Terabits of data containing sometimes mundane, sometimes pointed analyses of the disposition of our allies and potential foes flood through the arteries of the US intelligence network every day. Securing these networks is critical to maintaining a strategic advantage.Computer hackers' defiant infiltration of some of the most protected networks has led the government to develop local area networks restricted from public access. Still, someone with enough persistence and initiative can compromise them. Even if the military has its own network, spies can splice into the system to recover information. To address this problem, a fiber optic system that formerly was used for perimeter security has been tested and approved for a Protected Distribution System that secures voice, computer and fax lines.Traditionally, a secure local area network was implemented by hooking a cypher, encoder and decoder to the line, but this is very expensive. It also requires continued maintenance and slows down the data rate. However, the Fiber Defender, a fiber optic line sensor from Fiber SenSys Inc., can be embedded in the coaxial cable where it detects attempts to cut into the line, without affecting data rates.The device relies on intermodal mixing to detect pressure, vibration and motion caused by an intruder. Intermodal mixing occurs when a disturbance alters the modes of light launched down a multimode fiber. Changes in the phase relationship are compared against attributes defined by preset parameters and determine whether the disturbance is from tampering or benign effects, such as road noise or heat. Intrusion or compromise of the system is indicated via a relay output.Joe Cardyn of Security Solutions has used the Fiber Defender on fences and in underground applications. He said, however, that the "breakthrough" application of this technology is its use in classified communications, which he installed for the US Air Force.Now the cables can be installed in walls and ceilings rather than underground without danger of having the messages compromised.