Fiber Optic Connector Ferrule Design

In the earliest days of optical fiber jointing the sophisticated manufacturing techniques now associated with demountable connectors were not available. Instead existing components or technology had to be used or modified to provide an acceptable level of insertion loss.

The most obvious method was to use a machined V-groove as an alignment tool and to secure the two fiber ends within metal tubes or ferrules in such a way as to ensure acceptable performance. It is relatively easy to produce V-grooves and tubes to the required tolerances: the difficult task was to align the fibers within the tubes themselves.

These basic ferrule designs resorted to watch jewels made from synthetic ruby or sapphire which were inserted into the tubes (see the following figure).

These jewelled ferrules are still used as methods of connection to test equipment as they are simple to terminate and their performance is limited by the hole diameter, concentricity and fitting tolerances of the jewel within the tube.

Basic ferrule designs resorted to watch jewels

The first true demountable connector was the SMA (subminiature ‘A’) which was loosely based upon the design of the electrical connector of the same name.

It featured an all-metal ferrule together with a rear body which allowed stable connection to the incoming cable. The rear body provided connection to the alignment tool (a threaded tube) by means of a captive nut.

MTP-MPO Patch Cords

Connector. The complete assembly attached to the fiber optic cable. In general these are of male configuration requiring a female component to allow the jointing of two connectors. In technical literature the connector may be termed a plug.

Adaptor. The female component used to join two connectors. The adaptor is responsible for providing the alignment of the connectors and the fibers within them. Other terms frequently used are: uniters, couplers, sockets, receptacles.

Termination. The process of attaching a connector is to a fiber optic cable element (and also the name given to the completed assembly).

Most fiber optic connectors comprise a ferrule (or in the case of multi-element devices, a number of ferrules) which is responsible for the control of fiber alignment and a rear body which is responsible for the attachment of the connector to the cable. The rear body can be complex in construction and has additional responsibilities for the connection of the connector to the adaptor.

The mechanics of the ferrules and rear bodies vary from connector to connector and will be discussed for each connector type. The ferrule is usually made of a ceramic material, because of its hardness and thermal stability, although polymer ferrules are now accepted as being nearly as good.

The particular ceramic is known as zirconia (zirconium dioxide ZrO2) chosen because its hardness and thermal coefficient of expansion is very similar to glass. It also polishes well. To be more specific the material is usually tetragonal zirconia poly-crystal, which is zirconia and yttria (yttrium oxide Y2O3).

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