Optical fiber sensor
The optical fiber sensor design addresses the challenges of cost and miniaturization by using a dual-fiber structure with optimized core diameters and numerical apertures to enhance optical coupling efficiency and eliminate unnecessary components, resulting in a compact and efficient magnetic field measurement system.
Patent Information
- Application Number
- JP2024078535
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-14
- Publication Date
- 2025-11-27
Smart Images
Figure 2025173127000001_ABST
Abstract
Description
[Technical Field]
[0001] The present invention relates to an optical fiber sensor. [Background technology]
[0002] There is a known method for measuring magnetic fields based on optically detected magnetic resonance (ODMR) that utilizes lattice defects that emit fluorescence, such as color centers. In particular, there is a known method for measuring magnetic fields that uses nitrogen-vacancy complexes (NV centers), which are a type of color center formed in diamond (Non-Patent Document 1). The NV center has a structure in which one carbon atom is replaced by nitrogen and the next one is a vacancy, and it is in a neutral charge state, NV 0 Capture one electron from NV - Then, the electron has a magnetic quantum number m s The NV center forms a spin triplet state of -1, 0, +1. The principle of magnetic field detection by the NV center is as follows. - has two relaxation processes depending on the spin state before photoexcitation. s In the =0 state, electrons excited by green light emit red fluorescence and s When microwaves with a resonant frequency (2.87 GHz) are irradiated, electron spin resonance (ESR) occurs, and the s = ±1 state occurs, but m s Some of the electrons photoexcited from m = ±1 return to the ground state through a non-radiative transition and do not emit light. Therefore, by sweeping the frequency of the microwave irradiated to the NV center, electron spin resonance can be detected at the point where the red light intensity decreases. This is called ODMR. Here, m s =±1 undergoes Zeeman splitting in proportion to the external magnetic field strength, so the external magnetic field strength can be detected from the frequency at which the red light intensity drops. [Prior art documents] [Non-patent literature]
[0003] [Non-Patent Document 1] Yang Gao, Chaoqun Xu, Kui Huang, Yuting Gao, Nankai Wu, Zhong Yi, “Research and Experiment on the System of Miniaturized Diamond NV Center Ensemble Magnetometer Based on Fiber Coupling”, 2021 IEEE 15th International Conference on Electronic Measurement & Instruments(ICEMI) Summary of the Invention [Problem to be solved by the invention]
[0004] The structure in Non-Patent Document 1 is a structure in which excitation light is incident on a diamond substrate via an optical fiber, but fluorescence is also incident on this optical fiber. This structure, in which excitation light and fluorescence are propagated through a single optical fiber, has the following problems. First, it requires an optical system, such as an expensive wavelength-selective beam splitter to separate the excitation light from the fluorescence, and a lens to collimate the light emitted from the light source and the end of the optical fiber, which increases costs and requires space to install the optical system, making it difficult to miniaturize. Furthermore, if the area in the diamond from which fluorescence is emitted is large, there is a problem of low optical coupling efficiency with the optical fiber.
[0005] In view of the above circumstances, the present invention aims to provide an optical fiber sensor that is low-cost, easy to miniaturize, and has high optical coupling efficiency, even when it has a structure in which excitation light is irradiated via an optical fiber onto an element having lattice defects that emit fluorescence when excitation light is incident thereon. [Means for solving the problem]
[0006] The optical fiber sensor of the present invention is an optical fiber sensor comprising: a light source that irradiates excitation light; a first optical fiber that is arranged on the optical path of the excitation light and into which the excitation light is incident at one end; an element that is in contact with the other end of the first optical fiber and has lattice defects that emit fluorescence when the excitation light is incident via the first optical fiber; and a measurement unit that measures the intensity of the fluorescence. The sensor further comprises a second optical fiber that is arranged in contact with the element so as to sandwich the element together with the first optical fiber, and into which the fluorescence is incident and which emits the incident fluorescence toward the measurement unit, and the core diameter and numerical aperture of the other end of the first optical fiber that is in contact with the element are smaller than the core diameter and numerical aperture of the end of the second optical fiber that is in contact with the element. [Effects of the Invention]
[0007] According to the present invention, it is possible to provide an optical fiber sensor that is low cost, easy to miniaturize, and has high optical coupling efficiency, even in a structure in which excitation light is irradiated via an optical fiber onto an element having lattice defects that emit fluorescence when the excitation light is incident thereon. [Brief explanation of the drawings]
[0008] [Figure 1] 1 is a diagram showing an outline of an optical fiber sensor according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a diagram schematically showing the crystal structure of a diamond substrate having an NV center. [Figure 3] 1A and 1B are diagrams for explaining the principle of measuring magnetic field strength using an optical fiber sensor. [Figure 4] 10 is a graph showing the relationship between the point at which the intensity of fluorescence decreases during a microwave frequency sweep, the microwave frequency, and the magnetic field strength. [Figure 5] FIG. 2 is an enlarged view of the portion in FIG. 1 where the diamond substrate is in contact with the first optical fiber and the second optical fiber. [Figure 6] FIG. 1 is a diagram showing an outline of a conventional optical fiber sensor. [Figure 7] FIG. 7 is a diagram showing the portion of FIG. 6 where the diamond substrate and the first optical fiber are in contact with each other. [Figure 8] FIG. 4 is a diagram showing an outline of an optical fiber sensor according to a second embodiment of the present invention. [Figure 9] FIG. 9 is an enlarged view of the portion in FIG. 8 where the diamond substrate is in contact with the first optical fiber and the second optical fiber. [Figure 10] FIG. 10 is an enlarged view of a portion where the diamond substrate is in contact with the first optical fiber and the second optical fiber in the optical fiber sensor according to the third embodiment of the present invention. [Figure 11] FIG. 11 is a diagram showing a state in which the diamond substrate is not in contact with the first optical fiber and the second optical fiber in FIG. [Figure 12] FIG. 1 is a diagram showing an example of a portion where a diamond substrate and a first optical fiber are in contact with each other in a conventional optical fiber sensor. [Figure 13] FIG. 10 is a diagram showing another example of the contact portion between the diamond substrate and the first optical fiber in a conventional optical fiber sensor. [Figure 14] FIG. 12 is a diagram showing a modified example of the optical fiber sensor according to the third embodiment of the present invention, and corresponds to FIG. [Figure 15] FIG. 12 is a diagram showing another modified example of the optical fiber sensor according to the third embodiment of the present invention, and corresponds to FIG. DETAILED DESCRIPTION OF THE INVENTION
[0009] The present invention will be described below in accordance with preferred embodiments. Note that the present invention is not limited to the embodiments shown below and can be modified as appropriate without departing from the spirit of the present invention. In addition, in the embodiments shown below, some components are omitted from illustration and description, but for the details of the omitted technologies, publicly known or well-known technologies are applied as appropriate within the scope of not causing any contradictions with the content described below.
[0010] First, an outline of an optical fiber sensor according to a first embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a diagram showing an outline of an optical fiber sensor 1 according to the first embodiment of the present invention. Fig. 1 illustrates a sensor that measures the strength of a surrounding magnetic field as the optical fiber sensor 1. The optical fiber sensor 1 shown in Fig. 1 comprises a light source 3, a first optical fiber 9, a diamond substrate 2 (element), a photodetector 4 (measurement unit), and a second optical fiber 11. The optical fiber sensor 1 also comprises a wavelength-selective filter 13, a microwave irradiator 6, a magnetic field generator 7, and a control unit 5.
[0011] The light source 3 is a semiconductor laser or the like that emits green excitation light GL (Green Light). The first optical fiber 9 is a waveguide that transmits the excitation light GL emitted from the light source 3 and may be made of quartz or resin. The first optical fiber 9 is installed on the optical path of the excitation light GL, and the excitation light GL is incident on one end 15. The diamond substrate 2 is a substrate that has NV centers, a type of color center that emits red fluorescence RL (Red Light) when the excitation light GL is incident on it, as lattice defects. The diamond substrate 2 is installed in contact with the other end 9c of the first optical fiber 9, and the excitation light GL is incident through the first optical fiber 9. The photodetector 4 is a measurement unit that measures the intensity of the fluorescence RL and is installed on the optical path of the fluorescence RL. The photodetector 4 shown in Figure 1 is installed on the opposite side of the first optical fiber 9 across the diamond substrate 2. The second optical fiber 11 is a waveguide that transmits the fluorescence RL emitted by the diamond substrate 2 and is installed on the optical path of the fluorescence RL in contact with the diamond substrate 2 so that the diamond substrate 2 is sandwiched between the first optical fiber 9 and the second optical fiber 9. 1 is provided between the diamond substrate 2 and the photodetector 4, receives the excitation light GL and fluorescence RL, and emits the incident excitation light GL and fluorescence RL toward the photodetector 4. The wavelength-selective filter 13 is a filter that transmits only the fluorescence RL out of the excitation light GL and fluorescence RL emitted from the second optical fiber 11, and is provided between the photodetector 4 and a rear end 23 of the second optical fiber 11 from which the excitation light GL and fluorescence RL are emitted.
[0012] The microwave irradiator 6 is an irradiation unit that irradiates the diamond substrate 2 with microwaves while sweeping the frequency. The magnetic field generator 7 is a device that generates a DC magnetic field around the diamond substrate 2. The control unit 5 is a computer that measures the magnetic field strength and controls the frequency of the microwaves irradiated by the microwave irradiator 6. Specifically, the control unit 5 controls the driving of the light source 3, photodetector 4, microwave irradiator 6, and magnetic field generator 7, and measures the surrounding magnetic field strength from the relationship between the frequency of the microwaves irradiated by the microwave irradiator 6 and the intensity of the fluorescence RL measured by the photodetector 4. In addition, the control unit 5 intentionally generates a magnetic field of a predetermined strength using the magnetic field generator 7 as needed to expand the magnetic field measurement range.
[0013] In this way, the optical fiber sensor 1 irradiates the NV center with excitation light GL via the first optical fiber 9, sweeps the frequency of microwaves, and mainly measures the magnetic field strength of the measurement target from the intensity of fluorescence RL emitted from the NV center. This measurement principle will be described with reference to Figs. 2 to 4.
[0014] Figure 2 is a diagram showing a schematic diagram of the crystal structure of a diamond substrate 2 having an NV center. As shown in Figure 2, the NV center is a complex impurity defect consisting of a pair of nitrogen (N) that has entered a substitutional position of carbon in the diamond lattice and a vacancy (V) that has been removed from the carbon atom adjacent to this nitrogen. This NV center is in a neutral charge state NV 0 Capture one electron from NV - Then, the magnetic quantum number m s = -1, 0, +1 electron spin triplet states are formed.
[0015] FIG. 3 is a diagram for explaining the principle of measuring magnetic field intensity B using the optical fiber sensor 1. As shown in FIGS. 2 and 3, the NV center emits red fluorescence RL when irradiated with green excitation light GL. The intensity of this fluorescence RL depends on the magnetic quantum number m of the electron spin of the NV center electron. s Compared with the case where the electron is excited from the state of =0, the level at which electron spin resonance occurs (magnetic quantum number m s =±1 state), it becomes smaller.
[0016] For example, when a microwave MW with a resonant frequency (approximately 2.8 GHz) is irradiated onto an NV center when the magnetic field strength B is 0, no Zeeman splitting occurs, and the level at which electron spin resonance (ESR) occurs is a single degenerate level. Some of the electrons photoexcited from this level return to the ground state via non-radiative transition and do not contribute to light emission. Therefore, when electrons in the NV center are excited from the level at which electron spin resonance occurs, the intensity of the fluorescence RL decreases.
[0017] Figure 4 is a graph showing the relationship between the intensity drop points of the fluorescence RL during frequency sweep of the microwave MW and the frequency of the microwave MW and the magnetic field strength B. As shown in this graph, when the magnetic field strength B is 0, there is only one point where the intensity of the fluorescence RL drops, whereas when the magnetic field strength B is a value B1, B2, B3 (B3>B2>B1>0) greater than 0, there are two points where the intensity of the fluorescence RL drops. The two points where the intensity of the fluorescence RL drops are the m S = ±1 are the frequencies of microwaves MW corresponding to the levels m S The levels of =±1 undergo Zeeman splitting so that the energy difference increases in proportion to the magnitude of the external magnetic field strength. Therefore, the frequency difference Δf (=f2-f1) between the two points increases in proportion to the magnetic field strength B. Therefore, the magnetic field strength B can be calculated from the magnitude of the frequency difference Δf. In this way, the optical fiber sensor 1 can measure the magnetic field strength B from the relationship between the intensity of the fluorescence RL and the frequency of the microwave MW. This is the principle of measuring magnetic field strength B using the optical fiber sensor 1.
[0018] Next, with reference to FIGS. 1 and 5, the detailed configurations of the first optical fiber 9 and the second optical fiber 11, which are characteristic parts of the optical fiber sensor 1, will be described. FIG. 5 is an enlarged view of the part in FIG. 1 where the diamond substrate 2 is in contact with the first optical fiber 9 and the second optical fiber 11. The first optical fiber 9 shown in FIG. 1 has a core 9a and a clad 9b. The second optical fiber 11 also has a core 11a and a clad 11b. As shown in FIG. 5, the core diameter dt (diameter) of the other end 9c of the first optical fiber 9 that contacts the diamond substrate 2 is smaller than the core diameter dr (diameter) of the end 11c of the second optical fiber 11 that contacts the diamond substrate 2. Furthermore, the numerical aperture NAt of the other end 9c of the first optical fiber 9 that contacts the diamond substrate 2 is smaller than the numerical aperture NAr of the end 11c of the second optical fiber 11 that contacts the diamond substrate 2.
[0019] The reason why the second optical fiber 11 is provided so as to sandwich the diamond substrate 2 together with the first optical fiber 9 and the core diameter dt and numerical aperture NAt of the first optical fiber 9 are made smaller than the core diameter dr and numerical aperture NAr of the second optical fiber 11 will be explained with reference to Fig. 1 and Figs. 5 to 7. Fig. 6 is a diagram showing an outline of an optical fiber sensor 100 of the prior art. Fig. 7 is a diagram showing the portion in Fig. 6 where the diamond substrate 2 and the first optical fiber 9 are in contact.
[0020] First, consider the case where excitation light GL and fluorescence RL are irradiated onto a diamond substrate 2 in a conventional optical fiber sensor 100, in which excitation light GL and fluorescence RL are propagated through a first optical fiber 9, as shown in FIG. 6. In this case, the region A on the diamond substrate 2 into which the excitation light GL is incident can be illustrated as a truncated cone, as shown in FIG. 7(a). Fluorescence RL emitted from the NV centers in this region A is irradiated in all directions, as shown in FIG. 7(b). In the structure of FIG. 6, as shown in FIG. 7(c), the fluorescence RL irradiated toward the first optical fiber 9 must be incident into the first optical fiber 9 and emitted from one end 15 toward the photodetector 4. In this case, excitation light GL also enters the first optical fiber 9 from one end 15, but this excitation light GL must be incident on the diamond substrate 2. Therefore, in the optical fiber sensor 100, a beam splitter 21 is required on the optical paths of the excitation light GL and fluorescence RL near the one end 15 of the first optical fiber 9 to separate the fluorescence RL from the excitation light GL without blocking the excitation light GL from entering the first optical fiber 9. The beam splitter 21 shown in FIG. 6 is provided between the first optical fiber 9 and the light source 3 and reflects the fluorescence RL and transmits the excitation light GL. Meanwhile, as shown in FIG. 1, the optical fiber sensor 1 according to the first embodiment allows the fluorescence RL emitted from the NV center and irradiated toward the second optical fiber 11 to be incident on the photodetector 4. At this time, the excitation light GL also enters the second optical fiber 11. However, since the excitation light GL incident on the second optical fiber 11 has passed through the diamond substrate 2, it does not need to be incident on the diamond substrate 2. Therefore, when separating the fluorescence RL and the excitation light GL incident on the second optical fiber 11, the optical path of the excitation light GL may be blocked by a wavelength-selective filter 13. Therefore, the second optical fiber 11 shown in FIG. 1 does not require the beam splitter 21. Furthermore, in the conventional optical fiber sensor 100 shown in FIG. 6, the excitation light GL, which is light emitted from the light source 3, needs to be collimated by a lens 17, and the fluorescence RL, which is light emitted from one end 15 of the first optical fiber 9, also needs to be collimated by a lens 19. However, the optical fiber sensor 1 according to the first embodiment shown in FIG. 1 does not need to collimate the excitation light GL and the fluorescence RL, and therefore does not need the lenses 17 and 19.In this way, the optical fiber sensor 1 does not require optical systems such as the beam splitter 21 and lenses 17 and 19, and therefore can reduce the number of parts and cost more than the optical fiber sensor 100. Furthermore, since the optical fiber sensor 1 does not require space to install optical systems such as the beam splitter 21 and lenses 17 and 19, it can be easily made smaller and space can be saved.
[0021] Furthermore, in the conventional optical fiber sensor 100, the fluorescence RL irradiated toward the first optical fiber 9 from a region of the region A shown in FIG. 7(a) that has a smaller diameter than the core 9a of the first optical fiber 9, such as the region A1 shown in FIG. 7(c), enters the core 9a. Therefore, the fluorescence RL irradiated toward the first optical fiber 9 from the region A1 can be efficiently captured by the first optical fiber 9. However, as shown in FIG. 7(d), in the region A, the fluorescence RL irradiated toward the first optical fiber 9 from the region A2, which is an outer periphery of the region A1, enters the outer periphery of the core 9a and therefore cannot be efficiently captured by the first optical fiber 9. Therefore, of the fluorescence RL irradiated toward the first optical fiber 9, only the fluorescence RL irradiated toward the first optical fiber 9 from the region A1 can be efficiently captured by the first optical fiber 9, resulting in a decrease in optical coupling efficiency. On the other hand, in the optical fiber sensor 1 of the first embodiment, as shown in FIG. 1, the fluorescence RL is incident on the second optical fiber 11, which is separate from the first optical fiber 9 through which the excitation light GL propagates. 5, the core diameter dt of the other end 9c of the first optical fiber 9 is smaller than the core diameter dr of the end 11c of the second optical fiber 11 that contacts the diamond substrate 2. In addition, in the optical fiber sensor 1, the numerical aperture NAt of the other end 9c of the first optical fiber 9 is smaller than the numerical aperture NAr of the end 11c of the second optical fiber 11, and therefore the maximum angle ω of the light ray emitted from the first optical fiber 9 is t Therefore, the maximum angle ω of the light beam that can be incident on the second optical fiber 11 is ris larger. This allows the diameter of region A to be smaller than the core diameter dr of the second optical fiber 11, and more fluorescence RL irradiated toward the second optical fiber 11 from region A2, which has a larger diameter than the core 9a of the first optical fiber 9, can be incident on the core 11a of the second optical fiber 11 than in the past. Therefore, the fluorescence RL irradiated toward the second optical fiber 11 from region A can be efficiently taken into the second optical fiber 11, increasing the amount of fluorescence taken into the second optical fiber 11 and improving the optical coupling efficiency. The above is the reason why the second optical fiber 11 is provided so as to sandwich the diamond substrate 2 together with the first optical fiber 9, and the core diameter dt and numerical aperture NAt of the first optical fiber 9 are made smaller than the core diameter dr and numerical aperture NAr of the second optical fiber 11.
[0022] The following are specific examples of cases in which the core diameter dt and numerical aperture NAt of the first optical fiber 9 are smaller than the core diameter dr and numerical aperture NAr of the second optical fiber 11. For example, a single-mode optical fiber (cutoff wavelength 470 nm, core diameter dt = 4 μm, numerical aperture NAt = 0.1) is used as the first optical fiber 9, and a multimode optical fiber (core diameter dr = 400 μm, numerical aperture NAr = 0.5) is used as the second optical fiber 11. This improves the optical coupling efficiency of the fluorescence RL. Furthermore, the optical coupling efficiency improves as the numerical aperture NAr of the second optical fiber 11 increases, but the maximum numerical aperture of commercially available optical fibers is approximately 0.5. Meanwhile, the numerical aperture NAt of the first optical fiber 9 is not particularly limited as long as it is smaller than the numerical aperture NAr of the second optical fiber 11. However, when a single-mode fiber with a small core diameter is used, the numerical aperture is approximately 0.1. Furthermore, it is preferable that the core diameter dr of the second optical fiber 11 satisfy the condition shown in the following formula (1). dr≧2×(t×NAr / n)+dt …(1)
[0023] Equation (1) can be obtained as follows: First, as shown in Fig. 5, the difference between the radius of the core 9a of the first optical fiber 9 and the radius of the core 11a of the second optical fiber 11 is a, and the thickness of the diamond substrate 2 is t. In this case, the relationship between t, a, and the maximum angle ω of light that can be incident from the first optical fiber 9 to the core 11a of the second optical fiber 11 can be expressed by the following equation (2). a=t×tanω …(2) Since ω is very small (ω<0.2), we can consider tanω≒sinω. Therefore, equation (2) can be expressed as equation (3) below. a = t × tanω ≒ t × sinω …(3) Furthermore, the core diameter dr of the second optical fiber 11 and the core diameter dt of the first optical fiber 9 satisfy the relationship shown in the following formula (4). dr=2×a+dt=2×t×sinω+dt …(4)
[0024] In order to maximize the optical coupling efficiency, it is necessary to allow all of the light incident from the first optical fiber 9 to enter the core 11a of the second optical fiber 11. To achieve this, ω≧ω r , that is, sinω≧sinω r Furthermore, the numerical aperture NAr must be equal to n×sinω. r Therefore, sinω r =NAr / n. Therefore, in order to maximize the optical coupling efficiency, the relationship shown in the following formula (5) must be established. sinω≧sinω r =NAr / n …(5) Equation (1) is obtained from equations (5) and (4).
[0025] As described above, according to the first embodiment, the optical fiber sensor 1 includes a first optical fiber 9, a diamond substrate 2, and a second optical fiber 11. The second optical fiber 11 is disposed so as to sandwich the diamond substrate 2 together with the first optical fiber 9. The first optical fiber 9 of the optical fiber sensor 1 has a core diameter dt and a numerical aperture NAt at the other end 9c that contacts the diamond substrate 2, which are smaller than the core diameter dr and numerical aperture NAr of the end 11c of the second optical fiber 11 that contacts the diamond substrate 2. In this configuration, excitation light GL enters the diamond substrate 2 through the first optical fiber 9, and fluorescence RL emitted from the NV center enters the photodetector 4 through the second optical fiber 11, which has a core diameter dr and a numerical aperture NAr larger than those of the first optical fiber 9. This allows the diameter of the region A to be smaller than the core diameter dr of the second optical fiber 11, and therefore allows more fluorescence RL irradiated from the region A2, which has a larger diameter than the core 9a of the first optical fiber 9, toward the second optical fiber 11 to enter the core 11a of the second optical fiber 11 than in the past. Therefore, the fluorescence RL irradiated from the region A toward the second optical fiber 11 can be efficiently taken into the second optical fiber 11, and the amount of fluorescence taken into the second optical fiber 11 can be increased, thereby improving the optical coupling efficiency. Furthermore, when the optical coupling efficiency is improved, the output when the light source 3 emits the excitation light GL can be reduced, so that the power consumption of the light source 3 can be reduced and heat generation from the light source 3 can also be suppressed.
[0026] Next, a second embodiment will be described with reference to Figures 8 and 9. In the second embodiment, the first optical fiber 9 and the second optical fiber 11 are provided in a position in which the optical axis of the second optical fiber 11 is radially shifted from the optical axis of the first optical fiber 9. In the second embodiment, elements that perform the same functions as in the first embodiment are assigned the same numbers, and the following description will mainly focus on the parts that are different from the first embodiment.
[0027] FIG. 8 is a schematic diagram of an optical fiber sensor according to a second embodiment of the present invention. FIG. 9 is an enlarged view of the contact area between the diamond substrate 2 and the first and second optical fibers 9 and 11 in FIG. 8 . In the optical fiber sensor 1a according to the second embodiment shown in FIG. 8 , the optical axis CL2 of the second optical fiber 11 is offset from the optical axis CL1 of the first optical fiber 9 in the radial direction of the first and second optical fibers 9 and 11. More specifically, the optical axis CL2 is offset from the optical axis CL1 so that the core 11a of the second optical fiber 11 is positioned outside the optical path of the excitation light GL emitted from the first optical fiber 9. Furthermore, the optical axis CL2 is offset from the optical axis CL1 so that at least a portion of the fluorescence RL is incident on the core 11a of the second optical fiber 11. In this configuration, the excitation light GL that has passed through the first optical fiber 9 and the diamond substrate 2 is irradiated onto the cladding 11b of the second optical fiber 11 and attenuated there, or is irradiated outside the second optical fiber 11 without entering the core 11a of the second optical fiber 11. Therefore, the excitation light GL is not optically coupled to the second optical fiber 11, and only the fluorescence RL propagates through the second optical fiber 11 and enters the photodetector 4. Therefore, the fluorescence RL and the excitation light GL can be separated without providing a wavelength-selective filter 13 that blocks the excitation light GL between the second optical fiber 11 and the photodetector 4.
[0028] Specifically, the amount D by which the optical axis CL2 is shifted from the optical axis CL1 can be exemplified as one that satisfies the following formula (6). D>{(dr+dt) / 2}+(t×NAt / n) …(6) Equation (6) can be obtained as follows: First, as shown in Fig. 9, if the radial distance between the outer periphery of the core 9a of the first optical fiber 9 and the outer periphery of the core 11a of the second optical fiber 11 is b, D can be expressed by the following equation (7). D = {(dr + dt) / 2} + b … (7) On the other hand, if the maximum incident angle when the excitation light GL is incident from the first optical fiber 9 to the core 11a of the second optical fiber 11 is ω, the relationship shown in the following equation (8) holds between b, t and ω. b = t × tan ω …(8) Since ω is very small (ω<0.2), we can consider tanω≒sinω. Therefore, equation (8) can be expressed as equation (9) below. b≒t×sinω …(9)
[0029] In order to prevent the excitation light GL from being incident on the core 11a of the second optical fiber 11 from the first optical fiber 9, ω>ω t , that is, sinω>sinω t Furthermore, the numerical aperture NAt=n×sinω t Therefore, sinω t =NAt / n. Therefore, in order to prevent the pumping light GL from being incident from the first optical fiber 9 into the core 11a of the second optical fiber 11, the relationship shown in the following formula (10) must be established. sinω>sinω t =NAt / n …(10) Equation (6) is obtained from equations (7), (9), and (10).
[0030] As D increases, the radial distance between the region A on the diamond substrate 2 where the excitation light GL is incident and the second optical fiber 11 increases, and the angle of incidence of the fluorescence RL incident from the region A into the second optical fiber 11 also increases. Therefore, as D increases, the amount of fluorescence that can be taken in by the second optical fiber 11 decreases. Therefore, it is preferable that D is as small as possible within a range that satisfies formula (6).
[0031] Moreover, whether to adopt the first or second embodiment can be selected appropriately taking into consideration the advantages of each. For example, the first embodiment is advantageous in that the optical coupling efficiency between the fluorescence RL and the second optical fiber 11 is higher than that of the second embodiment because most of the fluorescence RL irradiated from the diamond substrate 2 toward the second optical fiber 11 can be taken in by the second optical fiber 11. On the other hand, the second embodiment is advantageous over the first embodiment in that the wavelength-selective filter 13 is not required.
[0032] As described above, the optical fiber sensor 1a according to the second embodiment includes a first optical fiber 9, a diamond substrate 2, and a second optical fiber 11, and the second optical fiber 11 is disposed so as to sandwich the diamond substrate 2 together with the first optical fiber 9. Furthermore, the core diameter dt and the numerical aperture NAt of the other end 9c of the first optical fiber 9 that contacts the diamond substrate 2 are smaller than the core diameter dr and the numerical aperture NAr of the end 11c of the second optical fiber 11 that contacts the diamond substrate 2. Therefore, the second embodiment has the same effects as the first embodiment.
[0033] Furthermore, in the optical fiber sensor 1a according to the second embodiment, the optical axis CL2 of the second optical fiber 11 is shifted in the radial direction of the first optical fiber 9 and the second optical fiber 11 relative to the optical axis CL1 of the first optical fiber 9. In this configuration, the excitation light GL does not enter the core 11a of the second optical fiber 11, and therefore the excitation light GL is not optically coupled to the second optical fiber 11, and only the fluorescence RL propagates through the second optical fiber 11 and enters the photodetector 4. Therefore, the fluorescence RL and the excitation light GL can be separated without providing a wavelength-selective filter 13 between the second optical fiber 11 and the photodetector 4.
[0034] Next, a third embodiment will be described with reference to Figures 10 to 15. In the third embodiment, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 in the first embodiment are made convex. In the third embodiment, elements that perform the same functions as in the first embodiment are assigned the same numbers, and differences from the first embodiment will mainly be described.
[0035] First, the configuration of the optical fiber sensor according to the third embodiment will be described with reference to FIGS. 10 and 11. FIG. 10 is an enlarged view of a portion of the optical fiber sensor 1b according to the third embodiment where the diamond substrate 2 is in contact with the first optical fiber 9 and the second optical fiber 11. FIG. 11 shows the state in FIG. 10 where the diamond substrate 2 is not in contact with the first optical fiber 9 and the second optical fiber 11. As shown in FIGS. 10 and 11, the diamond substrate 2 has a contact surface 33 that contacts the other end 9c of the first optical fiber 9 and a contact surface 35 that contacts the end 11c of the second optical fiber 11. Furthermore, when the first optical fiber 9 is not in contact with the diamond substrate 2, the other end 9c of the first optical fiber 9 (the contact surface of the first optical fiber 9 with the diamond substrate 2) is convex. Furthermore, when the second optical fiber 11 is not in contact with the diamond substrate 2, the end 11c of the second optical fiber 11 (the contact surface of the second optical fiber 11 with the diamond substrate 2) is convex. Furthermore, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are butted against the diamond substrate 2 as indicated by the white arrows in FIG. 11, resulting in a state of contact as shown in FIG. 10. When the first optical fiber 9, the second optical fiber 11, and the diamond substrate 2 are in contact with each other, the convex portions of the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are butted against each other and deformed into a flat shape. Mechanical processing such as polishing can be used as a means of making the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 convex. The term "convex" as used herein refers to a shape in which the cross section parallel to the butting direction is arc-shaped, for example.
[0036] Thus, in the optical fiber sensor 1b, the other end 9c of the first optical fiber 9 and the contact surface 33 of the diamond substrate 2 are butted together, and the convex portion formed on the other end 9c is pressed by the butt contact and deformed into a flat shape. Also, in the optical fiber sensor 1b, the end 11c of the second optical fiber 11 and the contact surface 35 of the diamond substrate 2 are butted together, and the convex portion formed on the end 11c is pressed by the butt contact and deformed into a flat shape. In this configuration, the first optical fiber 9, the second optical fiber 11, and the diamond substrate 2 can be tightly attached simply by butting them together, so air gaps are unlikely to form at the connections between the diamond substrate 2 and the first optical fiber 9 and the second optical fiber 11. Therefore, the optical fiber sensor 1b can suppress reflection loss at the connections between the diamond substrate 2 and the first optical fiber 9 and the second optical fiber 11, and stabilize the optical coupling state at the connections between the diamond substrate 2 and the first optical fiber 9 and the second optical fiber 11. Furthermore, in the optical fiber sensor 1b, no other substances such as adhesives are used in the connection between the diamond substrate 2 and the first and second optical fibers 9 and 11. Therefore, the diamond substrate 2 and the first and second optical fibers 9 and 11 can be connected in a shorter time than when adhesives or the like are used. Also, problems of expansion / contraction due to heat are less likely to occur than when adhesives or the like are used, and the optically coupled state between the diamond substrate 2 and the first and second optical fibers 9 and 11 can be stabilized.
[0037] The reason why the optical coupling state is stable will be specifically explained with reference to FIGS. 11, 12, and 13. FIG. 12 shows an example of a contact area between the diamond substrate 2 and the first optical fiber 9 in a conventional optical fiber sensor 100. FIG. 13 shows another example of a contact area between the diamond substrate 2 and the first optical fiber 9 in a conventional optical fiber sensor 100. As shown in FIG. 12, when both the contact surface 33 of the diamond substrate 2 and the other end 9c of the first optical fiber 9 are flat, if the optical axis of the first optical fiber 9 is tilted with respect to the normal direction of the diamond substrate 2, only the outer end of the first optical fiber 9 is in close contact with the diamond substrate 2. As a result, an air gap G is generated between the non-contact area. When the gap G is generated, the boundary between the diamond substrate 2 and the gap G becomes the interface between the diamond and air. Furthermore, the boundary between the gap G and the first optical fiber 9 becomes the interface between the air and the first optical fiber 9. Because a portion of the excitation light GL and the fluorescence RL is reflected at the interface, the generation of the gap G causes Fresnel reflection loss and reduces the optical coupling efficiency between the first optical fiber 9 and the diamond substrate 2. On the other hand, in the optical fiber sensor 1b of the third embodiment, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are convex, as shown in Fig. 11. Therefore, even if the optical axis of the first optical fiber 9 is tilted with respect to the normal direction of the diamond substrate 2, the convex portions are pressed against each other and deformed into a flat shape, resulting in close contact without creating a gap G. Therefore, the optical fiber sensor 1b can stabilize the optical coupling state between the diamond substrate 2 and the first optical fiber 9.
[0038] On the other hand, when the first optical fiber 9 is fixed to the diamond substrate 2 with an adhesive 41 as shown in FIG. 13 , no gap G occurs. However, manufacturing the optical fiber sensor 100 requires labor and time for applying and curing the adhesive 41, which increases the manufacturing time. Furthermore, when the adhesive 41 is used, the relative position between the diamond substrate 2 and the first optical fiber 9 changes due to thermal expansion / contraction of the adhesive 41. Therefore, the wider the temperature range in which the first optical fiber 9 is used, the more difficult it is to stabilize the optical coupling state between the first optical fiber 9 and the diamond substrate 2. On the other hand, the optical fiber sensor 1b of the third embodiment does not include other substances such as the adhesive 41 in the connection between the diamond substrate 2 and the first optical fiber 9 and the second optical fiber 11. Therefore, the diamond substrate 2 can be connected to the first optical fiber 9 and the second optical fiber 11 in a shorter time than when the adhesive 41 is used. Furthermore, compared to when the adhesive 41 is used, the problem of thermal expansion / contraction is less likely to occur, and the optical coupling state at the connection between the diamond substrate 2 and the first optical fiber 9 and the second optical fiber 11 can be stabilized regardless of changes in the ambient temperature.
[0039] The structure for butting the diamond substrate 2 with the first and second optical fibers 9 and 11 is not particularly limited as long as it applies an appropriate stress sufficient to deform the convex portions into a flat shape when butted. For example, the optical fiber sensor 1b may include a spring-like elastic pressing member on at least one of the diamond substrate 2 and the first and second optical fibers 9 and 11. Alternatively, the optical fiber sensor 1b may be configured such that the first and second optical fibers 9 and 11 themselves function as springs, for example, by bending the first and second optical fibers 9 and 11 into a springy shape, such as a mountain shape. The curvature of the convex portions is not particularly limited because it depends on the material, but it may be sufficient as long as it is pressed by butting and deforms into a flat shape. For example, a radius of curvature of approximately 25 mm is sufficient. In this case, if the first and second optical fibers 9 and 11 are made of glass (SiO2), a pressing force of 0.5 kgf or more is sufficient to flatten the convex portions when butted.
[0040] Furthermore, in the state where the first optical fiber 9 and the second optical fiber 11 are butted against the diamond substrate 2, the entire convex portion does not necessarily have to be deformed into a flat shape.
[0041] FIG. 11 illustrates a structure in which the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are convex when the first optical fiber 9 and the diamond substrate 2 are not in contact with each other, and the contact surfaces 33 and 35 of the diamond substrate 2 are flat. However, the optical fiber sensor 1b is not limited to the structure illustrated in FIG. 11. The optical fiber sensor 1b may have at least one of the other end 9c, the end 11c, the contact surface 33, and the contact surface 35 convex when the first optical fiber 9 and the second optical fiber 11 are not in contact with each other. This point will be explained with reference to FIGS. 14 and 15. FIG. 14 is a diagram illustrating a modified example of the optical fiber sensor 1b according to the third embodiment of the present invention, and corresponds to FIG. 11. FIG. 15 is a diagram illustrating another modified example of the optical fiber sensor 1b according to the third embodiment of the present invention, and corresponds to FIG. 11. As shown in Fig. 14, in the optical fiber sensor 1b, when the first optical fiber 9 and the diamond substrate 2 are not in contact with each other, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 may be flat, and the contact surfaces 33, 35 of the diamond substrate 2 may be convex. Alternatively, as shown in Fig. 15, in the optical fiber sensor 1b, when the first optical fiber 9 and the diamond substrate 2 are not in contact with each other, the contact surfaces 33, 35 of the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 and the diamond substrate 2 may be convex. Note that mechanical processing such as polishing can be used as a means for making the contact surfaces 33, 35 of the diamond substrate 2 convex.
[0042] When the first optical fiber 9 and the diamond substrate 2 are not in contact with each other, whether the other end 9c, the end 11c, or the contact surfaces 33 and 35 are made convex, or whether all of them are made convex, can be appropriately selected taking into account the advantages of each. For example, in a structure in which the other end 9c and the end 11c are made convex and the contact surfaces 33 and 35 are flat, as shown in FIG. 11, the first optical fiber 9 and the second optical fiber 11, which have a lower hardness than the diamond substrate 2, are processed into a convex shape, which is advantageous in that wear on the processing tools can be reduced. On the other hand, in a structure in which the other end 9c and the end 11c are flat and the contact surfaces 33 and 35 are convex, as shown in FIG. 14, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are not processed into a convex shape. Therefore, this structure is advantageous in that the optical properties of the first optical fiber 9 and the second optical fiber 11 are not affected by heat, stress, etc., during processing. Furthermore, in a structure in which the other end 9c, the end 11c, and the contact surfaces 33 and 35 are all convex, the two convex portions can be deformed flat even if the deformation amount of each of them is reduced compared to when there is only one convex portion. Therefore, even if the pressing force when butting is weakened, the convex portions can be flattened and tightly attached, which is advantageous in that it increases the options for the pressing structure.
[0043] As described above, the optical fiber sensor 1b according to the third embodiment includes a first optical fiber 9, a diamond substrate 2, and a second optical fiber 11, and the second optical fiber 11 is disposed so as to sandwich the diamond substrate 2 together with the first optical fiber 9. Furthermore, the core diameter dt and the numerical aperture NAt of the other end 9c of the first optical fiber 9 that contacts the diamond substrate 2 are smaller than the core diameter dr and the numerical aperture NAr of the end 11c of the second optical fiber 11 that contacts the diamond substrate 2. Therefore, the third embodiment has the same effects as the first embodiment.
[0044] Furthermore, in the optical fiber sensor 1b according to the third embodiment, when the first optical fiber 9 and the second optical fiber 11 are not in contact with the diamond substrate 2, at least one of the other end 9c, the end 11c, and the contact surfaces 33, 35 is convex. In this configuration, the first optical fiber 9 and the second optical fiber 11 are butted against the diamond substrate 2, and are in contact with each other with the convex portions pressed by the butt contact and deformed into a flat shape. Therefore, the first optical fiber 9 and the second optical fiber 11 can be tightly attached to the diamond substrate 2 without any gaps simply by butting them against each other. Therefore, reflection loss at the connection between the diamond substrate 2 and the first optical fiber 9 can be suppressed.
[0045] In particular, in the third embodiment, when the first optical fiber 9 and the second optical fiber 11 are not in contact with the diamond substrate 2, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are convex, and the contact surfaces 33, 35 of the diamond substrate 2 are flat. In this configuration, the first optical fiber 9 and the second optical fiber 11 are butted against the diamond substrate 2, and the convex portions formed on the first optical fiber 9 and the second optical fiber 11 are pressed against the diamond substrate 2 during butting, and are deformed into a flat shape. In this configuration, the first optical fiber 9 and the second optical fiber 11, which have a lower hardness than the diamond substrate 2, are processed into a convex shape, so wear on the processing tools can be suppressed.
[0046] Furthermore, in the third embodiment, when the first optical fiber 9 and the second optical fiber 11 are not in contact with the diamond substrate 2, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 may be flat, and the contact surfaces 33, 35 of the diamond substrate 2 may be convex. In this configuration, the first optical fiber 9 and the second optical fiber 11 are butted against the diamond substrate 2, and the convex portions formed on the diamond substrate 2 are pressed against the first optical fiber 9 and the second optical fiber 11 during butting, and are deformed into a flat shape. In this configuration, the other end 9c of the first optical fiber 9 and the end 11c of the second optical fiber 11 are not machined into a convex shape, so there is no risk of the optical properties of the first optical fiber 9 and the second optical fiber 11 being changed by heat, stress, etc. applied when machining the other end 9c of the first optical fiber 9.
[0047] In the third embodiment, the other end 9c, the end 11c, and the contact surfaces 33 and 35 may be convex when the first optical fiber 9 and the second optical fiber 11 are not in contact with the diamond substrate 2. In this configuration, the first optical fiber 9 and the diamond substrate 2 are butted together, and the other end 9c and the contact surface 33, and the end 11c and the contact surface 35 are pressed against each other and deformed into a flat shape. In this configuration, the two convex portions are butted together to deform, so the amount of deformation of each of the two convex portions can be reduced compared to when one convex portion is pressed against a flat portion to deform. Therefore, even if the pressing force when butting them together is weak, the convex portions can be flattened and tightly attached, which increases the options for the pressing structure.
[0048] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and modifications may be made within the scope of the spirit of the present invention, and publicly known or well-known technologies may be combined as appropriate.
[0049] For example, in the first to third embodiments, a substrate having an NV center is exemplified as the diamond substrate 2, but the diamond substrate 2 is not limited to a substrate having an NV center as long as it has lattice defects that emit fluorescence RL when irradiated with excitation light GL. For example, the diamond substrate 2 may be a diamond substrate having an SnV color center consisting of tin (Sn) and vacancies V, an SiV color center consisting of silicon (Si) and vacancies V, or a GeV color center consisting of germanium (Ge) and vacancies V.
[0050] Furthermore, in the first to third embodiments, magnetic sensors are exemplified as the optical fiber sensors 1, 1a, and 1b, but the present invention can also be applied to a fluorescent temperature sensor that generates a temperature signal from the center position of two resonance frequencies.
[0051] Furthermore, in the first to third embodiments, the diamond substrate 2 is exemplified as an element, but the element is not limited to the diamond substrate 2. For example, SiC having spin defects called silicon vacancies may be used as the element. [Explanation of symbols]
[0052] 1, 1a, 1b: Optical fiber sensor 2: Diamond substrate (element) 3:Light source 4: Photodetector (measurement unit) 5: Control section 9: First optical fiber 9c: Other end (contact surface) 11: Second optical fiber 11a: Core 11c: End (contact surface) 15: One end 33, 35: Contact surface CL1, CL2: Optical axis dr, dt: Core diameter GL: Excitation light NAr, NAt: Numerical aperture RL: Fluorescent
Claims
1. an optical fiber sensor including a light source that irradiates excitation light; a first optical fiber that is provided on an optical path of the excitation light and into one end of which the excitation light is incident; an element that is in contact with the other end of the first optical fiber and has a lattice defect and that emits fluorescence when the excitation light is incident through the first optical fiber; and a measurement unit that measures the intensity of the fluorescence, a second optical fiber that is provided in contact with the element so as to sandwich the element together with the first optical fiber, the second optical fiber receiving the fluorescence and emitting the fluorescence toward the measuring unit; The first optical fiber has a core diameter and a numerical aperture at the other end thereof that contacts the element smaller than the core diameter and the numerical aperture at the end of the second optical fiber that contacts the element. An optical fiber sensor characterized by:
2. The first optical fiber and the second optical fiber are 2. The optical fiber sensor according to claim 1, wherein the optical axis of the second optical fiber is radially shifted from the optical axis of the first optical fiber so that the core of the second optical fiber is positioned away from the optical path of the excitation light emitted from the first optical fiber and so that at least a portion of the fluorescence is incident on the core of the second optical fiber.
3. When the first optical fiber and the second optical fiber are not in contact with the element, at least one of a contact surface of the first optical fiber with the element, a contact surface of the second optical fiber with the element, a contact surface of the element with the first optical fiber, and a contact surface of the element with the second optical fiber is convex, 3. The optical fiber sensor according to claim 1, wherein when the first optical fiber and the second optical fiber are butted against and in contact with the element, at least a portion of the convex portion is deformed into a flat shape by the butting.
4. 4. The optical fiber sensor according to claim 3, wherein when the first optical fiber and the second optical fiber are not in contact with the element, the contact surface of the first optical fiber with the element and the contact surface of the second optical fiber with the element are convex, and the contact surface of the element with the first optical fiber and the second optical fiber is flat.
5. 4. The optical fiber sensor according to claim 3, wherein a contact surface of the first optical fiber with the element and a contact surface of the second optical fiber with the element are flat, and a contact surface of the element with the first optical fiber and the second optical fiber are convex.
6. 4. The optical fiber sensor according to claim 3, wherein a contact surface of the first optical fiber with the element, a contact surface of the second optical fiber with the element, a contact surface of the element with the first optical fiber, and a contact surface of the element with the second optical fiber are convex.