Intrusion detection system for use on single mode optical fiber using a polarimeter

• US 20060153520A1
• Filed: 06/15/2005
• Published: 07/13/2006
• Est. Priority Date: 01/12/2005
• 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 7,206,469 B2
• Time in Patent Office: Days
• US Class Current: 385/147
• CPC Class Codes: G02B 6/276 Removing selected polarisation component of light, i.e. polarizers
• CPC Class Codes: H04B 10/071 using a reflected signal, e.g. using optical time domain reflectometers [OTDR]
• CPC Class Codes: H04B 10/532 Polarisation modulation
• CPC Class Codes: H04B 10/85 Protection from unauthorised access, e.g. eavesdrop protection
• CPC Class Codes: H04L 63/1416 Event detection, e.g. attack signature detection

Abstract

A telecommunications optical fiber is secured against intrusion by detecting manipulation of the optical fiber prior to an intrusion event. This can be used in a non-locating system where the detection end is opposite the transmit end or in a locating system which uses Fresnel reflections and Rayleigh backscattering to the transmit end to detect and then locate the motion. The Rayleigh backscattering time sliced data can be stored in a register until an intrusion event is detected. The detection is carried out by a polarization detection system which includes an optical splitter which is manufactured in simplified form for economic construction. This uses a non-calibrated splitter and less than all four of the Stokes parameters. It can use a polarimeter type function limited to linear and circular polarization or two linear polarizers at 90 degrees.

Claims

1. A method for securing a telecommunications optical fiber against intrusion comprising:

providing a telecommunications optical fiber having a transmit end and a receive end;

providing for the optical fiber a transmitter of optical pulses at the transmit end and a receiver of the transmitted optical pulses at the receive end such that data can be transmitted along the fiber in a telecommunications system;

detecting manipulation of the optical fiber causing movement of a portion thereof along the length thereof by;

injecting polarized light pulses into an end of the optical fiber;

detecting at an end of the optical fiber a series of received light signals which have been transmitted along the fiber and which include reflected and/or backscattered components;

comparing in a polarimeter at least some of the received light signals relative to data obtained from previously received ones of the received light signals to detect changes of polarization of the received light signals relative to the previously received light signals;

analyzing the changes in polarization to determine any changes which are indicative of manipulation of the optical fiber causing movement of a portion thereof along the length thereof;

and generating an alarm in response to the detection of any such changes which are indicative of manipulation of the optical fiber causing movement of a portion thereof along the length thereof.

2. The method according to claim 1 including determining the location along the fiber of the said manipulation by:

detecting the signals at the same end as the light pulses are injected such that the signals contain reflected and/or Rayleigh backscattered components;

detecting polarization of a series of the light signals from the Rayleigh backscattering components in discrete time steps to generate for each time step data relating to the polarization;

such that the stored data is time dependent and thus indicative of a time of travel of the light signals and thus of a location of the position from which the Rayleigh backscattering components have originated along the fiber;

storing the data in a register for a period of time and discarding the data after the period of time and replacing it with fresh data;

and, in the event that movement is detected of the optical fiber, extracting the data from the register and analyzing the polarization of the series of signals to detect the location of the movement.

3. The method according to claim 2 wherein the register is a FIFO.

4. The method according to claim 2 wherein the scattering signal level is typically orders of magnitude lower than the Fresnel Reflections and the Fresnel Reflections are typically infrequent and wherein the reflections are integrated along with the scattering such that the Fresnel Reflections integrate into a manageable signal and the total integrated signal is monitored for indication of fiber manipulation.

5. The method according to claim 2 wherein the scattering signal level is typically orders of magnitude lower than the Fresnel Reflections and the Fresnel Reflections are typically infrequent and wherein the large Fresnel reflections are sampled using a storage technique, this stored sample is compared to other dynamic or stored samples and this comparison is monitored for indication of fiber manipulation.

6. The method according to claim 1 wherein the changes in polarization are detected by splitting the received light signals into at least first and second paths and using the first path to detect an amplitude of the light in the path when linearly polarized and using the second path to detect an amplitude of the light in the path when circularly polarized.

7. The method according to claim 6 wherein the circularly polarized light in the second path is isolated by a quarter wave retarder and a linear polarizer.

8. The method according to claim 6 wherein the light signals are split into the separate paths by a splitter.

9. The method according to claim 8 wherein the splitter is selected such that the state of polarization (SOP) of the signals in the separate paths are NOT necessarily maintained relative to an absolute reference as would be required in a standard polarimeter.

10. The method according to claim 6 wherein the light signals are split into the separate paths by an optical switch which separates the signals in time division such that the paths are selected sequentially.

11. The method according to claim 10 wherein there is provided a second optical switch or a coupler for supplying the signals from the separate paths to a single receiving system for analyzing the amplitude.

12. The method according to claim 10 wherein the light in the second path is circularly polarized by a quarter wave retarder and a linear polarizer and wherein the second path is fed back to the optical switch so as to use a single linear polarizer for both the first and second paths.

13. The method according to claim 6 wherein an absolute value is obtained of the change in amplitude in each path between a signal and previous signals and the absolute values are summed together to provide an output for analysis.

14. The method according to claim 1 wherein the signals are detected at the opposite end of the optical fiber from which the pulses are injected.

15. The method according to claim 1 wherein the polarimeter is a full four path polarimeter providing the four Stokes parameters.

16. The method according to claim 1 wherein the polarimeter has less than four paths providing less than the four Stokes parameters.