As is known, radio and unprotected point-to-point optical communication systems do not guarantee privacy since it is simple to detect and decode the radiated signal. Propagation by electrical signals in a wire, coaxial cable or waveguide likewise do not guarantee privacy since there is always some stray radiation and since it is relatively simple to tap into the electrical system. On the other hand, optical waveguides (i.e., fiber optics) offer greater privacy since there is no stray electromagnetic radiation. However, it is still possible to deliberately penetrate a cladding around the optical signal-carrying fiber or fibers and extract a signal unless the complete length of fiber cable is maintained secure. For example, the cladding may be removed at a point along the length of the cable without detection and a signal extracted by a probe. Various types of probes can be used such as a Tee-connection, a prism, the core of an optical fiber or an electro-optical detector.
In the past, encryption (i.e., secret coding) of the signal has been required to avoid interpretation. Encryption equipment, however, is costly and introduces additional data processing delays.