Optical fibers go into homes - how to join them

As mentioned above, almost every installation company will sooner or later face the task of installing fiber optic links or implementing entire systems based on optical medium. Currently, many of the companies commission these jobs to external specialist companies employing qualified staff equipped with professional equipment. However, the specialist companies take over a significant part of the profits. So, despite the problems that a novice fiber-optic installer faces when entering the world of fiber optic systems, such as the lack of elementary theoretical and practical knowledge and the costs of the necessary tools that seem to be much higher than in the case of "classic" low-voltage installations, the game is worth the candle. The problem of the lack of knowledge can be solved by taking part in the appropriate training courses. For any installer, it is important to choose courses with practical skills components. The theoretical knowledge of the propagation of light and phenomena that accompany optical transmission within fibers is important, but it must be supported by a solid dose of practical expertise. Every installer has to distinguish various types of optical cables and fibers, know how to join and terminate them, install them in distribution frames and installation boxes. The latter ability is especially important in the process of the implementation of fiber optic installations.
Those who follow the fiber optic market certainly noticed that the rise in popularity of this technology is directly linked to the general decline in prices. Everything becomes cheaper - optical cables, passive components, active devices. What is more, it turns out that the today's market is able to offer novice installers everything they need to start working in this field at an acceptable price.

The installation of almost any fiber optic system can be divided into 3 stages: cable routing, placing the end sections (with spare lengths) of the cables in distribution frames and installation boxes, joining the fibers of the cables with pigtails (splicing). The highest costs are generated by the latter stage. The splicing operations take a considerable amount of time, require application of appropriate tools and high level of precision. Currently, the market is dominated by two splicing/welding methods: mechanical and thermal (fusion). Each of these is characterized by both advantages and disadvantages, which should be known for every novice installer. The advantage of mechanical splicing is relatively lower initial cost of the equipment needed to start working. After getting basic tools, i.e.: scissors for cutting Kevlar(R), fiber stripper, fiber cleaver, dust-free wipes, and IPA solvent, the installer can use mechanical splices to join any kinds of optical fibers. The necessary testing tool is visual fault locator, allowing for the verification of the splicing process and thus better results.
The disadvantages of this solution include the unit cost of such splice, well above the cost of a thermal weld, and much worse repeatability of parameters of the connections (attenuation, return loss etc.). Though the declared attenuation of most commercially available mechanical splices is 0.15 dB, close to typical level assumed for fusion splicing, this parameter may in practice reach much higher values. The expertise of the installer is here the decisive factor. It is worth mentioning that the higher attenuation does not always mean trouble. In the case of comparatively short links (several hundred meters or so) based on media converters or SFP modules, a reserve of power can often "forgive" imprecise splices, sometimes with attenuation at the level of several decibels. Generally, such situations are unacceptable in FTTH systems. In many countries the maximum attenuation of the whole route from the optical fiber distribution frame to the termination in the user outlet is strictly regulated (e.g. 1.2 dB in Poland). Such a restriction is not applied without reason - the power budget in xPON passive networks, continually gaining increased popularity, is an important issue. A single improperly made splice may block the service for one or a number of users.

Ultimode TB-02H box with L3551 pigtail,
L5550 mechanical splice, and L4211 adapter

The second method of joining fibers is based on fusion splicing, i.e. thermal welding. From a technical point of view, the use of a welder/splicer, e.g. EasySplicer L5810, has only advantages. The time needed to splice the fibers is much shorter and the parameters of the splice are considerably better with respect to mechanical connections. A common myth saying that the cost of such a device is close to the price of a new car is the reason for the continuous popularity of the services of external specialist companies, despite a considerable price drop. Similarly to the decline of prices for the components of fiber optic systems, the same process takes place in the case of the installation equipment. Of course, the top-class equipment still means major expenditure, but most installation companies do not need it to perform simple fiber optic systems. Currently, there are relatively cheap models of fusion splicers, suitable for companies/installers implementing small and medium-size projects. It turns out that the cheapest versions can be purchased at prices only slightly above the costs of good meters of TV signals.

EasySplicer L5810 belongs to the group of the cheapest fusion splices on the market

Of course, the price is only one of the factors determining the final choice of the welding machine. The purchase should be preceded by the answer to a number of questions concerning the future use of the device. The key factor is the number of splices performed during one installation, as well as the number of them on a monthly / yearly basis. Installers performing regularly from tens to hundreds of splices during a single installation should direct their attention to more expensive devices with batteries of higher capacity and shorter time needed for completion of a single splice. At first glance, the latter feature may seem insignificant, but every second saved during the making of a single connection will grow to many minutes in the case of installing a larger distribution frame. Beside the easier and faster operation, more expensive models offer additional features such as automated collection of scrap fibers after cleaving, which also saves the time of the installer. These facilities are not essential for the installer performing a small number of welds, say up to a hundred per month. This category of installers should choose between the cheaper models, because the time savings will not be significant in relation to the total working time.
An important issue is the working environment. Some welding machines (not necessarily the cheapest fusion splicers) have a mirror that can evaporate when working in difficult field conditions, which completely makes the splicing operations impossible.

All the above mentioned parameters and welding properties may be important, but the greatest impact on the price of a fusion splicer and the quality of the splices has the method of positioning the fibers implemented in the welding machine. There are two basic methods of positioning, to the core and to the cladding.
Using the former one, the device makes a series of enlargements looking for the cores of both fibers, and then using the built-in motors positions the fibers so that the both cores are perfectly opposite each other. The splicers with the alignment of real core positions are the most professional and at the same time the most expensive solutions. The devices using this method can automatically recognize the type of welded fibers and arrange them according to their real cores. Such welds are characterized by the best performance parameters and should be used in networks of large telecommunications operators. In smaller systems, one can accept other methods. One of them is used in splicers with aligning geometric core positions - this method is used by the majority of the cheapest splicers (usually manufactured in China) and many manufacturers and retailers wrongly describe them as devices with aligning (actual) core positions; the core geometry is obtained by optical means (visible light). The intensity of light in the fiber decreases as a function of depth, and with a given sensitivity and simple image analysis, it is possible to determine the approximate positions of fiber cores. This method has been optimized for good quality fibers, where the geometric position of the core is equivalent to its real position. When the core is embedded eccentrically, the parameters of the splice can be worse than in the case of fiber cladding alignment. It is worth mentioning that in the case of splicing multimode fibers (with greater risk of the difference between the "geometric" and real positions of the cores), some of the welding machines with automatic mode selection choose the positioning to the cladding as the better option.

The positioning method based on mutual alignment of the fibers' cladding gets fully satisfactory results in installations where the transmission path has a few welds. This does not mean that the results are always worse than in the case of core alignment. In the vast majority of situations, the results will be practically identical to those obtained with the use of much more expensive method of positioning the real cores of the joined fibers. Larger differences may only occur in situations where the installer will attempt to connect very old fibers or the cheapest fibers of dubious quality. Nowadays, the risk of such situations is comparatively low. The technological process associated with the production of optical fibers is so advanced that it is increasingly difficult (hopefully) come across cables of poor quality. Given this bottom line and the fact that the splicers with alignment to the core have extremely attractive prices, it is worth to consider purchasing this type of device. So, the splicers based on fiber cladding alignment can be successfully used in FTTH systems, in which, according to standards, the attenuation of any path in a building cannot exceed 1.2 dB, as well as in any fiber-optic networks where the lines between active devices have up to several splices. The examples are LANs, CCTV networks, or point-to-point links up to several kilometers. In the case of hundreds or thousands of splices per month, it is advisable to invest in the most expensive equipment with the alignment of the real core positions, automatic operations, capable of performing great number of splicing cycles per battery charge etc.

Regardless of the choice of the welding machine, the fusion splicing process looks very similar. It consists of a series of steps. The first of them is the removal of the fiber coating, 250 μm or sometimes 900 μm. Independently from the type of the spliced fibers, the bare cladding has diameter of 125 μm, the size needed for further operations. Then, it is necessary to clean the fibers with isopropyl alcohol (normally with dust-free wipes or cotton swabs), and cleave them using a suitable cleaver.

The latter operation is very critical - it is important to ensure both the appropriate length of the fiber ends (in most cases the installer does not have anything to measure, because the construction of the cleaver allows for easy selection of the suitable length) and perfect angles of the cleavage planes. In practice, the welders accept some deviation from the right angle (about 1 degree), but more precise cleaving will result in better performance of the fusion splices. Low quality cleaver or insufficient accuracy of this operation are the main reasons of lower quality of the splices.

View of the fibers prepared for splicing in EasySplicer L5810

After placing the fibers in the welding machine, the results of the previous operations are verified. In the case of an excessive dirt on a fiber end face or improper cleaving, the device will notify the user to repeat the fiber end preparation process. When everything is in order, the fibers are welded with the use of an electrical arc, and the fusion splice is tested for strength and transmission parameters (loss).

After splicing: estimation of the splice loss (0.03 dB)

The estimation of the splice loss is based only on image analysis. In the case of sensitive applications, the installer should test each splice in both directions by professional means, i.e. using a precision light source and optical power meter. Of course, in the case of smaller systems or those in which the attenuation levels are not so critical, the simplified estimation can be sufficient.
The process of joining fibers usually takes several to about ten seconds. The next and last step is the protection of the splice with a heat-shrinkable sleeve, with the use of a heat oven, which is often integrated with the fusion splicer. Newly made splices are fragile and always require additional protection. Heat-shrinkable sleeves protect them mechanically and allow for easy placement of the connected fibers in splice cassettes.