Fiber tapping is a method to extract signal from an optical fibre without breaking the connection. Tapping of optical fibre allows diverting some of the signal being transmitted in the core of the fibre into another fibre or a detector. Fibre to the home (FTTH) systems use splitters to allow many users to share one backbone fibre connecting to a central office, cutting the cost of each connection to the home. Test equipment can simply put a bend in the fibre and extract sufficient light to identify a fibre or determine if a signal is present.

Similar techniques can be used to surreptitiously tap fibre for surveillance, although this is rarely done since all electronic equipment used in telecommunications is required to allow access to any phone line for tapping by legal authorization. Tapping the fibre means that all signals from every communications source being routed through the fibre are presented and must be sorted for relevant data, an immense task when thousands of sources of data or voice may be present.

According to reports, tapping fibre was used by the US government for surveillance following the September 11, 2001 attacks

September 11, 2001 attacks

The September 11 attacks The September 11 attacks The September 11 attacks (also referred to as September 11, September 11th or 9/119/11 is pronounced "nine eleven". The slash is not part of the pronunciation...

 and a nuclear submarine

Nuclear submarine

A nuclear submarine is a submarine powered by a nuclear reactor . The performance advantages of nuclear submarines over "conventional" submarines are considerable: nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for...

, the Jimmy Carter

USS Jimmy Carter (SSN-23)

USS Jimmy Carter , the third and last Seawolf-class submarine, is the first ship of the United States Navy to be named for former President Jimmy Carter, who served in the United States Navy as a Communications Officer, Sonar Officer, Electronics Officer, Weapons Officer, and Supply Officer while...

, was modified to allow tapping undersea cables.

How to detect fiber tapping

One way to detect fiber tapping is by noting increased attenuation

Attenuation

In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

 added at the point of tapping. There are, however, tappers which allow tapping without significant added attenuation.

In either case there should be a change of scattering

Scattering

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...

 pattern in that point in line which, potentially, can be detectable. However once the tapper has been detected it may be too late since a part of the information has been already eavesdropped

Eavesdropping

Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...

.

See the NCS reference below for a more complete explanation.

Counter-measures

One counter-measure is encryption

Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

of data. This does not prevent tapping but does make the stolen data unintelligible to the thief.