Optical fibers with end caps, fiber arrays, light source devices, and wavelength beam coupling devices

Abstract

The present invention provides an end capped optical fiber capable of reducing the divergence angle of a collimated light beam. [Solution] The optical fiber with an end cap comprises an optical fiber having a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, and an end cap having a first surface connected to the end face of the second portion and a second surface located opposite the first surface, wherein the diameter of the first core is constant along the optical axis of the first core, the diameter of the second core is larger closer to the end cap, the end cap includes a convex lens, the second surface includes the convex lens surface of the convex lens, and the focal point of the convex lens is located inside the optical fiber and away from the end face of the second portion.

Description

Technical Field

[0001] The present disclosure relates to an optical fiber with an end cap, a fiber array, a light source device, and a wavelength beam combining device.

Background Art

[0002] An optical fiber with an end cap is an optical element including an optical fiber and an end cap fused to an end face of the optical fiber. The end cap may include a function as a collimating lens that collimates and emits light propagating through the optical fiber depending on the application. Patent Document 1 discloses an example of an optical fiber with an end cap in which the end cap includes a collimating lens.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] To provide an optical fiber with an end cap capable of reducing the divergence angle of a collimated light beam.

Means for Solving the Problems

[0005] An end capped optical fiber of the present disclosure, in one embodiment, comprises an optical fiber having a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, and an end cap having a first surface connected to the end face of the second portion and a second surface located opposite the first surface, wherein the diameter of the first core is constant along the optical axis of the first core, the diameter of the second core is larger closer to the end cap, the end cap includes a convex lens, the second surface includes the convex lens surface of the convex lens, and the focal point of the convex lens is located inside the optical fiber and away from the end face of the second portion.

[0006] An end capped optical fiber of the present disclosure, in one embodiment, comprises an optical fiber having a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, and an end cap having a first surface connected to the end face of the second portion and a second surface located opposite the first surface, wherein the diameter of the first core is constant along the optical axis of the first core, the diameter of the second core is larger closer to the end cap, the end cap includes a convex lens, the second surface includes the convex lens surface of the convex lens, and the thickness of the end cap on the optical axis of the convex lens is less than the distance from the apex of the convex lens to the focal point of the convex lens.

[0007] In one embodiment, the fiber array of the present disclosure comprises a plurality of end-capped optical fibers, each of which is an end-capped optical fiber, wherein the second faces of the end caps in the plurality of end-capped optical fibers face the same side.

[0008] In one embodiment, the light source device of the present disclosure includes: a fiber array, wherein the plurality of end-capped optical fibers include a first end-capped optical fiber and a second end-capped optical fiber; a first laser light source that emits a first laser beam, wherein the first laser beam is coupled to the optical fiber included in the first end-capped optical fiber from the side opposite to the end cap; and a second laser light source that emits a second laser beam, wherein the second laser beam is coupled to the optical fiber included in the second end-capped optical fiber from the side opposite to the end cap.

[0009] In one embodiment, the wavelength beam coupling apparatus of the present disclosure comprises the above-described light source apparatus and a diffraction grating, wherein the first laser light has a first peak wavelength and the second laser light has a second peak wavelength shorter than the first peak wavelength, the first end cap optical fiber emits a first light beam collimated with the first laser light from the second surface of the end cap, the second end cap optical fiber emits a second light beam collimated with the second laser light from the second surface of the end cap, and the diffraction grating wavelength-combines the first light beam and the second light beam. [Effects of the Invention]

[0010] According to embodiments of this disclosure, an end capped optical fiber can be realized that can reduce the divergence angle of a collimated light beam. [Brief explanation of the drawing]

[0011] [Figure 1A] Figure 1A is a schematic diagram showing the configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. [Figure 1B] Figure 1B schematically shows another configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. [Figure 1C]Figure 1C schematically shows yet another configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. [Figure 1D] Figure 1D schematically shows yet another configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. [Figure 2A] Figure 2A schematically illustrates a modified example of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. [Figure 2B] Figure 2B schematically illustrates another modification of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. [Figure 3A] Figure 3A is a schematic top view showing the configuration of a fiber array according to an exemplary embodiment of the present disclosure. [Figure 3B] Figure 3B is a schematic front view showing the configuration of a fiber array according to an exemplary embodiment of the present disclosure. [Figure 4A] Figure 4A schematically shows a modified example of a fiber array according to an exemplary embodiment of the present disclosure. [Figure 4B] Figure 4B schematically shows another modification of the fiber array according to an exemplary embodiment of the present disclosure. [Figure 5] Figure 5 is a schematic diagram showing the configuration of a light source device according to an exemplary embodiment of the present disclosure. [Figure 6] Figure 6 is a schematic diagram showing the configuration of a wavelength beam coupling apparatus according to an exemplary embodiment of the present disclosure. [Figure 7A] Figure 7A is a micro-mirror image of the optical fiber with end caps shown in Figure 1C. [Figure 7B] Figure 7B is a graph showing the change in the radial size of the core and cladding in the optical axis direction of an optical fiber contained in an end-capped optical fiber. [Figure 8] Figure 8 shows a calculation example representing the deviation from the reference plane of the focal position of a convex lens where the divergence angle of the light beam is minimized, with respect to the radius of curvature R. [Figure 9]FIG. 9 is a calculation example showing the deviation of the focal position of a convex lens at which the divergence angle of the light beam is minimized with respect to the spreading angle θ from the reference plane. [Figure 10A] FIG. 10A is a graph showing the relationship between the divergence angle of the light beam and the spreading angle of the side surface of the second core when the focal position of the convex lens coincides with the reference plane. [Figure 10B] FIG. 10B is a graph showing the relationship between the minimum value of the divergence angle of the light beam and the spreading angle of the side surface of the second core in the example shown in FIG. 9. [Figure 11A] FIG. 11A is a diagram schematically showing an example of light rays propagating through an optical fiber in mode A. [Figure 11B] FIG. 11B is a diagram schematically showing an example of light rays propagating through an optical fiber in mode B. [Figure 12A] FIG. 12A is a diagram schematically showing an example of light rays propagating through an optical fiber in mode C. [Figure 12B] FIG. 12B is a diagram schematically showing an example of light rays propagating through an optical fiber in mode D.

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[0012] Hereinafter, an end-cap-attached optical fiber, a fiber array, a light source device, and a wavelength beam combining device according to embodiments of the present disclosure will be described with reference to the drawings. Parts denoted by the same reference numerals in the plurality of drawings indicate the same or equivalent parts.

[0013] Furthermore, the embodiments described below are illustrative for embodying the technical idea of the present invention, and do not limit the present invention thereto. In addition, the description of the size, material, shape, relative arrangement, etc. of the components is not intended to limit the scope of the present invention thereto, but is intended to be illustrative. The sizes and positional relationships of the members shown in each drawing may be exaggerated for ease of understanding.

[0014] (Embodiment) [End-cap-attached optical fiber] An end capped optical fiber according to one embodiment of the present disclosure comprises an optical fiber having a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, and an end cap having a first surface connected to the end face of the second portion and a second surface located opposite the first surface, wherein the diameter of the first core is constant along the optical axis of the first core, the diameter of the second core is larger closer to the end cap, the end cap includes a convex lens, the second surface includes the convex lens surface of the convex lens, and the focal point of the convex lens is located inside the optical fiber and away from the end face of the second portion.

[0015] An end capped optical fiber according to one embodiment of the present disclosure comprises an optical fiber having a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, and an end cap having a first surface connected to the end face of the second portion and a second surface located opposite the first surface, wherein the diameter of the first core is constant along the optical axis of the first core, the diameter of the second core is larger closer to the end cap, the end cap includes a convex lens, the second surface includes the convex lens surface of the convex lens, and the thickness of the end cap on the optical axis of the convex lens is smaller than the distance from the apex of the convex lens to the focal point of the convex lens.

[0016] In the end capped optical fiber of this disclosure configured as described above, the divergence angle of the collimated light beam can be reduced.

[0017] First, an example of the configuration of an end-capped optical fiber according to an embodiment of the present disclosure will be described with reference to Figure 1A. Figure 1A is a schematic diagram showing the configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure.

[0018] The optical fiber 100A with an end cap shown in Figure 1A comprises an optical fiber 10 that propagates light and an end cap 20 connected to the optical fiber 10 that emits the light propagating through the optical fiber 10 as a collimated light beam L. The optical fiber 10 and the end cap 20 are fused together. The light propagating through the optical fiber 10 may be laser light or LED light. The region enclosed by the dashed line in Figure 1A is where the intensity of the light beam L is 1 / e of its maximum intensity. 2 This refers to the region greater than or equal to the value above. e is the base of the natural logarithm. Optical fiber 10 may be, for example, a multimode fiber. Optical fiber 10 may also be a single-mode fiber, but a multimode fiber is advantageous in that it can propagate high-power light. The core of a multimode fiber may be formed from, for example, quartz, which is less susceptible to damage from high-power light.

[0019] The optical fiber 10 comprises the following first part 10a and second part 10b. The first part 10a has a first core 10a1 and a first cladding 10a2 surrounding the first core 10a1. Since the refractive index of the first core 10a1 is higher than that of the first cladding 10a2, the first core 10a1 can propagate light by totally internal reflection at the boundary between the first core 10a1 and the first cladding 10a2. The second part 10b has a second core 10b1 and a second cladding 10b2 surrounding the second core 10b1. Since the refractive index of the second core 10b1 is higher than that of the second cladding 10b2, the second core 10b1 can propagate light by totally internal reflection at the boundary between the second core 10b1 and the second cladding 10b2. The second core 10b1 is adjacent to the first core 10a1, and the second cladding 10b2 is adjacent to the first cladding 10a2. The refractive index of the second core 10b1 is the same as that of the first core 10a1, and the refractive index of the second cladding 10b2 is the same as that of the first cladding 10a2. As the optical fiber 10, an optical fiber having a double-cladding structure in which the first cladding 10a2 and the second cladding 10b2 are surrounded by other cladding may be used.

[0020] Before fusing the optical fiber 10 and the end cap 20, the diameter of the core of the optical fiber 10 is constant along the optical axis of the core. However, when the optical fiber 10 and the end cap 20 are fused, the shape near the end face 12 of the optical fiber 10 deforms, and the diameter of the core of the optical fiber 10 is no longer constant along the optical axis of the core.

[0021] The differences between the first part 10a and the second part 10b due to the fusion splicing described above are as follows: The diameter of the first core 10a1 is constant along the optical axis of the first core 10a1, while the diameter of the second core 10b1 is larger closer to the end cap 20. The length of the second part 10b on the optical axis of the second core 10b1 may be, for example, 0.01 mm or more and 0.5 mm or less. The end face 12 of the optical fiber 10 is also the end face 12 of the second part 10b. In the example shown in Figure 1A, the second core 10b1 spreads out in a curved shape in the plane containing the optical axis of the second core 10b1. The radius of curvature of the side surface of the second core 10b1 may be, for example, 0.01 mm or more. In the example shown in Figure 1A, the shape of the spread of the second core 10b1 is symmetrical with respect to the optical axis of the second core 10b1. The length of the second portion 10b and the shape of the spread of the second core 10b1 depend on the fusion conditions, such as the heating temperature and pressure, when fusing the optical fiber 10 and the end cap 20.

[0022] The diameter of the second core 10b1 is not constant along the optical axis, and is larger closer to the end cap 20. Therefore, the pseudo-emitting surface that emits the propagating light within the optical fiber 10 is located not at the end face 12 of the second portion 10b, but inside the second core 10b1 away from the end face 12, or at the boundary between the first core 10a1 and the second core 10b1. The conditions under which the pseudo-emitting surface is located inside the second core 10b1 away from the end face 12, and the conditions under which the pseudo-emitting surface is located at the boundary between the first core 10a1 and the second core 10b1, will be explained in the calculation examples described later.

[0023] The end cap 20 includes a convex lens. The end cap 20 has a first surface 22a connected to the end face 12 of the second portion 10b, and a second surface 22b located opposite the first surface 22a and including the convex lens surface of the convex lens. The dotted line shown in Figure 1A represents the optical axis 24 of the convex lens. As shown in Figure 1A, the optical axis of the first core 10a1, the optical axis of the second core 10b1, and the optical axis of the light beam L coincide with the optical axis 24 of the convex lens. However, some misalignment of these optical axes is acceptable as long as a collimated light beam is obtained. The positional misalignment of the optical axis may be, for example, 200 μm or less, preferably 50 μm or less. The angular misalignment of the optical axis may be, for example, 1.0° or less, preferably 0.2° or less.

[0024] If the refractive index of the end cap 20 is close to that of the second core 10b1 of the second portion 10b contained in the optical fiber 10, Fresnel reflection at the end face 12 of the second portion 10b can be reduced. If the refractive indices of both are the same, no Fresnel reflection will occur at the end face 12 of the second portion 10b.

[0025] The area of ​​the first surface 22a is larger than the area of ​​the end face 12 of the second portion 10b. Therefore, it is easy to fuse the optical fiber 10 and the end cap 20 such that the end face 12 of the second portion 10b is located near the optical axis of the convex lens on the first surface 22a. Fusion can be suitably performed, for example, by a laser fusion method described later. The minimum dimension of the first surface 22a in the direction perpendicular to the optical axis of the convex lens may be, for example, 0.05 mm or more and 20 mm or less. The maximum dimension of the end face 12 of the second portion 10b in the direction perpendicular to the optical axis of the convex lens may be, for example, 0.05 mm or more and 2.0 mm or less.

[0026] The convex lens contained in the end cap 20 has a focal point F. Light rays passing through the focal point F of the convex lens become parallel to the optical axis of the convex lens after passing through it. The focal point F of the convex lens is located inside the optical fiber 10 and away from the end face 12 of the second portion 10b. More specifically, the focal point F of the convex lens is located inside the second core 10b1 or at the boundary between the first core 10a1 and the second core 10b1. The back focus of the convex lens is greater than 0. The thickness d of the end cap 20 on the optical axis of the convex lens contained in the end cap 20 is less than the distance from the vertex of the convex lens to the focal point F. The vertex of the convex lens is located where the optical axis of the convex lens and the surface of the convex lens intersect. The distance from the vertex of the convex lens to the focal point F may be, for example, between 0.5 mm and 100 mm. The distance from the vertex of the convex lens to the focal point F is longer than the focal length f of the convex lens. Here, focal length means the back focal length.

[0027] Unlike the optical fiber 100A with an end cap according to this embodiment, in the configuration where the focal point F of the convex lens in the end cap 20 is located on the end face 12 of the second portion 10b, the focal point F of the convex lens is far from the pseudo-light-emitting surface in the optical fiber 10. As a result, the laser light propagating through the optical fiber 10 is not sufficiently collimated by the convex lens and is emitted from the second surface 22b of the end cap 20.

[0028] In contrast, in the end cap-equipped optical fiber 100A according to this embodiment, the focal point F of the convex lens is located inside the optical fiber 10 and away from the end face 12 of the second portion 10b. More specifically, the focal point F of the convex lens is located inside the second core 10b1, or at the boundary between the first core 10a1 and the second core 10b1. Therefore, the focal point F of the convex lens can be brought closer to the pseudo-light-emitting surface in the optical fiber 10, and more preferably, the focal point F of the convex lens can be positioned on the pseudo-light-emitting surface. As a result, the propagating light in the optical fiber 10 is emitted from the second surface 22b of the end cap 20 as a light beam L that is sufficiently collimated by the convex lens. In this specification, "collimated light beam L" includes not only a light beam L that is perfectly parallel light, but also a light beam L with reduced divergence. The divergence angle (total angle) of a sufficiently collimated light beam L may be, for example, 0.4° or less.

[0029] Thus, with the end cap-equipped optical fiber 100A according to this embodiment, it is possible to reduce the divergence angle of the collimated optical beam L.

[0030] As a result of misalignment in the fusion splicing of the optical fiber 10 and the end cap 20, the focal point F of the convex lens may be located inside the optical fiber 10 and away from the end face 12 of the second portion 10b, but not inside the second core 10b1 or at the boundary between the first core 10a1 and the second core 10b1. Even in that case, if the focal point F of the convex lens is closer to the pseudo-emitting surface than when it is located at the end face 12, it is possible to reduce the divergence angle of the collimated light beam L.

[0031] In the example above, the end cap-equipped optical fiber 100A emits the light propagating within the optical fiber 10 as a collimated light beam L from the end cap 20, but this is not the only example. The end cap-equipped optical fiber 100A can also capture the collimated light beam L from the second surface 22b of the end cap 20 and propagate it into the optical fiber 10. When the light beam L is captured from the second surface 22b of the end cap 20, the above-mentioned pseudo-emitting surface can be reinterpreted as a pseudo-receiving surface. In this way, the end cap-equipped optical fiber 100A can effectively capture the collimated light beam L from the end cap 20.

[0032] Figure 1B schematically shows another configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. The end-capped optical fiber 100B shown in Figure 1B has the same structure as the end-capped optical fiber 100A shown in Figure 1A, except for the shape near the end of the optical fiber 10 to which the end cap 20 is fused.

[0033] In the example shown in Figure 1B, the second core 10b1 spreads out linearly in the plane containing the optical axis of the second core 10b1. The angle of spread of the side surface of the second core 10b1 with respect to the optical axis of the second core 10b1 (i.e., half-angle at half maximum) can be, for example, between 0.01° and 45°.

[0034] Figure 1C schematically shows yet another configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. The end-capped optical fiber 100C shown in Figure 1C differs from the end-capped optical fiber 100A shown in Figure 1A in the following respect: namely, when viewed from the optical axis 24 side of the convex lens, the size of the first surface 22a is smaller than the size of the second surface 22b. This allows the size of the first surface 22a to be closer to the size of the end face 12 of the second portion 10b of the optical fiber 10. Therefore, it becomes easier to fuse the optical fiber 10 and the end cap 20 so that the optical axis of the optical fiber 10 and the optical axis of the convex lens are close to or coincide. The size of the first surface 22a may be, for example, 0.95 times or more and 1.5 times or less, or 0.95 times or more and 1.1 times or less, the size of the end face 12.

[0035] Figure 1D schematically shows yet another configuration of an end-capped optical fiber according to an exemplary embodiment of the present disclosure. The end-capped optical fiber 100D shown in Figure 1D has the same structure as the end-capped optical fiber 100C shown in Figure 1C, except for the shape near the end of the optical fiber 10 to which the end cap 20 is fused. In the example shown in Figure 1D, similar to the example shown in Figure 1B, the second core 10b1 spreads out linearly in the plane containing the optical axis of the second core 10b1.

[0036] Next, a method for fusing the optical fiber 10 and the end cap 20 will be briefly described. The optical fiber 10 and the end cap 20 can be fused using, for example, a laser fusion method or an arc discharge method.

[0037] Laser fusion splicing is an advantageous method for fusing two components with different diameters. In laser fusion splicing, a CO2 laser is irradiated onto the connection surface of the larger-diameter component, causing it to melt. The connection surface of the smaller-diameter component is then pressed against the melted connection surface, and the heat from the melted surface is used to fuse the two components together. This method reduces the melting of surfaces unrelated to the fusion, thus reducing component degradation. Laser fusion splicing can be suitably used, for example, for the end-capped optical fibers 100A and 100B shown in Figures 1A and 1B.

[0038] The arc discharge method is advantageous for fusing two components with similar diameters. In the arc discharge method, the connection surfaces of each component are exposed to a plasma region generated by applying a voltage between two electrodes, causing the connection surfaces to melt. The two components are then fused together by bringing the molten connection surfaces into contact. The arc discharge method can be suitably used, for example, for optical fibers 100C and 100D with end caps, as shown in Figures 1C and 1D.

[0039] Next, a modified example of the end cap-equipped optical fiber 100A according to this embodiment will be described with reference to Figure 2A. Figure 2A is a schematic diagram illustrating a modified example of the end cap-equipped optical fiber 100A according to this embodiment.

[0040] The end capped optical fiber 110A shown in Figure 2A differs from the end capped optical fiber 100A shown in Figure 1A in the following two respects. First, the end capped optical fiber 110A further comprises an end cap 21 connected to the optical fiber 10 on the opposite side of the end cap 20. The optical fiber 10 and the end cap 21 are fused together. In this specification, the end cap 21 is also referred to as the "other end cap". Second, the optical fiber 10 further comprises a third portion 10c located on the opposite side of the second portion 10b with respect to the first portion 10a.

[0041] The third part 10c has a third core 10c1 and a third cladding 10c2 surrounding the third core 10c1. Since the refractive index of the third core 10c1 is higher than that of the third cladding 10c2, the third core 10c1 can propagate light by totally internal reflection at the boundary between the third core 10c1 and the third cladding 10c2. The third core 10c1 is adjacent to the first core 10a1, and the third cladding 10c2 is adjacent to the first cladding 10a2. The refractive index of the third core 10c1 is the same as that of the first core 10a1, and the refractive index of the third cladding 10c2 is the same as that of the first cladding 10a2.

[0042] The differences between the first part 10a and the third part 10c due to the fusion splicing of the optical fiber 10 and the end cap 21 are as follows: The diameter of the first core 10a1 is constant along the optical axis of the first core 10a1, while the diameter of the third core 10c1 is larger closer to the end cap 21. In the example shown in Figure 2A, the third core 10c1 spreads out in a curved shape in the plane containing the optical axis of the third core 10c1. The radius of curvature of the side surface of the third core 10c1 may be, for example, 0.01 mm or more. In the example shown in Figure 2A, the shape of the spreading of the third core 10c1 is symmetrical with respect to the optical axis of the third core 10c1. In the example shown in Figure 2A, the third core 10c1 may also spread out in a straight line.

[0043] The diameter of the third core 10c1 is not constant along the optical axis, and is larger closer to the end cap 21. Therefore, the pseudo-emitting surface or pseudo-receiving surface in the optical fiber 10 is located not on the end face 13 of the third portion 10c, but inside the third core 10c1 away from the end face 13, or at the boundary between the first core 10a1 and the third core 10c1.

[0044] The end cap 21 includes a convex lens. In this specification, the convex lens of the end cap 21 is also referred to as the "other convex lens". The end cap 21 has a third surface 23a connected to the end face 13 of the third portion 10c, and a fourth surface 23b located opposite the third surface 23a and including the convex lens surface of the convex lens. The area of ​​the third surface 23a is larger than the area of ​​the end face 13 of the third portion 10c. Therefore, it is easy to fuse the optical fiber 10 and the end cap 21 such that the end face 13 of the third portion 10c is located near the optical axis of the convex lens on the third surface 23a. The minimum dimension of the third surface 23a in the direction perpendicular to the optical axis of the convex lens may be, for example, 0.05 mm or more and 20 mm or less. The maximum dimension of the end face 13 of the third portion 10c in the direction perpendicular to the optical axis of the convex lens may be, for example, 0.05 mm or more and 2.0 mm or less.

[0045] The convex lens contained in the end cap 21 has a focal point F'. The focal point F' of the convex lens contained in the end cap 21 is located inside the optical fiber 10 and away from the end face 13 of the third portion 10c. More specifically, the focal point F' of the convex lens is located inside the third core 10c1, or at the boundary between the first core 10a1 and the third core 10c1. The thickness d' of the end cap 21 on the optical axis of the convex lens contained in the end cap 21 is less than the distance from the vertex of the convex lens to the focal point F'. The distance from the vertex of the convex lens to the focal point F' may be, for example, between 0.5 mm and 100 mm. The distance from the vertex of the convex lens to the focal point F' is longer than the focal length f' of the convex lens.

[0046] In the end capped optical fiber 110A, the focal point F' of the convex lens is located inside the optical fiber 10 and away from the end face 13 of the third portion 10c, so that the focal point F' of the convex lens can be brought closer to a pseudo-emitting surface or pseudo-receiving surface in the optical fiber 10. More preferably, the focal point F' of the convex lens can be positioned on the pseudo-emitting surface or pseudo-receiving surface. As a result, the light propagating through the optical fiber 10 is emitted from the fourth surface 23b of the end cap 21 as a sufficiently collimated optical beam L. The divergence angle of the collimated optical beam L is reduced.

[0047] Alternatively, the collimated optical beam L is captured from the fourth surface 23b of the end cap 21 and propagates through the optical fiber 10. Therefore, in the optical fiber 110A with an end cap, the collimated optical beam L can be captured from one of the end caps 20 and 21 and emitted from the other.

[0048] Figure 2B is a schematic diagram illustrating a modified example of the end-capped optical fiber 100B according to this embodiment. The end-capped optical fiber 110B shown in Figure 2B differs from the end-capped optical fiber 100B shown in Figure 1B in the two points described above in relation to the end-capped optical fiber 110A shown in Figure 2A. The end-capped optical fiber 110B shown in Figure 2B has the same structure as the end-capped optical fiber 110A shown in Figure 2A, except for the shape near the end of the optical fiber 10 to which the end caps 20 and 21 are fused.

[0049] In the example shown in Figure 2B, similar to the example shown in Figure 1B, the second core 10b1 spreads out linearly in the plane containing the optical axis of the second core 10b1. In the example shown in Figure 2B, the third core 10c1 also spreads out linearly in the plane containing the optical axis of the third core 10c1. The spreading angle of the side surface of the third core 10c1 with respect to the optical axis of the third core 10c1 may be, for example, between 0.01° and 45°. In the example shown in Figure 2B, the third core 10c1 may also spread out in a curved shape.

[0050] Similar modifications can be made to the end-capped optical fibers 100C and 100D shown in Figures 1C and 1D, as can be made to the end-capped optical fibers 110A and 110B shown in Figures 2A and 2B.

[0051] [Fiber Array] A fiber array according to one embodiment of the present disclosure comprises a plurality of end-capped optical fibers, each of which is an end-capped optical fiber, wherein the second surfaces of the end caps in the plurality of end-capped optical fibers face the same side.

[0052] In the fiber array of this disclosure configured as described above, it becomes possible to input light into multiple end-capped optical fibers and emit multiple collimated light beams from the multiple end-capped optical fibers.

[0053] Below, an example of the configuration of a fiber array according to an embodiment of the present disclosure will be described with reference to Figures 3A and 3B. Figures 3A and 3B are schematic top and front views, respectively, showing the configuration of a fiber array according to an exemplary embodiment of the present disclosure.

[0054] The fiber array 200 shown in Figures 3A and 3B comprises multiple end-capped optical fibers 100A. Instead of end-capped optical fibers 100A, end-capped optical fibers 100B, 100C, or 100D may be used.

[0055] In the examples shown in Figures 3A and 3B, there are three end-capped optical fibers 100A, but the number is not limited to this example. The number of end-capped optical fibers 100A may be two or four or more.

[0056] The fiber array 200 further includes a support member 42 for supporting a plurality of end caps 20, a fixing member 44 for securing the plurality of end caps 20, and screws 46 for maintaining the distance between the fixing member 44 and the support member 42. The support member 42 has a plurality of recesses 43 for stably positioning the plurality of end caps 20, as shown in Figure 3B. Each recess 43 corresponds to one end cap 20. The recesses 43 may be, for example, V-shaped grooves. Figure 3A shows through to a plurality of end caps 20 located below the fixing member 44.

[0057] In multiple end-capped optical fibers 100A, the second surfaces 22b of the end caps 20 face the same side. More specifically, in any two end-capped optical fibers 100A, the angle formed by the normal directions of the second surfaces 22b of the end caps 20 is 45° or less. Preferably, the angle formed by the normal directions of the second surfaces 22b of the end caps 20 in any two end-capped optical fibers 100A is 5° or less. The normal direction of the second surface 22b is in a direction that coincides with the optical axis of the convex lens included in the end cap 20 and is away from the end cap 20.

[0058] From the above, the fiber array 200 according to this embodiment makes it possible to input light into multiple end-capped optical fibers 100A and emit multiple collimated optical beams L from the multiple end-capped optical fibers 100A. The wavelengths of the light input into the multiple end-capped optical fibers 100A may all be the same, or some or all of them may be different.

[0059] Next, a modified example of the fiber array 200 according to this embodiment will be described with reference to Figures 4A and 4B. In the fiber array 200 according to this embodiment, all of the end-capped optical fibers 100A have the same structure. If the wavelengths of light input to the multiple end-capped optical fibers 100A are different, the multiple end-capped optical fibers 100A may have different structures depending on the wavelength of the input light. End-capped optical fibers 100B, 100C, or 100D may be used instead of end-capped optical fibers 100A.

[0060] Figure 4A is a schematic diagram showing modification 1 of the fiber array 200 according to this embodiment. In Figure 4A, the support member 42, the fixing member 44, and the screw 46 are omitted. The fiber array 210 shown in Figure 4A differs from the fiber array 200 shown in Figure 3A in the following respects. The plurality of end-capped optical fibers 100A include an end-capped optical fiber 100A1 for a first wavelength λ1 (i.e., a first end-capped optical fiber 100A1), an end-capped optical fiber 100A2 for a second wavelength λ2 shorter than the first wavelength λ1 (i.e., a second end-capped optical fiber 100A2), and an end-capped optical fiber 100A3 for a third wavelength λ3 shorter than the second wavelength λ2 (i.e., a third end-capped optical fiber 100A3).

[0061] The shorter the wavelength of light propagating through the optical fiber 10, the higher the refractive index of the end cap 20, resulting in a longer optical path length. Based on this, in the end-capped optical fibers 100A1 to 100A3, the length of the second portion 10b in the optical fiber 10 is the same, while the thicknesses d1 to d3 of the end cap 20 on the optical axis of the convex lens are all different. The shape of the convex lens is the same, and the shape near the end face of the optical fiber 10 is also the same. This allows for a uniform optical path length.

[0062] More specifically, the thickness d2 of the end cap 20 in the second end cap optical fiber 100A2 on the optical axis of the convex lens is smaller than the thickness d1 of the end cap 20 in the first end cap optical fiber 100A1 on the optical axis of the convex lens. The thickness d3 of the end cap 20 in the third end cap optical fiber 100A3 on the optical axis of the convex lens is smaller than the thickness d2 of the end cap 20 in the second end cap optical fiber 100A2 on the optical axis of the convex lens. That is, d1 > d2 > d3.

[0063] Based on the above, the end-capped optical fibers 100A1 to A3 included in the fiber array 210 make it possible to reduce the divergence angles of the collimated optical beams La to Lc with wavelengths λ1 to λ3, respectively.

[0064] Figure 4B schematically shows a second modification of the fiber array 200 according to this embodiment. The fiber array 220 shown in Figure 4B differs from the fiber array 210 shown in Figure 4A in the following respects. Specifically, in the end capped optical fibers 100A1 to 100A3, the thickness of the end cap 20 on the optical axis of the convex lens is all the same, while the lengths t1 to t3 of the second portion 10b in the optical fiber 10 are all different. The shape of the convex lens is all the same.

[0065] More specifically, the length t2 of the second portion 10b of the optical fiber 10 contained in the second end cap optical fiber 100A2 is smaller than the length t1 of the second portion 10b of the optical fiber 10 contained in the first end cap optical fiber 100A1. The length t3 of the second portion 10b of the optical fiber 10 contained in the third end cap optical fiber 100A3 is smaller than the length t2 of the second portion 10b of the optical fiber 10 contained in the second end cap optical fiber 100A2. That is, t1 > t2 > t3.

[0066] Based on the above, the end-capped optical fibers 100A1 to A3 included in the fiber array 220 make it possible to reduce the divergence angles of the collimated optical beams La to Lc with wavelengths λ1 to λ3, respectively.

[0067] The fiber arrays 210 of Modification 1 and 220 of Modification 2 can be suitably used when the difference between the longest and shortest wavelengths of the light coupled to them is between 0.1 nm and 50 nm. By adjusting the thickness of the end cap 20 on the optical axis of the convex lens or the length of the second portion 10b, taking into account the wavelength dependence of the convex lens, the chromatic aberration of the end capped optical fibers 100A1 to 100A3 can be effectively reduced.

[0068] [Light source device] Next, an example of the configuration of a light source device according to an embodiment of the present disclosure will be described with reference to Figure 5. Figure 5 is a schematic diagram showing the configuration of a light source device according to an exemplary embodiment of the present disclosure. The light source device 300 shown in Figure 5 comprises a plurality of laser light sources 30 and a fiber array 200 comprising a plurality of end-capped optical fibers 100A. Each end-capped optical fiber 100A corresponds to one laser light source 30. In Figure 5, the support member 42, fixing member 44, and screw 46 included in the fiber array 200 are omitted.

[0069] In the example shown in Figure 5, there are three laser light sources 30, but this is not the only example. The number of laser light sources 30 may be two or three or more. The number of end-capped optical fibers 100A is the same as the number of laser light sources 30.

[0070] The laser light emitted from each laser light source 30 is coupled to the optical fiber 10 contained in the corresponding end-capped optical fiber from the side opposite the end cap 20. As a result, each end-capped optical fiber 100A emits a collimated optical beam L from the second surface 22b of the end cap 20.

[0071] The peak wavelengths of the laser light emitted from multiple laser light sources 30 may all be the same, or some or all of them may be different. The optical fiber 10 and end cap 20 in each end capped optical fiber 100A are appropriately designed according to the peak wavelength of the laser light emitted from the corresponding laser light source 30.

[0072] The multiple laser light sources 30 include a first laser light source 30a that emits a first laser beam with a first peak wavelength, a second laser light source 30b that emits a second laser beam with a second peak wavelength, and a third laser light source 30c that emits a third laser beam with a third peak wavelength. These three peak wavelengths may be, for example, 200 nm to 1100 nm, preferably 240 nm to 700 nm, and more preferably 360 nm to 570 nm.

[0073] If the first peak wavelength, second peak wavelength, and third peak wavelength are the first wavelength λ1, second wavelength λ2, and third wavelength λ3 described above, respectively, then the second peak wavelength is shorter than the first peak wavelength, and the third peak wavelength is shorter than the second peak wavelength. In that case, the light source device 300 may use the fiber array 210 shown in Figure 4A or the fiber array 220 shown in Figure 4B instead of the fiber array 200.

[0074] The first laser beam is coupled to the optical fiber 10 contained in the first end-capped optical fiber 100A1 from the side opposite the end cap 20. The second laser beam is coupled to the optical fiber 10 contained in the second end-capped optical fiber 100A2 from the side opposite the end cap 20. The third laser beam is coupled to the optical fiber 10 contained in the third end-capped optical fiber 100A3 from the side opposite the end cap 20.

[0075] The first end cap optical fiber 100A1 emits a first optical beam La, which is collimated with the first laser beam, from the second surface 22b of the end cap 20. The second end cap optical fiber 100A2 emits a second optical beam Lb, which is collimated with the second laser beam, from the second surface 22b of the end cap 20. The third end cap optical fiber 100A3 emits a third optical beam Lc, which is collimated with the third laser beam, from the second surface 22b of the end cap 20.

[0076] Based on the above, the light source device 300 according to this embodiment can emit multiple collimated light beams L from multiple laser light sources 30 via the fiber array 200.

[0077] [Wavelength-beam coupling device] A wavelength beam coupling apparatus according to one embodiment of the present disclosure comprises the above-described light source apparatus and a diffraction grating, wherein the first laser light has a first peak wavelength and the second laser light has a second peak wavelength shorter than the first peak wavelength, the first end cap optical fiber emits a first light beam collimated with the first laser light from the second surface of the end cap, the second end cap optical fiber emits a second light beam collimated with the second laser light from the second surface of the end cap, and the diffraction grating wavelength-combines the first light beam and the second light beam.

[0078] In the wavelength beam coupling apparatus of this disclosure configured as described above, a first light beam and a second light beam emitted from a light source device can be wavelength-combined by a diffraction grating to form a high-power coupled beam.

[0079] Next, with reference to Figure 6, an example of the configuration of a wavelength beam coupling device according to an embodiment of the present disclosure will be described. The wavelength beam coupling device comprises a light source device 300 that emits a plurality of optical beams L having different peak wavelengths, and a diffraction grating. The diffraction grating wavelength-combines a plurality of optical beams L, which include at least a first optical beam La with a first peak wavelength and a second optical beam Lb with a second peak wavelength shorter than the first peak wavelength. As a result, a high-power coupled beam is formed.

[0080] Figure 6 is a schematic diagram showing the configuration of a wavelength beam coupling device according to an exemplary embodiment of the present disclosure. The wavelength beam coupling device 400 shown in Figure 6 comprises a light source device 300 that emits a plurality of unpolarized light beams L having different peak wavelengths, a first optical member 52a, a second optical member 52b, a first diffraction grating 53a, a second diffraction grating 53b, and optical fibers 100A, 100B, 100C, or 100D with end caps. The first optical member 52a and the second optical member 52b have the same structure. The first diffraction grating 53a and the second diffraction grating 53b have the same structure and are arranged parallel to each other.

[0081] Figure 6 schematically shows the mutually orthogonal X, Y, and Z axes for reference. The direction of the arrow on the X axis is referred to as the +X direction, and the opposite direction is referred to as the -X direction. When the ±X directions are not distinguished, they are simply referred to as the X direction. The same applies to the Y and Z directions. This does not restrict the orientation when using the wavelength beam coupling device 400; the orientation of the wavelength beam coupling device 400 is arbitrary.

[0082] The unpolarized light beam L includes a first light beam La with a first peak wavelength, a second light beam Lb with a second peak wavelength, and a third light beam Lc with a third peak wavelength. The first, second, and third peak wavelengths are the first wavelength λ1, second wavelength λ2, and third wavelength λ3, respectively. That is, λ1 > λ2 > λ3.

[0083] The first optical member 52a includes a cube-shaped first polarizing beam splitter 52a1 having a first polarization plane 52as, a first prism 52a2, and a first polarization conversion element 52a3 which is a half-wave plate. The second optical member 52b includes a cube-shaped second polarizing beam splitter 52b1 having a second polarization plane 52bs, a second prism 52b2, and a second polarization conversion element 52b3 which is a half-wave plate.

[0084] In the example shown in Figure 6, the first polarizing beam splitter 52a1 and the first prism 52a2 are in contact, and the first prism 52a2 and the first polarization conversion element 52a3 are in contact, but the example is not limited to this. The first polarizing beam splitter 52a1 and the first prism 52a2 may be separated, and the first prism 52a2 and the first polarization conversion element 52a3 may be separated. The same applies to the second polarizing beam splitter 52b1 and the second prism 52b2, and the second prism 52b2 and the second polarization conversion element 52b3.

[0085] The double arrows in Figure 6 represent so-called P-polarization, where the polarization direction is parallel to the XZ plane, while the symbols enclosed in small circles in Figure 6 represent so-called S-polarization, where the polarization direction is parallel to the Y direction. The solid lines in Figure 6 represent the unpolarized state, the dashed lines represent the S-polarized state, and the dashed-dotted lines represent the P-polarized state.

[0086] The first optical member 52a extracts multiple collimated first polarized (S-polarized) beams L1 and multiple collimated second polarized (S-polarized) beams L2 from multiple unpolarized light beams L propagating in the +Z direction, as follows. The reason for extracting multiple first polarized (S-polarized) beams L1 and second polarized (S-polarized) beams L2 from multiple unpolarized light beams L is that the diffraction efficiency of S-polarized light is higher than that of P-polarized light in the first diffraction grating 53a and the second diffraction grating 53b.

[0087] In the first optical element 52a, the first polarization beam splitter 52a1 uses the first polarization plane 52as to separate a plurality of unpolarized light beams L traveling in the +Z direction into a plurality of first-polarized (S-polarized) beams L1 traveling in the -X direction and a plurality of collimated third-polarized (P-polarized) beams L3 traveling in the +Z direction. The first prism 52a2 performs total internal reflection of the plurality of third-polarized (P-polarized) beams L3 traveling in the +Z direction in the -X direction. The first polarization conversion element 52a3 converts the plurality of third-polarized (P-polarized) beams L3 into a plurality of second-polarized (S-polarized) beams L2.

[0088] The first diffraction grating 53a is arranged so that multiple first polarized beams L1 and multiple second polarized beams L2 are all incident at an incident angle α (e.g., 45 degrees). The first diffraction grating 53a is further arranged so that multiple first polarized beams L1 incident parallel to each other at the same incident angle α are diffracted at different diffraction angles β1 to β3 according to their respective wavelengths λ1 to λ3, and incident into the first region 53b1 of the opposing second diffraction grating 53b. The first diffraction grating 53a is further arranged so that multiple second polarized beams L2 incident parallel to each other at the same incident angle α are diffracted at different diffraction angles β1 to β3 according to their respective wavelengths λ1 to λ3, and incident into the second region 53b2 of the opposing second diffraction grating 53b. The diffraction angle β2 corresponding to wavelength λ2 is smaller than the diffraction angle β1 corresponding to wavelength λ1, and the diffraction angle β3 corresponding to wavelength λ3 is smaller than the diffraction angle β2 corresponding to wavelength λ2. That is, β1 > β2 > β3.

[0089] The second diffraction grating 53b, having the same structure as the first diffraction grating 53a, is positioned to emit reflected diffracted light from the first diffraction grating 53a, which is incident at different incident angles β1 to β3 depending on the wavelength λ1 to λ3, at the same diffraction angle α (e.g., 45 degrees). As a result of the wavelength combination and coaxial coupling of multiple first polarization beams L1 in this manner, the first region 53b1 emits a collimated first combined beam CL1 of S polarization traveling in the -X direction. Similarly, as a result of the wavelength combination and coaxial coupling of multiple second polarization beams L2, the second region 53b2 emits a collimated second combined beam CL2 of S polarization traveling in the -X direction.

[0090] The second optical member 52b forms an unpolarized, collimated third coupled beam CL3 traveling in the +Z direction from a first coupled beam CL1 with S polarization traveling in the -X direction and a second coupled beam CL2 with S polarization traveling in the -X direction, as follows: In the second optical member 52b, the second polarization conversion element 52b3 converts the first coupled beam CL1 with S polarization into a collimated fourth coupled beam CL4 with P polarization. The second prism 52b2 totally reflects the fourth coupled beam CL4 traveling in the -X direction in the +Z direction. The second polarization beam splitter 52b1 uses the second polarization plane 52bs to polarize and combine the second coupled beam CL2 traveling in the -X direction and the fourth coupled beam CL4 traveling in the +Z direction to form an unpolarized third coupled beam L3 traveling in the +Z direction.

[0091] The end-capped optical fibers 100A, 100B, 100C, or 100D effectively capture the unpolarized, collimated third coupled beam L3. The optical axis of the third coupled beam L3 is parallel to the optical axis of the convex lens contained in the end cap 20. The end-capped optical fibers 100A, 100B, 100C, or 100D further emit the captured unpolarized third coupled beam L3 outside the wavelength beam coupling device 400.

[0092] As described above, in the wavelength beam coupling device 400 of this embodiment, multiple collimated, unpolarized light beams L with different peak wavelengths are emitted from the light source device 300, and the multiple light beams L are wavelength-combined via the first optical member 52a, the first diffraction grating 53a, the second diffraction grating 53b, and the second optical member 52b in that order. As a result, an unpolarized, collimated, high-power third coupled beam CL3 can be formed. The output of the third coupled beam CL3 increases as the number of light beams L increases. The unpolarized, collimated third coupled beam CL3 is effectively captured by end-capped optical fibers 100A, 100B, 100C, or 100D and emitted outside the wavelength beam coupling device 400.

[0093] (Examples) An embodiment of the end-capped optical fiber 100C according to this embodiment will be described below with reference to Figures 7A and 7B. Figure 7A is a microscopic image of the end-capped optical fiber 100C shown in Figure 1C. Figure 7B is a graph showing the change in the radial size of the core and cladding in the optical fiber 10 contained in the end-capped optical fiber 100C along the optical axis. The vertical axis of the graph shown in Figure 7B represents the distance of the core from the optical axis. The minus sign preceding the numerical value on the vertical axis of the graph indicates that the distance is measured below the optical axis. The horizontal axis of the graph represents the distance of the core along the optical axis. A value of zero on the horizontal axis means the position of the end face of the optical fiber 10 to which the end cap 20 is fused. A negative value on the horizontal axis means a position inside the optical fiber 10. The larger the absolute value of the "distance" indicating the position of interest, the further that position is from the end face of the optical fiber 10 towards the inside of the optical fiber 10.

[0094] The black and white triangles in Figure 7B represent the distances (radii) from the optical axes of the upper and lower sides of the core, respectively, in the plane containing the optical axis of the core shown in Figure 7A. The black and white circles in Figure 7B represent the distances (radii) from the optical axes of the upper and lower sides of the cladding, respectively, in the same plane.

[0095] As shown in Figure 7B, when the distance from the end face of the optical fiber 10 is greater than 0.10 mm, the diameter of the core is constant. In contrast, when the distance from the end face of the optical fiber 10 is 0.10 mm or less, the diameter of the core increases as it approaches the end cap 20. Therefore, the portion of the optical fiber 10 that is greater than 0.10 mm from the end face corresponds to the first portion 10a shown in Figure 1C, and the portion that is 0.10 mm or less from the end face corresponds to the second portion 10b shown in Figure 1C.

[0096] (Calculation example) The following describes a calculation example of an end cap optical fiber 100C according to this embodiment, with reference to Figure 8. In the calculation example, the wavelength of light propagating through the optical fiber 10 is 459 nm. The refractive indices of the first core 10a1 and the second core 10b1 are 1.464904, and the refractive indices of the first cladding 10a2 and the second cladding 10b2 are 1.449774. The critical angle θc of the optical fiber 10, which is the maximum propagation angle of light rays that can propagate through the first core 10a1 of the optical fiber 10, is 8.3°. The diameter of the first core 10a1 is 0.11 mm. The refractive index of the end cap 20 is 1.464904. The thickness of the end cap 20 on the optical axis of the convex lens is 23.878 mm. The focal length of the convex lens is 16.3 mm. Zemax OpticStudio was used for the calculation.

[0097] Figure 8 shows a calculation example representing the deviation from the reference plane of the focal position of the convex lens where the divergence angle of the optical beam L is minimized, with respect to the radius of curvature R. The reference plane is the end face, i.e., the fusion surface, where the optical fiber 10 and the end cap 20 are joined. In the example shown in Figure 8, the radius of curvature R = 0° represents, for convenience, the case where the diameter of the core in the optical fiber 10 is constant along the optical axis of the core. In the example shown in Figure 8, the radii of curvature R of the curved side surface of the second core 10b1 are 0.05 mm and 0.10 mm.

[0098] The horizontal axis in Figure 8 represents the radius of curvature R (mm), and the vertical axis in Figure 8 represents the deviation (mm) of the focal position of the convex lens from the reference plane. A negative sign on the vertical axis indicates a position within the second core 10b1, away from the reference plane. On the horizontal axis, R=0mm is plotted at the origin. R=0.05mm and 0.1mm represent the difference from the origin.

[0099] As shown in Figure 8, a tendency was observed for the displacement to increase as the radius of curvature increased. This is thought to be because, as the length of the second portion 10b increased, the pseudo-luminescent surface moved away from the end face of the optical fiber 10 and became located inside the optical fiber 10.

[0100] Next, with reference to Figure 9, a calculation example of the end capped optical fiber 100D according to this embodiment will be described. Figure 9 is a calculation example showing the deviation from the reference plane of the focal position of the convex lens where the divergence angle of the optical beam L is minimized, with respect to the divergence angle θ. In the example shown in Figure 9, the divergence angle θ = 0 (°) represents, for convenience, the case in the optical fiber 10 where the diameter of the core is constant along the optical axis of the core. In the example shown in Figure 9, the divergence angles θ of the linearly spreading side surface of the second core 10b1 are 0.2°, 0.5°, 1.0°, 2.0°, 5.0°, and 10.0°. The length of the second core 10b1 is fixed at 0.15 mm.

[0101] The horizontal axis in Figure 9 represents the divergence angle θ (°), and the vertical axis in Figure 9 represents the deviation (mm) of the focal position of the convex lens from the reference plane. A negative sign on the vertical axis indicates a position within the second core 10b1, away from the reference plane. On the horizontal axis, θ = 0° is plotted at the origin. θ = 0.2°, 0.5°, 1.0°, 2.0°, 5.0°, and 10.0° represent the deviation from the origin.

[0102] As shown in Figure 9, a tendency was observed for the deviation to increase as the spread angle increased. Note that the results for θ=0.2° and 0.5° were almost identical to the results for θ=0°, suggesting that they may contain calculation errors.

[0103] Figure 10A is a graph showing the relationship between the divergence angle of the light beam L and the divergence angle θ of the side surface of the second core 10b1, as exemplified in Figure 9, when the focal position of the convex lens coincides with the reference plane. As shown in Figure 10A, it can be seen that the divergence angle increases as the divergence angle θ becomes greater than 0°.

[0104] Figure 10B is a graph showing the relationship between the minimum value of the divergence angle of the optical beam L and the divergence angle θ of the side surface of the second core 10b1 in the example shown in Figure 9. As shown in Figure 10B, as the divergence angle θ of the side surface of the second core 10b1 increases, the minimum value of the divergence angle of the optical beam L increases and then decreases. In Figure 10B, the position indicated by the thick dashed line is the critical angle θc of the optical fiber 10 set in the simulation, and its value is 8.3°. As shown in Figure 10B, when the divergence angle is greater than 0° and less than θc, the minimum value of the divergence angle is greater than when the divergence angle θ is 0°. Conversely, when the divergence angle θ is greater than θc, the minimum value of the divergence angle becomes the same as when the divergence angle is 0°, and then becomes the lowest again.

[0105] From these results, it was confirmed that it is preferable to make the spreading angle θ larger than the critical angle θc of the optical fiber 10, and to position the focal point F of the convex lens of the end cap 20 inside the optical fiber 10.

[0106] To qualitatively understand the results from Figures 8 to 10B, the behavior of the light beam L is categorized into the following four modes. (Mode A) When the divergence angle < θc and the focal point F (and focal plane) of the convex lens coincides with the end face 12 of the optical fiber 10. (Mode B) When the divergence angle < θc and the focal point F (and focal plane) of the convex lens coincides with the boundary between the first core 10a1 and the second core 10b1. (Mode C) When the divergence angle > θc and the focal point F (and focal plane) of the convex lens coincides with the end face 12 of the optical fiber 10. (Mode D) When the divergence angle > θc, and the focal point F (and focal plane) of the convex lens coincides with the boundary between the first core 10a1 and the second core 10b1.

[0107] Figures 11A and 11B schematically show the propagation of light rays through the optical fiber 10 in modes A and B, respectively. Figures 12A and 12B schematically show the propagation of light rays through the optical fiber 10 in modes C and D, respectively. In each figure, dashed lines represent light rays. θc is the critical angle of the optical fiber 10.

[0108] Mode A, shown in Figure 11A, is a mode in which the divergence angle θ of the side surface of the second core 10b1 is 5.0° (<θc), and the focal point F of the convex lens coincides with the end face 12 of the optical fiber 10. In Mode A, although the divergence angle θ < θc, the optical beam L does not reflect off the side surface of the second core 10b1 until it reaches the end face 12, or it reflects in the vicinity of the end face 12. In this case, the optical emission surface becomes larger than the cross-section of the first core 10a1 (the plane perpendicular to the optical axis), so the divergence angle of the optical beam L becomes larger.

[0109] Mode B, shown in Figure 11B, is a mode in which the divergence angle θ of the side surface of the second core 10b1 is 5.0° (<θc), and the focal point F of the convex lens coincides with the boundary between the first core 10a1 and the second core 10b1. In Mode B, when the light beam L is reflected off the side surface of the second core 10b1, the pseudo-emission surface is the area (solid line) where the reflection positions shown in Figure 11B are connected by a straight line. In other words, this is because the pseudo-core diameter at the focal point is larger than the size of the first core 10a1, so the pseudo-emission surface at the focal point is larger than the cross-section of the first core 10a1. Therefore, the divergence angle of the light beam L is large. This can also be seen from the fact that, as shown in Figure 11B, if we extend the dashed line toward the boundary line between the first core 10a1 and the second core 10b1 and represent this with a dashed line, the distance between the two dashed lines is larger than the diameter of the first core 10a1.

[0110] Mode C, shown in Figure 12A, is a mode in which the divergence angle θ of the side surface of the second core 10b1 is 10.0° (>θc), and the focal point F of the convex lens coincides with the end face 12 of the optical fiber 10. In Mode C, the pseudo-emitting surface is located at the boundary between the first core 10a1 and the second core 10b1. However, since the focal point F does not coincide with the pseudo-emitting surface, the divergence angle of the optical beam L becomes large.

[0111] Mode D, shown in Figure 12B, is a mode in which the divergence angle θ of the side surface of the second core 10b1 is 10.0° (>θc), and the focal point F of the convex lens coincides with the boundary between the first core 10a1 and the second core 10b1. In mode D, since the divergence angle of the side surface of the second core 10b1 is greater than the critical angle θc of the light ray, the light ray is not reflected by the side surface of the second core 10b1. Therefore, the light emission surface at this boundary is the same as the cross-section of the first core 10a1. Also, since the position of the light emission surface and the focal point F coincide, the divergence angle is smaller compared to modes A to C.

[0112] From the behavior of modes A through D, it can be seen that modes B and D are the ones that can minimize the divergence angle. Mode D minimizes the divergence angle the most. That is, it is preferable that the divergence angle θ > θc and that the focal point F (and focal plane) of the convex lens coincides with the boundary between the first core 10a1 and the second core 10b1.

[0113] If the side surface of the second core 10b1 curves outwards, the angle of spread of the side surface of the second core 10b1 may be reinterpreted as the tangent angle of the side surface of the second core 10b1. In this case, the tangent angle of the side surface of the second core 10b1 is the angle between the tangent at a certain point on the side surface of the second core 10b1 and the optical axis of the second core 10b1. As this point moves away from the boundary between the first core 10a1 and the second core 10b1, the tangent angle will eventually become larger than the critical angle θc. Therefore, the curved shape of the side surface of the second core 10b1 is preferable because it makes it easier to achieve a tangent angle larger than the critical angle θc. Also, as the radius of curvature decreases, the curvature increases, making it easier for the tangent angle to become larger than θc. Therefore, even if the length of the second core 10b1 is relatively short, the tangent angle can easily be made larger than the critical angle θc.

[0114] This disclosure includes end-capped optical fibers, fiber arrays, light source devices, and wavelength beam coupling devices as described in the following items.

[0115] [Item 1] An optical fiber comprising a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, An end cap having a first surface connected to the end face of the second portion and a second surface located on the opposite side of the first surface, Equipped with, The diameter of the first core is constant along the optical axis of the first core. The diameter of the second core increases as it approaches the end cap. The end cap includes a convex lens, The second surface includes the convex lens surface of the convex lens, An end capped optical fiber, wherein the focal point of the convex lens is located inside the optical fiber and away from the end face of the second portion.

[0116] [Item 2] An optical fiber comprising a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, An end cap having a first surface connected to the end face of the second portion and a second surface located on the opposite side of the first surface, Equipped with, The diameter of the first core is constant along the optical axis of the first core. The diameter of the second core increases as it approaches the end cap. The end cap includes a convex lens, The second surface includes the convex lens surface of the convex lens, An end capped optical fiber, wherein the thickness of the end cap on the optical axis of the convex lens is less than the distance from the apex of the convex lens to the focal point of the convex lens.

[0117] [Item 3] The optical fiber with an end cap according to claim 1 or 2, wherein the focal point of the convex lens is located inside the second core or at the boundary between the first core and the second core.

[0118] [Item 4] An end capped optical fiber according to any one of items 1 to 3, wherein the second core extends in a curved shape in a plane containing the optical axis of the second core.

[0119] [Item 5] An end capped optical fiber according to any one of items 1 to 3, wherein the second core extends linearly in a plane containing the optical axis of the second core.

[0120] [Item 6] An end capped optical fiber according to any one of items 1 to 5, wherein the area of ​​the first surface is greater than the area of ​​the end face of the second portion.

[0121] [Item 7] The optical fiber is a multimode fiber, and is an end-capped optical fiber as described in any one of items 1 to 6.

[0122] [Item 8] The optical fiber further comprises other end caps connected to the optical fiber, The optical fiber further comprises a third portion located on the opposite side of the second portion with respect to the first portion, the third portion having a third core and a third cladding surrounding the third core, the third core being adjacent to the first core and the third cladding being adjacent to the first cladding. The other end cap has a third surface to which the end face of the third portion is connected, and a fourth surface located on the opposite side of the third surface. The diameter of the third core increases as it approaches the other end caps. The aforementioned other end cap includes other convex lenses, The fourth surface includes the convex lens surface of the other convex lens, The optical fiber with an end cap according to any one of items 1 to 7, wherein the focal point of the other convex lens is located inside the optical fiber and away from the end face of the third portion.

[0123] [Item 9] The optical fiber with an end cap as described in item 8, wherein the focal point of the other convex lens is located inside the third core or at the boundary between the first core and the third core.

[0124] [Item 10] The system comprises multiple end-capped optical fibers, each of which is an end-capped optical fiber as described in any one of items 1 through 9. A fiber array in which the second surfaces of the end caps in the plurality of end-capped optical fibers face the same side.

[0125] [Item 11] A fiber array as described in item 10, wherein the plurality of end-capped optical fibers include a first end-capped optical fiber and a second end-capped optical fiber, A first laser light source that emits a first laser beam, wherein the first laser beam is coupled to the optical fiber contained in the first end capped optical fiber from the side opposite to the end cap, A second laser light source that emits a second laser beam, wherein the second laser beam is coupled to the optical fiber contained in the second end capped optical fiber from the side opposite to the end cap, A light source device equipped with the following features.

[0126] [Item 12] The first laser light has a first peak wavelength, The second laser light has a second peak wavelength that is shorter than the first peak wavelength. The light source device according to item 11, wherein the thickness of the end cap included in the second end capped optical fiber on the optical axis of the convex lens is smaller than the thickness of the end cap included in the first end capped optical fiber on the optical axis of the convex lens.

[0127] [Item 13] The light source device described in item 11 or 12, Diffraction grating and, Equipped with, The first laser light has a first peak wavelength, The second laser light has a second peak wavelength that is shorter than the first peak wavelength. The optical fiber with the first end cap emits a first optical beam, which is collimated with the first laser light, from the second surface of the end cap. The optical fiber with the second end cap emits a second optical beam, in which the second laser light is collimated, from the second surface of the end cap. The diffraction grating is a wavelength beam coupling device that combines the wavelengths of the first light beam and the second light beam. [Industrial applicability]

[0128] The end-capped optical fibers, fiber arrays, light source devices, and wavelength beam coupling devices of this disclosure can be widely used in applications using collimated optical beams. [Explanation of Symbols]

[0129] 10: Optical fiber 10a: First part 10a1: First core 10a2: First cladding 10b: Second part 10b1: Second core 10b2: Second cladding 10c: Third part 10c1: Third core 10c2: Third cladding 12, 13: End faces 20, 21: End caps 22a: First surface 22b: Second surface 23a: Third surface 23b: Fourth surface 24: Optical axis of convex lens 30, 30a~30c: Laser light source 42: Support member 43: Recess 44: Fixing member 46: Screw 52a: First optical element 52a1: First polarizing beam splitter 52a2: First prism 52a3: First polarization conversion element 52as: First polarization plane 52b: Second optical element 52b1: Second polarization beam splitter 52b2: Second prism 52b3: Second polarization conversion element 52bs: Second polarization plane 53a: First diffraction grating 53b: Second diffraction grating 53b1: First region 53b2: Second region 100A, 100A1~100A3, 100B, 100C, 100D, 110A, 110B: Optical fibers with end caps 200, 210, 220: Fiber array 300: Light source device 400: Wavelength beam coupling device CL1~CL4: Coupled beam F, F': Focal point L, La~Lc: Optical beam L1~L3: Polarized beam

Claims

1. An optical fiber comprising a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, An end cap having a first surface connected to the end face of the second portion and a second surface located on the opposite side of the first surface, Equipped with, The diameter of the first core is constant along the optical axis of the first core. The diameter of the second core increases as it approaches the end cap. The end cap includes a convex lens, The second surface includes the convex lens surface of the convex lens, An end capped optical fiber, wherein the focal point of the convex lens is located inside the optical fiber and away from the end face of the second portion.

2. The optical fiber with an end cap according to claim 1, wherein the focal point of the convex lens is located inside the second core or at the boundary between the first core and the second core.

3. The optical fiber with an end cap according to claim 1 or 2, wherein the second core extends in a curved shape in a plane including the optical axis of the second core.

4. The optical fiber with an end cap according to claim 1 or 2, wherein the second core extends linearly in a plane including the optical axis of the second core.

5. The optical fiber with an end cap according to claim 1 or 2, wherein the area of ​​the first surface is larger than the area of ​​the end face of the second portion.

6. The optical fiber with an end cap according to claim 1 or 2, wherein the optical fiber is a multimode fiber.

7. The optical fiber further comprises other end caps connected to the optical fiber, The optical fiber further comprises a third portion located on the opposite side of the second portion with respect to the first portion, the third portion having a third core and a third cladding surrounding the third core, the third core being adjacent to the first core and the third cladding being adjacent to the first cladding. The other end cap has a third surface to which the end face of the third portion is connected, and a fourth surface located on the opposite side of the third surface. The diameter of the third core increases as it approaches the other end caps. The aforementioned other end cap includes other convex lenses, The fourth surface includes the convex lens surface of the other convex lens, The optical fiber with an end cap according to claim 1 or 2, wherein the focal point of the other convex lens is located inside the optical fiber and away from the end face of the third portion.

8. The optical fiber with an end cap according to claim 7, wherein the focal point of the other convex lens is located inside the third core or at the boundary between the first core and the third core.

9. An optical fiber comprising a first portion having a first core and a first cladding surrounding the first core, and a second portion having a second core and a second cladding surrounding the second core, wherein the second core is adjacent to the first core and the second cladding is adjacent to the first cladding, An end cap having a first surface connected to the end face of the second portion and a second surface located on the opposite side of the first surface, Equipped with, The diameter of the first core is constant along the optical axis of the first core. The diameter of the second core increases as it approaches the end cap. The end cap includes a convex lens, The second surface includes the convex lens surface of the convex lens, An end capped optical fiber, wherein the thickness of the end cap on the optical axis of the convex lens is less than the distance from the apex of the convex lens to the focal point of the convex lens.

10. The invention comprises a plurality of end-capped optical fibers, each of which is an end-capped optical fiber according to claim 1 or 2, A fiber array in which the second surfaces of the end caps in the plurality of end capped optical fibers face the same side.

11. A fiber array according to claim 10, wherein the plurality of end-capped optical fibers include a first end-capped optical fiber and a second end-capped optical fiber, A first laser light source that emits a first laser beam, wherein the first laser beam is coupled to the optical fiber contained in the first end capped optical fiber from the side opposite to the end cap, A second laser light source that emits a second laser beam, wherein the second laser beam is coupled to the optical fiber contained in the second end capped optical fiber from the side opposite to the end cap, A light source device equipped with the following features.

12. The first laser light has a first peak wavelength, The second laser light has a second peak wavelength that is shorter than the first peak wavelength. The light source device according to claim 11, wherein the thickness of the end cap included in the second end capped optical fiber on the optical axis of the convex lens is smaller than the thickness of the end cap included in the first end capped optical fiber on the optical axis of the convex lens.

13. The light source device according to claim 11, Diffraction grating and, Equipped with, The first laser light has a first peak wavelength, The second laser light has a second peak wavelength that is shorter than the first peak wavelength. The optical fiber with the first end cap emits a first optical beam, which is a collimated first laser beam, from the second surface of the end cap. The optical fiber with the second end cap emits a second optical beam, in which the second laser light is collimated, from the second surface of the end cap. The diffraction grating is a wavelength beam coupling device that combines the wavelengths of the first light beam and the second light beam.

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