Fiber laser microphones with graphene diaphragms

1. An acoustic sensor, comprising:

a sensing head, comprising:

a ferrule,

an optical fiber inserted into the ferrule, the optical fiber defining an optical path therethrough, the optical fiber having an optical fiber tip at a longitudinal end of the optical fiber, the optical fiber tip extending beyond an end face of the optical path such that the end face of the optical path is spaced apart from the optical fiber tip by a distance, the optical fiber tip located axially inwards of a longitudinal end of the ferrule, and

a graphene diaphragm disposed on and contacting the end face of the optical fiber tip axially inwards of the longitudinal end of the ferrule such that a portion of the graphene diaphragm is spaced apart from the end face of the optical path defined by the optical fiber by the distance, an outer peripheral edge of the graphene diaphragm being coupled to the ferrule, the graphene diaphragm configured to vibrate in response to an acoustic signal; and

a fiber laser optically coupled to the sensing head, the fiber laser comprising a first set of fiber Bragg gratings and a second set of fiber Bragg gratings such that a gap is present between the first set and the second set of fiber Bragg gratings, the fiber laser configured to generate a sensing optical signal having a first intensity in response to an excitation optical signal, the sensing optical signal impinging on the graphene diaphragm such that a feedback optical signal is reflected from the graphene diaphragm towards the fiber laser, the feedback optical signal having a second intensity due to modulation of the sensing optical signal by the vibration of the graphene diaphragm that corresponds to the acoustic signal, the feedback optical signal having sufficient power for being detectable by an intensity detector after travelling through a fiber optic communication cable having a length in a range of 1 km to 10 km.

2. The acoustic sensor of claim 1 , wherein at least a portion of the graphene diaphragm is spaced apart from the optical fiber tip by a distance.

3. The acoustic sensor of claim 1 , wherein the sensing optical signal has a wavelength in a range from 1,530 to 1,565 nm.

4. The acoustic sensor of claim 1 , wherein the excitation optical signal has a wavelength of about 980 nm.

5. The acoustic sensor of claim 1 , wherein the fiber laser comprises a Distributed Bragg Reflector (DBR) fiber laser, and wherein the gap between the first set and the second set of fiber Bragg gratings is in a range of 1 mm to 10 mm.

6. The acoustic sensor of claim 5 , wherein a spacing between each grating of the first set of fiber Bragg gratings and the second set of fiber Bragg gratings is about 530 nm.

7. The acoustic sensor of claim 1 , wherein the fiber laser comprises a Distributed Feedback Bragg (DFB) laser, and wherein the gap between the first set and the second set of fiber Bragg gratings is about 265 nm.

8. The acoustic sensor of claim 7 , wherein a spacing between each grating of the first set of fiber Bragg gratings and the second set of fiber Bragg gratings is about 530 nm.

9. An acoustic sensing assembly, comprising:

a sensing head, comprising:

a ferrule;

an optical fiber inserted into the ferrule, the optical fiber defining an optical path therethrough, the optical fiber having an optical fiber tip, the optical fiber tip extending beyond an end face of the optical path such that the optical path is spaced apart from the optical fiber tip by a distance, the optical fiber tip located axially inwards of a longitudinal end of the ferrule, and

a graphene diaphragm disposed on and contacting the end face of the optical fiber tip axially inwards of the longitudinal end of the ferrule such that a portion of the graphene diaphragm is spaced apart from the end face of the optical path defined by the optical fiber by the distance, an outer peripheral edge of the graphene diaphragm being coupled to the ferrule, the graphene diaphragm configured to vibrate in response to an acoustic signal;

a fiber laser optically coupled at a first end thereof to the sensing head and comprising a first set of fiber Bragg gratings and a second set of fiber Bragg gratings such that a gap is present between the first set and the second set of fiber Bragg gratings, the fiber laser configured to generate a sensing optical signal having a first intensity in response to an excitation optical signal, the sensing optical signal impinging on the graphene diaphragm such that a feedback optical signal is reflected from the graphene diaphragm towards the fiber laser, the feedback optical signal having a second intensity due to modulation of the sensing optical signal by the vibration of the graphene diaphragm that corresponds to the acoustic signal;

a fiber optic communication cable having a length in a range of 1 km to 10 km coupled to a second end of the fiber laser opposite the first end;

a signal generator configured to generate the excitation optical signal; and

an intensity detector configured to detect the response optical signal and determine the acoustic signal therefrom,

wherein the feedback optical signal has sufficient power for being detectable the intensity detector after travelling through the fiber optic communication cable.

10. The acoustic sensing assembly of claim 9 , wherein the sensing optical signal has a wavelength in a range of 1,530 nm to 1,565 nm.

11. The acoustic sensing assembly of claim 9 , wherein the excitation optical signal has wavelength of about 980 nm.

12. The acoustic sensing assembly of claim 9 , wherein the fiber laser comprises a Distributed Bragg Reflector (DBR) laser, and wherein the gap between the first set and the second set of fiber Bragg gratings is in a range of 1 mm to 10 mm.

13. The acoustic sensing assembly of claim 12 , wherein a spacing between each grating of the first set of fiber Bragg gratings and the second set of fiber Bragg gratings is about 530 nm.

14. The acoustic sensing assembly of claim 9 , wherein the fiber laser comprises a Distributed Feedback Bragg (DFB) laser, and wherein the gap between the first set and the second set of fiber Bragg gratings is about 265 nm.

15. The acoustic sensing of claim 14 , wherein a spacing between each grating of the first set of fiber Bragg gratings and the second set of fiber Bragg gratings is about 530 nm.

16. The acoustic sensing assembly of claim 9 , further comprising a wavelength-division multiplexer coupled to each of the signal generator and the intensity detector.

17. A method, comprising:

transmitting an excitation optical signal to a fiber laser of an acoustic sensor, the acoustic sensor comprising:

a sensing head, comprising:

a ferrule;

an optical fiber inserted into the ferrule, the optical fiber defining an optical path therethrough, the optical fiber having an optical fiber tip, the optical fiber tip extending beyond an end face of the optical path such that the end face of the optical path is spaced apart from the optical fiber tip by a distance, the optical fiber tip located axially inwards of a longitudinal end of the ferrule, and

a graphene diaphragm disposed on the optical fiber tip axially inwards of the longitudinal end of the ferrule such that a portion of the graphene diaphragm is spaced apart from the end face of the optical path by the distance, an outer peripheral edge of the graphene diaphragm being coupled to the ferrule, the graphene diaphragm configured to vibrate in response to an acoustic signal; and

a fiber laser optically coupled to the sensing head and comprising a first set of fiber Bragg gratings and a second set of fiber Bragg gratings having a gap therebetween, the fiber laser configured to generate a sensing optical signal having a first intensity in response to the excitation optical signal;

detecting a feedback optical signal reflected from the graphene diaphragm, the feedback optical signal having a second intensity due to modulation of the sensing optical signal by the vibration of the graphene diaphragm that corresponds to the acoustic signal, the feedback optical signal having sufficient power for being detectable by an intensity detector after travelling through a fiber optic communication cable having a length in a range of 1 km to 10 km;

determining optical parameters of the feedback optical signal; and

determining acoustic parameters of the acoustic signal from the optical parameters.

18. The method of claim 17 , wherein the fiber laser comprises a Distributed Bragg Reflector (DBR) laser, and wherein the gap between the first set and the second set of fiber Bragg gratings is in a range of 1 mm to 10 mm.

19. The method of claim 17 , wherein the fiber laser comprises a Distributed Feedback Bragg (DFB) laser, and wherein the gap between the first set and the second set of fiber Bragg gratings is about 265 nm.