Guide to fibre-optic testing

The deployment of multifibre push-on cabling systems is rising as businesses try to address the growing data requirements. This article provides an overview of issues related to testing the cabling for these systems.

Typically, the new-age data centre applications are for 40G/100G ethernet. Even if the immediate requirement is 40G, cabling may be installed for 100G ethernet in order to allow for incremental bandwidth upgrades. The 24 fibres used for 100G ethernet can alternatively be used to carry three 40G systems. This offers a choice of upgrade paths; for example, to incrementally add extra 40G channels or replace with a single 100G channel. Standards are also being developed for 400G ethernet.

The incremental cost of installing 24-fibre 100G cabling, instead of 12-fibre 40G cabling is actually quite modest, particularly given the cost of future rewiring and downtime. The major additional complexity may be a patch panel to give adequate flexibility for future deployment options.

Standards compliance

Internationally recognised standards define just about everything on data centre design. Just in the area of fibre-optic cabling, there are standards for fibre types (typically multimode OM3, OM4 etc), cable types (fire retardance, bend resistance, riser cables etc), cable installation (fire stops, ducting etc), connectors (LC, MPO etc), labelling conventions, ethernet, optical wavelengths (CWDM etc), optical safety (mandatory) and finally: cable acceptance testing.

There is one aspect of fibre test standards that everyone can agree on: they are currently a work in progress, and this is particularly true of MPO testing.

The cable acceptance test requirements should be reviewed and refined at the project definition stage based on the latest and relevant standards and transmission equipment requirements.

So at the project definition stage, it is advisable to review and refine the fibre cable acceptance test requirements that end up as a combination of standards-based requirements and the mandatory requirements of the particular transmission equipment in use.

A small acceptable optical loss

The majority of LAN transmission systems using MPO connectors have extremely tight end-to-end optical loss requirements, typically around 1.2 dB. This tiny loss figure creates unique challenges for precision loss measurement for cable certification. Most of the currently available test equipment and testing procedures are incapable of achieving the required repeatability (accuracy) to give confidence that the specification has been achieved.

The two reasons for this tight specification are to minimise per port electronics cost and because these systems are dispersion limited, not power limited. These systems use 850 nm and 50 µm core fibre, and end-to-end length is usually limited (by dispersion) to around 300 metres.

Inspection and cleaning

Connector condition and cleanliness is a major consideration, and it is the most common cause of problems. It’s essential to have a good quality microscope with an MPO specific adaptor, a supply of suitable specific cleaning materials (more than one type is good), and adequate time and know-how to use them. The default MPO cleaners tend to do a reasonable job of cleaning the fibre ends only, but extra cleaning materials are available to clean alignment pins, and sometimes the whole connector end face. Inspection and cleaning should be performed every time before a connector is mated.

Cabling disturbance

Because of the combination of low loss requirements and fibre complexity, it is essential that testing and commissioning is accomplished with the lowest possible level of disturbance to connections. Also, once loss testing is done, any disturbance may require the loss testing to be repeated. This means that an unusually strict test regime is appropriate, so that each acceptance level is completed with confidence.

Continuity testing

This is the most basic test: does light get from end to end? As a minimum, this test is easily accomplished with a VFL source and a one low-quality breakout cable. If a numbered breakout cable is used each end, this could also double as a polarity test phase. Continuity testing usually doesn’t need documenting.

The continuity test phase is usefully accompanied by thorough cleaning and inspection of the connectors with a microscope. This achieves a few objectives. Bad connectors are the main cause of installation failures. The inspection and cleaning enables the installer to identify and rework bad connectors at the earliest point. It also ensures that bad connectors do not contaminate the test leads and degrade other connectors.

If the connector quality is inadequate, there is little point proceeding to the next stages until this issue is fixed. Simple continuity testing doesn’t identify if fibres are swapped back to front.

Polarity testing

Typically, the cable installation requires polarity testing to determine that the correct 1, n arrays of fibre at one end of a system, are mapped to the correct 1, n arrays of fibre on the other end. If the installation has a patching cable section so that connections can be split off to particular transmission equipment, niggling polarity mistakes can occur here. If the polarity test isn’t 100% accurate, a great deal of time will be spent in finding swapped fibres. This will require a basic level of documentation, sufficient to record which fibres were tested and the general array direction.

Labelling

The system is now physically in place with the light going to and fro in the right combinations. This is the perfect time to check and rectify so that all associated labelling is fully correct, otherwise the next phase of loss testing could result in incorrect reassembly and non-working systems. A methodical approach is essential.

Accuracy issues

As mentioned, the acceptable maximum loss figure may only be 1.2 dB. For example, if the test uncertainty is 0.3 dB, then all measured losses above 0.9 dB would have to be considered marginal. Every small improvement in the uncertainty figure yields a suitably larger maximum loss level, which will reduce installation cost. This explains the importance of proper planning of test leads, cleaning etc. In practice, accuracy is optimised by:

• use of low-loss MTP Elite test leads, and ensuring they are in top condition;
• use of an encircled flux compliant test source, followed by a mandrel wrap;
• use of accurate loss test equipment, remembering that you are working on the limits of achievable loss test accuracy;
• rigorous inspection and cleaning every time a connector is mated;
• performing test lead verification at the start of the job;
• use of appropriate data recording systems, since there will be plenty of test data.

Test leads

A major loss test consideration is the test leads. Elite MPO/MTP leads are available with per connector/fibre loss of below 0.35 dB. Here are the USConnec specifications for these connectors.

	MM MT Elite multimode MT ferrule	Standard multimode MT ferrule	SM MT Elite singlemode MT ferrule	Standard singlemode MT ferrule
Insertion loss	0.1 dB typical (all fibres) 0.35 dB maximum (single fibre)	0.20 dB typical (all fibres) 0.60 dB maximum (single fibre)	0.10 dB typical (all fibres) 0.35 dB maximum (single fibre)	0.25 dB typical (all fibres) 0.75 dB maximum (single fibre)
Optical return loss	>20 dB	>20 dB	>60 dB (8° angle polish)	>60 dB (8° angle polish)

Testing will require a combination of straight patch leads and breakout leads. It is also importance to ensure that connector pin polarities are correct, since swapping in the field is not viable. Spare leads will be required, in case one gets degraded or damaged.

The typical application environment uses a mix of LC and multifibre connectors. LC-MPO breakout test leads are an obvious choice. However, SC-MPO breakout leads are typically more robust, easier to handle and offer slightly improved optical performance.

Number the leads and ends

Test leads require performance verification before each use. The performance verification test is quite specific, and failure to do this preliminary test may result in invalidating all the test results. If the actual loss of each connection is recorded for each fibre during this test, then this loss can be used to compensate the following test results and improve accuracy.

General loss test equipment choices

There are three choices to be made - they will depend on the scale of expected work:

• Use a basic source and metre. This has minimal equipment cost but is tricky to perform (and prone to error), since both instruments need to be moved for each fibre to be tested. It’s suitable for small projects.
• Use a basic source and special MPO compatible metre so that only the source needs to be moved for each fibre. For a modest equipment cost, this greatly speeds up and simplifies testing. This works for any fibre type, or wavelength, or number of fibres per connector, so it has excellent flexibility, and the same equipment can be used for other work. This is a suitable choice for mid-size projects or for cabling contractors.
• A special MPO test set. However, these sets only work at 850 nm and 12 fibres. They are expensive and inflexible, making them suitable for large projects.

Whatever the test equipment, a good reporting software will be required to prevent tampering.