Patent ID: 7376293

Claim:
A method of detecting intrusion into a multimode optical fiber comprising: launching a monitoring light signal into one end of the optical fiber to be monitored from a source of light so as to establish a non-uniform mode field power distribution in the optical fiber; capturing a portion of higher order signal modes passing through the fiber to provide a sample signal; monitoring the captured portion of the signal for transient changes in the mode field power distribution that are characteristic of fiber handling; and activating an alarm in response to detection of said changes; wherein the launch is localized to a desired point by use of a single mode jumper fiber up to that location such that locally launching the light eliminates sensitivity to intrusion of all lead-in fiber; wherein the captured sample signal output is launched into a delivery multi-mode fiber connected to a detection system such that delivering the captured signal using multi-mode fiber eliminates sensitivity to intrusion of the lead-out fiber.