Method and apparatus for acoustic sensing using multiple optical pulses

• US 20080088846A1
• Filed: 12/27/2006
• Published: 04/17/2008
• Est. Priority Date: 10/13/2006
• Status: Active Grant

• Current Assignee: AT&T Corporation (AT&T, Inc.)
• Original Assignee: AT&T Corporation (AT&T, Inc.)
• Inventors: Lewis, Andrew B., Hayward, Justin, Russell, Stuart
• Granted Patent: US 7,764,363 B2
• Time in Patent Office: Days
• US Class Current: 356/446
• CPC Class Codes: G01D 5/353 influencing the transmission properties of an optical fibre
• CPC Class Codes: G01D 5/35358 using backscattering to detect the measured quantity
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01M 11/3109 Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR
• CPC Class Codes: G01M 11/3127 using multiple or wavelength variable input source
• CPC Class Codes: H04B 10/071 using a reflected signal, e.g. using optical time domain reflectometers [OTDR]

Abstract

An improved technique for acoustic sensing involves, in one embodiment, launching into a medium, a plurality of groups of pulse-modulated electromagnetic-waves. The frequency of electromagnetic waves in a pulse within a group differs from the frequency of the electromagnetic waves in another pulse within the group. The energy scattered by the medium is detected and, in one embodiment, may be used to determine a characteristic of the environment of the medium. For example, if the medium is a buried optical fiber into which light pulses have been launched in accordance with the invention, the presence of acoustic waves within the region of the buried fiber can be detected

Claims

1. A method comprising:

launching into a medium at least four pulse modulated electromagnetic waves, each of said pulse-modulated electromagnetic-waves having an associated electromagnetic-wave frequency, the frequency separation between the first and second of said waves being different from the frequency separation between the third and fourth of said waves; and

detecting a signal associated with the electromagnetic waves that are scattered by the medium.

24. The method of claim 1 wherein the frequency of the electromagnetic waves are in the optical spectrum, the medium is a buried optical fiber, and the frequency separations are between 1 MHz and 25 MHz.

2. A method comprising:

launching into a medium a plurality of pulse modulated electromagnetic waves, each of said pulse-modulated electromagnetic-waves having an associated electromagnetic-wave frequency

a frequency separation between the electromagnetic waves in two pulse-modulated electromagnetic-waves being different from a frequency separation between the electromagnetic waves in two other pulse-modulated electromagnetic-waves; and

detecting a signal associated with the electromagnetic waves that are scattered by the medium.

25. The method of claim 2 wherein the frequency of the electromagnetic waves are in the optical spectrum, the medium is a buried optical fiber, and the frequency separations are between 1 MHz and 25 MHz.

3. A method comprising:

launching into a medium a plurality of groups of pulse-modulated electromagnetic-waves,

each of said groups comprising a plurality of pulse-modulated electromagnetic-waves,

each of said pulse-modulated electromagnetic-waves within a group having an associated electromagnetic-wave frequency,

the associated electromagnetic wave frequency of one of said pulse-modulated electromagnetic-waves within a group being different from the associated electromagnetic-wave frequency of another of said pulse-modulated electromagnetic-waves within the same group, thereby resulting in a frequency separation between the electromagnetic waves in two pulse-modulated electromagnetic-waves within the group

a frequency separation between the electromagnetic waves in two pulse-modulated electromagnetic-waves within a first group being different from a frequency separation between the electromagnetic waves in two pulse-modulated electromagnetic-waves within a second group; and

detecting a signal associated with the electromagnetic waves that are scattered by the medium.

26. The method of claim 3 wherein the frequency of the electromagnetic waves are in the optical spectrum, the medium is a buried optical fiber, and the frequency separations are between 1 MHz and 25 MHz.

4. A method comprising:

launching into a medium a plurality of groups of pulse-modulated electromagnetic-waves, a frequency separation between the electromagnetic waves in two pulses within a first group being different from a frequency separation between the electromagnetic waves in two pulses within a second group; and

detecting a signal associated with the electromagnetic waves that are scattered by the medium.

5. The method of claim 4 wherein the frequency of the electromagnetic waves are in the optical spectrum.

6. The method of claim 5 wherein the medium is an optical fiber.

7. The method of claim 6 wherein the optical fiber is a buried optical fiber.

8. The method of claim 7 wherein the frequency separations are between 1 and 25 MHz.

9. The method of claim 8 wherein the phase of the signal is detected.

10. The method of claim 9 wherein the phase of the signal is weighted by the amplitude of the signal

11. The method of claim 10 wherein the phase of the signal for a first of said plurality of pairs is added coherently to the phase of the signal for the second of said plurality of pairs.

12. The method of claim 11 wherein a phase unwrap algorithm is applied to said phase signals to increase the dynamic range of the resultant signal.

13. The method of claim 12 wherein the resultant signal is related to an acoustic disturbance in the environment of the fiber.

27. The method of claim 4 wherein the frequency of the electromagnetic waves are in the optical spectrum, the medium is a buried optical fiber, and the frequency separations are between 1 MHz and 25 MHz.

14. An apparatus comprising:

means for launching into a medium a plurality of groups of pulse-modulated electromagnetic-waves, the frequency separation between the electromagnetic waves in two pulses within a first group being different from the frequency separation between the electromagnetic waves in two pulses within a second group; and

means for detecting a signal associated with the electromagnetic waves that are scattering by the medium

15. The apparatus of claim 14 wherein the frequency of the electromagnetic waves are in the optical spectrum.

16. The apparatus of claim 15 wherein the medium is an optical fiber.

17. The apparatus of claim 16 wherein the optical fiber is a buried optical fiber.

18. The apparatus of claim 17 wherein the frequency separations are between 1 MHz and 25 MHz.

19. The apparatus of claim 18 wherein the phase of the signal is detected.

20. The apparatus of claim 19 wherein the phase of the signal is weighted by the amplitude of the signal

21. The apparatus of claim 20 wherein the phase of the signal for a first of said plurality of pairs is added coherently to the phase of the signal for the second of said plurality of pairs.

22. The apparatus of claim 21 wherein a phase unwrap algorithm is applied said phase signals to increase the dynamic range of the resultant signal.

23. The apparatus of claim 22 wherein the resultant signal is related to an acoustic disturbance in the environment of the fiber.