Page 141 - Optical Communications Essentials

Connectors and Splices

Connectors and Splices 131

receptacle. The center of the ferrule has a hole that precisely matches the size
of the fiber cladding diameter. Typically, the hole size is 125.0
1.0µm. The
fiber is secured in the hole with epoxy, and the end of the ferrule is polished flat
to a smooth finish. The mechanical challenges in fabricating a ferrule include
maintaining both the dimensions of the hole diameter and the position of the
hole relative to the ferrule outer surface.
Around 95 percent of the ferrules used in optical connectors are made of
ceramic material due to some of the desirable properties they possess. These
include low insertion loss required for optical transmission, remarkable strength,
small elasticity coefficient, easy control of product characteristics, and strong
resistance to changes in environmental conditions such as temperature. This is
also why ferrules commonly are referred to as ceramic ferrules.
As shown in Fig. 8.11, since both ends of a cable have the same ferrule struc-
ture, an adapter is used to align the two fiber ends. Two popular adapter designs
are the straight-sleeve and the biconical (or tapered-sleeve) mechanisms. In
the straight-sleeve connector, the length of the sleeve and a guide ring on the fer-
rule determine the end separation of the two fibers. The biconical connector uses
a tapered sleeve to accept and guide tapered ferrules. Again, the sleeve length and
the guide rings on the ferrule maintain a given fiber end separation. Of these two
coupling mechanisms, the straight-sleeve connector is in much wider use.
Note that an adapter also can be used to mate one type of connector to
another. The only precaution in any case is to make sure that either the two
fibers being joined have similar characteristics or that one is not trying to cou-
ple light from a multimode fiber into a single-mode fiber. However, it is not a
problem to have light from a single-mode fiber couple into a multimode fiber.
For example, this often is done at the receiving end of a link where a multimode
flylead on a photodetector receives light from a single-mode transmission fiber.
Normally a connector also has some type of strain relief mechanism called a
boot, which shields the junction of the connector body and the cable from bends
and pulls. The connector figures in Sec. 8.6 show examples of boots.

8.6. Optical Connector Types
Since it is taking some time to establish standards in the optical connector
industry, several large companies developed their own particular design. This
has resulted in numerous connector styles and configurations. Typically, the dif-
ferent connector types are designated by combinations of two or three letters.
The main ones are ST, SC, FC, LC, MU, MT-RJ, MPO, and variations on MPO.
Therefore the vendors refer to these connectors as, for example, SC-type con-
nectors or simply SC connectors. Each of these connector types is described in
greater detail below. However, to understand the purpose of the seemingly arbi-
trary letter designations, let us first take a brief look at their origins:

■ ST is derived from the words straight tip, which refers to the ferrule configu-
ration.

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