Installed copper cabling & equipment creates a significant electrical safety, fire and smoke hazard. Installed fiber cable only creates a passive fire & smoke hazard, so fiber cable safety is only about choosing a low smoke / fire retardant cable, and ensuring that fire stops are good. So some testing can be eliminated, and some key design issues no longer exist e.g.: earth bonding is irrelevant, and fiber cables can be laid right next to electrical power cables.

The network designer usually specifies the required installation acceptance test requirements, however these may be more application specific than you are used to, as we will see.

Typically, fiber cable acceptance will be based on permanent link attenuation, length /signal delay, and connector quality. OTDR testing is sometimes done. Note that frame / data testing is not usually required at this level.

Although fundamentally length is dispersion limited, in order to lower Tx/Rx equipment cost, a very tight optical loss may be specified, which creates significant optical testing challenges.

This discussion is about passive Ethernet optical cable, with optical connectors & separate transmitters. Where “active cable” is used, there are deliberately no fiber connectors, the fiber is hard attached to the Tx/Rx device, so no optical testing can be done.

Fiber connector cleaning and inspection is an important new area which you’ll need to learn first up. Every time a connection is made, the connector tip should be cleaned and inspected prior to mating. Just get a good microscope, and a quick play with one will show you why. Most (totally avoidable) problems are caused by inadequate cleaning and inspection. This process will correctly take up a fair bit of site time, but is absolutely worth it. All technicians who make connections will need training and quick access to good cleaning & inspection resources.

On some jobs, it will also be a requirement to take pictures of connector tips and record these with your test results. “Automated pass / fail” inspection devices are available, but they are expensive, and the consistency is questionable. Sometimes it may be more practical to get hold of the visual standards, use cheaper equipment, and just make the visual pass / fail judgement yourself.

Whatever you do, make sure your staff get some practice with your chosen solutions so you can minimise site rework.

For Occupational Health and safety reasons, keep everything on site at a Class 1 power level by default. This means no more than 2 mW or +3 dBm emitted from any optical port, including any VFLs. Above this, you’ll need specialist laser safety knowledge.

A copper cable certifier is a (usually expensive) instrument used to test cabling to carry data conforming to a particular standard, typically e.g. 100M / 1G / 10G Ethernet data rates. This sort of testing may only loosely apply to fiber, as it doesn’t always have the same value here.

Qualification usually describes mid-level cable testing for a particular data use, e.g. a subset of a standard. You’ll note that this seems to regularly describe the fiber situation, although this term isn’t used much by the fiber industry. This may explain a lot of confusion.

Verification usually describes “craft tool testing” such as continuity etc. Fiber test tools also exist here, although they are quite different since it’s not really possible to “probe” a fiber cable.

However, another common issue is securing the test data / report: it should not be possible to tamper with either the test results or the report. Use of equipment with insecure reporting software may lead to rejection of test results. Broadly, the test equipment needs to come with a complete & secure reporting package, or results are subject to the possibility of tampering. There is a very limited choice of solutions that meet this requirement.

These are the very broad issues. There will be other considerations such as: test wavelength(s), Encircled Flux compliance, and so on.

• Test equipment connectors may not match the application. However, there are some tests where the power meter connector style must match the application, which may affect equipment choice.

• Fiber connectors are prone to wear-out, so the instrument connector is typically not used to directly test the cabling, to avoid degrading the instrument: the test lead serves as a disposable wear item.

• It’s possible that mixed fiber core diameters have been accidentally used in the system. A good test procedure should pick this up. You may note that more relaxed procedures are acceptable where there is only one fiber piece per link (no splices), e.g. this mix-up can’t happen.
• When OTDR testing, a test spool is usually specified after the instrument, or this length may be built into some instruments. You will also need to know how and when to change the various set-up parameters, in order of importance: the set fiber length must be longer than the link (or there will be strange ghosting effects), the averaging time should be “medium” for most approval testing, then the pulse width should be just long enough for test noise to be acceptable.
• When doing source and meter testing, make sure you understand how and when to take a reference.
• Note that the standards usually require testing at 1300 nm on multimode fiber. This is to try and identify sharp bends which could cause later breakage. This may not show up at 850 nm, but may show up at 1300 nm. This has particular relevance to MPO connectors, since some MPO specific test equipment only tests one wavelength.

Unstable light sources. When you nudge a connector on a source, or just reconnect it, best practice is that the connected power should typically not change by more than about 0.1 dB. Worse than this will cause problems.

Power meter detector types:  Silicon detectors are best for 850 nm only, InGaAs detectors are best for for 850 - 1625 nm.

Power Meters with a small detector and no optics: for a simple power meter head, a 1 mm diameter detector is very poor: 3 mm is best. These small detectors are just inaccurate, since some light misses the detector, so they are sensitive to beam geometry.

Broadly, test uncertainty goes a bit like this: Your acceptable maximum loss is say 1.9 dB, and your test uncertainty is say 0.5 dB, so any loss test result over 1.4 dB is marginal. That’s tight! Obviously any reduction in uncertainty will be welcome, which will require a combination of good equipment, good test leads, good test method, good attention to cleaning & inspection, and so on. A weakness in any of these may have a serious impact on this common scenario.

At the “high end”, there are various attractive looking MPO specific cable test solutions which should offer very good test throughput, however their technical measurement performance is often quite poor, resulting in a lot of re-testing and frustration. These solutions are also typically tied to only one test wavelength, and only one connector type (e.g. 12 x 1 @ 850 nm only). At the “low end” are any standard sources and power meters, however their throughput is just too low. A mid-level solution comprises MPO-optimized power meters, so that the power meter end doesn’t need moving as each fiber is tested, which greats speeds up & simplifies this procedure. This mid-level solution is easily adapted to a wide range of fiber types, wavelengths and fiber counts.

• Cleaning materials. For LC this is easy, MPO is a bit more difficult, since some solutions do not seem too good, or need too much care in use. Each technician needs these consumables.
• Inspection Scopes: Each technician should have “something”, which could be an inexpensive optical scope, and some may want electronic scopes with more capability. Good MPO scopes are less common.

• Test Leads, and a test lead maintenance plan.
• Loss test equipment with excellent accuracy / stability, and secure reporting software.
• Power meter ports should preferably accommodate duplex LC connectors.
• Test sources should be Encircled Flux compliant, or use a suitable conditioning lead.
• Some way of arriving at “cable length” which could be “by inspection”, or with some other length tester, or with an OTDR.
• OTDR and launch spool when specified.
• Try out the test procedure & equipment in a test environment before going on-site, early enough so that you can rectify obvious equipment deficiencies.
• Send a sample report (format) for general approval by your customer.
• Craft tools such as a VFL for fault finding / identification / polarity testing.