12.2   FIBER OPTIC CONNECTORS

In many situations we need to have a demountable connection between fibers, between
a fiber and a source or a detector, or between different fiber optic instruments.
Demountable connections are achieved using fibers with connectorized terminations.
Different designs of connectors have been developed over many years with the aim
of reducing the loss or for ease of connection. The connector should ensure that the
fiber core is aligned with the laser or the detector or the fiber to which it is being
connected. Due to the smallness of the core of the fiber, connectors are high-precision
devices with tolerances of less than 1
m. Connector losses between two fibers will
be minimized if the fibers are identical and are perfectly aligned and touch each other.
Any gap between the two fibers will lead to a small loss but also to reflections from
the fiber ends. This reflection can lead to problems, especially when they enter a laser
diode. Hence, back reflection loss should be minimized in a connector. It is possible
to reduce back reflection by using angle-polished end faces, since reflection from the
ends do not couple back into the fiber provided that the angle of the polish is chosen
appropriately. In a connector the fiber ends are polished and fixed. Any dirt on the
fiber end can lead to losses when the connector is used.

Each connector consists of four major components: the ferrule, the connector
body, the cable, and the coupling device. The ferrule is a long, thin cylindrical device,
usually made of metal or ceramic, with a hole drilled through its center. The hole
diameter is slightly larger than the cladding diameter so the fiber can be positioned
snugly within the ferrule. Connectors with different ferrule shapes or finishes are
available. Most currently used ferrules allow for physical contact between the two
fibers being connected. Better physical contact is achieved by using a convex surface
for the ferrules.

The connector body is usually made out of metal or plastic and holds the ferrule that
carries the fiber. The ferrule extends beyond the connector body so as to be able to connect
to another connector. The cable jacket and strength members are usually bonded
or crimped to the connector body. The cable is attached to the body of the connector
and acts as the point of entry for the fiber. The coupling device helps in the alignment of
two fibers. Figure 12.1 shows a picture with different standard connectors: ST (straight
terminus), SC (subscriber connector), FC (fiber connector), and LC (lucent connector).
The connector losses are typically 0.15 to 0.3 dB, and the return loss (which is
the ratio of the power reflected to the power in the forward direction) is about 55 dB.