Why Splice?

Up to this point, we have not addressed how to connect or mate two optical fibers together. Nor have we discussed why we would want to do this. This chapter discusses splicing. Splicing is one way to join two optical fibers together so the light energy from one optical fiber can be transferred into another optical fiber.

A splice is a permanent connection of two optical fibers. Once the two optical fibers are joined with a splice, they cannot be taken apart and put back together as they can if you join them using connectors. A splice is typically employed for one of four reasons: to repair a damaged cable, to extend the length of a cable, to join two different cable types, or to attach a pigtail. Pigtails are covered in-depth in Chapter 9, “Connectors.”

Intrinsic Factors

Even when fibers are manufactured within specified tolerances, slight variations still exist from one optical fiber to another. These variations can affect the performance of the splice even though the optical fibers are perfectly aligned when mated. The variations between two optical fibers that affect splice performance are referred to as intrinsic factors.

Let's look at the most common types of variations.

Numerical Aperture (NA) Mismatch

A numerical aperture (NA) mismatch occurs when the NA of one optical fiber is different from the NA of the other optical fiber. If the NA of the transmitting fiber is larger than the NA of the receiving optical fiber, a loss may occur. However, a loss will not occur if the NA of the transmitting optical fiber is less than the NA receiving optical fiber.

In Chapter 5, “Optical Fiber Characteristics,” we showed you how to calculate the NA of an optical fiber using the refractive indexes of the core and cladding. Recall that in order for light to be contained within a multimode optical fiber it must reflect off the boundary between the core and the cladding, rather than penetrating the boundary and refracting through the cladding. Light must enter within a specified range defined by the acceptance cone or cone of acceptance.

Light entering the core of an optical fiber from outside of the acceptance cone will either miss the core or enter at an angle that will allow it to pass through the boundary with the cladding and be lost, as shown in Figure 8.1. Light entering the core of the optical fiber at an angle greater than the acceptance angle will not propagate the length of the optical fiber. For light to propagate the length of the optical fiber, it must enter at an angle that does not exceed the acceptance angle.

The exact loss from an NA mismatch is difficult to calculate. Factors such as light source type, light source launch condition, optical fiber length, and bends in the optical fiber all affect the potential loss. It is possible to have an NA mismatch between two optical fibers and no loss resulting from the mismatch.

Core Diameter Mismatch

Core diameter mismatch occurs when there is a difference in the core diameters of the two optical fibers. A core diameter mismatch loss results when the core diameter of the transmitting optical fiber is greater than the core diameter of the receiving optical fiber, as shown in Figure 8.2. A loss occurs when light at the outer edge of the transmitting optical fiber core falls outside the diameter of the receiving optical fiber core. This light is lost in the cladding of the receiving optical fiber. Core diameter mismatch loss is typically only a concern with multimode optical fiber.

It is not uncommon for two multimode optical fibers with different core diameters to be spliced together. As you saw in Chapter 5, ANSI/TIA-568-C.3 recognizes multimode optical fibers with 50μm cores and 62.5μm cores. We can calculate the worst-case loss percentage for a splice that joins a 50μm core and 62.5μm core using this formula:

1. Loss = (d₁² – d₂²) / d₂²

where d₁ is the diameter of the transmitting core and d₂ is the diameter of the receiving core.

1. 62.5² – 50² = 1406.25
2. 1406.25/50² = 0.5625 or 56.25%

Using the following formula, the decibel loss can be calculated:

1. dB = 10Log₁₀(P_out / P_in)

Note that the decibel formula is a ratio where the power output (P_out) is divided by the power input (P_in). In this case, the power input is the power being put into the receiving optical fiber. We know that the receiving optical fiber will not accept 56.25 percent of the light from the transmitting optical fiber. Subtracting 56.25 percent from 100 percent, as shown next, results in a difference of 43.75 percent, which can be written as 0.4375.

1. 100% – 56.25% = 43.75% or 0.4375
2. 10Log₁₀ (0.4375 / 1) = –3.59dB
3. Loss = 3.59dB

The 3.59dB loss that we just calculated is the maximum loss possible from the core diameter mismatch. The actual loss may vary depending on factors such as light source type, light source launch condition, optical fiber length, and bends in the optical fiber.

Mode Field Diameter Mismatch

Mode field diameter is only a concern in single-mode optical fibers. It describes the diameter of the light beam traveling through the core and a portion of the inner cladding. Values for the mode field diameter are defined by wavelength with longer wavelengths typically having a larger diameter than shorter wavelengths. A typical mode field diameter nominal range is 8.6 to 9.55μm at 1310nm and 8 to 11μm at 1550nm.

A mode field diameter mismatch occurs when there is a difference in the mode field diameters of two single-mode optical fibers. A mode field diameter mismatch loss results when the mode field diameter of the transmitting optical fiber is greater than the mode field diameter of the receiving optical fiber, as shown in Figure 8.3. A loss occurs when optical fiber with the smaller mode field diameter will not accept all of the light from the optical fiber with the larger mode field diameter.

Cladding Diameter Mismatch

Cladding diameter mismatch occurs when the cladding diameters of the transmit and receive optical fibers are not the same. Cladding diameter mismatch loss occurs when the cores of the optical fiber are not aligned because of the cladding diameter mismatch, as shown in Figure 8.4. A cladding diameter mismatch can cause the light exiting the core of the transmitting optical fiber to enter the cladding of the receiving optical fiber. The light entering the cladding is lost, causing attenuation.

Concentricity

Ideally, the core and cladding of an optical fiber are perfectly round and concentric, which means that they share a common geometric center. However, optical fibers are not perfect and there will be concentricity variations. These concentricity variations can cause the optical fiber cores to misalign, as shown in Figure 8.5, causing a loss when the light exiting the core of the transmitting optical fiber enters the cladding of the receiving optical fiber.

The illustration in Figure 8.5 is greatly exaggerated to clearly show how a concentricity loss may occur. The core and cladding concentricity differences are typically less than 1μm.

Noncircularity

Just as the core and cladding of an optical fiber may not be perfectly concentric, they may also not be perfectly circular. The noncircularity of the core will cause a loss when light from the core of the transmitting optical fiber enters the cladding of the receiving optical fiber. In Figure 8.6, the core is elliptical to show how core noncircularity loss occurs.

Cladding noncircularity may cause loss when it causes part of the core of the transmitting optical fiber to align with the cladding of the receiving optical fiber. Any light that enters the cladding of the receiving optical fiber will be lost, causing attenuation.

Note that the amount of loss depends on the alignment of the ellipticities of the two cores. Maximum loss occurs when the long or major axes of the cores are at right angles to one another, and minimum loss occurs when the axes of the cores are aligned. After reading about all the possible intrinsic losses, you may find it hard to believe that two optical fibers can be spliced together with virtually no loss. Optical fiber manufacturing produces optical fibers with little variation, as shown in Tables 8.1, 8.2, 8.3, and 8.4. These tables list the attributes and tolerances for different single-mode optical fibers as defined in ITU standards ITU-T G.652, ITU-T G.655, and ITU-T G.657.

G.652.A, G.652.B, G.652.C, and G.652.D
Fiber attributes	Detail	Value
Mode field diameter	Wavelength	1310nm
	Range of nominal values	8.6–9.5μm
	Tolerance	±0.6μm
Cladding diameter	Nominal	125.0μm
	Tolerance	±1μm
Core concentricity error	Maximum	0.6μm
Cladding noncircularity	Maximum	1%

ITU-T G.655.C
Fiber attributes	Detail	Value
Mode field diameter	Wavelength	1550nm
	Range of nominal values	8–11μm
	Tolerance	±0.7μm
Cladding diameter	Nominal	125.0μm
	Tolerance	±1μm
Core concentricity error	Maximum	0.8μm
Cladding noncircularity	Maximum	2%

ITU-T G.655.D
Fiber attributes	Detail	Value
Mode field diameter	Wavelength	1550nm
	Range of nominal values	8–11μm
	Tolerance	±0.6μm
Cladding diameter	Nominal	125.0μm
	Tolerance	±1μm
Core concentricity error	Maximum	0.6μm
Cladding noncircularity	Maximum	1%

ITU-T G.657 Categories A and B
Fiber attributes	Detail	Value
Mode field diameter	Wavelength	1310nm
	Range of nominal values	8.6–9.5μm
	Tolerance	±0.4μm
Cladding diameter	Nominal	125.0μm
	Tolerance	±0.7μm
Core concentricity error	Maximum	0.5μm
Cladding noncircularity	Maximum	1%

Extrinsic Factors

Extrinsic factors that affect optical fiber splice performance are factors related to the condition of the splice itself, external to the optical fiber. In an ideal splice, the optical fibers are identical and they are aligned so that cores are perfectly centered on each other and the core axes are perpendicular to the endfaces being joined, as shown in Figure 8.7. However, there is no such thing as an ideal splice, only a real splice. In a real splice, intrinsic and extrinsic factors affect splice performance.

In this section, we will examine common extrinsic factors that affect splice performance. As you read about these extrinsic factors, keep in mind that many times they are caused by dirt and contamination. Microscopic particles of dirt can cause the misalignment of one or both optical fibers, creating a high-loss splice.

Lateral Misalignment

Lateral misalignment occurs when the two optical fibers are offset, as shown in Figure 8.8. Lateral misalignment loss occurs when light from the core of the transmitting optical fiber enters the cladding of the receiving optical fiber, creating a loss. As the lateral misalignment increases, less light from the core of the transmitting optical fiber makes its way into the core of the receiving optical fiber, increasing the loss of the splice.

End Separation

Even if the optical fibers are perfectly aligned, the splice still may still experience loss from end separation. End separation is simply a gap between the transmitting and receiving optical fibers, as shown in Figure 8.9.

Two different types of losses can be generated from end separation. The first is through Fresnel reflection, which takes place when light passes from the higher refractive index of the core in the transmitting optical fiber into the lower refractive index of the air, and then back into the core of the receiving optical fiber. Each change in the refractive index causes a certain amount of light to be reflected and therefore lost.

One way to overcome the effects of Fresnel reflections in separated optical fibers is to use an index matching gel, which is a transparent gel having a refractive index close to that of the core of the optical fibers being spliced. The gel fills the gap and reduces or eliminates the Fresnel reflection. Index matching gel is typically used in all mechanical splices.

End separation also causes loss because when light exits the transmitting optical fiber it spreads out like the light from a flashlight. Some of the light leaving the core of the transmitting optical fiber may enter the cladding of the receiving optical fiber, causing a loss. How much the light spreads out depends on several variables, including the distance between the optical fibers, light source type, launch type, length and NA of the transmitting optical fiber, and the bends in the transmitting optical fiber.

Angular Misalignment

If the optical fibers in a splice meet each other at an angle, a loss from angular misalignment may occur, as shown in Figure 8.10. The amount of loss depends on the severity of the angular misalignment and the acceptance cones of the transmitting and receiving optical fibers. Because the NA of a multimode optical fiber is greater than the NA of a single-mode optical fiber, multimode splices tolerate angular misalignment better than single-mode splices.

The loss from angular misalignment occurs when light from the core of transmitting optical fiber enters the cladding of the receiving optical fiber or enters the core of receiving optical fiber at an angle exceeding the acceptance angle. Light entering the core of the receiving optical fiber at an angle exceeding the acceptance angle may not propagate the length of the receiving optical fiber.

Angular misalignment also prevents the endfaces from contacting each other, resulting in Fresnel reflections exactly like those described in the “End Separation” section earlier in this chapter. Again, this is why index-matching gel is used with every mechanical splice.

Splicing Hazards

Let's look at some of the hazards directly related to splicing.

Electrical

Unlike mechanical slicing, fusion splicing uses a high-voltage electric arc between two electrodes to heat and melt the fiber ends as shown in Figure 8.11. Touching the electrodes while the arc is present can cause severe electrical shock. Remember that high voltage causes most of its damage by making muscles seize up, including the heart and lungs.

Fusion splicers like the one shown in Figure 8.12 have a cover that prevents access to the splicing area during the splicing process. This device typically has a safety interlock that interrupts the splicing process if lifted. Do not attempt to gain access to the splicing area until after the arc on the fusion splicer display shown in Figure 8.13 is extinguished. The arc is extinguished when it no longer appears on the display, as shown in Figure 8.14.

Explosion

Prior to fusion splicing, ensure you are not in a potentially explosive atmosphere. Remember the arc generated during the splicing process is essentially a continuous spark capable of igniting flammable vapors such as those produced by an open container of isopropyl alcohol or the refueling of a vehicle. The ignition of these vapors will cause an explosion.

If you are using isopropyl alcohol during fusion splicing, keep it a safe distance from the fusion splicer and work in a well-ventilated area. Do not initiate the fusing process in the presence of a high concentration of isopropyl alcohol vapors.

In Chapter 7, “Fiber-Optic Cables,” you learned that the aerospace and avionics industries are incorporating more optical fiber in aircraft. This is evident with the Boeing 787 Dreamliner, which has 110 fiber-optic links and over 1.7km of fiber-optic cable. As more commercial and military aircraft install optical fiber, there will be a need for fusion splicing.

A fueled aircraft is a hazardous environment for fusion splicing and special precautions must be taken prior to splicing. Only a fusion splicer like the one shown in Figure 8.15, developed specifically for explosion-proof operation, can be used in hazardous environments such as near a fueled aircraft.

Cleaning Materials

One of the keys to performing a successful fiber-optic splice is cleanliness. The optical fiber needs to be properly cleaned prior to cleaving and splicing. In addition, the splicing area should be kept as clean as possible. As discussed earlier in the chapter, a microscopic piece of dirt could cause lateral displacement, end separation, or angular misalignment.

Prior to starting any splice, ensure you have lint-free wipes designed for optical fiber; a solvent designed to clean and remove the coating residue from the optical fiber; and if you will be fusion splicing, swabs to clean the V-groove. Lint-free wipes like those shown in Figures 8.16 and 8.17 are engineered to lift away oils, grime, and dust from the surface of the optical fiber.

When handling a lint-free wipe, be sure to only touch one side of the wipe because the wipe will absorb the oil from your skin. Use the side of the wipe that you did not touch for cleaning the optical fiber. You do not want to contaminate the optical fiber during the cleaning process. Cleaning of the optical fiber is covered in depth later in the “Splicing Procedures” section.

When you select an optical fiber cleaning solvent, ensure it has been engineered to clean optical fibers. The goal is to select a solvent that leaves little or no residue on the surface of the optical fiber. If you choose isopropyl alcohol, make sure that it is virtually free of water and has been filtered to remove impurities, like the isopropyl alcohol shown in Figure 8.18. Do not use the isopropyl alcohol commonly available at your local pharmacy; this alcohol contains a large percentage of water and will leave a residue on the optical fiber.

Isopropyl alcohol poses a problem for air travel because of its flammability. If the splicing job requires air travel, you will not be able to bring isopropyl alcohol on the aircraft. For jobs requiring air travel, you need to select a cleaning fluid that is approved for air travel. Remember from Chapter 6 that isopropyl alcohol vapors are highly flammable and can ignite if exposed to a spark or flame in high enough concentrations.

Cleaning fluids like the one shown in Figure 8.19 are approved for air travel and engineered specifically to clean the optical fibers. These cleaning fluids typically contain little or no isopropyl alcohol and have been filtered to remove microscopic contaminates.

As will be discussed later in this chapter, many fusion splicers have a precision V-groove that holds the optical fiber. Dirt or contamination in a V-groove can cause the optical fibers to misalign resulting in a high-loss fusion splice. When the V-grooves become contaminated, they can be cleaned using a fiber-optic swab, as shown in Figure 8.20.

MicroCare, ITW Chemtronics, Corning Cable Systems, and AFL Telecommunications manufacture some of the most commonly used cleaning materials.

1. MicroCare www.microcare.com
2. ITW Chemtronics www.chemtronics.com
3. Corning www.corningcablesystems.com
4. AFL Telecommunications www.afltele.com

Cleavers

A cleaver is required for all splice types. Cleavers are available for single optical fibers or both single and ribbon fibers. Cleavers that support both single and ribbon fibers have removable adapters designed specifically for single fiber or ribbon fiber cleaving. An adapter is not inserted in the cleaver shown in Figure 8.21. A single fiber adapter is inserted in the cleaver shown in Figure 8.22.

Cleaving is typically a two-step process, and it takes place after the optical fiber has been properly prepared and cleaned. The first step in the process is scoring the optical fiber. This is accomplished with a scoring blade. The scoring blade should be held perpendicular to the optical fiber, as shown in Figure 8.23. When the cleaver is operated, the scoring blade lightly contacts the optical fiber, creating a small surface flaw.

After the optical fiber is scored, the next step is to bend the optical fiber. The optical fiber is bent slightly, as shown in Figure 8.23. Slightly bending the optical fiber places tensile stress on the surface of the optical fiber where it has been scored. This causes the small surface flaw to open and split the fiber.

After the optical fiber is cleaved, the endface should be perpendicular to the optical fiber without any surface defects and the cleave angle should not exceed 1.0°. Cleave angles can be evaluated only with a microscope. This is typically only done when a fusion splice is performed. Many fusion splicers will measure the cleave angle of the optical fiber prior to splicing.

Figure 8.24 is a photograph of a cleaved optical fiber with defects. In an ideal cleave, the mirror area would cover the entire endface. However, in this example the mirror area represents only a small portion of the endface. The rest of the endface looks like it was hacked with a sharp object many times. These irregular defects are commonly referred to as hackles.

Mechanical Splice

Many manufacturers offer mechanical splices. Mechanical splices like the ones shown in Figures 8.25 and 8.26 are typically permanent. They align the cleaved optical fibers and hold them in place. Index matching gel inside the mechanical splice reduces or eliminates Fresnel reflections.

Mechanical splices can be used for both multimode and single-mode optical fiber. Mechanical splices do not outperform fusion splices. However, they will outperform mated connector pairs. The key advantage to a mechanical splice over a fusion splice is the low cost of the equipment required to perform the mechanical splice.

The assembly tool that holds the optical fibers and the mechanical splice is relatively inexpensive when compared to the price of a fusion splicer. However, the actual mechanical splices shown in Figures 8.25 and 8.26 cost considerably more than the protective sleeve required for a fusion splice.

Specialty Mechanical Splice

Up to this point, we have only discussed splicing optical fibers that have been cleaved so the endface is perpendicular to the optical fiber. However, angled mechanical splices that offer better return loss performance than traditional mechanical splices are being introduced.

Angled mechanical splices require a special cleaver, angle cleaved fiber holders, and a special mechanical splice. The special cleaver like the one shown in Figure 8.27 places a 7° angle on the fiber endface while the angle cleaved fiber holders maintain the orientation of the cleave. Maintaining the orientation of the cleaved fibers is critical so that they align properly inside the mechanical splice.

The angled mechanical splice is an alternative to fusion splicing for analog video transmission, which typically requires lower return loss than a digital transmission. The 7° angle on the fiber endface reflects light into the cladding where it is absorbed. This reduces the amount of light energy that could be reflected back toward the light source, minimizing the back reflection.

Like the traditional mechanical splice, the angled mechanical splice is filled with index matching gel. The combination of the 7° angle and the index matching gel create a mechanical spice that meets or exceeds the return loss requirements for analog video transmission.

3M and Corning Cable Systems manufacture some of the most commonly used mechanical splices.

1. 3M www.3m.com
2. Corning www.corningcablesystems.com

Fusion Splice

As you learned earlier in this chapter, fusion splicing uses a high-voltage electric arc between two electrodes to heat and melt the fiber ends as shown in Figure 8.11. This creates a permanent splice that is very fragile and must be protected from the outside environment and bending. This is typically accomplished using heat-shrink tubing and a small metal rod commonly referred to as a protective sleeve.

Prior to performing the splice, the optical fiber is passed through the center of the protective sleeve and the sleeve is positioned so it will not interfere with the fusion splicing process. After the fusion splice is successfully completed, the optical fiber is removed from the fusion splicer and the protective sleeve is positioned directly over the slice area.

The protective sleeve is then placed in an electric oven that is typically built into the fusion splicer. The oven heats the tubing, shrinking it around the fusion splice. The end result looks like the protected fusion splice shown in Figure 8.28.

Fusion splicers are more expensive than the assembly tool required for a mechanical splice. However, they provide the lowest-loss splice possible. In addition, fusion splices do not produce Fresnel reflections.

Fusion splicing is the most accurate and durable method for joining two optical fibers. After the optical fibers are stripped and cleaved, they are placed into the fusion splicer, where the optical fibers are aligned between two electrodes, as shown in Figure 8.29. Alignment may be accomplished several ways depending on the type of fusion splicer used.

Several alignment techniques are used to align the optical fibers. A common alignment technique is to use a fixed V-groove, as shown in Figure 8.30. Each optical fiber is placed in a precision-machined V-groove. The V-grooves align the cladding of the optical fibers. The fusion splicer moves the optical fibers axially into the electric arc, melting the optical fibers together. This type of fusion splicer may be referred to as a passive alignment fusion splicer or a passive v-groove fusion splicer.

Fusion splicers with this type of alignment system have the lowest cost and are typically the smallest and lightest. However, they align the cladding of the optical fiber instead of the core and can create higher loss splices than fusion splicers that align the cores of the optical fibers.

A fusion splicer that aligns the cores of the optical fibers will produce the lowest-loss splice. However, aligning the cores of the optical fibers is more difficult than aligning two optical fibers placed in V-grooves. To align the cores, the fusion splicer must be able to detect the cores and have the ability to move the optical fibers on each axis. This type of fusion splicer may be referred to as an active alignment fusion splicer.

Several patented technologies are available to detect the cores of optical fibers. Because they are patented, they will not be discussed in detail in this chapter.

Fusion splicers on the market today typically feature a display that allows you to see the optical fibers on two different axes. The cameras in the fusion splicer magnify the optical fibers so that the endfaces can be evaluated. You cannot look at an optical fiber with the naked eye and evaluate the endface. Without magnification, optical fiber with a perfect cleave will look just like an optical fiber that has been broken. Figure 8.31 is a photograph of a fusion splicer display showing a two-axis view of properly cleaved and aligned optical fibers. Shown in Figure 8.32 is a two-axis view of two broken optical fibers. This photograph clearly shows that a broken optical fiber can have a jagged endface.

Most fusion splicers available today also have the ability to approximate the loss for the splice after it is completed. Fusion splicers are also available that have the ability to measure the loss for the splice after it is completed. Figure 8.33 is a photograph of a fusion splicer display showing the estimated splice loss. The splice loss is displayed on the bottom of the screen, and different axis views of the spliced optical fiber are shown at the top of the screen. Note that the spliced optical fibers appear as a single optical fiber with no flaws.

You can find out more about specific features of fusion splicers through their manufacturers' websites. Some of the major manufacturers of fusion splicers are as follows:

1. Corning www.corningcablesystems.com
2. Aurora Optics www.aurora-optics.com
3. AFL Telecommunications www.afltele.com
4. Fitel www.fitel.com

Next let's look at the procedures used with each type of splicer.

Mechanical Splicing Procedure

To prepare for mechanical splicing, make sure that the work area is clean, dry, and well lit. Do all your work over a fiber-optic mat and place any scrap optical fibers in their proper container, as described in Chapter 6. Assemble the following tools before you begin:

• Mechanical splice assembly tool
• Mechanical splice
• Buffer and coating removal tool
• Optical fiber cleaning fluid
• Lint-free wipes
• Cleaver

Once your materials are assembled, clean the cleaver as directed by the manufacturer, and then proceed with the following steps:

1. Remove the 3M Fibrlok II mechanical splice from its protective packaging and load the splice into the assembly tool by pressing firmly at the ends of the splice. Do not depress the raised section on the mechanical splice.
2. Strip approximately 3cm of buffer and/or coating from the optical fiber using a stripper, as shown in Figure 8.34.
3. Clean the optical fiber by pulling the fiber through a lint-free wipe soaked in optical fiber cleaning fluid, as shown in Figure 8.35.
4. Place the optical fiber in the cleaver, as shown in Figure 8.36, to the length specified by the mechanical splice manufacturer.
5. Cleave the optical fiber.
6. If a gauge is provided, check the cleave length with the gauge. If the optical fiber has touched the gauge, repeat step 3.
7. Repeat steps 2 through 6 for the other fiber end to be spliced.
8. Push one cleaved optical fiber into one end of the mechanical splice until it stops moving.
9. Push the other cleaved optical fiber into the other end of the mechanical splice until it stops moving.
10. Place both optical fibers in the clamping mechanisms on the opposite sides of the splicing tool, forming a modified loop in the optical fiber, as shown in Figure 8.37.
11. Pivot the splicing tool handle down until it contacts the top of the splice; then squeeze the tool handle to complete the assembly and lock the spliced ends in place.
12. Remove the optical fibers from the clamping mechanisms and lift the mechanical splice from the tool.

Fusion Splicing Procedure

The equipment required for fusion splicing is far more complex and expensive than the equipment required for mechanical splicing. However, many of the steps required to perform a fusion splice are identical to the steps required to perform a mechanical splice.

Many fusion splicers contain a feature that automatically positions the fiber ends in proper relationship with each other and with the electrodes for the best possible splice. All that is required of the operator is to prepare the fibers properly and place them in the fusion splicer as outlined by the manufacturer.

The fusion-splicing procedures outlined in this chapter describe the typical steps required to fusion-splice single optical fibers and ribbon optical fibers. These steps are not product specific and modifications may be required depending on the fusion splicing equipment being used. Always review the manufacturer's operating documentation prior to performing a splice.

To prepare for fusion splicing, as with mechanical splicing, make sure that the work area is clean, dry, and well lit. Do all your work over a fiber-optic mat and place any scrap optical fibers in their proper container, as described in Chapter 6. Assemble the following tools before you begin:

• Fusion splicer
• Buffer and coating removal tool
• Optical fiber cleaning fluid
• Lint-free wipes
• Cleaver
• Heat-shrink protective covering

Once your materials are assembled, clean the cleaver and the fusion splicer as directed by the manufacturer, and then proceed with the following steps. First, we'll look at single optical fiber fusion splicing:

1. Power on the fusion splicer and select the appropriate splicing program or profile for the optical fiber you will be splicing.
2. Slide the protective heat-shrink tubing over one optical fiber end and move it far enough up the optical fiber to place it out of the way.
3. Strip approximately 3cm of buffer and/or coating from the optical fiber using a stripper, as shown in Figure 8.38.
4. Clean the optical fiber by pulling the fiber through a lint-free wipe soaked in optical fiber cleaning fluid, as shown in Figure 8.39.
5. Place the optical fiber in the cleaver, as shown in Figure 8.40, to the length specified by the mechanical splice manufacturer.
6. Cleave the optical fiber.
7. Place the optical fiber in the fusion splicer following the manufacturer's instructions. Position the endface of the optical fiber between the electrodes.
8. Repeat steps 3 through 7 for the other fiber end to be spliced. The properly placed fibers should be slightly separated between the electrodes, as shown in Figure 8.41.
9. Close the electrode cover.
10. Begin the fusion-splicing process.
11. Carefully remove the splice and position the heat-shrink tubing from step 2 over it. Place the splice and tubing in the heat-shrink oven to seal and protect the splice.

Next we'll look at ribbon fiber fusion splicing:

1. Power on the fusion splicer and select the appropriate splicing program or profile for the optical fiber you will be splicing.
2. Slide the protective heat-shrink tubing over one ribbon fiber end and move it far enough up the optical fiber to place it out of the way.
3. Strip approximately 3cm of Mylar tape and coating from the ribbon fiber, as shown in Figure 8.42.
4. Clean the ribbon fiber by pulling the fiber through a lint-free wipe soaked in optical fiber cleaning fluid, as shown in Figure 8.43.
5. Place the ribbon fiber in the cleaver, as shown in Figure 8.44, to the length specified by the manufacturer of splicer you are using.
6. Cleave the ribbon fiber.
7. Place the ribbon fiber in the fusion splicer, as shown in Figure 8.45. Position the ribbon fiber as described in the manufacturer's instructions.
8. Lock the ribbon fiber in place as described in the manufacturer's instructions.
9. Repeat steps 3 through 8 for the other fiber end to be spliced.
10. Close the electrode cover.
11. Begin the fusion-splicing process.
12. Carefully remove the splice and position the heat-shrink tubing from step 2 over it. Place the splice and tubing in the heat-shrink oven to seal and protect the splice.

Fusion Splicing Troubleshooting

Following the fusion splicing procedures outlined earlier typically results in a low-loss splice that outperforms the attenuation requirements for inside or outside plant splices. However, occasionally you will encounter a splice that exceeds maximum attenuation requirements. As you've learned, how well a splice performs depends on many variables. As with mechanical splicing, these variables can be broken into two groups: intrinsic factors and extrinsic factors.

Intrinsic factors are the result of slight variations from one optical fiber to another. These variations can affect the performance of the splice even though the optical fibers are perfectly aligned when mated. However, the optical fibers available today have very little variation, which means that a small percentage of the splice loss may be from intrinsic factors.

Extrinsic factors are related to the condition of the splice itself, external to the optical fiber. Many times, they are caused by dirt and contamination. Microscopic particles of dirt can cause the misalignment of one or both optical fibers, creating a high-loss splice.

Next we will examine some of the common causes for a high-loss fusion splice.

Cleave Angle

As discussed earlier, the endface of a properly cleaved optical fiber should be perpendicular to the optical fiber without any surface defects and the cleave angle should not exceed 1.0°. Cleave angles can be evaluated only with a microscope. Fortunately many fusion splicers are equipped with a 2-axis microscope that allows you to observe the cleave and the measured cleave angle values. If you or the fusion splicer detects a bad cleave, as shown in Figure 8.46, perform the following steps:

1. Remove the optical fibers from the fusion splicer.
2. Prepare the optical fibers for cleaving adhering to the manufacturer's instructions.
3. Ensure the optical fiber and cleaver is clean.
4. Re-cleave both optical fibers.
5. Verify the cleave dimensions as stated in the manufacturer's instructions.
6. Place the optical fibers in the fusion splicer following the manufacturer's instructions. Position the endface of the optical fibers between the electrodes.
7. Perform the fusion splice.

Cleave Length

Cleave length is just as important as cleave angle. Each fusion splice manufacturer defines how much bare optical fiber should be exposed after the cleaving process is completed. This dimension is typically provided in millimeters with maximum and minimum values. Exposing too much optical fiber or not enough optical fiber can create a high-loss fusion splice. If you suspect improper cleave length, follow these steps:

1. Remove the optical fibers from the fusion splicer.
2. Prepare the optical fibers for cleaving, adhering to the manufacturer's instructions.
3. Ensure the optical fiber and cleaver is clean.
4. Re-cleave both optical fibers.
5. Verify the cleave dimensions as stated in the manufacturer's instructions.
6. Place the optical fibers in the fusion splicer, following the manufacturer's instructions. Position the endface of the optical fibers between the electrodes.
7. Perform the fusion splice.

Bubbles

Dirt or entrapped air may create a bubble or bubbles, resulting in a high-loss fusion splice like the one shown in Figure 8.47. To prevent bubbles

1. Ensure you are using the types of cleaning products and the cleaning processes required by the fusion splicer and cleaver manufacturer or described in this chapter.
2. Verify that you have selected the appropriate splicing program or profile for the optical fiber you are splicing.

Necking

Necking is a term used to describe a fusion splice where the diameter of the fused optical fiber is smaller near the electrodes than it was prior to being fused, as shown in Figure 8.48. Necking is typically the result of too much heat during the prefuse, causing glass to be transported away from the splice area and thus producing a high-loss splice. To prevent necking

1. Ensure you are using the types of cleaning products and the cleaning processes required by the fusion splicer and cleaver manufacturer or described in this chapter.
2. Verify that you have selected the appropriate splicing program or profile for the optical fiber you are splicing.
3. Ensure the optical fibers are properly cleaved.

Contamination

Contamination on the optical fiber or cleaver that is invisible to the unaided eye can cause fusion splices to exceed attenuation requirements. Because you cannot see the contamination, make sure that you are using the types of cleaning products and the cleaning process required by the fusion splicer and cleaver manufacturer or described in this chapter. Never let a cleaned optical fiber touch any surface after cleaving. If you have any cleaning-related questions, you can always reach out to a cleaning product manufacturer such as MicroCare or ITW Chemtronics for help.

1. MicroCare www.microcare.com
2. ITW Chemtronics www.chemtronics.com

Determine if the Splice Loss Is from an Intrinsic Factor

Even when fibers are manufactured within specified tolerances, there are still slight variations from one optical fiber to another. These variations can affect the performance of the splice even though the optical fibers are perfectly aligned when mated.

Two optical fibers are about to be fusion spliced together, as shown here. Determine what type of problem exists with the optical fibers about to be spliced.

A close inspection of the graphic reveals that the core diameter of the left optical fiber is slightly smaller than the right optical fiber. This core diameter mismatch is the result of an intrinsic factor.

Determine if the Splice Loss Is from an Extrinsic Factor

Extrinsic factors that affect optical fiber splice performance are factors related to the condition of the splice itself, external to the optical fiber.

Two optical fibers are about to be fusion spliced together, as shown here. Determine what type of problem exists with the optical fibers about to be spliced.

A close inspection of the graphic reveals that there is some angular misalignment. The angular misalignment is the result of an extrinsic factor.

Calculate the Potential Splice Loss from a Core Diameter Mismatch

A core diameter mismatch loss results when the core diameter of the transmitting optical fiber is greater than the core diameter of the receiving optical fiber.

Calculate the worst-case loss for a splice where the transmitting optical fiber has a core diameter of 51μm and the receiving optical fiber has a core diameter of 49.5μm.

We can calculate the worst-case loss percentage for a splice that joins a 50.5μm core and 49.5μm core using the following formula:

1. Loss = (d₁² – d₂)² / d₂²

where d1 is the diameter of the transmitting core and d2 is the diameter of the receiving core.

1. 50.5² – 49.5² = 100
2. 100/49.5² = 0.0408 or 4.08%

Using this formula, the decibel loss can be calculated:

1. dB = 10Log₁₀(P_out / P_in)

Note that the decibel formula is a ratio, where the power output is divided by the power input. We know that the receiving optical fiber will not accept 4.08 percent of the light from the transmitting optical fiber. Subtracting 4.08 percent from 100 percent, as shown here, results in a difference of 95.92 percent, which can be written as 0.9592.

1. 100% – 4.08% = 95.92% or 0.9592
2. 10Log₁₀ (0.9592 / 1) = –0.181dB
3. Worst-case loss = 0.181dB

Troubleshoot a Fusion Splice

How well a splice performs depends on many variables. These variables can be broken into two groups: intrinsic factors and extrinsic factors.

Two optical fibers have been fused together as shown here. Determine the type of problem and potential causes.

A close inspection of the of the fusion splicer display reveals that the optical fiber ends that have been joined are larger in diameter than the optical fiber. This is typically caused by dirt or entrapped air creating a bubble or bubbles. The result is a high-loss fusion splice. To prevent bubbles, verify that you have selected the appropriate splicing program or profile for the optical fiber you are splicing and ensure you are properly cleaning the fusion splicer and cleaver.