Light receiving device and light receiving method

Abstract

A light receiving device is provided that has high coupling efficiency of light received at the side of an optical fiber to the core. A light receiving device A includes a light diffusing fiber 10 having a core and a clad covering the core. The light diffusing fiber 10 includes a light diffusing portion 101 in which a light scatterer is added to the clad.

Description

[Technical Field]

[0001] The present invention relates to a light receiving device and a light receiving method. [Background technology]

[0002] In the field of free-space optical communications, a light-receiving device is used to receive an optical signal propagating through space. For example, Patent Document 1 discloses such a light-receiving device in which an optical signal is collected by a collecting lens and received by a light-receiving element. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2024-013025 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a light receiving device that has high coupling efficiency of light received at the side surface of an optical fiber to the core. [Means for solving the problem]

[0005] The present invention is a light-receiving device equipped with a light-diffusing fiber having a core and a clad covering the core, the light-diffusing portion having a light-scattering material added to the clad.

[0006] The present invention is a light receiving method using a light diffusing fiber having a core and a cladding covering the core, and including a light diffusing portion in which a light scatterer is added to the cladding, in which light is received at a side surface of the light diffusing portion of the light diffusing fiber. [Effects of the Invention]

[0007] According to the present invention, by using a light diffusing fiber including a light diffusing portion in which a light scatterer is added to the cladding as an optical fiber, it is possible to obtain a high coupling efficiency of light received at the side surface of the light diffusing portion to the core. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a configuration of a light receiving device according to a first embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a light diffusing portion of a light diffusing fiber. [Figure 3A] FIG. 2 is a diagram showing the configuration of a first stress-applying structure. [Figure 3B] FIG. 10 is a diagram showing the configuration of a second stress-applying structure. [Figure 3C] FIG. 10 is a diagram showing the configuration of a third stress-applying structure. [Figure 4] FIG. 10 is a diagram showing a configuration in which a light reflecting structure is added to the third stress-applying structure. [Figure 5] FIG. 10 is a diagram illustrating the configuration of a light receiving device according to a second embodiment. [Figure 6] FIG. 10 is a diagram illustrating a configuration of a light receiving device according to a third embodiment. [Figure 7] FIG. 10 is a diagram illustrating the configuration of a light receiving device according to a fourth embodiment. [Figure 8] FIG. 10 is a diagram illustrating the configuration of a light receiving device according to a fifth embodiment. [Figure 9A] FIG. 1 is a first explanatory diagram showing a measurement method according to an embodiment. [Figure 9B] FIG. 2 is a second explanatory diagram showing the measurement method of the embodiment. [Figure 10] 10 is a graph showing the relationship between the distance from the photodetector to the light irradiation position and the light transmission power measured by the photodetector. [Figure 11] 10 is a graph showing the relationship between the amount of depression of the stress-applying member and the peak value of the pulsed light measured by the photodetector. DETAILED DESCRIPTION OF THE INVENTION

[0009] The embodiments will be described in detail below.

[0010] (Embodiment 1) 1 shows a light-receiving device A according to embodiment 1. This light-receiving device A can be applied to, for example, free-space optical communications that receive signal light at any position and transmit the signal light over long distances without a power source, and highly sensitive optical sensors that have high-speed response to received light.

[0011] The light receiving device A of embodiment 1 comprises a light diffusing fiber 10, photodetectors 20 connected to both ends of the fiber, and an optical filter 30 interposed between the light diffusing fiber 10 and the photodetector 20.

[0012] As shown in FIG. 2, the light diffusing fiber 10 has a core 11 having a circular cross section located in the center of the fiber cross section, a cladding 12 having an annular cross section located to cover the core 11, and a coating layer 13 having an annular cross section located to further cover the cladding 12.

[0013] The core 11 has a relatively high refractive index and is formed from a material such as glass material (e.g., quartz) or an organic material (e.g., acrylic resin or fluororesin). The cladding 12 has a relatively low refractive index and is formed from an organic material (e.g., silicone resin, acrylic resin, fluororesin), or a glass material (e.g., quartz). The coating layer 13 can be formed from an organic material (e.g., acrylic resin, nylon resin, fluororesin). There are no particular limitations on the material as long as it is transparent to the target light. From the viewpoints of environmental resistance for outdoor use and manufacturability, it is preferable that the light diffusing fiber 10 has the core 11 formed from quartz, the cladding 12 formed from silicone resin, and the coating layer 13 formed from fluororesin.

[0014] The diameter of the core 11 is, for example, 10 μm or more and 2000 μm or less. The outer diameter of the cladding 12 is, for example, 100 μm or more and 10000 μm (10 mm) or less. The outer diameter of the coating layer 13 is, for example, 200 μm or more and 20000 μm (20 mm) or less.

[0015] The light diffusing fiber 10 includes a light diffusing portion 101 of a predetermined length in which light scatterers 14 are added to the cladding 12, and light transmitting portions 102 provided on both ends of the light diffusing portion 101 in the longitudinal direction and in which no light scatterers 14 are added to the cladding 12. Such a light diffusing fiber 10 can be obtained by connecting an optical fiber in which light scatterers 14 are added to the cladding 12 with an optical fiber in which no light scatterers 14 are added to the cladding 12. These optical fibers are preferably made of the same material from the viewpoint of easy fusion splicing.

[0016] In the light diffusing section 101, fine particles of the light scatterers 14 are dispersed in the clad 12. Examples of materials for forming the light scatterers 14 include organic and inorganic materials. Examples of organic materials include resin materials. Examples of resin materials include polymethyl methacrylate resin (PMMA) and polystyrene resin (PS). Examples of inorganic materials include quartz, ceramics, and metals. Examples of metals include aluminum and gold. The light scatterers 14 may be formed of microbubbles. The material for forming the light scatterers 14 preferably includes one or more of these materials. When the clad 12 is formed of a silicone-based resin, it is more preferable to include an organic material such as polymethyl methacrylate resin (PMMA) or polystyrene resin (PS) because these materials have high transparency, low light absorption, and excellent dispersibility. The diameter of the light scatterers 14 is, for example, 1 μm or more and 10 μm or less. From the viewpoint of the balance between light scattering and attenuation, the content of light scatterer 14 is preferably 0.01 mass % or more and 20 mass % or less with respect to the mass of clad 12. Note that light scatterer 14 may be added not only to clad 12 but also to core 11 and / or coating layer 13.

[0017] Light from the light diffusing fiber 10 is incident on the photodetector 20. Examples of the photodetector 20 include an optical semiconductor element, a solar cell, and a high-speed electron multiplier. Examples of the optical semiconductor element include a small photodiode with a high response speed.

[0018] The optical filter 30 removes ambient light (environmental light) and blocks light of unnecessary wavelengths, transmitting only light components in a specific wavelength range.

[0019] In the light-receiving device A according to the first embodiment, when light is received at the side surface of the light diffusing section 101 of the light diffusing fiber 10, the light is scattered by the light scatterers 14 added to the cladding 12, and enters and couples into the core 11. The light coupled into the core 11 propagates through the core 11 and exits from each fiber end of the light transmitting section 102 on both sides of the light diffusing section 101. Of the light exiting each fiber end, only a component of a specific wavelength passes through the optical filter 30 and enters the photodetector 20.

[0020] According to the light-receiving device A of the first embodiment, a light diffusing fiber 10 including a light diffusing portion 101 having a cladding 12 doped with light scatterers 14 is used as an optical fiber. Light received at the side of the light diffusing portion 101 is scattered by the light scatterers 14 doped in the cladding 12, enters the core 11, and is coupled thereto, thereby achieving high coupling efficiency. Furthermore, since an optical component such as a lens is not required to couple light into the core 11, alignment is not required, and there is no restriction on the angle of incidence, such as the aperture angle of a lens. Furthermore, in the light diffusing fiber 10, the position of the light diffusing portion 101 is limited, and light transmitting portions 102 having cladding 12 without doping light scatterers 14 are provided on both sides of the light diffusing portion 101. This prevents light coupled to the core 11 by the light diffusing portion 101 from dissipating to the outside in the light transmitting portions 102. Furthermore, since the light diffusing fiber 10 can be freely wired, even a large beam can be received by arranging the light diffusing fiber 10 according to the shape of the light so that the light is received at the side of the light diffusing portion 101. Furthermore, any desired location for receiving light can be set as the light diffusion section 101. Furthermore, since the optical filter 30 is provided in front of the photodetector 20, high light receiving sensitivity (S / N) can be obtained.

[0021] In the light receiving device A according to the first embodiment, it is preferable that the light diffusing portion 101 is provided with a stress applying structure in order to increase the efficiency of coupling light received by the side surface of the light diffusing portion 101 to the core 11.

[0022] As an example of the stress-applying structure, as shown in FIG. 3A , there is a first stress-applying structure in which stress is applied to the light diffusion portion 101 by pressing the light diffusion portion 101 using stress-applying members 40. In this first stress-applying structure, the light diffusion portion 101 is sandwiched between a pair of stress-applying members 40. Each of the pair of stress-applying members 40 has a member body 41 in the shape of an elongated rectangular plate and multiple cylindrical pressing portions 42. The multiple pressing portions 42 are integrally provided on the fiber-facing side of the member body 41 at intervals in the longitudinal direction and extend in the width direction. The pair of stress-applying members 40 are arranged such that one pressing portion 42 and the other pressing portion 42 are arranged in a zigzag pattern in the longitudinal direction, sandwiching the light diffusion portion 101 between them. Each pressing portion 42 provided on the pair of stress-applying members 40 applies stress to the light diffusion portion 101 by pressing it from the side.

[0023] The first stress-applying structure is suitable when the light diffusion section 101 is arranged to extend linearly. The first stress-applying structure is also suitable when restricting the light diffusion section 101 from which strong light reception is desired or when propagation loss becomes a problem because the light diffusion section 101 is far from the photodetector 20. Furthermore, the first stress-applying structure can apply a larger stress to the light diffusion section 101 than the second stress-applying structure described below, thereby more significantly improving the coupling efficiency of light received at the side surface of the light diffusion section 101 to the core 11. The first stress-applying structure can control the coupling efficiency of light received at the side surface of the light diffusion section 101 to the core 11 by adjusting the magnitude of the stress applied by pressing the light diffusion section 101. From the viewpoint of achieving a high coupling efficiency of light received at the side surface of the light diffusion section 101 to the core 11, the stress-applying member 40 is preferably formed of a light-transmitting material.

[0024] Another example is a second stress-applying structure, as shown in FIG. 3B , which applies stress by bending the light diffusion unit 101 to a predetermined bending radius. The second stress-applying structure allows the light diffusion unit 101 to have a wide light-receiving surface in a two-dimensional plane, eliminating the need for precise alignment adjustment when receiving optical signals. This makes it suitable for applications such as spatial optical communication. Furthermore, the second stress-applying structure is simpler and easier to design than the first stress-applying structure, and enlarging the light-receiving surface of the light diffusion unit 101 does not decrease the response speed to light. This is an advantage over optical semiconductor elements such as photodiodes, whose response speed decreases as the light-receiving surface increases due to increased capacitance. The second stress-applying structure can control the coupling efficiency of light received by the side surface of the light diffusion unit 101 to the core 11 by adjusting the bending radius of the bending deformation that applies bending stress to the light diffusion unit 101. The bending shape of the second stress-applying structure may be a full ring shape as shown in FIG. 3B , a half ring shape, or a quarter ring shape.

[0025] Furthermore, a third stress-applying structure can be exemplified by combining the first and second stress-applying structures, as shown in FIG. 3C , in which a stress-applying member 40 is used to press the light diffusion portion 101 and bend it to a predetermined bending diameter to apply stress. In this third stress-applying structure, the light diffusion portion 101 is disposed on the stress-applying member 40. The stress-applying member 40 has a member main body 41 in the shape of an elongated rectangular plate and multiple cylindrical pressing portions 42. The multiple pressing portions 42 are erected on the fiber-arrangement side of the member main body 41 in a zigzag arrangement spaced apart in the longitudinal direction. The light diffusion portion 101 is routed on the member main body 41 of the stress-applying member 40 so as to weave through the zigzag-arranged pressing portions 42 to form a sine curve. As a result, the light diffusion portion 101 is wrapped around each pressing portion 42 and is subjected to pressure and bending deformation, thereby applying stress.

[0026] Although the third stress applying structure is slightly more complicated than the first and second stress applying structures, it can receive light with high sensitivity on the wide light receiving surface of the light diffusing section 101. The third stress applying structure is also effective in detecting weak light, and is therefore useful for free-space optical communications, as well as for highly sensitive optical sensors.

[0027] The first to third stress-applying structures described above are highly effective in increasing the coupling efficiency of light received by the side surface of the light diffusion section 101 to the core 11, and the magnitude of the stress applied to the light diffusion section 101 can be easily controlled. However, the stress-applying structures are not limited to these first to third stress-applying structures. The light diffusion section 101 may be arranged in a linear or ring-like configuration, or in any other configuration, such as a circular spiral configuration, a square spiral serpentine configuration, a triangular spiral configuration, or a character shape. The stress-applying structure is not particularly limited as long as it applies stress to the light diffusion section 101. For example, the stress-applying structure may be a structure that presses the light diffusion section 101, a structure that pulls the light diffusion section 101 like a hook, a structure that fits the light diffusion section 101 into a groove, a structure that adheres the light diffusion section 101 to a substrate, or a structure that wraps the light diffusion section 101 around a rod-shaped body. From the viewpoint of increasing the efficiency of coupling light received at the side surface of the light diffusing portion 101 to the core 11, it is preferable that these stress-applying members be made of a light-transmitting material.

[0028] In order to improve the coupling efficiency of light received by the side surface of the light diffusion section 101 to the core 11, the light receiving device A according to the first embodiment preferably includes, in addition to the stress applying structure, a light reflecting structure that reflects light in a direction perpendicular to the stress applying direction to the light diffusion section 101, changes the light path in the stress applying direction, and guides the light to the light diffusion section 101. Specifically, as shown in FIG. 4, for example, a light reflecting structure may be provided in which a pair of light reflecting members 50 is added to the third stress applying structure shown in FIG. 3C. In this light reflecting structure, the pair of light reflecting members 50 are each formed of an elongated columnar body having a cross-sectional shape of a right-angled isosceles triangle. The pair of light reflecting members 50 are provided on the member body 41 of the stress applying member 40, along the multiple pressing portions 42, sandwiching them from both sides, and with the reflective surfaces of the inclined surfaces corresponding to the hypotenuses of the right-angled isosceles triangles facing the pressing portions 42. As a result, light in a direction perpendicular to the stress-applying direction to the light diffusion section 101 is reflected by the reflective surfaces of the inclined surfaces of each light reflecting member 50, and the light path is changed to the stress-applying direction and is guided to the light diffusion section 101.

[0029] In the light receiving device A according to the first embodiment, a photodetector 20 is provided at each of both fiber ends of the light diffusing fiber 10, but this is not particularly limited to this, and the light detector 20 may be provided at only one fiber end of the light diffusing fiber 10, and in this case, a reflective structure such as a mirror may be provided at the other fiber end.

[0030] In the light-receiving device A according to the first embodiment, the light-diffusing fiber 10 is configured such that the light-transmitting portions 102 are provided on both sides of the light-diffusing portion 101, but the present invention is not particularly limited to this, and the light-diffusing portion 101 may be configured such that the light-scattering material 14 is added to the cladding 12 over the entire length, or the light-diffusing portions 101 of predetermined lengths and the light-transmitting portions 102 of predetermined lengths are alternately provided along the length direction. When a plurality of light-diffusing portions 101 are provided, the plurality of light-diffusing portions 101 can receive light of different wavelengths and couple the light to the core 11.

[0031] (Embodiment 2) 5 shows a light receiving device A according to embodiment 2. Note that parts with the same names as those in embodiment 1 are denoted by the same reference numerals as those in embodiment 1.

[0032] In the light-receiving device A according to the second embodiment, the light-diffusing fiber 10 includes a first light-transmitting section 102 formed of a single optical fiber on one side of the light-diffusing section 101 in the longitudinal direction, and a second light-transmitting section 102 formed of an optical fiber with a branched structure on the other side. A photodetector 20A is connected to the fiber end of the first light-transmitting section 102, with an optical filter 30A interposed therebetween. A photodetector 20B is connected to one of the branched fiber ends of the second light-transmitting section 102, with an optical filter 30B interposed therebetween. A photodetector 20C is connected to the other fiber end, with an optical filter 30C interposed therebetween. Each of the optical filters 30A, 30B, and 30C transmits only light in a specific wavelength range, specifically, light in the green wavelength range, light in the blue wavelength range, and light in the red wavelength range, respectively. The other components are identical to those of the first embodiment.

[0033] In the light-receiving device A according to the second embodiment, when light is received at the side surface of the light diffusing portion 101 of the light diffusing fiber 10, the light is scattered by the light scatterers 14 added to the cladding 12, enters the core 11, and is coupled therein. The light coupled to the core 11 propagates through the core 11 and is output from the fiber end of the first light transmission portion 102 and one and the other branched fiber ends of the second light transmission portion 102. Only the green wavelength component of the light output from the fiber end of the first light transmission portion 102 passes through the optical filter 30A and enters the photodetector 20A, only the blue wavelength component of the light output from one fiber end of the second light transmission portion 102 passes through the optical filter 30B and enters the photodetector 20B, and only the red wavelength component of the light output from the other fiber end of the second light transmission portion 102 passes through the optical filter 30C and enters the photodetector 20C.

[0034] According to the light receiving device A of the second embodiment, light in a required specific wavelength range can be obtained in each of the photodetectors 20A, 20B, and 20C. Other functions and effects are the same as those of the first embodiment.

[0035] (Embodiment 3) 6 shows a light receiving device A according to embodiment 3. Note that parts with the same names as those in embodiment 1 are denoted by the same reference numerals as those in embodiment 1.

[0036] In the light receiving device A according to the third embodiment, the light diffusing fiber 10 has light diffusing portions 101 and light transmitting portions 102 arranged alternately along the length direction. The light detector 20 is provided so as to be connected to the light transmitting portion 102 at one fiber end of the light diffusing fiber 10, with the optical filter 30 interposed therebetween, and the light source 60 is provided so as to be connected to the light transmitting portion 102 at the other fiber end.

[0037] The light source 60 emits light that is incident on the light diffusing fiber 10. Examples of the light source 60 include a semiconductor laser, a light emitting diode, a solid-state laser, and a gas laser. A semiconductor laser is suitable because it is small, portable, easy to control, and has high optical coupling efficiency with the light diffusing fiber 10. The wavelength of the light emitted by the light source 60 can be selected appropriately depending on the application, but is within the transmission wavelength range of the light diffusing fiber 10 (ultraviolet to infrared regions). The output form of the light source 60 can be continuous light (CW) or pulsed light, but is pulsed light for signal transmission. The output power of the light emitted by the light source 60 varies depending on the application, but is generally on the order of several mW to several tens of W. The configuration of the other components is the same as that of embodiment 1.

[0038] In the light-receiving device A according to the third embodiment, when light is received at the side surface of each light diffusing portion 101 of the light diffusing fiber 10, the light is scattered by the light scatterers 14 added to the clad 12, enters the core 11, and is coupled. The light coupled to the core 11 propagates through the core 11 and exits from one fiber end formed by the light transmission portion 102. Of the light exiting from one fiber end, only a component of a specific wavelength passes through the optical filter 30 and enters the photodetector 20. Furthermore, when light is emitted from the light source 60, the light enters the light diffusing fiber 10 and propagates through the core 11. When the light propagating through the core 11 of the light diffusing fiber 10 leaks into the clad 12 in the light diffusing portion 101, the light is scattered by the light scatterers 14 and exits to the external space.

[0039] The light receiving device A according to the third embodiment can function not only as a light receiving device but also as a light emitting device. Other advantages are the same as those of the first embodiment.

[0040] In the light-receiving device A according to the third embodiment, a stress-applying structure may be configured in each light diffusing portion 101 of the light diffusing fiber 10. In this case, taking into consideration light attenuation and from the viewpoint of uniformity in the coupling efficiency of light to the core 11 in the plurality of light diffusing portions 101, it is preferable to increase the stress applied to the light diffusing portion 101 depending on the distance from the photodetector 20. On the other hand, from the viewpoint of uniformity in the efficiency of emitting light from the light source 60 to the outside in the plurality of light diffusing portions 101, it is preferable to increase the stress applied to the light diffusing portion 101 depending on the distance from the light source 60.

[0041] In the light receiving device A according to the third embodiment, if the wavelength of the light received by the light diffusing unit 101 is different from the wavelength of the light emitted, and the optical filter 30 blocks the light from the light source 60 from entering the photodetector 20, then the light receiving device A can receive and output the signal light via space. If the optical filter 30 is not provided, the timing of the signal light reception and the signal light output can be shifted.

[0042] In the light receiving device A according to the third embodiment, the photodetector 20 is provided at one fiber end of the light diffusing fiber 10, and the light source 60 is provided at the other fiber end. However, the present invention is not limited to this. Since the light coupled to the core 11 also propagates to the other fiber end, a branching structure may be formed there using a mirror, an optical waveguide coupler, or the like, and another photodetector may be connected to detect the light. Alternatively, a branching structure may be similarly formed at one fiber end using a mirror, an optical waveguide coupler, or the like, and another light source may be connected to emit light.

[0043] (Embodiment 4) 7 shows a light receiving device A according to embodiment 4. Note that parts with the same names as those in embodiments 1 and 3 are denoted by the same reference numerals as those in embodiments 1 and 3.

[0044] In the light-receiving device A according to the fourth embodiment, a light diffusing fiber 10 includes a first light transmission section 102 of a predetermined length, a second light diffusing section 101 of a predetermined length, and a second light transmission section 102 of a predetermined length, which are provided in this order along the length direction on both sides of a first light diffusing section 101 of a predetermined length. A light source 60 is provided facing the first light diffusing section 101, a photodetector 20 is provided facing the second light diffusing section 101, and an optical filter 30 is interposed between the second light diffusing section 101 and the photodetector 20. The other component configurations are the same as those of the first and third embodiments.

[0045] In the light-receiving device A according to the fourth embodiment, when light is emitted from the light source 60, the light is received by the side surface of the first light diffusing section 101 of the light diffusing fiber 10, scattered by the light scatterers 14 added to the clad 12, and enters and couples into the core 11. The light coupled into the core 11 propagates through the cores 11 of the first light transmitting sections 102 on both sides of the first light diffusing section 101, and when it leaks into the clad 12 in each second light diffusing section 101, it is scattered by the light scatterers 14 and is emitted into the external space. Of the light emitted into the external space from each second light diffusing section 101, only a component of a specific wavelength passes through the optical filter 30 and enters the photodetector 20. Other functions and effects are the same as those of the first embodiment.

[0046] In the light receiving device A of embodiment 4, the light from the light source 60 is received on the side surface of the first light diffusion section 101, and the light emitted from the second light diffusion section 101 is incident on the photodetector 20 via the optical filter 30, but this is not particularly limited to this, and the light from outside other than the light source 60 may be received on the side surface of the first light diffusion section 101, or the light emitted from the second light diffusion section 101 may be used for external light emission.

[0047] When stress is applied to the light diffusing fiber 10 and a reflective structure is also added to it, the light reflection due to the presence of an air layer can be reduced by filling the gaps between the light diffusing fiber 10, the stress applying member, and the reflective structure with a transparent material, thereby improving the efficiency of light reception and emission. There are no particular restrictions on the transparent material, but silicone resin, urethane resin, etc. are suitable as they can easily penetrate into the gaps.

[0048] (Embodiment 5) 8 shows a light receiving device A according to embodiment 5. Note that parts with the same names as those in embodiment 1 are denoted by the same reference numerals as those in embodiment 1.

[0049] The light-receiving device A according to the fifth embodiment includes a plurality of light-diffusing fibers 10, each including a light-diffusing portion 101 of a predetermined length and a light-transmitting portion 102 provided on each side of the light-diffusing portion 101 in the longitudinal direction. The plurality of light-diffusing fibers 10 extend parallel to each other, in close proximity or contact with each other, and the light-diffusing portions 101 are arranged adjacent to each other. The plurality of light-diffusing fibers 10 may be arranged two-dimensionally in parallel, or may be arranged three-dimensionally in a square or hexagonal close-packed cross-sectional structure. The light-diffusing portions 101 of the plurality of light-diffusing fibers 10 are embedded in an embedding member 70 formed of a translucent resin. The configuration of each light-diffusing fiber 10 is the same as that of the first embodiment.

[0050] In the light-receiving device A according to the fifth embodiment, when light is incident on one side, the other side, or both sides of the core 11 of at least one first light diffusing fiber 10 among the plurality of light diffusing fibers 10, the light propagates through the core 11 of the first light diffusing fiber 10. When the light propagating through the core 11 of the first light diffusing fiber 10 leaks into the cladding 12 in the light diffusing section 101, it is scattered by the light scatterers 14 and emitted to the external embedded member 70. When the light emitted to the embedded member 70 is received by the side surface of the light diffusing section 101 of another adjacent second light diffusing fiber 10, the light is scattered by the light scatterers 14 added to the cladding 12, enters the core 11, is coupled, and propagates through the core 11 of the second light diffusing fiber 10. From the viewpoint of facilitating the light emitted from the first light diffusing fiber 10 to be received by the side surface of the second light diffusing fiber 10 and coupled to the core 11, it is preferable that the refractive index of the embedding member 70 is close to the refractive index of the coating layer 13 of the light diffusing fiber 10. Specifically, for example, when the coating layer 13 of the light diffusing fiber 10 is made of a fluororesin, it is preferable that the embedding member 70 is made of a silicone resin.

[0051] In the light-receiving device A according to the fifth embodiment, a stress-applying structure may be configured in each light diffusing portion 101 of the plurality of light diffusing fibers 10. In this case, the light diffusing fiber 10 that inputs light can improve the efficiency of emitting light scattered by the light scatterers 14 in the cladding 12 to the outside, and the light diffusing fiber 10 that receives light can improve the efficiency of coupling light scattered by the light scatterers 14 in the cladding 12 to the core 11.

[0052] In the light receiving device A of embodiment 5, the light diffusing fiber 10 is configured to include a light diffusing section 101 and a light transmitting section 102, but this is not particularly limited to this, and the light diffusing section 101 may be configured in such a way that a light scatterer 14 is added to the clad 12 over the entire length.

[0053] In the light receiving device A of embodiment 5, the light diffusion portions 101 of the multiple light diffusion fibers 10 are configured to be embedded in the embedding member 70, but this is not particularly limited to this, and the light diffusion portions 101 of the multiple light diffusion fibers 10 may be configured to be exposed to the external space. [Example]

[0054] Four types of light diffusing fibers, Samples A to D, were prepared, each having a quartz core with a diameter of 250 μm, a silicone resin cladding with an outer diameter of 400 μm, and a fluororesin coating layer with an outer diameter of 0.9 mm, with a light diffusing section in which light scatterers were added to the cladding over its entire length. Samples A to D varied the content of light scatterers in the cladding. The content of light scatterers in the cladding of Samples A to D was 6 mass%, 0.6 mass%, 0.2 mass%, and 0.02 mass%, respectively, relative to the mass of the cladding.

[0055] For each of samples A to D, as shown in Fig. 9A, a photodetector 20 was connected to one fiber end, and continuous light was irradiated onto the side from an LED of light source 60, and the relationship between the distance L from the light irradiating position to photodetector 20 and the light transmission power measured by photodetector 20 was investigated. The results are shown in Fig. 10.

[0056] 10, it can be seen that the greater the content of light scatterers in the cladding, the higher the light transmission power, i.e., the higher the coupling efficiency of light to the core. For example, sample A has a light transmission power that is 10 to 50 times higher than sample D, which has a light scatterer content in the cladding that is two orders of magnitude lower. Note that although light scatterers are added to the cladding in all of samples A to D, the measurement results of their light transmission power suggest that the light transmission power would be very low if light scatterers were not added to the cladding.

[0057] It can also be seen that the greater the content of light scatterers in the cladding, the greater the attenuation of the transmission power associated with light transmission. For example, sample A has very high light transmission power when the distance L from the photodetector is within 500 mm, but when the distance L is longer than 500 mm, the attenuation of the light transmission power is significant, and at distance L around 1200 mm, it falls below the light transmission power of sample B. This is thought to be because, when the content of light scatterers in the cladding is high, the reciprocity of light (reverse propagation) becomes advantageous, making it easier for light to couple to the core, but there is also a higher probability that light propagating through the core will leak into the cladding, be scattered by the light scatterers, and be emitted as a diffused light.

[0058] Furthermore, for each of Samples A to D, stress was applied using a pair of stress-applying members 40 having a configuration similar to that shown in the first stress-applying structure of Embodiment 1, as shown in FIG. 9B . Each of the pair of stress-applying members 40 had five pressing portions 42 provided on a member body 41, and the pair of stress-applying members 40 was arranged so that one pressing portion 42 and the other pressing portion 42 were arranged in a zigzag pattern with a 4 mm interval P in the longitudinal direction. Each of the ten pressing portions 42 of the pair of stress-applying members 40 pressed the light diffusing fiber 10 from the side so that the pressing depth d was 300 μm, thereby applying stress. Then, as described above, light was irradiated onto the side surface of the light diffusing fiber 10 from the LED of the light source 60, and the relationship between the distance L from the light irradiating position to the photodetector and the light transmission power measured by the photodetector was investigated. The results are shown in FIG. 10 .

[0059] As can be seen from Figure 10, the application of stress increases the light transmission power by 10 to 100 times. However, it is noteworthy that a high content of light scatterers in the cladding does not necessarily result in high light transmission power. Sample A has the highest content of light scatterers in the cladding and has the highest light transmission power when no stress is applied, but when stress is applied, the light transmission power is equal to or lower than that of Sample D, which has the lowest content of light scatterers in the cladding. In this regard, while the application of stress makes the reciprocity (regression) of light more pronounced, it also strengthens the tendency for light to be diffused and emitted to the outside. Therefore, it is possible that the effect of applying stress is excessive when the content of light scatterers in the cladding is high.

[0060] Next, pulsed light from an LED (repetition frequency 1.4 kHz, duty ratio 50%) was used as the light source 60, and the peak value of the pulsed light transmitted through the core was measured. In this measurement, the push-in depth d of the stress-applying member 40 was varied to 200 μm, 300 μm, and 400 μm. The measurement samples were A, B, C, and D, plus samples E, F, and G, with light-scattering material contents of 1 mass%, 2.2 mass%, and 3 mass%. The distance between the photodetector 20 and the stress-applying member 40 was fixed at 500 mm. The results are shown in Figure 11.

[0061] Figure 11 shows that the pulse height of the pulsed light increases with increasing indentation depth d for all samples. Furthermore, while this change increases steadily with indentation depth d for samples with a low light-scattering content, it changes significantly with increasing content, showing a tendency toward saturation. Furthermore, a comparison of the magnitude of the pulse height shows that there is no significant difference between the contents of 0.02% to 1% by mass, but decreases when the content exceeds 2.2% by mass. This is likely due to the excessive stress application effect when the light-scattering content is high, as mentioned above. While the details vary depending on the distance from the light detection position to the photodetector, these data suggest that the optimal range for the light-scattering content in a stress-applying structure is approximately 0.01% to 2% by mass.

[0062] Considering the above series of characteristics, an optimal design is possible. For example, in an application in which light is detected at a position far from a photodetector, a light diffusing fiber having a short light diffusing portion and a high content of light scatterers in the cladding is arranged at the detection position, and the other portions are light transmitting portions without light scatterers in the cladding. This makes it possible to obtain high coupling efficiency of light to the core while minimizing attenuation of the transmission power associated with the transmission of the light. Furthermore, when a structure is used in which stress is applied to the light diffusing portion of the light diffusing fiber for the same purpose, by selecting a content of light scatterers in the cladding of the light diffusing portion that increases the light transmission power, it is possible to obtain high coupling efficiency of light to the core while also obtaining high transmission power of the light.

[0063] Furthermore, in applications where light is detected simultaneously at multiple locations, for example, by placing short light diffusing sections with increasing light scattering material content depending on the distance from the photodetector at the detection position and connecting these sections with a light transmitting section in which no light scattering material is added to the cladding, the coupling power of distant light can be increased and attenuation during transmission can be suppressed, enabling highly sensitive light detection at multiple locations. Furthermore, when a stress-applying structure is provided for the same purpose, by increasing the pressing depth d of the pressing section depending on the distance from the photodetector, the coupling power of distant light can be increased and attenuation during transmission can be suppressed. These two methods (weighting the content by distance and weighting the pressing depth by distance) can be implemented in combination, which is an even more effective method. [Industrial Applicability]

[0064] The present invention is useful in the technical fields of a light receiving device and a light receiving method. [Explanation of symbols]

[0065] A Photodetector 10. Light diffusing fiber 101 Light diffusion section 102 Optical transmission section 11 cores 12 Clad 13 Covering layer 14 Light scatterer 20, 20A, 20B, 20C Photodetector 30, 30A, 30B, 30C Optical Filters 40 Stress applying member 41 Component body 42 Pressing section 50 Light reflecting member 60 light source 70 Buried materials

Claims

1. A light-receiving device comprising a light-diffusing fiber having a core and a clad covering the core, the light-diffusing fiber including a light-diffusing portion in which a light-scattering material is added to the clad.

2. 2. The light receiving device according to claim 1, The light-receiving device includes a light-transmitting section in which the light-diffusing fiber is connected to the light-diffusing section and in which no light-scattering material is added to the clad.

3. 2. The light receiving device according to claim 1, The light receiving device further comprises a photodetector into which light from the light diffusing fiber is incident.

4. 2. The light receiving device according to claim 1, The light receiving device further comprises a light source that emits light that is incident on the light diffusing fiber.

5. A light receiving method using a light diffusing fiber having a core and a cladding covering the core, the light diffusing fiber including a light diffusing portion in which a light scatterer is added to the cladding, wherein light is received at a side surface of the light diffusing portion of the light diffusing fiber.

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