Fiber Connectors and Splices

Connectors and splices link the ends of two fibers both optically and mechanically. The two are not interchangeable. A connector is mounted on the end of a cable or optical device so it can be attached to other cables or devices. Like electrical connectors, fiberoptic connectors can be plugged and unplugged. In contrast, splices are permanent junctions between a pair of fiber ends. The cables attached to your television and stereo have connectors on the end so they can be plugged into other components. Splices are the optical equivalent of permanent solder joints. This chapter starts by explaining their applications and their common operating principles, then describes connector properties and types, and splicing. Chapter 14 will cover couplers, which in fiber optics are quite different from connectors and splices.

Fiber Connector Types Overview

In the present fiber connector market, there are about 100 fiber optic cable connectors in total. And the most commonly used fiber connector types includes LC, SC, MU, ST, FC, MTRJ, NID, E2000 and MTP/MPO connector. However, no matter what the fiber connector types are, they have the same function and similar basic components—ferrule, connector body and coupling device. And these fiber connectors are widely used in the termination of optical fibers like fiber optic pigtails. Here are five common optical cable connectors.

—LC Connector (Lucent Connector)

Developed by Lucent Technologies, the LC fiber optic connector has become the ubiquitous fiber connector for today’s optical telecom applications, especially for connections with SFP and SFP+ fiber transceivers. As one popular SFF (small form factor) connector, LC fiber connector has a ferrule of 1.25mm, which make it perfect for high density cabling. There are single mode LC connector and multimode LC connector. And based on the connector construction, LC connector also can be divided into LC duplex and simplex connector.

—SC Connector

Different from LC connector, SC fiber connectors use a round 2.5mm ferrule to hold a single mode fiber (SMF). And it has a “square shaped” connector body, which is the source of name “square connector”. Due to its excellent performance, fiber optic SC connector remains the second most common connector for polarization maintaining applications. SC fiber connector is ideally suited for datacoms and telecoms applications including point to point and passive optical networking.

—ST Connector

ST fiber connector was created and licensed by AT&T and is still one of the most popular connectors. It has an insertion loss of about 0.25dB and holds the fiber with a ceramic, spring-loaded 2.5mm ferrule that stays in place with a half-twist bayonet mount. ST fiber connector is usually used in both long and short distance applications such as campuses and building multimode fiber applications, corporate network environments and military applications.

—FC Connector

FC fiber connector was the first optical fiber connector to use a ceramic ferrule, but unlike the plastic bodied SC and LC connector, it utilizes a round screw-type fitment made from nickel-plated or stainless steel. The FC 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, FC connector is still the connector of choice for precise measuring equipment such as OTDRs, as well as the choice for single mode fiber.

—MPO/MTP Fiber Connector

MPO/MTP fiber connector is a multi-fiber connector which combines fibers from 12 to 24 fibers in a single rectangular ferrule. It’s often used in 40G and 100G optical parallel connections. Compared with other fiber connectors mentioned above, MPO/MTP fiber connectors are more complicated. Because there are key-up and key-down, male and female MPO/MTP connectors.

Simplex vs Duplex Fiber Connector

Simplex connection means signals are sent in one direction. For example, a signal is transmitted through two simplex connectors and a simplex fiber cable from device A to device B. It cannot return from device B to device A via the same route. But the revise transmission can be achieved through duplex connectors and duplex fiber cable, which is called duplex connection. In addition, simplex fiber connector is often connected with one strand of glass or plastic fiber, while duplex fiber connector needs to connect with two strand of fibers. The following picture shows the comparison of LC duplex connector and SC duplex connector.

Fiber Connector Polishing: APC Connector vs UPC Connector

According to the polishing type, optical fiber cable connectors can be divided into APC connector and UPC connector. The main difference lies in APC connector vs UPC connector is the fiber endface. APC fiber connectors have an 8 angle cut in their ferrules, which makes connections between optical fibers very tight. But their return loss can be up to -60dB. While UPC connectors have a convex end-face, but have a finer surface finish. Their return loss goes from -40dB from -55dB, which makes them reliable for transmitting TV and data signals. Besides, the color code for APC fiber connector is green, UPC connector for blue. That’s why we see different colors of LC APC connector, SC APC connector and other fiber connectors.

Single Mode vs Multimode Fiber Connectors

As we have known, the basic difference between single mode and multimode fiber cable is the number of light modes which can be transmitted at a time. Single mode fiber allows only one light mode to pass through at a time. While multimode fiber can propaganda multiple modes at a time. That’s also the basic difference lies in single mode fiber connectors and multimode fiber connectors. Because they are combined with the corresponding type of optical fibers. However, with technologies getting advanced, fiber connectors like SC, LC and FC, provided by fiber optic connector factories are compatible with single mode and multimode fiber cables.

Fiber Splicing

Fiber optic splicing is an important method of joining two fiber optic cables together. It is a preferred solution when an available fiber optic cable is not sufficiently long enough for the required distance. Splicing is also designed to restore fiber optic cables when they are accidentally broken. Nowadays, fiber optic splicing is widely deployed in telecommunications, LAN (Local Area Network) and networking projects. Typically, fiber optic splices can be undertaken in two ways: fusion splices and mechanical splices. Americom specializes primarily in the higher quality more trusted, and requested fusion method.

Fusion Splicing Method - Fusion splicing is a permanent connection of two or more optical fibers by welding them together using an electronic arc. It is the most widely used method of splicing as it provides for the lowest loss, less reflectance, strongest and most reliable joint between two fibers. When adopting this method, fusion splicing machines are often used. Generally, there are four basic steps in fusion splicing process:

Step 1: strip the fiber
The splicing process begins with the preparation for both fibers ends to be fused. So you need to strip all protective coating, jackets, tubes, strength members and so on, just leaving the bare fiber showing. It is noted that the cables should be extremely clean.

Step 2: cleave the fiber
A good fiber cleaver is crucial to a successful fusion splice. The cleaver merely nicks the fiber and then pulls or flexes it to cause a clean break rather than cut the fiber. The cleave end-face should be perfectly flat and perpendicular to the axis of the fiber for a proper splice.

Step 3: fuse the fiber
When fusing the fiber, there are two important steps: aligning and melting. First of all, aligning the ends of the fiber within the fiber optic splicer. Once proper alignment is achieved, utilizing an electrical arc to melt the fibers to permanently welding the two fiber ends together.

Step 4: protect the fiber
A typical fusion splice has a tensile strength between 0.5 and 1.5 lbs and it is not easy to break during normal handling. However, it still requires protection from excessive bending and pulling forces. By using heat shrink tubing, silicone gel and/or mechanical crimp protectors will keep the splice protected from outside elements and breakage.