth applications of 10/40/100Gbps.

Installers, consultants and end users need to be aware that this will affect the overall situation for each party.

To date, regular maintenance and calibration of test equipment with more precise reference test cords, cleaning and inspections has come at a price. On the other hand it is far more cost effective to invest beforehand rather than to pay later for troubleshooting, correction and associated downtime.

Fibre is the key technology for many aspects of IT networks. It is well worthwhile to spend time on installation and cleaning and invest in the right tools such as testers, cleaning kits, hand-held fibre microscopes or digital microscope cameras.