Intrusion detection system for use on an optical fiber using a translator of transmitted data for optimum monitoring conditions

• US 20060002650A1
• Filed: 05/26/2005
• Published: 01/05/2006
• Est. Priority Date: 07/18/2003
• Status: Active Grant

• Current Assignee: Network Integrity Systems, Inc.
• Original Assignee: Network Integrity Systems, Inc.
• Inventors: Vokey, David E., Murphy, Cary R., Bridges, Mark K.
• Granted Patent: US 7,092,586 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G02B 6/262 Optical details of coupling light into, or out of, or between fibre ends, e.g. special fibre end shapes or associated optical elements
• CPC Class Codes: G02B 6/4469 Security aspects
• CPC Class Codes: H04B 10/0771 Fault location on the transmission path

Abstract

A method and system of intrusion detection system for a multimode fiber optic cable. A light signal is launched into the cable fiber to establish a narrow spectral width, under-filled non-uniform mode field power distribution in the cable. A small portion of the higher order signal modes arriving at the remote end of the cable is sampled by use of a coupler and monitored for transient changes in the mode field power distribution. The power distribution changes with physical disturbance of the cable. When those changes are detected as being characteristic of fiber intrusion, the system activates an alarm. This method can sense and alarm any attempt to access the optical fibers in a fiber optic communication cable. In preferred embodiments, the active signal of a multimode optical fiber is monitored for both signal degradation and transient power disturbance patterns that could indicate fiber damage or physical intrusion. A translator can be provided in an existing optical fiber system in which the data signals are translated in wavelength and/or launch conditions to optimize the monitoring signals in an otherwise non-optimized system.

Claims

1. A method of detecting intrusion into a multimode optical fiber comprising:

launching a light signal into a proximal end of the fiber from a source of light that is offset from the center of a core of the optical fiber so as to establish a non-uniform mode field power distribution in the cable;

capturing a portion of higher order signal modes arriving at a distal end of the cable to provide a sample signal;

monitoring the sample signal for transient changes in the mode field power distribution that are characteristic of cable handling; and

activating an alarm in response to detection of said changes.

2. A method according to claim 1 wherein the source of light includes a single mode fiber which is coupled to the multimode optical fiber at the proximal end at a position which is offset from the center of the core.

3. A method according to claim 1 wherein the light signal is less-divergent than the numerical aperture of the multimode fiber.

4. A method according to claim 1 wherein the light signal establishes a narrow spectral width, under-filled, non-uniform mode field power distribution in the multimode optical fiber.

5. A method according to claim 1 wherein the higher order signal modes at the distal end are captured by a tap coupler.

6. A method according to claim 5 wherein the tap coupler taps off a small portion of the higher order modes for the sample signal and provides a larger separate signal forming a bulk optical launch distribution.

7. A method according to claim 6 wherein the tap coupler is made by a cladding-to-cladding fusion which provides a predetermined tap-coupling ratio.

8. A method according to claim 6 wherein the larger separate signal is delivered to a second detector circuit where an absolute throughput power of the launched light signal is calculated therefrom.

9. A method according to claim 6 wherein there is provided a microprocessor which compares the response in the sample signal and the separate larger signal and acts to calculate any power change as well as changes in modal power distribution such that excessive power change levels produce an alarm indication.

10. A method according to claim 9 wherein the microprocessor is arranged such that a significant change in the sample signal and the separate larger signal is used to indicate a problem with the source or the optical fiber while a transient and steady state change in the high order mode power distribution from the sample signal only would is used to provide an indication of an intrusion attempt.

11. A method according to claim 6 wherein the source is modified to communicate data and the separate larger signal is used to extract the data.

12. A method according to claim 6 wherein there is provide a second tap coupler connected to the larger separate signal forming a bulk optical launch distribution which taps off a second sample signal.

13. A method according to claim 1 wherein the light signal is launched into a proximal end of a first portion of the fiber which is connected to a remaining portion of the fiber by a coupler.

14. A method according to claim 1 wherein the light signal is launched into a proximal end of a first portion of the fiber which is connected to a remaining portion of the fiber by a coupler and wherein the coupler is a bulk optical wavelength division multiplexer which maintains the modal distribution of the light from the fiber, and combines into the remaining portion of the fiber both the light signal and a data signal from a data source where the data source is at a different wavelength from the light signal.