How to Take Care of Fiber Optic Connectors

>> Fiber Optic Connectors Are Fragile

All fiber optic connectors should be taken care of as a high quality lens of a good camera. Even a small fingerprint can lead to big trouble. The weak link in many fiber optic communication systems, and in optical instrument reliability, is the use and care of the fiber optic connector.

After so many years of improvement, current fiber connectors are so easy to use, that people tend to be vigilance in connector care and cleaning. However, remember this, it only takes ONE time of a missed cleaning for a piece of grit to permanently damage the glass and ruin the connector.

Fiber optic connectors are susceptible to damage that is not immediately obvious to the naked eye. This damage can have significant effects on measurements being made. Keep it in mind: any degradation of a ferrule or fiber endface, any stray particles or finger oil on the endface, can have a significant effect on connector performance.

>> The Effect of Improper Use of Fiber Optic Connector

Fiber optic connector maintenance is a big topic. The following figure shows a close-up of a clean connector endface.

In contrast, the following figure shows a connector end that was used with a cable end that avoided the alignment key. This may have been caused by a temporary ferrule without a key, or possibly a mismatched long ferrule put into a short connector assemble. Material is smeared and ground into the endface causing light scattering and poor reflection.

Even worse, the following figure shows that if the connector was used continuously this way, it will grind off the glass face and destroy the connector.

Repeated connections without removing the particles, or using improper tools can lead to physical damage of the glass fiber endface as shown in the following pictures. Where the damage is severe, the damage on one connector can be transferred to another good connector that comes in contact with it.

The cure is disciplined connector care. Soft swabs should be used – never use metal or sharp objects. Some connectors have a soft endface material that can be very easily scratched and damaged.

In the <a> picture above, for example, the results of using a pin on the connector surface is seen. In <b>, a close-up of this damaged connector, not only is there gouging in the endface around the fiber, but the glass fiber itself has been chipped off and broken. Broken or damaged glass can lead to a number of problems that are often ascribed to other causes. This connector will probably damage any other fiber that is mated to it.

In the <c> and <d> pictures above, another connector is seen with severe abuse. In this example, the soft ferrule core was pushed away from the glass and there is pitting in the fiber end. In addition, the glass fiber is sticking up almost 3um above the uneven surface (from severe wear of the soft metal that was once flush with the glass fiber).

>> Matching Gel and Oils Contamination

Another potential problem comes with matching gel and oils. While these often work well on first insertion, they are great dirt magnets. Damage is often caused as the oil or gel grabs and holds grit that is then ground into the fiber endface. Also, some early gels were designed for the FC, noncontacting connectors, using small glass spheres. When used in contacting connectors, these glass balls can scratch and pit the fiber.

Index matching oils is also discouraged. It runs and migrates into unwanted areas, attracting and holding dust. If an index matching gel or oil must be used, apply it to a freshly cleaned connector, make the measurement, and immediately clean the gel or oil off. Never use a gel for long-term connections and never use it to improve a damaged connector. The gel can mask the damaged end and continued use of a damaged fiber can transfer damage to another connector.

>> Not-Too-Tight Mating of Fiber Optic Connectors

Another tip for good fiber connector mating is: not too tight.Unlike electrical connections, tighter is not better.

The connector’s job is to bring the endfaces of two fibers together. Once they touch, tighter only causes a greater force to be applied to the delicate endface. In some connectors, the end can cock off-axis with a tight connection (due to the curved face) resulting in a worse return loss. Many measurements are actually improved by backing off the connector pressure. Also, if a piece of grit does happen to get by the cleaning procedure, a tighter connection is more likely to damage the glass. Tighten just until the two fibers touch.

>> Fiber Optic Cleaning Procedures

The basics of cleaning are simple. Use a pure grade of isopropyl alcohol on a clean cotton swab to wipe off the endface and ferrule. (While other devices and methods also work, this one is easy, problem free, and inexpensive.)

After the wet scrub, wipe off the wet endface with a dry swab, or blow it off with filtered, nonresidue compressed air. As the following figure shows, leaving the fluid to air dry will leave behind the dissolved oils in little puddles which will interfere with the correct physical contact. Then when reinserting the cable into the connector, insert it gently, in as straight as possible. Tipping, and inserting with an angle, can scrape off material from inside the connector or even break the inside sleeve of some connectors made with ceramic material.

It is pretty often that a cable or connector may require more than one cleaning. If the performance seems poor, clean again. Often the second cleaning will restore the proper performance where the first did not. The idea is that the first step is to gently remove any grit and oil using the alcohol as a solvent. Remove the solvent by blowing it off or using a gentle dry wipe. Then if there is a caked-on layer of material still present (such as can happen if the beryllium-copper sides of the ferrule retainer gets scraped and deposited on the endface during insertion of the cable), the second cleaning should be harder, with a scrubbing action. But if scrubbed first, grit can be caught in the swab and become a gouging element.

A system is only as good the weakest link. Don’t let the connector become a failure because of poor attention. Use fusion splicers on the more permanent critical nodes. Choose the best connector possible. Check and measure the connector’s return loss often, for degradation. Clean every connector, every time.

>> Use of Wipes and Cleaning Cassettes

Fabric and/or composite material wipes provide combined mechanical action and absorbency to remove contamination. Wipes should be used with a resilient pad in order avoid potential scratching of the connector end-face. This method is appropriate for cleaning connectors with exposed ferrules or termini but cannot be used to clean connector end-faces within alignment sleeves. The wipe should be constructed of material that is lint free and non-debris producing during the cleaning process. Please note that dry wipes have been shown to leave a static charge on the end-face of the connector which can thereafter attract particulate contamination. Therefore it is recommended that a static dissipative solvent be used with a dry wipe to eliminate this condition.

1. Technique for Cleaning with Wipes or Cleaning Cassette

As mentioned above it is recommended to use a lint-free, non-debris generating wipe with a static dissipative solvent. The user should dampen a portion of the wipe with the solvent, place the connector end-face into the damp area of the wipe and draw the connector into the dry area of the wipe. A physical wipe of 2 – 5 cm should be sufficient. This may be repeated in a different area of the wipe if desired, however 1 or 2 strokes should be sufficient for most common contaminants.

Upon inspection, if the connector is not clean after the first cleaning, the process can be repeated perhaps with slightly more pressure on the connector to increase the mechanical action and perhaps making several stokes from the damp to dry sections of the cleaning material.

If the connector is still not clean and contamination has not moved it should be considered nonremovable contamination and depending on the location and size of the contamination the user should pass or reject use of the connector.

2. Swabs or Port Cleaning Device

Use of purpose built swabs or mechanical port cleaning devices provides mechanical action and absorbency to remove contamination. However since the area within a port is very confined and limits mechanical action, it is recommended that a wet/dry cleaning process be utilized for cleaning connector end-faces within alignment sleeves. A static dissipative solvent will add chemical action to the cleaning process as well eliminate any residual static charge that may be on connector. The cleaning end of the swab or cleaning material used in the port cleaning device should be lint free, non-debris generating material.

Select a swab or port cleaning device that is manufactured for the size connector you are cleaning. Do not touch or contaminate the cleaning end of the swab or the port cleaning device. It is recommended that the user dampen the swab or port cleaning device with a static dissipative solvent. The swab or port cleaning device should be damp and not wet. (The process is analogous to wiping a smooth surface with a sponge. A damp sponge will pick up contaminates, whereas a wet sponge will only spread them around.) If a very fast drying solvent is used then a follow-up cleaning with a dry swab or an addition activation of the port cleaning device may not be necessary.

3. Swab Cleaning Detail

Place the dampened cleaning end of the swab into the port and rotate the swab while applying some pressure to the connector end-face. Usually pushing so that the compression spring in the connector is slightly activated is ideal for 2.5mm connectors. Rotating the swab 6 – 12 times is sufficient. The swab should only be used once and then disposed. If the user is cleaning angled polished connectors – APC (typically color coded with a green connector housing or bulkhead adaptor), then using a ¼ turn back and forth rotation may help the swab end-face conform to the 8 degree angle and thereby clean the entire surface of the connector end-face.

If a very fast drying solvent was used, you are now ready to inspect. If not, repeat the above process with a dry swab.

4. Port Device Cleaning Detail

Insert the solvent dampened device into the alignment sleeve and active the cleaner to perform the mechanical clean either by pushing the device or by pressing on a button on the device. An additional activation of the device assures that any excess solvent has been removed. Upon inspection, if the connector is not clean after the first cleaning, the process can be repeated.

5. About Solvents

Solvents used to clean fiber optics should be static-dissipative and residue-free. Many solvents are flammable and/or packaged so that transportation of the solvent is considered a hazardous material increasing cost of shipment and storage of the solvent. However, there are solvents available that are non-flammable and non-hazardous and packaged so that shipping requires no additional fees or paperwork.

Historically, 99% pure isopropyl alcohol (IPA) has been used as a solvent. However, IPA is easily contaminated. The solvent is hygroscopic and therefore absorbs moisture from the atmosphere including any contaminates present contaminating the solvent and potentially leaving a haze on the connector end-face which can be a problem with higher power laser networks. In addition, IPA is flammable and is considered a hazardous material. Typically IPA must be repackaged for use and is therefore very subject to contamination.

Also, it is important to select a solvent that is compatible with the cleaning wipe, swab, or device. You do not want a solvent will dissolve a binder or glue used in a wipe or swab and generate additional contamination.

>> Acceptable & Unacceptable Fiber Connector End-Face Finishes

For measurement of the performance criteria, Back-reflection and Insertion Loss, there are meters available which are generally familiar to polishers. The geometrical criteria, Apex Offset, Radius of Curvature, and Fiber Undercut are confirmed by using an Interferometer.

Visual inspection will always play an important role in evaluating the polished surface (see diagram below), but the now increasingly used interferometer is needed to confirm geometry. Interferometers are available from a number of sources, ranging from those which provide a monitor from which the user determines product acceptability, to computer aided programs that provide a printed readout which includes all performance and geometric characteristics of the connector.