Parsing Fiber Optic Connectors

The network cabling industry’s fiber optic manufacturers over the last few decades have been on a constant mission to develop the better fiber connector. This means lower cost, lower dB losses, easier to terminate out in the field. There have been over 100 connectors developed over the years but a select few have stood the test of time and beat out their competition. Now, let’s talk about the most common fiber connectors as following:

A fiber optic connector terminates at the end of a fiber optic cable is used when you need a means to connect and disconnect the fiber cable quickly. A fiber splice would be used in a more permanent application. the connectors provide a mechanical connection for the two fiber cables and align both cores precisely so the light can pass through with little loss. There are many different types of connectors but many share similar features. Many connectors are spring loaded. This will push the fiber ends very close other so as to eliminate airspace between them, which would result in higher dB losses.

There are generally five main components to a fiber connector: the ferrule, the body, the coupling structure, the boot and the dust cap.

Ferrule: The ferrule is the small round cylinder that actually makes contact with the glass and holds it in place. These are commonly made of ceramic today but also are made of metal and plastic.

Body: This sub assembly holds the ferrule in place. It then fits into the connector housing.

Connector Housing: This holds all sub assembly parts in place and has the coupling that will connect to the customer’s equipment. The securing mechanism is usually bayonet, snap-in or screw on type.

Boot: This will cover the transition from the connector to the fiber optic cable. Provides stress relief.

Dust Cap: Just as it implies will protect the connector from accumulating dust.

There are many types of connectors on the market. The major differences are the dimensions and the method of connection to equipment. Most companies will settle on one type of connector and keep that as a standard across the board. It makes sense because all equipment has to be ordered with that specific connector type and to have 2 or 3 different connector types can get messy. For typical network cabling projects today LC is fast becoming the shining star of fiber connectors. LC is a small form factor connector which means it requires a much smaller footprint in your IT closet. Thus you can fit many more LC connectors into you fiber panels then say ST or SC connectors.

LC Connector

The LC connector was developed by Lucent Technologies, hence the LC. It is a Single Form Factor Connector that has a 1.25mm ferrule. The attaching mechanism is similar to an RJ-45 connector with the retaining clip. It is a smaller square connector, similar to the SC. LC connectors are often held together with a duplex plastic retainer. They are also very common in single mode fiber applications.

ST Connector

The ST connector was the first popular connector type to be used as a standard for many organizations in their fiber network applications. It has first developed by AT&T. Often called the “round connector” it has a spring loaded twist bayonet mount with a 2.5mm round ferrule and a round body. The ST connector is fast being replaced with the smaller, denser SFF connectors.

SC Connector

The SC connector is a push in/pull-out type connector that also has a 2.5 mm ferrule. It is very popular for its excellent performance record. The SC connector was standardized in TIA-568-A, and has been very popular for the last 15 years or so. It took a while to surpass the ST because of price and the fact that users were comfortable with the ST. Now it’s much more competitive with pricing and it is very easy install, only requiring a push in and pull out connection. This is very helpful in tight spaces. Simplex and duplex SC connectors are available. The SC was developed by the Japanese and some say stands for Standard Connector.

FC Connector

The FC connector you may find in older single mode installations. It was a popular choice that has been replaced by mostly ST or SC type connectors. It also has a 2.5mm ferrule. They have a screw on retaining mechanism but you need to be sure the key and slot on the connector are aligned correctly. FC connectors can also be mated to ST & SC’s through the use of an adaptor.

MT-RJ Connector

MTRJ stands for Mechanical-Transfer Registered Jack and was developed by Amp/Tyco and Corning. MTRJ is very similar to an RJ type modular plug. The connector is always found in duplex form. The body assembly of the connector is usually made from plastic and clips and locks into place. There are small pins present that guide the fiber for correct alignment. MTRJ’s also are available in male or female orientation. They are only used for multi-mode applications. They can also be difficult to test because many testers on the market do not accept a direct connection. You usually need to rig up a patch cord adaptor kit to make testing possible.

MU Connector

MU looks a miniature SC with a 1.25 mm ferrule. It’s more popular in Japan.

MT Connector

MT is a 12 fiber connector for ribbon cable. It’s main use is for preterminated cable assemblies and cabling systems. Here is a 12 fiber MT broken out into 12 STs.

MT connector is sometimes called a MTP or MPO connector which are commercial names.

Hopefully this guide may help you get an idea of what options are out there for your fiber optic connector needs.

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A comprehensive understanding of fiber optic connectors

Fiber connector has traditionally been the biggest concern in using fiber optic systems. While connectors were once unwiedy and difficult to use, connector manufacturers have standardized and simplified connectors greatly. This increases the user use convenient increase in the use of optical fiber systems; It is also emphasising taken proper care of and deal with the optical connector. This article covers connector basics including the parts of a fiber optic connector, installing fiber optic connectors, and the cleaning and handling of installed connectors. For information on connector loss, see Connector Loss Test Measurement.

Optical fiber to fiber optic interconnection can be made by a joint, a permanent connection, or a connector, and is different from the plug in it can be to disconnect and reconnect. Fiber optic connector types are as various as the applications for which they were developed. Different connector types have different characteristics, different advantages and disadvantages, and different performance parameters. But all connectors have the same four basic components.
  • The Ferrule: The fiber is installed in a long, thin cylinder, the ferrule, which act as a fiber alignment mechanism. The ferrule is bored through the center at a diameter that is slightly larger than the diameter of the fiber cladding. The end of the fiber is located at the end of the ferrule. Ferrules are typically made of metal or ceramic, but they may also be constructed of plastic.
  • The Connector Body: Also known as the connector housing, the body holds the ferrule. It is usually constructed of metal or plastic and includes one or more assembled pieces which hold the fiber in place. The details of these connector body assemblies vary among connectors, but the welding and/or crimping is commonly used to attach strength members and cable jackets to the connector body. The ferrule extends past the connector body to slip into the couping device.
  • The Cable: The cable is attached to the connector body. It acts as the point of entry for the fiber. Often, a strain relief boot is added over the junctioni between the cable and the connector body, providing extra stength to the junction.
  • The Coupling Device: Most fiber optic connectors do not use the male-female configuration common to electronic connectors. Instead, a coupling device such as an alignment sleeve is used to mate the connectors. Similar devices may be installed in fiber optic transmitters and receivers to allow these devices to be mated via a connector. These devices are also known as feed-through bulkhead adapters.
Table 1 illustrates some types of optical connectors and lists some specifications. Each connector type has strong points.
Table 1- Types Of Optical Connectors

Installing Fiber Optic Connectors
The method for attaching fiber optic connectors to optical fibers varies among connector types. While not intended to be a definitive guide, the following steps are given as a reference for the basic of optical fiber interconnection.
  • Cut the cable one inch longer than the required finished length.
  • Carefully strip the outer jacket of the fiber with “no nick” fiber strippers. Cut the exposed strength members, and remove the fiber coating. The fiber coating can be removed in two ways: a. by soaking the fiber for two minutes in paint thinner and wiping the fiber clean with a soft, lint-free cloth; b. by carefully stripping the fiber with afiber stripper. Be sure to use strippers made specifically for use strippers made specifically for use with fiber rather than metal wire strippers as damage can occur, weakening the fiber.
  • Thoroughly clean the bared fiber with isopropyl alcohol poured onto a soft, lint-free cloth such as kimwipes. NEVER clean the fiber with a dry tissue.

Note: Use only industrial grade 99% pure isopropyl alcohol. Commercially available medicinal and isopropyl alcohol is light mineral oil dilution water. Industrial grade isopropyl alcohol should be dedicated.

  •  The connector may be connected by applying epoxy or by crimping. If using expoxy, fill the connector with enough epoxy to allow a small bead of epoxy to form at the tip of the connector. Insert the clean, stripped fiber into the connector. Cure the epoxy according to the instructions provided by the epoxy manufacturer.
  • Anchor the cable strength members to the connector body. This prevents direct stress on the fiber. Slide the back end of the connector into place (where applicable).
  • Prepare fiber face to achieve a good optical finish by cleaving and polishing the fiber end. Before the connection is made, the end of each fiber must have a smooth finish that is free of defects such as hackles, lips, and fractures. These defects, as well as other impurities and dirt change geometry transmission patterns of light and scattered.
Cleaving
Cleaving involves cutting the fiber end flush with the end of the ferrule. Cleaving, also called the scrible-and-break method of fiber end face preparation, takes some skill to achieve optimum results. Properly handled, the cleave produces a perpendicular, mirror-like finish. Incorrect cracks will cause the lips and the comb as shown in Figure 2. While cleaving may be done by hand, a cleaver tool, available from such manufacturers as Fujikura and FiberStore, 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 3.)

Figure 3 – A Well-cleaved Multimode Fiber

Polishing

After 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 computerized fiber end against polishing paper in a figure-eight pattern approximately sixty times.

To increase the ease and repeatability of connector installation, some companies provide the connector kits. Some kits are specific to the type of connector to be installed while others supply the user with general tools and informationi 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.
Care and Handling of Fiber Optic Connectors
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. Increase the optical output power of modern lasers may 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 milliatts at 850nm 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.
Cleaning
Another important thing to remember in handling fiber optic connector 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! 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.
Handling
  • Never touch the fiber end face of the connector.
  • 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.
  • 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 back reflection, often dramatically. 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.