Optical waveguide

Abstract

The present invention keeps bend loss at a low level in an optical waveguide in which a core includes a bend section. This optical waveguide (1) comprises: a core (11) that includes a bend section (11b); and a cladding (12) that surrounds the core (11) and has a lower refractive index than the core (11). The refractive index of the cladding (12) on the opposite side of the bend section (11b) from the curvature center (o) side of the bend section (11b) is lower than the refractive index of the cladding (12) on the curvature center (o) side of the bend section (11b).

Description

Technical Field

[0001] The present invention relates to an optical waveguide whose core includes a curved portion.

Background Art

[0002] Optical waveguides are widely used for efficiently transmitting light. Silicon waveguides and optical fibers are examples of optical waveguides. Further, Patent Document 1 describes P a three-dimensional optical waveguide made of a plastic material is disclosed. This three-dimensional optical waveguide includes a core having two crank-shaped bent curved portions.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of Invention

Problems to be Solved by the Invention

[0004] In an optical waveguide, bending loss occurs when the core includes a curved portion. There are the following two reasons for the occurrence of bending loss.

[0005] First reason: In the straight portion, the symmetry axis of the guided mode is located on the central axis of the core, whereas in the curved portion, the guided mode of sym name metry axis is located on the outer side of the central axis of the core (the side farther from the center of curvature of the curved portion). This guided mode mismatch causes loss at the boundary between the straight portion and the curved portion.

[0006] Second reason: The smaller the bending radius of the curved portion, the stronger the mode coupling between the guided mode and the radiation mode. Therefore, when the bending radius is small, this mode coupling causes loss in the curved portion.

[0007] As a method to suppress bending loss, a technique is known in which the curved portion of the core is shifted inward (towards the side closer to the center of curvature of the curved portion), thereby reducing the mismatch of the waveguide modes mentioned above. However, if this technique is adopted, a step is created between the curved portion and the straight portion of the core. but This can result in losses due to this step. Furthermore, this technology cannot suppress the losses caused by mode coupling to the radiated modes as described above.

[0008] Another method for suppressing bending losses is to reduce the mismatch of waveguide modes described above by making the shape of the curved portion of the core a clothoid curve, as shown in Figure 6(b). However, while this technique can reduce losses at the boundary between the straight and curved portions, losses occur throughout the curved portion as the radius of curvature gradually decreases. Furthermore, if the change in the radius of curvature is made gradual in order to reduce losses throughout the curved portion, the curved portion becomes longer, resulting in increased waveguide losses in the curved portion. Moreover, this technique cannot suppress the losses due to mode coupling to the radiated modes described above.

[0009] One aspect of the present invention has been made in view of the above-mentioned problems, and its object is to minimize bending loss in an optical waveguide in which the core includes a curved portion. [Means for solving the problem]

[0010] An optical waveguide according to Embodiment 1 of the present invention comprises a core including a curved portion, and a cladding surrounding the core having a refractive index lower than that of the core, wherein the refractive index of the cladding on the side opposite to the center of curvature of the curved portion is lower than the refractive index of the cladding on the side of the center of curvature of the curved portion. [Effects of the Invention]

[0011] According to the present invention, bending loss can be kept to a minimum in an optical waveguide in which the core includes a curved portion. [Brief explanation of the drawing]

[0012] [Figure 1] These are a plan view and a cross-sectional view showing the configuration of an optical waveguide according to one embodiment of the present invention. [Figure 2] Figure 1 is a graph showing the refractive index distribution in an optical waveguide. [Figure 3] Figure 1 shows a graph illustrating a modified refractive index distribution in an optical waveguide. [Figure 4] Figure 1 is a perspective view showing an optical device that includes an optical waveguide as an edge coupler. [Figure 5] (a) is a side view of an optical fiber with a connector, including the optical waveguide shown in Figure 1. (b) is a cross-sectional view of the same optical fiber with a connector. [Figure 6] This is a plan view showing the configuration of an optical waveguide related to the reference configuration. [Modes for carrying out the invention]

[0013] (Configuration of optical waveguides) The configuration of the optical waveguide 1 according to one embodiment of the present invention will be described with reference to Figure 1. Figure 1(a) is a plan view showing the configuration of the optical waveguide 1, and Figure 1(b) is a cross-sectional view showing the configuration of the optical waveguide 1.

[0014] As shown in Figure 1, the optical waveguide 1 comprises a core 11 and a cladding 12.

[0015] The core 11 is a region with a refractive index higher than that of the cladding 12 and is responsible for guiding light. In this embodiment, the cross-sectional shape of the core 11 is square. However, the cross-sectional shape of the core 11 is arbitrary and may be rectangular, circular, or elliptical, for example. The core 11 includes a straight portion 11a whose central axis 11c is straight and a curved portion 11b whose central axis 11c is arc-shaped.

[0016] The cladding 12 is a region having a lower refractive index than the core 11, and serves the function of confining light in the core 11. The core 12 is embedded in the cladding 12, and the cladding 12 extends in a direction other than the extending direction of the core 11 o and others surrounds the core 11.

[0017] In the present embodiment, the materials of the core 11 and the cladding 12 are polymers whose refractive index can be controlled by laser irradiation, and more specifically, are polyvinyl alcohol to which titanium oxide is added. The concentration of titanium oxide in the polyvinyl alcohol is not particularly limited, and is, for example, 20% by volume or more and 60% by volume or less.

[0018] The refractive index of the polyvinyl alcohol to which titanium oxide is added has, for example, an energy density of 2 J / mm 2 , can be increased by irradiating a femtosecond laser with a wavelength of 515 nm (hereinafter also referred to as "low energy density laser"). The reason why the refractive index increases by irradiation with the low energy density laser is as follows.

[0019] That is, in the region irradiated with the low energy density laser, multiphoton absorption occurs and the temperature locally rises. As the temperature rises, dehydration condensation of polyvinyl alcohol proceeds, so that the density of the region increases, and as a result, the refractive index of the region increases. At this time, the amount of increase in the refractive index can be adjusted by changing the power and / or the irradiation time of the low energy density laser to be irradiated.

[0020] Conversely, the refractive index of the polyvinyl alcohol to which titanium oxide is added has, for example, an energy density of 20 J / mm 2 , can be decreased by irradiating a femtosecond laser with a wavelength of 515 nm (hereinafter also referred to as "high energy density laser"). The reason why the refractive index decreases by irradiation with the high energy density laser is as follows.

[0021] In other words, in a region irradiated with a high-energy-density laser, laser ablation occurs, causing localized volatilization of polyvinyl alcohol. As polyvinyl alcohol volatilizes, vacancies are formed, reducing the density in that region and consequently lowering its refractive index. The amount of this reduction in refractive index can be adjusted by changing the power and / or irradiation time of the high-energy-density laser.

[0022] (Refractive index distribution of optical waveguide) Next, the refractive index distribution of the optical waveguide 1 described above will be explained with reference to Figure 2. Figure 2(a) is a graph showing the refractive index distribution n(x) on the line segment AA' (see Figure 1) perpendicular to the central axis 11c of the core 11 in the straight portion 11a of the core 11, and Figure 2(b) is a graph showing the refractive index distribution n'(x) on the line segment BB' (see Figure 1) perpendicular to the central axis 11c of the core 11 in the curved portion 11b of the core 11.

[0023] Furthermore, the refractive index distribution on any line segment perpendicular to the central axis 11c of the core 11 in the straight portion 11a of the core 11 coincides with or approximately coincides with the refractive index distribution n(x) on line segment AA'. Hereinafter, this refractive index distribution n(x) will be referred to as the "refractive index in the straight portion 11a". distribution It is also written as n(x). Similarly, the refractive index distribution on any line segment perpendicular to the central axis 11c of the core 11 in the curved portion 11b of the core 11 coincides with or approximately coincides with the refractive index distribution n'(x) on the line segment BB'. Hereafter, this refractive index distribution n'(x) will also be written as "refractive index distribution n'(x) in the curved portion 11b".

[0024] In this embodiment, the refractive index distribution n(x) in the straight section 11a is a top-hat type refractive index distribution defined by the following equation (a). Here, the x-axis is taken to be perpendicular to the central axis 11c of the core 11, and the central axis 11c of the core 11 is taken as the origin. Also, d is the width when the core 11 is viewed from above (the width in the direction perpendicular to the central axis 11c of the core 11).

number

number

number

[0025] Furthermore, the refractive index distribution n'(x) in the curved portion 11b is not limited to that expressed by equation (b) above. That is, the refractive index distribution n'(x) in the curved portion 11b is acceptable as long as it is expressed by equation (d) below.

number

[0026] Furthermore, the refractive index distribution n'(x) in the curved portion 11b is not limited to that expressed by equation (d) above. That is, the refractive index distribution n'(x) in the curved portion 11b only needs to satisfy the condition (hereinafter also referred to as "condition A") that the refractive index of the cladding 12 on the opposite side (outside) of the curvature center o with respect to the curved portion 11b of the core 11 is lower than the refractive index of the cladding 12 on the side (inside) of the curvature center o. This makes it possible to suppress the mismatch in refractive index distribution that may occur at the boundary between the straight portion 11a and the curved portion 11b, and also suppresses the coupling of light guided by the core 11 into a radiation mode that leaks out on the opposite side of the curvature center o. As a result, the loss caused by the inclusion of the curved portion 11b in the core 11 (i.e., bending loss) can be kept small.

[0027] An example of the refractive index distribution n'(x) in the curved section 11b that satisfies the above condition A is shown in Figure 3.

[0028] In the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(a), the refractive index n0out (a constant value) of the cladding 12 on the opposite side of the curvature center o to the curved portion 11b of the core 11 is lower than the refractive index n0in (a constant value) of the cladding 12 on the curvature center o side, thus satisfying the above condition A. Therefore, the loss caused by the inclusion of the curved portion 11b in the core 11 (i.e., bending loss) can be kept to a minimum.

[0029] In the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(b), the maximum refractive index n0out of the cladding 12 on the opposite side of the curvature center o relative to the curved portion 11b of the core 11 is lower than the refractive index n0in (a constant value) of the cladding 12 on the curvature center o side, thus satisfying the above condition A. Therefore, the loss caused by the inclusion of the curved portion 11b in the core 11 (i.e., bending loss) can be kept to a minimum.

[0030] Furthermore, in the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(b), the condition (hereinafter also referred to as "condition B") is satisfied, in which the refractive index of the cladding 12 on the side opposite to the curvature center o relative to the curved portion 11b of the core 11 gradually decreases as it moves away from the curvature center o. Therefore, the mismatch in refractive index distribution that may occur at the boundary between the straight portion 11a and the curved portion 11b can be further suppressed in the cladding 12 on the side opposite to the curvature center o.

[0031] In Figure 3(b), an example is shown in which the refractive index of the cladding 12 on the side opposite to the center of curvature o relative to the curved portion 11b of the core 11 decreases continuously as it moves away from the center of curvature o, but the invention is not limited to this. An example may also be adopted in which the refractive index of the cladding 12 on the side opposite to the center of curvature o relative to the curved portion 11b of the core 11 decreases stepwise as it moves away from the center of curvature o.

[0032] In the refractive index distribution n'(x) at the curved portion 11b shown in Figure 3(c), the refractive index n0out (a constant value) of the cladding 12 on the opposite side of the curvature center o to the curved portion 11b of the core 11 is lower than the refractive index n0in (a constant value) of the cladding 12 on the curvature center o side, thus satisfying the above condition A. Therefore, the loss caused by the inclusion of the curved portion 11b in the core 11 (i.e., bending loss) can be kept to a minimum.

[0033] Furthermore, the refractive index distribution n'(x) in the curved section 11b shown in Figure 3(c) satisfies the condition (hereinafter also referred to as "condition C") that the refractive index of the core 11 gradually decreases as it moves away from the center of curvature o. Therefore, the mismatch in refractive index distribution that may occur at the boundary between the straight section 11a and the curved section 11b can be further suppressed in the core 11.

[0034] In Figure 3(c), an example is shown in which the refractive index of the core 11 decreases continuously as it moves away from the center of curvature o, but the model is not limited to this. An example may also be adopted in which the refractive index of the core 11 decreases in steps as it moves away from the center of curvature o.

[0035] In the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(d), the maximum value n0out of the refractive index of the cladding 12 on the opposite side of the curvature center o relative to the curved portion 11b of the core 11 is lower than the refractive index n0in (a constant value) of the cladding 12 on the curvature center o side, thus satisfying the above condition A. Therefore, the loss caused by the inclusion of the curved portion 11b in the core 11 (i.e., bending loss) can be kept to a minimum.

[0036] Furthermore, the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(d) satisfies conditions B and C above. Therefore, the mismatch in refractive index distribution that may occur at the boundary between the straight portion 11a and the curved portion 11b can be further suppressed in the cladding 12 and core 11 on the side opposite to the curvature center o.

[0037] In Figure 3(d), an example is shown in which the refractive index of the cladding 12 on the side opposite to the center of curvature o relative to the curved portion 11b of the core 11 decreases continuously as it moves away from the center of curvature o, but the invention is not limited to this. An example may also be adopted in which the refractive index of the cladding 12 on the side opposite to the center of curvature o relative to the curved portion 11b of the core 11 decreases in steps as it moves away from the center of curvature o.

[0038] Furthermore, while Figure 3(d) illustrates a configuration in which the refractive index of the core 11 decreases continuously as it moves away from the center of curvature o, the model is not limited to this configuration. A configuration in which the refractive index of the core 11 decreases gradually as it moves away from the center of curvature o may also be adopted.

[0039] Furthermore, in the refractive index distribution n'(x) in the curved portion 11b shown in Figure 2(b), the maximum value n0out of the refractive index of the cladding 12 on the opposite side of the curvature center o relative to the curved portion 11b of the core 11 is lower than the minimum value n0in of the refractive index of the cladding 12 on the curvature center o side, thus satisfying the above condition A. For this reason, the loss caused by the inclusion of the curved portion 11b in the core 11 (i.e., bending loss) can be kept to a minimum.

[0040] Furthermore, in the refractive index distribution n'(x) in the curved portion 11b shown in Figure 2(b), in addition to conditions B and C above, the condition (hereinafter also referred to as "condition D") is satisfied, which states that the refractive index of the cladding 12 on the curvature center o side of the core 11 gradually increases as it approaches the curvature center o. This further suppresses the mismatch in refractive index distribution that may occur at the boundary between the straight portion 11a and the curved portion 11b in the cladding 12 on the opposite side of the curvature center o, the core 11, and the cladding 12 on the curvature center o side.

[0041] (Examples of application) The optical waveguide 1 described above can be used, for example, as a coupler interposed between two optical waveguides to optically couple the cores of these two optical waveguides.

[0042] Figure 4 is a perspective view of the optical device 100, which includes the optical waveguide 1 as an edge coupler 120.

[0043] The optical device 100 includes an edge coupler 120, a silicon waveguide 110, and a multicore fiber 130. The edge coupler 120 is interposed between the silicon waveguide 110 and the multicore fiber 130 and is used to optically couple the silicon waveguide 110 and the multicore fiber 130.

[0044] The silicon waveguide 110 comprises a substrate 111, a lower cladding layer 112 formed on the upper surface of the substrate 111, a plurality of cores 113 formed on the upper surface of the lower cladding layer 112, and an upper cladding layer (not shown) formed on the upper surface of the lower cladding layer 112 so as to surround each of the plurality of cores 113. The substrate 111 and the cores 113 are made of silicon, while the lower cladding layer 112 and the upper cladding layer are made of silica. The refractive index of the silica constituting the lower cladding layer 112 and the upper cladding layer is lower than the refractive index of the silicon forming each of the plurality of cores 113. This enables the confinement of light into each of the plurality of cores 113.

[0045] The multicore fiber 130 comprises multiple cores 131 and a cladding 132 surrounding each of the multiple cores 131. The cores 131 and cladding 132 are made of quartz glass. The cores 131 and cladding 132 of the multicore fiber 130 are dopanted such that the refractive index of the cladding 132 is lower than that of the cores 131. This enables the confinement of light into each of the multiple cores 131.

[0046] The edge coupler 120 comprises a plurality of cores 121 and a cladding 122 surrounding each of the plurality of cores 121. The cores 121 and cladding 122 are examples of the optical waveguide 1 described above and are made of titanium dioxide-doped polyvinyl alcohol. Laser light is shone onto the cores 121 and cladding 122 of the edge coupler 120 such that the refractive index of the cladding 122 is lower than that of the cores 121. This achieves light confinement into each of the plurality of cores 121.

[0047] The core 121 of the edge coupler 120 is connected at one end face to the core 113 of the silicon waveguide 110, and at the other end face to the core 131 of the multicore fiber 130. This enables optical coupling between the core 113 of the silicon waveguide 110 and the core 131 of the multicore fiber 130.

[0048] At the end face of the silicon waveguide 110, the cores 113 are arranged in a straight line. On the other hand, at the end face of the multicore fiber 130, the cores 131 are arranged in a 2x3 matrix. Therefore, in the edge coupler 120, the cores 121 meander three-dimensionally inside the cladding 122. When manufacturing the edge coupler 120 using three-dimensional stereolithography technology, it is easy to make the cores 121 meander three-dimensionally inside the cladding 122.

[0049] In the edge coupler 120, the size of the core 121 may or may not be constant. As an example of the latter, if the size of the core 131 of the multicore fiber 130 is larger than the size of the core 113 of the silicon waveguide 110, the following configuration can be adopted.

[0050] Configuration 1: The size of the core 121 of the edge coupler 120 is reduced at the end facing the silicon waveguide 110 (for example, tapered). That is, the size of the core 121 of the edge coupler 120 is matched to the size of the core 113 of the silicon waveguide 110 at the end facing the silicon waveguide 110, and the remaining portion is matched to the size of the core 131 of the multicore fiber 130.

[0051] Configuration 2: The size of the core 121 of the edge coupler 120 is increased at the end on the multicore fiber 130 side (for example, by increasing it in a reverse taper shape). That is, the size of the core 121 of the edge coupler 120 can be matched with the size of the core 131 of the multicore fiber 130 at the end on the multicore fiber 130 side, and matched with the size of the core 113 of the silicon waveguide 110 in the remaining portion.

[0052] When configuration 1 or configuration 2 is adopted, the connection loss at the connection point between the edge coupler 120 and the silicon waveguide 110, and the connection loss at the connection point between the edge coupler 120 and the multicore fiber 130 can be kept to a minimum.

[0053] (Other application examples) For example, the optical fiber MF with connector may include the optical waveguide 1 described above, a connector that accommodates one end face of the optical waveguide 1, including the core, so as to be connectable to another optical waveguide, and an optical fiber that is optically connected to the other end face of the optical waveguide 1, including the core.

[0054] In Figure 5, (a) is a side view of an optical fiber MF with a connector attached via a ferrule F to a connector housing CH, and (b) is a cross-sectional view of the optical fiber MF with a connector. Here, the connector of the optical fiber MF with a connector is composed of a connector housing CH that accommodates one end face including the core of an optical waveguide 1 so as to be connectable to another optical waveguide. For example, the optical waveguide 1 of the optical fiber MF with a connector may have one core, in which case the optical fiber of the optical fiber MF with a connector may be a single-core fiber. Alternatively, for example, the optical waveguide 1 of the optical fiber MF with a connector may have multiple cores, in which case the optical fiber of the optical fiber MF with a connector may be a multi-core optical fiber or a bundled optical fiber. Figure 5(b) shows, as an example, multiple cores a1 to a4 of the optical fiber MF with a connector.

[0055] For example, the silicon waveguide 110, edge coupler 120, and multicore fiber 130 provided in the optical device 100 described above correspond to other optical waveguides connected to the optical fiber MF with connector, optical waveguide 1 provided in the optical fiber MF with connector, and the optical fiber provided in the optical fiber MF with connector, respectively. That is, optical waveguide 1 provided in the optical fiber MF with connector optically couples the optical waveguide 1 with the optical fiber MF with connector optically couples the optical waveguide 1 with the optical fiber MF with connector optically couples the other optical waveguide with the optical fiber MF with connector, similar to how the edge coupler 120 optically couples the silicon waveguide 110 and the multicore fiber 130. However, the other optical waveguide and the optical fiber MF with connector are detachably connected by the connector described above.

[0056] This enables coupling of optical waveguides with multiple cores to multicore optical fibers or bundled optical fibers.

[0057] For example, the shortest distance between each core in one end face of the optical waveguide 1 of the optical fiber MF with connector may be smaller than the shortest distance between each core in the other end face of the optical waveguide 1. This enables coupling between a multicore fiber and a silicon waveguide for silicon waveguides where the shortest distance between cores is smaller than that of a multicore fiber.

[0058] Furthermore, for example, the shortest distance between each core included in one end face of the optical waveguide 1 of the optical optical fiber MF with connector may be greater than the shortest distance between each core included in the other end face of the optical waveguide 1. This enables coupling between multicore fibers and single-core fibers for multiple single-core fibers where the shortest distance between cores is greater than that of a multicore fiber.

[0059] Furthermore, for example, the diameter of the core included in one end face of the optical waveguide 1 of the optical fiber MF with connector may be smaller than the diameter of the core included in the other end face of the optical waveguide 1. This enables coupling between a multicore fiber and a silicon waveguide for silicon waveguides whose core diameter is smaller than that of a multicore fiber.

[0060] Furthermore, for example, a lens may be provided on one end face of the optical waveguide 1 of the optical fiber MF with connector. That is, the lens is interposed between the optical waveguide 1 and the other optical waveguide to which the optical fiber MF with connector is connected. Here, the lens may be fabricated, for example, by the ImpFab method described later. This makes it possible to realize collimated light that prevents light diffusion in the gap between the end faces where the optical waveguide of the optical fiber with connector and the other optical waveguide are connected. In addition, a conversion of the mode field diameter in the gap between the end faces is realized.

[0061] (Additional notes) The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means included in the embodiments described above are also included within the technical scope of the present invention.

[0062] For example, the optical waveguides according to each embodiment described above can be fabricated by a photopolymerization method that creates a desired shape pattern or refractive index distribution in the gel by exposing a swollen gel to light. An example of such a photopolymerization method is Implosion Fabrication (hereinafter abbreviated as ImpFab) (see, for example, References 1 and 2 below). In the ImpFab method, after exposing a gel (e.g., hydrogel) using a high-magnification, high-NA objective lens, the gel is dried and shrunk to create a micrometer-scale or nanometer-scale pattern. The ImpFab method performs photopolymerization by applying the principle of one-photon absorption or multi-photon absorption. Therefore, in addition to photopolymerization in the in-plane direction parallel to the main surface of the gel, photopolymerization can be performed in the thickness direction perpendicular to the main surface of the gel.

[0063] Here, the dried gel obtained by exposing the swollen gel as described above and then drying it is not particularly limited as long as it can swell and shrink with a solvent, for example, poly(meth)acrylic acid, polyvinyl alcohol, poly Echi It may contain lenglycol and other materials, and may contain at least one of polymethacrylic acid and polyacrylic acid. The dried gel does not need to consist of a single monomer unit, but may be a copolymer of multiple types of monomers. in The dry gel can be made of materials such as polyacrylic acid or a copolymer of acrylic acid and acrylamide. The dry gel can also contain a crosslinking agent; for example, the copolymer of acrylic acid and acrylamide can be crosslinked with a crosslinking agent such as N,N'-methylenebisacrylamide. Furthermore, the dry gel can be a block polymer or block copolymer, formed by linear polymerization of these materials, in which case a network structure can be formed by chemical or physical crosslinking with a crosslinking agent.

[0064] (Reference 1) U.S. Patent Application Publication No. 2017 / 0081489 (Reference 2) Daniel Oran et. al.,Science 362, 1281-1285 (2018) 14 December 2018 〔summary〕 An optical waveguide according to Embodiment 1 of the present invention comprises a core including a curved portion, and a cladding surrounding the core having a refractive index lower than that of the core, wherein the refractive index of the cladding on the side opposite to the center of curvature of the curved portion is lower than the refractive index of the cladding on the side of the center of curvature of the curved portion.

[0065] According to the above configuration, it is possible to suppress the mismatch in refractive index distribution that may occur at the boundary between the straight and curved sections, and to suppress the coupling of light guided through the core into radiation modes that leak out on the opposite side of the curvature center. Therefore, bending losses can be suppressed.

[0066] An optical waveguide according to aspect 2 of the present invention is an optical waveguide according to aspect 1, wherein the refractive index of the cladding on the side opposite to the center of curvature with respect to the curved portion gradually decreases as it moves away from the center of curvature.

[0067] According to the above configuration, the mismatch in refractive index distribution that may occur at the boundary between the straight section and the curved section can be further suppressed in the cladding on the side opposite to the center of curvature relative to the curved section.

[0068] An optical waveguide according to aspect 3 of the present invention is an optical waveguide according to either aspect 1 or 2, wherein the refractive index of the core gradually decreases as it moves away from the center of curvature.

[0069] According to the above configuration, the mismatch in refractive index distribution that may occur at the boundary between the straight section and the curved section can be further suppressed in the core.

[0070] An optical waveguide according to aspect 4 of the present invention is an optical waveguide according to any one of aspects 1 to 3, wherein the refractive index of the cladding on the curvature center side with respect to the curved portion gradually increases as it approaches the curvature center.

[0071] According to the above configuration, the mismatch in refractive index distribution that may occur at the boundary between the straight section and the curved section can be further suppressed in the cladding on the curvature side of the curved section.

[0072] An optical waveguide according to aspect 5 of the present invention is an optical waveguide according to any one of aspects 1 to 4, wherein the core includes a straight portion, and the refractive index distribution n'(x) on a line segment perpendicular to the central axis of the core in the curved portion is represented by the following formula (1).

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[0073] According to the above configuration, it is possible to eliminate the mismatch in refractive index distribution that may occur at the boundary between the straight section and the curved section.

[0074] The optical waveguide according to aspect 6 of the present invention is an optical waveguide according to aspect 5, wherein the refractive index distribution n(x) on a line segment perpendicular to the central axis of the core in the straight portion is expressed by the following equation (2), and the refractive index distribution n'(x) on a line segment perpendicular to the central axis of the core in the curved portion is expressed by the following equation (3).

number

number

[0075] An optical waveguide according to aspect 7 of the present invention is an optical waveguide according to any one of aspects 1 to 6, wherein the core meanders three-dimensionally within the cladding.

[0076] With the above configuration, it is possible to realize an optical waveguide with low bending loss, even though the core meanders three-dimensionally within the cladding.

[0077] An optical waveguide according to aspect 8 of the present invention is a coupler interposed between a first optical waveguide and a second optical waveguide for optically coupling the core of the first optical waveguide and the core of the second optical waveguide. This is an optical waveguide relating to any one of the embodiments 1 to 7.

[0078] According to the above configuration, the first optical waveguide and the second optical waveguide are connected via an optical waveguide with low bending loss. of It can be coupled with an optical waveguide.

[0079] An optical waveguide according to aspect 9 of the present invention is an optical waveguide according to aspect 8, wherein the end of the core of the optical waveguide on the first optical waveguide side is reduced in size to match the core of the first optical waveguide, or the end of the core of the optical waveguide on the second optical waveguide side is enlarged to match the core of the second optical waveguide.

[0080] With the above configuration, connection losses with the first optical waveguide or with the second optical waveguide can be kept to a minimum.

[0081] An optical fiber with a connector according to aspect 10 of the present invention comprises an optical waveguide according to any one of aspects 1 to 9, a connector that houses one end face of the optical waveguide, including the core, so as to be connectable to another optical waveguide, and an optical fiber that is optically connected to the other end face of the optical waveguide, including the core.

[0082] According to the above configuration, it is possible to connect an optical waveguide with a connector to another optical waveguide via an optical waveguide with low bending loss.

[0083] An optical fiber with a connector according to aspect 11 of the present invention is an optical fiber with a connector according to aspect 10, wherein the optical waveguide has a plurality of cores and the optical fiber is a multicore optical fiber or a bundled optical fiber.

[0084] According to the above configuration, an optical waveguide having multiple cores can be coupled with a multicore optical fiber or a bundled optical fiber.

[0085] An optical fiber with a connector according to embodiment 12 of the present invention is characterized in that the shortest distance between each core included in one end face of the optical waveguide is smaller than the shortest distance between each core included in the other end face of the optical waveguide, embodiment 1 1 This is an optical fiber with a connector.

[0086] With the above configuration, it is possible to couple a silicon waveguide with a minimum distance between cores that is shorter than that of a multicore fiber.

[0087] An optical fiber with a connector according to embodiment 13 of the present invention is characterized in that the shortest distance between each core included in one end face of the optical waveguide is greater than the shortest distance between each core included in the other end face of the optical waveguide, embodiment 1 1 This is an optical fiber with a connector.

[0088] With the above configuration, multiple single-core fibers whose shortest distance between cores is greater than that of a multi-core fiber can be coupled with a multi-core fiber.

[0089] An optical fiber with a connector according to embodiment 14 of the present invention is an optical fiber with a connector according to embodiment 10, wherein the diameter of the core included in one end face of the optical waveguide is smaller than the diameter of the core included in the other end face of the optical waveguide.

[0090] With the above configuration, it is possible to couple a silicon waveguide with a core diameter smaller than that of a multicore fiber with a silicon waveguide.

[0091] The optical fiber with a connector according to aspect 15 of the present invention is an optical fiber with a connector according to aspect 10, wherein a lens is provided on one end face of the optical waveguide.

[0092] According to the above configuration, collimated light can be realized by preventing light diffusion in the gap between the end faces where the optical waveguide of the connector-equipped optical fiber connects to another optical waveguide. Furthermore, the mode field diameter in the gap between the end faces can be converted.

[0093] The optical waveguide according to aspect 16 of the present invention is an optical waveguide according to any one of aspects 1 to 9, comprising a dried gel obtained by exposing a swollen gel to light and then drying it.

[0094] With the above configuration, micrometer-scale patterns or nanometer-scale patterns can be fabricated using stereolithography.

[0095] An optical waveguide according to aspect 17 of the present invention is an optical waveguide according to aspect 16, wherein the dried gel contains poly(meth)acrylic acid, or at least one of polymethacrylic acid and polyacrylic acid.

[0096] According to the above configuration, micrometer-scale patterns or nanometer-scale patterns can be more effectively fabricated using stereolithography. [Explanation of Symbols]

[0097] 1 Optical waveguide 11 cores 11a Straight section 11b Curved section 11c center axis 12 clad

Claims

1. A core including a curved section, The system comprises a cladding surrounding the core, having a refractive index lower than that of the core, With respect to the curved portion, the refractive index of the cladding on the side opposite to the center of curvature of the curved portion is lower than the refractive index of the cladding on the side of the center of curvature of the curved portion. optical waveguide.

2. The refractive index of the cladding on the side opposite to the center of curvature with respect to the curved portion gradually decreases as it moves away from the center of curvature. The optical waveguide according to claim 1.

3. The refractive index of the core gradually decreases as it moves away from the center of curvature. The optical waveguide according to claim 1.

4. With respect to the curved portion, the refractive index of the cladding on the side of the curvature center gradually increases as it approaches the curvature center. The optical waveguide according to claim 1.

5. The core includes a straight section, In the curved portion, the refractive index distribution n'(x) on the line segment perpendicular to the central axis of the core is represented by the following equation (1): The optical waveguide according to claim 1. [Math 1] Here, n(x) is the refractive index distribution on a line segment perpendicular to the central axis of the core in the straight portion, and R is the radius of curvature of the central axis of the curved portion.

6. In the aforementioned straight section, the refractive index distribution n(x) on the line segment perpendicular to the central axis of the core is expressed by the following equation (2): In the curved portion, the refractive index distribution n'(x) on the line segment perpendicular to the central axis of the core is expressed by the following equation (3): The optical waveguide according to claim 5. [Math 2] [Math 3]

7. The core is three-dimensionally meandering within the cladding. An optical waveguide according to any one of claims 1 to 6.

8. A coupler interposed between a first optical waveguide and a second optical waveguide, for optically coupling the core of the first optical waveguide and the core of the second optical waveguide. An optical waveguide according to any one of claims 1 to 6.

9. The end of the core of the optical waveguide on the first optical waveguide side is reduced in size to match the core of the first optical waveguide, or the end of the core of the optical waveguide on the second optical waveguide side is enlarged to match the core of the second optical waveguide. The optical waveguide according to claim 8.

10. An optical waveguide according to any one of claims 1 to 6, A connector that houses one end face of the optical waveguide, including the core, so as to be connectable to another optical waveguide, The optical waveguide comprises an optical fiber optically connected to the other end face of the optical waveguide, including the core, Optical fiber with connector.

11. The optical waveguide has multiple cores, and the optical fiber is a multicore optical fiber or a bundled optical fiber. The optical fiber with connector according to claim 10.

12. The shortest distance between each core included in one end face of the optical waveguide is smaller than the shortest distance between each core included in the other end face of the optical waveguide. The optical fiber with connector according to claim 11.

13. The shortest distance between each core included in one end face of the optical waveguide is greater than the shortest distance between each core included in the other end face of the optical waveguide. The optical fiber with connector according to claim 11.

14. The diameter of the core included in one end face of the optical waveguide is smaller than the diameter of the core included in the other end face of the optical waveguide. The optical fiber with connector according to claim 10.

15. A lens is provided on one end face of the optical waveguide. The optical fiber with connector according to claim 10.

16. The optical waveguide consists of a dried gel obtained by exposing a swollen gel to light and then drying it. An optical waveguide according to any one of claims 1 to 6.

17. The dried gel contains poly(meth)acrylic acid, or at least one of polymethacrylic acid and polyacrylic acid. The optical waveguide according to claim 16.

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