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Fiber tapping uses a network tap method that extracts signal from an optical fiber 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 beam 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 surreptitiously tap fibre for surveillance, although this is rarely done where electronic equipment used in telecommunication 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 and a nuclear submarine, the Jimmy Carter, was modified to allow tapping undersea cables. 
Detecting fiber taps
One way to detect fiber tapping is by noting increased attenuation 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 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.
See the NCS reference below for a more complete explanation.
One counter-measure is encryption  to make the stolen data unintelligible to the thief. However, encryption can be an expensive solution, and there are also concerns about network bandwidth when it is used.
Another counter-measure is to deploy a fiber-optic sensor into the existing raceway, conduit or armored cable. In this scenario, anyone attempting to physically access the data (copper or fiber infrastructure) is detected by the alarm system. A small number of alarm systems manufacturers provide a simple way to monitor the optical fiber for physical intrusion disturbances. There is also a proven solution that utilizes existing unused fiber (dark fiber) in a multi-strand cable for the purpose of creating an alarm system.
In the alarmed cable scenario, the sensing mechanism uses optical interferometry in which modally dispersive coherent light traveling through the multi-mode fiber mixes at the fiber's terminus, resulting in a characteristic pattern of light and dark splotches called a speckle pattern. The laser speckle is stable as long as the fiber remains immobile, but flickers when the fiber is vibrated. A fiber-optic sensor works by measuring the time dependence of this speckle pattern and applying digital signal processing to the Fast Fourier Transform (FFT) of the temporal data.
The U.S. government has been concerned about the tapping threat for many years, and it also has a concern about other forms of intentional or accidental physical intrusion. In the context of classified information Department of Defense (DOD) networks, Protective distribution system (PDS) is a set of military instructions and guidelines for network physical protection. PDS is defined a system of carriers (raceways, conduits, ducts, etc.) that are used to distribute Military and National Security Information (NSI) between two or more controlled areas or from a controlled area through an area of lesser classification, i.e., outside the Sensitive Compartmented Information Facility (SCIF) or other similar area. National Security Telecommunications and Information Systems Security Instruction (NSTISSI 7003), Protective Distribution Systems (PDS), provides guidance for the protection of SIPRNet wire line and optical fiber PDS to transmit unencrypted classified National Security Information (NSI).
- National Communications Systems Technical Information Bulletin 00-7 (2000) "All Optical Networks" (pdf), includes a tutorial on how to tap fiber and detect it.