Bragg grating fiber hydrophone with a bellows amplifier including a fluid cavity and an optical fibre in which the bragg grating is integrated

1. A Bragg grating fibre hydrophone comprising:

a fluid cavity ( 1 ); and

an optical fibre ( 2 ) in which a Bragg grating ( 3 ) is integrated, wherein

said optical fibre ( 2 ) extends through said fluid cavity ( 1 ) along a longitudinal axis (X) and in such a manner that said Bragg grating ( 3 ) is positioned inside the fluid cavity,

said fluid cavity ( 1 ) is filled with a compressible fluid and is delimited by a casing ( 4 ) comprising two ends ( 5 a , 5 b ) integral with the optical fibre ( 2 ),

said casing ( 4 ) comprises at least one part formed by an extensible and compressible tube ( 6 a , 6 b ) extending along the longitudinal axis (X), wherein said part formed by the extensible and compressible tube ( 6 a , 6 b ) comprises an outer end coinciding with one of the two casing ends ( 5 a , 5 b ), and is configured to be longitudinally deformed by a difference in the pressures applied to its walls, causing a length variation of the optical fibre ( 2 ) measured by a variation of wavelength of a luminous flux extracted from the optical fibre ( 2 ), and

said casing ( 4 ) comprising two parts formed by the extensible and compressible tube ( 6 a , 6 b ), and a rigid intermediate part ( 7 ) separating said parts formed by the extensible and compressible tube ( 6 a , 6 b ), wherein each of the outer ends of the parts formed by the extensible and compressible tube ( 6 a , 6 b ) coincide with one of the two ends ( 5 a , 5 b ) of the casing ( 4 ), respectively.

2. The Bragg grating fibre hydrophone according to claim 1 , wherein the intermediate part ( 7 ) of the casing comprises a wall ( 16 ) positioned at the middle thereof, and passed through by the optical fibre ( 2 ).

3. The Bragg grating fibre hydrophone according to claim 2 , wherein the optical fibre ( 2 ) is integral with said wall ( 16 ).

4. The Bragg grating fibre hydrophone according to claim 1 , wherein said optical fibre ( 2 ) is a pre-stressed optical fibre, wherein said optical fibre ( 2 ) is kept tensioned between the two casing ends ( 5 a , 5 b ).

5. The Bragg grating fibre hydrophone according to claim 1 , further comprising at least one hydrostatic filter ( 10 a , 10 b ) cooperating with the fluid cavity ( 1 ), wherein each hydrostatic filter ( 10 a , 10 b ) is provided with an orifice ( 13 a , 13 b ) capable of providing a fluid communication between the inside and the outside of the fluid cavity ( 1 ).

6. The Bragg grating fibre hydrophone according to claim 5 , wherein each hydrostatic filter ( 10 a , 10 b ) comprises a tank ( 11 a , 11 b ) delimited by a deformable outer casing ( 12 a , 12 b ) and by the outer wall of one of the extensible and compressible tubes ( 6 a , 6 b ), wherein said tank ( 11 a , 11 b ) is in fluid communication with the fluid cavity ( 1 ) through said orifice ( 13 a , 13 b ) and wherein said deformable outer casing ( 12 a , 12 b ) is tightly fastened to the casing ( 4 ).

7. The Bragg grating fibre hydrophone according to claim 1 , further comprising a flexible and tight outer casing, surrounding the unit formed by the casing ( 4 ) and the hydrostatic filters ( 10 a , 10 b ), wherein said outer casing is filled with a fluid so as to transmit the pressure variations external to said fluid cavity ( 1 ).

8. The Bragg grating fibre hydrophone according to claim 1 , wherein the compressible fluid of the fluid cavity ( 1 ) has a compressibility modulus lower than 1.5 Gpa.

9. The Bragg grating fibre hydrophone according to claim 2 , wherein said optical fibre ( 2 ) is a pre-stressed optical fibre, wherein said optical fibre ( 2 ) is kept tensioned between the two casing ends ( 5 a , 5 b ).

10. The Bragg grating fibre hydrophone according to claim 2 , further comprising at least one hydrostatic filter ( 10 a , 10 b ) cooperating with the fluid cavity ( 1 ), wherein each hydrostatic filter ( 10 a , 10 b ) is provided with an orifice ( 13 a , 13 b ) capable of providing a fluid communication between the inside and the outside of the fluid cavity ( 1 ).

11. The Bragg grating fibre hydrophone according to claim 2 , further comprising a flexible and tight outer casing, surrounding the unit formed by the casing ( 4 ) and the hydrostatic filters ( 10 a , 10 b ), wherein said outer casing is filled with a fluid so as to transmit the pressure variations external to said fluid cavity ( 1 ).

12. The Bragg grating fibre hydrophone according to claim 2 , wherein the compressible fluid of the fluid cavity ( 1 ) has a compressibility modulus lower than 1.5 Gpa.

13. The Bragg grating fibre hydrophone according to claim 1 , wherein the compressible fluid is perfluorohexane.

14. The Bragg grating fibre hydrophone according to claim 1 , wherein the hydrophone has a sensitivity characterized by the ratio ε x /P e where ε x is the axial lengthening of the optical fibre and P e is external pressure.

15. The Bragg grating fibre hydrophone according to claim 14 , wherein the optical fibre has a theoretical sensitivity of ε x /P e =2ν/E=4.7*10 −12 where E and ν are the Young modulus and the Poisson ratio of silica, respectively.

16. The Bragg grating fibre hydrophone according to claim 1 , wherein The fluid cavity ( 1 ), via its compressibility, tends to balance its pressure with the external pressure by a variation of its volume:

−1/χ s dV/V =( Pe−Pi )

where χ s is the adiabatic compressibility of the fluid, V is the volume of the fluid cavity ( 1 ), and dV is the variation of volume of the cavity.