Optical fiber sensor and pressure detecting device

Abstract

Provided are an optical fiber sensor and a pressure detecting device capable of stably performing highly accurate measurement by eliminating the effects of temperature.　This optical fiber sensor 100 comprises first and second optical fibers 11, 12, a hetero-core portion HP which is positioned between the first and second optical fibers 11, 12 and which has a core diameter different from that of the first and second optical fibers 11, 12, a flexible member 21 which flexes and deforms under pressure, and first and second support members 25, 26 which have base end portions that are fixed to the flexible member 21 and which fix and support the first and second optical fibers 11, 12 at the tip ends thereof, the optical fiber sensor 100 detecting pressure on the basis that the transmission loss of light passing through the hetero-core portion HP changes in response to a change in the bending state of the hetero-core portion HP caused by the flexing of the flexible member 21, wherein the first and second support members 25, 26 are arranged in positions at which the output of the hetero-core portion HP is an output whereby the effect of temperature on pressure detection is at most equal to a prescribed standard.

Description

Optical fiber sensor and pressure detection device

[0001] The present invention relates to an optical fiber sensor using a heterocore portion and a pressure detection device using the same.

[0002] Conventionally, optical fiber sensors that use a heterocore portion to detect the displacement of a deforming member or a moving member are known. This optical fiber sensor utilizes the fact that the loss of light passing through the optical fiber changes in response to changes in the state of the optical fiber, such as bending or stretching. The optical fiber is arranged so that the state of the optical fiber changes in response to the displacement of the deforming member or the moving member, and the sensor is configured to detect the displacement of the deforming member or the moving member based on changes in the intensity of the light passing through the heterocore portion. Then, based on the detected displacement of the deforming member or the moving member, the load, pressure, etc. applied to the deforming member or the moving member can be measured (see, for example, Patent Document 1).

[0003] JP 2024-46401 A

[0004] The optical fiber sensor described in Patent Document 1 has high detection sensitivity by providing a hetero-core portion in which the core diameter of the optical fiber is partially changed in a sensor portion in which the curvature of the optical fiber changes. In other words, the optical fiber sensor can detect minute displacements of a diaphragm, which is a deformable member, and can measure loads, pressures, etc. with high accuracy.

[0005] However, if the detection sensitivity of the optical fiber sensor is improved, it will also detect displacement due to thermal expansion of the diaphragm in addition to the load, pressure, etc. that are being measured, which creates the problem that the measurement results may vary depending on various conditions such as the time and location of the measurement.

[0006] In view of the above circumstances, the present invention aims to provide an optical fiber sensor and a pressure detection device that can eliminate the influence of temperature and perform highly accurate measurements stably.

[0007] The optical fiber sensor of the present invention comprises first and second optical fibers having a core and a cladding, a heterocore portion located between the first and second optical fibers, bendable in response to changes in stress and having a core diameter different from that of the first and second optical fibers, a flexible member that bends and deforms under pressure, a first support member whose base end is fixed to the flexible member and fixes and supports the first optical fiber at its tip, and a second support member whose base end is fixed to the flexible member and fixes and supports the second optical fiber at its tip, wherein the bending state of the heterocore portion changes as the spacing between the tip ends of the first and second support members changes in response to the rate of change of the bending amount of the flexible member, and the optical fiber sensor detects pressure based on the change in transmission loss of light that is incident on the first optical fiber, passes through the heterocore portion, and then exits the second optical fiber in response to changes in the bending state of the heterocore portion, characterized in that the first and second support members are positioned at output positions where the effect of temperature on the pressure detection is below a predetermined standard.

[0008] According to the optical fiber sensor of the present invention, by appropriately adjusting the arrangement of the first and second support members, the influence of temperature on pressure detection can be reduced to a predetermined level or less. Specifically, the optical fiber sensor of the present invention is configured to change the distance between the tip ends of the first and second support members in response to bending deformation of the flexible member under pressure, thereby changing the bending state of the hetero core portion.

[0009] The distance between the tips of the first and second support members is determined by the tilt angle of the flexible member at the positions where the first and second support members are arranged, i.e., the rate of change of the amount of deflection and the height of the support members. When pressure is applied to the flexible member, the tilt angle at the center of the deflection becomes zero due to the balance of force and moment, and within a certain range from the center of the deflection, the tilt angle increases with increasing distance from the center of the deflection.

[0010] Therefore, by adjusting the arrangement of the support members, specifically the distance from the center of the bending, it is possible to adjust the inclination angle of the first and second support members in accordance with the bending deformation of the flexible member, and to adjust the amount of change in the bending state of the heterocore portion due to changes in the spacing between the tip ends of the first and second support members.

[0011] In other words, by adjusting the arrangement of the first and second support members, the change in the bending state of the heterocore portion due to the bending deformation of the flexible member caused by the application of pressure can be made sufficiently larger than the change in the bending state of the heterocore portion due to the expansion or contraction of the flexible member, etc., caused by a change in temperature. This increases the detection sensitivity of the bending deformation of the flexible member by the heterocore portion, thereby relatively reducing the detection sensitivity of the expansion or contraction of the flexible member, etc., caused by a change in temperature. As a result, the first and second support members can be positioned at an output position where the effect of temperature on pressure detection is below a predetermined standard (e.g., a thermal sensitivity of 0.1% FS / °C or less), thereby reducing the thermal sensitivity to a practical level without using correction by a temperature sensor.

[0012] In addition, in the optical fiber sensor of the present invention, it is preferable that the first and second support members are arranged at positions spaced apart in opposite directions from the center of the bending of the flexible member, and that the tip ends of the support members are configured to swing in directions away from or towards each other due to the bending deformation of the flexible member.

[0013] In this way, the first and second support members swing in directions away from or towards each other, which further amplifies the detection sensitivity of the deflection of the flexible member, thereby facilitating the design of the thickness of the flexible member and the arrangement of the first and second support members, etc.

[0014] Furthermore, in the optical fiber sensor of the present invention, it is preferable that the flexible member is composed of a diaphragm provided in an opening of a base, and the first and second support members are arranged at positions spaced apart in opposite directions from the center of the bending of the flexible member, and are arranged so that the tip ends swing in directions away from or towards each other due to the bending deformation of the flexible member.

[0015] In this way, since the flexible member is composed of a diaphragm provided in the opening of the base, the state of the flexible member's bending deformation when pressure is applied can be stabilized, and changes in thermal sensitivity due to increases and decreases in pressure can be reduced.

[0016] In the optical fiber sensor of the present invention, the flexible member has a linear expansion coefficient of 30×10 －６ It is preferable that the material be a material having a temperature of 1000KJ / °C or less.

[0017] This reduces the effect of temperature on pressure detection because the flexible member has a low coefficient of linear expansion, which reduces temperature-related changes in the flexible member. Furthermore, the flexibility in the thickness of the flexible member and the arrangement of the first and second support members is increased, which simplifies design.

[0018] In the optical fiber sensor of the present invention, the flexible member is preferably made of brass, phosphor bronze, beryllium copper, stainless steel or ceramics.

[0019] This allows the flexible member to have good spring properties and reduce the mechanical hysteresis of the flexible member, thereby enabling more accurate pressure detection.

[0020] In the optical fiber sensor of the present invention, it is preferable that the flexible member is made of brass, phosphor bronze or beryllium copper, and that the surface thereof is coated with an anti-rust material.

[0021] This makes it possible to prevent the flexible member from rusting even in a corrosive environment such as underwater, so that the environment in which it can be used is not limited.

[0022] Furthermore, the pressure detection device of the present invention comprises the optical fiber sensor of the present invention, a measurement unit that measures the light emitted from the second optical fiber, a conversion unit that converts the output from the measurement unit into a pressure value, and a temperature sensor that detects the temperature of the measurement unit, and is characterized in that the conversion unit corrects the output from the measurement unit based on the output of the temperature sensor.

[0023] According to this, since the optical fiber sensor of the present invention reduces the effects of temperature by arranging the first and second support members, temperature compensation means in this part is not required, and the device can be configured with only temperature compensation means in the measurement section, thereby simplifying the configuration of the device.

[0024] The optical fiber sensor and pressure detection device of the present invention can provide the excellent effect of being able to eliminate the influence of temperature and perform highly accurate measurements stably.

[0025] 1 is a schematic diagram of a pressure detection device according to an embodiment of the present invention; FIG. 1 is a cross-sectional view of an optical fiber; FIG. 2 is a cross-sectional view of the chamber taken along line A-A in FIG. 1; FIG. 3 is a view of the chamber from the rear side of FIG. 1 with the rear frame removed; FIG. 4 is an exaggerated view of a diaphragm deflected by application of water pressure; FIG. 5 is a diagram showing the positional relationship between the first and second support members and the state of the optical fiber between them when the diaphragm is deflected; FIG. 6 is a diagram showing the positional relationship between the first and second support members and the state of the optical fiber between them when the diaphragm is deflected; FIG. 7 is a graph showing the distribution trend of the amount of deflection of the diaphragm on the x-axis when the center of the diaphragm is the origin; FIG. 8 is a graph showing the distribution trend of the rate of change of the amount of deflection of the diaphragm on the x-axis when the center of the diaphragm is the origin; FIG. 9 is a graph showing the trend of change in the sensitivity of the hetero core portion when the distance between the first and second support members is changed; and FIG. 10 is a diagram showing an example of another form of the first and second support members.

[0026] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that the dimensions and shapes in the drawings are not necessarily accurate, and some descriptions have been omitted or simplified to facilitate understanding.

[0027] 1 is a diagram schematically illustrating a pressure detection device 1 according to one embodiment of the present invention. The pressure detection device 1 of this embodiment is placed underwater in various tanks, pipes, etc., and measures a water level WL by detecting a water pressure (hydrostatic pressure) P. As shown in FIG. 1, the pressure detection device 1 is composed of an optical fiber sensor 100 and a measurement controller 200.

[0028] The optical fiber sensor 100 comprises an optical fiber 10 having a heterocore portion HP, and a chamber 20 in which the heterocore portion HP is disposed and which has a diaphragm (flexible member) 21 that flexes and deforms in response to water pressure P.

[0029] Fig. 2 is a cross-sectional view of the optical fiber 10. As shown in Fig. 2, the optical fiber 10 is composed of a first optical fiber 11 at an incident end side to which light is incident from the measurement controller 200, a second optical fiber 12 at an output end side from which the light incident from the measurement controller 200 is output, and a heterocore portion HP inserted between the first optical fiber 11 and the second optical fiber 12 and capable of bending together with the first and second optical fibers in response to changes in stress.

[0030] The heterocore portion HP is a short optical fiber strand having a core 13 through which light is transmitted (passes) and a cladding 14 that covers the outer periphery of the core 13. The first and second optical fibers 11 and 12 are long optical fiber strands that connect the heterocore portion HP and the measurement controller 200, and each have a core 15 through which light is transmitted and a cladding 16 that covers the outer periphery of the core 15.

[0031] The hetero core portion HP and the first and second optical fibers 11, 12 are all composed of single-mode optical fibers. The diameter of the core 13 in the hetero core portion HP is, for example, 5 μm, and the diameter of the cladding 14 is, for example, 125 μm. On the other hand, the diameter of the core 15 in the first and second optical fibers 11, 12 is, for example, 9 μm, and the diameter of the cladding 16 is, for example, 125 μm.

[0032] That is, the heterocore portion HP is configured to locally provide a portion in the optical fiber 10 with a smaller core diameter than the first and second optical fibers 11, 12, thereby allowing a portion of the transmitted light to leak appropriately into the cladding 14, and the amount of light leakage varies depending on the bending state of the heterocore portion HP and its vicinity. Specifically, the smaller the radius of curvature of the heterocore portion HP and the first and second optical fibers 11, 12 in its vicinity, the greater the amount of light leakage (transmission loss). This amount of light leakage (transmission loss) is adjusted by setting the length Lh of the heterocore portion HP.

[0033] In this embodiment, the hetero core portion HP and the first and second optical fibers 11, 12 are joined approximately coaxially by discharge fusion or the like so that the cores 13 and 15 are joined at interfaces 17 perpendicular to the longitudinal direction.

[0034] The heterocore portion HP may have a larger core diameter than the first and second optical fibers 11 and 12, and in this case, the amount of light leakage can be changed depending on the bending state of the heterocore portion HP. The heterocore portion HP may be made of a single material having a refractive index equivalent to that of the cladding 16 or the core 15 of the first and second optical fibers 11 and 12. In this case, the heterocore portion HP can be considered to have a heterocore structure in which the diameter of the core 13 of the heterocore portion HP is set to 0 or the same as the diameter of the cladding 16. The heterocore portion HP may be formed by, for example, melt drawing, and the diameters of the core 13 and the core 15 may be gradually changed.

[0035] Furthermore, the heterocore portion HP and the first and second optical fibers 11, 12 are preferably made of single-mode optical fiber strands in terms of being less susceptible to external influences, but may also be made of multi-mode optical fiber strands.

[0036] As shown in Fig. 1, the chamber 20 is a hollow rectangular parallelepiped member having an enclosed space S formed therein, and is formed by combining a front frame 22 on the front side of Fig. 1 with a rear frame 23 (see Fig. 3A) on the rear side of Fig. 1. In this embodiment, the chamber 20 is made of brass. The surface of the chamber 20 is coated with an appropriate anti-corrosion material, which prevents rust from occurring even in a corrosive environment such as underwater.

[0037] 3A is a cross-sectional view of the chamber 20 taken along line AA in FIG. 1, and FIG. 3B is a view of the chamber 20 (i.e., the front frame 22) from the rear side of FIG. 1 with the rear frame 23 removed.

[0038] The front frame 22 and the rear frame 23 are fastened together in a watertight manner with four bolts (not shown) with an appropriate sealant interposed therebetween, thereby preventing water from entering the sealed space S. The sealed space S is also connected to the atmosphere via a vent pipe 24, thereby maintaining the internal pressure at approximately atmospheric pressure. One end of the vent pipe 24 is watertightly fixed to a hole provided in the rear frame 23 with an appropriate adhesive or the like, and the other end is open to the atmosphere (see FIG. 1).

[0039] The front side frame 22 comprises a rectangular frame-shaped base 22a, a thin diaphragm 21 arranged to cover the opening of the base 22a, a first support member 25 whose base end is fixed to the inner surface of the diaphragm 21 and which fixes and supports the first optical fiber 11 at its tip end, and a second support member 26 whose base end is fixed to the inner surface of the diaphragm 21 and which fixes and supports the second optical fiber 12 at its tip end.

[0040] The base 22a supports the outer peripheral edge of the diaphragm 21 and is connected to the rear frame 23. The base 22a has screw holes (not shown) formed at appropriate positions, and a first groove 22a1 through which the first optical fiber 11 is inserted and a second groove 22a2 through which the second optical fiber 12 is inserted are formed on a mating surface 22a3 with the rear frame 23.

[0041] In this embodiment, the first and second grooves 22a1 and 22a2 are provided on the same side of the sealed space S, thereby facilitating the routing of the optical fiber 10. Specifically, the first and second grooves 22a1 and 22a2 are provided so as to communicate the sealed space S with the outside of the chamber 20 by extending substantially along the y direction on one side of the y direction (the lower side in FIG. 3B ) among the x, y, and z directions which are orthogonal to each other.

[0042] The diaphragm 21 is configured as a rectangular flat plate with rounded corners. The x-direction dimension Lx, y-direction dimension Ly, and thickness (z-direction dimension) Lz of the diaphragm 21 are set to dimensions that allow it to bend and deform appropriately in response to the water pressure P to be detected.

[0043] The first and second support members 25, 26 are configured as rectangular pillars protruding from the inner surface of the diaphragm 21 in the z direction. The first and second support members 25, 26 are positioned at equal distances (L1 / 2) in opposite directions in the x direction from the center C of the diaphragm 21. This allows the tips of the first and second support members 25, 26, to which the first and second optical fibers 11, 12 are fixed, to swing toward or away from each other in response to the flexural deformation of the diaphragm 21, thereby changing the bending state of the heterocore portion HP disposed therebetween and the first and second optical fibers 11, 12 in the vicinity thereof. Therefore, the dimensions and shapes of the first and second support members 25, 26 are set to behave as substantially rigid bodies so that the change in the distance between their tips during swing accurately reflects the change in the flexural deformation of the diaphragm 21.

[0044] In addition, as will be described in detail later, in this embodiment, the height H of the first and second support members 25, 26 and the distance L1 between them are appropriately set to eliminate the effect of temperature on the detection of water pressure P. Furthermore, in this embodiment, the distance Le in the x direction between the first and second support members 25, 26 and the base 22a is set to a dimension that does not inhibit the flexural deformation of the diaphragm 21, thereby minimizing the x direction dimension of the chamber 20.

[0045] The rear frame 23 is composed of a rectangular rear base 23a and a blocking portion 23b that is provided to block the opening of the rear base 23a. Unlike the diaphragm 21, the blocking portion 23b is set to a thickness that causes almost no bending deformation even when water pressure P is applied.

[0046] In this embodiment, the front frame 22 and the rear frame 23 are both made of brass, but the front frame 22 and the rear frame 23 may be made of other metals, ceramics, resin, etc. The front frame 22 and the rear frame 23 may also be made of different materials. If the front frame 22 and the rear frame 23 are made of a corrosion-resistant material such as stainless steel or ceramics, the coating with a rust-preventive material may be omitted.

[0047] In this embodiment, the front frame 22 is integrally formed by machining a brass block, but the diaphragm 21, base 22a, and first and second support members 25, 26 may be formed separately and then connected by welding, adhesive, or the like. In this case, the diaphragm 21, base 22a, and first and second support members 25, 26 may be made of different materials. The same applies to the rear frame 23.

[0048] However, the material of the diaphragm 21 is preferably made of brass, phosphor bronze, beryllium copper, stainless steel or ceramics in order to provide good spring properties and reduce the mechanical hysteresis of the flexible member, thereby improving the accuracy of detecting the water pressure P.

[0049] 3B, the optical fiber 10 is arranged in the chamber 20 in a state in which it is bent in only one direction in the xy plane with a predetermined radius of curvature R1, i.e., in a state in which it is bent in a U-shape. The heterocore portion HP is located at the apex of the U, and is arranged between the first and second support members 25 and 26.

[0050] The first and second optical fibers 11, 12 of the optical fiber 10 are arranged along the tip surfaces of the first and second support members 25, 26 and are fixed by resin 27 attached to the tip surfaces. As a result, the radius of curvature of the hetero core portion HP between the first and second support members 25, 26 and the first and second optical fibers 11, 12 changes in the xy plane as the first and second support members 25, 26 swing, while the arc length A1 remains constant, i.e., the chord length changes.

[0051] The radius of curvature R1 of the heterocore portion HP and its vicinity is not particularly limited, but it is preferable that the radius be such that it is not only easy to detect changes in the amount of light leakage that accompany changes in the bending state, but also takes into consideration the likelihood of breakage or the like occurring during assembly or use.

[0052] The first and second optical fibers 11 and 12 are also inserted into the first and second grooves 22a1 and 22a2, respectively, and are fixed to the base 22a by being embedded in the resin 27 that fills these grooves. As a result, the first and second optical fibers 11 and 12 both extend from the chamber 20 toward one side in the y direction. Furthermore, by filling the first and second grooves 22a1 and 22a2 with the resin 27, the airtightness of the chamber 20 is ensured.

[0053] Although not shown, it is preferable to provide an appropriate protective member to protect the optical fiber 10 extending from the chamber 20 from unexpected deformation, breakage, etc., depending on the environment in which the pressure detection device 1 is used. In this case, the protective member may protect only the optical fiber 10, or may protect the optical fiber 10 as well as the chamber 20 and the ventilation pipe 24.

[0054] As shown in FIG. 1, the measurement controller 200 includes a measurement unit 210 to which the optical fiber 10 is connected, a temperature sensor 220 that measures the temperature of the measurement unit 210, an amplifier unit 230 that performs A / D conversion and amplifies the signal transmitted from the measurement unit 210, and a conversion unit 240 that converts the signal transmitted from the amplifier unit 220 into water pressure P or a water level WL based thereon.

[0055] The measuring unit 210 includes a light source 211 (e.g., a light-emitting diode or the like) that inputs light into the first optical fiber 11, and a light-receiving unit 212 (e.g., a photodiode or the like) that detects light emitted from the second optical fiber 12, and is configured to transmit an analog signal corresponding to the intensity (power) of the light detected by the light-receiving unit 212 to the amplifying unit 230. The temperature sensor 220 is configured to measure the temperature of the measuring unit 210, and transmit a signal corresponding to the measured temperature to the amplifying unit 230.

[0056] The amplifier unit 230 is configured to convert the analog signal sent from the measurement unit 210 into a digital signal, amplify the signal with a predetermined gain, and send the digital signal to the conversion unit 240. The amplifier unit 230 is also configured to perform temperature compensation on the analog signal from the measurement unit 210 or the digital signal sent to the conversion unit 240, as necessary, based on the signal sent from the temperature sensor 220.

[0057] The conversion unit 240 is configured to calculate the water pressure P or the water level WL based on the digital signal sent from the amplification unit 230, and to send a digital signal corresponding to the calculated water pressure P or water level WL to an external computer, storage device, etc. Note that various known configurations can be applied to each unit of the measurement controller 200, and therefore detailed explanations will be omitted.

[0058] Next, the operation of the optical fiber sensor 100 will be described.

[0059] Fig. 4A is an exaggerated view of the state of diaphragm 21 deflected by application of water pressure P, and is a cross-sectional view of chamber 20 taken along line A-A in Fig. 1. Figs. 4B and 4C are views showing the positional relationship between first and second support members 25, 26 and the state of optical fiber 10 therebetween when diaphragm 21 deflects, and are views of the vicinity of first and second support members 25, 26 as viewed from the rear side of Fig. 1.

[0060] 4A, when subjected to a water pressure P higher than atmospheric pressure, the diaphragm 21 is flexed and deformed so that the center C is most recessed. This flexural deformation of the diaphragm 21 causes the first and second support members 25, 26 to tilt at an angle θ with respect to the z-axis in the xz plane, and the distance in the x-direction between the tip ends of the two members to which the optical fibers 10 are fixed increases from distance L1 to distance L2.

[0061] 4B , the optical fiber 10 between the first and second support members 25, 26 has a constant arc length A1 and a chord length that increases from distance L1 to distance L2, and therefore the radius of curvature in the xy plane increases from radius R1 to radius R2. This reduces the amount of leakage (transmission loss) of light incident on the first optical fiber 10 from the light source 211 in the hetero core portion HP, and the intensity of the light emitted from the second optical fiber 12 and received by the light receiving unit 212 increases compared to when water pressure P is not applied. Note that the change in the radius of curvature of the optical fiber 10 in the xz plane is negligibly small.

[0062] The optical fiber sensor 100 of this embodiment is configured to change the bending state of the optical fiber 10 between the first and second support members 25, 26 by tilting them in accordance with the water pressure P, thereby changing the transmission loss of light in the hetero core portion HP. The measurement controller 200 is configured to calculate the water pressure P or the water level WL based on the water pressure P based on a change in the intensity of light received by the light receiving portion 212 (i.e., the output of the hetero core portion HP) that accompanies a change in the transmission loss of light in the hetero core portion HP.

[0063] Here, the displacement ΔLp of the tip portions of the first and second support members 25, 26 in the x direction due to the inclination of these members due to the water pressure P is expressed by the following equation (1).

[0064] ΔLp=H sinθ (1) The amount of change ΔL in the distance between the first and second support members 25, 26 is the sum of the displacements ΔLp of the first and second support members 25, 26, as shown in FIG. 4B, and is therefore expressed by the following equation (2).

[0065] ΔL=L2−L1=2ΔLp (2) However, depending on the environment in which the optical fiber sensor 100 is used, the temperature of the chamber 20 may change due to seasonal changes, etc., causing the chamber 20 to expand or contract. The amount of change ΔLt in the gap between the first and second support members 25, 26 due to this temperature change is expressed by the following equation (3), where α is the linear expansion coefficient of the material of the chamber 20 and ΔT is the amount of change in temperature.

[0066] ΔLt = αL1ΔT (3) When water pressure P is applied in an environment where a temperature change ΔT occurs, the amount of change ΔL in the spacing between the first and second support members 25, 26 is the sum of the amount of change 2ΔLp due to the water pressure P and the amount of change ΔLt due to the temperature change ΔT, as shown in Figure 4C, and is expressed by the following equation (4).

[0067] ΔL=L2-L1=2ΔLp+ΔLt (4) The optical fiber 10 does not inherently require temperature compensation because its transmission loss is low in sensitivity to temperature changes, but the presence of the heterocore portion HP increases the sensitivity of its transmission loss to changes in the bending state, and so if the ratio of the change ΔLt caused by the temperature change ΔT to the change 2ΔLp caused by the water pressure P is large, the effect that temperature changes have on changes in transmission loss becomes significant, and temperature compensation becomes necessary. In other words, even when using an optical fiber 10 that does not inherently require temperature compensation, it may end up in a situation where temperature compensation is required.

[0068] This problem can be solved by making the change 2ΔLp caused by the water pressure P sufficiently larger than the change ΔLt caused by the temperature change ΔT, thereby relatively reducing the effect of temperature change on the change in transmission loss. As a specific measure, for example, as described in Patent Document 1, one that can be considered is to increase the height H of the first and second support members 25, 26.

[0069] In this case, however, as shown in formula (2), the amount of change 2ΔLp increases in proportion to the height H, and therefore, for example, to double the amount of change 2ΔLp, the height H must be doubled, which increases the dimension of the chamber 20 in the z direction and compromises the compactness of the optical fiber sensor 100. Therefore, in this embodiment, by adjusting the positions of the first and second support members 25, 26 of the diaphragm 21 in the x direction, it is possible to make the amount of change 2ΔLp sufficiently larger than ΔLt without compromising the compactness of the optical fiber sensor 100.

[0070] Fig. 5A is a graph showing the tendency of the distribution on the x-axis of the deflection amount δ (displacement in the z-direction) of the diaphragm 21 (i.e., the distribution on the straight line along which the first and second support members 25, 26 are arranged) when the center C of the diaphragm 21 is the origin. Fig. 5B is a graph showing the tendency of the distribution on the x-axis of the rate of change (slope) δ' (= tan θ) of the deflection amount δ of the diaphragm 21 when the center C of the diaphragm 21 is the origin.

[0071] Considering the deflection of a beam fixed at both ends under a uniformly distributed load and the deflection of a flat plate fixed all around under a uniformly distributed load, the diaphragm 21, when subjected to water pressure P, will bend and deform in a quartic curve on the line along which the first and second support members 25, 26 are arranged, as shown in Figure 5A. As the water pressure P increases from P0, for example, to twice (2P0) or three times (3P0), the absolute value of the deflection amount δ at each position in the x direction will also increase.

[0072] Furthermore, the rate of change δ' of the deflection amount δ at each position on the line along which the first and second support members 25, 26 are arranged is obtained by differentiating the deflection amount δ, and therefore varies in a cubic curve as shown in Fig. 5B. That is, at the center C of the diaphragm 21, the rate of change δ' of the deflection amount δ is 0 due to the balance of forces and moments, and the absolute value of the rate of change δ' of the deflection amount δ increases with increasing distance from the center C up to the position where the rate of change δ' of the deflection amount δ is at its extreme value.

[0073] The inventors focused on the characteristics of the flexural deformation of the diaphragm 21 and discovered that by positioning the first and second support members 25, 26 at a position appropriately spaced from the center C of the diaphragm 21, it is possible to increase the change 2ΔLp caused by the water pressure P more efficiently than by increasing the height H.

[0074] Specifically, by adjusting the positions of the first and second support members 25, 26 in the x direction, the change 2ΔLp caused by the water pressure P can be increased at a higher rate than by adjusting the height H by appropriately separating the first and second support members 25, 26 from the center C.

[0075] Furthermore, because a relatively large moment is applied near the periphery of diaphragm 21, sufficient flexural deformation is possible even over a short distance, and therefore even if the distance from center C of first and second support members 25, 26 is set large, there is no need to set interval Le (see FIG. 3B) large accordingly. In other words, by adjusting the positions of first and second support members 25, 26 in the x direction, the change 2ΔLp caused by water pressure P can be increased without substantially increasing the dimension of chamber 20 in the x direction.

[0076] 5B, as the water pressure P increases from P0 to, for example, twice (2P0) or three times (3P0), the absolute value of the rate of change δ' of the deflection δ at each position in the x direction also increases. However, as shown by the change in the spacing between the curves in FIG. 5B, by appropriately separating the first and second support members 25, 26 from the center C, it is possible to increase the rate of change of the change 2ΔLp with increasing water pressure P. In other words, by adjusting the positions of the first and second support members 25, 26 in the x direction, it is possible to adjust not only the change 2ΔLp at each water pressure P, but also the rate of change of the change 2ΔLp with changes in water pressure P.

[0077] In addition, when the diaphragm 21 actually flexes and deforms, the portions where the first and second support members 25, 26 are provided behave as approximately rigid bodies, and therefore this point must be taken into consideration when adjusting the positions of the first and second support members 25, 26 in the x direction.

[0078] Furthermore, when adjusting the positions of the first and second support members 25, 26 in the x-direction, it is necessary to take into consideration that the sensitivity of the hetero core portion HP decreases as the first and second support members 25, 26 are moved away from the center C of the diaphragm 21 and the distance L1 between them is increased.

[0079] Figure 6 is a graph showing the tendency of change in the sensitivity of the heterocore portion HP when the distance L1 between the first and second support members 25, 26 is changed, and shows the radius of curvature R2 when the radius of curvature R1 of the heterocore portion HP and its vicinity is fixed to a provisional value R0, and the change in the distance between the first and second support members 25, 26, ΔL = L2 - L1, is fixed to a provisional value ΔL0.

[0080] 6, from the relationship between the radius, arc length, and chord length of the circle, the smaller the spacing L1 between the first and second support members 25, 26, the larger the radius of curvature R2 for the same spacing change ΔL, and the larger the change in optical transmission loss. Also, the larger the spacing L1 between the first and second support members 25, 26, the smaller the radius of curvature R2 for the same spacing change ΔL, and the smaller the change in optical transmission loss. This tendency appears in a similar form depending on the size of the radius of curvature R1.

[0081] That is, the smaller the distance L1 between the first and second support members 25, 26 relative to the radius of curvature R1, the higher the sensitivity of the heterocore HP; the larger the distance L1 between the first and second support members 25, 26 relative to the radius of curvature R1, the lower the sensitivity. Therefore, if the distance L1 between the first and second support members 25, 26 is too small relative to the radius of curvature R1, the change ΔLt due to the temperature change ΔT will be detected sensitively, which is undesirable. Furthermore, if the distance L1 between the first and second support members 25, 26 is too large relative to the radius of curvature R1, the change 2ΔLp due to the water pressure P will be large, but the detection of this change will be insensitive, which is undesirable. According to the inventors' research, the distance L1 between the first and second support members 25, 26 is preferably between 60% and 120% of the radius of curvature R1.

[0082] In this embodiment, the first and second support members 25, 26 are positioned to balance the rate of change δ' of the deflection amount δ of the diaphragm 21 and the sensitivity of the heterocore portion HP. This allows the temperature effect on the detection of the water pressure P, i.e., the thermal sensitivity, to be kept below 0.1% FS / °C. In other words, by positioning the first and second support members 25, 26 at an output from the heterocore portion HP where the temperature effect on the detection of the water pressure P is below a predetermined standard (thermal sensitivity of 0.1% FS / °C or less), the influence of temperature is eliminated, enabling highly accurate detection of the water pressure P or stable measurement of the water level WL. Furthermore, the measurement controller 200 is provided with only a temperature compensation means for the measurement unit 210, which operates electrically and is difficult to eliminate the influence of temperature, thereby simplifying the device configuration.

[0083] It goes without saying that the smaller the linear expansion coefficient of the material of the chamber 20, the smaller the effect of temperature on the detection of the water pressure P. Therefore, the linear expansion coefficient of the material of the chamber 20 is 30×10 －６ / °C or less, and －６ / °C or less is more preferable. Furthermore, it is preferable that expansion or contraction due to temperature changes occurs as uniformly as possible in the chamber 20 so as not to cause bending deformation of the diaphragm 21. Therefore, it is preferable that at least the diaphragm 21 and the base 22a are made of the same material.

[0084] Next, a modification of this embodiment will be described.

[0085] 7 is a diagram showing an example of another embodiment of the first and second support members 25, 26, and is a cross-sectional view of the chamber 20 taken along line A-A in FIG. 1. In this example, the first and second support members 25, 26 are configured to extend obliquely with respect to the z-axis. By shifting the positions of the distal and proximal ends of the first and second support members 25, 26 in the x-direction in this manner, it is possible to individually set the interval L1, which is the chord length of the optical fiber 10, and the positions of the first and second support members 25, 26. This allows for more fine adjustment of the balance between the rate of change δ' of the deflection amount δ of the diaphragm 21 and the sensitivity of the heterocore portion HP, thereby minimizing the effect of temperature on the detection of the water pressure P.

[0086] It goes without saying that the first and second support members 25, 26 may be configured in a shape other than that shown in Figure 7, so that the interval L1, which is the chord length of the optical fiber 10, and the positions of the first and second support members 25, 26 can be set individually.

[0087] Alternatively, although not shown, an optical coupler may be provided midway through the first optical fiber 11 to branch off another optical fiber, and a reflecting section with a mirror formed by silver deposition or the like may be provided at the end of the shortened second optical fiber 12, and the end of the branched optical fiber may be connected to the light receiving section 212. In this case, it is possible to extend only the first optical fiber 11 from the chamber 20, which makes it easier to handle the optical fiber 10.

[0088] Furthermore, an OTDR (Optical time-domain reflectometer) device may be connected to the end of the second optical fiber 12 as the light receiving unit 212, and the backward Rayleigh scattered light of the sensor light incident from the OTDR device may be measured.

[0089] Furthermore, the chamber 20 may be one in which the inside of the sealed space S is maintained at a pressure other than atmospheric pressure by connecting a pump or the like to the ventilation pipe 24 or by sealing the chamber 20 without the ventilation pipe 24. Furthermore, the chamber 20 may be one that is fixed to other members or various devices and used.

[0090] Furthermore, the pressure detection device 1 is not limited to detecting the water pressure P or measuring the water level WL, but may be configured to detect the pressure or liquid level of various liquids other than water, or to detect the pressure of various gases. Furthermore, the pressure detection device 1 is not limited to detecting a pressure higher than the pressure in the sealed space S, but may be configured to detect a pressure lower than the pressure in the sealed space S. In this case, the direction of the flexural deformation of the diaphragm 21 and the direction of the inclination of the first and second support members 25, 26 due to the flexural deformation of the diaphragm 21 are reversed, but even in this case, the above-mentioned effect can be achieved by adjusting the positions of the first and second support members 25, 26 in the x direction.

[0091] The above describes an embodiment of the present invention, but the optical fiber sensor and pressure detection device of the present invention are not limited to the above-described embodiment, and various modifications can be made within the scope that does not deviate from the gist of the present invention.

[0092] Furthermore, the actions and effects shown in the above-described embodiments are merely a list of the most preferable actions and effects resulting from the present invention, and the actions and effects of the present invention are not limited to these.

[0093] REFERENCE SIGNS LIST 1 Pressure detection device 11 First optical fiber 12 Second optical fiber 13, 15 Core 14, 16 Cladding 21 Diaphragm (flexible member) 22a Base 25 First support member 26 Second support member 100 Optical fiber sensor 210 Measuring section 220 Temperature sensor 230 Conversion section C Center of diaphragm HP Heterocore section

Claims

1. An optical fiber sensor comprising: first and second optical fibers having a core and a cladding; a heterocore portion located between the first and second optical fibers, bendable in response to changes in stress, and having a core diameter different from that of the first and second optical fibers; a flexible member that flexes and deforms when subjected to pressure; a first support member whose base end is fixed to the flexible member and which fixes and supports the first optical fiber at its tip; and a second support member whose base end is fixed to the flexible member and which fixes and supports the second optical fiber at its tip, wherein the bending state of the heterocore portion changes as the spacing between the tip ends of the first and second support members changes in response to a rate of change in the amount of bending of the flexible member, and the optical fiber sensor detects pressure based on changes in transmission loss of light that is input to the first optical fiber, passes through the heterocore portion, and is output from the second optical fiber in response to changes in the bending state of the heterocore portion, wherein the first and second support members are positioned at output positions where the effect of temperature on pressure detection is below a predetermined standard.

2. An optical fiber sensor as claimed in claim 1, characterized in that the first and second support members are arranged at positions spaced apart in opposite directions from the centre of the bending of the flexible member, and are arranged so that the tip ends thereof swing in directions moving away from or towards each other as the flexible member bends.

3. An optical fiber sensor as claimed in claim 1, wherein the flexible member is made up of a diaphragm provided in an opening in a base, and the first and second support members are arranged at positions spaced apart in opposite directions from the centre of the bending of the flexible member, and are arranged so that the tip ends thereof swing in directions moving away from or towards each other due to the bending deformation of the flexible member.

4. In the optical fiber sensor according to claim 1, the flexible member has a linear expansion coefficient of 30×10 －６ An optical fiber sensor characterized by being made of a material having a temperature of 100°C or less.

5. An optical fiber sensor according to claim 1, wherein the flexible member is made of brass, phosphor bronze, beryllium copper, stainless steel or ceramics.

6. An optical fiber sensor according to claim 1, wherein the flexible member is made of brass, phosphor bronze or beryllium copper, and the surface is coated with an anti-corrosion material.

7. A pressure detection device comprising: the optical fiber sensor according to claim 1; a measurement unit that measures the light emitted from the second optical fiber; a conversion unit that converts the output from the measurement unit into a pressure value; and a temperature sensor that detects the temperature of the measurement unit, wherein the conversion unit corrects the output from the measurement unit based on the output of the temperature sensor.

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