Distributed fiber optic acoustic sensor

1. A coherent optical time domain reflectometer device, comprising:

a coherent light source that produces coherent probe light pulses at an optical wavelength;

an optical coupling unit coupled to one end of a fiber link under test to direct the coherent probe light pulses into the fiber link under test and to router reflected probe light pulses from the fiber link under test to an optical detection network;

the optical detection network including an optical interferometer to process the reflected probe light pulses along two different optical paths to generate different optical output signals arising from interference between the reflected probe light pulses along different optical paths, and optical detectors respectively in the different optical paths to receive the optical output signals from the optical interferometer; and

circuitry to control optical elements in the optical detection network and to process detector signals from the optical detectors to extract information on spatial distribution of acoustic- or vibration- or strain-dependent characteristics as a function of distance along the fiber link under test.

2. The device as in claim 1 , wherein the optical detection network includes:

different optical polarization controllers in the different optical paths, respectively; and

optical polarizers respectively in the different optical paths downstream from corresponding optical polarization controllers to produce polarized output signal to the optical detectors, respectively.

3. The device as in claim 1 , wherein the optical detection network includes a phase modulator coupled to the optical interferometer to adjust or modulate an optical phase difference between two different optical paths within the optical interferometer.

4. The device as in claim 1 , wherein the circuitry is configured to obtain measurements of optical power levels of reflected probe light pulses at different times and an average power level of two or more reflected probe light pulses.

5. The device as in claim 1 , wherein the optical coupling unit includes an optical circulator that includes a first optical port to receive light from the coherent light source, a second optical port coupled to the fiber link under test, and a third optical port coupled to the optical detection unit.

6. The device as in claim 1 , wherein the optical interferometer includes a Mach-Zehnder interferometer.

7. The device as in claim 1 , wherein the optical interferometer includes a polarization modulator in one of the two different optical paths.

8. The device as in claim 1 , wherein the optical interferometer includes a phase modulator in one of the two different optical paths.

9. The device as in claim 1 , wherein the optical interferometer includes a polarization modulator in one of the two different optical paths and a phase modulator in another one of the two different optical paths.

10. The device as in claim 1 , wherein the coherent light source includes a laser that produces a laser beam, an optical modulator that receives the laser beam from the laser and modulates the received laser beam to produce a modulated laser beam, an optical fiber amplifier that receives and amplifies the modulated laser beam to produce an amplified modulated laser beam, and an optical filter that further filters the amplified modulated laser beam.

11. The device as in claim 1 , wherein the optical detection network includes an optical fiber amplifier coupled to the optical coupling unit to receive and amplify the reflected probe light pulses, and an optical filter to further filter the amplified reflected probe light pulses from the optical fiber amplifier for further processing by the optical interferometer.

12. The device as in claim 1 , wherein the optical interferometer includes a first optical Faraday rotator mirror in one of the two different optical paths and a second optical Faraday rotator mirror in another one of the two different optical paths.

13. A method for operating the device as in claim 1 to obtain a high spatial resolution coherent measurement of acoustic and/or vibration and/or strain signals, comprising using a metal tube to enclose the fiber link under test to direct the coherent probe light pulses.

14. A method for operating the device as in claim 1 to obtain a high spatial resolution coherent measurement of acoustic and/or vibration and/or strain signals, comprising wrapping the fiber link under the test around a pipeline.

15. A method for operating the device as in claim 1 to obtain a high spatial resolution coherent measurement of acoustic and/or vibration and/or strain signals, comprising providing fiber Bragg gratings (FBGs) along the fiber link.

16. A method for operating the device as in claim 1 to obtain a high spatial resolution coherent measurement of acoustic and/or vibration and/or strain signals, comprising discrete Fresnel reflections along the fiber link.

17. The device as in claim 1 , wherein the optical interferometer includes an optical delay adjustment device in one of the two different optical paths.

18. The device as in claim 1 , wherein the optical interferometer includes a 3×3 coupler to combine light from the two different optical paths.