Method and apparatus for acoustic sensing using multiple optical pulses

• US 20080144016A1
• Filed: 10/03/2007
• Published: 06/19/2008
• 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., Russell, Stuart
• Granted Patent: US 7,742,157 B2
• Time in Patent Office: Days
• US Class Current: 356/73.1
• CPC Class Codes: G01D 5/35364 using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering
• 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 11/3127 using multiple or wavelength variable input source
• CPC Class Codes: G01M 11/3172 Reflectometers detecting the back-scattered light in the frequency-domain, e.g. OFDR, FMCW, heterodyne detection
• 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, the beat signal 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 a 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 characteristic of a beat signal between a given pair of electromagnetic waves that are scattered by the medium.

23. 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 characteristic of a beat signal between a given pair of electromagnetic waves that are scattered by the medium.

24. 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 characteristic of a beat signal between a given pair of electromagnetic waves that are scattered by the medium.

25. 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, 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

detecting a characteristic of a beat signal between a given pair of 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 separation between the waves in a given pair is between 1 MHz and 25 MHz.

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

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

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

12. The method of claim 11 wherein a phase unwrap algorithm is used on said phase 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.

26. 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. A 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 characteristic of a beat signal between a given pair of e electromagnetic waves that are scattered 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 optical fiber is a buried optical fiber.

17. The apparatus of claim 34 wherein the frequency separation between the waves in a given pair is between 16 MHz and 25 MHz.

18. The apparatus of claim 17 wherein the phase of the beat signal is detected.

19. The apparatus of claim 18 wherein the phase of the beat signal is weighted by the amplitude of the beat signal

20. The apparatus of claim 19 wherein the phase of the beat signal for a first of said plurality of pairs is added coherently to the phase of the beat signal for the second of said plurality of pairs.

21. The apparatus of claim 20 wherein a phase unwrap algorithm is used on said phase to increase the dynamic range of the resultant signal.

22. The apparatus of claim 21 wherein the resultant signal is related to an acoustic disturbance in the environment of the fiber.