Fiber Optic Hydrophone

Abstract

To obtain an optical fiber hydrophone for suppressing an influence of temperature on sensitivity for detecting sound waves.SOLUTION: An optical fiber hydrophone includes: an optical fiber coil around which an optical fiber is wound cylindrically; a flexible tube disposed inside the optical fiber coil; and a pair of lid parts for covering both the opened ends of the optical fiber coil and the flexible tube. At least one lid part has an opening for connecting a space between the flexible tube and the optical fiber coil and a space outside the one lid part. The flexible tube has a member that shrinks a cavity part in a space including a center shaft toward the center shaft when pressure in the outer space becomes higher than pressure in the cavity part of the space including the center shaft of the flexible tube, and has a smaller temperature change in characteristics concerning elasticity than that in resin.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a fiber optic hydrophone for detecting acoustic waves in liquids. [Background technology]

[0002] High-pressure-resistant optical fiber hydrophones have been proposed in the past (see, for example, Patent Document 1). The optical fiber hydrophone disclosed in Patent Document 1 has an elastic cylinder placed inside an optical fiber coil, lids that close both ends of the optical fiber coil, and an opening provided on one of the lids that maintains pressure balance between the inside and outside of the optical fiber coil. Patent Document 1 discloses that an elastic body made of a resin material is used as the elastic cylinder. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-68087 Summary of the Invention [Problem to be solved by the invention]

[0004] In the optical fiber hydrophone disclosed in Patent Document 1, a resin material is used for the elastic cylinder, but the hardness of resin materials changes with temperature. Therefore, the bulk modulus of the elastic cylinder changes depending on the temperature of the water where the optical fiber hydrophone is installed. When the bulk modulus of the elastic cylinder changes, the sensitivity of the optical fiber hydrophone changes. Therefore, optical fiber hydrophones that use an elastic resin cylinder have the problem of changing sensitivity when the temperature of the installation location changes. [Means for solving the problem]

[0005] The optical fiber hydrophone according to the present invention comprises an optical fiber coil in which an optical fiber is wound cylindrically, a flexible tube disposed inside the optical fiber coil, and a pair of lids covering the open ends of the optical fiber coil and the flexible tube, at least one of the pair of lids having an opening connecting a space between the flexible tube and the optical fiber coil to a space outside the one lid, the flexible tube contracting the hollow portion of the space including the central axis toward the central axis when the pressure in the outside space becomes higher than the pressure in the hollow portion of the space including the central axis of the flexible tube, and the flexible tube comprising a material whose temperature change in elastic properties is smaller than that of resin. A cross section taken along a plane perpendicular to the central axis has a shape in which mountains and valleys are alternately repeated along the circumference of a circle centered on the central axis. [Effects of the Invention]

[0006] According to the present invention, a flexible tube is provided inside the optical fiber coil. When the pressure of the liquid surrounding the optical fiber coil becomes higher than the pressure in the hollow portion of the space including the central axis of the flexible tube, the hollow portion of the space including the central axis of the flexible tube contracts. The flexible tube also contains a material whose elastic properties change less with temperature than resin. Therefore, the sensitivity change due to temperature changes in the liquid where the optical fiber hydrophone is installed is smaller than in the case of a conventional elastic resin cylinder. As a result, the effect of temperature on the sensitivity of sound wave detection can be suppressed. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a diagram showing an example of the configuration of an acoustic wave detection device having an optical fiber hydrophone according to a first embodiment. [Figure 2] FIG. 1 is a diagram showing an example of the configuration of an optical fiber hydrophone according to a first embodiment. [Figure 3] 3 is a diagram for explaining the operation of the optical fiber hydrophone shown in FIG. 2. FIG. [Figure 4] FIG. 10 is a diagram showing an example of the configuration of an optical fiber hydrophone according to a second embodiment. [Figure 5] 5 is a diagram for explaining the operation of the optical fiber hydrophone shown in FIG. 4. FIG. [Figure 6]FIG. 10 is a diagram showing an example of the configuration of an optical fiber hydrophone according to a third embodiment. [Figure 7] FIG. 7 is a diagram for explaining the operation of the optical fiber hydrophone shown in FIG. 6. DETAILED DESCRIPTION OF THE INVENTION

[0008] Embodiment 1 A description will be given of a sound wave detection device having an optical fiber hydrophone according to the present embodiment 1. Fig. 1 is a diagram showing an example of the configuration of a sound wave detection device having an optical fiber hydrophone according to the first embodiment.

[0009] The acoustic wave detection device 50 comprises a sensing interferometer 21 that detects acoustic waves in water, a light source 22 that provides light to the sensing interferometer 21, a light receiving unit 23 that receives the interfered light, and a demodulation unit 24 that demodulates the interfered light into acoustic waves. The sensing interferometer 21 comprises an optical fiber hydrophone 1, an optical coupler 2, a mirror 3, and a mirror 4. The sensing interferometer 21 is immersed in water when detecting acoustic waves in water.

[0010] The optical fiber hydrophone 1 has an optical fiber coil 6 in which an optical fiber 5d is wound cylindrically. One end of the optical fiber 5d extends to the optical coupler 2, and the other end of the optical fiber 5d extends to the mirror 4. The optical fiber coil 6 modulates the phase of the light propagating through the optical fiber 5d by mechanical strain caused by sound pressure.

[0011] The light source 22 emits light to the optical coupler 2 via the optical fiber 5a. The optical coupler 2 branches the light incident from the light source 22 via the optical fiber 5a, and emits one light to the optical fiber 5c and the other light to the optical fiber 5d. The light emitted from the optical coupler 2 to the optical fiber 5c is reflected by the mirror 3, then propagates through the optical fiber 5c, and returns to the optical coupler 2. On the other hand, the light emitted from the optical coupler 2 to the optical fiber 5d propagates through the optical fiber coil 6, is reflected by the mirror 4, and then propagates through the optical fiber coil 6 again, and returns to the optical coupler 2. The light returning from the optical fiber 5c to the optical coupler 2 is called reference light, and the light returning from the optical fiber coil 6 to the optical coupler 2 is called sensing light.

[0012] The optical coupler 2 combines the reference light incident from the optical fiber 5c with the sensing light incident from the optical fiber coil 6, and outputs the combined interference light to the light receiving unit 23 via the optical fiber 5b. Mechanical strain caused by sound pressure causes a change in the difference in length between the path of the optical fiber coil 6 and the path of the optical fiber 5c. This causes a change in the phase difference between the reference light and the sensing light, and interference light of two types of light with a changed phase difference is output from the optical coupler 2 to the light receiving unit 23.

[0013] When the interference light is incident on the light receiving unit 23 from the optical coupler 2, the light receiving unit 23 performs O / E (Optical / Electronic) conversion to convert the interference light into an electrical signal. The light receiving unit 23 outputs the electrical signal after O / E conversion to the demodulation unit 24. When the demodulation unit 24 receives the electrical signal from the light receiving unit 23, it demodulates the sound pressure of the sound wave detected by the optical fiber coil 6 from the electrical signal.

[0014] (composition) Next, the configuration of the optical fiber hydrophone 1 shown in Figure 1 will be described. Figure 2 is a diagram showing an example of the configuration of the optical fiber hydrophone according to the first embodiment. Figure 2(a) is a diagram showing a schematic diagram of the internal structure of the optical fiber hydrophone 1 with part of the optical fiber coil 6 removed, Figure 2(b) is a cross-sectional view taken along A-A' in Figure 2(a), and Figure 2(c) is a cross-sectional view taken along B-B' in Figure 2(a). For ease of explanation, Figure 2(a) shows three axes indicating directions in three-dimensional space.

[0015] The optical fiber hydrophone 1 includes an optical fiber coil 6, a flexible tube 8 disposed inside the optical fiber coil 6, a pair of lids 7a and 7b that cover the open ends of the optical fiber coil 6 and the flexible tube 8, and a support 9 that connects the pair of lids 7a and 7b. Of the pair of lids 7a and 7b, an opening 10 is formed in lid 7a. The opening 10 may be formed in lid 7b instead of lid 7a.

[0016] The opening 10 serves to connect the space S1 between the flexible tube 8 and the optical fiber coil 6 with the space outside the optical fiber hydrophone 1. When the optical fiber hydrophone 1 is submerged in water, water flows into the space S1 from the outside through the opening 10, making the pressure of the water around the optical fiber coil 6 equal to the pressure of the water in the space S1. As a result, the optical fiber coil 6 expands and contracts in response to the sound pressure of incoming sound waves with a period shorter than the response time of the flow at the opening 10, but is prevented from being mechanically distorted by the pressure of the surrounding water.

[0017] Both ends of the flexible tube 8 are connected to a pair of lids 7a and 7b. The flexible tube 8 cooperates with the lids 7a and 7b to seal a space S2 including the central axis Ax of the flexible tube 8. The central axis Ax shown in FIG. 2(c) is a coordinate axis parallel to the Z axis shown in FIG. 2(a). As shown in FIG. 2(c), the cross section of the flexible tube 8 taken along a plane perpendicular to the central axis Ax has a shape in which peaks and valleys alternately repeat in a wavy manner along the circumference of a circle centered on the central axis Ax. The flexible tube 8 is configured so that when the pressure in the space outside the optical fiber hydrophone 1 becomes higher than the pressure in the hollow portion of space S2, space S2 contracts toward the central axis Ax.

[0018] The flexible tube 8 is made of a material whose temperature change in elastic properties, such as Young's modulus or Poisson's ratio, is smaller than that of resin. Specifically, a metal material is suitable for the flexible tube 8. As a metal material, steel, which has a large elastic limit, is suitable for applications where a wide operating pressure range is important, while aluminum alloys or titanium alloys are suitable for applications where lightweight flexible tube 8 is important. In the first embodiment, the flexible tube 8 is made of a metal material. The wall thickness of the flexible tube 8, the cross-sectional shape of the flexible tube 8, and the diameter of the support 9 are designed taking into consideration at least the following three factors. The first factor is that the bulk modulus of the flexible tube 8, including the space S2, is smaller than the bulk modulus of the liquid into which the optical fiber hydrophone 1 is introduced. The second factor is that the flexible tube 8 comes into contact with the support 9 before the strain generated in the flexible tube 8 due to its contraction reaches its elastic limit, thereby preventing strain exceeding the elastic limit from occurring in the flexible tube 8. The third requirement is to prevent the flexible tube 8 from coming into contact with the support 9 within the operating pressure range of the optical fiber hydrophone 1. Using a metal material for the flexible tube 8 increases its strength compared to resin. Therefore, according to the first embodiment, by making the wall thickness of the flexible tube 8 thinner than that of resin, it is possible to obtain sensitivity equivalent to that of resin.

[0019] In the normal state of the optical fiber hydrophone 1, the hollow portion of space S2 is filled with a gas such as air. However, if the mechanical resonance of the flexible tube 8 increases the sensitivity frequency deviation of the optical fiber hydrophone 1 and affects sound wave detection, a material such as rubber or sponge in which a gas is dispersed in a solid can be inserted into space S2. In this way, by providing a material such as rubber that has a higher mechanical resistance than a gas such as air in space S2 and bringing the flexible tube 8 into contact with this material with a higher mechanical resistance, the mechanical resistance to vibration of the flexible tube 8 is increased, and sensitivity changes near the resonance frequency can be reduced.

[0020] (operation) Next, the operation of the optical fiber hydrophone 1 of the first embodiment will be described. Figure 3 is a diagram for explaining the action of the optical fiber hydrophone shown in Figure 2. Figure 3(a) is a cross-sectional view taken along line A-A' in Figure 2(a), and Figure 3(b) is a cross-sectional view taken along line B-B' in Figure 2(a). Figure 3(a) shows three axes indicating directions in three-dimensional space, just like Figure 2(a).

[0021] When the sensing interferometer 21 including the optical fiber hydrophone 1 of the first embodiment is immersed in water for sound wave detection, water flows into the space S1 from the outside through the opening 10 shown in FIG. 2(b). The pressure in the space S1 becomes equal to the pressure of the water surrounding the optical fiber coil 6. As a result, the water pressure inside and outside the optical fiber coil 6 becomes equal, and deformation of the optical fiber coil 6 due to water pressure from the outside is suppressed. In this way, the optical fiber hydrophone 1 achieves high water pressure resistance.

[0022] After the sensing interferometer 21 is immersed in water, the light source 22 emits light to the optical coupler 2 via the optical fiber 5a. The optical coupler 2 splits the light incident from the light source 22, and emits one light to the optical fiber 5c and the other light to the optical fiber 5d. The light emitted from the optical coupler 2 to the optical fiber 5c is reflected by the mirror 3, propagates through the optical fiber 5c, and returns to the optical coupler 2.

[0023] On the other hand, the light emitted from the optical coupler 2 to the optical fiber 5d propagates through the optical fiber coil 6, is reflected by the mirror 4, and then propagates through the optical fiber coil 6 again and returns to the optical coupler 2. When a sound wave arrives at the optical fiber hydrophone 1, the optical fiber coil 6 expands and contracts due to the sound pressure of the sound wave. This modulates the phase of the light propagating through the optical fiber coil 6.

[0024] When the pressure of the water surrounding the optical fiber coil 6 becomes higher than the pressure in the hollow portion of the space S2, the flexible tube 8 contracts toward the support 9, as shown in Figures 3(a) and 3(b). However, the cross-sectional shape of the flexible tube 8 has alternating peaks and valleys, like a wave. This prevents the flexible tube 8 from buckling due to the pressure of the water surrounding the optical fiber coil 6, and prevents the hollow portion of the space S2 from becoming smaller. Therefore, the bulk modulus of the flexible tube 8, including the space S2, becomes smaller than the bulk modulus of the water surrounding the optical fiber coil 6. As a result, high sensitivity to sound waves arriving at the optical fiber coil 6 is obtained.

[0025] The light whose phase is modulated by the expansion and contraction of the optical fiber coil 6 becomes sensing light and returns from the optical fiber coil 6 to the optical coupler 2. The optical coupler 2 combines the reference light incident from the optical fiber 5c and the sensing light incident from the optical fiber coil 6 to generate interference light, which is output to the light receiving unit 23 via the optical fiber 5b. When the interference light is incident from the optical coupler 2, the light receiving unit 23 outputs an electrical signal obtained by O / E converting the interference light to the demodulation unit 24. When the demodulation unit 24 receives the electrical signal from the light receiving unit 23, it demodulates the sound pressure of the sound wave detected by the optical fiber coil 6 from the electrical signal. In this way, sound waves arriving at the optical fiber hydrophone 1 are detected underwater.

[0026] When the pressure of the water surrounding the optical fiber coil 6 exceeds the operating pressure range, the flexible tube 8 comes into contact with the support 9, causing the optical fiber hydrophone 1 to lose sensitivity. However, when the pressure of the water surrounding the optical fiber coil 6 returns to the operating pressure range, the optical fiber hydrophone 1 resumes detecting sound waves with high sensitivity.

[0027] (effect) The effects of the optical fiber hydrophone 1 of this embodiment 1 will be described. The optical fiber hydrophone 1 of this embodiment 1 has a flexible tube 8 that contracts the hollow portion of space S2 toward the central axis Ax when the pressure of the liquid surrounding the optical fiber coil 6 becomes higher than the pressure in the hollow portion of space S2. The flexible tube 8 is made of a material whose elastic properties change less with temperature than resin. Therefore, the change in sensitivity due to temperature changes in the liquid where the optical fiber hydrophone 1 is installed is smaller than that of a conventional elastic resin cylinder. As a result, the effect of temperature on the sensitivity of sound wave detection can be suppressed.

[0028] Furthermore, the optical fiber hydrophone 1 of the first embodiment is configured so that the flexible tube 8 comes into contact with the support 9 when the pressure of the liquid surrounding the optical fiber coil 6 exceeds the operating pressure range. This prevents the flexible tube 8 from being deformed due to strain exceeding its elastic limit, and suppresses changes in sensitivity after the pressure of the surrounding water returns to the operating pressure range.

[0029] The advantages of the optical fiber hydrophone 1 of the first embodiment will be explained below in comparison with conventional optical fiber hydrophones. For example, in the optical fiber hydrophone disclosed in Patent Document 1, if a metal material is used instead of a resin material for the elastic cylinder, the temperature change in sensitivity is reduced, but the sensitivity is significantly reduced. If an attempt is made to maintain the same sensitivity as in the case of a resin by reducing the thickness of the metal elastic cylinder, the elastic cylinder is more likely to buckle due to water pressure. As a result, water pressure resistance is reduced. In contrast, the optical fiber hydrophone 1 of the first embodiment is configured with a flexible tube 8 whose cross section perpendicular to the central axis Ax has alternating undulating peaks and valleys. This prevents buckling of the elastic member that holds the hollow portion of the sealed space S2 and achieves high sensitivity.

[0030] Furthermore, consider the case where the optical fiber hydrophone disclosed in Patent Document 1 is used to detect the direction of sound waves using a configuration in which multiple hydrophones are arranged in an array. If the sensitivity of all hydrophones arranged in the array is not uniform, the detection error of the direction of sound waves will increase. In the case of an optical fiber hydrophone using an elastic resin cylinder, there is a risk that the detection error of the direction of sound waves will increase due to differences in the water temperature at the installation location. In contrast, according to the first embodiment, the flexible tube 8 is made of a material with elastic properties that change little with temperature, so the sensitivity change due to differences in the water temperature at the installation location of the hydrophone is small. Therefore, the sensitivity change due to differences in water temperature between the installation locations of multiple hydrophones is reduced, and the detection error of the direction of sound waves can be prevented from increasing.

[0031] Embodiment 2 In the second embodiment, the flexible tube has a plurality of springs as members whose elasticity-related characteristics change with temperature less than that of resin. In the second embodiment, the same components as those described in the first embodiment are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. In the second embodiment, the components and operations that differ from the first embodiment will be described in detail, and descriptions of the components and operations that are the same as those in the first embodiment will be omitted.

[0032] Fig. 4 is a diagram showing an example of the configuration of an optical fiber hydrophone according to embodiment 2. An acoustic wave detection device 50 in this embodiment 2 has a configuration in which the optical fiber hydrophone 1 in the sensing interferometer 21 shown in Fig. 1 is replaced with an optical fiber hydrophone 1a shown in Fig. 4. Therefore, in the following, detailed explanations of the configurations of the optical coupler 2, light source 22, light receiving unit 23, and demodulation unit 24 shown in Fig. 1 will be omitted.

[0033] (composition) The configuration of the optical fiber hydrophone 1a of the second embodiment will be described with reference to Fig. 4. Fig. 4(a) is a diagram showing a schematic diagram of the internal structure of the optical fiber hydrophone 1a with a portion of the optical fiber coil 6 removed, Fig. 4(b) is a cross-sectional view taken along A-A' in Fig. 4(a), and Fig. 4(c) is a cross-sectional view taken along B-B' in Fig. 4(a). For ease of explanation, Fig. 4(a) shows three axes indicating directions in three-dimensional space.

[0034] The optical fiber hydrophone 1a includes an optical fiber coil 6, a flexible tube 8a disposed inside the optical fiber coil 6, and a pair of lids 7a and 7b. The flexible tube 8a includes multiple springs 11 arranged along the central axis Ax, and a covering 12 that cooperates with the lids 7a and 7b to seal a space S2 containing the multiple springs 11. The covering 12 is, for example, a sheet-like or cylindrical member. The covering 12 may be made of a metal material, but need not be made of a metal material. The covering 12 may also be made of a resin.

[0035] The covering 12 is attached to each of the lids 7a and 7b by adhesive or the like. This prevents liquid from entering the hollow portion of the space S2 from around the optical fiber hydrophone 1a. Each spring 11 is made of the same material and has the same shape. The multiple springs 11 are arranged apart from each other but are arranged so that their center of gravity overlaps the central axis Ax. In the configuration example shown in Figure 4(c), the eight springs 11 are arranged so that the central angles of two adjacent springs 11 are the same on the circumference of an imaginary circle centered on the central axis Ax. Although Figure 4(c) shows a case where there are eight springs 11, the number of springs 11 is not limited to eight. The springs 11 are made of a material whose temperature change in elastic properties, such as Young's modulus or Poisson's ratio, is smaller than that of resin. Specifically, a metal material is suitable for the springs 11. As a metal material, steel with a large elastic limit is suitable for applications where a wide operating pressure range is important, while aluminum alloy or titanium alloy is suitable for applications where lightweight flexible tube 8a is important. In the second embodiment, spring 11 is made of a metal material.

[0036] The flexible tube 8a is designed taking into consideration the following factors. The first factor is that the bulk modulus of the flexible tube 8a, including the space S2 surrounded by the covering portion 12 and the pair of lid portions 7a and 7b, is smaller than the bulk modulus of the liquid into which the optical fiber hydrophone 1a is poured. The second factor is that the multiple springs 11 come into contact with each other before the strain generated in the springs 11 due to contraction of the flexible tube 8a reaches its elastic limit, so that strain exceeding the elastic limit is not generated in the springs 11. The third factor is that the multiple springs 11 do not come into contact with each other within the operating pressure range of the optical fiber hydrophone 1.

[0037] Furthermore, when the pressure of the liquid around the optical fiber hydrophone 1a increases, the pressure in the space S1 increases, causing the covering 12 and the multiple springs 11 to contract toward the central axis Ax. At this time, the strength of the sheet constituting the covering 12 and the springs 11 is designed so that the deformation caused by the bending of the multiple springs 11 is greater than the deformation caused by the extension of the circumferential length of the covering 12.

[0038] In the normal state of the optical fiber hydrophone 1a, the hollow portion of the space S2 is filled with a gas such as air. However, if the mechanical resonance of the spring 11 increases the sensitivity frequency deviation of the optical fiber hydrophone 1a and affects sound wave detection, a material such as rubber or sponge in which a gas is dispersed in a solid can be inserted into the space S2. In this way, by providing a material such as rubber with a higher mechanical resistance than the above-mentioned gas such as air in the space S2 and bringing the spring 11 into contact with this material with a higher mechanical resistance, the mechanical resistance to the vibration of the spring 11 is increased, thereby suppressing sensitivity changes near the resonant frequency.

[0039] (operation) Next, the operation of the optical fiber hydrophone 1a of the second embodiment will be described. Figure 5 is a diagram for explaining the operation of the optical fiber hydrophone shown in Figure 4. Figure 5(a) is a cross-sectional view taken along line A-A' in Figure 4(a), and Figure 5(b) is a cross-sectional view taken along line B-B' in Figure 4(a). Figure 5(a) shows three axes indicating directions in three-dimensional space, just like Figure 4(a).

[0040] The operations of the light source 22, the light receiving unit 23, the demodulation unit 24, the optical coupler 2, the mirror 3 and the mirror 4 are the same as those described in the first embodiment, and therefore detailed description thereof will be omitted in the second embodiment.

[0041] When the sensing interferometer 21 including the optical fiber hydrophone 1a of the second embodiment is immersed in water for sound wave detection, water flows into the space S1 from the outside through the opening 10 shown in FIG. 4(b). The pressure in the space S1 becomes equal to the pressure of the water surrounding the optical fiber coil 6. As a result, the water pressure inside and outside the optical fiber coil 6 becomes equal, preventing the optical fiber coil 6 from being deformed by the water pressure from the outside. In this way, the optical fiber hydrophone 1a achieves high water pressure resistance.

[0042] Furthermore, when the pressure of the water surrounding the optical fiber coil 6 becomes higher than the pressure in the hollow portion of the space S2, the multiple springs 11 and the covering portion 12 contract toward the central axis Ax, as shown in Figures 5(a) and 5(b). The cross-sectional shape of the covering portion 12 of the flexible tube 8a has peaks in the parts that are in contact with the springs 11 and valleys in the parts that are not in contact, resulting in a wavy shape in which peaks and valleys are alternately repeated.

[0043] When the pressure of the water surrounding the optical fiber coil 6 exceeds the operating pressure range, the multiple springs 11 of the optical fiber hydrophone 1a come into contact with each other, causing a decrease in sensitivity. However, when the pressure of the water surrounding the optical fiber coil 6 returns to the operating pressure range, the optical fiber hydrophone 1a resumes detecting sound waves with high sensitivity.

[0044] (effect) The effects of the optical fiber hydrophone 1a of this second embodiment will be explained. In the optical fiber hydrophone 1a of this second embodiment, the spring 11 of the flexible tube 8a is made of a material whose elastic properties change little with temperature. Therefore, the change in sensitivity due to temperature changes in the liquid where the optical fiber hydrophone 1a is installed is smaller than in the case of a conventional elastic cylinder made of resin. As a result, the effect of temperature changes on the sensitivity of sound wave detection can be suppressed.

[0045] Furthermore, according to the second embodiment, the spring 11 of the flexible tube 8b is made of a material with small temperature changes in elastic properties, so that sensitivity changes due to differences in water temperature at the installation location of the hydrophone are small. Therefore, even if multiple optical fiber hydrophones 1a of the second embodiment are combined and arranged in an array, sensitivity changes due to differences in water temperature between the installation locations of the multiple hydrophones can be reduced, and detection errors in the direction of sound wave arrival can be prevented from increasing.

[0046] Furthermore, for example, in the optical fiber hydrophone disclosed in Patent Document 1, if the elastic cylinder is made of a metal material, the temperature change in sensitivity is reduced, but the sensitivity is significantly reduced. In contrast, according to the second embodiment, the elasticity of the spring 11 is used, which is softer than the compressive elasticity of the elastic cylinder, so high sensitivity can be obtained even when a hard metal material is used.

[0047] Furthermore, in the second embodiment, when the pressure of the liquid around the optical fiber coil 6 exceeds the operating pressure range of the optical fiber hydrophone 1a, the spring 11 comes into contact with other springs 11, preventing strain exceeding the elastic limit from occurring in the spring 11. As a result, when the pressure of the liquid around the optical fiber coil 6 returns to the operating pressure range of the optical fiber hydrophone 1a, the optical fiber hydrophone 1a can resume detecting sound waves with its original sensitivity.

[0048] Embodiment 3 The third embodiment further reinforces the flexible tube described in the second embodiment. In the third embodiment, the same components as those described in the first and second embodiments are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. In the third embodiment, the components and operations that are different from those in the first and second embodiments will be described in detail, and descriptions of the components and operations that are similar to those in the first and second embodiments will be omitted.

[0049] Fig. 6 is a diagram showing an example of the configuration of an optical fiber hydrophone according to embodiment 3. An acoustic wave detection device 50 according to embodiment 3 has a configuration in which the optical fiber hydrophone 1 in the sensing interferometer 21 shown in Fig. 1 is replaced with an optical fiber hydrophone 1b shown in Fig. 6. Therefore, in the following, detailed descriptions of the configurations of the optical coupler 2, light source 22, light receiving unit 23, and demodulation unit 24 shown in Fig. 1 will be omitted.

[0050] (composition) The configuration of the optical fiber hydrophone 1b of the third embodiment will be described with reference to Fig. 6. Fig. 6(a) is a diagram showing a schematic diagram of the internal structure of the optical fiber hydrophone 1b with a portion of the optical fiber coil 6 removed, Fig. 6(b) is a cross-sectional view taken along A-A' in Fig. 6(a), and Fig. 6(c) is a cross-sectional view taken along B-B' in Fig. 6(a). For ease of explanation, Fig. 6(a) shows three axes indicating directions in three-dimensional space.

[0051] The optical fiber hydrophone 1b has an optical fiber coil 6, a flexible tube 8b arranged inside the optical fiber coil 6, and a pair of lids 7a and 7b. The flexible tube 8b has a plurality of springs 11, a reinforcing coil 13 surrounding the plurality of springs 11, and a covering 12 that seals the hollow portion of the space S2 that contains the plurality of springs 11 and the reinforcing coil 13. The reinforcing coil 13 is arranged outside the plurality of springs 11 and inside the covering 12.

[0052] The optical fiber hydrophone 1b of the third embodiment differs from the optical fiber hydrophone 1a described in the second embodiment in three ways. The first difference is that a reinforcing coil 13 is provided between the multiple springs 11 and the covering portion 12. The wire used for the reinforcing coil 13 is one that is easy to bend but does not stretch easily. Specifically, piano wire, which is a high-tensile wire made of hard steel, or aramid fiber wire is suitable for the wire. The reinforcing coil 13 is shaped by winding wire around the multiple springs 11 arranged parallel to the central axis Ax in a direction perpendicular to the central axis Ax so as to cover the side surfaces of all of the springs 11.

[0053] The second difference is the provision of a support 9 connecting the pair of lids 7a and 7b. The diameter of the support 9 and the distance from each spring 11 to the support 9 are designed so that within the operating pressure range of the optical fiber hydrophone 1b, each spring 11 does not come into contact with the support 9 or with the other springs 11, and so that the springs 11 come into contact with the support 9 before the strain of the springs 11 exceeds their elastic limit, preventing strain in the springs 11 that exceeds their elastic limit.

[0054] The third difference is that the support 9, the multiple springs 11, the reinforcing coil 13, and the pair of lids 7a and 7b are wrapped in the covering 12. A specific example of a method for sealing these components with the covering 12 will be described. When, for example, a thermoplastic resin is used as the material for the covering 12, these components are placed in a sheet-shaped, tubular, or bag-shaped covering 12 material, and then open portions such as the outer periphery are welded to seal these components and the hollow portion of the space S2. When a thermosetting resin is used as the material for the covering 12, these components can be wrapped in the uncured material and then cured to form the covering 12.

[0055] (operation) Next, the operation of the optical fiber hydrophone 1b of the third embodiment will be described. Figure 7 is a diagram for explaining the operation of the optical fiber hydrophone shown in Figure 6. Figure 7(a) is a cross-sectional view taken along line A-A' in Figure 6(a), and Figure 7(b) is a cross-sectional view taken along line B-B' in Figure 6(a). Figure 7(a) shows three axes indicating directions in three-dimensional space, similar to Figure 6(a).

[0056] The operations of the light source 22, the light receiving unit 23, the demodulation unit 24, the optical coupler 2, the mirror 3 and the mirror 4 are the same as those described in the first embodiment, and therefore detailed description thereof will be omitted in the third embodiment.

[0057] When the sensing interferometer 21 including the optical fiber hydrophone 1b of the third embodiment is immersed in water for sound wave detection, water flows into the space S1 from the outside through the opening 10 shown in FIG. 6(b). The pressure in the space S1 becomes equal to the pressure of the water surrounding the optical fiber coil 6. As a result, the water pressure inside and outside the optical fiber coil 6 becomes equal, preventing the optical fiber coil 6 from being deformed by the water pressure from the outside. In this way, the optical fiber hydrophone 1b achieves high water pressure resistance.

[0058] Furthermore, when the pressure of the water around the optical fiber coil 6 becomes higher than the pressure in the hollow portion of the space S2, the multiple springs 11, the reinforcing coil 13, and the covering portion 12 contract toward the support 9, as shown in Figures 7(a) and 7(b). The cross-sectional shape of the covering portion 12 of the flexible tube 8b has peaks in the parts that are in contact with the springs 11 and valleys in the parts that are not in contact, resulting in a wavy shape in which peaks and valleys are alternately repeated.

[0059] When the pressure of the water surrounding the optical fiber coil 6 exceeds the operating pressure range, the multiple springs 11 come into contact with the support posts 9, causing the optical fiber hydrophone 1b to lose sensitivity. However, when the pressure of the water surrounding the optical fiber coil 6 returns to the operating pressure range, the optical fiber hydrophone 1b resumes detecting sound waves with high sensitivity.

[0060] (effect) The following describes the effects of the optical fiber hydrophone 1b of this embodiment 3. The optical fiber hydrophone 1b of this embodiment 3 not only provides the same effects as the optical fiber hydrophone 1a described in embodiment 2, but also provides the following effects.

[0061] In the second embodiment, when high pressure is applied to the flexible tube 8a and the covering 12 stretches, and then the pressure on the flexible tube 8a is reduced, the covering 12 does not quickly return to its original shape, which may result in the covering 12 being pinched by the spring 11 and being damaged. In contrast, in the third embodiment, the reinforcing coil 13 is provided between the covering 12 and the spring 11, which prevents the covering 12 from being pinched by the spring 11 when it stretches. This prevents the covering 12 from being damaged, further improving the water pressure resistance.

[0062] In the second embodiment, if a higher pressure is applied to the springs 11 after they bend toward the central axis Ax and are in contact with each other, the flexible tube 8a may be deformed so that the distance from each spring 11 to the central axis Ax becomes different. In this case, even if the pressure applied to the flexible tube 8a decreases, a malfunction may occur in which all of the springs 11 do not return to their original positions. In contrast, in the third embodiment, the support posts 9 are arranged in a direction in which each spring 11 deforms due to external pressure, and each spring 11 comes into contact with the support posts 9 when deformed by pressure. Therefore, the distance from each spring 11 to the central axis Ax becomes uniform, further improving water pressure resistance.

[0063] In the second embodiment, when high pressure is applied to the flexible tube 8a, the covering 12 may lose its ability to seal the hollow portion of the space S2 at the ends connected to one or both of the lids 7a and 7b. In contrast, in the third embodiment, the covering 12 encases the support 9, the springs 11, the reinforcing coil 13, and the pair of lids 7a and 7b, and has a closed shape with no ends connected to other components. This improves the sealing of the hollow portion of the space S2, further improving water pressure resistance.

[0064] (Variation) In the above-described first and third embodiments, the flexible tube 8 or spring 11 comes into contact with the support 9 when the pressure of the liquid around the optical fiber coil 6 exceeds the operating pressure range of the optical fiber hydrophone. However, this is not limited to this case. For example, a configuration may be adopted in which the flexible tube 8 or spring 11 comes into contact with the support 9 connected to only one of the pair of lids 7a and 7b, thereby suppressing deformation due to high pressure. Also, a component (not shown) may be provided to be attached to one of the multiple springs 11, and other adjacent springs 11 may come into contact with the component, thereby suppressing deformation due to high pressure.

[0065] Furthermore, in the second and third embodiments, the case where a plurality of springs 11 are used has been described, but the number of springs 11 is not limited to a plurality, and may be one. When there is one spring 11, for example, a configuration is made in which a support post 9 is provided along the central axis Ax, and the spring 11 is provided in a coil shape so as to surround the support post 9. In this case, as in the second and third embodiments, the hollow portion of the space S2 including the support post 9 and the spring 11 is sealed.

[0066] Furthermore, in the above-described first to third embodiments, the sensing interferometer 21 is configured as a Michelson interferometer using the optical coupler 2 and the mirrors 3 and 4, but this is not the only possible example. The sensing interferometer 21 may be configured as another type of interferometer, such as an interferometer using two fiber Bragg gratings (FBGs).

[0067] In the above-described first to third embodiments, a configuration example has been described in which one of the pair of lids 7a and 7b is provided with one opening 10, but an opening 10 may be provided in each of both lids 7a and 7b, or multiple openings 10 may be provided. Furthermore, in the first to third embodiments, a configuration example has been described in which the central axis Ax of the flexible tubes 8, 8a, and 8b overlaps with the central axis of the cylinder of the optical fiber coil 6, but these axes do not have to overlap. [Explanation of symbols]

[0068] 1, 1a, 1b Fiber Optic Hydrophone 2 Optical Coupler 3, 4 mirror 5a~5d Optical fiber 6 Optical fiber coil 7a, 7b Lid 8, 8a, 8b flexible tube 9 pillars 10 Opening 11 Spring 12 Covering part 13 Reinforcing coil 21 Sensing Interferometer 22 Light source 23 Light receiving part 24 Demodulation section 50 Ultrasonic detector Ax center axis S1, S2 space

Claims

1. an optical fiber coil in which an optical fiber is wound in a cylindrical shape; a flexible tube disposed inside the optical fiber coil; a pair of lids for covering the open ends of the optical fiber coil and the flexible tube, At least one of the pair of lids has an opening that connects a space between the flexible tube and the optical fiber coil to a space outside the one lid, When the pressure in the outer space becomes higher than the pressure in a hollow portion of a space including a central axis of the flexible tube, the hollow portion of the space including the central axis contracts toward the central axis, the flexible tube includes a material whose temperature change in elastic properties is smaller than that of resin, and a cross section taken along a plane perpendicular to the central axis has a shape in which peaks and valleys are alternately repeated along the circumference of a circle centered on the central axis. Fiber optic hydrophone.

2. a support pillar disposed on the central axis and connecting the pair of lid portions; the flexible tube is configured so that an inner surface of the flexible tube comes into contact with the support column before the pressure in the outer space becomes equal to the pressure at which the strain of the flexible tube reaches its elastic limit. The fiber optic hydrophone of claim 1 .

3. An optical fiber coil in which an optical fiber is wound in a cylindrical shape; a flexible tube disposed inside the optical fiber coil; a pair of lids for covering the open ends of the optical fiber coil and the flexible tube, At least one of the pair of lids has an opening that connects a space between the flexible tube and the optical fiber coil to a space outside the one lid, When the pressure in the outer space becomes higher than the pressure in a hollow portion of a space including a central axis of the flexible tube, the hollow portion of the space including the central axis contracts toward the central axis, the flexible tube includes a material whose elasticity-related temperature change is smaller than that of resin; the member is one or more springs provided as the member along the central axis; the flexible tube has a covering portion that seals a hollow portion of a space including the one or more springs; Fiber optic hydrophone.

4. The covering portion is configured to enclose at least the one or more springs and the pair of lid portions.

4. The fiber optic hydrophone of claim 3.

5. The coil spring further includes a reinforcing coil disposed inside the covering and surrounding the one or more springs.

5. The optical fiber hydrophone according to claim 3 or 4.

6. the flexible tube has the plurality of springs spaced apart from one another; The plurality of springs come into contact with each other before the pressure in the outer space becomes equal to the pressure at which the strain of the springs reaches their elastic limit. The optical fiber hydrophone according to any one of claims 3 to 5.

7. The member is made of a metal material. The optical fiber hydrophone according to any one of claims 1 to 6.

8. the flexible tube has a member having a mechanical resistance greater than that of air in a hollow portion of a space including the central axis; The optical fiber hydrophone according to any one of claims 1 to 7.

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