Optical fiber sensor and optical fiber sensor device using the same

Abstract

The present invention provides an optical fiber sensor with a simple structure and a wide measurable range, and an optical fiber sensor device using the same. [Solution] The optical fiber sensor of the present invention comprises an optical fiber having an optical transmission section for transmitting incident light, and an optical response section is provided in the middle of the optical transmission section, which includes a resonant film formed on the surface of the optical fiber and causing loss mode resonance, and whose response to incident light changes in accordance with changes in the surrounding conditions, wherein the thickness of the resonant film is changed according to the measurement target of the optical fiber sensor and / or the wavelength of light passing through the optical transmission section.

Description

Technical Field

[0001] The present invention relates to an optical fiber sensor and an optical fiber sensor device using the sensor.

Background Art

[0002] As sensors for performing various measurements, sensors using optical fibers, so-called optical fiber sensors, have been proposed. For example, in Patent Document 1, a hydrogen detection optical fiber sensor hydrogen sensor is proposed in which a metal film is formed on the outer periphery of an optical fiber, a dielectric film is formed on the outer periphery of the metal film, and a hydrogen storage metal film is further formed on the outer periphery of the dielectric film.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the optical fiber hydrogen sensor as shown in Patent Document 1, it has a two-layer structure of a metal film and a dielectric film, and it is necessary to perform sputtering twice to form these films, so the structure of the sensor is complex and the manufacturing process is complex.

[0005] Also, in the optical fiber sensor as shown in Patent Document 1, hydrogen is measured by observing the refractive index change of the hydrogen storage metal film using the surface plasmon resonance phenomenon. However, the refractive index range measurable by the structure composed of the metal film and the dielectric film is up to about 1.40 RIU. Considering the detection of other substances and the use for refractive index measurement in addition to the detection of hydrogen, one of the problems is that the measurable range is narrow.

[0006] The object of the present invention is to provide an optical fiber sensor with a simple structure and a wide measurable range, and an optical fiber sensor device using the same. [Means for solving the problem]

[0007] The present invention provides an optical fiber sensor comprising an optical fiber having an optical transmission section for transmitting incident light, wherein the optical transmission section includes a resonant film formed on the surface of the optical fiber and causing loss-mode resonance, and an optical response section is provided in the middle of the optical transmission section, the response when receiving the incident light changes in accordance with changes in the surrounding conditions, wherein the thickness of the resonant film is determined according to the measurement target of the optical fiber sensor and / or the wavelength of light passing through the optical transmission section. [Effects of the Invention]

[0008] The optical fiber sensor of the present invention is a sensor that includes an optical fiber and a thin film arranged around the optical fiber to induce loss mode resonance. By adjusting the thickness of the thin film, the resonance wavelength of the loss mode resonance occurring in the thin film can be changed, thereby changing the detection range and improving the detection accuracy. With such a configuration, it is possible to provide an optical fiber sensor with a simple structure and a wide measurable range, as well as an optical fiber sensor device using the same. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic cross-sectional view of the optical fiber sensor of Example 1. [Figure 2] This is an explanatory diagram showing an example of the configuration of a measuring device. [Figure 3A] This spectral graph shows the change in the transmitted light spectrum due to the change in refractive index when the coating film 7 is an ITO film with a thickness of 200 nm. [Figure 3B] This spectral graph shows the change in the transmitted light spectrum due to the change in refractive index when the coating film 7 is an ITO film with a thickness of 340 nm. [Figure 4]This graph shows the change in the position of the light absorption peak as the film thickness changes. [Figure 5] This graph shows the change in light loss at a wavelength of 850 nm as the refractive index changes. [Figure 6] This graph shows the change in optical loss over time when there is a change in refractive index. [Figure 7] This is a schematic cross-sectional view of the optical fiber sensor of Example 2. [Figure 8] This is a magnified view of a portion of Figure 7. [Figure 9] This graph shows the change in optical loss in the response section in response to changes in the external atmosphere. [Figure 10] schematic cross-sectional view of the optical fiber sensor of Example 3 [Figure 11] This is an explanatory diagram showing an example of the configuration of a measuring device. [Modes for carrying out the invention]

[0010] First, we will describe the overview and operating principle of the optical fiber sensor of the present invention.

[0011] The optical fiber sensor of the present invention is a sensor that uses an optical fiber consisting of a core and a cladding surrounding the core. In its basic structure, a thin film (hereinafter also referred to as a resonant film) that generates loss mode resonance (LMR) (hereinafter also simply referred to as LMR or LMR phenomenon) is formed on the outer circumference of a portion of the optical fiber in the longitudinal direction, surrounding the optical fiber, and in the portion where the thin film is formed, light is leaked out to the outer circumference of the optical fiber. In other words, it is a structure that leaks light from the cladding of the optical fiber.

[0012] In the optical fiber sensor of the present invention, due to a change in the dielectric constant or refractive index of a substance arranged in contact with or in proximity to the surface of the thin film that causes the above-described loss mode resonance, the resonance wavelength of the loss mode resonance in the thin film changes. As a result, the amount of loss of the light leaking from the clad in the thin film changes. Measurement of the refractive index using the optical fiber sensor of the present invention is performed using this change in the amount of loss.

[0013] Due to the change in the loss mode resonance occurring in the thin film, the above-described change in the amount of loss of light occurs, and the intensity of the light passing through from one end to the other end of the optical fiber, in other words, the light transmitted from one end to the other end of the optical fiber (hereinafter also referred to as transmitted light or propagating light) changes. In the optical fiber sensor of the present invention, the refractive index of the measurement target is measured by measuring the intensity of this transmitted light.

[0014] In the optical fiber sensor of the present invention, in particular, the optical fiber has a hetero-core structure in which the core diameter of the portion of the optical fiber where the resonance film is formed is smaller than the core diameter of the other portions of the optical fiber. In other words, the optical fiber sensor of the present invention has a configuration in which the core diameter becomes smaller in the middle and then becomes larger again.

[0015] By making the optical fiber used in the optical fiber sensor have the above-described hetero-core structure, light can be actively leaked from the core to the outside in the portion of the optical fiber where the core diameter is smaller than the other portions. As a result, the change in the loss of light, that is, the change in the intensity of the transmitted light, which changes depending on the dielectric constant or refractive index of the substance in contact with the thin film, appears more prominently than in a normal optical fiber with a constant core diameter.

[0016] As described above, as an optical fiber sensor, it is desirable to have a hetero-core structure. Basically, in the portion where the resonance film is formed, it is sufficient if the propagating light that has propagated from the core of the optical fiber to the outer periphery can leak.

[0017] The optical fiber sensor device using the above optical fiber sensor may measure the refractive index from the change in intensity of the emitted light emitted from the other end of the optical fiber, by injecting light of a single wavelength from one end of the optical fiber. Alternatively, the optical fiber sensor device using the above optical fiber sensor may measure the change in refractive index from the change in intensity of a specific wavelength of light among the emitted light emitted from the other end, or from the change in the spectrum of said emitted light, by injecting light with a broad spectrum, such as that obtained from a white light source using a halogen light source, from one end of the optical fiber.

[0018] Furthermore, by forming a sensitive film on the surface of the resonant film of the optical fiber sensor, which changes its refractive index in response to changes in the magnetic field or adsorption of substances, various measurements and detections, such as magnetic field measurement and substance detection, can be performed. In such cases, as with optical fiber sensor devices using optical fiber sensors without the sensitive film described above, the refractive index can be measured by incident light with a single wavelength or broad spectrum from one end of the optical fiber and observing the change in intensity of the emitted light from the other end.

[0019] The materials used for the resonant film that produces the above LMR phenomenon include semiconductor films, specifically Si, as well as ITO, IZO, IGZO, ZnO, SnO2, CuO, and TiO2. 2、 Examples include metal oxide semiconductor films such as WO3, dielectric films, or conductive polymers, specifically polyaniline and polypyrrole. The resonant film material is required to have a higher refractive index than the optical fiber cladding.

[0020] In the optical fiber sensor described above, the thicker the resonant film, the more the wavelength at which the absorption peak, i.e., loss mode resonance, occurs, tends to shift to longer wavelengths. In other words, by changing the thickness of the resonant film, it is possible to shift the resonance wavelength, and consequently, the wavelength range of high sensitivity.

[0021] This property makes it possible to create optical fiber sensors with characteristics tailored to the wavelength range of light introduced into the optical fiber sensor and the target of measurement by setting the thickness of the resonant film according to the wavelength range of light introduced and the target of measurement. [Examples]

[0022] An optical fiber sensor 1 according to Embodiment 1 of the present invention will be described with reference to the drawings. Note that the drawings are stylized to clarify the optical fiber sensor 1 and its components, and do not represent the actual proportions.

[0023] Figure 1 shows a cross-sectional view of the optical fiber sensor 1 cut along the length of the optical fiber. As shown in Figure 1, the optical fiber sensor 1 consists of an optical fiber 4 having a heterocore portion 3 of a predetermined length sandwiched between optical transmission portions 2, 2. The optical transmission portion 2 is a multimode optical fiber consisting of a core 5a as a first core or a third core, and a cladding 6a as a first cladding or a third cladding that covers the outer surface of the core 5a, i.e., the outer surface of the core 5a.

[0024] The heterocore section 3 is a single-mode optical fiber consisting of a second core 5b, which is a core with a smaller diameter than core 5a, and a second cladding 6b, which is a cladding that covers the outer surface of core 5b. The heterocore section 3 is an optical leakage section that leaks a portion of the light transmitted from the optical transmission section 2.

[0025] In this embodiment, a multimode fiber (MMF) (FutureGuide-MM50, manufactured by Fujikura Ltd.), which is a communication optical fiber with a core 5a having a diameter of 50 μm and a cladding 6a having a diameter of 125 μm, was used as the optical transmission unit 2.

[0026] Furthermore, a single-mode fiber (SMF) (manufactured by F-SA Newport, USA) with a core 5b diameter of 3 μm and a cladding 6b diameter of 125 μm was used as the heterocore section 3. The length of the heterocore section 3 was set to 15 mm. The two optical transmission sections 2 and the heterocore section 3 are joined to each other by fusion splicing. The length of the heterocore section 3 can be, for example, several millimeters to several tens of millimeters.

[0027] The coating film 7, which acts as a resonant film, is a thin film of ITO formed on the outer surface of the cladding 6b of the heterocore portion 3. In this embodiment, the coating film 7 is a thin film of ITO with a thickness of 340 nm. The coating film 7 can be formed on the surface of the cladding 6b, for example, by vacuum deposition or sputtering. Naturally, the thickness of the coating film 7 is not limited to 340 nm. In this embodiment, the coating film 7 was formed by forming an ITO film on the surface of the cladding 6b using sputtering.

[0028] In the optical fiber sensor 1, the heterocore portion 3 and the coating film 7 formed on the heterocore portion 3 together constitute the sensing portion 10, which is the optical response portion.

[0029] As described above, by joining the optical fiber constituting the optical transmission section 2 and the optical fiber constituting the heterocore section 3, which has a core diameter smaller than that of the optical transmission section 2, by fusion splicing, it is possible to leak more light in the heterocore section 3 than in other parts without reducing the overall diameter of the optical fiber including the cladding. In other words, it is possible to leak more light in the heterocore section 3 than in other parts without reducing the mechanical strength of the sensing section 10.

[0030] In the optical fiber sensor 1, when light is incident from the end of the optical transmission section 2, the core diameter in the heterocore section 3 becomes smaller than that of the optical transmission section 2. As a result, light leaks from cores 5a and 5b into the cladding 6b and propagates through the cladding 6b. In other words, a large amount of light in the so-called radiation mode is generated, leaking from core 5b into the cladding 6b, and this light propagates through the cladding 6b.

[0031] As a result, light in this emission mode is emitted from the outer surface of the cladding 6b and reaches the coating film 7. Of the light that reaches the coating film 7, some light with a specific wavelength and angle of incidence is confined within the coating film, generating standing waves. Because the coating film has a small extinction coefficient, this portion of light is absorbed within the coating film and does not return much to the optical fiber.

[0032] Furthermore, the specific wavelength and incident angle confined within the coating film 7 change depending on the dielectric function or refractive index of the substance in contact with the surface of the coating film 7, i.e., the substance being measured or detected (hereinafter also simply referred to as the substance being measured). In other words, in the coating film 7 of the sensing part 10, loss mode resonance (LMR) occurs, in which the manner of optical loss changes depending on the change in the dielectric function or refractive index of the environment outside the sensing part 10.

[0033] Furthermore, in the optical fiber sensor 1, when light is incident from the end of the optical transmission section 2, the core diameter of the heterocore section 3 becomes smaller than that of the optical transmission section 2. As a result, light leaks from cores 5a and 5b into the cladding 6b and propagates through the cladding 6b. Consequently, evanescent light seeps out from the outer surface of the cladding 6b. This evanescent light excites surface plasmon resonance in the coating film 7.

[0034] Since the optical fiber sensor in this embodiment primarily uses the loss mode resonance described above for measurement, the material used for the thin film as the coating film 7 is a material that causes loss mode resonance with respect to the light emitted from the cladding surface of the optical fiber.

[0035] The coating film 7 of the optical fiber sensor 1 in this embodiment is a semiconductor film that generates loss mode resonance, specifically Si, as well as ITO, IZO, IGZO, ZnO, SnO2, CuO, and TiO2. 2、 Metal oxide semiconductor films such as WO3, dielectric films, and conductive polymers, specifically polyaniline and polypyrrole, can be used.

[0036] Figure 2 shows the configuration of the measuring device 20 as an optical fiber sensor device using the optical fiber sensor 1 described above.

[0037] The light source device 21 is a light source device that uses near-infrared (NIR: 700~1400nm) LED elements. The light source device 21 is coupled to one end of the optical fiber 4 of the optical fiber sensor 1 and is configured to allow light to be incident into the optical fiber 4 from that end. An LD element may also be used as the light source of the light source device 21.

[0038] The light-receiving device 22, acting as a light-receiving unit, is a device having the configuration of a so-called optical spectrum analyzer, which includes a spectroscopic element that spectrally analyzes the light emitted from the other end of the optical fiber 4, and a light-receiving element that receives the light spectrally analyzed by the spectroscopic element. The light-receiving device 22 outputs the measurement result of the spectrum of the received light. Note that if a light source with a very narrow spectrum and a single wavelength is used as the light source, a spectroscopic element is not necessarily required.

[0039] The analysis device 23, acting as the detection unit, is a device that receives the measurement results of the light spectrum output from the light receiving device 22 and records and displays them. The analysis device 23 is, for example, a personal computer with data logger software installed. The analysis device 23 may also have an application installed that can calculate the refractive index of the substance to be measured in contact with the coating film 7 of the optical fiber sensor 1, using the loss of light propagated through the optical fiber sensor 1, which is identified based on the above measurement results. Of course, the analysis device 23 may be a general-purpose computer or a computer specialized for a specific application.

[0040] To summarize the above, the measuring device 20 includes a light source that is optically coupled to the optical transmission unit 2 and causes light to be incident on the optical transmission unit 2, a light receiving device 22 that receives light from the optical transmission unit 2 which is optically coupled to the heterocore unit 3, and an analysis device 23 that analyzes the received signal generated by the light received by the light receiving device 22 and outputs the optical loss and the dielectric function or refractive index of the above material.

[0041] As described above, in the optical fiber sensor 1, light propagating through the core 5a of the optical transmission section 2 leaks into the cladding 6b in the heterocore section 3 and reaches the coating film 7, where loss mode resonance occurs and light attenuation occurs.

[0042] The amount of light attenuation in the coating film 7, or the amount of light attenuation for each wavelength, changes depending on the dielectric function or refractive index of the material being measured. As a result, the light intensity of the propagating light emitted from the light source device 21, propagating through the light transmission unit 2, and reaching the light receiving device 22 changes. By detecting this change in light intensity with the light receiving device 22 and analyzing it with the analysis device 23, it is possible to measure the dielectric function or refractive index of the material being measured, or to detect changes in these functions.

[0043] In this embodiment, an ITO film was used as the coating film 7 because the ITO film enables high sensitivity in measurements using inexpensive near-infrared (NIR: wavelength 700-1400 nm) LEDs.

[0044] Furthermore, the fact that ITO is a material that exhibits both loss-mode resonance due to radiant light and surface plasmon resonance due to evanescent light is also advantageous from the perspective of sensor applications. That is, by using an ITO film, the optical fiber sensor 1 can perform refractive index measurement and material detection and measurement using the change in the amount of light attenuation due to surface plasmon resonance, and it can also perform refractive index measurement and material detection and measurement utilizing both surface plasmon resonance and loss-mode resonance.

[0045] The reason the thickness of the ITO film of the coating film 7 is set to 340 nm is to make the optical fiber sensor 1 suitable for using near-infrared light (NIR: wavelength 700-1400 nm), particularly light with a wavelength of 850 nm, which is commonly used as optical fiber propagation light, as the propagating light incident on the optical fiber 4 of the optical fiber sensor 1, i.e., introduced. Specifically, this is to increase the change in the loss of the 850 nm propagating light with respect to changes in the dielectric function and refractive index of the material in contact with the surface of the coating film 7.

[0046] In this way, by using an optical fiber sensor suitable for near-infrared light, it becomes possible to construct the device using inexpensive near-infrared LEDs, thereby reducing the device cost.

[0047] In the process of arriving at the above conclusion, we investigated how the loss pattern of propagating light in the optical fiber sensor changes depending on the thickness of the ITO film when fabricating the optical fiber sensor 1. Specifically, we measured the change in light loss at a wavelength of 850 nm that occurred in the optical fiber sensor 1 when coating films 7 made of ITO films with different thicknesses were immersed in liquids with different refractive indices. This change can be understood as a change in optical loss in the heterocore portion 3 due to the difference in the thickness of the coating film and the refractive index of the liquid.

[0048] The measurements were performed using the measuring device 20 described above. Specifically, the optical fiber sensor 1 of Example 1 and the coating film 7 of the sensing part 10 of the optical fiber sensor 1, which was made of ITO film with a thickness of 200 nm, were attached to the measuring device 20, and the sensing part 10 was immersed in target substances having multiple different refractive indices to measure the optical spectrum.

[0049] In this measurement, instead of the near-infrared light source described above, the light source device 21 used was a white light source device AQ4305 (manufactured by Yokogawa Electric Corporation) consisting of a halogen light source emitting light with a wavelength of 400-1600 nm, and the light receiving device 22 used a CCS200 / M compact spectrometer (manufactured by Thorlabs).

[0050] The target substances were air (RIU: 1.000), water (RIU: 1.333), 20w% glycerin aqueous solution (RIU: 1.369), 40w% glycerin aqueous solution (RIU: 1.392), 60w% glycerin aqueous solution (RIU: 1.420), and 80w% glycerin aqueous solution (RIU: 1.447).

[0051] Figures 3A and 3B show graphs illustrating the light spectra propagated from one end to the other of the optical fiber sensor 1 when the coating film 7 was exposed to or immersed in the target substance, respectively. Figure 3A is the graph for the case where the coating film 7 is an ITO film with a thickness of 200 nm, and Figure 3B is the graph for the case where the coating film 7 is an ITO film with a thickness of 340 nm. The vertical dashed line in the figures represents the line drawn at a wavelength of 850 nm.

[0052] As shown in Figure 3A, when the ITO film thickness is 200 nm, the bottoming-out point where the light intensity is low, i.e., the peak of light loss or absorption, appears at a wavelength of approximately 500 nm to 550 nm. This indicates that loss mode resonance is occurring for light with wavelengths of 500 nm to 550 nm.

[0053] Furthermore, the change in optical loss in the heterocore 3 due to the change in refractive index is significant, and the sensor sensitivity is highest in the wavelength range of approximately 500 nm to 550 nm. In contrast, around 850 nm, the optical loss from sources other than air is almost the same, and the change in optical loss in the heterocore 3 due to the change in refractive index is hardly discernible.

[0054] As shown in Figure 3B, when the ITO film thickness is 340 nm, the bottoming-out point where the light intensity is low, and the peak of light loss, appear at wavelengths of approximately 750 nm to 850 nm. This indicates that loss mode resonance is occurring for light with wavelengths of 750 nm to 850 nm. In other words, increasing the film thickness shifts the wavelength of light that causes loss mode resonance to the longer wavelength side.

[0055] Furthermore, the change in optical loss in the heterocore 3 due to the change in refractive index is significant, and the optical loss increases in proportion to the magnitude of the refractive index from approximately 850 nm to 900 nm. It can also be seen that, in both film thickness cases, the peak of optical loss shifts to longer wavelengths as the refractive index increases.

[0056] Based on the above results, setting the thickness of the ITO film of the coating film 7 to 340 nm increases the sensor sensitivity when the light introduced into the optical fiber 4 during measurement using the optical fiber sensor 1 is 850 nm, that is, when the light source of the light source device 21 is a light source with an output light wavelength of 850 nm. In other words, setting the thickness of the ITO film of the coating film 7 to 340 nm makes the optical fiber sensor 1 suitable for use with a light source with an output light wavelength of 850 nm.

[0057] Furthermore, the same measurements as described above were performed while varying the thickness of the ITO thin film of coating film 7 to confirm the optical loss and absorption. Figure 4 shows the position of the optical absorption peak in the near-infrared region when the same measurements as described above were performed while varying the thickness of coating film 7 to 300 nm, 320 nm, and 340 nm.

[0058] As shown in Figure 4, it can be seen that as the thickness of the coating film 7 increases, the wavelength at which the absorption peak, i.e., loss mode resonance, occurs tends to shift to the longer wavelength side.

[0059] By changing the thickness of the coating film 7, it is possible to cause a shift in the resonance wavelength, and consequently, a shift in the wavelength range with high sensitivity. By changing the thickness of the coating film 7 according to the wavelength range of the light introduced into the optical fiber sensor and the object to be measured, it is possible to create an optical fiber sensor with characteristics that match the wavelength range of the light introduced and the object to be measured.

[0060] For example, specifically, when a sensitive film is formed on the surface of the coating film 7, whose refractive index changes depending on the substance being measured, a fiber optic sensor suitable for measuring that substance can be created by changing the thickness of the coating film 7 according to the characteristics of the sensitive film.

[0061] Furthermore, the higher the refractive index range of the substance being measured, the thicker the coating film 7 can be made, shifting the resonance wavelength and the region of high sensitivity to the longer wavelength side, thereby making the optical fiber sensor suitable for the refractive index range of the substance being measured.

[0062] Furthermore, when extending the wavelength range of the introduced light, increasing the film thickness of the coating film 7 shifts the resonance wavelength and the region of high sensitivity to the longer wavelength side, thereby creating a sensor with high refractive index sensitivity in the wavelength range of the introduced light.

[0063] [Performance Verification] The performance of the optical fiber sensor 1 of Example 1, in which an ITO film was deposited on the surface of the cladding 6b of the heterocore portion 3 as the coating film 7 described above, was compared with that of the optical fiber sensor of Comparative Example 1, in which an Au film and a Ta2O5 film were deposited in that order instead of an ITO film in the sensing portion, to verify the performance of the optical fiber sensor 1.

[0064] The optical fiber sensor of Comparative Example 1 generates optical loss through surface plasmon resonance caused by evanescent light leaking into the Au film from the interface between the Au film and the optical fiber sensor 1, and measures the refractive index based on the change in this loss.

[0065] This performance verification was performed using the measurement device 20 with the optical fiber sensor 1 of Example 1 and the measurement device 20 with the optical fiber sensor of Comparative Example 1 installed instead of the optical fiber sensor 1. In this verification, the light source device 21 of the measurement device 20 used a light source that emits light with a wavelength of 850 nm.

[0066] Specifically, verification experiments were conducted in which the sensing parts of the optical fiber sensor of Example 1 and the optical fiber sensor of Comparative Example 1 were exposed to or immersed in the same substances as the target substances described above: air (RIU: 1.000), water (RIU: 1.333), 20w% glycerin aqueous solution (RIU: 1.356), 40w% glycerin aqueous solution (RIU: 1.384), 60w% glycerin aqueous solution (RIU: 1.411), and 80w% glycerin aqueous solution (RIU: 1.441), and the optical loss was measured.

[0067] The results of the above verification experiment are shown in Figure 5. As shown in Figure 5, the change in optical loss with respect to the increase in refractive index is significantly greater for the optical fiber sensor 1 of Example 1 than for the optical fiber sensor of Comparative Example 1.

[0068] Furthermore, looking at the graph in Figure 5, in Comparative Example 1, the change in loss tends to saturate when the RIU reaches around 1.42 and 1.44, but in Example 1, the loss continues to increase steadily with increasing refractive index even up to 1.44 RIU. From this, it can be understood that the optical fiber sensor 1 of Example 1 is capable of measuring refractive indices well beyond 1.44 RIU.

[0069] Table 1 shows the sensitivity [dB / RIU] and coefficient of determination R of the optical fiber sensor in Example 1 and the comparative example, calculated from the graph in Figure 5. 2 The values ​​are shown. As shown in Table 1, sensitivity and R 2 In both cases, the optical fiber sensor 1 of Example 1, which uses a single layer of ITO, shows significantly higher values ​​than the optical fiber sensor of Comparative Example 1, which uses a two-layer film of Au and Ta2O5.

[0070] [Table 1]

[0071] These results show that by using an ITO film of an appropriate thickness, it is possible to achieve higher performance as a refractive index sensor compared to conventional optical fiber sensors using Au thin films and high refractive index materials such as Ta2O5.

[0072] This difference is thought to be due to the very small emission angle of light leaking out from the side of the core in the heterocore portion 3 of the optical fiber sensor. Specifically, the emission angle of light exiting from the core of the heterocore portion 3 of the heterocore optical fiber is around 10°, and the incidence angle to the outer surface of the cladding is also around 10°.

[0073] As described above, in the conventional optical fiber sensor of Comparative Example 1, the refractive index is measured based on the optical loss due to resonant plasmon resonance caused by evanescent light leaking into the Au thin film. However, as described above, the angle of incidence of the leaked light to the outer surface of the cladding is very small, so not much evanescent light is generated. Therefore, the optical loss due to resonant plasmon resonance caused by evanescent light is also small.

[0074] In contrast, in the optical fiber sensor of Embodiment 1 of the present invention, the refractive index is measured based on the optical loss due to loss mode resonance that occurs in the coating film 7 formed by the ITO film, caused by light emitted in radiation mode from the core through the cladding.

[0075] Loss-mode resonance is more suitable for light with a small incident angle, and therefore occurs well under the condition that the incident angle of light entering the cladding is small, as described above. For this reason, the optical loss due to loss-mode resonance is more significant than the optical loss due to resonance plasmon resonance in conventional optical fiber sensors, and it is thought that the optical fiber sensor of Example 1 provides higher performance as a refractive index sensor than the optical fiber sensor of Comparative Example 1.

[0076] Furthermore, the optical fiber sensor 1 of Example 1 also exhibits good time response to changes in the dielectric function or refractive index of the external environment. Therefore, a response evaluation test was conducted to assess the time response characteristics of optical loss to changes in the refractive index of the external environment. This evaluation test was performed using a measuring device 20 with a light source emitting light at a wavelength of 850 nm as the light source device 21. In the evaluation test, the refractive index of the sensing element 10 in the external environment was changed by alternately immersing the sensing element 10 in liquids with various refractive indices and water, and the change in optical loss was observed.

[0077] Figure 6 is a graph showing the change in optical loss in the sensing element 10 in response to a change in the refractive index of the external environment. From this graph, it can be seen that the rise and fall of the optical loss are very steep. Therefore, it can be seen that the optical fiber sensor 1 of Example 1 has very high responsiveness to changes in the refractive index of the external environment. In other words, it can be seen that the optical fiber sensor 1 of Example 1 enables detection and measurement with high time responsiveness.

[0078] As described above, with the optical fiber sensor of Example 1, by using a semiconductor film or dielectric film, specifically ITO, as the coating film 7 as the resonant film, high sensitivity was achieved in measurements using an inexpensive near-infrared light source such as a near-infrared (NIR: wavelength 700~1400nm) LED with a single-layer resonant film.

[0079] [Manufacturing method] Next, a method for manufacturing the optical fiber sensor 1 of Example 1 of the present invention will be described.

[0080] First, the optical fiber 4 is prepared. Next, a coating film 7 made of ITO is formed on the surface of the cladding 6b of the heterocore portion 3 of the optical fiber 4. This completes the optical fiber sensor 1.

[0081] First, the optical fiber sensor 1 was fabricated. Specifically, as described above, a multimode fiber (MMF) (FutureGuide-MM50, manufactured by Fujikura Ltd.), which is a communication optical fiber with a core 5a of 50 μm in diameter and a cladding 6a of 125 μm in diameter, was used as the optical transmission section 2.

[0082] As the heterocore section 3, a single-mode fiber (SMF) (manufactured by F-SA Newport) was used, with a core 5b having a diameter of 3 μm and a cladding 6b having a diameter of 125 μm. The length of the heterocore section 3 was set to 15 mm. The two optical transmission sections 2 and the heterocore section 3 were joined by fusing the two optical transmission sections 2 to each end of the heterocore section 3.

[0083] Next, a 340 nm ITO film is deposited on the portion of the optical fiber 4 including the heterocore 3 by sputtering, so that the entire circumference of the cladding is covered, and the optical fiber sensor 1 is completed.

[0084] In the sputtering of the ITO film of this coating 7, the inflow gas was Ar99% + O21%, that is, an inert gas mixed with only 1% oxygen. By mixing oxygen into the inflow gas during sputtering in this way, it is possible to form a good ITO film with little oxygen deficiency. When the inflow gas was Ar99% + O21%, the average film thickness of the ITO film 10,000 seconds after the start of film formation was 423 nm.

[0085] When we varied the oxygen ratio and formed an ITO film by sputtering with an inflow gas of 80% Ar + 20% O2, the average film thickness of the ITO film after 10,000 seconds from the start of deposition was 109 nm. From this, it can be seen that simply increasing the O2 concentration will decrease the film deposition efficiency.

[0086] The inventors have found that sputtering with an inflow gas of Ar99% + O21% is preferable from the viewpoint of film quality and film formation speed when sputtering an ITO film onto the cladding 6b surface of the heterocore portion 3. [Examples]

[0087] The optical fiber sensor 30 according to Embodiment 2 of the present invention, which applies the optical fiber sensor 1 of Embodiment 1 to a hydrogen sensor, will be described below with reference to the drawings. Note that the drawings are simplified to clarify the optical fiber sensor 30 and its components, and do not represent actual proportions. The optical fiber sensor 30 of Embodiment 2 has the same configuration as the optical fiber sensor 1 of Embodiment 1, except that a sensitive film 8 is provided so as to cover the coating film 7.

[0088] When the optical fiber sensor of Example 1 is applied to a hydrogen sensor, a hydrogen-sensitive film, such as a hydrogen-absorbing metal, is placed near the surface of the resonant film. When this sensitive film absorbs hydrogen or otherwise reacts to hydrogen, a change in the dielectric function or refractive index occurs in the sensitive film, and this causes a change in the resonance wavelength of the loss mode resonance occurring in the thin film. This change is used to detect hydrogen and measure the hydrogen concentration.

[0089] Specifically, for example, when a hydrogen-absorbing metal is used in the sensitive film, the amount of hydrogen absorbed by the sensitive film changes depending on the hydrogen concentration, which in turn changes the dielectric function or refractive index of the sensitive film. This change in dielectric function or refractive index changes the resonance conditions for loss mode resonance (LMR) that occurs in the aforementioned resonant film.

[0090] The change in loss mode resonance occurring in the thin film described above causes a change in the intensity of light passing from one end of the optical fiber to the other, in other words, light transmitted from one end of the optical fiber to the other (hereinafter also referred to as transmitted light or propagated light). In the hydrogen detection device using the optical fiber sensor of this embodiment, hydrogen is measured by measuring the intensity of this transmitted light.

[0091] As described in the description of Example 1, the optical fiber sensor 1 using a single layer of ITO in Example 1 has higher sensitivity and R than conventional optical fiber sensors using a two-layer film of Au and Ta2O5. 2 All of these factors are significantly improved. Therefore, forming a hydrogen-sensitive film on the optical fiber sensor for hydrogen detection in this embodiment results in a much higher detection accuracy than forming a sensitive film on a conventional optical fiber sensor.

[0092] The specific configuration is described below. Figure 7 shows a cross-sectional view of the optical fiber sensor 30 cut along the length of the optical fiber. The sensitive film 8 of the optical fiber sensor 30 in Example 2 is a film formed to cover the surface of the coating film 7. The sensitive film 8 is a laminate in which an intermediate layer made of polylysine and a hydrogen reaction layer containing palladium (Pd) nanoparticles as hydrogen reaction fine particles are alternately stacked on the surface of the coating film 7 using a layer-by-layer method.

[0093] Figure 8 is a magnified view of region A in Figure 7. In this embodiment, the intermediate layer 8a and the hydrogen reaction layer are treated as one layer, and six such layers are formed. That is, the intermediate layer 8a and the hydrogen reaction layer 8b are alternately stacked six times. The intermediate layer 8a is the first to be formed on the surface of the coating film 7 and the cladding 6b. Note that the number of repetitions of the intermediate layer 8a and the hydrogen reaction layer 8b is not limited to six; there may be only one pair of the intermediate layer 8a and the hydrogen reaction layer 8b.

[0094] The reason for stacking the intermediate layer 8a and the hydrogen reaction layer 8b using the Layer-by-Layer method is to ensure good formation of the hydrogen reaction layer 8b. Specifically, in fabricating the optical fiber sensor 1 of this embodiment, negatively charged Pd nanoparticles were used. However, the surfaces of the ITO coating film 7 and cladding 6b are negatively charged, and if left as is, these surfaces and the Pd nanoparticles will have the same polarity and repel each other, preventing the formation of a Pd nanoparticle layer properly.

[0095] When a polylysine layer, which is a cationic polymer, is formed on the surface of the coating film 7 and cladding 6b, the surface becomes positively charged. This creates an attractive force between the surface of the polylysine layer and the negatively charged Pd nanoparticles, enabling the formation of a good Pd nanoparticle layer. In other words, the first intermediate layer 8a can be considered a buffer layer for forming the hydrogen reaction layer 8b.

[0096] In this example, poly-L-lysine hydrobromide (MW 70000-150000, manufactured by Sigma-Aldrich) was used to form the intermediate layer 8a, and the total thickness of the intermediate layer 8a in the entire sensitive film was 5 nm. Furthermore, Pd nanoparticles with an average particle size of 4 nm (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) were used to form the hydrogen reaction layer 8b, and the total thickness of the hydrogen reaction layer 8b in the entire sensitive film 8 was approximately 54 nm. While the intermediate layer 8a is described as being formed from polylysine, it may also be formed from other cationic polymers such as polydiallyldimethylammonium chloride (PDDA). Also, while Pd nanoparticles are used for the hydrogen reaction layer 8b, other hydrogen storage metals may be used.

[0097] In the optical fiber sensor 30, when hydrogen is absorbed into the hydrogen reaction layer 8b, the refractive index of the sensitive film 8 changes. From the change in optical loss, the change in the intensity of the propagated light, and the change in the spectrum corresponding to the change in the refractive index of the sensitive film 8, it is possible to detect hydrogen and calculate the hydrogen concentration.

[0098] Furthermore, in the optical fiber sensor 30 described above, a problem may arise in which the Pd nanoparticles degrade due to long-term and repeated use, reducing the amount of hydrogen that can be absorbed and thus lowering the sensitivity of the sensor. Therefore, in order to suppress the degradation of the Pd nanoparticles, a Pt thin film may be formed to cover the Pd nanoparticles.

[0099] Furthermore, by covering the surface of Pd nanoparticles with a Pt thin film, the Pd nanoparticles act as a catalyst that promotes the decomposition of hydrogen molecules into hydrogen atoms, thereby mitigating the localized accumulation of hydrogen that causes degradation. In other words, it is expected that sensor degradation will be suppressed by covering Pd nanoparticles with a Pt thin film. When covering Pd nanoparticles with a Pt thin film, for example, the Pt thin film is deposited on the Pd nanoparticles using RF magnetron sputtering.

[0100] Furthermore, the optical fiber sensor 30 of Example 2 exhibits good time response in detecting hydrogen in the external environment. Therefore, a response evaluation test was conducted to assess the time response characteristics of the optical loss in detecting hydrogen in the external environment.

[0101] This evaluation test was conducted using a measuring device 20 that employed a light source device 21 emitting light with a wavelength of 850 nm. In the evaluation test, the part including the sensing element 10 was placed in a chamber, and the atmosphere inside the chamber was alternately changed between a pure nitrogen atmosphere of 100% nitrogen and a hydrogen-mixed atmosphere of 96% nitrogen + 4% hydrogen, and the change in light loss was observed.

[0102] Figure 9 is a graph showing the change in light loss in the sensing unit 10 in response to changes in the external atmosphere. In this graph, the time periods with a colored background are the time periods when the atmosphere is hydrogen-contaminated. From the graph in Figure 9, it can be seen that the rise and fall of light loss are very steep, and that the start timing of the hydrogen-contaminated atmosphere coincides with the rise in loss, and the end timing of the hydrogen-contaminated atmosphere coincides with the fall in loss.

[0103] Therefore, it can be seen that the optical fiber sensor 30 of Example 2 has a very high time response to the presence or absence of hydrogen in the external environment. In other words, it can be seen that the optical fiber sensor 30 of Example 2 enables highly time-responsive hydrogen detection and measurement.

[0104] A third embodiment of the present invention, an optical fiber sensor 101, will be described with reference to the drawings. Note that the drawings are simplified to clarify the optical fiber sensor 101 and its components, and do not represent actual proportions. Furthermore, configurations similar to those in the first embodiment will be described using the same notations.

[0105] Figure 10 shows a cross-sectional view of the optical fiber sensor 101 according to the third embodiment, cut along the length of the optical fiber. As shown in Figure 10, the optical fiber sensor 101 includes an optical fiber 4 having a heterocore portion 3 of a predetermined length joined to the end of the optical transmission portion 2. The optical transmission portion 2 is a multimode optical fiber consisting of a core 5a as a first core and a cladding 6a as a first cladding covering the outer surface of the core 5a.

[0106] The heterocore section 3 is a single-mode optical fiber consisting of a core 5b, which is a second core with a smaller diameter than the core 5a, and a cladding 6b, which is a second cladding covering the outer surface of the core 5b. In Embodiment 2, the heterocore section 3 is fused to one end of the optical transmission section 2, and the optical fiber 4 is terminated at the heterocore section 3.

[0107] In this embodiment, as in Embodiment 1 above, a multimode fiber (MMF) (FutureGuide-MM50, manufactured by Fujikura Ltd.), which is a communication optical fiber with a core 5a having a diameter of 50 μm and a cladding 6a having a diameter of 125 μm, was used as the optical transmission unit 2.

[0108] Furthermore, a single-mode fiber (SMF) (manufactured by F-SA Newport, USA) with a core 5b diameter of 3 μm and a cladding 6b diameter of 125 μm was used as the heterocore section 3. The length of the heterocore section 3 was set to 15 mm. The optical transmission section 2 and the heterocore section 3 are joined by fusion splicing. Note that the length of the heterocore section 3 can be, for example, several millimeters to several tens of millimeters.

[0109] The coating film 7 is a film that covers the outer circumferential surface of the cladding 6b of the heterocore portion 3 to the end face 3a opposite to the end face joined to the optical transmission portion 2 of the heterocore portion 3. In this embodiment, the coating film 7 is a 340 nm thin film of ITO. The coating film 7 can be formed, for example, on the surface of the cladding 6b and the end face 3a by vacuum deposition or sputtering. The material exemplified in Example 1 above can be used for the coating film 7.

[0110] In the optical fiber sensor 101 of this embodiment, as in the optical fiber sensor 1 of Embodiment 1, when light is incident from the end of the optical transmission section 2, the core diameter of the heterocore section 3 becomes smaller than that of the optical transmission section 2, causing light to leak from core 5a or core 5b to the cladding 6b and propagate through the cladding 6b. As a result, loss mode resonance (LMR) occurs in the coating film 7 of the sensing section 10, which changes with changes in the dielectric function or refractive index of the environment outside the sensing section 10.

[0111] Figure 11 shows a measuring device 40 using the optical fiber sensor 101 described above. As shown in the figure, the measuring device 40 has a main optical fiber 41 and individual optical fibers 42 branched from the main optical fiber 41. In the measuring device 40, the optical fiber sensor 101 is joined to the tip of an individual optical fiber 42.

[0112] The main optical fiber 41 and the individual optical fibers 42 are made of multimode optical fibers similar to the optical transmission section 2 of Example 1. The individual optical fiber 42 may also serve as the optical transmission section 2. That is, the heterocore section 3 may be directly fused to the end of the individual optical fiber 42.

[0113] The reflected / backscattered light intensity distribution detection device 43 (hereinafter also simply referred to as the scattering intensity distribution detection device) is, for example, an optical time domain reflectometer (OTDR) connected to the main optical fiber 41. It emits optical pulses towards multiple optical fiber sensors 101 via the main optical fiber 41 and individual optical fibers 42, and measures the position and intensity of the backscattered light at each optical fiber sensor 101.

[0114] The analysis device 23 is a device that receives the measurement results of the position and intensity of backscattered light output from the scattering intensity distribution detection device 43, and records and displays them. The analysis device 23 is, for example, a personal computer with data logger software installed.

[0115] The analysis device 23, acting as the detection unit, is a device that receives the measurement results of the light spectrum output from the light receiving device 22 and records and displays them. The analysis device 23 is, for example, a personal computer with data logger software installed. The analysis device 23 may also have an application installed that can calculate the refractive index of the substance to be measured in contact with the coating film 7 of the optical fiber sensor 1, using the loss of light propagated through the optical fiber sensor 1, which is identified based on the above measurement results. Of course, the analysis device 23 may be a general-purpose computer or a computer specialized for a specific application.

[0116] Furthermore, by forming the sensitive film 8 described in Example 2 on the surface of the coating film 7 of the optical fiber sensor 101, the optical fiber sensor 101 can be used as a hydrogen sensor.

[0117] In the above-described embodiment, the case in which a heterocore portion 3 with a smaller core diameter than other portions is formed in the sensing portion 10 of the optical fiber 4 was mainly explained. However, in order to function as a hydrogen sensor, it is sufficient that light leaks out of the fiber in the sensing portion 10, and the sensing portion 10 of the optical fiber 4 does not necessarily have to be a heterocore portion 3. In other words, the optical fiber 4 may have a uniform core diameter including the sensing portion 10 without having a heterocore portion 3.

[0118] For example, in a fiber with a uniform core diameter, the portion corresponding to the sensitive part 10 of the present invention may be an unclad fiber without cladding, or a fiber with cladding that is thinner than the rest of the fiber.

[0119] Even in this case, since a lot of light leaks out of the core into the fiber at the sensing unit 10, it is possible to measure the refractive index and detect or measure materials by observing the light loss that changes with the change in refractive index of the ambient light at the sensing unit 10.

[0120] Furthermore, as mentioned above, if a portion is created by eliminating or thinning the cladding, the diameter of the fiber itself in that portion becomes smaller, resulting in a decrease in mechanical strength. In contrast to such a configuration, the structure of the present invention, which includes the heterocore portion 3, reduces only the core diameter in the sensing portion 10, eliminating the need to reduce the fiber diameter including the cladding. This makes it possible to maintain mechanical strength while more actively leaking light in the sensing portion 10 than in other portions.

[0121] Furthermore, the core diameter of the sensing portion 10 of fiber 4 may be larger than that of other portions. Even in this case, since a large amount of light leaks out of the core to the outside of the fiber at the sensing portion 10, it is possible to measure the refractive index and detect or measure materials by observing the light loss that changes with the change in refractive index of the outside environment at the sensing portion 10.

[0122] Furthermore, in the above embodiment, the optical fiber sensor 30 of Embodiment 2, which has a sensitive layer containing a hydrogen-absorbing metal, was described as an application of the optical fiber sensor 1 to a hydrogen sensor. However, the optical fiber sensor 1 can be applied to various measurements by changing the material of the sensitive layer. For example, the optical fiber sensor 1 can be configured to measure magnetic fields by providing a sensitive layer made of magnetic fluid. In this case, the refractive index of the magnetic fluid changes according to the magnetic field strength it receives, and the amount of light absorbed in the sensitive part 10 changes, so the magnetic field can be measured by detecting this change.

[0123] Furthermore, although a coating film 7 is formed in the above embodiment, the hydrogen sensor can also function with a configuration in which the sensitive film 8 is directly formed on the surface of the fiber of the heterocore portion 3 without forming the coating film 7. In this case as well, the light leaking out from the heterocore portion 3 changes depending on the presence or absence of hydrogen and changes in its concentration, and the loss of transmitted light changes.

[0124] The various configurations and dimensions in the above-described embodiments are merely examples and can be modified as appropriate depending on the application. [Explanation of symbols]

[0125] 1,30,101… Fiber optic sensor 2…Optical transmission section 3... Heterocore section 4… Fiber optics, 5a, 5b... Core 6a, 6b... clad 7...Coating film 8…Sensing membrane 10...Sensing part 20.40… Measuring device 21…Light source 22...Detection device 23…Analysis device 41…Main optical fiber 42... Individual optical fibers, 43…Intensity distribution detection and analysis device

Claims

1. It comprises an optical fiber having an optical transmission section for transmitting incident light, An optical fiber sensor is provided in the middle of the optical transmission section, which includes a resonant film formed on the surface of the optical fiber and causing loss mode resonance, and an optical response section whose response to incident light changes in accordance with changes in the surrounding conditions, The thickness of the resonant film is determined according to the wavelength of light passing through the optical fiber sensor and / or the optical transmission section.

2. An optical fiber sensor according to claim 1, wherein the film is an ITO film.

3. An optical fiber sensor according to claim 2, characterized in that the ITO film is formed by sputtering.

4. An optical fiber sensor according to claim 2, wherein a sensitive layer containing hydrogen-reactive fine particles is formed on the outer circumferential surface of the optical response portion.

5. An optical fiber sensor according to claim 4, wherein the sensitive layer is formed by alternately laminating a hydrogen reaction layer containing hydrogen reaction fine particles and an intermediate layer made of a cationic polymer.

6. An optical fiber sensor according to claim 4, wherein the hydrogen-reactive fine particles are palladium nanoparticles.

7. An optical fiber sensor according to claim 5, wherein the intermediate layer is made of polylysine.

8. The optical response unit according to claim 1 is an optical fiber sensor comprising an optical fiber having a core smaller in diameter than the core of the optical fiber of the optical transmission unit and a cladding covering the outer surface of the core.

9. An optical fiber sensor according to claim 8, wherein the optical fiber of the transmission section is a multimode fiber and the optical fiber of the optical response section is a single-mode fiber.

10. A light source that is optically coupled to the optical fiber of the transmission unit according to claim 1 and causes light to be incident on the optical fiber of the transmission unit, A light receiving unit that receives light transmitted via the aforementioned light response unit, A detection unit that detects changes in the optical response unit based on the light received signal generated by the light received by the light receiving unit, A fiber optic sensor device having the following features.

11. An optical fiber sensor device according to claim 10, wherein the optical fiber of the transmission section is a multimode fiber, the optical fiber of the optical response section is a single-mode fiber, the light source is a near-infrared light source, and the thickness of the resonant film is determined according to the object to be measured by the optical fiber sensor.

12. It comprises an optical fiber having an optical transmission section for transmitting incident light, An optical fiber sensor is provided in the middle of the optical transmission section, which includes a resonant film formed on the surface of the optical fiber and causing loss mode resonance, and an optical response section whose response to incident light changes in accordance with changes in the surrounding conditions, An optical fiber sensor having a sensitive layer containing hydrogen-reactive fine particles formed on the outer surface of the light-responding portion.

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