sábado, 17 de julio de 2010

Fusion splicing

Fusion splicing is the most permanent and lowest loss method of connecting optic fibers. In essence, the two fibers are simply aligned then joined by electric-arc welding. The resulting connection has a loss of less than 0.05 dB, about 1% power loss. Most fusion splicers can handle both single mode and multimode fibers in a variety of sizes but, due to the losses involved, we only splice multimode to multimode or singlemode to singlemode. There are also splicers that can automatically splice multicore and ribbon cable up to 12 fibers at a time.

Preparation of the fiber

The fibers must first be stripped, cleaned and cleaved as we have seen in Chapter 9. To allow spare fiber for easy access and to allow for several attempts, a length of at least five meters of jacket should be removed. The primary buffer is only stripped to about 25 mm. The exact length is determined by the fusion splicer in use.
The quality of the cleave is of paramount importance. However much money we spend on buying the most sophisticated splicing apparatus, it will all be wasted if we cannot cleave the fiber accurately. Both cleavers and splicers come in a range of prices with splicers being the more expensive by a factor of at least ten and sometimes a hundred. It is never a good idea to save money by buying an inadequate cleaver — it is always better to buy the cleaver you have confidence in, then, if necessary, recover the money by buying a slightly cheaper version of the splicer. Most splicers nowadays measure the accuracy of the cleave and if found wanting, the fiber is rejected until you have redone it to a satisfactory standard. Most splicers consider an end angle of better than about 3° as satisfactory.

Protecting the fiber

Splice protector

In the preparation phase, we have stripped the fiber of all its mechanical and waterproof protection. Once the fiber has been spliced, some protection must be restored since the splicing process will have reduced the fiber strength to less than 30% of its former value. This is achieved by a device called a splice protector. It consists of a short length (about 60 mm) of heatshrink sleeving enclosing some hot-melt glue and a stainless steel wire rod as seen in Figure 10.1.

Prior to joining the fiber, the splice protector is slid onto the fiber. After thesplice is completed, the protector is centered over the splice and heated, usually in a purpose-built oven although a hot-air gun can be used. The oven is a simple tray with a lid, a heater and a timer which are normally built-in features of the splicers. The hot-melt glue keeps the protector in position whilst the stainless steel rod provides proof against any bending that may occur. The outer sleeve offers general mechanical and water protection to replace the buffer that has been removed. To ensure that the fiber is fully protected along its length, at least 10 mm of the protector must overlap the primary buffer at each end of the splice.

Enclosures (termination enclosures)

After the splice is completed, we are left with a length of fiber deprived of its outer jacket. The fiber must be protected from mechanical damage, and from water. This is achieved by an enclosure (Figure 10.2).

The design, and cost, of the enclosures depend on the environment in which the fiber is going to live. Obviously something to protect fiber under water has to be superior to a plastic box in an air-conditioned office.
The significant feature is a means of ensuring that the fiber is well supported within the container in such a way that bending loss is avoided. This is done by having something to wind the fiber around, like a reel, referred to as a cassette, or at least a few clips to support the fiber and the splices. They are readily available in different sizes to hold everything from 4 to 240 fibers. Each fiber must be identified, otherwise a simple job could become a real nightmare. This is achieved by attaching labels to the fibers or splice protectors and by using colored splice protectors.
There are some other factors to consider which may not immediately spring to mind such as:
  • Security of the data. With the outer jacket removed a simple live fiber detector can be clipped onto the fiber and all the data being passed can be copied.
  • Access to an enclosure is the easy way to sabotage a communication system.
  • There is also a problem with light, again with no jacket and as bends are inevitable, there is a risk of light entering the system so the container should be light proof.
  • Unpleasant environments. Salt spray, acids, high temperatures, crushing and all sorts of other nasties.
  • Access for repairs or for testing purposes.
We stripped off five meters of outer jacket to enable the fiber to be lifted out of the enclosure with enough spare fiber to be easily connected to test equipment or a fusion splicer.

Holding and moving the fibers in the splicer

The fibers are held in vee-grooves cut into steel or ceramic blocks (Figure 10.3). As usual, cleanliness is all-important. The fiber is cleaned and cleaved then the vee-groove is cleaned by a lint free cloth, tissue or a 'cotton bud' moistened with isopropyl alcohol. Do not use compressed air cleaners as any contamination will turn it into a grit blaster and damage the critical dimensions of the veegroove. The fiber is gently pressed into the vee-groove by a magnetic or gravity clamp.

Once the fibers are safely clamped into their vee-grooves, they are moved, veegrooves and all, until the fibers are aligned with each other and positioned directly under the electrodes from which the electric arc will be produced. We are aiming to achieve positioning with an accuracy of better than 1 μm. In the least accurate splicers, suitable only for multimode fibers, this can be achieved by simple microgears operated manually. More precision is required for single mode fiber since the core is so much smaller. A 1 μm error in positioning an 8 μm core causes a lateral misalignment of 12.5% whereas the same error on the larger 62.5 μm core in a multimode fiber would represent a lateral misalignment of less than 2%. The extra precision is provided by using stepper motors. A stepper motor is an electric motor that behaves in a different manner to a 'normal' electric motor. We usually picture electric motors spinning round as power is applied. Stepper motors, instead, turn a set number of degrees and then lock in that position. The amount it turns can be precisely controlled by digital input signals and, in conjunction with a gear train, is able to provide extremely accurate alignment of the fibers.

Observing the alignment

All fusion splicers are fitted with some means to observe the fiber positioning and the condition of the electrodes. This is achieved by either a microscope or by a CCD camera (CCD = charge coupled device — a semiconductor light sensor) and a liquid crystal display (LCD ). The trend is towards CCD cameras since they are more pleasant to use and have the safety advantage of keeping our eyes separated from the infrared light which can, of course, cause irreparable damage to the eyes if we accidentally observe an active fiber through the microscope. The optic system always allows viewing from two angles as fibers can otherwise hide one behind the another and appear to be aligned. One way of achieving this is shown in Figure 10.4.

Automatic positioning

There are two methods.

PAS — the profile alignment system — Figure 10.5
This is the standard method of aligning the fibers in modern fusion splicers. The idea is very simple. A light is shone through the fiber and is detected by a CCD camera. The change of light intensity at the edge of the cladding and at the core due to the changes in refractive indices allows the system to detect their positions. Several readings are taken from each fiber and averaged out to reduce any slight errors.
Once the positions are detected, small stepper motors are activated to bring thetwo fibers into alignment. The viewing angle is switched through 90° to allow

the system to check in both planes and further small adjustments are made until the splicer is quite satisfied. The whole operation is usually automatic but we can follow the process on the liquid crystal display. As the system is able to detect the core position as well as the cladding, any eccentricity error in the core can be compensated for.

LID — light injection and detection system — Figure 10.6
This system makes use of bend loss, the light leakage that occurs when moderately tight bends are introduced into a fiber. Remember that the light can go into the fiber at a bend as well as being able to escape from it.
A bend is introduced at the input side of the splicer and light is injected into the fiber through the primary buffer. The light travels down the fiber and jumps the gap into the other fiber. A similar bend in the other length of fiber allows light to escape.
A stepper motor is used to move one of the fibers horizontally and the output light is monitored to detect the point of maximum light transfer. This means that the cores are aligned, at least in one plane. The fiber is then moved in the vertical plane until, once again, the point of maximum light transfer is discovered. The whole process is repeated once or twice making finer and finer adjustments until it homes in on the point of best light transfer. Once this has been achieved, the fibers are spliced.

There are one or two slightly worrying aspects with this design. The first is the severity of the bends introduced. The radius of these bends is generally tighter than the fiber specification allows. This means that if the fiber fails, immediately or at a later date, the fiber manufacturer will not be interested since you exceeded the fiber limits. It does not usually break, of course, but there is still a slight feeling of unease. There is another thought, too. If the primary buffer happens to be opaque, the light cannot penetrate. LID systems are sometimes offered as a bolt-on goody to the standard PAS splicer.

Fusion splicing of the fibers

The arc that occurs between the two electrodes is about 7000 volts with an adjustable current up to 25 mA — a very unpleasant and dangerous combination. Safety precautions are built in to splicers to ensure that we cannot accidentally come into contact with the arc. There are two approaches to this problem, the first is to be sure we know where our fingers are before the arc starts. This is done by involving two buttons both of which must be held down during the operation and are spaced so that both hands must be used. The alternative is to enclose the dangerous area with a cover that must be closed before the arc can be energized.


The first stage of the fusing process is to align the fibers with an end gap equal to one fiber diameter and apply a short, relatively low power arc. This is called a prefuse. Its purpose is to clean and dry the end surfaces of the cleaves so that nothing untoward gets trapped inside the splice. It can remove very slight tangs from the cleave but don't expect miracles, it won't repair a poorly prepared fiber.

Main fuse

The fibers are then brought together and some additional end pressure is applied. The additional pressure allows the fibers to move towards each other slightly as they melt. How far they move, called the overfeed, autofeed or stuffing distance is critical. Too much or too little and the splice will not be satisfactory. The reason for introducing the overfeed is illustrated in Figure 10.7. In the figure, the cleaved end of the fiber is shown greatly magnified and, even with a good cleaver the surface is never completely smooth. When two fibers are brought together there are some small air gaps present which means that within

the area marked A there is actually less glass than in a similar length, shown as B. When the electric arc melts the end of the fiber, the glass tends to collapse inwards, filling the air gaps. This produces some waisting as shown on the page of disasters (see Figure 10.9). The main fusing arc is more powerful and lasts for a longer period of time, between 10 and 20 seconds. If the splicer is fitted with a microscope, it is important to check in the instruction book to see if the fiber can be viewed during splicing. The arc emits ultraviolet light and as the microscope concentrates the light there is a danger of suffering from arc-eye, a temporary or permanent damage to the eyes. This is the result of electric arc welding without eye protection. Most viewing microscopes have a UV filter (and an infrared filter) to prevent eye damage, but check the manual first. This is another advantage of choosing one with a CCD camera and monitor.

Once fusing is completed, have a good look at the splice. If it is difficult to see where the splice is, then it's probably a good one (Figure 10.8). We are looking for the outer edges of the cladding to be parallel, just like a new continuous length of fiber. Sometimes a small white line appears across the core but this is not important and can be ignored. See Figure 10.9 for all the

Setting up the splicer

Different types of fibers require particular values of arc current and lengths of time since this determines the temperature to which the fiber rises. There are several automatic programs installed as well as user-defined settings that we can employ in the light of experience.
The equipment is powered by nicad batteries giving up to 300 splices per charge, somewhat less (50 or so) if the oven is used to shrink the splice protector. Automatic splice testing
When the splice is completed, the fiber positions are rechecked and an estimated splice loss is calculated and displayed. Although the estimate is reasonably accurate, it is only an estimate and not a measurement. To be really certain, the loss must be measured and this must be done after the splice protector has been fitted just in case the splice is damaged during this operation.

Kevin M Contreras H
CI 18.255.631


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