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The role of splicing in FTTH
14 May 2008, Telecoms, Datacoms, Wireless, IoT

While FTTH (fibre to the home) is being heralded as the way of the future in terms of delivering high quality communications to consumers, it brings a whole new raft of challenges for existing and potential fibre cable installers.

One of the biggest issues to address is the need for reliable drop cable connections and whereas fusion splicing has long been the de facto approach for fibre installation, mechanical splicing is now coming to the fore. So what are the relative pros and cons of each and which is best for FTTH?

Dependable broadband services via FTTH depend on subscriber cable drops - in other words, the final link to the home from the telephone network - that are stable and efficiently installed. Furthermore, due to the commercial pressures of FTTH, these cable drops also need to be operationally flexible, so that customers may be added or services changed, as well as affordable in terms of mass deployment. There are three main splicing options available to FTTH installers and each has its pros and cons: fusion splicing, pre-terminated patch cords and mechanical splicing.

The main splicing options for FTTH cable drops

Traditionally, fusion splicing - whereby the glass of the fibre is jointed by fusing it together - has been the method used for fibre feeder and distribution. In the past, fusion splicing machines have been an expensive investment, with sophisticated features such as aligning the light-carrying cores with each other before completion of the splice, or testing the splice both mechanically and optically after splicing. They are not necessarily the most cost-effective and rapid solution for mass-market FTTH deployment, but the emergence of more simple handheld fusion splicers has been viewed as a suitable solution for FTTH splicing. Compared with traditional machines they are a less expensive and more portable solution.

However, handheld fusion splicers are not without their limitations: they rarely have the optical measurement capability, by which the average loss of the splice can be lowered. Furthermore, the typical cost per handheld fusion-splicer is R40 000 or more, plus there are the costs of buying the necessary preparation tools. Take into account the fact that each installation crew must have a set and the overall investment can escalate.

Further complications include the need to have a local power source, such as a battery (with associated maintenance costs and possible downtime) and since an electric arc is used to fuse the two fibres, fusion splicers work best in low humidity environments. Also, fusion splicers can take several minutes to set up, even if only one splice is required.

Pre-terminated patch-cords

The advent of factory-terminated pigtails as customer drops offer an alternative, because they eliminate the need for installation equipment and are quick and easy to install. However, having to install lots of different cable lengths can be an inventory headache, plus there are additional material costs associated with slack cable and an entire pre-terminated cable will need to be replaced if one connector is damaged in the field. Storage of slack aerial drop cable can also be an issue where local councils are concerned about aesthetic appearances.

Mechanical splicing

The limitations of both fusion splicing and pre-terminated pigtails have led installers - particularly in Asia where FTTH is more widespread than in most markets - to re-examine the benefits of mechanical splicing. This is where the fibres are joined together using mechanical means, such as a 'v' groove, which is a simple way of aligning two 'rods' end to end accurately. Traditionally, there were reservations about mechanical splicing, both in terms of cost and performance; however they now provide an effective alternative to fusion splicers, both in terms of cost and performance.

The two relevant optical parameters are insertion loss (how much light is lost across the joint) and return loss (how much light is reflected from the joint).

Most mechanical splices and the kind of more cost-effective handheld fusion splicers that are being used for FTTH deployment have comparable insertion loss, since they both align the fibres using the cladding diameter (outer glass surface of the fibre) as a reference (as opposed to optical alignment before splicing, which is seen in the most expensive fusion splicing systems).

In fusion splicing, the glass fibre is joined back together again, meaning that there are no surfaces from which the light can reflect. With mechanical jointing, there are two fibre ends butted up together. This joint is surrounded by 'index matching' material (a gel) which protects the splice and lowers the reflections.

Up until recently, this gel gave its best performance at mid-temperature ranges and was less effective at the high and low extremes of temperature, but new technical improvements, such as angle cleaving, have removed this problem, again making mechanical splicing a more viable option for FTTH installation.

Furthermore, mechanical splicers have become a cost-effective option, with a tool set for mechanical fibre preparation and splice actuation available at a reasonable price, including the fibre stripper and cleaver. This makes it feasible to equip multiple crews for intensive drop cable work at modest costs compared to handheld fusion splicing.

Rapidity of installation is another benefit: for drop applications, mechanical splice and connector terminations can generally be completed in about half of the time required for fusion splicing. When thousand of splices must be finished quickly, with only two or three splices per location, mechanical splicing is more efficient than fusion splicing. Additionally, since mechanical splicing is a simpler process than fusion splicing, technicians have less chance of error or damage to sensitive components.

It is for all these reasons that a number of service providers in Asia have settled on mechanical splicing for FTTH drop cables. So let us take a closer look at what mechanical splicing involves and the experiences of those operators.

The mechanical splice concept explained

A typical mechanical fibre-optic splice consists of a small plastic housing with an aluminium alloy element to precisely align and clamp fibres. As previously mentioned, an index-matching gel pre-installed at the fibre connection point maintains a low-loss optical interface, which results in a median insertion loss of less than 0,1 dB, comparable to fusion splicing (where the two fibre ends are fused or melted back together and then covered with a sleeve to protect the joint and the bare fibre). With both fusion and mechanical splicing, the process depends on the quality of the fibres. If the fibres are poor, the losses will be worse.

The mechanical splice process involves four steps: stripping the fibre coating from the glass, cleaning the glass, cleaving the fibre, inserting the fibres in the mechanical splice, and activating the splicing tool. This tool holds the mechanical splice in place and retains the fibres so that their ends are in contact.

A splice is completed by pulling down the tool handle to force a plastic cap down, which presses the sides of the metal element together and clamps the fibre ends. This hand-operated mechanical splicing tool can be used on any flat surface and requires only a small work area. Field-assembled mechanical splice connectors use the same metal element concept (a folded piece of metal that the fibres sit in) and the same assembly and actuation sequence (closing down the splice to complete it). As is the case with fusion splicing - and indeed, just about any other optical fibre installation - using the appropriate tools and paying attention to on-site cleanliness is essential to achieving optimum mechanical splice performance.

The Japanese experience

So that is the theory; so far, so good, but what does all this mean in practice? Probably the best source of information is to look at the experiences of operators in Asia, such as Japan, which leads the way in FTTH, with millions of homes already equipped and a target of 30 million users by 2010. Between them, the local service providers are installing at a rate of 200 000 cable drops per month. Quite simply, this means that Japanese operators have more experience in FTTH than anywhere else in the world.

In the past few years, Japanese service providers have carried out extensive laboratory and field tests that confirm mechanical splicing as a viable long-term answer to outside plant fibre-optic requirements. They report that mechanical splicing reduced initial tool capital expenditures by 90%, doubled splicing speeds, and decreased installed costs by 50% relative to fusion splicing.

Subsequently, the Japanese providers abandoned fusion splicing in favour of full scale mechanical splicing deployment of the FTTH drop. Millions of splices and field-assembled no-polish connectors using mechanical splice technology have been completed in Japan, with more being installed daily, making Japan the leader to date in implementing mechanical fibre-optic splicing. Field performance has been excellent, with one supplier's products having a success rate of greater than 99,98%.

Transition strategy

However, changing from fusion to mechanical splicing required some planning. Service providers in Japan recognised the need for a 250 μm fibre splice that could be used in existing fusion splice trays for FTTH drop splices. To meet this need, a compact splice component was developed, using the same splice element design as a conventional mechanical splice. For several years, this device has been used in Japan in widely varying environmental conditions, yet has managed to achieve very high reliability. Usability has been improved too, because this compact tool can be handheld and used even while the installer is on a ladder or standing at the side of a house; a level work surface is not required.

Making installation easier

One of the biggest challenges facing service providers is the need for skilled installers. As readers will be aware, handling fibre is not as simple as copper cable and requires far greater precision, yet this can be difficult to achieve when large volumes of connections are being deployed in a relatively short space of time, especially when there is a lack of even work surfaces, or if the installer only has one hand free.

To address this, a field-assembled mechanical splice fibre-optic connector was developed by 3M especially for the Japanese market, although it is now being rolled out in Europe as well. Both the connector and socket have precisely aligned and factory-polished ferrule assemblies that are connected to internal stub fibres. These - together with internal metal elements - are then used to make fibre drop cable connections in the same manner as the basic mechanical splice.

No polishing or other special preparation is required for a complete, low-loss pluggable connection, and a handheld mechanical splicing tool can be used with field-assembled mechanical connectors when a suitable work surface is not available. This custom plug-and-socket connector pair is used in Japan with a special tight-bend fibre, which handles a 15 mm radius with low-loss at long wavelengths, instead of the 25 or 30 mm bend radius of standard fibres. This fibre is encased in a strippable jacket that enables the connector to be mounted directly on to the cable. This fibre is easier to store, less likely to break and allows for more compact packaging than standard FTTH drop cables.

Users have also reported reduced tool costs of 90% and improved productivity of 50%, using this approach to mechanical splicing.

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