Connectorized Couplings

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Quite often it is desirable to have a means of connecting two fibers together through a temporary mating device or connector. Figure 7.4 shows a common way to implement such a connector. Each fiber is placed in a ferrule whose function is to provide the mechanical support for the fiber and hold it in place tightly. The ferrule can be made out of plastic, metal, or ceramic materials. The central piece of the connector itself is an alignment sleeve. The two ferrules are inserted in the sleeve, and proper alignment between the cores is ensured because of the tight mechanical tolerances of the ferrules and the sleeve. The gap between the two fibers can be controlled by a mechanical stop which determines the exact stopping positions of the fibers. In some variations, the alignment sleeve is tapered to improve connector mating and demating.

Well-designed connectors provide low coupling loss, in the order of 0.1 dB or less. However, as shown in Fig. 7.5, a number of underirable situations can reduce the coupling efficiency. Figure 7.5a shows a case of two fibers with different core diameters. In general, whenver the numberical apertures of two fibers are different, the potential for power loss exists. In this case, light coupling from a narrower fiber core to a wide fiber core is easier and more efficient compared to coupling in the other direction. Figure 7.5b shows an example of poor concentricity. Fibers that do not provide a tigh concentricity tolerance may show large coupling variations depending on the orientation or from one pair of fibers to the next.

fiber connector

A large air gap, shown in Fig 7.5c, is another reason for loss of power. An air gap can result from incomplete insertion of the fiber or from mechanical problems inside the sleeve. It is also common for microscopic dust particles to get into fiber optic connector, preventing them from making proper conatact, or even scratching and damaging the fiber facets.

More dramatic power reduction results when dust particles land on the fiber core, blocking the light path. As a result, constant monitoring and cleaning of fiber facets are important to prevent such probems. Angular or lateral displacement, the mechanical tolerances are not tigh enough or when the dimensions of the sleeve and the ferrule do not match.

A fiber connector is characterized by several important parameters. As noted before, the most important factor is insertion loss, or simply connector loss. Another important factor is repeatability. If the same two fibers are connected through the same connector a number of times, each time the coupling will be slightly different. A good connector assmbly provides a small standard deviation for coupling efficiency across multiple insertions. Another desiralbe specification of a fiber connector is low return loss, i.e., a low back reflection. Return loss is defined as the ratio of the reflected power from the connector to the input power. For example, a return loss f 30 dB means 0.001 of the input power is reflected back from the connector. A conector must also be resistant and show a minimal coupling variation in the presence of normal mechanical forces such as axial and lateral forces. This is a practical requirement because in a normal environment it is likely for the connector to encounter a range of mechanical forces.

A wide range of connectors have been designed and are in use in the industry. Here we give an overview of some of the most popular types.

Straight tip or ST connectors are one of the more common type of connectors and in wide use in many applications. The ferrule diameter in an ST connector is 2.5 mm. ST connectors are spring loaded and enaged by a twist-and-lock mechanism.

Fixed connector or FC connectors use an alignment key and a threaded (screw-on) socket and are similar to the popular SMA connectors used in electronics. They are in wide use in single-mode applications and provide low insertion loss and high repeatability.

Subscriber connector, or SC, is another common type of connector. The advantage of SC connectors is that they are engaged by a push-and-snap mechanism, without the need for any roation. This make plugging and unplugging them very easy and also reduces wear out. Moreover, a higher connector density is achieved. Many transceivers provide either an SC receptacle connector, or a pigtail SC connector, as their optical interface. The push-and-snap feature of SC connectors thus provides very convenient and easy way of connecting to optical trasceivers. SC connectors are avaiable in simplex and duplex variations. The ferrule diameter in an SC connector is 2.5 mm.

SC fiber optic patch cable is one of the earliest stype and one of the most commonly used fiber optic cable, it is convenient to use and cost saving, SC fiber optic patch cord is widely uesed in fiber optic networks. SC fiber patch cable is with zirconia sleeve and plastic housing. The common type of SC connector patch cord, there are SC to SC fiber patch cord,  SC to LC Fiber Optic Patch Cable, SC to ST Fiber Optic Patch Cable,  SC to FC Fiber Optic Patch Cable, ect.

The LC or small form factor connector is similar to the SC, but with half the size. The diameter of the ferrule in an LC connector is 1.25 mm, vs 2.5 mm for most other connectors. This allow for twice the connector density for a given space. Because of their compactness, LC connectors have become more popular and are used in many high-end transceivers such as SFPs and XFPs.  LC fiber optic patch cable is with a small form factor (SFF) connector and is ideal for high density applications. LC fiber optic patch cord connector has a zirconia ceramic ferrule measuring 1.25mm O.D. with a PC or APC endface, and provides optimum insertion and return loss.

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