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Fiber Connectors - what's the difference?

Posted by Shaun Trezise

Given the variety of splice options available to fiber network planners today identifying the best connector for FTTH can be overwhelming. Consequently often not much thought is given to connector selection with choice driven by cost, availability or what’s been used before. However each connector has its own unique design and therefore, pros and cons. Over time or depending on project size this can have a dramatic impact on deployment speeds and costs.

So what are the differences and what do they mean to your implementation? This table of common connectors gives an overview of strengths and weaknesses, with more detail in the accompanying descriptions:

Fiber Connectors

1. SC Connector

SC fiber connectorThe SC was developed by the laboratories at Nippon Telegraph and Telephone (NTT) in the mid-eighties, and was one of the first connectors to hit the market following the advent of ceramic ferrules. Sometimes referred to as the ‘square connector’ the SC has a push-pull coupling end face with a spring loaded ceramic ferrule. Initially intended for Gigabit Ethernet networking, it was standardized into the telecommunications specification TIA-568-A in 1991 and slowly grew in popularity as manufacturing costs came down. Due to its excellent performance it dominated fiber optics for over a decade with only the ST rivalling it. Thirty years on, it remains the second most common connector for polarization maintaining applications. The SC is ideally suited for datacoms and telecoms applications including point to point and passive optical networking.

2. LC Connector

LC fiber connectorConsidered by some to be the modern replacement of the SC connector; its introduction was less successful, in part due to initially high license fees from inventor Lucent Corporation. Also a push-pull connector, the LC utilizes a latch as opposed to the SC locking tab and with a smaller ferrule it is known as a small form factor connector. Having half the footprint of the SC connector gives it huge popularity in datacoms and other high-density patch applications, as its combination of small size and latch feature make it ideal for densely populated racks/panels. With the introduction of LC compatible transceivers and active networking components, its steady growth in the FTTH arena is likely to continue.

3. FC Connector

FC fiber connectorThe FC was the first optical fiber connector to use a ceramic ferrule, but unlike the plastic bodied SC and LC, it utilizes a round screw-type fitment made from nickel-plated or stainless steel. The connector end face relies on an alignment key for correct insertion and is then tightened into the adaptor/jack using a threaded collet. Despite the additional complexity both in manufacturing and installation, it’s still the connector of choice for precise measuring equipment such as OTDRs.

Initially intended for datacoms and telecoms applications, its use has reduced since the introduction of the SC and LC. These deliver similar performance to the FC but both have less expensive components and are quicker to connect. However, the screw-on collet of the FC does make it particularly effective in high vibration environments, ensuring that the spring-loaded ferrule is firmly mated.

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4. ST Connector

ST fiber connectorThe ST connect was developed by AT&T shortly after the arrival of the FC. At a glance they can be mistaken for one another but the ST uses a bayonet fitment rather than a screw thread. Usage has declined in recent decades, for the same reasons as the FC. Additionally it cannot be terminated with an angled polish, which limits use in single mode fiber and FTTH applications.

Deployed predominately in multi-mode datacoms it is most common in network environments such as campuses, corporate networks and in military applications where the quick connecting bayonet had its advantages at the time. It is typically installed into infrastructures that were built at the turn of the century; when retro-fitting, STs are typically swapped out for more cost effective SC and LC connectors.

5. MTP/MPO connector

MTP fiber connectorThe MT ferrule connector is another of NTT’s inventions and has been around since the 1980s, although the technology has only recently become popular under branded versions of the Multiple Fiber Push-On/Pull-Off connector, such as MTP and MPO. It is larger than the other connectors but for good reason - it can support up to 24 fibers in a single ferrule.

Multi-fiber connectors are not currently designed for field-fit applications so must be lab terminated. In high density patch environments such as datacenters they are used extensively, both at single mode and multi-mode wavelengths. On a ‘per-fiber’ basis the costs are relatively inexpensive. However as might be expected, the attenuation loss can be higher than a single ceramic ferrule connector. That being said, it is possible to order ‘low loss’ MTP/MPO connectors which have comparable insertion loss performances. These are more costly however.

Network planners should also consider that whilst still using a uniter/adaptor much like other connectors, the MTP/MPO must also be mated to an opposing male or female connector. This may require more than one connector specification or type within inventory, adding to cost and complexity.

Because the sequence of the fibers cannot physically be changed after termination, the connector is often supplied with a fan-out assembly at the opposing end (such as LC, SC FC etc.). This allows the operator to change channels simply by re-patching the fanned-out side of the cable. The consequence of this is that the small form high density design of the MTP/MPO will only benefit one side of the assembly.

More common in datacoms, these connectors are starting to appear in FTTH applications. They should therefore be considered if drivers include quick deployment of aggregate fibers, high density patching or where smaller ODFs and nodes might be crucial.

The differences between types of connectors can easily overlooked in the complex planning around fiber deployments. However taking the time to select the right one for the job can deliver big benefits when it comes to speed and cost, so take the time to investigate your options before making your connector choice.

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