Most fiber optic connectors are manufactured

Most fiber optic connectors are manufactured so that the fiber is mounted on a protruding splint, called a "male" style connector. Two male connectors are attached using a coupling adapter that keeps the splints aligned and allows them to be in the center. If connectors like these are being tested and the test equipment has interfaces that fit these connectors, the single wire reference (OFSTP-14 Method B) can be used. This method is the simplest method and is generally considered the preferred method since connections are not included when establishing the 0 dB reference.

After establishing a reference, the launch cable is disconnected from the meter, but not from the source. The reference launch cable should never be disconnected from the source after the reference is established, in order to ensure that the launch power remains constant. The receiving cable is connected to the meter and then both reference cables are connected to the cable to be tested. The loss reading will include both connections to the cable under test and the loss of the fiber and any other component in the cable.

If the test equipment has an interface for a connector of another style, so that the connectors on the cables being tested cannot be attached to the instruments, a two-wire reference method can be used (OFSTP-14 Method A ). The reference cables must be hybrid cables with connectors at one end to fit the interface of the instruments and the other end to fit the connectors on the cable to be tested. The 0 dB reference is established by joining the two reference cables to the instruments and connecting the other ends with a coupling adapter. After setting the reference,

The loss reading will include both connections to the cable being tested and the loss of the fiber and any other component in the cable minus the loss of the connection between the two reference cables when establishing the reference. Therefore, the measured loss obtained with the two-wire reference will be less than the single-wire reference for the connection that is included when establishing the reference. The uncertainty of this loss of connection included in the reference also adds to the uncertainty of the loss measurement of any cable that is tested in this way.

Some fiber optic connectors have "male" and "female" style connectors, in which one has a splint that protrudes while the other has a female plug or receptacle. Some have alignment pins only on one side, such as the MTP connector where the pins are used on the side of the female socket. They are generally used with male plugs on both ends of the connecting cables and female plugs or receptacles on permanently installed cables that terminate in racks or sockets.

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Either of these two connector styles can only be coupled to an appropriate connector style, which makes it difficult to make a reference of one or two wires. The solution is a three-wire reference (OFSTP-14 Method C), in which the hybrid cables connected to the instruments for the reference cables terminate in male plugs and a third cable terminated in female plugs is inserted between them to create a 3 wire reference. After setting the reference, the two reference cables are disconnected from the third cable in the middle, and the cable to be tested is inserted between them instead of the reference cable.

As mentioned earlier, the loss reading will include both connections to the cable being tested and the loss of the fiber and any other component in the cable minus the loss of the two connections between the third reference cable and the two reference cables to the set the reference. Since generally, the third cable is only a short length fiber with connections at each end, fiber loss can be ignored. Therefore, the loss measured with the three-wire reference will be less than the single cable reference for the two connections included when setting the reference. The uncertainty of these two connection losses included in the reference also adds to the uncertainty of the loss measurement of any cable that is tested in this way.

Although this three-wire method has the greatest uncertainty, it is the only method that works for any connector and test equipment. Therefore, it has become the preferred method in several international standards.