Optical fiber sensor and material detection device using it

Abstract

This invention provides a highly durable optical fiber sensor and a material detection device using it, which enable the monitoring of light up to wavelengths of around 5.5 μm on the longer wavelength side using absorption spectroscopy. [Solution] The optical fiber 12 has a core 16 and a cladding 18 covering the core 16. The optical fiber 12 has a core 16 made of fluoride glass and is capable of transmitting infrared light in the mid-infrared region. The optical fiber sensors 10 and 20 have a detection unit 14 in the middle of the optical path of the core 16, and the cladding 18 on which the detection unit 14 is located is formed to be thinner than other parts or the core 16 is exposed. When light passes through the core 16 of the detection unit 14, a portion of the light passing through the core 16 leaks out of the core 16 of the detection unit 14, and light with a spectrum specific to the object being measured that is in contact with the detection unit 14 is absorbed. The system includes a light source 32 and a spectral analyzer 34 capable of detecting the spectrum of light transmitted through the optical fiber 12.

Description

Technical Field

[0001] This invention relates to an optical fiber sensor for directly detecting carbon dioxide or the like using an optical fiber, and a substance detection device using the same.

Background Art

[0002] Conventionally, for example, as disclosed in Patent Document 1, an optical fiber sensor using a hetero-core optical fiber including an optical transmission part having a core and a clad, and a hetero-core part having a core and a clad that are respectively continuous with the core and the clad of the optical transmission part, where the core of the hetero-core part is smaller in diameter than the core of the optical transmission part, has been proposed. This hetero-core optical fiber sensor is a hydrogen sensor including a metal film capable of exciting surface plasmon resonance (Surface Plasmon Resonance SPR) or localized surface plasmon resonance formed on the outer peripheral surface of the clad of the hetero-core part, a dielectric film formed on the outer peripheral surface of the metal film, and a hydrogen storage metal film formed on the outer peripheral surface of the dielectric film.

[0003] The hetero-core optical fiber hydrogen sensor disclosed in Patent Document 1 allows light propagating from the core of the optical transmission part to leak from the core of the hetero-core part to the outside, and excites surface plasmon resonance (SPR) in a metal film formed on the outer peripheral surface of the clad of the hetero-core part. When the hetero-core optical fiber hydrogen sensor is placed in a hydrogen atmosphere, the hydrogen storage metal film contains hydrogen, its dielectric function and refractive index change, and the resonance wavelength of surface plasmon resonance (SPR) in the metal film changes due to the change in the dielectric function. Therefore, hydrogen can be measured from the amount of change in the resonance wavelength.

[0004] However, the hydrogen sensor disclosed in Patent Document 1 has a metal film, a dielectric film, etc. formed on the outer periphery of the optical fiber. Since the metal film, the dielectric film, etc. deteriorate over time, the usable period of the hydrogen sensor depends on the lifespan of the metal film, the dielectric film, etc., and its durability is low. Therefore, an optical fiber sensor with high durability is required.

[0005] As an optical fiber sensor with excellent durability, the optical fiber sensor disclosed in Patent Document 2 has been proposed. This optical fiber sensor comprises a core, a cladding covering the core, and a detection unit. The detection unit is provided in the path of the optical fiber, and the cladding in the portion where the detection unit is formed is thinner than the rest of the cladding. This optical fiber sensor has excellent durability due to its simple structure, which includes a detection unit with a thin portion formed in part of the cladding. This is because the structure is such that only the cladding of the detection unit is formed thinly, and it does not have a sensing layer such as a metal film provided in conventional fiber sensors, so there is no decrease in durability due to deterioration of the sensing layer.

[0006] Furthermore, the heterocore optical fiber hydrogen sensor disclosed in Patent Document 1 is for detecting hydrogen and not for detecting other chemical species, such as carbon dioxide. As a heterocore optical fiber sensor for detecting carbon dioxide, there is an optical fiber sensor disclosed in Patent Document 3. This heterocore optical fiber carbon dioxide sensor also comprises an optical transmission section having a core and cladding, and a heterocore section having a core and cladding connected to the core and cladding of the optical transmission section, respectively, and includes an optical fiber in which the core of the heterocore section has a smaller diameter than the core of the optical transmission section. The outer circumferential surface of the cladding of the heterocore section is provided with a metal film capable of exciting surface plasmon resonance or localized surface plasmon resonance, and the outer circumferential surface of the metal film is provided with an ionic liquid layer capable of adsorbing and desorbing carbon dioxide.

[0007] The heterocore optical fiber carbon dioxide sensor disclosed in Patent Document 3 also works in which light propagating within the core of the optical transmission section leaks from the core of the heterocore section to the outside world and undergoes total internal reflection at the interface between the cladding of the heterocore section and the metal film formed on the outer surface of the cladding, thereby exciting an evanescent wave in the metal film, and this evanescent wave excites surface plasmon resonance (SPR) in the metal film. Furthermore, the ionic liquid layer formed on the outer surface of the metal film is capable of adsorbing and desorbing carbon dioxide, and when carbon dioxide is adsorbed, the dielectric function of the ionic liquid changes, and the resonance wavelength of surface plasmon resonance (SPR) in the metal film changes in accordance with this change in dielectric function. As a result, a shift occurs in the reflection spectrum itself in total internal reflection, and the light intensity of the light propagating in the optical transmission section changes, so carbon dioxide can be detected by detecting this change in light intensity. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2014-59300 [Patent Document 2] Japanese Patent Publication No. 2022-164411 [Patent Document 3] Japanese Patent Publication No. 2021-117002 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Generally, to detect substances in the gaseous or liquid phase, a membrane that reacts with the substance to be detected is used. For example, the hydrogen sensor disclosed in Patent Document 1 indirectly achieves hydrogen monitoring by using a hydrogen-sensitive membrane. The carbon dioxide sensor disclosed in Patent Document 3 indirectly detects carbon dioxide by having an ionic liquid layer capable of adsorbing and desorbing carbon dioxide on the outer surface of a metal film. Other than that, existing carbon dioxide sensors such as optical fiber sensors only indirectly detect dissolved carbon dioxide from the pH change of a fluorescent reagent.

[0010] On the other hand, absorption spectroscopy, which utilizes the absorption spectrum unique to the object being measured, can directly capture the liquid and gas phases. However, when using optical fibers for general communications, the material is silica, and its transmission wavelength is about 2 μm, making measurement in the mid-infrared region impossible. In contrast, fluoride optical fibers can propagate light in the mid-infrared region with low loss. Furthermore, the optical fiber sensor disclosed in Patent Document 2 also has an operating wavelength range of 2.5 μm to 3.8 μm. However, since carbon dioxide absorption occurs at a wavelength of around 4.2 μm, the optical fiber sensor disclosed in Patent Document 2 cannot detect longer wavelength objects, including carbon dioxide. Moreover, the carbon dioxide sensor disclosed in Patent Document 3 has a metal film cladding to the heterocore portion of the optical fiber, resulting in low durability, a complex structure, difficulty in manufacturing, and high costs, as mentioned above.

[0011] This invention has been made in view of the above-mentioned prior art, and aims to provide an optical fiber sensor and a material detection device using the same that enable monitoring of light up to wavelengths of around 5.5 μm on the longer wavelength side using absorption spectroscopy, and that also have high durability. [Means for solving the problem]

[0012] This invention is an optical fiber sensor that enables measurement from the ultraviolet to the mid-infrared region around 5.5 μm wavelength by absorption spectroscopy utilizing the absorption spectrum unique to the object being measured. The sensor uses a fluoride optical fiber that enables low-loss light propagation in the ultraviolet to mid-infrared region (wavelengths from 310 nm to 5.5 μm). Furthermore, by using a light source and an optical spectrum detector, it is a substance detection device that enables monitoring of gaseous and liquid-phase carbon dioxide and other substances in the ultraviolet to mid-infrared region.

[0013] This invention relates to an optical fiber sensor comprising an optical fiber having a core and a cladding covering the core, wherein the optical fiber has a core made of fluoride glass, is capable of transmitting infrared light in the mid-infrared region, and has a detection unit formed in the middle of the optical path of the core, wherein the cladding on which the detection unit is located is formed to be thinner than other parts or the core is exposed, and when light passes through the core, a portion of the light passing through the core leaks out of the core from the detection unit and light with a spectrum unique to the object to be measured that is in contact with the detection unit is absorbed, and the optical fiber sensor detects the object to be measured by detecting a change in the spectrum of the light passing through the detection unit.

[0014] The optical fiber comprises a core made of fluoride glass whose component is indium fluoride (InF3), and the detection portion is tapered so that the detection portion is thinner than the rest of the optical fiber, and the cladding of the tapered portion and the detection portion is thinner than the rest of the optical fiber. In particular, the optical fiber is preferably capable of transmitting light up to a wavelength of approximately 5.5 μm.

[0015] The detection unit has a cladding thickness of 20 μm or less at its thinnest point. In particular, the optical fiber is capable of transmitting light up to a wavelength of approximately 5.5 μm and comprises a core made of ZrF4-based fluoride glass. The detection unit is configured such that a portion of the cladding surrounding the core where the detection unit is located is removed, exposing the core, or the cladding thickness is 20 μm or less. Furthermore, it is preferable that the thickness of the cladding thickness at its thinnest point is less than or equal to the peak wavelength of the absorption spectrum specific to the object being measured.

[0016] Furthermore, this invention relates to an optical fiber having a core and a cladding covering the core, wherein the optical fiber comprises a core made of fluoride glass, an optical fiber sensor capable of transmitting infrared light in the mid-infrared region, the optical fiber sensor having a detection unit formed in the middle of the optical path of the core, the detection unit having a cladding where the detection unit is located formed thinner than other parts or the core is exposed, and when light passes through the core, a portion of the light passing through the core leaks out of the core from the detection unit, and light with a spectrum specific to the object to be measured that is in contact with the detection unit is absorbed, causing the spectrum of the light passing through the detection unit to change, and the invention relates to a substance detection device comprising a light source that irradiates and transmits light to the core of the optical fiber, and a spectral analyzer capable of detecting the spectrum of light transmitted through the optical fiber, transmitting light through the optical fiber, bringing the detection unit into contact with the object to be measured, detecting the absorption spectrum specific to the object to be measured, and detecting the object to be measured.

[0017] The optical fiber of the substance detection device of this invention comprises a core made of fluoride glass whose component is indium fluoride (InF3), and the detection portion is tapered so that the detection portion is thinner than the other portion, and the tapered portion and the cladding of the detection portion are formed to be thinner than the other portion of the optical fiber.

[0018] In particular, this substance detection device transmits light up to a wavelength of approximately 5.5 μm through the optical fiber and detects carbon dioxide by detecting the absorption spectrum unique to the carbon dioxide being measured in the detection unit. [Effects of the Invention]

[0019] The optical fiber sensor and substance detection device using the same of this invention utilize a fluoride optical fiber capable of low-loss propagation of light in the ultraviolet to mid-infrared region (wavelengths from 310 nm to 5.5 μm), enabling easy and accurate detection and measurement of the target substance by absorption spectroscopy. The detection unit has a simple structure, high durability and environmental resistance, and can detect and measure the target substance even in harsh environments.

Brief Description of the Drawings

[0020] [Figure 1] It is a longitudinal schematic cross-sectional view (a) showing an optical fiber sensor according to an embodiment of the present invention, and a cross-sectional view (b) at the grinding position of a device for grinding a cladding. [Figure 2] It is a schematic cross-sectional view showing an optical fiber sensor according to another embodiment of the present invention. [Figure 3] It is a schematic diagram showing a substance detection device of an optical fiber sensor according to an embodiment of the present invention. [Figure 4] It is a graph showing the measurement results of the absorption spectrum for water using the polished optical fiber sensor of the embodiment shown in FIG. 1. [Figure 5] It is a graph showing the measurement results of the absorption spectrum for methanol using the polished optical fiber sensor of the embodiment shown in FIG. 1. [Figure 6] It is a graph showing the measurement results of the absorption spectrum at 100% carbon dioxide using the polished optical fiber sensor of the embodiment shown in FIG. 1. [Figure 7] It is a graph showing the measurement results of the absorption spectrum for glycerin using the polished optical fiber sensor of the embodiment shown in FIG. 1. [Figure 8] It is a graph showing the measurement results of the absorption spectrum for water using the tapered optical fiber sensor of the embodiment shown in FIG. 2. [Figure 9] It is a graph showing the measurement results of the absorption spectrum at 100% carbon dioxide using the tapered optical fiber sensor of the embodiment shown in FIG. 2.

Embodiments for Carrying Out the Invention

[0021] The following describes an optical fiber sensor and a material detection device using the same according to one embodiment of the present invention, based on the drawings. As shown in Figures 1 and 2, the optical fiber sensors 10 and 20 of this embodiment consist of an optical fiber 12, with a detection unit 14 formed in the middle of the optical fiber 12. The optical fiber 12 consists of a central core 16 and a cladding 18 covering the core 16, and the outside of the cladding is protected by a resin coating 26. The optical fiber 12 of this embodiment is a fluoride-based optical fiber that transmits light in the wavelength range from ultraviolet to mid-infrared, with wavelengths from 310 nm to 5.5 μm. The length of the optical fiber 12 of the optical fiber sensors 10 and 20 is not particularly limited and can be set as appropriate depending on the object to be detected and the conditions of the device.

[0022] The optical fiber sensors 10 and 20 detect and analyze the spectrum of light transmitted through the optical fiber 12 using a spectral analyzer 34 (described later) to detect and measure the components or concentrations of the object to be measured that the detection unit 14 comes into contact with. The object to be measured is not particularly limited, but includes fluids containing gases or liquids, such as various hydrocarbon molecules having resonance lines in the mid-infrared region, carbon dioxide, ammonia, water, and water vapor.

[0023] The detection unit 14 is formed in the middle of the optical fiber 12, and the cladding 18 in that portion is formed thinner than or removed from the cladding 18 in the portion other than where the detection unit 14 is formed. In the polished optical fiber sensor 10 of the embodiment shown in Figure 1, the cladding 18 of the detection unit 14 is removed and the core 16 is exposed. The diameter D1 of the core 16 is 100 μm to 400 μm, and the diameter D2 of the cladding 18 is 125 μm to 600 μm. Specifically, in the case of the polished optical fiber sensor 10 shown in Figure 1, for example, the diameter D1 of the core 16 is 400 μm, the diameter D2 of the cladding 18 is 500 μm, and the diameter D3 of the resin coating 26 is 600 μm. When a thin cladding portion 24 is left in the detection unit 14, its thickness is greater than 0, preferably 5 μm or more and 20 μm or less, and preferably less than or equal to the peak wavelength of the absorption spectrum specific to the object being measured.

[0024] Furthermore, in another embodiment of the tapered optical fiber sensor 20 shown in Figure 2, the detection unit 14 is formed to be thinner in the middle of the optical fiber 12 via a tapered section 22. The detection unit 14 is provided as a thin-layer cladding section 24 in which the cladding 18 of the optical fiber 12 is formed to be thin. The thickness of the thin-layer cladding section 24 is greater than 0, preferably 5 μm to 20 μm, and preferably less than or equal to the peak wavelength of the absorption spectrum specific to the object being measured. Because the thickness of the cladding of the thin-layer cladding section 24 in the detection unit 14 is greater than 0, the core 16 is protected, and as will be described later, it has high durability even in harsh environments. In the optical fiber sensor 20 of the embodiment shown in Figure 2, the optical fiber 12 has a core diameter D1 of 100 μm to 200 μm and a cladding diameter D2 of 125 μm to 300 μm. Specifically, in the case of the optical fiber sensor 20, for example, the diameter D1 of the core 16 other than the detection unit 14 is 200 μm, the diameter D2 of the cladding is 290 μm, the detection unit 14 is stretched by about 5 mm via the tapered portion 22, the diameter of the cladding 18 is 160 μm or less, and the diameter of the core 16 is about 100 μm.

[0025] There is no lower limit to the length of the detection section 14 in the optical axis direction of the optical fiber 12, but it is preferably 0.5 mm or more, and more preferably 1 mm or more. There is also no particular upper limit to the length of the detection section 14, but it is preferably 5 mm to 30 mm, and the length of the detection section 14 is, for example, about 10 mm. The length of the optical fiber 12 equipped with the detection section 14 is not particularly limited, and a length necessary for detecting the object to be measured is selected, which may range from several meters to several kilometers or more.

[0026] The core 16 of the optical fiber 12 is the part through which light emitted from the light source 32 of the material detection device 30 (described later) passes, and it is preferable that it transmits light in the wavelength range of 310 nm to 5.5 μm. In the case of the polished optical fiber sensor 10 shown in Figure 1, the core 16 of the optical fiber 12 is, for example, a ZrF4-based fluoride glass such as ZBLAN(ZrF4-BaF2-LaF3-AlF3-NaF) glass, ZBLAN(ZrF4-BaF2-LaF3-AlF3) glass, ZBYA(ZrF4-BaF2-YF3-AlF3) glass, etc. Other fluoride glasses such as indium fluoride glass (InF3) may also be used. Among these, it is preferable to use InF3-based glass. In the case of the tapered optical fiber sensor 20 of the embodiment shown in Figure 2, fluoride glass such as indium fluoride glass (InF3) is preferred.

[0027] The optical fiber 12 is preferably a multimode fiber in which light of mid-infrared wavelengths is transmitted in multiple modes, but a single-mode fiber may also be used. The cladding 18 of the optical fiber 12 is made of glass having a lower refractive index than the core 16, and is made from the same material as the core 16.

[0028] Next, the principle by which the fiber sensors 10 and 20 having the above configuration detect an object to be measured that comes into contact with the detection unit 14 will be explained. When light passes through the core 16 of the optical fiber 12 from the light source 32, as the light propagates within the core 16, the refractive index of the core 16 is higher than that of the cladding 18. Therefore, normally, if the angle of incidence is greater than or equal to the critical angle, the light undergoes total internal reflection and is guided in the direction of the optical axis. However, in the case of the polished optical fiber sensor 10 shown in Figure 1, where the cladding 18 has been removed, some of the light leaks out of the core 16 in the detection unit 14. Also, in the case of the detection unit 14 where a thin layer of cladding 18 remains, as in the tapered optical fiber sensor 20 shown in Figure 2, some of the light that passes through the core 16 of the detection unit 14 penetrates the cladding 18 as evanescent light. Furthermore, when an object to be measured comes into contact with the thin cladding 24 of the detection unit 14, light with a wavelength corresponding to the resonance frequency of the object to be measured leaks out of the detection unit 14 as leaked light or evanescent light. Light leaking from the detection unit 14 of the optical fiber sensors 10 and 20 is absorbed by the object to be measured. The spectral analyzer 34 detects this light absorption due to interaction at the detection unit 14, allowing the object to be detected and measured.

[0029] As shown in Figure 3, the substance detection device 30 of this embodiment includes a light source 32 and a general-purpose spectral analyzer 34 that detects the spectrum of light irradiated from the light source 32 and passed through the detection unit 14 of the optical fiber 12. The light source 32 only needs to be capable of outputting light in a band covering the optical absorption wavelength of the substance to be measured, and a light source capable of irradiating light in the wavelength range of 310 nm to 5.5 μm that transmits well through the optical fiber 12 of this embodiment should be appropriately selected. In particular, it is preferable to use a white light source or the like that has a certain frequency bandwidth that includes the frequency of the absorption spectrum of the substance to be measured. For example, an ASE (Amplified Spontaneous Emission) light source may also be used. An ASE light source is one in which broadband spontaneously emitted light, such as rare earth metal ions absorbed from excitation light irradiated from an excitation light source, is amplified by stimulated emission. For example, when detecting carbon dioxide, a light source that emits light in the wavelength range up to the mid-infrared region covering the absorption wavelength of carbon dioxide at 4257 nm should be selected.

[0030] As shown in Figure 3, the substance detection device 30 has one end 12a of an optical fiber 12 connected to a light source 32, and the other end 12b of the optical fiber 12 is connected to a spectral analyzer 34 for detecting light emitted from the optical fiber 12. The detection unit 14 of the optical fiber 12 is placed within the detection area 36. The spectral analyzer 34 detects the spectrum of light in the mid-infrared region with wavelengths up to, for example, 5.5 μm that has passed through the optical fiber 12, and analyzes the spectrum of the light emitted from the other end 12b of the optical fiber 12 in relation to the light from the light source 32 that has been incident on the one end 12a of the optical fiber 12, thereby detecting the type of substance to be measured or the concentration of the substance to be measured that has come into contact with the detection unit 14 within the detection area 36 (measurement chamber in this experimental apparatus).

[0031] Next, a method for manufacturing the polished optical fiber sensor 10 according to the embodiment shown in Figure 1 will be described. First, a polishing rod, such as a rubber polishing member (not shown), is prepared to polish the cladding 18 of the optical fiber 12. The polishing rod has a shape such as a cylindrical diameter, and an abrasive material is applied or mixed inside its surface. Then, at the position where the detection unit 14 is formed along the path of the optical fiber 12, the polishing rod is positioned so that its central axis is perpendicular to the optical axis direction of the optical fiber 12.

[0032] Next, the resin coating 26 at the position where the detection unit 14 is to be formed along the longitudinal path of the optical fiber 12 is removed to a predetermined length, and as shown in Figure 1(b), the resin coating 26 on both sides of the detection unit 14 is fitted and fixed to a pair of positioning jigs 28. In this state, the polishing rod is brought into contact with the cladding 18 of the optical fiber 12 between the positioning jigs 28 while rotating at a predetermined rotational speed, and pressed against the position where the detection unit 14 is to be formed, thereby grinding the cladding 18 of the detection unit 14 of the optical fiber 12. The grinding thickness of the cladding 18 is such that it is close to or exposed to the core 16. The optical axis length of the detection unit 14 is ground to a predetermined length as needed, as described above.

[0033] Furthermore, in the manufacturing method of the tapered optical fiber sensor 20 of the embodiment shown in Figure 2, the optical fiber 12 is heated and melted at the position of the detection part 14, and the optical fiber 12 is stretched in the longitudinal direction, for example by about 5 mm, to reduce the diameter of the detection part 14. As a result, as described above, the optical fiber 12 of the optical fiber sensor 20 of the embodiment shown in Figure 2 is formed with a core diameter D1 of 100 μm to 200 μm and a cladding diameter D2 of 125 μm to 300 μm. Specifically, for example, the diameter D1 of the core 16 other than the detection part 14 is 200 μm, the diameter D2 of the cladding is 290 μm, the detection part 14 is melted and stretched by about 5 mm to form a tapered part 22, and the cladding diameter 18 is 160 μm or less, and the core diameter 16 is melted and stretched to about 100 μm via the tapered part 22.

[0034] In this embodiment, the optical fiber sensors 10 and 20 are used as shown in Figure 3. The detection unit 14 of the optical fiber sensors 10 and 20 is placed at a predetermined position within the detection area 36 (measurement chamber in this experimental setup) where the substance to be detected is present, and light from the light source 32 is incident from one end 12a of the optical fiber 12. Carbon dioxide is injected and filled into the measurement chamber in the detection area 36. When the light incident from the light source 32 passes through the detection unit 14, the detection unit 14 comes into contact with the object to be measured, causing absorption of a predetermined spectrum of light. The spectrum of the light incident from the other end 12b of the optical fiber 12 is then analyzed by the spectrum analyzer 34 to detect the object to be measured, and its quantity and concentration are measured. In practice, the detection unit 14 is placed in the area where the object to be measured is present, and is set at an appropriate distance from the spectrum analyzer 34, ranging from a few meters to tens of kilometers.

[0035] By using the optical fiber sensors 10 and 20 of this embodiment, a fluoride optical fiber 12 capable of low-loss propagation of light from the ultraviolet to mid-infrared region (wavelengths from 310 nm to 5.5 μm) can be used to easily and accurately detect objects to be measured within a detection area 36 using absorption spectroscopy with a simple device configuration in a substance detection device 30. The detection unit 14 has a simple structure, high durability and environmental resistance, and can measure substances that are the target of measurement in harsh environments. This is because the detection unit 14 does not have a sensitive layer provided in conventional optical fiber sensors, so there are no durability problems due to the degradation of the sensitive layer. In particular, in the optical fiber sensor 20, the thickness of the cladding of the thin cladding part 24 in the detection unit 14 is greater than 0, protecting the core 16 and providing high durability even in harsh environments.

[0036] Furthermore, the optical fiber 12 is capable of spectral measurement of detection targets that have absorption in the mid-infrared region, and stable measurement is possible in real time. Since the detection unit 14 does not require a sensitive layer that depends on the detection target or measurement target, adjustment or replacement of the detection unit 14 or optical fiber 12 depending on the measurement target is unnecessary, and it is possible to measure multiple types of measurement targets, regardless of whether they are gases or liquids. In particular, the optical fiber sensors 10 and 20 of this embodiment are capable of detecting many molecules as detection targets, such as various hydrocarbon molecules and carbon dioxide, which have a resonance frequency in the mid-infrared region.

[0037] Furthermore, the optical fiber sensor and material detection device of this invention are not limited to the above-described embodiments. The bandwidth and type of light source of the light source 32 can be set as appropriate, and the material and structure of the optical fiber 12 can be selected to transmit light in the infrared region according to the object to be measured. [Examples]

[0038] An example demonstrating the performance of the optical fiber sensor and material detection device of this invention will be described below. First, an example of measurement using the side-polished fluoride optical fiber sensor 10 shown in Figure 1 will be presented. In this example, the optical fiber sensor 10 has a core diameter of 400 μm, a cladding diameter of 500 μm, and an outer diameter of 600 μm including the resin coating 26. Approximately 100 μm of the side surface of the cladding 18 is polished to expose the core 16.

[0039] Figures 4 to 7 show the results of measuring the wavelength and transmittance of light for water, methanol, carbon dioxide, and glycerin using a polished optical fiber sensor 10 with the experimental apparatus shown in Figure 3. As shown in the figures, the unique absorption spectra corresponding to each substance were confirmed. The solid line in Figure 4 shows the case where the detection unit 14 is placed in water, showing absorption in the wavelength range of the infrared absorption spectrum for water, and confirming the difference from the general infrared absorption spectrum in air shown by the dashed line. The graph in Figure 5 shows the infrared absorption spectrum when the detection unit 14 is placed in methanol, showing absorption at specific molecular structures in each absorption wavelength range. The graph in Figure 6 shows the infrared absorption spectrum when the detection unit 14 is placed in 100% carbon dioxide, showing the absorption spectrum at wavelengths unique to carbon dioxide. In addition, the solid line graph in Figure 7 shows the infrared absorption spectrum when the detection unit 14 is placed in glycerin, and confirming the difference from the general infrared absorption spectrum in air shown by the dashed line. Based on the above, it was confirmed that substances such as carbon dioxide can be reliably detected by absorption spectroscopy using the polished optical fiber sensor 10.

[0040] Next, the tapered optical fiber sensor 20 shown in Figure 2, whose detection unit 14 was fabricated using a CO2 laser fusion splicing device, was similarly verified. The optical fiber sensor 20 was created by extending the optical fiber 12 of the detection unit 14 by about 5 mm using a fluoride glass fiber with a core diameter 16 of 200 μm and a cladding diameter 18 of 300 μm. Using this optical fiber sensor 20, the wavelength and transmittance of light were measured for water and carbon dioxide using the experimental apparatus shown in Figure 3. Figure 8 shows the case where the detection unit 14 is placed in water, showing absorption in the wavelength range of the infrared absorption spectrum in water. The graph in Figure 9 shows the infrared absorption spectrum when the detection unit 14 is placed in 100% carbon dioxide, showing the absorption spectrum at wavelengths specific to carbon dioxide. From the above, it was confirmed that the tapered optical fiber sensor 20 can reliably detect substances such as carbon dioxide. [Industrial applicability]

[0041] In addition to numerous excellent features such as small diameter, light weight, broad bandwidth, environmental resistance, and remote monitoring, optical fibers can guide light to the observation site. Therefore, the optical fiber sensor and material detection device of this invention can be used as one of the material measurement devices in the construction of marine monitoring technology, for example. As such, it can be used in technologies for understanding the dynamics of marine chemicals such as carbon dioxide and nutrients, for injecting carbon dioxide into the seabed, and for other carbon capture and storage technologies such as CCS, geothermal power generation, and carbon recycling technologies. [Explanation of Symbols]

[0042] 10,20 Fiber Optic Sensors 12 Optical Fibers 14 Detection unit 16 cores 18 Clad 22 Tapered section 24 Thin cladding section 26 Resin coating 30 Substance detection device 32 light source 34. Spectral analyzer 36 Detection Area

Claims

1. It consists of an optical fiber having a core and a cladding covering the core, The optical fiber comprises a core made of fluoride glass, is capable of transmitting infrared light in the mid-infrared region, and has a detection unit formed in the middle of the optical path of the core. The detection unit is formed such that the cladding on which the detection unit is located is thinner than other parts, or the core is exposed. An optical fiber sensor that detects a change in the spectrum of light passing through the core by detecting that, when light passes through the core, a portion of the light passing through the core leaks out of the core of the detection unit, and light with a spectrum unique to the object to be measured that is in contact with the detection unit is absorbed, thereby enabling the detection of the object to be measured.

2. The optical fiber is composed of indium fluoride (InF ３ The core is made of fluoride glass of the following type: The optical fiber sensor according to claim 1, wherein the detection portion is thinner than the other portion of the optical fiber via a tapered portion, and the cladding of the tapered portion and the detection portion is formed to be thinner than the other portion.

3. The optical fiber sensor according to claim 1 or 2, wherein the optical fiber is capable of transmitting light up to a wavelength of approximately 5.5 μm.

4. The optical fiber sensor according to claim 1 or 2, wherein the detection unit has a thickness of 20 μm or less at the thinnest part of the cladding.

5. The optical fiber has a component of ZrF ４ The core is made of a fluoride glass system, The optical fiber sensor according to claim 1, wherein the detection unit is such that a portion of the cladding surrounding the core on which the detection unit is located is removed, exposing the core, or the thickness of the cladding is 20 μm or less.

6. The optical fiber sensor according to claim 1 or 2, wherein the thickness of the thinnest part of the cladding is less than or equal to the peak wavelength of the absorption spectrum specific to the object to be measured.

7. The optical fiber has a core and a cladding covering the core, The optical fiber comprises the core made of fluoride glass and an optical fiber sensor capable of transmitting infrared light in the mid-infrared region. The optical fiber sensor has a detection unit formed in the middle of the optical path of the core, The detection unit is configured such that the cladding on which the detection unit is located is thinner than other parts or the core is exposed, and when light passes through the core, a portion of the light passing through the core of the detection unit leaks out of the core, and light with a spectrum specific to the object being measured that is in contact with the detection unit is absorbed, thereby changing the spectrum of the light passing through the detection unit. The optical fiber comprises a light source that irradiates and transmits light through the core of the optical fiber, and a spectral analyzer capable of detecting the spectrum of the light transmitted through the optical fiber. A substance detection device characterized by transmitting light through the optical fiber, bringing the detection unit into contact with the object to be measured, and detecting the object by detecting the absorption spectrum unique to the object to be measured.

8. The optical fiber is composed of indium fluoride (InF ３ The core is made of fluoride glass of the following type: The substance detection device according to claim 7, wherein the detection portion is thinner than the other portion via a tapered portion, and the cladding of the tapered portion and the detection portion is formed to be thinner than the other portion of the optical fiber.

9. A substance detection device according to claim 7 or 8, which transmits light up to a wavelength of approximately 5.5 μm through the optical fiber and detects carbon dioxide by detecting the absorption spectrum specific to carbon dioxide, which is the object to be measured, in the detection unit.

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