Optical fiber sensor and optical fiber sensor device using the same

Abstract

The present invention provides an optical fiber sensor with extremely high sensitivity 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 in the middle of the optical transmission section, there is an optical response section which includes a resonance film made of a metal film formed on the surface of the optical fiber and which generates surface plasmon resonance when excited by evanescent light, or a metal particle film containing metal nanoparticles which generate localized surface plasmon resonance when excited by evanescent light, and an auxiliary film containing graphene oxide provided to cover the resonance film, and whose response when it receives the incident light changes in accordance with changes in the surrounding conditions.

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 a sensor for performing various measurements, a sensor using an optical fiber, so-called an optical fiber sensor, has been proposed. For example, in Patent Document 1, an optical fiber sensor hydrogen sensor for detecting hydrogen 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 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, one of the problems is that the detection accuracy of the refractive index of the sensor having a structure composed of the metal film and the dielectric film is very low in practical use.

[0005] An object of the present invention is to provide an optical fiber sensor with very high sensitivity and an optical fiber sensor device using the same.

Means for Solving the Problems

[0006] According to the present invention, an optical fiber having an optical transmission unit for transmitting incident light is provided. An optical fiber sensor is provided, which includes an optical response section in the middle of the optical transmission section, comprising a resonant film made of a metal film formed on the surface of the optical fiber and which generates surface plasmon resonance when excited by evanescent light, or a metal particle film containing metal nanoparticles that generate localized surface plasmon resonance when excited by evanescent light, and an auxiliary film containing graphene oxide provided to cover the resonant film, wherein the response when it receives the incident light changes in accordance with changes in the surrounding conditions. [Effects of the Invention]

[0007] The optical fiber sensor of the present invention is a sensor comprising an optical fiber, a resonant film arranged around the optical fiber to induce surface plasmon resonance, and an auxiliary film containing graphene oxide, and has a configuration that can utilize the changes in surface plasmon resonance that occur in the resonant film. With such a configuration, it is possible to provide an optical fiber sensor with very high sensitivity and an optical fiber sensor device using the same. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional view of the optical fiber sensor of Example 1. [Figure 2] This is a magnified section of Figure 1. [Figure 3] This is an explanatory diagram showing an example of the configuration of a measuring device. [Figure 4A] This is a spectral graph showing the change in the transmitted light spectrum as the refractive index changes. [Figure 4B] This is a spectral graph showing the change in the transmitted light spectrum as the refractive index changes. [Figure 4C] This is a spectral graph showing the change in the transmitted light spectrum as the refractive index changes. [Figure 5] This figure shows the change in optical loss due to a change in refractive index. [Figure 6] This is a schematic cross-sectional view of the optical fiber sensor of Example 2. [Figure 7A]This is a spectral graph showing the change in the transmitted light spectrum as the refractive index changes. [Figure 7B] This is a spectral graph showing the change in the transmitted light spectrum as the refractive index changes. [Figure 7C] This is a spectral graph showing the change in the transmitted light spectrum as the refractive index changes. [Figure 8] schematic cross-sectional view of the optical fiber sensor of Example 3 [Figure 9] This is an explanatory diagram showing an example of the configuration of a measuring device. [Modes for carrying out the invention]

[0009] First, we will describe the overview and operating principle of the optical fiber sensor of the present invention.

[0010] 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 resonant film is formed on the outer circumference of a portion of the optical fiber in the longitudinal direction, which is a metal film that generates surface plasmon resonance (SPR) or a film containing metal particles that generate localized surface plasmon resonance (LSPR) and surrounds the optical fiber. An auxiliary film containing graphene oxide is formed to cover the resonant film. Furthermore, the optical fiber sensor of the present invention has a structure that allows light to leak out to the outer circumference of the optical fiber in the portion where the resonant film and auxiliary film are formed. In other words, it has a structure that allows light to leak from the cladding of the optical fiber.

[0011] According to the optical fiber sensor of the present invention, it is possible to measure the refractive index of a substance in contact with the auxiliary film of the optical fiber sensor. By applying the optical fiber sensor of the present invention, various measurements such as detection of a substance at the location where the optical fiber sensor is placed, measurement of the amount or concentration of a substance, measurement of a magnetic field, etc. can be performed based on the measurement of the refractive index. In the optical fiber sensor of the present invention, depending on the object to be specified, a sensitive film or sensitive layer sensitive to a substance, magnetic field, etc. may be formed on the surface of a thin film that generates the above-mentioned SPR or LSPR.

[0012] In the optical fiber sensor of the present invention, due to the change in the refractive index of the substance in contact with the above-mentioned auxiliary film, the resonance wavelength of SPR or LSPR in the resonance film changes. Thereby, the amount of light lost in the resonance film changes. In the measurement of the refractive index using the optical fiber sensor of the present invention, this change in the light loss is used.

[0013] Due to the change in the resonance wavelength of SPR or LSPR generated in the above-mentioned resonance film, a change in the amount of the above-mentioned light loss 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 object 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 above-mentioned resonance film is formed is smaller than the core diameter of the optical fiber in other portions. 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-mentioned hetero-core structure, light can be actively leaked from the core in the portion of the optical fiber where the core diameter is smaller than in other portions. Thereby, the change in the light loss that changes depending on the refractive index of the target substance in contact with the above-mentioned auxiliary film, that is, the change in the intensity of the transmitted light, appears more prominently than in a normal optical fiber with a constant core diameter.

[0016] As described above, as the optical fiber sensor, it is desirable to have a hetero-core structure. Basically, in the portion where the resonance film is formed, it is only necessary that the propagated light that has propagated from 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 the intensity of the emitted light emitted from the other end by incident light of a single wavelength from one end of the optical fiber. Further, the optical fiber sensor device using the above optical fiber sensor may measure the change in the refractive index from the change in the intensity of the light of a specific wavelength among the emitted light emitted from the other end and the change in the spectrum of the emitted light by incident light having a broad spectrum such as obtained from a white light source using a halogen light source from one end of the optical fiber.

[0018] As the material of the thin film in which the above SPR phenomenon and LSPR phenomenon occur, metals, specifically, noble metals such as gold and silver, and copper can be used. The thin film that causes the SPR phenomenon is a thin film made of only a single metal or an alloy or a thin film mainly composed of these. The thin film that causes the LSPR phenomenon is a thin film containing single metal or alloy particles.

[0019] In the above optical fiber sensor, the auxiliary film is composed of a structure in which one or more layers of a combination of a layer of a cationic polymer and a layer of graphene oxide formed thereon are formed. The larger the number of layers of this auxiliary film, the more the resonance wavelength of SPR or LSPR and the peak wavelength of light absorption by SPR or LSPR tend to shift to the long wavelength side. That is, by changing the number of layers of the auxiliary film, a shift in the resonance wavelength, and thus a shift in the wavelength range with high sensitivity, can be caused.

[0020] 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 number of auxiliary film layers according to the wavelength range of light introduced and the target of measurement. Furthermore, a larger number of auxiliary film layers also means a larger thickness of graphene oxide within the auxiliary film layer 8. [Examples]

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] The coating film 7, acting as a resonant film, is a metal film formed on the outer surface of the cladding 6b of the heterocore portion 3. In this embodiment, the coating film 7 is a 40 nm thick gold (Au) thin film. 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 40 nm.

[0027] The auxiliary film 8 is a laminate in which a polymer layer made of polylysine and a graphene layer made of graphene oxide are alternately laminated on the surface of the coating film 7 using a layer-by-layer method.

[0028] Figure 2 is a magnified view of region A in Figure 1. In this embodiment, the polymer layer 8a and the graphene layer 8b are treated as one layer, and three such layers are formed. That is, the polymer layer 8a and the graphene layer 8b are alternately laminated three times. The polymer 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 polymer layer 8a and the graphene layer 8b is not limited to three; only one pair of polymer layer 8a and graphene layer 8b may be formed.

[0029] The reason for laminating the polymer layer 8a and the graphene layer 8b using the Layer-by-Layer method is to ensure good formation of the graphene layer 8b. Specifically, in forming the auxiliary film 8 of the optical fiber sensor 1 in this embodiment, negatively charged graphene oxide particles were used. However, the surfaces of the coating film 7 and cladding 6b, which are made of Au, are negatively charged, and if left as is, these surfaces and the graphene oxide particles will have the same polarity and repel each other, preventing the graphene oxide layer from being formed properly.

[0030] When a polylysine layer, 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 polylysine layer surface and the negatively charged graphene oxide, enabling the formation of a good graphene oxide layer. In other words, the first polymer layer 8a can be considered a buffer layer for forming the graphene oxide layer 8b.

[0031] In this example, poly-L-lysine hydrobromide (MW 70000-150000, manufactured by Sigma-Aldrich) was used to form the polymer layer 8a. Furthermore, thin flakes of graphene oxide with an average thickness of 1 nm (Graphene oxide dispersion, manufactured by Sigma-Aldrich) were used to form the graphene layer 8b. While the polymer layer 8a is described as being formed with polylysine, it may be formed with other cationic polymers.

[0032] In the optical fiber sensor 1, the heterocore portion 3, the coating film 7 formed on the heterocore portion 3, and the auxiliary film 8 together constitute the sensing portion 10 as an optical response portion.

[0033] 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.

[0034] 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.

[0035] As a result, evanescent light or evanescent waves are generated that seep out from the outer surface of the cladding 6b. This evanescent light excites surface plasmon resonances and localized surface plasmon resonances in the coating film 7.

[0036] As the coating film 7 of the optical fiber sensor 1 in this embodiment, a metal can be used, specifically a material that causes SPR or LSPR phenomena by evanescent light, such as precious metals like gold and silver as described above, or copper. The thin film may be a thin film that causes SPR phenomena with the above material as the base material, or it may be a metal particle film that causes LSPR phenomena in which the above material is included as metal nanoparticles in another base material.

[0037] [Measuring device] Figure 3 shows the configuration of the measuring device 20 as an optical fiber sensor device using the optical fiber sensor 1 described above.

[0038] 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.

[0039] 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 device, a spectroscopic element is not necessarily required.

[0040] 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.

[0041] 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.

[0042] 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 at the heterocore section 3, and this light generates evanescent light that seeps towards the coating film 7 at the interface between the cladding 6b and the coating film 7. This evanescent light then causes SPR or LSPR in the coating film 7, resulting in light attenuation.

[0043] The amount of light attenuation in the coating film 7, or the amount of light attenuation for each wavelength, changes depending on the refractive index of the substance 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 refractive index of the substance being measured or to detect changes in these refractive intensities.

[0044] In this invention, by forming the auxiliary film 8, high sensitivity can be achieved in measurements using inexpensive near-infrared (NIR: wavelength 700-1400 nm) LEDs. In other words, by forming the auxiliary film 8, it becomes possible to construct a device with high measurement sensitivity using inexpensive near-infrared LEDs, thereby reducing the cost of the device.

[0045] [Verification experiment] We investigated whether the characteristics of the optical fiber sensor 1 described above could be changed by altering the peak wavelength of optical loss by changing the number of layers of the auxiliary film 8.

[0046] This experiment was conducted using three samples of the optical fiber sensor 1 from Example 1: Sample 1 (GO1), in which the auxiliary film 8 was formed with one set of polymer layers 8a and graphene layer 8b; Sample 2 (GO2), in which the auxiliary film 8 was formed with two sets of polymer layers 8a and graphene layer 8b; and Sample 3 (GO3), in which the auxiliary film 8 was formed with three sets of polymer layers 8a and graphene layer 8b, similar to Example 1.

[0047] Specifically, the intensity spectra of transmitted light passing through the optical fiber sensor 1 were measured when these samples were exposed to air and when immersed in liquids with different refractive indices. By examining these spectra, it was confirmed whether the peak of optical loss shifted depending on the number of polymer layers 8a and graphene layers 8b in the auxiliary film 8.

[0048] The intensity spectra were measured using the measuring device 20 described above. Specifically, the fiber sensors of samples 1, 2, and 3 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 spectra.

[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 pure water (RIU: 1.333) (H2O in the figure), 10% glycerin aqueous solution (RIU 1.346) (gly10%) in the figure, 20w% glycerin aqueous solution (RIU: 1.361) (gly20%) in the figure, 30w% glycerin aqueous solution (RIU: 1.376) (gly30%) in the figure, 40w% glycerin aqueous solution (RIU: 1.391) (gly40%) in the figure, 50w% glycerin aqueous solution (RIU: 1.405) (gly50%) in the figure, and 60w% glycerin aqueous solution (RIU: 1.419) (gly60%) in the figure.

[0051] Figures 4A, 4B, and 4C show graphs illustrating the light spectra propagated from one end to the other of the optical fiber sensor 1 when the sensing element 10 was immersed in the target substance, for samples 1 to 3, respectively. Figure 4A is the graph for sample 1, Figure 4B for sample 2, and Figure 4C for sample 3. In each graph, the vertical axis represents normalized light intensity, and the horizontal axis represents wavelength.

[0052] As can be seen from the graphs in Figures 4A to 4C, the more the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 is increased, the more the loss peak, i.e., the resonance wavelength of the SPR occurring in the coating film 7, shifts to the longer wavelength side. From this result, it was confirmed that the characteristics of the optical fiber sensor 1 can be changed by shifting the loss peak to the longer wavelength side by increasing the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8.

[0053] As described above, by changing the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8, 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 number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 according to the wavelength range of 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 light introduced and the object to be measured.

[0054] For example, specifically, when a sensitive film is formed on the surface of the auxiliary film 8 whose refractive index changes in the presence of the substance to be measured, an optical fiber sensor suitable for measuring the substance can be created by changing the number of polymer layers 8a and graphene layers 8b of the auxiliary film 8 according to the characteristics of the sensitive film.

[0055] Furthermore, the higher the refractive index range of the substance being measured, the more the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 can be increased, 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.

[0056] Furthermore, when extending the wavelength range of the introduced light, the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 is increased to shift the resonance wavelength and the region with high sensitivity to the longer wavelength side, thereby creating a sensor with high refractive index sensitivity in the wavelength range of the introduced light.

[0057] [Comparative experiment] A comparative experiment was conducted to compare samples 1, 2, and 3 of the optical fiber sensor 1 described above with a conventional optical fiber sensor in which a tantalum oxide (Ta2O5) film, which is a metal oxide film, was formed on the sensing part 10 of the optical fiber sensor 1 instead of the auxiliary film 8 made of graphene oxide of Example 1. As Comparative Example 1, a film of Ta2O5 with a thickness of 20 nm was deposited on the coating film 7.

[0058] In the comparative experiment, the loss of transmitted light passing through the optical fiber sensor will be measured when these samples and comparative examples are immersed in seven of the eight target substances other than air. By observing this loss, the superiority of the optical fiber sensor of Example 1 over the optical fiber sensor of Comparative Example 1 will be confirmed. Furthermore, by comparing the losses of samples 1, 2, and 3 with each other, the shift in the high-sensitivity region will also be confirmed.

[0059] The loss was measured using the measuring device 20 described above. Specifically, the fiber sensors of Sample 1, Sample 2, Sample 3, and Comparative Example 1 were attached to the measuring device 20, and the sensing part was exposed to or immersed in target substances having multiple different refractive indices to measure the optical spectrum.

[0060] In this measurement, the light source device 21 used a near-infrared LED emitting 850 nm light as described above, and the light receiving device 22 used a CCS200 / M compact spectrometer (manufactured by Thorlabs).

[0061] Figure 5 shows graphs plotting the optical loss of optical fiber sensors for samples 1, 2, and 3, as well as Comparative Example 1, for each target substance. (a) shows the optical loss for sample 1, (b) for sample 2, and (C) for sample 3, with the optical loss of the fiber sensor for Comparative Example 1 also shown in each graph. In each graph, the vertical axis represents normalized loss and the horizontal axis represents refractive index.

[0062] As can be seen from the graph in Figure 5, the graphs for Samples 1 to 3 all show a steeper slope at practical refractive indices of RIU 1.37 or higher than the graph for the fiber sensor of Comparative Example 1. Samples 1 to 3 show a greater increase in loss with increasing refractive index than the optical fiber sensor of Comparative Example 1. From this, it can be seen that Samples 1 to 3 have higher sensitivity than the optical fiber sensor of Comparative Example 1 at practical refractive indices of RIU 1.37 or higher.

[0063] Furthermore, comparing samples 1 through 3, sample 1 showed the highest sensitivity in the RIU 1.405–1.419 range, while sample 3 showed the highest sensitivity in the RIU 1.376–1.405 range. As confirmed in the verification experiment described above, it was found that by changing the number of layers according to the measurement target, the refractive index range with high sensitivity can be shifted to match the refractive index range of the measurement target.

[0064] As described above, the optical fiber sensor of Example 1 achieves high sensitivity in measurements using an inexpensive near-infrared light source such as a near-infrared (NIR: wavelength 700-1400 nm) LED with a single-layer resonant film by forming an auxiliary film 8 containing graphene oxide on a coating film 7 which is a thin film containing metal as a resonant film. Furthermore, the optical fiber sensor of Example 1 allows for easy shifting of the high-sensitivity region to match the refractive index range of the measurement target by changing the number of layers of the auxiliary film 8 as described above.

[0065] [Manufacturing method] Next, the manufacturing method of the optical fiber sensor 1 according to Example 1 of the present invention will be described.

[0066] First, the optical fiber 4 is prepared. Next, a coating film 7 made of Au is formed on the surface of the cladding 6b of the heterocore portion 3 of the optical fiber 4. Then, an auxiliary film 8 containing graphene oxide is formed to cover the coating film 7. This completes the optical fiber sensor 1.

[0067] First, the optical fiber 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.

[0068] 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.

[0069] Next, a 40 nm thick Au 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.

[0070] Next, the heterocore portion 3 is alternately immersed in a polylysine solution and a graphene oxide nanosheet suspension, and the polylysine layer and graphene oxide layer are alternately stacked using the Layer-by-Layer method to complete the optical fiber sensor 1. [Examples]

[0071] Below, an optical fiber sensor 30 according to Example 2 of the present invention, which is obtained by adding an interlayer, which is a layer of metal oxide, to the optical fiber sensor 1 of Example 1, will be described with reference to the drawings. Note that the drawings are distorted to clarify the optical fiber sensor 30 and its components, and do not represent the actual proportions. The optical fiber sensor 30 of Example 2 has the same configuration as the optical fiber sensor 1 of Example 1, except that an interlayer 9 is provided between the coating film 7 and the auxiliary film.

[0072] Figure 6 shows a cross-sectional view of the optical fiber sensor 30 cut along the length of the optical fiber. The interlayer 9 of the optical fiber sensor 30 in Example 2 is a film formed to cover the surface of the coating film 7. The interlayer 9, as a metal oxide film, is formed by depositing Ta2O5 on the surface of the coating film 7. An auxiliary film 8 is formed on top of the interlayer 9 so as to cover its upper surface.

[0073] [Verification experiment] Regarding the optical fiber sensor 30 of Example 2 described above, we investigated whether the characteristics of the sensor could be changed by changing the peak wavelength of optical loss by changing the number of layers of the auxiliary film 8.

[0074] This experiment was conducted using three samples of the optical fiber sensor 30 from Example 2: Sample 4, in which the auxiliary film 8 was formed with one set of polymer layers 8a and graphene layer 8b; Sample 5, in which the auxiliary film 8 was formed with two sets of polymer layers 8a and graphene layer 8b; and Sample 6, in which the auxiliary film 8 was formed with three sets of polymer layers 8a and graphene layer 8b, similar to Example 1.

[0075] Specifically, the intensity spectra of transmitted light passing through the optical fiber sensor 30 were measured when these samples were exposed to air and when immersed in liquids with different refractive indices. By examining these spectra, it was confirmed whether the peak of optical loss shifted depending on the number of polymer layers 8a and graphene layers 8b in the auxiliary film 8.

[0076] The intensity spectra were measured using the measuring device 20 described above. Specifically, the fiber sensors of samples 4, 5, and 6 were attached to the measuring device 20, and the sensing part 10 was exposed to or immersed in target substances having multiple different refractive indices to measure the optical spectra.

[0077] In this measurement, as in the verification experiment in Example 1, the light source device 21 used 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).

[0078] Furthermore, the target substances were the same as those used in the above verification experiment, with air added: air (RIU: 1.000) (shown as "air" in the figure), pure water (RIU: 1.333) (shown as "H2O" in the figure), 10% glycerin aqueous solution (RIU: 1.346) (shown as "Gly10%" in the figure), 20w% glycerin aqueous solution (RIU: 1.361) (shown as "Gly20%" in the figure), 30w% glycerin aqueous solution (RIU: 1.376) (shown as "Gly30%" in the figure), 40w% glycerin aqueous solution (RIU: 1.391) (shown as "Gly40%" in the figure), 50w% glycerin aqueous solution (RIU: 1.405) (shown as "Gly50%" in the figure), and 60w% glycerin aqueous solution (RIU: 1.419) (shown as "Gly60%" in the figure).

[0079] Figures 7A, 7B, and 7C show graphs illustrating the light spectra propagated from one end to the other of the optical fiber sensor 30 when the sensing element 10 was immersed in the target substance for samples 4 through 6, respectively. Figure 7A is the graph for sample 4, Figure 7B for sample 5, and Figure 7C for sample 36. In each graph, the vertical axis represents normalized light intensity, and the horizontal axis represents wavelength.

[0080] As can be seen from the graphs in Figures 7A to 7C, the more sets of polymer layers 8a and graphene layers 8b in the auxiliary film 8 are increased, the more the loss peak, i.e., the resonance wavelength of the SPR occurring in the coating film 7, shifts to the longer wavelength side.

[0081] From these results, it was confirmed that, similar to the optical fiber sensor 1 of Example 1, the characteristics of the optical fiber sensor 30 can be changed by increasing the number of sets of polymer layer 8a and graphene layer 8b of the auxiliary film 8 in the optical fiber sensor 30 with the addition of the interlayer film 9, thereby shifting the loss peak to the longer wavelength side.

[0082] Therefore, even when using the optical fiber sensor 30, similar to the optical fiber sensor 1 in Example 1, it is possible to adjust the characteristics of the sensor to the wavelength range of light introduced into the optical fiber sensor and the target of measurement by changing the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 according to the wavelength range of light introduced into the optical fiber sensor and the target of measurement.

[0083] For example, specifically, when a sensitive film is formed on the surface of the auxiliary film 8 whose refractive index changes in the presence of the substance to be measured, an optical fiber sensor suitable for measuring the substance can be created by changing the number of polymer layers 8a and graphene layers 8b of the auxiliary film 8 according to the characteristics of the sensitive film.

[0084] Furthermore, the higher the refractive index range of the substance being measured, the more the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 can be increased, 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.

[0085] Furthermore, when extending the wavelength range of the introduced light, the number of polymer layer 8a and graphene layer 8b pairs in the auxiliary film 8 is increased to shift the resonance wavelength and the region with high sensitivity to the longer wavelength side, thereby creating a sensor with high refractive index sensitivity in the wavelength range of the introduced light.

[0086] Furthermore, comparing the graphs in Figure 4 and Figure 7, the optical loss peak of the optical fiber sensor 1 in Example 1 is sharper than that of the optical fiber sensor 30 in Example 2. This sharpness of the peak is thought to be due to the presence or absence of the interlayer film 8. [Examples]

[0087] A third embodiment of the present invention, specifically an optical fiber sensor 101, will be described below 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] The coating film 7 is a metal film that covers the outer circumferential surface of the cladding 6b of the heterocore portion 3 and 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 thin film with a thickness of 40 nm AU. The coating film 7 can be formed, for example, on the surface of the cladding 6b and the end face 3a of the heterocore portion 3 by vacuum deposition or sputtering.

[0093] The auxiliary film 8 is a laminate in which a polymer layer made of polylysine and a graphene layer made of graphene oxide are alternately stacked on the surface of the coating film 7 using a layer-by-layer method. The layer structure of the auxiliary film 8 is the same as that of the optical fiber sensor 1 in Example 1 above, so a description is omitted.

[0094] 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 in the heterocore section 3 becomes smaller than that of the optical transmission section 2, so light leaks from the core 5a or core 5b to the cladding 6b and propagates through the cladding 6b. This propagated light reaches the interface between the cladding 6b and the coating film 7, generating evanescent light that seeps towards the coating film 7. This evanescent light causes SPR or LSPR to occur in the coating film 7 of the sensing section 10, which changes depending on the change in refractive index of the environment outside the sensing section 10.

[0095] In addition, in the optical fiber sensor 101 described above, an intermediate film 9 may be formed between the coating film 7 and the auxiliary film 8, similar to the optical fiber sensor 30 of Example 2.

[0096] Figure 9 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] Furthermore, the core diameter of the sensing portion 10 of the optical 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 into the fiber at the sensing portion 10, it is possible to measure the refractive index and detect or measure materials by observing the optical loss that changes with the change in refractive index of the ambient light at the sensing portion 10.

[0106] Furthermore, in the above examples, optical fiber sensor 1 and optical fiber sensor 30 were described as refractive index sensors. However, optical fiber sensor 1 and optical fiber sensor 30 can be applied to various measurements by changing the material of the sensing layer.

[0107] For example, optical fiber sensors 1 and 30 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.

[0108] Furthermore, optical fiber sensors 1 and 30 can be reconditioned to detect hydrogen by providing a sensing layer made of a hydrogen-absorbing metal. In this case, hydrogen absorption by the hydrogen-absorbing metal in the sensing layer changes the refractive index of the sensing layer, which changes the amount of light absorbed in the sensing part 10, and this change can be detected to detect hydrogen.

[0109] The various configurations, materials, dimensions, etc., in the above-described embodiments are merely examples and can be modified as appropriate depending on the application. [Explanation of symbols]

[0110] 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…Auxiliary membrane 9…Interlayer 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 made of a metal film formed on the surface of the optical fiber and which generates surface plasmon resonance when excited by evanescent light, or a metal particle film containing metal nanoparticles which generate localized surface plasmon resonance when excited by evanescent light, and an auxiliary film containing graphene oxide provided to cover the resonant film, wherein the response when it receives the incident light changes in accordance with changes in the surrounding conditions.

2. An optical fiber sensor according to claim 1, wherein the auxiliary film is formed by alternately stacking graphene oxide and a cationic polymer multiple times.

3. An optical fiber sensor according to claim 2, characterized in that the cationic polymer is polylysine.

4. An optical fiber sensor according to any one of claims 1 to 3, characterized in that the resonant film includes a metal oxide film formed on the metal film or metal particle film.

5. An optical fiber sensor according to claim 4, characterized in that the metal oxide film is a film made of tantalum oxide.

6. An optical fiber sensor according to claim 1, wherein the optical response portion comprises an optical fiber having a core smaller in diameter than the core of the optical fiber of the optical transmission portion and a cladding covering the outer surface of the core.

7. An optical fiber sensor according to claim 6, wherein the optical fiber of the optical transmission section is a multimode fiber and the optical fiber of the optical response section is a single-mode fiber.

8. A light source that is optically coupled to the optical transmission section of the optical fiber sensor according to claim 1 and causes light to be incident on the optical transmission section, 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.

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