Intrusion detection system for secure fiber optics

• US 6,819,849 B1
• Filed: 07/29/1988
• Issued: 11/16/2004
• Est. Priority Date: 10/25/1985
• Status: Active Grant

• Current Assignee: DIRECTV, LLC (AT&T, Inc.)
• Original Assignee: Hughes Electronics Corporation (AT&T, Inc.)
• Inventors: Pedinoff, Melvin E., Tangonan, Gregory L.
• Primary Examiner(s): Moskowitz, Nelson
• Application Number:
  US07/228,793
• Time in Patent Office: 5,954 Days
• Field of Search: 350/96.06, 350/96.1, 350/96.23, 350/96.33, 350/96.31, 350/30, 356/237, 455/610, 455/612, 332/7.51, 385/12, 385/31, 385/124, 385/126, 385/127, 385/142, 398/142, 398/143
• US Class Current: 385/126
• CPC Class Codes: G02B 6/0288 Multimode fibre, e.g. graded index core for compensating modal dispersion

Abstract

Methods and apparatus are disclosed for an intrusion detection system for secure fiber optic communication systems. A guard signal is transmitted over the optical fiber communication link and both the modal power and modal phase delay of this signal are monitored. Intrusions into the link for the purpose of intercepting information being transmitted, cause changes in the modal phase delay and power of the guard signal. These changes, when detected by the monitoring system, indicate the presence of an intruder. A fiber profile which is optimal for both transmission and intrusion detection purposes is described.

Claims

1. A system for detecting intrusions into a fiber optic communication system comprising:

(a) an optical fiber having a cladding and a primary core having an index profile with at least first, second and third graded index regions with intermediate first and second step index regions interposed between first and second, and second and third graded index regions, respectively, said index profile being uniform along the length of said fiber, said index profile for enhancing modal delay variation;

(b) first means optically coupled to one end of said fiber to launch a guard signal modulated optical beam in an entry point of said fiber for transmission by said fiber, said guard beam being capable of experiencing fluctuation in the magnitudes of modal delay and power attenuation responsively to intrusion into said fiber; and

(c) second means optically coupled to the other end of said fiber for detecting the fluctuations in the magnitudes of the differential modal delay and power attenuation of said guard beam.

2. The system of claim 1 wherein said first means further includes means for varying said entry point of said optical beam into said fiber.

3. The system of claim 1 wherein said second means further includes an alarm for signalling detected intrusions.

4. The system of claim 1, wherein said fiber is the communication link of a point-to-point communication system.

5. The system of claim 1 wherein said fiber is the communication link of a local area network communication system.

6. The system of claim 5 wherein said local area network has a ring configuration and said second means is located at the same location as said first means.

7. An intrusion detection system for a fiber optic communication system comprising:

(a) an optical fiber having a cladding and a primary core having an index profile with at least first, second and third graded index regions with intermediate first and second step index regions interposed between first and second, and second and third graded index regions, respectively, said index profile being uniform along the length of said fiber, said index profile for enhancing modal delay variation;

(b) first means optically connected to one end of said fiber to launch a guard signal modulated optical beam into an entry point of said fiber; and

(c) second means optically coupled to the other end of said fiber to monitor the modal delays of said guard beam.

8. The system of claim 7 wherein said first means further includes means for varying said entry point of said optical beam into said fiber.

9. The system of claim 7 wherein said second means further includes an alarm for signalling detected intrusions.

10. An intrusion detection system for a fiber optic communications system comprising:

(a) an optical fiber operating as a single mode fiber at the data transmission wavelength;

(b) first means optically coupled to one end of said fiber to launch a guard signal modulated optical beam into an entry point of said fiber, said guard beam having a wavelength such that said fiber acts as a multi-mode fiber at said guard beam wavelength; and

(c) second means optically coupled to the other end of said fiber to monitor the modal delays of said guard beam.

11. The intrusion detection system of claim 10 wherein said second means monitors said modal delays in combination with power attenuation of said guard beam.

12. A method of detecting intrusion into a fiber optic communication system comprising the steps of:

(a) transmitting a guard signal modulated optical beam through a primary core of an optical fiber, said primary core having an index profile with at least first, second and third graded index regions with intermediate step index regions interposed between first and second, and second and third graded index regions, respectively;

(b) monitoring the differential modal phase delays of said guard signal modulated optical beam; and

(c) detecting changes in the differential modal phase delays to detect the presence of said intrusion.

13. The method of claim 12 including the step of signalling the presence of said detected intrusion.

14. The method of claim 13 including the further step of signalling the presence of said detected intrusion.

15. A method of selecting optical fibers, suitable for use in a secure optical communication system, from a plurality of optical fibers suitable for optical communication, comprising the steps of:

(a) launching a guard signal modulated optical beam into an entry point of each one of said plurality of optical fibers;

(b) measuring a first modal delay profile of each one of said plurality of optical fibers without perturbation;

(c) perturbing each one of said plurality of optical fibers by applying perturbation of the same minimal magnitude;

(d) measuring a second modal delay profile of each one of said plurality of optical fibers when subjected to said perturbation; and

(e) comparing the first and second modal delay profile of each one of said plurality of optical fibers so as to ascertain the extent of fluctuation in modal phase delay magnitude with said minimal perturbation; and

(f) selecting optical fibers exhibiting sufficient fluctuation in modal phase delay magnitude for purposes of intrusion detection.

16. An apparatus for testing fibers to determine their suitability for use in intrusion detection systems for secure optical communications comprising:

(a) said fiber;

(b) first means optically coupled to one end of said fiber to launch a guard signal modulated optical beam into an entry point of said fiber;

(c) second means optically coupled to the other end of said fiber to determine the modal delay profile of said fiber; and

(d) third means connected to said fiber at some intermediate point between said one end and said other end for perturbing said fiber.

17. An optical fiber for secure transmission of an optical beam therethrough and for enhanced sensitivity to intrusion, comprising:

a cladding, and

a primary core having a cross-sectional index profile, said index profile having at least first, second and third regions having a graded index, a first step index region interposed between said first and second regions, and a second step index region interposed between said second and third regions,

said index profile being uniform along the length of said fiber,

said index profile for enhancing modal phase delay variation in said beam responsively to said intrusion.