Intrusion detection system for a multimode optical fiber using a bulk optical wavelength division multiplexer for maintaining modal power distribution

• US 7,403,674 B2
• Filed: 08/02/2006
• Issued: 07/22/2008
• 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.
• Primary Examiner(s): CONNELLY, MICHELLE R
• Application Number:
  US11/461,905
• Publication Number:
  US 20070086693A1
• Time in Patent Office: 720 Days
• Field of Search: 385/12, 385/13, 385/29
• US Class Current: 385/12
• CPC Class Codes: G02B 6/2852 using tapping light guides arranged sidewardly, e.g. in a non-parallel relationship with respect to the bus light guides (light extraction or launching through cladding, with or without surface discontinuities, bent structures)
• CPC Class Codes: G02B 6/29385 Channel monitoring, e.g. by tapping channel monitoring in optical transmission systems H04B10/07
• CPC Class Codes: G02B 6/421 the intermediate optical component consisting of a short length of fibre, e.g. fibre stub
• CPC Class Codes: H04B 10/2581 Multimode transmission
• CPC Class Codes: H04B 10/85 Protection from unauthorised access, e.g. eavesdrop protection

Abstract

Intrusion detection of one section only of a multimode fiber uses a light signal launched into the fiber at a location spaced from the source through a single mode fiber to establish a narrow spectral width, under-filled non-uniform mode field power distribution in the fiber. A small portion of the higher order signal modes at the a second location also spaced from the destination is sampled by a tap coupler and monitored for transient changes in the mode field power distribution which are characteristic of intrusion to activate an alarm. A fiber being used for data transmission can be monitored for intrusion by introducing a monitor wavelength different from that of the data signal. Central to this invention is the use of a bulk optic (commonly referred to as a pass/reflect) wavelength division multiplexer, one which maintains the modal distribution within the fiber.

Claims

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

launching a monitoring light signal into one end of the optical fiber from a source of light so as to establish a non-uniform mode field power distribution in the optical fiber;

launching into said one end of the optical fiber a data signal at a different wavelength from the monitoring signal for transmission along the fiber with the monitoring signal;

using a coupler to separate the data signal from the monitoring signal;

capturing a portion of higher order signal modes of the monitoring signal passing through the fiber 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

wherein the coupler used for separating the data signal from the monitoring signal comprises a bulk optical wavelength division multiplexer.

2. The 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.

3. The method according to claim 1 wherein the signal is captured by a tap coupler which is made by a cladding-to-cladding fusion which provides a predetermined tap-coupling ratio.

4. The method according to claim 1 wherein the monitoring light signal and the data signal are launched into the said end of the optical fiber by a bulk optical wavelength division multiplexer.