Light guide plate and solar-pumped laser

Abstract

The present disclosure provides a fluorescent light guide plate and a solar-pumped laser. The fluorescent light guide plate includes a first surface, a second surface, an edge surface connecting a periphery of the first surface and a periphery of the second surface, and a dichroic mirror superposed on the first surface. A fluorescent material is dispersed in at least one of an inside of a space defined by the first surface, the second surface, and the edge surface, the first surface, or the second surface. The fluorescent light guide plate has a plate-like structure made of a material having a higher refractive index than an outside. The fluorescent light guide plate is configured so that, when an irradiation light is incident from the first surface, fluorescent light emitted from the fluorescent material is emitted from the edge surface. A reflected light wavelength band of a normally incident light beam reflected by the dichroic mirror is in a range of a wavelength longer than a peak wavelength of a fluorescent light wavelength band of the fluorescent material.

Description

Technical Field

[0001] This invention relates to a fluorescent light guide plate and a solar-pumped laser. Background Technology

[0002] As a countermeasure to address global warming and other environmental problems, various structures for collecting sunlight have been proposed to utilize solar energy. For example, Japanese Unexamined Patent Application Publication Nos. 2017-168662A, 2018-018981A, and 2020-065027A disclose a novel structure for a solar-pumped laser that uses a focusing mechanism without lenses or a solar tracking mechanism. In a solar-pumped laser 10 including this novel focusing mechanism, such as… Figure 6A Schematarily shown, an optical fiber portion 3, including optical fiber 3a, is wound around the edge surface 2c of a fluorescent light guide plate 1, in which fluorescent material FM is dispersed in a plate member made of a material with a higher refractive index than the outside. The fluorescent material FM absorbs sunlight SL and emits fluorescence FL. The fluorescence FL emitted from the fluorescent material FM is light in a wavelength band with relatively high sensitivity to the laser medium. Optical fiber 3a includes: a core in which the laser medium is dispersed (in... Figure 6D (represented by 3c); a cladding portion made of a fluorescent material (in) Figure 6D (represented by 3b); a reflector that reflects virtually all the light onto one end face of fiber 3a (in... Figure 6B (represented by 5); and a reflector through which a portion of the light emitted from the laser medium passes at the other end face of fiber 3a. Figure 6B(represented by 4 in the original text). In the structure of the solar-pumped laser 10, the fluorescent light guide plate 1 functions to collect sunlight. The solar-pumped laser 10 is configured as follows. When sunlight SL enters from one surface 2a of the fluorescent light guide plate 1, the fluorescence FL emitted from the fluorescent material FM is collected at the edge surface 2c and emitted, and the emitted fluorescence FL passes through the cladding portion 3b of the optical fiber 3a and reaches the core 3c. The laser medium is excited by the fluorescence FL that has reached the core 3c, resulting in laser oscillation. With the above structure, it is advantageous that large components, such as a condenser lens and a mechanism for tracking the position of the sun, or a mechanism for adjusting the focal position of the condenser lens, are not required. As an example of using a fluorescent light guide plate constructed in this way, Japanese Unexamined Patent Application Publication No. 2015-201464 (JP2015-201464A) describes a structure in which a plurality of reflective layers are disposed around the edge surface of the fluorescent light guide plate to propagate fluorescence generated by the excitation of fluorescent material inside the fluorescent light guide plate due to sunlight that has entered from one surface of the fluorescent light guide plate to a solar cell arranged on the edge surface of the fluorescent light guide plate. Summary of the Invention

[0003] There is room for improvement in the efficient collection of fluorescence in fluorescent light guide plates.

[0004] Similar to the fluorescent light guide plates used in solar-pumped lasers described in JP2017-168662A, JP2018-018981A and JP2020-065027A, the present invention provides a fluorescent light guide plate for collecting light, configured to further concentrate the energy of light applied to the fluorescent light guide plate.

[0005] The present invention also provides a fluorescent light guide plate, as in the case of the fluorescent light guide plate described above, wherein a dichroic mirror is stacked on a receiving surface for irradiating light, and the reflected light wavelength band of the dichroic mirror is configured to allow as much irradiating light as possible to pass through into the plate and to concentrate the energy of the light by capturing as much fluorescence as possible in the plate.

[0006] The present invention further provides a solar-pumped laser using the above-described fluorescent light guide plate.

[0007] A first aspect of the present invention provides a fluorescent light guide plate. The fluorescent light guide plate includes a first surface, a second surface, an edge surface connecting the periphery of the first surface and the periphery of the second surface, and a dichroic mirror stacked on the first surface. A fluorescent material that absorbs illumination light applied to the first surface to emit fluorescence is dispersed at at least one of the following locations: inside the space defined by the first surface, the second surface, and the edge surface; on the first surface; or on the second surface. The fluorescent light guide plate has a plate-like structure made of a material with a higher refractive index than the exterior. The fluorescent light guide plate is configured such that when illumination light enters from the first surface, fluorescence emitted from the fluorescent material exits from the edge surface. The reflected wavelength band of the perpendicularly incident light beam reflected by the dichroic mirror is in a wavelength range longer than the peak wavelength band of the fluorescence wavelength band of the fluorescent material.

[0008] A second aspect of the present invention provides a solar-pumped laser. The solar-pumped laser includes: a fluorescent light guide plate according to a first aspect, and an optical fiber circumferentially wound around an edge surface of the fluorescent light guide plate along a first surface and a second surface. The optical fiber includes: a core in which a laser medium is dispersed; a cladding portion, the surface and interior of which are made of a material through which the fluorescence passes, and the cladding portion having a lower refractive index than the core; a first reflector configured to reflect all light emitted from the laser medium at one end face of the optical fiber; and a second reflector configured to allow a portion of the light emitted from the laser medium to pass through at the other end face of the optical fiber and reflect the remaining portion of the light. The optical fiber is configured such that the fluorescence emitted from the edge surface of the fluorescent light guide plate passes through the surface of the cladding portion and reaches the core, a laser is generated due to excitation of the laser medium caused by the fluorescence, and the laser is emitted from the other end face of the optical fiber.

[0009] According to the present invention, the energy of light applied to the fluorescent light guide plate can be concentrated more effectively at the edge surface. Further advantageous use of sunlight is also anticipated. By using the structure of the fluorescent light guide plate according to the present invention for a light-pumped laser excited by irradiated light (such as sunlight), easier laser oscillation and an increase in the energy to be extracted as laser light are expected. Attached Figure Description

[0010] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which like reference numerals denote like elements, wherein:

[0011] Figure 1A This is a schematic perspective view of a fluorescent light guide plate according to an embodiment of the present invention;

[0012] Figure 1B yes Figure 1A A schematic cross-sectional view of the fluorescent light guide plate is shown in the figure;

[0013] Figure 2 The solar spectrum Is, the absorption spectrum Af of the fluorescent material, and the fluorescence spectrum If are shown.

[0014] Figure 3 The diagram shows the variation of the reflectivity of the DM with respect to the wavelength of light (simulation results). The top plot shows the case where the DM is surrounded by air on both sides, and the bottom plot shows the case where the DM has a substrate with a refractive index of 1.47 at the entrance side and air at the exit side. 10°, 20°, 30° and 40° represent the incident angles of the light beam entering the DM, and If is the fluorescence spectrum of a typical fluorescent material.

[0015] Figure 4 This is a schematic diagram showing the illumination light SL entering from the outside of the fluorescent light guide plate and the light path of the fluorescence FL emitted from the fluorescent material FM.

[0016] Figure 5 The relationship between the fluorescence spectrum of the fluorescent material and the wavelength characteristics of the reflectance of the DM superimposed on the light-receiving surface of the fluorescent light guide plate according to an embodiment of the present invention is shown.

[0017] Figure 6A This is a schematic perspective view of a solar-pumped laser using a fluorescent light guide plate according to an embodiment of the present invention;

[0018] Figure 6B This is a schematic plan view of a solar-pumped laser;

[0019] Figure 6C This is a schematic cross-sectional view of a solar-pumped laser; and

[0020] Figure 6D This is a schematic cross-sectional view of the optical fiber of a solar-pumped laser according to an embodiment. Detailed Implementation

[0021] First, an overview of the invention will be described.

[0022] In use in Figure 6AIn the fluorescent light guide plate of the solar-pumped laser 10 shown, a fluorescent material dispersed in the plate is excited by light entering the plate from its surface. The fluorescence emitted from the excited fluorescent material is repeatedly reflected on the surface of the plate and collected at the edge surface of the plate, and the energy of the light applied to the plate is concentrated at the edge surface and extracted. Using this structure, light beams with an incident angle (the angle formed between the normal direction of the plate and the incident direction of the beam) to the plate surface (the interface between the plate and the outside) less than a critical angle partially pass through the plate surface, causing loss. As a technique to reduce this fluorescence passing through the plate surface from the interior of the fluorescent light guide plate, it is conceivable to adhere or stack a dichroic mirror (hereinafter referred to as "DM") on the plate surface to reduce the amount of fluorescence emitted from the fluorescent material within the fluorescent light guide plate that passes through the plate surface. A dichroic mirror is a thin film having the property of reflecting light of a specific wavelength while allowing light of other wavelengths to pass through. The aforementioned “critical angle” is the minimum angle of incidence at which a light beam is completely reflected when it enters the interface from the fluorescent light guide plate side, provided there is an interface in direct contact with the air layer. (This also applies below.)

[0023] As described above, when a DM is arranged on the surface of a fluorescent light guide plate to reduce the amount of fluorescence emitted from the fluorescent material within the light guide plate passing through the plate surface, the DM should be prepared such that the wavelength band of the light reflected by the DM (reflected light wavelength band) overlaps with or includes the wavelength band of the fluorescence emitted from the fluorescent material within the light guide plate, and as much light beam as possible is reflected to the plate surface at an incident angle less than the critical angle (the reflected light wavelength band of the DM can be changed by adjusting the composition of the thin film during preparation). In this regard, research related to the present invention has found that as the incident angle of the light beam to the DM increases, the reflected light wavelength band of the DM shifts towards the shorter wavelength side. Furthermore, as in the case where the DM is stacked in a fluorescent light guide plate, in structures where the refractive index of the outer interface of the DM is greater than 1, it has been found that the amount of shift of the reflected light wavelength band towards the shorter wavelength side due to the increase in the incident angle to the DM increases (see...). Figure 3 In other words, even when a beam of a certain wavelength is reflected when it enters the DM perpendicularly ((incident angle) = 0°, perpendicular incident light), a beam with a large incident angle to the DM at the same wavelength is located outside the reflected light wavelength band and can pass through the DM (the fluorescence that has passed through the DM is lost). Therefore, when the DM is stacked on the surface of the fluorescent light guide plate to reduce the amount of fluorescence passing through the surface, it is conceivable to set the reflected light wavelength band of the DM such that the reflected light wavelength band shifted according to the incident angle to the DM overlaps with the fluorescence wavelength band.

[0024] Incidentally, the illumination light (such as sunlight) to the fluorescent light guide plate typically enters perpendicularly to the receiving surface of the fluorescent light guide plate or at a relatively small angle of incidence. To further increase the energy of the light to be concentrated in the fluorescent light guide plate, the amount of fluorescence should be increased by using more photoexcitation of the fluorescent material in the plate. For this purpose, the greater the amount of light entering the fluorescent light guide plate, the better. Therefore, it is desirable for the wavelength band of the light that can enter the plate to be as wide as possible. In this respect, typically, the absorption wavelength band (excitation wavelength band) of the fluorescent material overlaps with the emission wavelength band (fluorescence wavelength band) of the fluorescent material. Therefore, it is conceivable that a light beam with a relatively small angle of incidence and a wavelength less than or equal to the fluorescence wavelength band of the fluorescent material can pass through the surface of the fluorescent light guide plate. On the other hand, for fluorescence to be emitted from the fluorescent material in the fluorescent light guide plate, as the angle of incidence of the fluorescent light beam on the plate surface decreases, the number of reflections of the beam on the plate surface increases until the beam reaches the edge surface of the plate (see...). Figure 4 Furthermore, each time the beam is reflected at the surface of the plate, a portion of the beam passes through the surface, causing a loss. For this reason, instead of reducing the amount of beam reflected many times in this manner to pass through the surface, the beam with a relatively large angle of incidence to the surface of the plate and a small number of reflections is reliably reflected and reaches the edge surface of the plate. The result is that collection can be further efficient, i.e., a larger amount of fluorescence can be collected at the edge surface of the plate. In other words, it is conceivable that the reflected wavelength band of the beam with a relatively large angle of incidence to the surface of the fluorescent light guide plate overlaps with the fluorescence wavelength band. Based on the above, when the DM stacked in the fluorescent light guide plate is prepared such that the reflected wavelength band of the DM is within a wavelength range longer than the fluorescence wavelength band of the fluorescent material within a relatively small angle of incidence from the vertical angle of incidence, and overlaps with the fluorescence wavelength band of the fluorescent material within a relatively large angle of incidence, it is understandable that fluorescence can be collected more efficiently in the fluorescent light guide plate.

[0025] A first aspect of the present invention provides a fluorescent light guide plate. The fluorescent light guide plate includes a first surface, a second surface, an edge surface connecting the periphery of the first surface and the periphery of the second surface, and a dichroic mirror stacked on the first surface. A fluorescent material that absorbs illumination light applied to the first surface to emit fluorescence is dispersed at least in at least one of the following locations: inside the space defined by the first surface, the second surface, and the edge surface; on the first surface; or on the second surface. The fluorescent light guide plate has a plate-like structure made of a material having a higher refractive index than the exterior. The fluorescent light guide plate is configured such that when illumination light enters from the first surface, fluorescence emitted from the fluorescent material is emitted from the edge surface. The reflected wavelength band of the perpendicularly incident light beam reflected by the dichroic mirror is in a wavelength range longer than the peak wavelength band of the fluorescence of the fluorescent material.

[0026] In the above structure, the fluorescent light guide plate is typically a plate-like structure in which a transparent or translucent material (such as quartz glass, polycarbonate resin, acrylic resin, and silicone resin) with a higher refractive index than the external space is used as the substrate, and fluorescent materials (such as fluorescent pigments and quantum dots) are dispersed in the substrate or on the surface (first or second surface) of the plate-like structure. A dichroic mirror is a thin film with the property of reflecting light of a specific wavelength while allowing light of other wavelengths to pass through, and is generally a dielectric multilayer film prepared from SiO2, TiO2, etc. A perpendicularly incident beam is a beam of light entering in a direction perpendicular to the surface of the dichroic mirror (incident angle of 0°). The reflected light wavelength band of the dichroic mirror is the wavelength band where the reflectivity of the light entering the dichroic mirror increases. The fluorescence wavelength band of the fluorescent material is the wavelength band where the intensity of the fluorescence to be emitted from the fluorescent material increases significantly. The peak wavelength is the wavelength that imparts the maximum intensity or local maximum intensity in the fluorescence wavelength band. The light applied to the first surface (irradiation light) can typically be sunlight; however, the light is not limited to this. The light can be from any light source.

[0027] In the fluorescent light guide plate described above, when light (such as sunlight) is applied to the first surface and enters the plate, the fluorescent material dispersed on the interior or the first or second surface is excited, resulting in fluorescence emission from each particle of the fluorescent material along the radiation direction. Here, the refractive index inside the plate is higher than the refractive index outside (typically, air). Therefore, in the fluorescence beam from the fluorescent material, beams reaching the interface (first or second surface) with an incident angle greater than the critical angle undergo total internal reflection and reach the edge surface of the plate-like structure. Furthermore, fluorescence beams with an incident angle less than the critical angle partially pass through the interface, and the remaining beams undergo repeated reflection and reach the edge surface. Thus, the fluorescence beams reflected or totally internalized at the interface are collected at the edge surface while being trapped within the plate. Therefore, the energy of light applied to the wide surface (first surface) of the plate-like structure is concentrated at the edge surface of the plate-like structure (focusing function).

[0028] In the above structure, in a fluorescence beam with an incident angle to the interface less than the critical angle, the beam passing through the interface will not reach the edge surface, resulting in energy loss. Therefore, reducing the amount of beam passing through the interface reduces energy loss, and increasing the amount of illumination light passing through the interface increases the energy emitted as fluorescence within the plate. As described above, the present invention aims to block the passage of fluorescence from inside the plate by stacking a dichroic mirror on the first surface, which serves as the receiving surface of the illumination light, while simultaneously increasing the amount of light reaching the edge surface by allowing a larger amount of illumination light to pass through from outside the plate.

[0029] In this regard, as described above, it has been found from studies related to this invention that as the incident angle of the light beam to the dichroic mirror increases, the reflected light wavelength band of the dichroic mirror shifts towards the shorter wavelength side, and the amount of shift increases with the increase of the refractive index of the medium. In other words, when the reflected light wavelength band of the perpendicularly incident light beam in the dichroic mirror matches the fluorescence wavelength band of the fluorescent material, the wavelength band of the fluorescence beam with a relatively large incident angle falls outside the reflected light wavelength band (shifting towards the shorter wavelength side), and the light beam can pass through the dichroic mirror. On the other hand, as the incident angle decreases, the number of reflections at the interface increases until the light beam reaches the edge surface, and the chance of the light beam passing through the interface increases. Therefore, even when the wavelength band of the fluorescence beam with a relatively small incident angle matches the reflected light wavelength band, the amount of light lost due to crossing the interface increases by the amount of the increase in the number of reflections at the interface. Therefore, in order to increase the amount of fluorescence reaching the edge surface, it is conceivable to set the reflected light wavelength band of the dichroic mirror considering the shift of the reflected light wavelength band towards the shorter wavelength side, so as to reliably reflect the fluorescence beam with a relatively large incident angle. As the amount of light exciting the fluorescent material increases, the amount of fluorescence emitted from the fluorescent material also increases. Typically, the excitation wavelength band of the fluorescent material overlaps with its fluorescence wavelength band; therefore, it is conceivable to allow light from the wavelength band covering the fluorescence wavelength band to pass through a dichroic mirror as illumination light to increase the amount of fluorescence from the fluorescent material.

[0030] In the structure of the present invention, as described above, the reflected wavelength band of the perpendicularly incident beam in the dichroic mirror is set to be within a wavelength range longer than the peak wavelength of the fluorescence wavelength band of the fluorescent material. Using this structure, in the fluorescent beam traveling from the inside of the plate toward the dichroic mirror, the reflected wavelength band of the beam with a relatively large incident angle shifts from the reflected wavelength band of the perpendicularly incident beam toward the shorter wavelength side and overlaps with the fluorescence wavelength band. As a result, a larger amount of beam is reflected and trapped within the plate. Since the irradiation light traveling from the outside of the plate toward the inside of the plate typically enters the dichroic mirror perpendicularly or at a relatively small incident angle, irradiation light with wavelengths shorter than or equal to the peak wavelength of the fluorescence wavelength band passes through the dichroic mirror because the reflected wavelength band is within a wavelength range longer than the peak wavelength of the fluorescence wavelength band of the fluorescent material. Light in a broad wavelength band, including wavelengths in the excitation wavelength band overlapping with the fluorescence wavelength band, enters the plate and helps to excite the fluorescent material. Therefore, it is possible to increase the amount of fluorescence to be emitted inside the plate.

[0031] In a fluorescent light guide plate, a reflective film or mirror can be stacked or adhered to a surface (second surface) where no illumination light is applied. The reflective film or mirror reflects fluorescence from inside the plate and illumination light that has passed through the plate, regardless of its angle of incidence.

[0032] In the above structure, more specifically, the dichroic mirror can be fabricated such that the reflected wavelength band of a light beam with an incident angle greater than a first predetermined angle in the direction from the interior of the fluorescent light guide plate toward the first surface overlaps with the fluorescence wavelength band of the fluorescent material. The first predetermined angle can be an angle less than a critical angle, set as needed. Specifically, the first predetermined angle can be set adaptively, and is preferably, for example, about 30°; however, the first predetermined angle is not limited to this. Using the above structure, a larger amount of fluorescence beam traveling from the interior of the plate toward the first surface and with an incident angle greater than the first predetermined angle to the dichroic mirror is reflected on the dichroic mirror. Therefore, a larger amount of fluorescence from the fluorescent material is captured in the plate and collected at the edge surface.

[0033] In the above structure, the dichroic mirror can be fabricated such that the reflected light wavelength band of a light beam having an incident angle greater than a first predetermined angle in the direction from the interior of the fluorescent light guide plate toward the first surface includes the fluorescence wavelength band of the fluorescent material. Using this structure, substantially most of the fluorescence beams in the wavelength band with incident angles greater than the first predetermined angle are reflected by the dichroic mirror. Therefore, a greater amount of fluorescence from the fluorescent material is captured within the plate and collected at the edge surface.

[0034] In the above structure, the reflected light wavelength band of the dichroic mirror can be set such that a light beam with an incident angle smaller than a second predetermined angle and a wavelength capable of exciting the fluorescent material passes through the dichroic mirror in the direction from the outside of the fluorescent light guide plate toward the first surface. The second predetermined angle can be determined by adaptation and can be the same as the first predetermined angle; however, the second predetermined angle is not limited to this. As described above, since illumination light such as sunlight typically comes from above the fluorescent light guide plate, a relatively small incident angle is desirable. Therefore, since a light beam with an incident angle smaller than the second predetermined angle and a wavelength capable of exciting the fluorescent material (i.e., a light beam in the excitation wavelength band) passes through the dichroic mirror, even when the light beam is in the fluorescence wavelength band, the light beam contributes to the excitation of the fluorescent material, and the amount of fluorescence is further increased.

[0035] The fluorescent light guide plate according to the present invention can be applied to various applications (e.g., guiding light to photovoltaic cells, etc.) to concentrate and collect the energy of light with relatively low concentration (low energy intensity) (such as sunlight), and to use that energy. Alternatively, the fluorescent light guide plate can be used as a fluorescent light guide plate for solar-pumped lasers described in JP2017-168662A or JP2018-018981A, or as a focusing unit described in JP2020-065027A. A second aspect of the present invention provides a solar-pumped laser. The solar-pumped laser includes: the fluorescent light guide plate of the first aspect, and an optical fiber circumferentially wound around the edge surface of the fluorescent light guide plate along a first surface and a second surface. The optical fiber includes: a core in which a laser medium is dispersed; a cladding portion whose surface and interior are made of a fluorescence-permeable material and has a lower refractive index than the core; a first reflector configured to reflect all light emitted from the laser medium at one end face of the optical fiber; and a second reflector configured to allow a portion of the light emitted from the laser medium to pass through the other end face of the optical fiber and reflect the remaining portion of the light. The optical fiber is configured such that fluorescence emitted from the edge surface of the fluorescent light guide plate passes through the surface of the cladding portion and reaches the core, laser emission occurs due to excitation of the laser medium caused by the fluorescence, and the laser is emitted from the other end face of the optical fiber.

[0036] In the above structure, a single optical fiber can be an optical fiber used in so-called fiber lasers. Specifically, in the optical fiber used in the present invention, the surface and interior of the cladding portion are made of a material through which fluorescence emitted from the fluorescent material passes. Therefore, the fluorescence is configured to enter the outer surface (outer peripheral surface) of the cladding portion surrounding the core in the radiation direction (perpendicular to the extension direction (axial direction)) and reach the core. A single optical fiber can be such that multiple optical fibers are connected in series to form a single line. The laser medium dispersed in the core of the optical fiber can be a material commonly used in this field and capable of realizing laser oscillation in fiber lasers (such as neodymium ions and ytterbium ions), and the core can be made of glass (typically, quartz glass) doped with these ions. The first and second reflectors respectively disposed at both ends of the optical fiber can both be mechanisms that reflect light from fiber Bragg gratings (FBGs), etc., commonly used in fiber lasers. At the first reflector, the reflectivity can be set to 99.999% (in the above structure, the phrase "reflect all light" means that it is only necessary to reflect substantially all light, and for the purposes of the present invention, to reflect a permissible amount of light), and at the second reflector, which serves as the emitting end, the reflectivity can be set to 98%, etc. Thus, in short, the solar-pumped laser according to the present invention is configured such that a single optical fiber capable of operating as a fiber laser is wound around the edge surface of a fluorescent light guide plate.

[0037] According to the present invention, by utilizing the discovery that the wavelength band of reflected light from a dichroic mirror shifts towards a shorter wavelength side as the incident angle to the dichroic mirror increases, and that the amount of shift increases with the refractive index of the medium, a fluorescent light guide plate for collecting light can bring a larger amount of irradiated light into the plate and capture a larger amount of fluorescence within the plate. Therefore, the energy of the light applied to the fluorescent light guide plate can be concentrated more effectively at the edge surface. In the structure of the present invention, the fluorescent light guide plate absorbs sunlight, converts the wavelength, and then captures the light, thereby increasing the energy density and thus making advantageous use of sunlight. In other words, in the present invention, by utilizing the discovery that the wavelength band of reflected light from a dichroic mirror shifts towards a shorter wavelength side as the incident angle increases, the opposites are achieved: "using a large amount of energy by expanding the range of sunlight" and "capturing light." By using the structure of the fluorescent light guide plate of the present invention in a light-pumped laser excited by irradiated light (such as sunlight), it is desirable to achieve easier laser oscillation and an increase in the energy to be extracted as laser light.

[0038] Some embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals denote the same parts.

[0039] Basic structure and operation of fluorescent light guide plate

[0040] like Figure 1A and Figure 1B As shown, a fluorescent light guide plate 1 according to an embodiment of the present invention includes a body 2 having a plate-like structure defined by a receiving surface 2a (first surface) for receiving illumination light SL (such as sunlight), a rear surface 2d (second surface), and an edge surface 2c connecting the light receiving surface 2a and the rear surface 2d. A dichroic mirror (DM) 7 is stacked on the receiving surface 2a. A reflector 6 that reflects light without allowing it to pass through can be applied to the rear surface 2d. The fluorescent light guide plate body 2 has the following structure: a transparent or translucent material (e.g., resin, such as quartz glass, polycarbonate resin, PMMA, acrylic resin, silicone resin, fluoropolymer resin, and polyurethane resin) with a higher refractive index than the external space is used as a substrate, and a fluorescent material FM is dispersed in the interior 2b. The fluorescent material FM can be any material that emits fluorescence FL by absorbing illumination light SL, such as fluorescent pigments (rhodamine, lumogen, etc.), quantum dots (methylamine PbI3 (perovskite) quantum dots, PbS quantum dots, CdTe quantum dots, or Si quantum dots). Although not shown in the figure, the fluorescent material FM does not need to be dispersed throughout the entire interior 2b of the plate, and can be dispersed, for example, on the receiving surface 2a or the rear surface 2d. The shape of the fluorescent light guide plate body 2 in the surface direction is not limited to the circle shown in the figure, and can be any shape. DM7 can be a dielectric multilayer film prepared from SiO2, TiO2, etc. DM7 reflects light of a specific wavelength band and allows light of other wavelength bands to pass through.

[0041] In the basic operation of the fluorescent light guide plate 1, when an irradiation light SL, such as sunlight, passes through DM7 and is applied to the light receiving surface 2a, the fluorescent material FM in the plate is excited by the irradiation light SL, thereby emitting fluorescence FL. At this time, each individual particle of the fluorescent material FM emits fluorescence FL along the radiation direction; however, since the refractive index of the substrate of the plate structure is higher than that of the outside (usually air), when the fluorescence beam FL reaches the surface of the plate structure (receiving surface 2a or rear surface 2b), the beam with an incident angle greater than the critical angle undergoes total internal reflection. As a result, the beam is captured within the fluorescent light guide plate body 2, propagates while being repeatedly reflected, and is collected at the edge surface 2c. Similarly, for beams with an incident angle less than the critical angle, since the reflector 6 is arranged on the rear surface 2d, the fluorescence beam FL is reflected on the rear surface 2d and returns to the plate interior 2b. On the other hand, since the receiving surface 2a needs to allow the irradiation light SL to pass through and enter the plate interior, the reflector 6 cannot be arranged on the receiving surface 2a. However, by arranging DM7 such that light in the wavelength band of the irradiating light SL (i.e., light in the absorption wavelength band of the fluorescent material FM) passes through and reflects light in the wavelength band of the fluorescent material FM's fluorescence FL, the fluorescence beam FL propagating from the interior 2b of the plate to the receiving surface 2a can be reflected on DM7 and return to the interior 2b of the plate. In other words, when DM7 is appropriately selected, a fluorescence beam with an incident angle less than the critical angle on the surface of the plate structure can be captured in the interior 2b of the plate and collected at the edge surface 2c while being repeatedly reflected.

[0042] Dependence of the reflected light wavelength band of DM on the incident angle

[0043] As described above, when a DM7, which allows the illumination light SL to pass through and achieves the state of reflected fluorescence FL, is stacked on the receiving surface 2a of the fluorescent light guide plate 1, a large amount of fluorescence beam FL with an incident angle to the receiving surface 2a smaller than the critical angle can be collected at the edge surface 2c. However, in order to collect as much fluorescence as possible at the edge surface 2c more effectively under the condition of applying illumination light at a certain intensity, it is desirable to adjust the wavelength characteristics of the DM so that, in the illumination light SL, the DM allows the widest possible wavelength band of light to be absorbed into the fluorescent material to pass through, while reflecting the widest possible wavelength band of fluorescence from the fluorescent material FM. However, when sunlight is used as the illumination light SL, the wavelength band of sunlight is in the range of... Figure 2 In this context, Is represents a wide range. Typically, the wavelength band Af that absorbs light from the fluorescent material overlaps with the wavelength band If that reflects the fluorescence from the fluorescent material. Therefore, in DM, it is a conflicting requirement to extend the wavelength band as wide as possible so that the component of the irradiating light SL that will be absorbed into the fluorescent material can pass through, and to extend the wavelength band as wide as possible to reflect the fluorescent component from the fluorescent material.

[0044] Incidentally, as described in the overview, studies relating to this invention have revealed that the reflected light wavelength band of the DM shifts towards a shorter wavelength side as the incident angle of the beam to the DM increases, and the amount of shift increases with the refractive index of the medium. More specifically, based on simulations conducted in studies relating to this invention, firstly, as... Figure 3 As shown, this illustrates that as the incident angle of the beam to the DM increases from 10° to 40°, the reflected light wavelength band of the DM shifts towards the shorter wavelength side. Furthermore, it has been found that as the refractive index of the medium through which the incident light propagates increases, the amount of shift in the reflected light wavelength band towards the shorter wavelength side due to the increase in the incident angle of the beam to the DM also increases, as can be understood by comparing the top and bottom figures in the accompanying drawings. Figure 3 As understood from the bottom figure, it shows that when the refractive index of the medium is equal to the refractive index of the material used for the substrate of the fluorescent light guide plate body 2 (e.g., refractive index n = 1.47), and the incident angle of the light beam is 10°, even when the reflected light wavelength band of DM is set to cover the fluorescence wavelength band If of a certain fluorescent material, when the incident angle of the light beam becomes 40°, the fluorescence wavelength band If of the same fluorescent material can significantly deviate from the (after transfer) reflected light wavelength band of DM. In other words, it is understood that when Figure 3 In the example diagram at the bottom, when the DM is applied to a fluorescent light guide plate, fluorescent beams with an incident angle of up to about 20° are reflected on the DM; however, fluorescent beams with an incident angle of greater than or equal to 30° pass through the DM. When the reflected light wavelength band of the DM is set to the fluorescent wavelength band of the covering fluorescent material when the incident angle of the beam is relatively small, the illumination light SL in the band overlapping with the fluorescent wavelength band is also reflected on the DM and does not enter the interior of the plate, and the amount of fluorescence emitted is correspondingly reduced.

[0045] For the fluorescence beam FL emitted from the fluorescent material FM within the fluorescent light guide plate body 2, the loss before reaching the edge surface increases as the incident angle of the beam to the receiving surface 2a decreases. This is because, as in Figure 4The diagram schematically illustrates that in a fluorescent beam FL emitted along the radiation direction from a fluorescent material FM that has absorbed the illumination light SL, a beam with a smaller incident angle θ upon reaching the receiving surface 2a is reflected more times on the receiving surface 2a from the moment it is emitted from the fluorescent material FM until it reaches the edge surface 2c. Furthermore, when the beam is reflected on the receiving surface 2a, a portion of the beam passes through the receiving surface 2a, resulting in losses (as shown, a beam with an incident angle θ1 (< θ2) is reflected more times before reaching the edge surface than a beam with an incident angle θ2, and the losses can increase accordingly). Therefore, when the fluorescent beam is reflected by DM on the receiving surface of the fluorescent light guide plate and trapped in the plate, when the reflected wavelength band of DM is set such that beams with relatively large incident angles to the receiving surface are reliably reflected further and returned to the plate, the amount of fluorescence reaching the edge surface increases (even when beams with small incident angles are configured to be reflected on DM, the losses ultimately increase due to the large number of reflections before reaching the edge surface).

[0046] In this embodiment, the wavelength band of the reflected light of DM is set.

[0047] From the above discussion, in this embodiment, as Figure 5 As shown, a DM is fabricated and stacked on the receiving surface such that the reflected light wavelength band (R_0°) for a perpendicularly incident beam is within a wavelength range longer than the fluorescence wavelength band of the fluorescent material. Specifically, the lower limit (cutoff wavelength λ) of the reflected light wavelength band of the perpendicularly incident beam is... cut-off The peak wavelength λ is set within the fluorescence wavelength band. peak Within a long wavelength range. Using the above structure, for a fluorescence beam from the fluorescent material inside plate 2b, the reflected wavelength band of the DM, which is directed towards the receiving surface at a relatively large angle of incidence, shifts towards a wavelength shorter than the reflected wavelength band R_0° of the perpendicularly incident beam. As a result, the width of the reflected wavelength band of the DM, which overlaps with the fluorescence wavelength band If of the fluorescence beam from the fluorescent material, increases, or, as in the case of R to 30°, the reflected wavelength band of the DM covers the fluorescence wavelength band If. Therefore, beams with a relatively large angle of incidence to the receiving surface (having fewer reflections before reaching the edge surface) are further reliably reflected on the DM and trapped inside the plate. Thus, a large amount of fluorescence beam can reach the edge surface. On the other hand, for the irradiation light SL to be applied to the receiving surface, sunlight, etc., typically arrives at the receiving surface from above at a relatively small angle of incidence. Therefore, when the reflected light wavelength band of the DM for a beam with a small incident angle is within a wavelength range longer than the fluorescence wavelength band of the fluorescent material, the irradiation light component in the broad wavelength band (including light in the band overlapping with the fluorescence wavelength band) enters the interior of the plate and can increase the excitation amount of the fluorescent material, thus resulting in an increase in the amount of fluorescence produced.

[0048] In embodiments, for example, when the irradiation light is sunlight, the incident angle of the irradiation light is typically about 0° to 30°. The number of times the fluorescence beam in the fluorescent light guide plate is reflected on the receiving surface after being emitted from the fluorescent material and before reaching the edge surface is approximately one-third of that in the case of an incident angle of 10°, when the incident angle is 30°. Therefore, a reflected light wavelength band perpendicular to the incident beam can be prepared such that when, for example, the incident angle is greater than or equal to 30°, the fluorescence beam is reflected on the DM, and when the incident angle is less than 30°, the fluorescence beam passes through the DM.

[0049] Applications of fluorescent light guide plates

[0050] The fluorescent light guide plate of this embodiment can concentrate and collect the energy of the irradiated light by converting the irradiated light (such as sunlight) into fluorescence in the plate and collecting the fluorescence at the edge surface of the plate. The fluorescent light guide plate of this embodiment can be used to provide light to solar cells or photoelectric converters.

[0051] The fluorescent light guide plate of this embodiment can be used as the fluorescent light guide plate of the solar-pumped laser described in JP2017-168662A, JP2018-018981A, or JP2020-065027A. In the solar-pumped laser 10 using the fluorescent light guide plate of this embodiment, as... Figures 6A to 6D As shown, the fiber section 3, made of a single optical fiber 3a, is wound around the edge surface of the basically disc-shaped fluorescent light guide plate 1 (for illustration, the structure of each part in the figure is shown schematically, and the scaling ratio of the actual device can be significantly different).

[0052] exist Figures 6A to 6C In the structure shown, more specifically, such as Figure 1A and Figure 1B As shown, the fluorescent light guide plate 1 is defined by a sunlight-receiving surface 2a (surface) for receiving sunlight SL, a rear surface, and an edge surface 2c connecting the sunlight-receiving surface 2a and the rear surface, and is made of a material in which fluorescent material is dispersed and which has a higher refractive index than the external space. Specifically, the fluorescent material FM can be a material that absorbs sunlight to emit fluorescence in the absorption wavelength band of a laser medium doped in the core of the optical fiber 3a (described later). The dimensions of the fluorescent light guide plate 1 and the area between the sunlight-receiving surface 2a and the edge surface 2c are designed, as described later, to satisfy the conditions for laser oscillation.

[0053] In the optical fiber section 3, the optical fiber 3a is an optical fiber that can be used in a fiber laser, and as shown in the attached figure, it can be configured such that a single optical fiber 3a is wound around the edge surface 2c of the fluorescent light guide plate 1 along the circumference of the fluorescent light guide plate 1. The optical fiber 3a can be configured such that a single optical fiber 3a is wound multiple times around the edge surface 2c of the fluorescent light guide plate 1, and the optical fiber 3a can be configured such that the single optical fiber 3a is densely wound (so that adjacent surfaces are in contact with each other) around the edge surface 2c of the fluorescent light guide plate 1. Figure 6D As shown in the schematic cross-sectional diagram, the optical fiber 3a can have the following structure: a core 3c made of a glass material doped with the laser medium LM is surrounded on its outer periphery by a cladding portion made of a glass material having a lower refractive index than the core 3c, and Fabry-Perot resonators can be configured by placing reflectors at both ends 4 and 5 of the optical fiber 3a to reflect the light propagating in the fiber (at least the components of the wavelength band of the light to be emitted from the laser medium). Systems commonly used in fiber lasers, such as fiber Bragg gratings (FBGs), can be used as reflectors. The reflectivity of each reflector is adjusted so that a portion of the light propagating in the fiber passes through end 4, which serves as the laser emission end. Specifically, the reflectivity can be adjusted to 99.999% at end 5 to reflect all light (since the excitation light does not need to enter from the end, it is unnecessary for the excitation light to pass through), and adjusted to 98% at end 4 on the side where the laser is extracted, etc. Specifically, in the solar-pumped laser 10 of this embodiment, since the fluorescence emitted from the edge surface 2c of the fluorescent light guide plate 1 enters through the outer peripheral surface of the wound optical fiber 3a, the surface of the cladding portion is set to an uncoated state, or when coating is performed, a material that allows light with a wavelength of fluorescence dispersed in the fluorescent light guide plate 1 to pass through is used as the coating material. The cladding portion can be made of multiple layers. Other components, conditions, and operation of the solar-pumped laser 10 can be similar to JP2017-168662A, JP2018-018981A, and JP2020-065027A.

[0054] Although embodiments of the invention have been described above, many modifications and changes can be readily made by those skilled in the art, and the invention is not limited to the embodiments shown above. It is evident that the invention can be applied to various devices without departing from its spirit.

Claims

1. A fluorescent light guide plate, characterized in that... include: First surface; Second surface; An edge surface that connects the periphery of the first surface to the periphery of the second surface; as well as A dichroic mirror, which is stacked on the first surface, wherein: The fluorescent material that absorbs illumination light applied to the first surface to emit fluorescence is dispersed in at least one of the following locations: inside the space defined by the first surface, the second surface, and the edge surface; on the first surface; or on the second surface. The fluorescent light guide plate has a plate-like structure made of a material with a higher refractive index than the outside; The fluorescent light guide plate is configured such that when the irradiation light enters from the first surface, the fluorescence emitted from the fluorescent material exits from the edge surface; and The reflected light wavelength band of the perpendicularly incident beam reflected by the dichroic mirror is within a wavelength range longer than the peak wavelength band of the fluorescent material; wherein... The reflected light wavelength band of the first beam reflected by the dichroic mirror overlaps with the fluorescent wavelength band of the fluorescent material. The first beam is a beam with an incident angle greater than a first predetermined angle in the direction from the inside of the fluorescent light guide plate toward the first surface. The reflected light wavelength band of the first light beam reflected by the dichroic mirror includes the fluorescence wavelength band of the fluorescent material; The reflected light wavelength band of the dichroic mirror is set such that a second light beam passes through the dichroic mirror. The second light beam is a light beam with an incident angle of less than a second predetermined angle in the direction from the outside of the fluorescent light guide plate toward the first surface, and the second light beam is a light beam with a wavelength capable of exciting the fluorescent material.

2. A solar-pumped laser, characterized in that... include: The fluorescent light guide plate according to claim 1; as well as An optical fiber is wound circumferentially around the edge surface of the fluorescent light guide plate along the first and second surfaces, wherein: The optical fiber includes: The laser medium is dispersed in the core. The cladding portion, the surface and interior of which are made of a material through which the fluorescence passes, has a lower refractive index than the core portion. A first reflector, configured to reflect all light emitted from the laser medium from one end face of the optical fiber, and A second reflector is configured to allow a portion of the light emitted from the laser medium to pass through the other end face of the optical fiber, and to reflect the remaining portion of the light; and The optical fiber is configured such that the fluorescence emitted from the edge surface of the fluorescent light guide plate passes through the surface of the cladding portion and reaches the core portion, a laser is generated due to the excitation of the laser medium caused by the fluorescence, and the laser is emitted from the other end face of the optical fiber.

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