AU2017317614B2

AU2017317614B2 - Hydrophone with optimised optical fibre

Info

Publication number: AU2017317614B2
Application number: AU2017317614A
Authority: AU
Inventors: Raphaël LARDAT; François-Xavier LAUNAY
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2016-09-01
Filing date: 2017-08-30
Publication date: 2023-05-11
Anticipated expiration: 2037-08-30
Also published as: FR3055425A1; EP3507623B1; AU2017317614A1; FR3055425B1; EP3507623A1; WO2018041855A1

Abstract

The invention proposes an acoustic sensor device (1) with an optical fibre, comprising a casing (10) which delimits a cavity (3) and an optical fibre sensor (2), the optical fibre sensor comprising an optical fibre (12), the optical fibre extending through the casing between two points. Advantageously, the cavity (3) comprises a mixed fluid which comprises a liquid-based fluid and microballoons, the microballoons comprising gas bubbles.

Description

OPTIMIZED FIBER-OPTIC HYDROPHONE

Field of the invention

The invention generally relates to systems for measuring acoustic pressure, and in particular fiber-optic acoustic sensors and to processes for manufacturing such acoustic sensors.

Prior art

The detection of submarine acoustic waves is of major importance in the surveil lance of the submarine environment (surveillance of submarine seismic activity, detection of environmental noise, detection of submarines, ships, divers, etc.).

Fiber-optic sensors have allowed major advances to be made in the field of detection of submarine acoustic waves. Thus, fiber-optic acoustic sensors, such as optical hydrophones, are used in the submarine environment to detect varia tions in acoustic pressures induced by outside effects. Fiber-optic acoustic sensors have many advantages with respect to conventional piezoelectric sen sors.

A fiber-optic acoustic sensor comprises at least one optical fiber the optical properties of which are sensitive to the acoustic pressure to be measured. When light is injected into the optical fiber, a luminous signal the properties of which depend on acoustic pressure is generated by the hydrophone. Some fiber-optic acoustic sensors use Bragg gratings inscribed in the fiber. A Bragg grating is a reflector comprising alternated layers of different refractive indices, this causing a periodic variation in effective refractive index in the optical fiber. Bragg-grating optical acoustic sensors are used to measure acoustic pressure, which corre sponds to a stress applied to the sensor, and which induces a wavelength varia tion.

Bragg-grating fiber-optic acoustic sensors may be passive or active (Optical Fiber Laser Sensor CLFO).

A Bragg-grating fiber-optic acoustic sensor CLFO comprises a protective jacket, which is passed through at either end by at least one optical fiber. The jacket delineates an amplifying cavity into which a fluid is injected. The optical hydro phone is suitable for receiving light injected into the optical fiber (generally originating from a light-emitting diode) and to deliver a measurement signal indicating the incident acoustic pressure.

However, a CLFO has an insufficient sensitivity for the detection of small pressure variations.

To improve the sensitivity of fiber-optic acoustic sensors, one solution consists in amplifying small pressure variations, by providing a particular jacket around the fiber (e.g. amplifying cladding, resonant cavity, etc.) that allows an amplified mechanical stress to be applied to the optical fiber. However, this technology does not always allow the sensitivities required for the detection of small acoustic pressure variations to be achieved and requires jacket configurations that are quite complex, possibly having an impact on the operation of the optical fiber. Moreover, when a satisfactory sensitivity to acoustic pressure is achieved, immersion resistance is often sacrificed.

More generally, existing fiber-optic acoustic sensors may be configured: - either to optimize the sensitivity of the sensor to variations in pressure for a given size (high sensitivity) by sacrificing maximum operating immersion depth (the acoustic hydrophone is then operational only up to an immersion depth of a few meters, i.e. typically at a pressure lower than 1 bar); - or to preserve a very good immersion resistance compatible with submarine applications (typically pressures of as high as more than 1000 bars), but to the detriment of sensitivity.

However, there is no optical-acoustic-sensor configuration that allows both the constraint of sensitivity and the constraint of immersion resistance to be controlla bly met.

Throughout this specification the word "comprise", or variations such as "compris es" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims.

General definition of the invention The invention aims to improve the situation. To this end a fiber-optic acoustic sensing device is provided, said device comprising a jacket delimiting a cavity and a fiber-optic sensor, said fiber-optic sensor comprising an optical fiber, and a holding device securely fastened to the optical fiber, the holding device forming a rigid body that is passed through by the optical fiber at fastening points the jacket being passed through by the optical fiber between two points. Advantageously, the cavity comprises a hybrid fluid comprising a liquid-based fluid and microspheres, said microspheres comprising gas bubbles, each microsphere being a micron sized or submicron-sized bubble filled with gas, wherein the proportion of micro spheres injected in the cavity with respect to the volume of injected fluid is previously calculated as a function of the target compressibility of the injected hybrid fluid and of the volume of the cavity delimiting the jacket, the maximum compressibility of the hybrid fluid for a chosen target maximum immersion depth being given by:

V__ ext 1 Xmax - tot Pmax

where Vtet is the total volume of hybrid fluid injected inside the jacket, Vext is the volume of hybrid fluid contained in the jacket and outside the cavity, and Pmax is the pressure at said target maximum immersion depth..

In one embodiment, the proportion of microspheres with respect to the volume of fluid may be a function of the target compressibility of the injected hybrid fluid and the volume of the cavity.

The microspheres may comprise microspheres of different sizes.

The microspheres may comprise microspheres including various types of gas.

The liquid-based fluid may consist of a mixture of fluids of various types.

As a variant, the liquid-based fluid may be a liquid.

In one embodiment, the liquid-based fluid may comprise an oil.

In another embodiment, the liquid-based fluid may be a gel.

In one embodiment, the maximum compressibility of the hybrid fluid for a chosen target maximum immersion depth may be a function of the total volume of hybrid fluid injected into the interior of the jacket, of the volume of hybrid fluid contained in the jacket and outside the cavity, and of the pressure at said target maximum immersion depth.

In particular, the maximum compressibility Xmax of the hybrid fluid may be given by:

Vext 1 Xmax- Vt 0t max

where Vtet is the total volume of hybrid fluid injected into the interior of the jacket, Vext is the volume of hybrid fluid contained in the jacket outside the cavity and Pmax is the pressure at said target maximum immersion depth.

The invention furthermore proposes a process for manufacturing a fiber-optic acoustic sensor comprising a jacket delimiting a cavity and a fiber-optic sensor, the fiber-optic sensor comprising an optical fiber and a holding device securely fastened to the optical fiber, the holding device forming a rigid body that is passed through by the optical fiber at fastening points. Advantageously, the process comprises the following steps:

- assembling the jacket and the fiber-optic sensor so that the optical fiber passes through the jacket at two points, and

- injecting a mixture of fluid and of microspheres into the cavity, said microspheres comprising gas bubbles,

wherein each microsphere being a micron-sized or submicron-sized bubble filled with gas, wherein the proportion of microspheres injected in the cavity with respect to the volume of injected fluid is previously calculated as a function of the target compressibility of the injected hybrid fluid and of the volume of the cavity delimiting the jacket, the maximum compressibility of the hybrid fluid for a chosen target maximum immersion depth being a function of the total volume of hybrid fluid injected inside the jacket, of the volume of hybrid fluid contained in the jacket and outside the cavity, and of the pressure at said target maximum immersion depth.

In one embodiment, the proportion of microspheres with respect to the volume of fluid may be a function of the target compressibility of the fluid and of the volume of the cavity.

Embodiments of the invention thus allow the sensitivity of the sensor to be optimized for a given bulk while preserving an immersion resistance compatible with submarine applications, and without it being necessary to modify the general structure of the fiber-optic acoustic sensor. The invention thus provides a solution that is applicable to any fiber-optic hydrophone, whatever the arrangement of these elements. Furthermore, it allows a target sensitivity and target immersion resistance to be obtained by adjusting the proportion of microspheres injected into the cavity of the optical acoustic sensor.

Description of the figures

Other features and advantages of the invention will become apparent from the following description and from the appended drawings, in which: - figure 1 is a schematic showing a fiber-optic sensing device; - figure 2 is a flowchart showing the process for manufacturing a fiber-optic sensing device according to one embodiment of the invention; and - figure 3 is a graph showing the variation in the sensitivity and in the immersion resistance of an optical hydrophone as a function of the proportion of micro spheres in the cavity

The drawings and the addendums to the description will possibly not only serve to better understand the description, but also contribute to the definition of the invention, where appropriate.

Detailed description

The embodiments of the invention provide a fiber-optic acoustic sensor allowing the sensitivity and immersion resistance of the sensor to be simultaneously optimized, whatever the application of the invention.

Figure 1 shows an optical fiber acoustic sensor 1 (also called an "optical hydro phone" or "optical acoustic sensor" below), according to certain embodiments of the invention, for detecting variations in acoustic pressure in the medium in which the hydrophone is submerged.

The optical fiber hydrophone 1 comprises a jacket 10 delineating a cavity 3 and at least one fiber-optic sensor 2 housed in the cavity 3.

The optical fiber hydrophone 2 comprises an optical fiber 12 and a holding device 11 that is securely fastened to the optical fiber 12. In certain embodiments, a plurality of hydrophones 1 may be multiplexed on a given fiber 12.

The holding device 11 may be passed through by the optical fiber. The holding device 11 may be fastened thereto via fastening points 13 that are provided on said holding device. The holding device 11 is configured to hold the fiber in position in the cavity 3. It may furthermore comprise additional elements for mechanical amplification in certain acoustic applications for example.

The optical fiber 12 may comprise at least one Bragg grating 4 embedded in the fiber and configured to emit wavelengths that are sensitive to the mechanical stress applied to the optical fiber 12. The measurement of these wavelength variations allows the stress applied to the optical fiber 12 to be deduced and thus the acoustic pressure to be measured, using an interrogation unit.

Those skilled in the art will understand that the invention is not limited to an optical fiber 12 comprising an embedded Bragg grating and may be applied to other types of optical fiber, such as for example a coiled fiber or a CLFO (fiber laser sensor) equipped with a Bragg grating.

The function of the holding device 11 is to amplify the axial deformation applied to the optical-fiber element 12 by the dynamic pressure exerted by the outside medium.

The optical sensor 2 is the sensitive portion of the hydrophone and conventionally performs the following functions:

- a pressure-sensor function that exploits the variation in the emission frequency of the laser cavity associated with a stretching and/or retracting axial elastic defor mation;

- a function of transmitting the luminous signal that carries the measurement;

- a multiplexing function implemented on the basis of the wavelength selectivity made possible by the Bragg grating.

The jacket 10 may be configured to mechanically protect the hydrophone 1, in particular from shocks, against certain forces due to the environment, etc. The jacket 10 may for example take the form of a rigid body such as a cylindrical tube the straight-line generatrix of which coincides substantially with the general axis of the optical fiber 12. The jacket may be formed of a plurality of elements that are assembled with one another or may have an integrally formed structure.

The optical fiber 12 may be fastened to the jacket 10 level with feedthroughs 150 and 160 by any rigid attaching means such as for example by welding (e.g. laser welding) or adhesive bonding (e.g. adhesive bonding with a polyamide coating or with epoxy adhesive).

In addition, the device 11 for holding the sensor 2 may be fastened to the jacket 10 at attachment points 13.

According to one embodiment, the cavity 3 delineated by the jacket 10 is filled with a hybrid fluid 5 comprising a liquid-based fluid and a chosen proportion of micro spheres, each microsphere being a micron-sized or submicron-sized bubble filled with gas.

Advantageously, the proportion of microspheres with respect to the volume of liquid in the hybrid fluid injected into the cavity 3 may be calculated so as to achieve a target hybrid-fluid compressibility. The target hybrid-fluid compressibility (hybrid-fluid compressibility in the cavity 3) may be predefined or chosen depend ing on the application in which the hydrophone is used. Since the sensitivity and immersion resistance of a fiber-optic hydrophone are directly related to the compressibility of the fluid used, the proportion of microspheres injected into the cavity with respect to the volume of liquid allows both a target sensitivity and a target immersion resistance to be achieved, without having to sacrifice either one of these two constraints. For example, if the proportion of microspheres injected into the cavity is calculated so as to achieve a high target compressibility, the hydrophone will be more sensitive but its immersion resistance will be lower because the fluid cavity has a volume limited by the size of the jacket 10.

The composition of the hybrid fluid 5 injected into the cavity 3 may thus be optimized in order to improve the sensitivity of the hydrophone depending on the desired immersion depth and on the bulk defined by the cavity, whatever the configuration of the hydrophone.

The use of microspheres allows the proportion of gas in the fluid to be controlled and thus a target fluid-compressibility level corresponding to the desired sensitivi ty/desired immersion resistance to be achieved.

In one embodiment, the liquid-based fluid contained in the hybrid fluid may be oil. The hybrid fluid injected into the cavity is then an oil mixed with microspheres, the proportion of microspheres integrated into the oil being chosen to achieve a target hybrid-fluid compressibility.

More generally, the liquid-based fluid that is injected into the cavity may be any type of fluid whether viscous, partially viscous or non-viscous. The liquid-based fluid may in particular be a liquid or a gel of various types or composed of such liquids and gels. The liquid-based fluid injected into the cavity may itself be composite and formed from one or more types of liquids. For example, the liquid based fluid may consist of water and/or oil.

The rest of the description will mainly make reference to a liquid-based fluid of liquid type, such as for example an oil, by way of nonlimiting example.

The microspheres may include microspheres having the same size or the same dimensions. As a variant, the microspheres may include microspheres having different sizes or dimensions.

The jacket 10 is seal-tight in order to isolate the fluid that it contains from the outside medium.

The jacket 10 may include at least one orifice for the passage of fluid in a chosen location of the jacket, said orifice being configured to allow hybrid fluid to be injected into the cavity 3 and/or the air initially contained in the jacket before the filling with fluid to exit. In the embodiment of figure 1, two orifices 600 and 601 for the passage of fluid are used. The orifices for the passage of fluid are arranged in two chosen locations of the jacket. The orifices comprise an entrance orifice 600 configured to allow hybrid fluid to be injected into the cavity 3 and an exit orifice 601 configured to allow air initially contained in the jacket 10 before the filling with fluid to exit.

The holding device 11 forms a rigid body (for example of generally cylindrical shape, having the same axis as the jacket 10) that is passed through by the optical fiber at two fastening points 152 and 162. The holding device 11 delineates an internal cavity 30 in which the active portion of the optical fiber is placed. The holding device 11 may furthermore comprise at least one secondary passage orifice. In the example of figure 1, the holding device 11 comprises the following two secondary passage orifices:

- a secondary inlet orifice 610 in order to allow hybrid fluid to enter from the cavity 3 into the internal cavity 30,

- an outlet orifice 611 in order to allow the air initially contained in the internal cavity before the filling with fluid to exit,

The inlet and outlet orifices 610 and 611 also perform a pressure equilibrating function: they bring the inside medium contained in the internal cavity 30 perma nently into contact with the medium contained in the cavity 3, this allowing the pressures inside the cavity 30 and outside the cavity 30 to be brought into equilib rium. The jacket of the holding device 11 is thus in fluid communication with the cavity 3.

The dimensions of the microspheres may be smaller than the dimensions of the inlet apertures for passage of fluid 600 and 610.

In one particular embodiment, the proportion of microspheres injected into the cavity 3 with respect to the injected volume of liquid may be calculated beforehand depending on the target compressibility Xtarget of the hybrid fluid (in Pa 1 with Pa designating the unit of pascals) and on the volume Vcavity of the cavity 3 deline ated by the jacket 10. By target compressibility of the hybrid fluid what is meant is the relative variation in fluid volume under the effect of an applied pressure.

In one embodiment, the maximum compressibility Xmax that may be achieved may be a function: - of the total volume Vtet of fluid injected into the jacket 10, which corresponds to the sum of the volume Vint of fluid contained in the cavity 30 (delineated by the device 11) and of the volume Vet of fluid contained in the jacket and outside the cavity 30 (Vtet = Vext +Vit); - of the volume Vet of fluid contained in the jacket but outside of the cavity 30; and - of the pressure Pmax at a target maximum immersion depth.

In particular, the maximum compressibility Xmax may be given by the following equation, equation (1):

Xmax = VetA1 (1)

Vtot Pmax

Figure 2 illustrates the process for manufacturing the fiber-optic hydrophone 1, according to one embodiment.

In step 400, the elements of the optical hydrophone are assembled. In this step, the jacket 10 and the optical sensor 2 are assembled. The hydrophone 1 thus assembled is initially filled with air.

In step 402, the hybrid fluid is injected through the inlet orifice 600, this having the effect of: - flushing the air contained in the cavity 3 through the outlet orifice 601; - making the hybrid fluid pass into the internal cavity 30 through the secondary inlet orifice 610 while flushing the air from the internal cavity 30 via the outlet orifice 611.

The hybrid fluid may be directly injected into the hydrophone 1, in step 402, in a composite form including the chosen proportion of microspheres and the volume of liquid. In such an embodiment, a reservoir containing the hybrid fluid, mixed with the chosen or calculated proportion of microspheres with respect to the liquid, may be connected to the jacket via the inlet orifice 600 in order to inject the hybrid fluid.

As a variant, the liquid and the microspheres may be injected in succession or in alternation until the chosen volume of liquid and the chosen proportion of micro spheres is achieved

The fiber-optic sensing device 1 thus obtained may be used in the intended operating environment, this guaranteeing that the target sensitivity and the target pressure withstand (corresponding to the target compressibility) are achieved.

By virtue of the use of a hybrid fluid made up of optimized proportions of micro spheres and liquid, it is possible to produce a more compact hydrophone. Specifi cally, by injecting a chosen proportion of microspheres, it is possible to achieve a target sensitivity, this allowing the volume of the jacket to be decreased.

In comparison, in conventional acoustic hydrophones using a liquid, such as water or oil, to fill the cavity of the optical sensor, although the use of liquid in the cavity allows a very good immersion resistance to be obtained (resistance to up to more than 1000 bars), a low sensitivity to acoustic pressure is obtained. For example, for a prior-art optical hydrophone tolerating a decrease in fluid volume of 10%, the maximum operating immersion depth with such an oil-filled hydrophone is about 5000 meters. Such a value is very much higher than the required operating immersion depth.

Conversely, in conventional acoustic hydrophones using a gas to fill the cavity of the optical sensor, a high-sensitivity is obtained but such a conventional hydro phone is able to operate only up to an immersion depth of a few meters (pressure lower than 1 bar). For example, for a hydrophone tolerating a decrease in fluid volume of 10%, the maximum operating immersion depth with such an air-filled hydrophone is about 1 meter. Such a value is insufficient for submarine applica tions.

The various proposed embodiments thus allow the compressibility of the fluid filling the cavity 3 to be adjusted by choosing the proportion of microspheres injected into the cavity with respect to the volume of fluid forming the hybrid fluid. For example, a proportion of microspheres of about 10% allows a 10-times higher compressibility, and therefore an increase in sensitivity of 20 dB with respect to conventional approaches, to be obtained. The immersion resistance is then divided by 10. Thus, in the considered example, an immersion resistance of 500 meters is obtained, this generally being sufficient for the applications of acoustic hydrophones.

Figure 3 is a graph showing the variation in the compressibility 7 of a hybrid fluid injected into an optical hydrophone (representative of the variation in sensitivity and immersion resistance) as a function of the proportion of microspheres in the cavity 3, according to some embodiments. As shown in figure 3, the compressibil ity y of the hybrid fluid is a linear and increasing function of the proportion Tm of microspheres injected into the jacket.

The proposed embodiments thus allow the sensitivity of the sensor to be optimized for a cavity 3 of given bulk while preserving an immersion resistance compatible with submarine applications.

The invention is not limited to the embodiments described above by way of nonlimiting example. It encompasses any variant embodiment that those skilled in the art are able to envision. In particular, the invention is not limited to particular configurations of optical hydrophones 1. Furthermore, it is not limited to one particular acoustic-sensor application nor to one particular form of jacket 10. For example, the invention may be used for the surveillance of submarine seismic activity, the detection of submarines, of ships, of divers, etc. Moreover, although the invention has been described with reference to an embodiment in which the jacket 10 comprises a single fiber-optic sensor 2, it may also be applied to a plurality of sensors mounted in parallel (all of the sensors possibly for example being held by a common holding device 11) or mounted in series. The invention is also not limited to any particular technique for generating microspheres. The microspheres may consist of any type of gas.

Claims

1. A fiber-optic acoustic sensing device, comprising a jacket delimiting a cavity and a fiber-optic sensor, said fiber-optic sensor comprising an optical fiber and a holding device securely fastened to the optical fiber, the holding device forming a rigid body that is passed through by the optical fiber at fastening points, the jacket being passed through by the optical fiber between two points, wherein the cavity comprises a hybrid fluid comprising a liquid-based fluid and microspheres, said microspheres comprising gas bubbles, each microsphere being a micron-sized or submicron-sized bubble filled with gas, wherein the proportion of microspheres injected in the cavity with respect to the volume of injected fluid is previously calculated as a function of the target compressibility of the injected hybrid fluid and of the volume of the cavity delimiting the jacket, the maximum compressibility of the hybrid fluid for a chosen target maximum immersion depth being given by

V__ ext 1 Xmax - tot Pmax

where Vtet is the total volume of hybrid fluid injected inside the jacket, Vext is the volume of hybrid fluid contained in the jacket and outside the cavity, and Pmax is the pressure at said target maximum immersion depth.

2. The device as claimed in any one of the preceding claims, wherein the micro spheres comprise microspheres of different sizes.

3. The device as claimed in any one of the preceding claims, wherein the micro spheres comprise microspheres including various types of gas.

4. The device as claimed in any one of the preceding claims, wherein the liquid based fluid consists of a mixture of fluids of various types.

5. The device as claimed in any one of the preceding claims, wherein the liquid based fluid is a liquid.

6. The device as claimed in claim 5, wherein the liquid-based fluid comprises an oil.

7. The device as claimed in any one of the preceding claims, wherein the liquid based fluid is a gel.

8. The device as claimed in any one of the preceding claims, wherein the com pressibility of the hybrid fluid is a linear and increasing function of the proportion of microspheres injected into the jacket.

9. A process for manufacturing a fiber-optic acoustic sensor comprising a jacket delimiting a cavity and a fiber-optic sensor, said fiber-optic sensor comprising an optical fiber and a holding device securely fastened to the optical fiber, the holding device forming a rigid body that is passed through by the optical fiber at fastening points, wherein it comprises the following steps:

- assembling the jacket and the fiber-optic sensor so that the optical fiber passes through the jacket at two points,

wherein each microsphere being a micron-sized or submicron-sized bubble filled with gas, wherein the proportion of microspheres injected in the cavity with respect to the volume of injected fluid is previously calculated as a function of the target compressibility of the injected hybrid fluid and of the volume of the cavity delimiting the jacket, the maximum compressibility of the hybrid fluid for a chosen target maximum immersion depth being given by

__ ext 1 Xmax - tot Pmax'