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Industry Insights

Fiber Connections

Wednesday, February 22, 2017 | by Rudy Montgelas

With more than 100 fiber optic connector styles and types available today, the right choice for a particular application can often be difficult to figure out.¹ Design and performance of both the optical fibers and connectors has become more sophisticated over the years. Today’s fiber connectors provide end-to-end solutions that cover a wide range of applications and, in many cases, can be easily terminated in the field.

That was then...

Some early fiber optic connectors, used in telephone central offices, such as AT&T’s single mode Biconic and D4 style connectors were terminated with heat-cured epoxy adhesive and polished, at the factory, onto single mode optical fibers. They were used to connect high speed digital outside plant (OSP) fiber links.

These connectors have threaded nuts that tighten onto threaded equipment port receptacles. In the case of the D4 and FC, built-in keys provide repeatable optical coupling when mated to other FC/PC and FC/UPC bulkhead mounted connectors or to single mode laser equipment receptacles.

In addition, the SMA and AMP Optimate style connectors were some of the key industry fiber connector solutions for digital communications data links at the time. Those were soon followed up by the popular ST bayonet-style connector that is still often used today.
 

And, this is now...

The LC (SFF - Small Form Factor) and MPO connector are popular today for applications in data centers and building networks because of their small size and ability to provide high density patching and connectivity. Optical transceivers for most switches either have LC duplex ports, or use 12 or 24-Fiber MPO ports when employing parallel optical links for 40gbps or 100gbps speeds.

In addition, other types of connectors, such as MXC style and PRIZM expanded beam lens (EBL) connectors, have been developed for applications such as fiber links to on-board optics and PC board mounted components such as silicon photonics devices.²

Unlike the butt-coupled, physical contact method used for LCs and MPOs, the tiny expanded beam lenses located on the ends of EBL connectors reduce the connector’s sensitivity to lateral and axial misalignment which is useful for short distance ribbon multi-fiber optical connections. However, the need for careful alignment of the EBL connector lenses with the core of the fiber usually requires a factory termination.

So, how do we determine the right connector?

First - define the application.

It is important to determine where the connectors are being used. Figure out what type of communication equipment is being connected at each end of the link, and what the data rates, distances and numbers of connections are for the network. That will determine the optical signal power loss budget, and the loss performance the fiber connectors need to meet.

Consult the standards requirements which can also help define the cable and connector types. For instance, connectors in the data center should meet the requirements of ANSI/TIA, and also meet the FOCIS standard defining the approved connector footprint(s) and ensuring intermatability. The standards can often point to a particular connector design and help quantify the key parameters of maximum Insertion loss and minimum return loss performance. Standards can also provide insights into the type of equipment necessary for the application:

Once the application is defined, the active equipment’s optical transceiver modules and bulkhead receptacles are also determined. This selection usually sets the types of adapters and connectors that will be selected for connecting the transceivers and the backbone and patching fiber cables and harnesses. At this point, it will become evident whether or not pre-terminated cables can be used. For example, in some instances if MPO connectors are specified, a pre-terminated cable can be selected. For splicing applications, connectors do not need to be installed the field, since factory terminated pigtails, and the splice connector, are fusion spliced directly onto the fiber cables.

Second - Describe the operating environment - How can the connectors be applied to the cable?

There are a number of ways a fiber optic connector can be terminated to fiber optic cables. Take a look at the type of fiber and cable construction you are planning to work with in the application. One example is that OSP cable is often used for longer-distance single mode links where the fibers are in loose-tube sub-units. The fibers in these cables have acrylate-coated 250 micron diameters that require a 900 micron protective breakout-kit be applied to the cable sub-units in the field for proper connector strain relief. Some connector terminations also crimp onto the 3mm or 2mm sub-unit jackets, as well as the strength members, but those often require different boot and crimp ring kits.

The best way to select a particular connector is by looking at the operating environment where it is going to be used. Harsher environments usually make field termination more difficult and demand the use of heat-cured epoxy. Factory terminated fiber patch cords are pot and polished, for instance.

Heat-cured epoxy fiber connectors were some of the first commercially available connectors on the market. Later on anaerobic, two-part adhesives were developed so that AC supplied or battery-powered heat curing ovens were not required in the field. Adhesive-based connectors most often require field polishing to obtain a high quality ferrule end-face which is crucial for high performance physical contact fiber connectors.

Pre-polished mechanical gel-filled fiber connector

Time consuming polishing and epoxies, or liquids, were slow and messy to work with. So, the industry looked for ways to eliminate field polishing and to terminate connectors in the field using mechanical means rather than adhesives. That led to the development of mechanically spliced pre-polished fiber stub connector such as Legrand’s Infinium connector. However, all gel-filled mechanical type connectors are typically limited in their temperature operating range to indoor applications such as data centers and building networks.

Fusion splicing of a short factory-cleaved stub of fiber (Fusion Splice Connector), at the back of a connector, soon became of interest to provide reliable connector terminations in the field. Fusion splicing eliminates the possibility of gaps, and their associated higher losses, that can sometimes be encountered with pre-polished connectors.

What is the connector life and performance expectancy?

Epoxy terminated fiber connectors provide a lifetime of performance and are backed by manufacturer’s warranties. The gel in pre-polished connectors has been used for years in telecommunications splicing for a 40-year life.

 

The ferrule end-face polish and finishes dictate the type of performance that a connector can achieve. The higher loss and reflections encountered with the SMA connector flat polish is virtually eliminated today with PC radius polishes.


To sum it all up:

Choosing the right fiber optic connector requires proper planning and attention to detail to achieve the best possible fiber link performance in the field. Careful consideration and definition of the application and the environment for the connector will help to determine the best connector solution for the application.


References and further reading:


¹ The Fiber Optic Association (FOA) “Guide to Fiber Optics and Premises Cabling”- http://go.legrand.us/e/84502/tech-connID-htm/74yc7l/226013193
² Prizm™ is a registered trademark of USCONEC From their “Prism Light Turn Connector data sheet”