Fiber optic connectors made of ferrules are known as Ferrule connectors. Ferrule connectors have two cylindrical plugs. Other parts of a ferrule connector include an alignment sleeve, and sometimes axial springs to perform fiber alignment.
Ferrule connectors have high precision holes for insertion of fibers. These holes are usually drilled or molded through the center of the ferrule. The holes allow insertion of one coating stripped optical fiber. By allowing a single fiber insertion, precise alignment of fiber inside the hole can be ensured. There would not be much gap for the fiber to move inside the ferrule hole.
The central hole is very important for ferrule connectors as precise fiber alignment depends on the accuracy of the central hole of each ferrule. In order to fix the fiber inside the hole, an adhesive is also applied when inserting the fiber. This adhesive is generally an epoxy resin. This holds the fiber intact and does not allow it to move
Fiber insertion precision is so important in connector making process as it decides the connector loss. Low levels of connection loss are ensured by different methods. The fiber-end faces are polished until they are flush with the end of the ferrule. Actual alignment of optical fiber happens as the ferrules are inserted into the alignment sleeve. The inner diameter of the alignment sleeve aligns the ferrules. This helps the fiber to align properly. Some kind of coupling mechanism is used to lock the ferrules in the alignment sleeve. A threaded shell is a commonly used one.
The below diagram will give a basic idea how a ferrule connector will look like.
Fiber alignment depends on an accuracy of hole through the center of the ferrule. Two types of commonly employed ferrules are ceramic and metallic. Drilling method is used to make the central hole in metallic ferrule connectors. Drilling individual ferrules may not yield exactly same parameters technically. Therefore an accurate hole through the entire for a metal ferrule is difficult.
Some manufacturers employ precision watch-jeweled centering method to to improve fiber alignment. A watch jewel with a precision centered hole is placed in the tip of the ferrule in precision watch-jeweled centering. The central hole of the watch jewel centers the fiber with respect to the axis of the cylindrical ferrule. This method ensures better fiber alignment as the regulation of the hole tolerance of the watch jewel is easier than maintaining a precise hole diameter when drilling through an entire ferrule.
Contrary to the metallic ferrules, in ceramic ferrules, the center hole is made by forming the ferrule around a precision wire. This method gives out accurately centered hole in the ferrule. Most of the available cylindrical ferrule connectors now use ceramic ferrules.