The Connector Question: Mechanical or Splice-On for Field Installation?

For all of the high-tech gadgetry that characterizes the fiber optics industry, it is perhaps the humble connector that offers the most consternation. For the construction supervisor, it is staff training and the possibility of high installation scrap rates that threatens budgets and deadlines. For network owners and operators, it is the unfortunate status of connectors as the weak link in the passive network that causes exasperation. And, for all parties involved, the selection of a proper connector for field-installation can be a subject of mystery and debate.

There have been countless variations on connector installation techniques and

recommended procedures. Most recently, however, discussion has centered on the

subject of mechanical connectors and splice-on connectors. The mechanical connector is a product that evolved primarily out of the enterprise space, and offers a simpler and cleaner alternative to the epoxy-and-polish connectors which preceded it. In contrast, the fusion splice-on connector (or “SOC”) evolved in the telecommunications space and offers factory-quality connector performance without the pigtails, splice trays, and space requirements that characterized conventional installation practices. Improvements in both products have steered them into the FTTH market where they now represent competing alternatives.

The appeal of a good field-installable connector for a FTTH carrier is fairly obvious. For one thing, many FTTH applications involve the deployment of fiber distribution hubs with an accompanying large number of connectors. So, if nothing else, a field-installable option is necessary just to effect repairs. Beyond that, however, the implications for the FTTH drop installation are considerable. Field-installable connectors do not require the inventory, terminals, slack storage, up-front engineering, and up-front investment of a preterminated drop solution. Additionally, they do not require customer premise equipment with splice trays and pigtail assemblies like a conventional fusion-spliced drop solution. Those are important attributes as the FTTH industry looks toward smaller customer premise equipment to support the indoor installations which are characteristic of multi-dwelling units. Likewise, those same attributes may help to address the cost concerns associated with drop installations in rural deployments.

Both mechanical and fusion splice-on connectors make use of a pre-polished fiber stub in the connector ferrule. Mechanical connectors, as the name implies, use a mechanical method to align a cleaved fiber with the pre-polished stub and then use a cam, wedge, or crimp mechanism to secure the fibers together. In essence, it is a connector end-face and a mechanical splice in one package and within a few millimeters distance of one another. The craft-dependent alignment and presence of two optical discontinuities in such close proximity to one another has always been the weakness of the mechanical connector. To adjust for this intrinsic shortcoming, vendors have used index matching gels to reduce the reflectance and attenuation associated with the mechanical splice behind the pre-polished stub. However, the lifespan of the gel and the robustness of the mechanical splice have often been questionable.

Without a doubt, mechanical connectors have improved. New index matching gels and improved alignment mechanisms have made mechanical connectors viable in areas where they would have previously never been considered. At the same time, however, advances in fusion splice-on connector technology have yielded cost reductions and craft improvements that put the two installation options on competitive footing.

The key value proposition for fusion splice-on connectors relative to mechanical

connectors has always been the fusion splice itself. There is no serious debate in the

industry about the quality difference between the two approaches. A fusion splice

dramatically reduces attenuation, eliminates reflectance, and mitigates craft-induced error by introducing an automated alignment process. Additionally, when the fusion splice is protected by a hermitically-sealed heat shrink, a package is created which is as mechanically robust as it is optically superior. Thus, a splice-on connector yields a

factory-quality connection in a field-installable format. However, the quality advantage

has been a fact for over a decade. The cost and craft advantages which make the spliceon option competitive with mechanicals have only been realized in the past couple of years.

When splice-on connectors first came on the scene in the late 1990’s, fusion splicers were cumbersome, complicated, and priced in the sports car range. So, in order to enable the application, vendors created proprietary splicers which were engineered solely for the purpose of installing proprietary splice-on connectors. To make this solution cost-effective, these machines replaced view screens with a microscope; replaced automated alignment with a manual process; and replaced loss estimation with a field technician’s educated guess. As a result, the splice-on connector option required an investment in dedicated splicing equipment and usually yielded a high scrap rate due to the limitations of the splicing equipment.

Today’s splice-on connectors are engineered to work with the removable fiber holders that are common on most fusion splicers. Thus, rather than requiring an entirely different machine, the splice-on connector merely requires an additional holder in order to work with the same machine that would be used anywhere else in the network. It follows, therefore, that the technician installing one of these connectors can use anything ranging from the ultra-compact and affordable splicers developed for FTTH to the most sophisticated core-alignment machines. And, unlike a mechanical splice, the splice-on connector installed with today’s technology uses automated alignment and calibrated loss estimation features. So, not only is scrap reduced through the use of automated technology, but site revisits are dramatically reduced since the technician has a very reliable indicator of connector performance before leaving the installation site.

Over the past decade, fusion splicer technology has followed a market trajectory similar to computers in that consumers have benefited from a combination of steadily improving performance and decreasing costs. At the same time, connector performance requirements in both telecommunication networks and enterprise applications have been rising. Since the quality of a mechanical connector is largely

dependent on the quality of the cleaver used to prepare the fiber for the mechanical splice, the low-cost cleavers that accompanied most mechanical connector kits are being replaced out of necessity by higher cost and higher quality products. So, while equipment costs for fusion splicers have been going down, kit costs for mechanical connectors have been going up. Granted, the cost is still not equal, but it is getting close enough to warrant stronger consideration of the quality advantages for the splice-on option.

Mechanical connectors are a relatively easy option, and they have made significant

strides in quality over the past several years. However, the fact of the matter is that any entity with a significant amount of fiber is probably going to need to own a fusion splicer, and a FTTH carrier certainly falls into that category. So, in FTTH, the skill sets and equipment for fusion splicing are already a necessity and the cost to add splice-on connector capability to a fusion splicer kit are less than or equal to the mechanical alternative. When those facts are combined with the undisputed quality advantages of a fusion-spliced solution, the splice-on connector becomes the obvious FTTH field installable alternative.

Posted February 10th, 2010

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Fiber Attenuation – how are you managing it?

The main limit to performance in optical fiber is attenuation.  Optical fiber attenuation, commonly known as loss, refers to the weakening or degradation of the optical signal as it passes through the glass fiber over the total distance of the fiber.  Insertion loss and back reflection (return loss) are two tests that will enable a technician to correct attenuation problems on a fiber span.

Testing the fiber will show you where the weakened signals are located.  Physical characteristics of the fiber, increased signal levels, amplification nonlinearity, inconsistent or ‘dirty’ connector end-faces as well as components on the fiber such as splices and connector terminations can all be a factor that cause increased attenuation.  Attenuation (loss and back reflection) can be measured by using an Optical Power Meter and an Optical Light Source , or an Integrated Power Meter / Light Source for bidirectional testing (pair needed), or an Optical Time Domain Reflectometer (OTDR) and a Handheld Power Meter.  (It is recommended to utilize a fiber reference jumper for all scenarios).  When testing large fiber counts, data storage and the ability to download the information to a computer, is essential.

Using any of the above mentioned test scenarios; the light source will send a continuous wave signal, which simulates the operating wavelength of the emitter on the transmission equipment, down the fiber in question.  At the far end of this fiber the power meter will be connected.  The result of this test will be the loss of dB which is a relative reading and is equal to the transmitted power minus the received power.  This test gives a numerical value for the power received.  To obtain accurate loss measurements, a reference setting function must be available on the power meter.  The setting of a reference requires a test jumper be connected to power meter, and also the light source and connected by a coupler. A reference reading is than taken, and than the power meter/light source are attached to the different ends of the cable under test. The resultant reading will be the loss or attenuation of the fiber under test only.  Attenuation will differ depending on the direction in the fiber itself.  Different results can be obtained when measuring from A to B and from B to A.

Back reflection (return loss) is the ratio of the light backscattered or reflected in the reverse direction of the forward direction of travel.  Back reflection limits and/or degrades system performance.  Unlike attenuation, which can be reduced by cleaning the connector interfaces etc., the effects of back reflection can only be resolved by re-polishing connector faces, or even changing the type of connector interfaces such as UPC or APC connector polishes.

While every effort should be made to keep attenuation to a minimum such as effective fusion splicing techniques, proper bend radius consideration, proper fiber end face maintenance techniques, etc., sometimes attenuation must be added to a circuit because the receiver on the network element can not accept the signal level. An attenuator is a passive device used to reduce the amplitude of a light signal without significantly changing the waveform itself.  Primarily there are five different configurations of attenuators and each configuration has its own strength.

Hybrid Attenuator (Plug) (Male to Female) or (The Opposing Ends are a Different Connector Type)  The hybrid style is ideal for reducing the intensity of a signal just prior to going into a receiver.  This type of attenuator is typically available with similar connector ends with male/female configurations. They can also be available with different connector types on each end as well.  The high performance characteristics of this type of attenuator make it the perfect choice for DWDM systems, CATV, EDFA, with Instrumentation and other highly amplified systems, LAN and Telecommunication Networks and high-speed data-com.

Patch Cord Attenuator (In Line) (Attenuator is within a cable assembly) the attenuating fiber patch cord is ideal for high power applications and can be easily installed into fiber splice enclosures.  The Patch Cord style offers simplified system set-up and reduced installation costs by combining the functions of patch cords and fixed attenuators in one convenient package.

Bulkhead Attenuator (Female to Female) The bulkhead style is ideal when two male connectors need to be mated with a fixed attenuator.  Many times you will find this type at the patch panel.  This type of attenuator is ideal for applications where return loss is not as critical and price is a consideration.

Loopback Attenuator The loopback style is ideal for simulating losses associated with outside plant cable runs allowing BER testing on engineering and production test standards

Variable Optical Attenuated Jumper (VOA) The variable style is ‘in-line’ (patch cord) and allows the user to change the attenuation of the signal in the fiber as it is transmitted through the device, using a screw on the side of the housing.  The maximum specified attenuation is achieved within 10 turns of the adjustment screw.  VOAs are often used to balance the signal strengths in fiber circuits and for precisely attenuating an optical signal in order to evaluate the dynamic range of transmission equipment.

I recently watched my coworker disassembling a computer using only one tool.  Was it the right tool for the job?  Yes and no.  It was the tool he had… it worked, however, there is definitely more than one tool out there that would have made the task easier!  This situation is definitely one that many fiber optic installers know all too well.  As a gentle reminder, how many of you have used your Splicer’s Tool Kit (cable knife/scissors) to remove jacketing or even slit a buffer tube and then use the scissors to hack away at the Kevlar?  Did you nick the glass?  Did you accidentally cut through the glass and have to start over?

Correctly splicing and terminating fiber optic cable requires special tools and techniques.  Training is important and there are many excellent sources of training available.  Do not mix your electrical tools with your fiber tools.  Use the right tool for the job!  Being proficient in fiber work will become increasingly necessary as the importance of data transmission speeds, fiber to the home and fiber to the premise deployments continue to increase.

Many factors set fiber installations apart from traditional electrical projects.  Fiber optic glass is very fragile; it’s nominal outside diameter is 125um. The slightest scratch, mark or even speck of dirt will affect the transmission of light, degrading the signal.  Safety is important because you are working with glass that can sliver into your skin without being seen by the human eye.  Transmission grade lasers are very dangerous, and require that protective eyewear is a must.  This industry has primarily been dealing with voice and data grade circuits that could tolerate some interruption or slow down of signal.  The person speaking would repeat themselves, or the data would retransmit.  Today we are dealing with IPTV signals and customers who will not tolerate pixelization, or momentary locking of the picture.   All of the situations mentioned are cause for the customer to look for another carrier.  Each situation could have been avoided if proper attention was given to the techniques used when preparing, installing, and maintaining fiber optic cables.

With that being said, why don’t we review basic fiber preparation?  Jacket Strippers are used to remove the 1.6 – 3.0mm PVC outer jacket on simplex and duplex fiber cables.  Serrated Kevlar Cutters will cut and trim the kevlar strength member directly beneath the jacket and Buffer Strippers will remove the acrylate (buffer) coating from the bare glass.  A protective plastic coating is applied to the bare fiber after the drawing process, but prior to spooling. The most common coating is a UV-cured acrylate, which is applied in two layers, resulting in a nominal outside diameter of 250um for the coated fiber.  The coating is highly engineered, providing protection against physical damage caused by environmental elements, such as temperature and humidity extremes, exposure to chemicals, point of stress… etc. while also minimizing optical loss.  Without it, the manufacturer would not  be able to spool the fiber without breaking it. The 250um-coated fiber is the building block for many common fiber optic cable constructions.  It is often used as is, especially when additional mechanical or environmental protection is not required, such as inside of optical devices or splice closures.  For additional physical protection and ease of handling, a secondary coating of polyvinyl chloride (PVC) or Hytrel (a thermoplastic elastomer that has desirable characteristics for use as a secondary buffer) is extruded over the 250um-coated fiber, increasing the outside diameter up to 900um.  This type of construction is referred to as‘tight buffered fiber’. Tight Buffered may be single or multi fiber and are seen in Premise Networks and indoor applications.  Multi-fiber, tight-buffered cables often are used for intra-building, risers, general building and plenum applications.

‘Loose tube fiber’ usually consists of a bundle of fibers enclosed in a thermoplastic tube known as a buffer tube, which has an inner diameter that is slightly larger than the diameter of the fiber.  Loose tube fiber has a space for the fibers to expand.  In certain weather conditions, a fiber may expand and then shrink over and over again or it may be exposed to water.  Fiber Cables will sometimes have ‘gel’ in this cavity (or space) and others that are labeled ‘dry block’.  You will find many loose tube fibers in Outside Plant Environments.  The modular design of loose-tube cables typically holds up to 12 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers.  Loose-tube cables can be all-dielectric or optionally armored. The armoring is used to protect the cable from rodents such as squirrels or beavers, or from protruding rocks in a buried environment.  The modular buffer-tube design also permits easy drop-off of groups of fibers at intermediate points, without interfering with other protected buffer tubes being routed to other locations. The loose-tube design also helps in the identification and administration of fibers in the system. When protective gel is present, a gel-cleaner such as D-Gel will be needed.  Each fiber will be cleaned with the gel cleaner and 99% alcohol. Clean room wipers (Kim Wipes) are a good choice to use with the cleaning agent.  The fibers within a loose tube gel filled cable usually have a 250um coating so they are more fragile than a tight-buffered fiber.  Standard industry color-coding is also used to identify the buffers as well as the fibers in the buffers.

A ‘Rotary Tool’ or ‘Cable Slitter’ can be used to slit a ring around and thru the outer jacketing of ’loose tube fiber’.  Once you expose the durable inner buffer tube, you can use a ‘Universal Fiber Access Tool’ which is made for single central buffer tube entry. Used on the same principle as the Mid Span Access Tool, (which allows access to the multicolored buffer coated tight buffered fibers) dual blades will slit the tube lengthwise, exposing the buffer coated fibers. Fiber handling tools such as a spatula or a pick will help the installer to access the fiber in need of testing or repair.  Once the damaged fiber is exposed a hand- stripping tool will be used to remove the 250um coating in order to work with the bare fiber.  The next step will be cleaning the fiber end and preparing it to be cleaved.  A good cleave is one of the most important factors of producing a low loss on a splice or a termination.  A Fiber Optic Cleaver is a multipurpose tool that measures distance from the end of the buffer coating to the point where it will be joined and it precisely cuts the glass.  Always remember to use a fiber trash-can for the scraps of glass cleaved off of the fiber cable.

When performing fusion splicing you will need a Fusion Splicer, fusion splice protection sleeves, and isopropyl alcohol and stripping tools.  If you are using a mechanical splice, you will need stripping tools, mechanical splices, isopropyl alcohol and a mechanical splice assembly tool.  When hand terminating a fiber you will need 99% isopropyl alcohol, epoxy/adhesive, a syringe and needle,polishing (lapping) film, a polishing pad, a polishing puck, a crimp tool, stripping tools, fiber optic connectors ( or splice on connectors)  and  piano wire.

When a termination is complete you must inspect the end face of the connector with a Fiber Optic Inspection Microscope.  Making sure that light is getting through either the splice or the connection, a Visual Fault Locator can be used.  This piece of equipment will shoot a visible laser down the fiber cable so you can tell that there are no breaks or faulty splices.  If the laser light stops down the fiber somewhere, there is most likely a break in the glass at that point.  When there is more than a dull light showing at the connector point, the termination was not successful.  The light should also pass through the fusion splice, if it does not, stop and re- splice or re-terminate.

We will provide additional informational guides for other segments of the fiber optic industry in upcoming blogs.

Posted November 17th, 2009

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Different ways to terminate fiber optic cable

Posted November 11th, 2009

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