Connectors are inherently more difficult to design than mechanical splices. This is due to the added requirement of being able to be taken apart and replaced repeatedly. It is one thing to find a way to align two fibers but it is something altogether different if the fibers are to be disconnected and reconnected a thousand times and still need to perform well.
If two fibers are to be joined, each fiber has a connector attached and each is then plugged into an adapter. An adapter is basically a tube into which the two connectors are inserted. It holds them in alignment and the connectors are fixed onto the adapter to provide mechanical support. An adapter is shown as part of Figure 12.1.
Although different makes are sold as compatible, it is good practice to use the same manufacturer for the connector and for the adapter.
The design of connectors originated with the adaptation of those used for copper based coaxial cables and were usually fitted by the manufacturers onto a few meters of fiber called a pigtail which was then spliced into the main system.
Most connectors nowadays are fitted by the installer although pre-fitted ones are still available. The benefit of using the pre-fitted and pigtailed version is that it is much quicker and easier to fit a mechanical splice or perform a fusion splice than it is to fit a connector, so there is some merit in allowing the factory to fit the connector since this saves time and guarantees a high standard of workmanship.
When a connector is purchased, it always comes with a plastic dust cap to prevent damage to the polished end of the optic fiber. It is poor workmanship to leave fibers laying around without the caps fitted.
Before considering the details of the various types of connector, we will look at the main parameters met in their specifications so that we can make sense of manufacturers' data.
This is the most important measure of the performance of a connector. Imagine we have a length of fiber which is broken and reconnected by two connectors and an in-line adapter. If the loss of the system is measured and found to have increased by 0.4 dB then this is the value of the insertion loss. It is the loss caused by inserting a mated pair of connectors in a fiber. Be careful to ascertain whether the quoted loss for a connector is per mated pair or for each connector.
Typical value: 0.2�0.5 dB per mated pair.
This is a measure of the Fresnel reflection. This power is being reflected off the connector back towards the light source. The lasers and LEDs used for multimode working are not greatly affected by the reflected power and so the return loss is not usually quoted in this instance. In singlemode systems the laser is affected and produces a noisy output. The laser suppliers will always be pleased to advise on permitted levels of return loss.
Typical value: �40 dB.
Also called Insertion loss change. It is a measure of how much the insertion loss is likely to increase in use after it has been connected and disconnected a large number of times.
Typical value: 0.2 dB per 1000 matings.
These are, of course, compatible with the optic fiber cables.
Typical values: �25°C to +80°C.
Also called tensile strength or pull-out loading.
This is the loading that can be applied to the cable before the fiber is pulled out of the connector. It is similar in value to the installation tension on a lightweight cable.
Typical value: 200 N
This is a measure of how consistent the insertion loss is when a joint is disconnected and then remade. It is not a wear-out problem like mating durability but simply a test of whether the connector and adapter are designed so that the light path is identical each time they are joined.
This is an important feature of a connector but is not always quoted in specifications owing to the difficulty in agreeing a uniform method of measuring it. Some manufacturers do give a figure for it, some just use descriptive terms like 'high' or 'very high'. The quoted insertion loss should actually be the average insertion loss over a series of matings, thus taking repeatability into account.
Once installed, connectors can last for many years and so earlier designs are still met during maintenance work and are included in this chapter. These oldies are even now available in current catalogs.
For new installations, there are three recommended connectors. These are referred to as the 'SC', 'ST' and 'MIC/FDDI' connectors and are shown in Figures 12.4, 12.9 and 12.10.
Connectors are nearly always assembled using epoxy resin and are not reusable. There are many similarities between the various types of connectors and the early SMA (sub-miniature assembly) will serve as a suitable starting point.
SMA (sub-miniature assembly ― Figure 12.1)
The SMA connector has been superseded by the more modern designs but there are many still in use. To connect two fibers, we simply screw a connector onto each end of the adapter. It is only used for multimode systems as the losses are too high for singlemode use.
The air gap
The length of the adapter ensures that the ends of the two ferrules are separated by an air gap small enough to allow the light to jump the gap to the other fiber.
This gives rise to the first problem. How tight do we tighten up on the screw thread? Not enough and the losses will be too high. Too tight and we will grind the end faces of the fiber together and will cause the glass to crack or be scratched. If this happens, the connector must be removed and thrown away.
The ferrules have a hole through the center to take the bare fiber (primary buffer stripped off) and are made of stainless steel or ceramic. In the case of the stainless steel, the 127 μm hole has to be drilled. If the hole is slightly off-center or over-size, it can cause eccentricity loss. Of the two, ceramic is by far superior. The ceramic is 'grown' on a piece of wire of precise thickness. When the wire is removed, we are left with a much more accurate hole.
Owing to misalignment and slight variations in the assembly of the connector there are losses which vary in magnitude as the connector is revolved within the adapter. This has the advantage of allowing us to optimize the connection by monitoring the losses as the connector is revolved. This is only OK if everyone who uses the connector has the test equipment, the time and patience to make the final adjustment. It is far better to have a known loss each time the connectors are mated by ensuring that it can only go together in a single fixed position. The SMA suffers from poor repeatability because it can be assembled in any random position. It is normal practice to insert the connector, measure the loss then revolve it through 90° and take a new measurement. This is done four times and the results are averaged.
The simple screw thread offers very little protection against loosening when exposed to vibration.
The two versions, 905 and 906 (now obsolete)
The original SMA connector was called the SMA 905. To improve its performance the ferrule was modified and the new version was called the SMA 906 and is shown in Figure 12.2. A Delrin sleeve is a small plastic tube that slips
ST (straight tip ― Figure 12.4)
This was developed by the US company, AT&T, to overcome many of the problems associated with the SMA and is now the most popular choice of connector for multimode fibers. It is also available for singlemode systems.
The problem of repeatability is overcome by fitting a key to the connector and a corresponding keyway cut into the adapter. There is now only one position in which the connector can fit into the adapter.
The screw thread of the SMA has been replaced by a bayonet fitting so that there is no worry about the connector becoming loose when exposed to vibration. The ferrule is spring loaded so that the pressure on the end of the ferrule is not under the control of the person fitting the connector. There is no SMA worries about how tight to do up the nut.
Polishing styles ― Figure 12.5
The fiber through the center of the connector is polished during the assembly of the connector to improve the light transfer between connectors. The are three
A flat finish is simply polished to produce a smooth flat end to the fiber so that the light comes straight out of the connector within the acceptance angle of the other fiber.
In the case of the PC finish, the fiber is polished to a smooth curve. There are two benefits of a PC connector. As the name implies, the two fibers make physical contact and therefore eliminates the air gap resulting in lower insertion losses. The curved end to the fiber also reduces the return loss by reflecting the light out of the fiber.
The APC finish results in very low return losses, It is simply a flat finish set at an angle, typically 80. The effect of this is that when the Fresnel reflection occurs much of the reflected power is at an angle less than the critical angle and is not propagated back along the fiber.
Fiber connector, physical contact (FCPC ) ― Figure 12.6
The FCPC is a high quality connector designed for long-haul singlemode systems and has very low losses. It can also be used for high quality multimode work if required and is often found on test equipment.
It looks like an SMA connector but it is keyed for repeatability.
The ferrule can be steel or ceramic inset in steel and is spring loaded or 'floating'. The end of the fiber is polished into the curved PC pattern. It can be polished flat if required and in this case it loses its PC suffix and just becomes an FC connector.
Mini-BNC ― Figure 12.7
This was developed for the US market but has not proved popular and surviveson ly because it is specified for the IBM token-ring network. Apart from being very slightly smaller, it has nothing to offer compared with the STPC.
In appearance, with its metal ferrule, it is easily mistaken for a BNC plug as used in copper based electrical systems. It is for multimode use only, uses bayonet fittings and the ferrule is spring loaded and is a PC connector.
This is another connector, shown in Figure 12.8, which is also specified for the IBM token-ring network in its multimode form and is also widely used in the US for long haul singlemode telecommunications.
It is secured by screw thread rather than bayonet and has a spring loaded ferrule with a PC finish. When fitted to the adapter, the conical ferrule causes it to be self centering, thus providing low losses.
It is easily recognized by the unusual tapered ferrule and the exposed spring.
Subscriber connector (SC)
Also available in PC and APC versions and suitable for singlemode and multimode systems, it is illustrated in Figure 12.9.
This connector is designed for high performance telecommunication and cable television networks. There is a different feel about this connector when compared with the previous types. The body is of light plastic construction and has a more 'domestic' or 'office' feel about it. It has low losses and the small size and rectangular shape allows a high packing density in junction boxes. It plugs into the adapter with a very positive click action, telling us it's definitely
engaged. A very nice connector destined to succeed.
It uses STPC ceramic ferrules, otherwise it is another all plastic connector, with a similar feel to the SC. It is intended to be used in local area networks (LANs) to interconnect computer systems and other pieces of office equipment.
Compared with the other connectors, it seems quite bulky and is designed to be easily handled and plugged into the equipment often by the end user rather than the system installer. It is keyed to prevent accidental insertion in the wrong socket and is color coded for easy recognition.
Generally a system is designed to use the same type of connector throughout, and to ensure complete compatibility, and hence best performance, they are normally sourced from the same manufacturer.
Occasionally however, we meet two new problems.
The first is to connect two cables fitted with non-compatible connectors, say, an STPC connector to one fitted with a biconic connector. Such problems are easily solved by a wide range of 'something-to-something' type adapters. Some of these adapters do introduce a little extra insertion loss but not more than about 1 dB.
The second is to join a bare fiber to a system, quickly and easily, perhaps to connect a piece of test equipment or to try out a new light source. This is achieved by a bare fiber adapter. This is a misleading name since it is actually a connector as can be seen in Figure 12.11. It is really a bare fiber connector since it has to be plugged into an adapter. The only difference is that the fiber is held in position by a spring clip rather than by epoxy so that it can be readily re-used.
Fitting the bare fiber adapter
- Strip off the primary buffer for about 25 mm or so and clean the fiber.
- Press the cable grip and push in the fiber until it comes to a stop then release the cable grip. The primary buffer will not pass down the ferrule.
- Cleave off the bit that sticks out of the end of the ferrule.
The results depend on the quality of the cleave and are not as good as with a permanently fitted connector.
Plastic fiber connector ― Figure 12.12
Plastic fiber connectors are very quick and easy to fit but the insertion loss is higher than normal for glass fibers ― between 1 dB and 2 dB. The cables are connected in the usual method of having two connectors plugged into an inline adapter. Sometimes the end of the plastic fiber is polished using a simplified version of the techniques used on glass fiber and sometimes it is cleaved off as in the bare fiber adapter.
Fitting a plastic fiber connector
- The outer jacket (2.2 mm) is stripped off for about 25 mm.
- The fiber is pushed into the connector as far as it will go.
- The end is cleaved or polished according to the manufacturer's instructions.
Terminating a silica glass optic fiber (fitting the connector)
To avoid the job altogether, buy the connector already attached to a pigtail. Everything is done for us, all we have to do is to join it to the rest of the system by means of a fusion splice or mechanical splice.
The most usual method is called glue and polish. In essence, all that happens is that the fiber is stripped, glued into the connector and the end of the fiber is polished with abrasive film.
As usual, it is most important that we take some time out to read the instructions. It can save a lot of time and money in second attempts.
Fitting a connector on a silica fiber
- Strip off the outer jacket, cut the Kevlar, and remove the primary buffer to the dimensions supplied with the connector (Figure 12.13).
- Slip the flexible boot and the crimp ring onto the fiber. The crimp ring is a metal tube about 10 mm in length which will grip the Kevlar and the connector to provide the mechanical support.
- Clean the fiber with isopropyl alcohol in the way that was done prior to cleaving. Just as a practice run, carefully insert the stripped fiber into the rear of the ferrule and ease it through until the buffer prevents any further movement. If this proves difficult, it may help if the connector is twisted backwards and forwards slightly but be careful, the fiber must not break. Check that the fiber sticks out from the end of the ferrule. If it does break, the piece of fiber can be released with a 125 μm diameter cleaning wire which is available from suppliers.
- Mix some two-part epoxy and load it into a syringe. The epoxy is often supplied in a sealed polyethylene bag with the hardener and adhesive separated by a sliding seal. Remove the seal and mix the adhesive and hardener by repeatedly squeezing the bag between the fingers. The mixing process can be aided by the use of a grooved roller which is rolled to and fro across the packet.
- Insert the syringe into the connector until it meets the rear of the ferrule. Squeeze epoxy in slowly until a tiny bead is seen coming out of the front end of the ferrule. This shows that the ferrule is well coated with epoxy.
- Carefully insert the stripped fiber into the rear of the ferrule until the buffer prevents any further movement. Again, take care not to break the fiber. If it does break, the fiber must be prepared again, as the dimensions will now be incorrect. The epoxy is very difficult to remove from the ferrule and the cleaning wire is not guaranteed to work under these conditions. Acetone may be helpful. This is a job best avoided.
- Arrange the Kevlar over the spigot at the back end of the connector and slide the crimp ring over the Kevlar as shown in Figure 12.14. One end of the crimp ring should overlap the spigot and the other should cover the fiber jacket.
- Using a hand crimping tool, crimp the Kevlar to the spigot and at the other end of the crimp ring, grip the cable jacket. This ensures that stress is taken by the Kevlar strength members and not by the optic fiber.
- Put the connector into a small oven to set the epoxy. The oven, shown in Figure 12.15, is an electrically heated block of metal with holes to take the connectors. This will take about ten minutes at 80°C. When cured, the golden epoxy may have changed color to a mid to dark brown.
- When it has cooled down, fit the boot.
- Using a hand cleaver, gently stroke the fiber close to the end of the ferrule and lift off the end of the fiber (Figure 12.16). Store the broken end safely in a sealed receptacle for disposal.
- The end of the fiber must now be polished. The easy way is to insert the fiber into a portable polishing machine and switch on. After about one minute, it's all over, the fiber is polished. The alternative is to do it ourselves.
- Consult the instructions at this stage, each manufacturer has a recommended procedure for each type of connector and they usually know best. We will need a flat base of plate glass, hard rubber or foam about 200 mm square. A soft base is normally used for the PC finish. The abrasive sheet is called a lapping film. It is a layer of aluminum oxide on a colored plastic sheet. Silicon carbide and diamond films are also available. The roughness of the abrasive is measured by the size of the particles and is colored to aid recognition. Grades vary from the coarse 30 μm colored green down to the ultra smooth white at 0.3 μm. Beware ― not all types of film employ the same colors.
- Using a magnifying glass, observe the end of the ferrule to see how much glass is protruding above the tiny bead of epoxy (Figure 12.17). This unsupported glass is easily broken and should be abraded down to the epoxy level by using a strip of coarse grade film (9 μm) held in the hand and stroked gently over the fiber. Be very careful not to apply too much pressure and stop when the epoxy is reached. If the fiber is too long or too much pressure is applied at this stage, the course lapping film will send shock waves down the fiber and it will crack. The crack has the characteristic 'Y' shape shown in Figure 12.18. It runs vertically down into the fiber and no amount of polishing will do anything to help the situation. We have lost a connector and gained some experience.
- The fiber is supported perpendicular by a polishing tool called a dolly or a polishing guide (Figure 12.19). Each dolly is designed for a particular type of connector to ensure the correct dimensions and fitting mechanisms. The suppliers will always advise on the grades of film and methods to be used to be used. Once again it is worth reading the instructions carefully if an unfamiliar connector is being fitted. We start with the coarsest grade recommended, probably about 3 μm. Lay the film on the base material and attach the fiber in the dolly. Using only the weight of the dolly, slide the dolly in a figure of eight pattern
for about eight circuits.
- Using a microscope or magnifying eye glass, observe the end of the ferrule. There will be a large dark area which is the epoxy. If this is the case, repeat the above stage until the epoxy becomes lighter in color and eventually has a transparent feathered edge.
- When this happens, remove the 3 μm film and clean the whole area, including the dolly and the connector. Clean it very carefully with a tissue moistened in alcohol or demineralized water. Make quite certain that no trace of abrasive is transferred from one stage to the next.
- Using a fine grade, about 0.3 μm, repeat the figures of eight. The end of the fiber takes on a bluish hue with some pale yellow from any remaining resin. Black marks are, as yet, unpolished areas or possibly water on the surface of the ferrule (dry it off before checking). A little more polishing as the resin finally disappears. If the end of the fiber is not clear and blue with no marks or scratches, polishing should be continued.
- Fit a dust cap to protect the fiber.
Connectors are manufactured with hot-melt glue coating the inside as an alternativeto using epoxy. This is convenient since we can always remelt it if we need to reposition the fiber.
Some epoxy resin is cured by ultraviolet light rather than by heat.
When observing any possible defects, the core is obviously of primary importance. The cladding is split into two halves. The outer part of the cladding does not greatly affect the operation of the fiber and we can be more forgiving of any failings in this area.
Main failing points ― Figures 12.20 and 12.21
- Chips and cracks which extend into the inner part of the cladding.
- Cracks that extend for more than 25% of the circumference of the cladding.
- Scratches in the core area of a severity which is not consistent with the polishing techniques suggested by the manufacturer. This is what will happen if the grit from one stage in the polishing process is able to contaminate the finer grade lapping film.