Fiber Bragg grating element and manufacturing method thereof

Abstract

Provided are: a fiber bragg grating element in which a fiber bragg grating that suppresses power loss of light is formed in a small-diameter region; and a method for manufacturing the fiber bragg grating element. An original optical fiber (31) is prepared, the original optical fiber (31) being formed from a quartz glass in which a core (21) and a cladding (22) each contain a light-sensitive material. A tapered optical fiber 11 is prepared by heating and stretching an original optical fiber 31 in which Ge (germanium) having an increased refractive index is used as a light-sensitive material and the refractive index of a core 21 is higher than the refractive index of a cladding 22 by setting the concentration of the light-sensitive material with respect to the core 21 to be higher than that of the cladding 22. The narrow-diameter region of the tapered optical fiber 11 is irradiated with ultraviolet rays by a phase mask method, and periodic refractive index modulation is formed in the core 21 and the cladding 22.

Description

Technical Field

[0001] The present invention relates to an optical fiber Bragg grating element and a method for manufacturing the same. Background Art

[0002] There is known an optical fiber Bragg grating element in which an optical fiber Bragg grating having a periodically varying refractive index is formed in the core of an optical fiber. For example, by irradiating interference light of an ultraviolet laser to generate interference fringes on a part of the optical fiber having a core containing a photosensitive material, the interference fringe pattern is used to impart a refractive index change to the core, thereby preparing an optical fiber Bragg grating.

[0003] In addition, there is known a tapered optical fiber having a narrow-diameter region called a nanofiber. In this tapered optical fiber, for example, a part of an optical fiber having a normal diameter (e.g., 100 μm or more) is heated and stretched to reduce the diameter to submicrons to form a narrow-diameter region with a reduced diameter. Further, in the tapered optical fiber, a transition region in which the diameter continuously decreases toward the narrow-diameter region is formed between the normal-diameter region and the narrow-diameter region (see, for example, Patent Document 1). In the narrow-diameter region, the entire core and cladding function as a core for propagating light, and the atmosphere or vacuum space outside the narrow-diameter region functions as a cladding. Since the tapered optical fiber has a strong light confinement effect and a large evanescent field, it is applied to fields such as optical engineering and quantum optics.

[0004] There is known an element in which an optical fiber Bragg grating is formed in the narrow-diameter region of the above-mentioned tapered optical fiber (see Non-Patent Document 1). In Non-Patent Document 1, by causing the +1st-order diffracted light and the -1st-order diffracted light after diffraction of a laser beam by a spectroscopic element to interfere on the narrow-diameter region, and ablating the narrow-diameter region with the interference fringe pattern, a plurality of recesses are periodically arranged and formed in the narrow-diameter region, and this part functions as an optical fiber Bragg grating.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Laid-Open No. 2016-153850

[0008] Non-Patent Documents

[0009] Non-Patent Document 1: K.P. Nayak and K. Hakuta, “Photonic crystal formation on optical nanofibers using femtosecond laser ablation technique” Vol. 21, Issue 2, pp. 2480 - 2490 (2013) Summary of the Invention

[0010] Technical Problem to be Solved by the Invention

[0011] However, in a structure in which a recess is provided in a thin-diameter region as in the fiber Bragg grating described in Non-Patent Document 1, there is a problem of large power loss due to light scattering by the recess.

[0012] The present invention has been completed in view of the above circumstances, and an object thereof is to provide a fiber Bragg grating element and a method for manufacturing the same, in which a fiber Bragg grating that suppresses power loss of light is formed in a thin-diameter region.

[0013] Technical Solution for Solving the Technical Problem

[0014] The method for manufacturing a fiber Bragg grating element of the present invention includes: a preparation step of preparing an optical fiber having a layer structure composed of a core and one or more outer peripheral layers, the one or more outer peripheral layers including a cladding that surrounds the core and has a refractive index lower than that of the core and is formed of a transparent optical material, and at least any one of the one or more outer peripheral layers including a photosensitive material; a stretching step of forming a thin-diameter region with a reduced diameter and a pair of transition regions whose diameters continuously and gradually increase from both ends of the thin-diameter region by heating and stretching the optical fiber; and an exposure step of irradiating a part of the thin-diameter region with light to cause a photoinduced refractive index change, thereby forming a fiber Bragg grating in which the refractive index of the outer peripheral layer containing the photosensitive material changes periodically.

[0015] The method for manufacturing a fiber Bragg grating element of the present invention includes: a preparation step of preparing an optical fiber having a layer structure composed of a core and a cladding that surrounds the core and has a refractive index lower than that of the core, and the core and the cladding each contain a photosensitive material; a stretching step of forming a thin-diameter region with a reduced diameter and a pair of transition regions whose diameters continuously and gradually increase from both ends of the thin-diameter region by heating and stretching the optical fiber; and an exposure step of irradiating a part of the thin-diameter region with light to cause a photoinduced refractive index change, thereby forming a fiber Bragg grating in which the refractive index of the cladding changes periodically together with the core.

[0016] The fiber Bragg grating element of the present invention includes: a tapered optical fiber having a layer structure composed of a core and one or more outer peripheral layers, the one or more outer peripheral layers including a cladding that surrounds the core and has a refractive index lower than that of the core and is formed of an optical material, and at least any one of the one or more outer peripheral layers is formed to contain a photosensitive material, and a thin-diameter region is integrally provided at one end of a transition region where the diameter continuously and gradually decreases; and a fiber Bragg grating formed in the thin-diameter region, in which the refractive index of the outer peripheral layer containing the photosensitive material changes periodically.

[0017] The fiber Bragg grating element of the present invention has: a tapered fiber having a layer structure composed of a core and a cladding that surrounds the core and has a refractive index lower than that of the core, the core and the cladding being formed of an optical material containing a photosensitive material, and a fine-diameter region being integrally provided at one end of a transition region where the diameter continuously and gradually decreases; and a fiber Bragg grating formed in the fine-diameter region, the refractive index of the cladding varying periodically together with the core.

[0018] Advantages of the Invention

[0019] According to the present invention, in an optical fiber, at least any one of one or more outer peripheral layers including the cladding contains a photosensitive material. The optical fiber is heated and stretched to form a fine-diameter region, and interference light is irradiated onto the fine-diameter region to form a fiber Bragg grating in which the refractive index of the cladding varies periodically together with the core. Therefore, a fiber Bragg grating element with suppressed optical power loss can be easily manufactured. In addition, the fiber Bragg grating element manufactured in this way can suppress optical power loss.

[0020] According to the present invention, an optical fiber in which the core and the cladding contain a photosensitive material is heated and stretched to form a fine-diameter region, and interference light is irradiated onto the fine-diameter region to form a fiber Bragg grating in which the refractive index of the cladding varies periodically together with the core. Therefore, a fiber Bragg grating element with suppressed optical power loss can be easily manufactured. In addition, the fiber Bragg grating element manufactured in this way can suppress optical power loss. Description of the Drawings

[0021] Figure 1 is a cross-sectional view showing a fiber Bragg grating element of an embodiment.

[0022] Figure 2 is an explanatory diagram schematically showing a fiber Bragg grating formed in the fine-diameter region.

[0023] Figure 3 is a flowchart showing the preparation steps of the fiber Bragg grating element.

[0024] Figure 4 is an explanatory diagram showing a heating and stretching device when preparing the tapered fiber.

[0025] Figure 5 is an explanatory diagram showing the irradiation state of ultraviolet light by the phase mask method.

[0026] Figure 6 is an explanatory diagram schematically showing a cross-section of a three-layer-structured tapered fiber. Detailed Embodiments

[0027] In Figure 1In [the figure], the fiber Bragg grating element 10 is formed by forming a fiber Bragg grating 14 in the narrow-diameter region 12 of the tapered fiber 11. The tapered fiber 11 includes: a narrow-diameter region 12, a fiber region 16, and a transition region 18. In this example, the fiber regions 16 are integrally formed at both ends of the narrow-diameter region 12 via the transition regions 18. For example, the fiber regions 16 are used to incident light into the narrow-diameter region 12, and to emit and propagate light from the narrow-diameter region 12.

[0028] The fiber Bragg grating 14 reflects light of a specific wavelength in the propagated light. In this example, a pair of fiber Bragg gratings 14 with the same characteristics are provided at a prescribed interval on the narrow-diameter region 12, and the fiber Bragg grating element 10 constitutes a microfiber optical resonator. That is, the pair of fiber Bragg gratings 14 form a Fabry–Pérot type microresonator that repeatedly reflects light of a specific wavelength between them to form a standing wave.

[0029] Any cross-section of the tapered fiber 11 that is orthogonal to the axial direction (the direction of light propagation, Figure 1 the left and right directions) is circular, and the tapered fiber 11 has a layer structure in which the core 21 is disposed at the center and the cladding 22 surrounds the periphery of the core 21. The tapered fiber 11, that is, its core 21 and cladding 22, are made of a silica-based glass formed by adding a photosensitive material to silica glass. The core 21 and the cladding 22 are continuous in the axial direction of the tapered fiber 11. The refractive index of the core 21 is higher than that of the cladding 22. It should be noted that, in this example, the cladding 22 is an outer peripheral layer, and this single layer of the cladding 22 constitutes the outer peripheral layer portion. In addition, Figure 1 the size of the diameter of the core 21 relative to the diameter of the cladding 22, etc. are exaggeratedly depicted in [the figure], and in addition, to avoid complication of the drawings, the shading is omitted. Regarding Figure 2 and Figure 3 , it is the same. In this example, the tapered fiber 11 is made of a silica-based glass as the optical material, but the optical material may also be a plastic optical fiber or a fluoride glass optical fiber.

[0030] As will be described in detail later, the narrow-diameter region 12 and the transition region 18 are prepared by heating (softening) and stretching a part of the optical fiber, and this optical fiber has the same structure as a general single mode used for optical communication, etc. Regarding the optical fiber used when preparing the tapered fiber 11 in this way, it is not limited to a single mode type of optical fiber, and it may also be a multimode type of optical fiber such as SI (step index) type, GI (graded index) type, a polarization maintaining type of optical fiber, etc. The fiber region 16 may be set to have the same structure as the optical fiber used. In addition, in the tapered fiber 11, both the core 21 and the cladding 22 contain a photosensitive material, but as described above, the refractive index of the core 21 is higher than that of the cladding 22.

[0031] Regarding the optical fiber region 16, the outer diameter of its part (the outer diameter of the cladding 22) and the diameter of the core 21 are the same as those of a general single-mode optical fiber. For example, the outer diameter of the optical fiber region 16 is about 125 μm, and the diameter of the core 21 is about several μm.

[0032] The outer diameter of the narrow-diameter region 12 is thinner than that of the optical fiber region 16, for example, the same degree as or smaller than the wavelength of the propagating light. There is no particular limitation on the wavelength of the light propagating in the tapered optical fiber 11. For example, when the tapered optical fiber 11 is made of silica-based glass, it can be set within the range of 400 nm to 2000 nm generally used in such silica-based glass optical fibers. The ratio of the diameter of the core 21 to the outer diameter of the cladding 22 in the narrow-diameter region 12 and the transition region 18 is the same as their ratio in the optical fiber region 16. Therefore, the diameter of the core 21 in the narrow-diameter region 12 is about several nm. In such a narrow-diameter region 12 where the diameter becomes thinner, the entire narrow-diameter region 12, that is, the core 21 and the cladding 22, act as a core, and in addition, the space (gas, vacuum, or liquid) around the narrow-diameter region 12 acts as a cladding to propagate light.

[0033] The transition region 18 has a shape in which the diameter continuously and gradually decreases from the optical fiber region 16 toward the narrow-diameter region 12, that is, a tapered shape that tapers from the optical fiber region 16 toward the narrow-diameter region 12. This transition region 18 reduces the optical loss when light propagates between the optical fiber region 16 and the narrow-diameter region 12. The transition region 18 is preferably set to a shape that satisfies the adiabatic condition where the loss caused by the coupling between the fundamental mode and the higher-order modes is sufficiently small.

[0034] In Figure 2 , the fiber Bragg grating 14 provided on the narrow-diameter region 12 is formed as a refractive index modulation region, and the refractive index of the refractive index modulation region changes (increases and decreases) along the axial direction at a prescribed period in the core 21 and the cladding 22. As described above, in the narrow-diameter region 12, the entire narrow-diameter region 12, that is, the core 21 and the cladding 22, act as a core, but due to the periodic refractive index modulation formed in the core 21 and the cladding 22 in the fiber Bragg grating 14, the refractive index of the light propagating in the narrow-diameter region 12 is effectively modulated. Therefore, due to the fiber Bragg grating 14, the light of a specific wavelength corresponding thereto in the light propagating in the narrow-diameter region 12 is reflected. At this time, since the fiber Bragg grating 14 is realized by the refractive index modulation of the core 21 and the cladding 22, there is no light scattering in the case where no recess is formed in the narrow-diameter region 12, and the power loss is small.

[0035] The above-mentioned fiber Bragg grating 14 is formed by using a light-induced refractive index change, which is generated by irradiating ultraviolet light onto the core 21 and the cladding 22 containing a photosensitive material as described later. The fiber Bragg grating 14 is formed in the following manner: When the wavelength (Bragg wavelength) to be reflected by the fiber Bragg grating 14 is set to λ b , the effective refractive index n of the narrow-diameter region 12 eff , and the modulation period Λ of the refractive index satisfy "λ b = 2n eff ·Λ". In this example, ultraviolet light is used as the light that causes the light-induced refractive index change, but it is not limited thereto. For example, in the case of using a light-induced refractive index change such as a multi-photon absorption process including a two-photon absorption process, light having a wavelength longer than ultraviolet light can be used.

[0036] The preparation steps of the above-mentioned fiber Bragg grating element 10 will be described. As Figure 3 shown, a fiber (hereinafter referred to as the original fiber) 31 that contains a photosensitive material in the core 21 and the cladding 22 respectively is prepared (preparation process), and the Bragg grating element 10 is prepared from the original fiber 31. More specifically, the tapered fiber 11 is prepared by heating and stretching the original fiber 31 through a stretching process, and the fiber Bragg grating 14 is formed in the narrow-diameter region 12 of the tapered fiber 11 by using a light-induced refractive index change through an exposure process.

[0037] A part of the original fiber 31 directly becomes the fiber region 16, and another part is stretched to become the narrow-diameter region 12 and the transition region 18. Therefore, the original fiber 31 has a two-layer structure of the core 21 and the cladding 22, and the core 21 and the cladding 22 contain a photosensitive material. In addition, the core 21 and the cladding 22 of the original fiber 31 are made of a quartz-based glass formed by adding a photosensitive material to quartz glass. Furthermore, the diameter of the core 21 of the original fiber 31 and the outer diameter of the original fiber 31 (cladding 22) are the same as those of the fiber region 16. It should be noted that the original fiber 31 is the same as a general single-mode fiber except that the core 21 and the cladding 22 contain a photosensitive material.

[0038] In this example, the core 21 and the cladding 22 of the original fiber 31 contain the same photosensitive material, and the refractive indices of the core 21 and the cladding 22 are controlled by this photosensitive material. As the photosensitive material, Ge (germanium) whose refractive index becomes higher due to its addition is used. Therefore, the refractive indices of both the core 21 and the cladding 22 are higher than those in the case of being composed of pure quartz. The reason for including the photosensitive material not only in the core 21 but also in the cladding 22 is that in the narrow-diameter region 12, as described above, a light-induced refractive index change is caused not only in the core 21 but also in the cladding 22 to form a refractive index modulation.

[0039] In the original optical fiber 31, there is a difference in the concentration of photosensitive materials between the core 21 and the cladding 22, so that the refractive index of the core 21 is higher than that of the cladding 22, in order to transmit light in the same way as a common optical fiber. Therefore, in this example, the Ge concentration in the core 21 is higher than that in the cladding 22.

[0040] As long as the photosensitive material is a material that imparts photosensitivity, that is, a material that causes a photoinduced refractive index change, it is not limited to Ge. In addition, the photosensitive material may be a photosensitive material that reduces the refractive index by its addition. Examples of photosensitive materials that reduce the refractive index by addition include B (boron) and F (fluorine). A plurality of types of photosensitive materials may also be added to the core 21 and the cladding 22. For example, B and Ge may also be added together as photosensitive materials. In this case, the photosensitivity can be improved compared with the case of adding Ge alone. In addition, it may be that the added photosensitive material or the combination of photosensitive materials is different between the core 21 and the cladding 22.

[0041] Regarding the original optical fiber 31, as long as photosensitivity is imparted to the cladding 22 and the refractive index of the core 21 is higher than that of the cladding 22, a photosensitive material that imparts photosensitivity by addition but does not affect or hardly changes the refractive index can also be used. In addition, a refractive index control material may also be used together with the photosensitive material to increase or decrease the refractive index by adding the refractive index control material without imparting photosensitivity. For example, the original optical fiber 31 may also be an optical fiber obtained by adding a photosensitive material that increases the refractive index by addition to the core 21 and a photosensitive material that decreases the refractive index by addition to the cladding 22. In addition, it may be that only a photosensitive material is added to the cladding 22 so that the refractive index is lower than that of the core 21 by addition. In addition, the original optical fiber 31 may also be an optical fiber obtained by adding a photosensitive material or a refractive index control material that increases the refractive index to the core 21 and a photosensitive material that only imparts photosensitivity and hardly changes the refractive index to the cladding 22. It should be noted that for the photosensitive material and the refractive index control material, photosensitive materials and refractive index control materials corresponding to the optical materials of the original optical fiber 31 are used.

[0042] It should be noted that since the photosensitive material is added to the core 21 and the cladding 22 of the original optical fiber 31 as described above, the core 21 and the cladding 22 in the narrow diameter region 12, the optical fiber region 16, and the transition region 18 of the tapered optical fiber 11 also contain the photosensitive material in the same way as the original optical fiber 31.

[0043] In the stretching process, for example, the narrow diameter region 12 and the transition region 18 are formed by the flame ablation method. Specifically, for example, use Figure 4The heating and stretching device 35 shown heats the original optical fiber 31 while axially pulling both ends of the original optical fiber 31 to extend it. In the heating and stretching device 35, heating scanning for moving the heating position of the original optical fiber 31 is performed by reciprocating a welding torch 36 as a heating unit along the axis of the original optical fiber 31. Further, while performing this heating scanning, the original optical fiber 31 is axially pulled by moving a pair of jigs 37 in directions away from each other, thereby stretching a portion of the original optical fiber 31 softened by heating. The pair of jigs 37 respectively hold both end portions of the heating region sandwiching the original optical fiber 31.

[0044] For example, the moving range is controlled by making the moving speed of the welding torch 36, i.e., the moving speed of the heating position, constant, and while pulling both ends of the original optical fiber 31 at a constant speed with a pair of jigs 37, the heated range of the original optical fiber 31 is deformed into a thin-diameter region 12 having a desired length and thickness, and a pair of transition regions 18 having a specified shape satisfying, for example, heat insulation conditions. Details of the method for preparing the tapered optical fiber 11 by such a flame ablation method are described in, for example, Japanese Unexamined Patent Application Publication No. 2016-153850. It should be noted that the method for preparing the tapered optical fiber 11 from the original optical fiber 31 is not limited to the flame ablation method.

[0045] In the exposure process, for the tapered optical fiber 11 prepared by the stretching process, an optical fiber Bragg grating 14 is formed by, for example, the phase mask method. In the phase mask method, as Figure 5 shown, the thin-diameter region 12 of the tapered optical fiber 11 is disposed close to a phase mask 38 having grooves formed at a specified pitch (period). In this state, an ultraviolet laser beam is irradiated onto the thin-diameter region 12 via the phase mask 38. Thereby, interference fringes formed by interfering the +1st-order diffracted light and the -1st-order diffracted light of the ultraviolet laser beam diffracted by the phase mask 38 are formed at the position of the thin-diameter region 12, and the ultraviolet light (interference light) intensity-modulated in the axial direction of the thin-diameter region 12 with the above period Λ is irradiated onto the thin-diameter region 12. In this example, in order to efficiently cause a change in the optical refractive index in the Ge-added thin-diameter region 12 (cladding 22), a deep ultraviolet laser beam in the deep ultraviolet region (wavelength 100 to 280 nm) is irradiated onto the thin-diameter region 12 via the phase mask 38, but the wavelength of the ultraviolet light is not limited to this. It should be noted that in order to form a pair of optical fiber Bragg gratings 14, the irradiation position of the ultraviolet laser beam with respect to the thin-diameter region 12 is shifted, and the ultraviolet laser beam is irradiated in the same manner.

[0046] As described above, in the case of using light-induced refractive index change using a photon absorption process of, for example, two or more photons, in order to form interference fringes in the thin-diameter region 12, it is sufficient to irradiate a laser beam (interference light) with a longer wavelength.

[0047] Since Ge, which is a photosensitive material, is added to each of the core 21 and the cladding 22 in the narrow-diameter region 12, in the core 21 and the cladding 22 of the narrow-diameter region 12, due to the change in the photoinduced refractive index, the refractive index of the portion irradiated with ultraviolet light increases as the irradiation amount increases. Therefore, when irradiating the intensity-modulated ultraviolet light as described above, according to the intensity modulation, the refractive indices of the core 21 and the cladding 22 in the narrow-diameter region 12 are modulated. Therefore, the fiber Bragg grating 14 is formed in the narrow-diameter region 12, and thus the fiber Bragg grating element 10 is obtained.

[0048] It should be noted that in this example, the fiber Bragg grating 14 is formed by the phase mask method, but the light irradiation method is not particularly limited. For example, the interference fringes of ultraviolet light can also be formed at the position of the narrow-diameter region 12 by using the two-beam interference method, so as to form the refractive index modulation in the core 21 and the cladding 22 of the narrow-diameter region 12. In addition, like the point-by-point method (dot painting method), while moving the narrow-diameter region 12 in its longitudinal direction, ultraviolet light, for example, whose beam width is narrowed by a slit or a condenser lens, is intermittently irradiated onto the narrow-diameter region 12, so that the photoinduced refractive index change occurs in the irradiated portion, thereby forming the fiber Bragg grating 14.

[0049] As described above, since the fiber Bragg grating element 10 is prepared by using the original optical fiber 31 containing the photosensitive material in the core 21 and the cladding 22, the refractive index modulation can be formed in the core 21 and the cladding 22 of the narrow-diameter region 12 by the same method as in the case of forming the fiber Bragg grating on the ordinary-diameter optical fiber, and the formation of the fiber Bragg grating 14 becomes easy.

[0050] As described above, the fiber Bragg grating 14 only needs to effectively modulate the refractive index of the narrow-diameter region 12 for the light propagating in the narrow-diameter region 12 that functions as the core for propagating light as a whole. In the narrow-diameter region 12, since the cross-sectional area of the core 21 is quite small, the presence or absence of the refractive index modulation in the core 21 has almost no influence on the light to be propagated. Therefore, the tapered optical fiber 11 can be fabricated by using the original optical fiber 31 containing the photosensitive material only in the cladding 22, and the refractive index modulation is performed only in the cladding 22 of the narrow-diameter region 12 to form the fiber Bragg grating 14. In this case, for example, if the core 21 and the cladding 22 use the same optical material, only the photosensitive material (such as B, F, etc.) that reduces the refractive index needs to be used.

[0051] The tapered optical fiber 11 (the narrow-diameter region 12, the optical fiber region 16, and the transition region 18) in the above example has a two-layer structure including a core 21 and a cladding 22 as an outer peripheral layer, but may also have a layer structure of three or more layers. The core 21 may be composed of multiple layers, and the outer peripheral layer portion composed of an outer peripheral layer such as a cladding as described later may also be composed of multiple layers.

[0052] Figure 6 An example in which the tapered optical fiber 11 has a three-layer structure is shown. In this example, the tapered optical fiber 11 has a three-layer structure including a core 21, a first outer peripheral layer 41 provided around the core 21, and a second outer peripheral layer 42 provided around the first outer peripheral layer 41, and a two-layer outer peripheral layer portion 43 is formed by the first outer peripheral layer 41 and the second outer peripheral layer 42.

[0053] For example, the first outer peripheral layer 41 is an inner cladding having a refractive index lower than that of the core 21, and the second outer peripheral layer 42 is an outer cladding having a refractive index lower than that of the first outer peripheral layer 41, and the tapered optical fiber 11 has the same layer structure as a so-called double-clad optical fiber. In the tapered optical fiber 11 having such a layer structure, a fiber Bragg grating 14 can be formed by refractive index modulation of one or both of the first outer peripheral layer 41 and the second outer peripheral layer 42 in a tapered optical fiber in which the photosensitive material is included only in one or both of the first outer peripheral layer 41 and the second outer peripheral layer 42.

[0054] The outer peripheral layer portion 43 may have any structure as long as it can confine light in the original optical fiber 31 (optical fiber region 16). Regarding the refractive index of the outer peripheral layer that does not affect the confinement of light, there is no particular limitation on the relationship between the refractive index of the outer peripheral layer and the refractive index of the outer peripheral layer inside it. For example, when the first outer peripheral layer 41 is a cladding that confines light, the second outer peripheral layer 42 may be a protective layer made of pure quartz, for example, that does not have a cladding function.

[0055] The outer peripheral layer portion 43 may have a layer structure having three or more outer peripheral layers. When the outer peripheral layer portion 43 has a layer structure having a plurality (two or more) of outer peripheral layers, as long as one or more of the outer peripheral layers constituting the outer peripheral layer portion 43 contain a photosensitive material and effectively modulate the refractive index of the narrow-diameter region 12 for the light propagating in the narrow-diameter region 12. Regarding the outer peripheral layer that modulates the refractive index by including the photosensitive material, whether it is a cladding or other than the cladding, as long as one or more of the outer peripheral layers are selected, or all of the outer peripheral layers are selected, and one or more of the selected outer peripheral layers contain a photosensitive material to modulate the refractive index. Therefore, it is also possible to select only one or more claddings, or one or more outer peripheral layers other than the cladding, and make one or more of the selected outer peripheral layers contain a photosensitive material to modulate the refractive index.

[0056] For example, as described above, in the case where the first outer peripheral layer 41 is a cladding layer and the second outer peripheral layer 42 is a protective layer, only the first outer peripheral layer 41 serving as the cladding layer or only the second outer peripheral layer 42 serving as the protective layer is selected, and the photosensitive material is included only in the selected cladding layer or protective layer, and its refractive index is modulated. Of course, both the first outer peripheral layer 41 and the second outer peripheral layer 42 can be selected. In addition, in the outer peripheral layer portion having a three-layer structure including an inner cladding layer and an outer cladding layer provided as the first and second outer peripheral layers and a protective layer provided so as to surround the outer cladding layer, one or more outer peripheral layers are selected from the inner cladding layer, the outer cladding layer, and the protective layer, and the refractive index is modulated by including the photosensitive material in the selected outer peripheral layer.

[0057] It should be noted that as the outer peripheral layer containing the photosensitive material, it is preferable to select the outer peripheral layer having the largest ratio of the cross-sectional area in the cross-section orthogonal to the axial direction of the tapered optical fiber 11. In addition, as the outer peripheral layer containing the photosensitive material, it is preferable to select one or more outer peripheral layers so that the total cross-sectional area is 50% or more.

[0058] Although the above example of forming a pair of fiber Bragg gratings in the narrow diameter region has been described, the fiber Bragg gratings to be formed are not limited thereto. For example, one or any number of fiber Bragg gratings can be formed in the narrow diameter region. In addition, the fiber Bragg gratings are not limited to those having a constant refractive index modulation period. The profiles of the fiber Bragg gratings such as the magnitude of the refractive index modulation, the period of the gradient, and the length are arbitrary. The refractive index of the fiber Bragg grating does not need to change at a constant period. For example, the fiber Bragg grating formed in the narrow diameter region can be a chirped fiber Bragg grating in which the refractive index modulation period continuously changes in the longitudinal direction (light propagation direction) of the optical fiber, or a phase-shifted fiber Bragg grating in which a phase shift portion is provided in the refractive index modulation region.

[0059] In addition, the refractive index distribution of the fiber Bragg grating to be formed can also be set to a Gaussian apodization type or the like other than the uniform type having a constant amplitude of a sine wave. In addition, a micro-optical resonator can be constituted by forming, for example, a chirped fiber Bragg grating in the narrow diameter region, and the chirped fiber Bragg grating has a profile in which the refractive index modulation period continuously increases gradually and then continuously decreases gradually, or continuously decreases gradually and then continuously increases gradually in one direction of the longitudinal direction of the narrow diameter region. In addition, a micro-optical resonator can be formed by forming a fiber Bragg grating having a structure in which only the period of the grating is changed in a narrow region near the center of the Bragg grating, or the amplitude of the refractive index modulation in the narrow region near the center of the Bragg grating is reduced, or a structure in which these structures are combined.

[0060] Description of Reference Numerals

[0061] 10 Fiber Bragg grating element

[0062] 11 Tapered optical fiber

[0063] 12 Thin diameter region

[0064] 14 Fiber Bragg grating

[0065] 16 Optical fiber region

[0066] 18 Transition region

[0067] 21 Core

[0068] 22 Cladding

[0069] 31 Original optical fiber

[0070] 41 First outer peripheral layer

[0071] 42 Second outer peripheral layer

[0072] 43 Outer peripheral layer part

Claims

1. A manufacturing method of a fiber Bragg grating element, characterized in that, it has: a preparation process of preparing an optical fiber, the optical fiber having a layer structure composed of a core and one or more outer peripheral layers, the one or more outer peripheral layers including a cladding that surrounds the core and has a refractive index lower than that of the core and is formed of a transparent optical material, and at least any one of the one or more outer peripheral layers including a photosensitive material; a stretching process of forming a reduced-diameter region with a reduced diameter by heating and stretching the optical fiber, and a pair of transition regions where the diameter continuously and gradually increases from the reduced-diameter region at both ends of the reduced-diameter region; and an exposure process of irradiating light onto a part of the reduced-diameter region to cause a photoinduced refractive index change, and forming a fiber Bragg grating in which the refractive index of the outer peripheral layer including the photosensitive material changes periodically.

2. A manufacturing method of a fiber Bragg grating element, characterized in that, it has: a preparation process of preparing an optical fiber, the optical fiber having a layer structure composed of a core and a cladding that surrounds the core and has a refractive index lower than that of the core, and the core and the cladding respectively including a photosensitive material; a stretching process of forming a reduced-diameter region with a reduced diameter by heating and stretching the optical fiber, and a pair of transition regions where the diameter continuously and gradually increases from the reduced-diameter region at both ends of the reduced-diameter region; and an exposure process of irradiating light onto a part of the reduced-diameter region to cause a photoinduced refractive index change, and forming a fiber Bragg grating in which the refractive index of the cladding changes periodically together with that of the core.

3. The manufacturing method of a fiber Bragg grating element according to claim 2, characterized in that, the core and the cladding include the same photosensitive material with different concentrations, and due to the concentration difference of the photosensitive material, the refractive index of the core is higher than that of the cladding.

4. The manufacturing method of a fiber Bragg grating element according to claim 1, characterized in that, the optical fiber includes the photosensitive material in the cladding and does not include the photosensitive material in the core.

5. The manufacturing method of a fiber Bragg grating element according to claim 1, characterized in that, the optical fiber includes the photosensitive material in the cladding and in the outer peripheral layer other than the cladding.

6. The manufacturing method of a fiber Bragg grating element according to claim 1, characterized in that, the optical fiber includes the photosensitive material in the outer peripheral layer other than the cladding.

7. A fiber Bragg grating element, characterized in that, it has: a tapered optical fiber having a layer structure composed of a core and one or more outer peripheral layers, the one or more outer peripheral layers including a cladding that surrounds the core and has a refractive index lower than that of the core and is formed of an optical material, at least any one of the one or more outer peripheral layers including a photosensitive material and formed, and having a reduced-diameter region integrally provided at one end of a transition region where the diameter continuously and gradually decreases; and a fiber Bragg grating formed in the reduced-diameter region, in which the refractive index of the outer peripheral layer including the photosensitive material changes periodically.

8. A fiber Bragg grating element, characterized in that, it has: A tapered optical fiber having a layer structure composed of a core and a cladding that surrounds the core and has a refractive index lower than that of the core, wherein the core and the cladding are formed of an optical material containing a photosensitive material, and a reduced-diameter region is integrally provided at one end of a transition region where the diameter continuously and gradually decreases; and An optical fiber Bragg grating formed in the reduced-diameter region, wherein the refractive index of the cladding varies periodically together with that of the core.

9. The optical fiber Bragg grating element according to claim 8, characterized in that the core and the cladding contain the same photosensitive material with different concentrations, and due to the concentration difference of the photosensitive material, the refractive index of the core is higher than that of the cladding.

10. The optical fiber Bragg grating element according to claim 7, characterized in that the cladding is formed of an optical material containing the photosensitive material, and the core is formed of an optical material not containing the photosensitive material, and for the optical fiber Bragg grating, the refractive index of the cladding varies periodically.

11. The optical fiber Bragg grating element according to claim 7, characterized in that the cladding and an outer peripheral layer other than the cladding are formed of an optical material containing the photosensitive material, and for the optical fiber Bragg grating, the refractive indices of the cladding and the outer peripheral layer other than the cladding vary periodically.

12. The optical fiber Bragg grating element according to claim 7, characterized in that the outer peripheral layer other than the cladding is formed of an optical material containing the photosensitive material, and for the optical fiber Bragg grating, the refractive index of the outer peripheral layer other than the cladding varies periodically.

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