Does high backreflection affect the bandwidth carrying capability of OM3 and OM4 fibers, particularly in regard to quick-term mechanical connections?*

This is a question that customers have been asking more and more lately and they are not alone in searching for answers. In today’s market there are many termination choices, with quick-term solutions becoming ever more attractive and prevalent due to their ease of use and relatively short training curve.

 
The debate tends to heat up when mechanical quick-term connectors are discussed. Traditionally, quick-term connectors can introduce backreflection due to the method in which the fiber is mated, namely by mechanical means. In contrast, newer “fusion splice” quick-term connectors drastically reduce or eliminate this reflective event due to the method in which they are attached, which is by fusion splicing.

Is it better to use a “fusion” splice-on connector over a “mechanical” splice-on connector? Although there are many standards and studies related to the effect that backreflection has on singlemode systems, a limited number of definitive studies have been done on the effects of backreflection in 10-gig multimode networks. However, in the hope of hedging our bets, “fusion” splice quick-terms appear to be the safer solution.

 
Traditionally the CATV and Telco industries have had to come to grips with the effects of backreflection in 10-gig fiber networks. The majority of the providers have relied heavily on the use of fusion splice-on Angled Physical Contact (APC) or Ultra Physical Contact (UPC) connectorized pigtails in their networks to minimize the effects of optical return loss (ORL). This helps to eliminate errors in the transmission of data and leads to a limited use of mechanical splices in these systems except in a temporary emergency restoration situation. It has been shown by testing that putting a potentially high reflective event such as a mechanical splice near a transmission laser can either damage the light source or cause a deterioration of the signal bandwidth being sent down the fiber by introducing a higher bit error rate (BER) in the signal. Or, the reflective event may limit the overall distance that a 10-gig signal will travel in the fiber.

 
Every time you choose to use a fusion splice connector in your network (instead of a mechanical splice connector) you eliminate the possibility of the above effects from happening. Five years from now you may have helped to future proof your network against advancing technology by allowing your installed fiber to maximize its potential today.

 
A Few Things to Consider
1. What is the future potential of my network?
2. What are the specified loss and reflection characteristics of the network?
3. Do mechanical quick-term connectors provide a higher detectable BER than other methods?
4. Are current standards in tune with changing installation trends?

*A “mechanical” quick-term connector can be defined as a connector that attaches to optical fiber or cable by mechanical means such as gripping mechanisms. In contrast, a “fusion splice” quick-term connector uses a high-temperature “fusion” splicing process to fuse the connector to the optical fiber.

By Ray Wertz
FIS Technical
Support, R&D
Does High Back Reflection Influence
Bandwidth in 10 Gig Systems?

Q. I see a lot of fiber optic connector inspection scopes on the market that feature coaxial or oblique illumination. What’s the difference?

A. There are two types of lighting commonly used for termination inspection: coaxial and oblique. Each has its advantages, so FIS offers both
types of microscopes in its product line.

Some microscopes incorporate coaxial illumination to provide maximum light transmission and render the clearest view of polish quality. In short, you see scratches very clearly with this type of illumination. This design is far preferred by fiber specialists because they can see exactly what type of polish quality they have produced. This product is recommended only for those experienced with installing and maintaining in-service fiber, as an inexperienced installer can waste time over-polishing because they can see too much detail.

Other microscopes utilize oblique illumination. This lighting method provides a clear view of core condition and cleanliness. This product is very popular with field technicians who are polishing connectors for a moderate bandwidth system and are not critically concerned with small scratches.

These microscopes may be better suited for less-experienced technicians. Both coaxial and oblique microscopes provide an excellent view of the ferrule endface. Most technicians recommend 200x magnification for multimode and 400x magnification for single mode. If you frequently terminate fiber and have received professional training, you may prefer coaxial illumination. If you only occasionally terminate fiber, and are unsure, you might consider oblique lighted scopes. Some microscopes incorporate both illumination techniques, and these are mostly available in 200x magnifications.

This article will help you zero in on what you need to know.

A Critical Consideration

Whether a fiber optic connector must interface with a simple transmitter or the latest ROADM multiplexer, the connector interface is of critical importance because of its unique loss characteristics. To illustrate this point, consider the difference between fiber optic vs. copper wire connectors.

Power loss for both types of connectors are stated in decibels (dB). That’s about where the similarity ends, because copper connectors and fiber optic connectors have opposite loss characteristics.

Copper connectors produce negligible loss when compared to losses produced by the copper cabling to which they are attached. With fiber, the exact opposite is true. In a typical fiber optic system, fiber optic connectors produce far more loss than that produced by the fiber optic cabling. That’s why careful connector selection, particularly in regard to a connector’s loss specifications, is so crucial.
Other considerations that affect connector loss involve how the connector is joined to the field fiber, and how meticulously fiber optic connectors are cleaned and inspected prior to coupling.

Narrowing the Field

There are nearly 100 styles of fiber optic connectors, so choosing the right connector for a particular application might seem daunting. However, this connector guide simplifies the selection process by focusing on the most useful and popular connector styles currently available. A companion article on page 16 will also help you choose the right termination method.

In many cases, the types of connectors that you must use are dictated to you, especially if you are upgrading a legacy system. In that case, you may have to use the same type of connectors that are already in place in order to accommodate existing equipment and cabling. Even so, it’s a good idea to know the loss characteristics and other attributes of the connectors that you are working with. For example, a connector’s “insertion loss” specification relates to optical loss that results from differences in concentricity, endface geometry or other irregularities. Knowing the connector’s insertion loss specification can be useful when testing.

In some cases, such as a new install, connectors may or may not be specified. If connectors are not specified, you will likely be presented with a loss budget for cabling and connectors that you must adhere too. In this case, you have to give some serious thought to selecting the best connectors for the job. You also have to take into account the connector termination method (e.g. fusion splicing, epoxy, or mechanical termination) because this can have a significant impact on optical loss and back reflection characteristics.

Choosing The Right Connector
The following are considerations for selecting fiber optic connectors.

Talk Like a Pirate….ARRG!
ARRG stands for Alignment, Ruggedness, Repeatability and Geometry. When choosing connectors, this memory aid will help you recall desirable connector qualities. The following attributes apply to most connector styles.

Alignment – A quality connector will keep fiber properly aligned with the fiber to which it is mated. Proper alignment is especially critical for singlemode fibers which have a very small fiber core through which signals are transmitted. Always buy quality connectors and mating sleeves from recognized manufacturers to ensure that connectors are manufactured to high tolerances and provide optimal alignment.

Ruggedness – Will connectors be installed in high-traffic areas? If so, a good choice are epoxy-style connectors, which have the fiber bonded to the ferrule. This resists optical disconnects cause by tugging, temperature changes and other external forces. As added protection, consider a spring-loaded “non-optical disconnect” connector, such as the SC connector, which is specifically designed to prevent optical disconnects. For harsh outdoor environments, “hardened” connectors are available.

Repeatability – Will there be a number of occasions when your connector will be disconnected? If so, consider using a connector that is known for good “repeatability.” The term repeatability refers to the performance of any class of connectors that are known to provide consistent loss performance that varies by a relatively narrow margin. Such connectors are typically keyed,or contain a keyway feature that prevents endface rotation. Keyed connectors ensure that connectors that are decoupled from one another maintain the same endface orientation when they are recoupled, resulting in connector losses that are predictable, consistent and “repeatable”.

Geometry – The shape of the connector endface has a major affect on interface loss. For example, PC connectors have ferrules that have a domed endface surface to insure contact at the core of two mated fibers, which helps to reduce insertion loss. Other connectors have an angled endface (APC connectors) which helps to minimize back reflection by directing endface reflections away from the core of the fiber. Knowing how endface geometry affects loss is important when selecting connectors, especially if you plan to polish your own connectors. Polishing procedures vary for different endface geometries.

Now that you know the general qualities you are looking for, it’s time to choose a specific connector for your application. The following approach uses a simple 3-step process of elimination.

Step 1. Weed Out Connectors that Can’t Meet the Loss Budget – Loss budgets will usually have connectors and cabling losses broken out separately from the rest of the network. Except for very long fiber links, losses for fiber optic cabling are usually negligible, so you’ll want to focus most of your attention on choosing the right connectors. Begin by narrowing down your possible connector choices to those that can stay within the loss budget of your application. For each connector being considered, simply multiply the number of connectors required by the dB loss specified for that type of connector. Now add cabling loss to that number. If you are still within loss budget, great. You can proceed to Step 2.*

Step 2. Consider Installation Time, Material Costs, and Skills Required – After narrowing your list down in Step 1, it’s time to consider the costs associated with each type of connector, including installation skills required. Will you have to put your best installers on the job?

Step 3. Your Own Preferences – After completing Steps 1 and 2, let’s say that you have narrowed your connector list down to two possibilities. Now you can use your own personal preference to make the final decision. Simply choose the connector for which you are most comfortable and proficient. This will increase your speed and productivity on the jobsite and helps to ensure quality terminations. Tip: When trying new connectors and termination procedures for the first time, do enough of them in the shop to become proficient. Experimenting in the field is never a good idea.

Termination Options
Interface Loss + ‘Termination Loss’ = True Connector Loss

Important Note – The method that you choose to terminate fiber has greater impact on a loss budget than your choice of connector!
The connector charts on the following pages show dB values in terms of interface loss (i.e. loss at the connector endface). For a true estimate of connector loss you must add to that value the additional loss produced by the termination method that you choose, which for the sake of discussion we will call “termination loss.” This additional loss occurs at the point where the connector is joined to the field fiber.

Most fiber optic connectors fall into one of two categories; they are either “quick termination connectors” or they are “epoxy style” connectors.

Quick Term vs. Epoxy Connectors. Which is Better?

Quick Termination Connectors and Epoxy Connectors each have their own advantages and disadvantages, depending on the application and options you choose.

Quick Term Connector Options
“Quick-term” connectors provide a “quick” way to terminate fiber. These connectors are available with factory-polished endfaces, so no time is required for field polishing. Also, they do not require time to cure epoxy.

Quick term connectors offer two main options:

• Quick Term Option 1 – “Mechanical” Quick-Term Connector
  Most “mechanical” quick-term connectors use a mechanical device to hold or “splice” the field fiber to a fiber stub within the connector body.  These connectors are great when…
  • Speed is of the essence, e.g. emergency restoration
  • Installers do not have the skill or experience to assemble and hand polish connectors in the field
  • Fusion splice connectors and equipment are not available

Examples of “mechanical” quick-term connectors are:

• UNICAM Pretium from Corning
• Bobtail Connector from Fiber Instrument Sales (FIS)
• Fast Connectors from AFL
• Mechanical Field Connectors from Sumitomo

• Quick-Term Option 2 – “Splice-On” Connector (SOC)

Similar to “mechanical” quick term connectors, SOC quick-terms have a factory-polished fiber stub within the connector body. The difference is that SOC’s use a fusion splicing process to join the fiber stub to the field fiber instead of mechanical means, resulting in lower loss.
SOC’s typically have lower loss than field-polished epoxy connectors. The factory polish on an SOC is of higher precision than that which can be achieved by hand polishing in the field.

Insertion Loss Comparison (mated pair):

SOC Connector:
Mating Loss .4 dB + Per Splice Loss<.05 dB = .5 dB total loss
Field-Polished Connector:
Mating Loss .75 dB + 0 Splice Loss = .75 dB total loss

Back reflection considerations are greatly improved by factory UPC polish (-55 dB) versus typical hand-polish PC (-40 dB). Using fusion spliced APC connectors provide a distinct advantage of -65 dB back reflection, ensuring high datarate performance. Field mechanical APC mated connectors require a cleaved angle of 8 degrees that increases insertion loss but will reduce the back reflection. SOC’s also provide a significant advantage over another “fusion-splice” termination method, namely connectorized pigtails. Unlike pigtails, SOC’s do not require an external splice protection sleeve or splice tray, which saves rack space.

SOC’s are available from a various manufacturers including:
• Fiber Instrument Sales (Cheetah SOC)
• AFL (FuseConnect )
• Sumitomo (Lynx 2)
• Fitel and Seikoh Giken also offer SOC’s

Epoxy Connector Options

With epoxy connecters, the field fiber is permanently bonded to the connector ferrule, providing a very reliable connection. Epoxy connectors offer two options:

Epoxy Option 1 – Field Polished Connector

With this termination method, the field fiber is routed through the connector body and ferrule, then field-polished as part of the ferrule endface.  Since no splicing is involved, there is no “termination” loss as defined earlier.

Advantages:
• Capable of low insertion loss and low back reflection
• High quality and reliability when installed properly
• Lowest cost per connector

Disadvantages:
• Special tools for curing epoxy/hand- polishing
• Long assembly time (including epoxy cure time)
• High level of training is required
• Precise endface geometry difficult by hand polishing

Most field-polished epoxy connectors use industry standard epoxies. The 3M Hot Melt Connector uses a proprietary adhesive that does not require epoxy cure time.

Epoxy Option 2 – Factory Polished Connector

Epoxy connectors can be purchased with factory-polished endfaces and are typically sold as Pre-terminated Cable Assemblies – Custom cable
assemblies pre-terminated with connectors of your choice. Connectorized Pigtails – Factory-polished connectors can be purchased that have attached pigtails for splicing to the field fiber.

Advantages of Factory Polished Epoxy Connectors:
• The precision endface finish can provide lower loss than that achieved by hand-polishing
• Less training than for hand-polished connectors
• Quick installation

Disadvantages:
• More complicated cable management
• Pre-polished connectors cost considerably more per unit are higher but labor costs are lower)
• Connectorized pigtails have a higher equipment cost (a fusion splicer is required).

Here are some useful things to keep in mind as you perform a network upgrade:

• Many IT managers perceive that switching from copper to fiber must be done all at once. Explain that it can be done in stages, as budgets allow, using media converters.
• When possible, choose “auto-negotiation” media converters that will automatically negotiate speed and compatibility.
• When looking for system bottlenecks in a legacy system, good places to begin are those areas where many cections converge at servers or routers.
• Sometimes an existing infrastructure has adequate speed and capacity but there aren’t enough ports to connect new users or equipment. A simple upgrade is to replace existing Ethernet wall outlets with 4-port Fast Ethernet switched outlets.
• PC’s in older legacy systems may negate some of the performance improvements of an upgraded system. Ask your client to consider new computers as part of the upgrade.
• Rule of thumb – Increasing network bandwidth also increases speed. A network that is running Ethernet 10-Mbps will operate 10 times faster when upgraded to Fast Ethernet (100 Mbps).

There are several ways to clean fiber optic connector endfaces, a dry clean; using lint free fiber optic wipes, a wet clean; using Isopropyl Alcohol and lint free wipes or lint free swabs. There are several variations of fiber optic cleaning supplies. Beware because some of them are restricted for ground only with a few products that are air travel safe.

Have you been troubleshooting a fiber optic line and still getting an unacceptable signal? Unfortunately, many people believe that just because a fiber optic connector has a “dust cap” installed, it has been protected from contaminates and therefore does not need to be cleaned. As you now realize, nothing can be further from the truth. It is important to always clean a fiber optic connector endface immediately before mating it, regardless of whether or not it has been “protected” with a dust cap.

Your fiber optic system may contain the very best Corning fiber optics laser optimized for maximum performance. However, if you have dirty fiber optic connectors, you will experience sub-par performance. Proper connector cleaning is essential for the optimal maintenance of fiberoptic systems.

The name “Dust Cap” may be a bit misleading.   In fact a dust cap can create a static change on a connector that will actually draw dust to it when removed.   The real purpose of a dust cap is to protect the polished endface of the connector from being scratched or damaged.   A fiber optic connector must always be cleaned prior to making any connections.

Dust is everywhere. Smaller dust particles that have a diameter of 1um or less can remain suspended in air for very long periods of time, if not indefinitely. Dust can easily find its way into a dust cap, and stow away until it has the opportunity to jump aboard your fiber optic connector endface.

Even worse, dust caps often contain grease, gels or other compounds left over from when the dust cap was manufactured. For example, the plastic dust caps used on many fiber optic connectors may contain mold release residue. Even blasting air into the cap will not remove these tenacious compounds.

Q. Why is the new 50μm multimode fiber superior to 62.5 fiber for 10 Gig applications?

A. Compared to 62.5μm fibers, 50μm fibers have a smaller fiber core, which results in less modal dispersion. Less modal dispersion results in higher bandwidth, enabling the transmission of faster data rates over longer distances.

Q. How much additional bandwidth does 50μm optical fiber provide?

A. Compared to 62.5μm fibers, 50μm fibers can provide nearly ten times more bandwidth. Effective modal bandwidth at 850nm for Corning InfiniCor 300 62.5μm fiber is 220 MHz/km vs. 2000 MHz/km for InfiniCor SX+ laser optimized 50μm fiber for 10 Gig applications.

Mode Conditioning Patch Cord

What is a Mode Conditioning Patch Cord? A mode conditioning patch cord is a duplex multimode cord that has a small length of singlemode fiber at the start of the transmission leg. The basic principle behind the cord is that you launch your laser into the small section of single mode fiber. The other end of the singlemode fiber is coupled to multimode section of the cable with the core offset from the center of the multimode fiber. The laser light thus misses the “dip” and this new launch condition more closely mimics a standard LED launch. The bonus is that you still retain the speed advantages of using a laser.

A Mode Conditioning Patchcord (MCP) creates an offset in the launch from the laser into the multimode fiber, avoiding the index dip in the center of the fiber and allowing the signal to propagate properly through the fiber.   Newer “Laser Enhanced” multimode fibers do not exhibit this phenomenon due to the engineering of the fiber without an index dip in the core.

In legacy fiber optic systems, multimode fibers were not designed to properly transmit a SM laser source without the creation of dispersion problems, where the original signal becomes spread out over distance (>300m in most systems) and pulses can combine with each other creating an unreadable signal at the receiving end.   This was caused by a small dip in the Index of Refraction profile of the fiber in the center of the core in which the laser transmission becomes “mode locked”.

The solution is to test and know your signal strength, and use a fiber optic attenuator to adjust the signal strength. Here is how you do it. Simply measure the power of the signal being received, then subtract the maximum input power of the active equipment from the measured power of the signal. This formula will let you determine the amount of attenuation needed.

When it comes to getting a fiber optic attenuator you have several options. Before you go shopping for one be sure you know at what level you want to attenuate your signal and then choose what type will work best for you. Attenuators are available in either fixed or variable levels of attenuation, and as pigtailed devices, bulkhead adapters, or male-to-female hybrids. Taking the time to choose the right one cans save you big time.

In most cases you will find standard boots on the connectors. That being said, when examining the connector boots know that singlemode boots are generally blue or white in color. Multimode connector boots are usually beige or black. Looking at the boot color can be the first indication to you of that particular fiber optic connector mode type. You can save yourself further investigation time if you look for these obvious color differences in the boot. Be aware that, fiber optic connector boots can be custom ordered and there is a small chance you could be looking at a special ordered boot and this could throw off your investigation.

The main difference between singlemode and multimode fiber optic connectors is the ferrule of the connector. Singlemode connectors will almost always have a Zirconia (ceramic) ferrule while multimode connectors can be made of stainless steel (Nickel-Silver), composite (plastic), or Zirconia. identifying the material in which these ferrules are made of can also sale you time in investigating your connector types. Lastly, the size of the hole in the center of the ferrule will be the end all determining factor to identify the correct fiber type that the connector is designed for. A standard singlemode Zirconia ferrule will usually have a ferrule hole of about 126 microns, while standard multimode ferrules will be closer to 127-128 microns. The difference is very small, but can make a large impact on insertion loss if the larger multimode ferrule is used on singlemode fiber. We are talking about microns here so it could be hard to accurately identify the diameters of these holes with the naked eye.

If you find yourself faced with the task of identifying fiber optic mode types be sure to follow these steps and it might save you a bunch of time calling around for support or researching for information on the web. Always remember the best way to tell the two connectors apart is to keep them in their original packaging until ready for use.