Distributed fiber optic sensing using guided acoustic modes

1. A method for sensing a parameter of an environment surrounding an optical fiber, comprising performing a distributed analysis of one or more guided acoustic wave Brillouin scattering (GAWBS) processes taking place therein by spatially mapping a spectral linewidth of a GAWBS coefficient along the optical fiber.

2. The method according to claim 1 , wherein the one or more GAWBS processes involve the stimulation of one or more radial, torsional, or radial-torsional guided acoustic modes of the optical fiber.

3. The method according to claim 1 , wherein the performing step includes spatially mapping a resonance frequency of a guided acoustic mode along the optical fiber.

4. The method according to claim 1 , wherein a GAWBS process is used to estimate acoustic reflectivity at an outer boundary of the fiber cladding.

5. The method according to claim 1 , wherein a GAWBS process is used to estimate acoustic reflectivity at an outer boundary of a layer or layers of coating surrounding the fiber cladding.

6. The method according to claim 1 , wherein a GAWBS process is used to estimate mechanical impedance of media outside the fiber cladding.

7. The method according to claim 1 , wherein a GAWBS process is used to estimate mechanical impedance of media outside a layer or layers of coating that are applied to the fiber cladding.

8. The method according to claim 1 , wherein the sensed parameter is used to classify liquids under test.

9. The method according to claim 8 , wherein the environment comprises a liquid that is selected from water, aqueous solutions, alcohols, and oils.

10. The method according to claim 1 , wherein outer surfaces of the fiber cladding or of a coating surrounding the fiber cladding are modified to attach a chemical or biological reagent of interest, and whereby the attachment of said reagent is monitored through modifications to GAWBS processes.

11. The method according to claim 1 , wherein the GAWBS distributed analysis is performed using an optical fiber selected from among standard single-mode, non-standard single-mode, few-mode, multi-core, multi-mode, photonic-crystal and micro-structured fibers.

12. The method according to claim 1 , wherein GAWBS and backwards stimulated Brillouin scattering are used together.

13. The method according to claim 1 , wherein two optical tones are used to stimulate guided acoustic waves in an optical fiber.

14. The method according to claim 13 , wherein one of the following procedures is employed:

(a) the two tones are launched from an input end of the optical fiber;

(b) a first tone of the two tones is launched from an input end of the optical fiber, and a second tone is generated through the propagation of said first tone via spontaneous scattering from guided acoustic waves;

(c) The an amplitude of the two tones is modulated by one or more, periodic or aperiodic pulses;

(d) power levels of one tone or both tones change in propagation along the optical fiber due to GAWBS;

(e) an optical power of one tone or both tones is spatially mapped along the optical fiber;

(f) the optical power of one tone or both tones is spatially mapped based on measurements of Rayleigh back-scattering from the optical fiber;

(g) the optical power of one tone or both tones is spatially mapped based on measurements of point reflections;

(h) the optical power of each of one tone or both tones is spatially mapped based on measurements of reflections from a series of fiber Bragg gratings;

(i) the Rayleigh back-scattering contributions of the two tones are separately detected;

(j) the Rayleigh back-scattering contributions of the two tones are separated based on frequency-selective stimulated Brillouin scattering processes in the optical fiber;

(k) the Rayleigh back-scattering contributions of the two tones are separated through mixing with a local oscillator;

(l) the Rayleigh back-scattering contributions of the two tones are jointly processed following detection, using radio-frequency mixing, filtering and processing;

(m) the two tones stimulate guided acoustic waves, which are measured in turn through their interaction with a third optical wave that is launched from either end of the optical fiber.

15. The method according to claim 1 , which is used to perform one of the following:

(j) monitoring and analysis of oil and gas wells;

(k) monitoring of pipeline integrity;

(l) monitoring of high-power electricity cables;

(m) structural health monitoring;

(n) oceanography;

(o) monitoring of water desalination processes;

(p) monitoring of chemical or petro-chemical industry processes;

(q) monitoring hazardous materials;

(r) monitoring of production processes in the food and beverage industry.

16. A sensor for sensing a parameter of an environment surrounding an optical fiber, comprising circuitry adapted to perform a distributed analysis of one or more guided acoustic wave Brillouin scattering (GAWBS) processes taking place in said optical fiber by spatially mapping a spectral linewidth of a GAWBS coefficient along the optical fiber.

17. The sensor according to claim 16 , comprising circuitry adapted to stimulate and/or monitor one or more modes selected from radial, torsional, or radial-torsional guided acoustic modes of the optical fiber.