Randomly choosing fiber optic connectors is a lot like throwing darts blindfolded. You’ll miss the requirements of your target application if you leave things to chance.

Optical Loss - 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 connectors for fiber optic cable vs. copper cable.

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 twisted-pair cable 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 one 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.

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, ferrule 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 to. 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 choosing fiber optic connectors for your application.

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 single mode 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 caused by tugging, temperature changes and other external forces. As added protection, consider a spring-loaded “non-optical disconnect” connector, such as the SC connector or LC connector, which are 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 ferrule endface rotation. Keyed connectors ensure that connectors that are uncoupled from one another maintain the same ferrule endface orientation when they are recoupled, resulting in connector losses that are predictable, consistent and “repeatable”.

Geometry - The shape of the connector ferrule endface has a major affect on interface loss. For example, UPC 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 ferrule endface (APC connectors) which helps to minimize back reflection by directing endface reflections away from the core of the fiber. Knowing how ferrule 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 fiber-optic cable loss to that number. If you are still within loss budget, great. You can proceed to Step 2.***

***It is possible to be within the loss budget but still have connections that produce unacceptable levels of back reflection. An Optical Return Loss (ORL) Test Set can be used to measure the level of back reflection. Also, an OTDR is useful for identifying the location of high-ORL events such as defective splices and connectors so that corrective action can be taken.

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 with which you are most comfortable and proficient. This will increase your speed and productivity on the jobsite and help 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.

Most Popular Connector Styles


Name: SC Connector

• Mode: Singlemode and Multimode
• Applications: Wide variety of singlemode applications especially datacom and telecom including premises installation. Often found in older corporate networks. It was designed to replace the ST connector.
• Ferrule size: 2.5mm
• Ferrule construction (typical): Pre-radiused zirconia
• Connector body: Composite. Similar in appearance to LC connector, except the SC is larger. Color coded according to fiber type; blue or green for singlemode, beige or black for multimode.
• Styles available: Simplex and duplex
• Latching mechanism: Push-pull, snap-in design
• Optical loss:
    Insertion loss: SM 0.10 - 0.30 dB; MM 0.10 - 0.40 dB
    Repeatability: 0.20 dB
• Meaning of name: Subscriber Connector, Square Connector or Standard Connector
Advantages: An excellent performer. Non-optical disconnect design (an advantage over the ST connector which the SC is replacing). Minimum back reflection when ultra-polished. Push-pull design helps prevent endface damage during connection. Square shape allows connectors to be packed closely together. Can fit into smaller spaces where the ST or FC cannot. The SC’s push-pull design allows quick patching of cables into rack or wall mounts.
• Disadvantages: Smaller LC connectors are replacing SC connectors in high density applications where space is at a premium.



Name: LC Connector

• Mode: Singlemode and Multimode
• Applications: System rack mounts and high density applications involving large fiber counts.
• Ferrule size: 1.25mm
• Ferrule construction (typical): Pre-radiused zirconia
• Connector body: Compact, Small Form Factor (SFF) design
• Styles available: Simplex and duplex
• Latching mechanism: Push and click
• Optical loss:

    Insertion Loss: SM 0.10 - 0.30 dB; MM 0.10 - 0.40 dB

    Repeatability: 0.20 dB
• Introduced by: Lucent Technologies
• Termination tips: Easily terminated with a variety of adhesives
• Advantages: A solid performer, the LC has become the Small Form Factor (SFF) connector of choice for many singlemode applications. At half the size of the ST, the LC is ideal for high-density applications. Also, the duplex LC has the same small footprint as a copper RJ-45. This is useful, for example, when converting a copper LAN to a fiber network since fewer tools and adapters are required. Pull-resistant “non-optical disconnect” design provides stability in system rack mounts.
• Disadvantages: Some people with large fingers may have difficulty installing such a small connector.



Name: FC Connector

• Mode: Singlemode and Multimode
• Applications: Datacom and Telecom. Popular for many years, the FC is being replaced by SCs and LCs.
• Ferrule size: 2.5 mm
• Ferrule construction: Available in zirconia or stainless alloy
• Connector body: Keyed metal body
• Reliability of the connector: Threaded coupling provides durable connections
• Latching mechanism: Threaded, screw-on coupling
• Optical loss:
    Insertion loss: SM 0.10 - 0.30 dB; MM 0.10 - 0.40 dB
    Repeatability 0.10 dB
• Meaning of name: Fixed connection
• Termination tips: Before tightening, check to see that the key is correctly aligned in the slot.
• Advantages: Non-optical disconnect; low back reflection. Good isolation from vibration due to floating ferrule design and threaded coupling mechanism.
Disadvantages: Care must be taken when inserting ferrule into receptacle due to risk of scratching the endface. This is of less concern with push-pull type connectors, such as the SC and LC, which are replacing the FC in many applications. Like the ST, installers may have difficulty grasping the FC retainer nut with their fingers in applications where connectors are spaced closely together.



Name: ST Connector

• Mode: Singlemode and Multimode
• Applications: The ST is an older style connector, long the most popular connector for multimode LANs, including campuses. Also popular for industrial applications, security systems, CCTV and naval applications.
• Ferrule size: 2.5mm
• Ferrule construction: Most are ceramic; metal and composite also available
• Connector body: Keyed metal body
• Reliability of the connector: Reliable and durable field installation. Allows installers to crimp the back of the body directly onto the cable jacket and Kevlar® to eliminate pull-away.
• Latching mechanism: Bayonet twist-lock
• Optical loss:
    Insertion loss: SM 0.10 - 0.30 dB; MM 0.10 - 0.40 dB
    Repeatability: Typical Singlemode: 0.40 dB; Multimode: 0.20 dB
• Meaning of name: ST (Straight Tip). Connector sometimes referred to as BFOC (Bayonet Fiber Optical Connector)
• Introduced by: AT&T in the late 80s
• Termination tips: Take care that these spring-loaded connectors seat properly when installed. Reconnect them if experiencing high loss.
• Advantages: Easy to install and relatively inexpensive. Keyed body helps keep mated fibers in alignment when they are coupled and uncoupled.
• Disadvantages: Lacks the non-optical disconnect feature. APC finish not possible. Installers may have difficulty grasping the ST retainer nut with their fingers in applications where connectors are spaced closely together. The ST is being replaced by more compact Small Form Factor (SFF) connectors.