Coherent alarm for a secure communication system

• US 5,694,114 A
• Filed: 05/06/1994
• Issued: 12/02/1997
• Est. Priority Date: 09/04/1992
• Status: Expired due to Term

• Current Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Original Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Inventors: Udd, Eric
• Primary Examiner(s): Crosland, Donnie L.
• Application Number:
  US08/238,954
• Time in Patent Office: 1,306 Days
• Field of Search: 340/506, 340/555, 340/556, 340/531, 340/508, 340/557, 359/109, 359/119, 359/143, 359/151, 359/152, 359/174, 359/177, 359/164, 359/118, 359/173, 359/181-184, 359/188, 359/195, 250/227.15, 250/227.16, 250/227.19, 356/73.1, 356/35.5, 356/345, 356/350, 385/13
• US Class Current: 340/506
• CPC Class Codes: G01M 11/39 in which light is projected from both sides of the fiber or waveguide end-face
• CPC Class Codes: H04B 10/85 Protection from unauthorised access, e.g. eavesdrop protection

Abstract

A high speed secure fiber optic communication system that includes a coherent alarm system utilizes a pair of single mode fiber optic cables in combination with one or more light sources, phase modulators, detectors and polarization scrambling elements to form a Sagnac interferometer. The phase modulator is driven so that counter propagating light beams in the Sagnac loop experience a different optical path as they pass through the loop. When the two beams are recombined on the central beamsplitter of the Sagnac loop, the two beams interfere with each other and the data impressed as phase modulation on the light beams by the phase modulator is recovered as amplitude modulation on the output detector of the Sagnac interferometer. The coherent alarm system applies a relatively low frequency background signal to the Sagnac interferometer and watches for changes occurring to the background signal that indicate the presence of an intruder. Although the system is very secure, such coherent alarm systems with or without other alarms, intrusion control, random pathlength changes and the like can make undetected, unauthorized access to the system impossible with available interception techniques.

Claims

1. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on said counter propagating light beams so that said information appears in said interferometric output;

output light detector means connected to receive said interferometric output and to produce therefrom an output signal representative of said information; and

an alarm system connected to said Sagnac loop to indicate unauthorized access to said Sagnac loop, said alarm system including;

means to apply a sinusoidal signal as an input to said optical phase modulator at a second frequency lower than said first frequency whereby said sinusoidal signal is impressed on said counter propagating light beams so that said sinusoidal signal appears in said interferometric output as an alarm signal having;

at least 1st and 2nd harmonics; and

means to extract harmonics from said alarm signal, excessive energy in harmonics above said 2nd harmonic being an indication of unauthorized access to said Sagnac loop.

2. The alarmed communication system as defined in claim 1 wherein said means to extract harmonics from said alarm signal include:

a synchronous demodulator connected to receive said alarm signal and to produce therefrom;

a first alarm output signal representative of the level of said first harmonic of said alarm signal; and

a second alarm output signal representative of the level of said second harmonic of said alarm signal; and

means to ratio said first and second alarm output signals.

3. The alarmed communication system as defined in claim 1 wherein said means to extract harmonics from said alarm signal include:

an array of synchronous demodulators to demodulate 1 through n harmonics of said alarm signal and to produce therefrom;

1 through n alarm output signals representative of the levels of the 1 through n harmonics of said alarm signal;

means to sum said 1 through n alarm output signals into a combined harmonic output; and

means to produce an alarm indication if said combined harmonic output changes.

4. The alarmed communication system as defined in claim 3 wherein said alarm system further includes:

an alarm detector; and

an alarm beamsplitter positioned to transmit a portion of said interferometric output to said alarm detector, said array of synchronous demodulators being connected to receive said alarm signal from said alarm detector.

5. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on said counter propagating light beams so that said information appears in said interferometric output;

output light detector means connected to receive said interferometric output and to produce therefrom an output signal representative of said information; and

an alarm system connected to said Sagnac loop to indicate unauthorized access to said Sagnac loop, said alarm system including;

means to apply an alarm input signal to said Sagnac loop at a second frequency lower than said first frequency whereby said alarm input signal is impressed on said counter propagating light beams; and

extraction means to extract harmonics of said alarm input signal from said interferometric output, excessive energy in selected harmonics being an indication of unauthorized access to said Sagnac loop, said extraction means including;

a beamsplitter connected to said output light detector means to split a portion of said interferometric output; and

alarm detector means connected to said beamsplitter to convert said portion of said interferometric output into an electrical alarm signal.

6. The alarmed communication system as defined in claim 5 wherein said Sagnac loop includes:

a central beamsplitter; and

a center opposite said central beamsplitter, said optical phase modulator being positioned in said Sagnac loop spaced from said central beamsplitter and said center opposite said central beamsplitter.

7. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on said counter propagating light beams so that said information appears in said interferometric output;

output light detector means connected to receive said interferometric output and to produce therefrom an output signal representative of said information; and

an alarm system connected to said Sagnac loop to, indicate unauthorized access to said Sagnac loop, said Sagnac loop including;

a central beamsplitter; and

a center opposite said central beamsplitter, said optical phase modulator being positioned in said Sagnac loop spaced from said central beamsplitter and said center opposite said central beamsplitter, and said alarm system including;

an alarm optical phase modulator remote from said light source and in said Sagnac loop for impressing alarm signals at a second frequency on said counter propagating light beams so that alarm signals appear in said interferometric output.

8. The alarmed communication system as defined in claim 7 wherein said alarm system also includes:

signal generator means connected to said alarm optical phase modulator to supply alarm signals to said alarm optical phase modulator.

9. The alarmed communication system as defined in claim 8 wherein said signal generator means supplies half wave alarm signals to said alarm optical phase modulator at a frequency equal to X{2Ln/c} where X equals an odd integer, L is the distance in said Sagnac loop between said alarm optical phase modulator and said center, c is the speed of light in vacuum, and n is the index of refraction of said Sagnac loop on opposite sides of said center.

10. The alarmed communication system as defined in claim 8 wherein said alarm optical phase modulator is located at said center of said Sagnac loop.

11. An alarmed secure communication system including:

communication system including;

a first fiber optic beamsplitter having;

a first arm;

a second arm;

a third arm; and

a fourth arm;

a first light source that produces a first beam of light into said first arm, said first fiber optic beamsplitter splitting said first beam of light into second and third beams of light on said third and fourth arms respectively;

a first optical pathway connecting said third arm to said fourth arm, said first optical pathway conducting said second and third beams of light from said third and fourth arms to said fourth and third arms respectively, said first optical pathway having;

a center;

a first optical phase modulator in said first optical pathway spaced from said center thereof;

said first optical phase modulator having;

an input for receiving a first information signal that said first optical phase modulator uses to phase modulate said second and third beams of light, whereby upon the return of said second and third beams of light to said first fiber optic beamsplitter, said second and third beams of light combine into an amplitude modulated fourth beam of light conducted on said second arm whose amplitude varies with the first information signal; and

a first detector connected to receive said fourth beam of light from said second arm and to produce therefrom a first output signal representative of the first information signal; and

a coherent alarm system including;

meads to apply sinusoidal signal as an input to said first optical phase modulator input at a second frequency lower than the frequency of the first information signal whereby said sinusoidal signal is impressed on said counter propagating light beams so that said sinusoidal signal appears in said amplitude modulated fourth beam of light as an alarm signal; and

means to extract harmonics from said alarm signal, excessive energy in harmonics above the 2nd harmonic being an indication of unauthorized access to said secure communication system.

12. The alarmed communication system as defined in claim 11 wherein said coherent alarm system includes:

means to apply a sinusoidal signal as an input to said first optical phase modulator input at a second frequency lower than the frequency of said first information signal whereby said sinusoidal signal is impressed on said counter propagating light beams so that said sinusoidal signal appears in said amplitude modulated fourth beam of light as an alarm signal; and

means to extract harmonics from said alarm signal, excessive energy in harmonics above the 2nd harmonic being an indication of unauthorized access to said Sagnac loop.

13. The alarmed communication system as defined in claim 11 wherein said means to extract harmonics from said alarm signal include:

a synchronous demodulator connected to receive said alarm signal and to produce therefrom;

a first alarm output signal representative of the level of the first harmonic of said alarm signal; and

a second alarm output signal representative of the level of the second harmonic of said alarm signal; and

means to ratio said first and second alarm output signals.

14. The alarmed communication system as defined in claim 11 wherein said means to extract harmonics from said alarm signal include:

an array of synchronous demodulators to demodulate 1 through n harmonics of said alarm signal and to produce therefrom;

1 through n alarm output signals representative of the levels of the 1 through n harmonics of said alarm signal;

means to sum said 1 through n alarm output signals into a combined harmonic output; and

means to produce an alarm indication if said combined harmonic output changes.

15. The alarmed communication system as defined in claim 14 wherein said coherent alarm system further includes:

an alarm detector; and

an alarm beamsplitter positioned to transmit a portion of said amplitude modulated fourth beam of light to said alarm detector, said array of synchronous demodulators being connected to receive said coherent alarm signal from said alarm detector.

16. The alarmed communication system as defined in claim 11 wherein said alarm system includes:

means to apply an alarm input signal at a second frequency lower than said first frequency as an input to said optical phase modulator, whereby said alarm input signal is impressed on said counter propagating light beams so that said alarm input signal appears in said interferometric output as an alarm signal; and

means to extract at least one harmonic from said alarm signal including;

demodulator means to demodulate said at least one harmonic from said alarm signal and to produce therefrom;

at least one alarm output signal representative of the level said at least one harmonic of said alarm signal; and

means to produce an alarm indication if said at least one alarm output signal changes.

17. An alarmed secure communication system including:

communication system including;

a first fiber optic beamsplitter having;

a first arm;

a second arm;

a third arm; and

a fourth arm;

a first light source that produces a first beam of light into said first arm, said first fiber optic beamsplitter splitting said first beam of light into second and third beams of light on said third and fourth arms respectively;

a first optical pathway connecting said third arm to said fourth arm, said first optical pathway conducting said second and third beams of light from said third and fourth arms to said fourth and third arms respectively, said first optical pathway having;

a center;

a first optical phase modulator in said first optical pathway spaced from said center thereof;

said first optical phase modulator having;

an input for receiving a first information signal that said first optical phase modulator uses to phase modulate said second and third beams of light, whereby upon the return of said second and third beams of light to said first fiber optic beamsplitter, said second and third beams of light combine into an amplitude modulated fourth beam of light conducted on said second arm whose amplitude varies with the first information signal; and

a first detector connected to receive said fourth beam of light from said second arm and to produce therefrom a first output signal representative of the first information signal; and

a coherent alarm system including;

means to apply an alarm input signal to said first optical pathway at a second frequency lower than said first frequency whereby said alarm input signal is impressed on said counter propagating light beams; and

extraction means to extract harmonics of said alarm input signal from said interferometric output, excessive energy in selected harmonics being an indication of unauthorized access to said secure communication system, said extraction means including;

a beamsplitter connected to said output light detector means to split a portion of said interferometric output; and

alarm detector means connected to said beamsplitter to convert said portion of said interferometric output into an electrical alarm signal.

18. The alarmed communication system as defined in claim 17 wherein said Sagnac loop includes:

a central beamsplitter; and

a center opposite said central beamsplitter, said optical phase modulator being positioned in said Sagnac loop spaced from said central beamsplitter and said center opposite said central beamsplitter.

19. An alarmed secure communication system including:

communication system including;

a first fiber optic beamsplitter having;

a first arm;

a second arm;

a third arm; and

a fourth arm;

a first light source that produces a first beam of light into said first arm, said first fiber optic beamsplitter splitting said first beam of light into second and third beams of light on said third and fourth arms respectively;

a first optical pathway connecting said third arm to said fourth arm, said first optical pathway conducting said second and third beams of light from said third and fourth arms to said fourth and third arms respectively, said first optical pathway having;

a center;

a first optical phase modulator in said first optical pathway spaced from said center thereof;

said first optical phase modulator having;

an input for receiving a first information signal that said first optical phase modulator uses to phase modulate said second and third beams of light, whereby upon the return of said second and third beams of light to said first fiber optic beamsplitter, said second and third beams of light combine into an amplitude modulated fourth beam of light conducted on said second arm whose amplitude varies with the first information signal; and

a first detector connected to receive said fourth beam of light from said second arm and to produce therefrom a first output signal representative of the first information signal; and

a coherent alarm system including;

an alarm optical phase modulator remote from said first light source and in said first optical pathway for impressing alarm signals at a second frequency on said second and third beams of light so that alarm signals appear in said first output signal; and

signal generator means connected to said alarm optical phase modulator to supply alarm signals to said alarm optical phase modulator, said signal generator means supplying half wave alarm signals to said alarm optical phase modulator at a frequency equal to X{2Ln/c} where X equals an odd integer, L is the distance in said first optical pathway between said alarm optical phase modulator and said center, c is the speed of light in vacuum, and n is the index of refraction of said first optical pathway on opposite sides of said center.

20. The alarmed communication system as defined in claim 19 wherein said alarm optical phase modulator is located at said center of said first optical pathway.

21. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on said counter propagating light beams so that said information appears in said interferometric output;

output light detector means connected to receive said interferometric output and to produce therefrom an output signal representative of said information; and

an alarm system connected to said Sagnac loop to indicate unauthorized access to said Sagnac loop, said alarm system including;

means to apply an alarm input signal to said Sagnac loop at a second frequency lower than said first frequency whereby said alarm input signal is impressed on said counter propagating light beams; and

means to extract harmonics of said alarm input signal from said interferometric output, excessive energy in selected harmonics being an indication of unauthorized access to said Sagnac loop.

22. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on said counter propagating light beams so that said information appears in said interferometric output;

output light detector means connected to receive said interferometric output and to produce therefrom an output signal representative of said information; and

an alarm system connected to said Sagnac loop to indicate unauthorized access to said Sagnac loop, said alarm system including;

means to apply an alarm input signal at a second frequency lower than said first frequency as an input to said optical phase modulator, whereby said alarm input signal is impressed on said counter propagating light beams so that said alarm input signal appears in said interferometric output as an alarm signal; and

means to extract at least one harmonic from said alarm signal including;

demodulator means to demodulate said at least one harmonic from said alarm signal and to produce therefrom;

at least one alarm output signal representative of the level said at least one harmonic of said alarm signal; and

means to produce an alarm indication if said at least one alarm output signal changes.

23. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on said counter propagating light beams so that said information appears in said interferometric output;

output light detector means connected to receive said interferometric output and to produce therefrom an output signal representative of said information; and

an alarm system connected to said Sagnac loop to indicate unauthorized access to said Sagnac loop, said Sagnac loop including;

a central beamsplitter; and

a center opposite said central beamsplitter, said optical phase modulator being positioned in said Sagnac loop spaced from said central beamsplitter and said center opposite said central beamsplitter and said alarm system including;

signal generator means connected to said optical phase modulator to supply half wave alarm signals to said optical phase modulator at a frequency equal to X{2Ln/c} where X equals an odd integer, L is the distance in said Sagnac loop between said optical phase modulator and said center, c is the speed of light in vacuum, and n is the index of refraction of said Sagnac loop on opposite sides of said center.

24. An alarmed secure communication system including:

communication system including;

a first fiber optic beamsplitter having;

a first arm;

a second arm;

a third arm; and

a fourth arm;

a first light source that produces a first beam of light into said first arm, said first fiber optic beamsplitter splitting said first beam of light into second and third beams of light on said third and fourth arms respectively;

a first optical pathway connecting said third arm to said fourth arm, said first optical pathway conducting said second and third beams of light from said third and fourth arms to said fourth and third arms respectively, said first optical pathway having;

a center;

a first optical phase modulator in said first optical pathway spaced from said center thereof;

said first optical phase modulator having;

an input for receiving a first information signal that said first optical phase modulator uses to phase modulate said second and third beams of light, whereby upon the return of said second and third beams of light to said first fiber optic beamsplitter, said second and third beams of light combine into an amplitude modulated fourth beam of light conducted on said second arm whose amplitude varies with the first information signal; and

a first detector connected to receive said fourth beam of light from said second arm and to produce therefrom a first output signal representative of the first information signal; and

a coherent alarm system including;

means to apply an alarm input signal to said first optical pathway at a second frequency lower than the frequency of the first information signal whereby said alarm input signal is impressed on said second and third beams of light; and

means to extract harmonics of said alarm input signal from said amplitude modulated fourth beam of light, excessive energy in selected harmonics being an indication of unauthorized access to said Sagnac loop.

25. An alarmed secure communication system including:

communication system including;

a first fiber optic beamsplitter having;

a first arm;

a second arm;

a third arm; and

a fourth arm;

a first light source that produces a first beam of light into said first arm, said first fiber optic beamsplitter splitting said first beam of light into second and third beams of light on said third and fourth arms respectively;

a first optical pathway connecting said third arm to said fourth arm, said first optical pathway conducting said second and third beams of light from said third and fourth arms to said fourth and third arms respectively, said first optical pathway having;

a center;

a first optical phase modulator in said first optical pathway spaced from said center thereof;

said first optical phase modulator having;

an input for receiving a first information signal that said first optical phase modulator uses to phase modulate said second and third beams of light, whereby upon the return of said second and third beams of light to said first fiber optic beamsplitter, said second and third beams of light combine into an amplitude modulated fourth beam of light conducted on said second arm whose amplitude varies with the first information signal; and

a first detector connected to receive said fourth beam of light from said second arm and to produce therefrom a first output signal representative of the first information signal; and

a coherent alarm system including;

means to apply an alarm input signal at a second frequency lower than the first frequency as an input to said optical phase modulator, whereby said alarm input signal is impressed on said counter propagating light beams so that said alarm input signal appears in said interferometric output as an alarm signal; and

means to extract at least one harmonic from said alarm signal including;

demodulator means to demodulate said at least one harmonic from said alarm signal and to produce therefrom;

at least one alarm output signal representative of the level said at least one harmonic of said alarm signal; and

means to produce an alarm indication if said at least one alarm output signal changes.

26. An alarmed secure communication system including:

communication system including;

a first fiber optic beamsplitter having;

a first arm;

a second arm;

a third arm; and

a fourth arm;

a first light source that produces a first beam of light into said first arm, said first fiber optic beamsplitter splitting said first beam of light into second and third beams of light on said third and fourth arms respectively;

a first optical pathway connecting said third arm to said fourth arm, said first optical pathway conducting said second and third beams of light from said third and fourth arms to said fourth and third arms respectively, said first optical pathway having;

a center;

a first optical phase modulator in said first optical pathway spaced from said center thereof;

said first optical phase modulator having;

an input for receiving a first information signal that said first optical phase modulator uses to phase modulate said second and third beams of light, whereby upon the return of said second and third beams of light to said first fiber optic beamsplitter, said second and third beams of light combine into an amplitude modulated fourth beam of light conducted on said second arm whose amplitude varies with the first information signal; and

a first detector connected to receive said fourth beam of light from said second arm and to produce therefrom a first output signal representative of the first information signal; and

a coherent alarm system including;

signal generator means connected to said first optical phase modulator to supply half wave alarm signals to said first optical phase modulator at a frequency equal to X{2Ln/c} where X equals an odd integer, L is the distance between said first optical phase modulator and said center, c is the speed of light in vacuum, and n is the index of refraction of said third and fourth arms on opposite sides of said center.

27. An alarmed communication system including:

a Sagnac interferometer producing an interferometric output and having;

a Sagnac loop;

a light source that produces counter propagating light beams on said Sagnac loop;

an optical phase modulator remote from said light source and in said Sagnac loop for impressing information at a first frequency on the counter propagating light beams so that the information appears in an interferometric output;

output light detector means connected to receive the interferometric output and to produce therefrom an output signal representative of the information; and

an alarm system connected to said Sagnac loop to indicate unauthorized access to said Sagnac loop including;

means to check the output signal representative of the information for errors; and

means to produce an alarm signal when a predetermined error rate occurs.