High efficiency in optical fibre installations can be achieved only by precision manufactured connectors and adaptors.

Optical fibre is ubiquitous in today’s suite of communications media. Such cabling technology is indispensable in reliable, secure and high-speed throughput for long-haul communications.

It is also used in campus environments, backbones of buildings, data centres, ‘fibre to the desk’ and local area networks.

The performance of optical fibre channels is specified by the Standards, ensuring inter-operability of equipment and the future-proofing of applications. Individual component performance is crucial for achieving and sustaining the intended operation.

OPTICAL FIBRE MEDIA

Multi-mode and single-mode optical fibre media are defined by cabling Standards for commercial premises, data centres and outside plant.

At present, the highest bandwidth applications defined are 40Gb Ethernet and 100Gb Ethernet – IEEE 802.3ba for multimode and single mode, and IEEE 802.3bg for 40Gb Ethernet for single-mode serial.

These applications require very tight optical power budgets – and tough restrictions on connectivity and optical fibre attenuation and return loss.

The next high-speed application being developed is 400Gb Ethernet. Single-mode optical fibre is expected to be the medium of choice.

GRADES OF CONNECTING HARDWARE

The latest version of AS/NZS 3080 and ISO/IEC 11801 specify several optical fibre connecting hardware grades and maximum splice losses.

The connector insertion loss grades enable a range of application loss budgets to be estimated, from large to very tight. Three connector quality grades are specified. These specifications (grades) apply to single-mode and multi-mode connecting hardware and are based on random mated connector pairs, independent of connector type.

The quality level stems from the Monte Carlo statistical algorithm for randomly selected connectors.

These performance values are required to ensure the overall insertion loss budget for a given application is not exceeded.

Application loss budget allocation The maximum attenuation allocated for connectivity in the application Standards is 1.5dB (out of a total of 2.60dB for 10GBase SR/ SW and 1.90dB for 40/100 GBase SR4/SR10) for optical channels using OM3 multi-mode fibre. For 40GBase SR4 and 100GBase SR10 using OM4 multi-mode fibre, the maximum connectivity loss is 1.0dB.

LOSS BUDGET EXAMPLE

Consider the scenario in a typical data centre.

Servers communicate with each other (east-west) via top-of-rack switches connected by 100m of multi-mode optical fibre (OM3 or OM4) via cross-connect distributors.

The optical fibre insertion loss budget (ILB) is calculated as follows:

ILB = (fibre attenuation coefficient x fibre length) + (number of connectors x connector loss) + (number of splices x splice loss);

The fibre attenuation coefficient for OM3 and OM4 at 850nm is 3.5dB/km.

In all situations the application channel limits cannot be exceeded.

Applying the link loss budget calculation model for 10GBase SR/SW, 40/100 GBase SR/SW (OM3 and OM4), using the AS/NZS 3080 allocated values, the matrix in is obtained.

For 10GBase SR application, the maximum channel loss at 850nm is 2.60dB.

Using the AS/NZS 3080 IL criteria, only options (ii) and (iii) are suitable for the 10GBase SR/SW application. Option (ii) provides a margin of 0.25dB. Option (iii) gives the highest headroom (0.85dB) allowing for the ageing of the transmitter and receiver, which might have been subjected to accidental operation at higher temperature and humidity than specified for the equipment.

For 40/100GBase SR4/SR10, the maximum channel loss at 850nm is 1.90dB for OM3 fibre.

Using AS/NZS 3080 criteria, only option (iii) is suitable. This option (iii) gives headroom of only 0.15dB. This is probably insufficient for the ageing of the transmitter and receiver, which might have been subjected to accidental operation at higher temperature and humidity than specified for the equipment.

For 40/100GBase SR4/SR10, the maximum channel loss at 850nm is 1.50dB for OM4 fibre.

None of the options are suitable! It is clear that the power loss budget calculations for these applications on OM4 using the component loss values defined in AS/NZS 3080 and ISO/IEC 11801 demonstrate that the OM4 medium is unsuitable.

Again, this is an even bigger problem using OM4 for 40/100GBase SR4/SR10.

In all these cases, the margins – if any – would be insufficient if splices were required in the channel (say pigtails were used instead of direct terminations).

This is a crucial factor to consider at the design stage. It narrows the options for termination to connector polishing on site (requires highly skilled technicians and more time) or acquiring pre-terminated optical fibre links.

An important scenario to consider is the use of MPO/MTP (8/12/24 fibre array connectors) cabling solutions. These budget calculations are critical in this situation, where each cassette includes two connectors (one MPO/MTP and one SC/LC/ST).

Even in the 10Gb Ethernet application, it would not be possible to use more than two MPO/MTP cassettes where the individual connector insertion loss cannot exceed 0.50dB.

It is universally accepted that MPO/MTP technology is the only one used for the 40/100GBase SR4/SR10 application. In this case, the cassettes are replaced by MPO/MTP couplers. This implies that very high quality multi-fibre array connectors be specified to meet the tight application insertion loss budgets.

It is clear that for 40G/100 GBase SR4/SR10 on OM4, the most viable option would be to reduce the number of connections in the channel from four to two. This makes the design inflexible (replacing cross-connects by interconnects) and means introducing more switches, thereby increasing the overall cost.

An option for maintaining design flexibility is to adopt very high quality cabling component specifications that guarantee lower losses than specified in the Standards. This would be an improvement on the acceptable quality levels defined in AS/NZS 3080 and ISO/IEC 11801.

Vendors of high quality connectivity offer such options which, as expected, would carry a price premium. Such proprietary connector specifications follow the same methodology used by IEC for single-mode connectors.

CONNECTING HARDWARE PERFORMANCE

Optical fibre connecting hardware performance specifications are defined in international connector Standards including IEC 61753 (performance), IEC 61755 (optical), IEC 61754 (mechanical) and IEC 61300 (test and measurement).

IEC 61753-1 defines optical fibre connectivity performance grades for single mode and multi-mode for controlled environments.

IEC 61753-1 specifies the insertion loss and return loss limits for single-mode connecting hardware.

These performance limits are defined for random-mated connectors, meaning that the values apply to off-the-shelf connectors not ‘reference’ connectors. Optical fibre connectors need to be defined by two parameters – eg: B/1 (insertion loss grade B, return loss grade 1).

These single-mode connector specifications allow long communications channels with multiple connections between the transmitter and receiver.

Grade A/1 performance can be achieved only by angle-polished single-mode connectors. Such low insertion loss values and high return loss values are possible only when special connector ‘tuning’ procedures are used during the production process.

Connector tuning ensures very low insertion loss for random-mated connections within the relevant grade.

It should be noted that the couplers (adaptors) must also be of the same or better grade as the connector pairs being mated. Even with the bestquality connectors, performance will be compromised unless the end-faces are clean and protected from damage.

CONCLUSION

The main objective here is to demonstrate the need for careful consideration in planning the optical fibre physical layer in enterprise premises, campus environments and data centres.

High performance levels can be achieved only by precision-manufactured connectors and adaptors. The values are generally achieved using factory-polished terminations and rigorous cleaning procedures.

Commissioning optical fibre installations requires compliance to testing specifications, where pass/fail limits apply even smaller insertion loss budgets than used for design purposes.

Unfortunately, very little attention is given to this part of optical fibre cabling design. The consequences of using unsuitable optical fibre components in the communications cabling network can be serious – including unsatisfactory performance when the proposed applications are implemented.

For the past 27 years, Patrick Attard has been involved in the cabling industry at various levels, including cable design, training, participating in the development of standards and technical writing. He is currently the technical director of Layer One Engineering & Networks Pty Ltd.