Figure 2  - Fiber End Face Defects: Hackle (left), Lip (right)

Cleaving involves cutting the fiber end flush with the end of the ferrule. Cleaving, also called the scribe-and-break method of fiber end face preparation, takes some skill to achieve optimum results. Properly done, the cleave produces a perpendicular, mirror-like finish. Incorrect cleaving will result in lips and hackles as seen in Figure 2. While cleaving may be done by hand, a cleaver tool, available from such manufacturers as Fujikura, allows for a more consistent finish and reduces the overall skill required. The steps listed below outline one procedure for producing good, consistent cleaves such as the one shown in Figure 3.

1. Place the blade of the cleaver tool at the tip of the ferrule.

2. Gently score the fiber across the cladding region in one direction. If the scoring is not done lightly, the fiber may break, making it necessary to reterminate the fiber.

3. Pull the excess, cleaved fiber up and away from the ferrule.

4. Carefully dress the nub of the fiber with a piece of 12-micron alumina-oxide paper.

5. Do the final polishing. (See Figure 4.)

Polishing

After a clean cleave has been achieved, the fiber end face is attached to a polishing brush, and the fiber is ground and polished. The proper finish is achieved by rubbing the connectorized fiber end against polishing paper in a figure-eight pattern approximately sixty times.

Figure 4 - Polishing Technique

To increase the ease and repeatability of connector installation, some companies offer connector kits. Some kits are specific to the type of connector to be installed while others supply the user with general tools and information for connecting different types of connectors. Some connectors require the use of an alignment sleeve, also called an interconnection sleeve. This sleeve serves to increase repeatability from connection to connection.

A number of events can damage fiber optic connectors. Unprotected connector ends can experience damage by impact, airborne dust particles, or excess humidity or moisture. The increased optical output power of modern lasers also have the potential to damage a connector, an often overlooked factor in discussions about handling and caring for optical fibers and connectors. Most designers tend to think of the power levels in optical fibers as relatively insignificant. However, a few milliwatts at 850 nm will do permanent damage to a retina. Today, optical amplifiers can generate optical powers of 1 Watt of more into a single-mode fiber. This becomes quite significant when one considers that the optical power is confined in the optical core only a few microns in diameter. Power densities in a single-mode fiber carrying an optical power of 1 Watt (+30 dBm) can reach 3 megawatts/cm2 or 30 gigawatts/m2! To put it in everyday terms, sunlight at the surface of the Earth has a power density of about 1,000 Watts/m2. Most organic materials will combust when exposed to radiant energies of 100 kilowatts/m2. Clearly, power densities of 30 gigawatts/m2 deserve attention.

One should never clean an optical connector attached to a fiber that is carrying light. Optical power levels as low as +15 dBm, or 32 milliwatts, may cause an explosive ignition of the cleaning material when it contacts the end of the optical connector, destroying the connector. Typical cleaning materials, such as tissues saturated with alcohol, will combust almost instantaneously when exposed to optical power levels of +15 dBm or higher. The micro-explosions at the tip of the connector can leave pits in the end of the connector and crack the connector’s surface, destroying its ability to carry light with low loss. Figure 5 shows an optical connector that has been heavily damaged by high optical power levels. Usually the damage is limited to less severe pitting.

Another important thing to remember in handling fiber optic connectors is that the fiber end face and ferrule must be absolutely clean before it is inserted into a transmitter or receiver. Dust, lint, oil (from touching the fiber end face), or other foreign particles obscure the end face, compromising the integrity of the optical signal being sent over the fiber. From the optical signal’s point-of-view, dirty connections are like dirty windows. Less light gets through a dirty window than a clean one.

It is hard to conceive of the size of a fiber optic connector core. Single-mode fibers have cores that are only 8-9 µm in diameter. As a point of reference, a typical human hair is 50-75 µm in diameter, approximately 6-9 times larger! Dust particles can be 20 µm or larger in diameter. Dust particles smaller than 1 µm can be suspended almost indefinitely in the air. A 1 µm dust particle landing on the core of a single-mode fiber can cause up to 1 dB of loss. Larger dust particles (9 µm or larger) can completely obscure the core of a single-mode fiber. Fiber optic connectors need to be cleaned every time they are mated and unmated; it is essential that fiber optics users develop the necessary discipline to always clean the connectors before they are mated.

It is also important to cover a fiber optic connector when it is not in use. Unprotected connector ends are most often damaged by impact, such as hitting the floor. Most connector manufacturers provide some sort of protection boot. The best protectors cover the entire connector end, but they are generally simple closed-end plastic tubes that fit snugly over the ferrule only. These boots will protect the connector's polished ferrule end from impact damage that might crack or chip the polished surface. Many of the tight fitting plastic tubes contain jelly-like contamination (most likely mold release) that adheres to the sides of the ferrule. A blast of cleaning air or a quick dunk in alcohol will not remove this residue. This jelly-like residue can combine with common dirt to form a sticky mess that causes the connector ferrule to stick in the mating adapter. Often, the stuck ferrule will break off as one attempts to remove it. The moral of the story is always thoroughly clean the connector before mating, even if it was cleaned previously before the protection boot was installed.

Required Equipment:
• Kimwipes^® or any lens-grade, lint-free tissue. The type sold for eyeglasses work quite well. 

• Denatured alcohol. Note: Use only industrial grade 99% pure isopropyl alcohol. Commercially available isopropyl alcohol is for medicinal use and is diluted with water and a light mineral oil. Industrial grade isopropyl alcohol should be used exclusively.

• 30X microscope. 

• Canned dry air. 

1. Fold the tissue twice so it is four layers thick. 

2. Saturate the tissue with alcohol. 

3. First clean the sides of the connector ferrule. Place the connector ferrule in the tissue, and apply pressure to the sides of the ferrule. Rotate the ferrule several times to remove all contamination from the ferrule sides. 

4. Now move to a clean part of the tissue. Be sure it is still saturated with alcohol and that it is still four layers thick. Put the tissue against the end of the connector ferrule. Put your fingernail against the tissue so that it is directly over the ferrule. Now scrape the end of the connector until it squeaks. It will sound like a crystal glass that has been rubbed when it is wet. 

5. Use the microscope to verify the quality of the cleaning. If it isn’t completely clean, repeat the steps with a clean tissue. Repeat until you have a cleaning technique that yields good, reproducible results. 

6. Mate the connector immediately! Don’t let the connector lie around and collect dust before mating. 

7. Air can be used to remove lint or loose dust from the port of a transmitter or receiver to be mated with the connector. Never insert any liquid into the ports. 

1. Never touch the fiber end face of the connector.

2. Connectors not in use should be covered over the ferrule by a plastic dust cap. It is important to note that inside of the ferrule dust caps contain a sticky residue that is a by-product of making the dust cap. This residue will remain on the ferrule end after the cap is removed.

3. The use of index-matching gel, a gelatinous substance that has a refractive index close to that of the optical fiber, is a point of contention between connector manufacturers. Glycerin, available in any drug store, is a low-cost, effective index-matching gel. Using glycerin will reduce connector loss and backreflection, often dramatically. See article “"Backreflection – The Bane of Good Performance" for more information. However, the index-matching gel may collect dust or abrasives that can damage the fiber end faces. It may also leak out over time, causing backreflections to increase.