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Fiber Optic Technology - Part Four - Fiber Connectors & Termination Methods

Proper fiber optic termination is extremely important when installing a fiber optic network. A network will be unreliable if this function is not performed correctly. Therefore, much attention is given to this area, with more and more products appearing on the market to make fiber optic termination easier and more accurate than ever.

Following on from our previous article in the series which outlined the installation process, this article will provide an in-depth look at the different types of fiber connectors and termination methods.

What is Fiber Optic Termination? 

Fiber optic termination is the connection of fiber or wire to a device, such as a wall outlet or equipment, which allows for connecting the cable to other cables or devices. The purpose of fiber optic termination is to enable fiber cross connection and light wave signal distribution. Proper fiber optic termination will protect the fibers from dirt or damage while in use and prevent excessive loss of light, thus, making a network run more smoothly and efficiently.

There are two methods of fiber optic termination: The use of connectors that join two fibers to form a temporary joint and splicing which involves connecting two bare fibers directly without any connectors. Splicing is a permanent method of termination.

Fiber Optic Connectors 
Fiber Optic Mechanical Splices 
Fusion Splicer

Since fiber optics began, over 80 different styles of connectors have been used commercially. Most have faded from use or never became popular, so only a few connector styles dominate today's networks. Multimode installations generally use the SC, ST or LC connector.

SC is a snap-in connector that is widely used in singlemode systems for its excellent performance. It's a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration.

ST (an AT&T Trademark) is used for multimode networks, like most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. And because they are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference.

​LC is a connector that uses a 1.25 mm ferrule, similar to, but half the size of the SC. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode.

DINTEK have a range of quality Fiber Optic Connectors that exceed the ANSI/TIA/EIA-568-C standard and offer superior insertion loss performance. The range includes SC, ST and FC connectors in both singlemode and multimode.

​Most connectors work by simply aligning the two fiber ends as accurately as possible and securing them in a fashion that is least affected by environmental factors. 

The most common method is to have a cylindrical ferrule with a fiber-sized hole in the center, in which the fiber is secured with an adhesive.

​Note that fiber optic connectors are mainly "male" style with a protruding ferrule, since the end of the ferrule must be polished after the fiber is glued into it. Connectors have used metal, glass, plastic and ceramic ferrules to align the fibers accurately, but ceramic seems to be the best choice. It is the most environmentally stable material, closely matching the expansion co-efficient of glass fibers.

It is easy to bond to glass fiber with epoxy glues, and its hardness is perfect for a quick polish of the fiber. As volume has increased, ceramic costs have become the lowest cost material for connector ferrules.

​Fiber optic connectors can have several different ferrule shapes or finishes, usually referred to as polishes. Early connectors, because they did not have keyed ferrules and could rotate in mating adapters, always had an air gap between the connectors to prevent them rotating and grinding scratches into the ends of the fibers. The air gap between the fibers causes a reflection when the light encounters the change in refractive index from the glass fiber to the air in the gap. This was a performance problem. PC or physical contact ferrules are now used.

Beginning with the ST and FC which had keyed ferrules, the connectors were designed to contact tightly, what we now call physical contact (PC) connectors. Reducing the air gap reduced the loss and back reflection (very important to laser-based singlemode systems), since light has a loss of about 5% (~0.25 dB) at each air gap and light is reflected back up the fiber. While air gap connectors usually had losses of 0.5 dB or more and return loss of 20 dB, PC connectors had typical losses of 0.3 dB and a return loss of 30 to 40 dB.

​Soon thereafter, it was determined that making the connector ferrules convex would produce an even better connection. The convex ferrule guaranteed the fiber cores were in contact. Losses were under 0.3 dB and return loss 40 dB or better. The final solution for singlemode systems extremely sensitive to reflections, like CATV or high bitrate telco links, was to angle the end of the ferrule 8 degrees to create what we call an APC or angled PC connector. Then any reflected light is at an angle that is absorbed in the cladding of the fiber.

​Fiber optic connectors and splices are used to couple two fibers together. Splices, however, are used to connect two fibers in a permanent joint. Connectors are also used to connect fibers to transmitters or receivers, and, of course, connectors are designed to be demountable. While they share some common requirements, like low loss, high optical return loss and repeatability, connectors have the additional requirements of durability under repeated matings. Splices, meanwhile, are expected to last for many years through sometimes difficult environmental conditions, perhaps underground, underwater or suspended from aerial cables.

Splicing is generally needed only if the cable runs are too long for one straight pull or you need to mix a number of different types of cables (like bringing a 48 fiber cable in and splicing it to six 8 fiber cables - and could you have used a breakout cable instead?) Splicing is sometimes used instead of connectorizing where the necessity of changing connections is unlikely, like in remote patch panels, or where space is at a premium. And of course, we use splices for restoration, after the number one problem of outside plant cables, a dig-up and cut of a buried cable, usually referred to as "backhoe fade" for obvious reasons!

They may have different uses, but the basic specifications for splices are the same as for connectors:

Loss: Which we want to be minimal.

Repeatability: We want splices to be repeatable so we can predict losses for power budgets.

Environment: They have to withstand the environmental stress of their locations, which may be underground, under water or in the air.

Reliability: We want splices to last a long time, so they must be reliable. Fusion splices, properly made, are as strong as the fiber itself.

Back Reflection: Mechanical splices have some back reflection like a connector, but fusion splices have very little back reflection.

Ease of Termination: Fusion splicers are automated and easy to use, as long as you follow proper procedures. Mechanical splices are a bit more work and require care.

There are two types of splices: fusion and mechanical. Mechanical splices use an alignment fixture to mate the fibers and either a matching gel or epoxy to minimize back reflection. Some mechanical splices use bare fibers in an alignment bushing, while others closely resemble connector ferrules without all the mounting hardware.

While fusion splicing normally uses active alignment to minimize splice loss, mechanical splicing relies on tight dimensional tolerances in the fibers to minimize loss.

DINTEK's ezi-FIBERTM Mechanical Splice provides an inexpensive, quick alternative to terminating fibers without the need for a fusion splicer. Using V-groove technology, it aligns the fibers together and maintains physical contact between the fibers. No assembly tool is required to ensure the fibers are mated correctly. The resulting connection using the connector results in an average insertion loss of about 0.1dB.

Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as strong as the virgin fiber itself. The source of heat is usually an electric arc, but can also be a laser, or a gas flame, or a tungsten filament through which current is passed.

The process of fusion splicing involves using localized heat to melt or fuse the ends of two optical fibers together. The splicing process begins by preparing each fiber end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fiber. The fiber is then cleaved using the score-and-break method so that its end-face is perfectly flat and perpendicular to the axis of the fiber. The quality of each fiber end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fiber-end faces. The two end-faces of the fibers are aligned, then are fused together. The bare fiber area is protected either by recoating or with a splice protector. It is often desirable to perform a proof-test to ensure that the splice is strong enough to survive handling, packaging and extended use.

Splicing Equipment 

Fusion splicers are expensive, fully automated machines that do most of the work. The operator uses a high quality cleaver to prepare the fibers and inserts them into the jaws of the splicer. The machine automatically aligns the ends, makes the splice and even gives an estimate of the loss.

The operator then places the splice in a holder which also seals it and inserts it in a splice tray. Fusion splicers can splice one fiber at a time or all the fibers in a ribbon. While fusion splicers are expensive, each splice is cheap. So if you are doing lots of splices, fusion splicing is more cost effective.

DINTEK's Optical Fusion Splicer is a mini type fusion splicing machine utilizing the latest in fiber adjustment and splicing technology. It adopts an advanced PAS fiber adjustment technology, making use of four motor drives. 

Used for single and multimode fibers of all types, it provides one of the world's fastest splicing and heating times.

The cost issue is simple: Fusion has high capital equipment costs but low splice costs, so if you are doing lots of splices, like OSP installation of cables with lots of fibers, fusion is cheap. If you are only doing a few splices, the lower tool costs of mechanical splicing makes it cheaper.

Mechanical Splicing 
Optical Core Alignment

Optical Core Alignment (also called "Profile Alignment"), an optical alignment technique, is used by many models of fusion splicers. The two fibers are illuminated from two directions, 90 degrees apart. From the images in a video camera, software recognizes the core of the fibers and aligns them automatically using movable stages. The software also estimates splice loss after the fusion splicing is complete. Ribbon splicers typically use profile alignment.

​Splicing Protector
A splicing protector sleeve is for protecting and reinforcing a fusion splice of two or more optical fibers. The fusion splice protector includes a heat-shrinkable sleeve adapted to surround the fusion splice and adjacent portions of the fused optical fibers; a stress-relieving support element adjacent to the fusion splice; and a hot-melt adhesive contained within the sleeve for retaining the support element adjacent a longitudinal section of the sleeve. The splice protector should always be put on the fiber first before stripping. 
​Splicing Trays

Fiber optic fusion and mechanical splices are placed in mechanical closures that are referred to as "splice enclosures", "splicing trays" or "splicing organizers". Fiber optic splice trays are designed to provide a location to store and to protect the fiber cables and the splices.

Fiber optic splice trays are located at intermediate points along a route where cables are required to be joined or at the termination and patch panel points at the end of fiber cable runs. 

Splice trays normally hold up to 12 splices, and several trays are used together to splice a large fiber cable. Each tray provides space for mounting fiber splice protectors and excess fiber. Fiber buffer tubes enter the splice tray at one end only. At this end, the buffer tubes stop and are secured to the tray where the individual fibers are exposed.
​Fiber Cleavers

Cleavers come in several types, from the simple cleaver that looks like a stapler to the more complex units like DINTEK's High Precision Optical CleaverA good rule of thumb is that price is an indicator of the consistent quality of the cleave. It's quite possible to get good cleaves with the "stapler" cleaver, but it requires practice and a certain amount of finesse. This hand held cleaver is often supplied with manufacturer's kits for mechanical splices because it is inexpensive. It can provide satisfactory results but requires practice and skill to get good cleaves.

A desktop cleaver such as DINTEK's Optical Cleaver is the more sophisticated cleaver and is most commonly the type provided for fusion splicers. It requires little skill or practice and provides virtually 100% good cleaves. While it is much more expensive, it quickly pays for itself in higher yields and productivity.
  • Prepare the fiber (install the splice protection sleeve before stripping or cleaving!!!!)
  • Stripping fiber
  • Cleaning fiber
  • Cleave fiber
  • Insert fiber in splicer
  • Loading fibers
  • Run splicer program
  • Evaluating the sleeve
  • Evaluating the splice
Prepare the fiber

Remember to install the splice protection sleeve before stripping or cleaving!!! It is practically impossible to install after the fiber is stripped without damaging the fiber. 

The splice protection sleeve will be heated to seal the fiber splice after splicing is completed.

Stripping fiber

Stripping fiber is the most critical phase of splicing where fiber damage is most likely to occur. Try to avoid nicks or cuts as it weakens fiber and can cause long term reliability problems. Strip 900 micron buffer first, then 250 micron, both in one step. To minimize fiber nicks, strip in one step instead of little bites as done with connectors. Be careful cleaning the fiber and placing it in holders for cleaving or splicing too.

Cleaning fiber 

Place an Alco pad (or lint-free wipe with pure isopropyl alcohol) between your thumb and forefinger, and wipe the fiber between them. Careful - do not break the fiber! The process is the same for all splice types: strip, clean & cleave. Each fiber must be cleaned thoroughly before stripping for splicing.

Cleave fiber

First check to see the fiber is straight and in the middle of the pad indicated by the arrow. Move the scribe wheel to the front of the cleaver. Gently close the right clamp to hold the fiber. Open the cleaver clamp to remove the fiber, then pick up the fiber scrap with tweezers and dispose of properly. Open the left clamp and remove the fiber.

Insert fiber in splicer

Raise splicer hood located in the middle of the top of the unit. Release fiber clamps by pushing the activators toward the rear of the unit. Lift the clamp lever to raise both the bare fiber clamps and the coated fiber clamps simultaneously. Lower fiber gently into V-grooves so the cleaved end overhangs the V-groove and protrudes into the fusion area. The fiber end should be about halfway between the end of the V-groove and the electrodes. Align the end of the buffer coating on the fiber with the cleave length mark on the unit.


Close clamps GENTLY by pushing the clamp lever down. First press the clamp lever to lower the fiber coating clamp and press it down until it locks. Then gently lower the bare fiber clamp to properly seat the bare fiber in the V-groove. The fiber should now be resting in the V-grooves. Repeat for the other fiber. Close the hood and you are ready to splice.

Loading fiber

Gently lay the fiber in the guides on the splicer. Note the position of the end of the buffer coating - at the 16 mm mark. 

Check the position of the fiber end - it should be near the electrodes.

Run splicer program

The splicer will move the fiber into place and show the fiber on screen. 

During the process, the screen will show fiber placement and messages will display to show progress:

GAP ADJUST - the splicer is setting end gap.

FOCUS - adjusting the camera focus.

SPLATTERING - pre-fusing the fibers to polish the fiber ends.

FIBER END CHECK - checks the cleave angle and cleanliness.

FIELD CHANGE - changes from X to Y image.

CORE ALIGN/DIAMETER ALIGN - aligns the fibers according to the chosen program.

ARC FUSION - fuses the fibers by heating the ends in an arc and feeding them together.

INSPECTION - High-resolution Direct Core Monitoring (HDCM) to evaluate the splice quality using the camera.

ES LOSS - displays the estimated loss in dB and any observed defects.

Sleeve inspection

Inspect sleeve carefully as this is protection for the splice and will greatly affect splice reliability. You must have at least 6 mm (1/4 inch) of buffer inside sleeve. No bubbles can be near fiber. No bend is allowed in the bare fiber in the middle of the splice sleeve.

Splice inspection - good splices

Visually inspect the splice after the program has run, using both X and Y views. Some flaws that do not affect optical transmission are acceptable, as shown. Some fibers (e.g. fluorine-doped or titanium coated) may cause white or black lines in splice region that are not faults.

Splice inspection - bad splices

Some flaws are unacceptable and require starting the splicing process over. Some, like black spots or lines, can be improved by repeating the ARC step, but never more than twice. For large core offsets, bubbles or bulging splices, always redo.

Splice troubleshooting

​Here are some common problems and likely causes:

  • Current too high
  • Feed rate too slow
  • Pre-fusion time too long
  • Pre-fusion current too high
  • Gap too wide
  • Contaminated electrodes
  • Auto-feed too fast
  • Incorrect current
  • Contaminated fiber end faces
  • Poor cleave
  • Fusion current too high
  • Pre-fusion current or time too low
  • Contaminated electrodes
  • Fusion current much too high
  • Pre-fusion time much too long
  • Pre-fusion current much too high
  • Auto-feed too small
  • Gap too large
  • ​Fusion current too low
  • Pre-fusion time too short

Additional problems

Fusion splicers generally have stored programs for most fibers and the user can modify those program parameters or create new ones. Refer to the instruction manual or ask the manufacturer if there are any questions about using the splicer with the fiber you are installing.

It is sometimes necessary to splice older fibers, either in restoration or modifying networks. Older fibers may become brittle and hard to strip.

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