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

optical waveguide

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

[0002] Optical waveguides are widely used to efficiently transmit light. Silicon waveguides and optical fibers are examples of optical waveguides. Patent Document 1 discloses a three-dimensional optical waveguide made of a plastic material. This three-dimensional optical waveguide includes a core that includes two crank-shaped curved portions.

[0003] Japanese Patent Publication No. 2004-29613

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

[0005] First, in the straight section, the axis of symmetry of the guided mode is located on the central axis of the core, whereas in the curved section, the axis of symmetry of the guided mode is located outside the central axis of the core (farther from the center of curvature of the curved section). This mismatch of the guided mode causes loss at the boundary between the straight section and the curved section.

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

[0007] As a method for suppressing bending loss, a technique is known in which the curved portion of the core is shifted inward (toward the center of curvature of the curved portion) to reduce the mismatch of the waveguide mode described above, as shown in Figure 6(a). However, when this technique is adopted, a step is created between the curved portion of the core and the straight portion of the core, resulting in loss due to this step. Furthermore, this technique cannot suppress the loss due to mode coupling to the radiation mode described above.

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

[0009] One aspect of the present invention has been made in view of the above problems, and its object is to reduce bending loss in an optical waveguide whose core includes a curved portion.

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

[0011] According to the present invention, bending loss can be reduced in an optical waveguide in which the core includes a curved portion.

[0012] 1A and 1B are a plan view and a cross-sectional view showing the configuration of an optical waveguide according to one embodiment of the present invention; FIG. 2 is a graph showing the refractive index distribution in the optical waveguide shown in FIG. 1; FIG. 3 is a graph showing a modified example of the refractive index distribution in the optical waveguide shown in FIG. 1; FIG. 4 is a perspective view showing an optical device including the optical waveguide shown in FIG. 1 as an edge coupler; (a) is a side view of a connectorized optical fiber including the optical waveguide shown in FIG. 1; (b) is a cross-sectional view of the connectorized optical fiber; and FIG. 5 is a plan view showing the configuration of an optical waveguide according to a reference embodiment.

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

[0014] As shown in FIG. 1, the optical waveguide 1 includes a core 11 and a clad 12 .

[0015] The core 11 is a region having a refractive index higher than that of the cladding 12, and functions to guide 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, for example, rectangular, circular, or elliptical. The core 11 includes a straight portion 11a whose central axis 11c is linear, 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 to confine light within the core 11. The core 11 is embedded in the cladding 12, and the cladding 12 surrounds the core 11 in directions other than the direction in which the core 11 extends.

[0017] In this embodiment, the material of the core 11 and the clad 12 is a polymer whose refractive index can be controlled by laser irradiation, more specifically, polyvinyl alcohol to which titanium oxide has been added. The concentration of titanium oxide in the polyvinyl alcohol is not particularly limited, but is, for example, 20% by volume or more and 60% by volume or less.

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

[0019] That is, in the region irradiated with the low-energy-density laser, multiphoton absorption occurs, causing a local temperature rise. As the temperature rises, dehydration condensation of polyvinyl alcohol progresses, increasing the density of that region, and as a result, increasing the refractive index of that region. In this case, the amount of increase in the refractive index can be adjusted by changing the power and / or irradiation time of the low-energy-density laser.

[0020] On the other hand, the refractive index of polyvinyl alcohol to which titanium oxide is added is, for example, 2 The refractive index can be reduced by irradiating a 515 nm femtosecond laser (hereinafter also referred to as a "high energy density laser"). The reason why the refractive index is reduced by irradiation with a high energy density laser is as follows.

[0021] That is, in the region irradiated with the high-energy-density laser, laser ablation occurs, causing the polyvinyl alcohol to volatilize locally. When the polyvinyl alcohol volatilizes, voids are formed, reducing the density of the region and, as a result, reducing the refractive index of the region. The amount of reduction in the 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 above-mentioned optical waveguide 1 will be described with reference to Fig. 2. Fig. 2(a) is a graph showing the refractive index distribution n(x) on a line segment AA' (see Fig. 1) that is perpendicular to the central axis 11c of the core 11 in the straight portion 11a of the core 11, and Fig. 2(b) is a graph showing the refractive index distribution n'(x) on a line segment BB' (see Fig. 1) that is perpendicular to the central axis 11c of the core 11 in the curved portion 11b of the core 11.

[0023] In addition, the refractive index distribution on any line segment in the straight portion 11a of the core 11 that is perpendicular to the central axis 11c of the core 11 matches or nearly matches the refractive index distribution n(x) on the line segment AA'. Hereinafter, this refractive index distribution n(x) will also be referred to as the "refractive index n(x) on the straight portion 11a." Similarly, the refractive index distribution on any line segment in the curved portion 11b of the core 11 that is perpendicular to the central axis 11c of the core 11 matches or nearly matches the refractive index distribution n'(x) on the line segment BB'. Hereinafter, this refractive index distribution n'(x) will also be referred to as the "refractive index distribution n'(x) on the curved portion 11b."

[0024] In this embodiment, the refractive index distribution n(x) in the linear portion 11a is a top-hat refractive index distribution defined by the following formula (a): where the x-axis is taken to be perpendicular to the central axis 11c of the core 11 and has the central axis 11c of the core 11 as the origin. Also, d is the width of the core 11 when viewed in plan (the width in the direction perpendicular to the central axis 11c of the core 11). In this embodiment, the refractive index distribution n'(x) in the curved portion 11b is defined by the following formula (b): where the x-axis is taken to be perpendicular to the central axis 11c of the core 11 and has the central axis 11c of the core 11 as the origin. R is the radius of curvature of the central axis 11c of the core 11 in the curved portion 11b of the core 11. As a result, the refractive index distributions are matched at the boundary between the straight portion 11 a and the curved portion 11 b. This is because the effective refractive index distribution n″(x) in the curved portion 11 b is given by the following formula (c) and coincides with the refractive index distribution n(x) in the straight portion 11 a. As described above, the optical waveguide 1 according to this embodiment can eliminate mismatching of the refractive index distribution that may occur at the boundary between the straight portion 11 a and the curved portion 11 b, thereby minimizing loss (i.e., bending loss) caused by the inclusion of the curved portion 11 b in the core 11.

[0025] The refractive index distribution n'(x) in the curved portion 11b is not limited to that expressed by the above formula (b). That is, the refractive index distribution n'(x) in the curved portion 11b may be that expressed by the following formula (d): If the refractive index distribution n′(x) in the curved portion 11 b is expressed by the above formula (d), then it is possible to eliminate mismatching of the refractive index distribution that may occur at the boundary between the straight portion 11 a and the curved portion 11 b, regardless of the refractive index distribution n(x) in the straight portion 11 a. Therefore, it is possible to minimize the loss (i.e., bending loss) that occurs due to the inclusion of the curved portion 11 b in the core 11.

[0026] Furthermore, the refractive index distribution n′(x) in the curved portion 11b is not limited to that expressed by the above formula (d). 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 curved portion 11b of the core 11 from the center of curvature o is lower than the refractive index of the cladding 12 on the opposite side (inside) of the curvature center o. This can suppress mismatching of the refractive index distributions that may occur at the boundary between the straight portion 11a and the curved portion 11b, and can also suppress coupling of light guided through the core 11 to a radiation mode leaking to the opposite side of the center of curvature o. As a result, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

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

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

[0029] 3B, the refractive index profile n'(x) in the curved portion 11b of the core 11 has a maximum refractive index n0out of the cladding 12 on the opposite side of the curvature center o with respect to the curved portion 11b of the core 11 lower than the refractive index n0in (constant value) of the cladding 12 on the curvature center o side, thereby satisfying the above-mentioned condition A. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

[0030] 3B, the refractive index distribution n′(x) in the curved portion 11b satisfies the condition (hereinafter also referred to as “Condition B”) that the refractive index of the cladding 12 on the opposite side to the center of curvature o with respect to the curved portion 11b of the core 11 gradually decreases with increasing distance from the center of curvature 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 opposite side to the center of curvature o.

[0031] 3B illustrates an example in which the refractive index of the cladding 12 on the opposite side of the curved portion 11b of the core 11 from the center of curvature o decreases continuously with increasing distance from the center of curvature o, but the present invention is not limited to this. An example in which the refractive index of the cladding 12 on the opposite side of the curved portion 11b of the core 11 from the center of curvature o decreases stepwise with increasing distance from the center of curvature o may also be employed.

[0032] 3C, the refractive index profile n'(x) in the curved portion 11b of the core 11 has a refractive index n0out (constant value) of the cladding 12 on the opposite side of the curvature center o relative to the curved portion 11b of the core 11 lower than a refractive index n0in (constant value) of the cladding 12 on the curvature center o side, thereby satisfying the above-mentioned condition A. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

[0033] 3C, the refractive index distribution n′(x) in the curved portion 11b satisfies the condition that the refractive index of the core 11 gradually decreases with increasing distance from the center of curvature o (hereinafter also referred to as “Condition C”). Therefore, mismatch in the 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 core 11.

[0034] 3C illustrates an example in which the refractive index of the core 11 decreases continuously with increasing distance from the center of curvature o, but the present invention is not limited to this. An example in which the refractive index of the core 11 decreases stepwise with increasing distance from the center of curvature o may also be adopted.

[0035] 3D, the refractive index profile n'(x) in the curved portion 11b of the core 11 satisfies the above-mentioned condition A because the maximum refractive index n0out of the cladding 12 on the side opposite to the center of curvature o with respect to the curved portion 11b of the core 11 is lower than the refractive index n0in (constant value) of the cladding 12 on the side of the center of curvature o. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

[0036] 3D, the refractive index distribution n′(x) in the curved portion 11b satisfies the above-mentioned conditions B and C. 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 the core 11 on the opposite side to the center of curvature o.

[0037] 3D illustrates an example in which the refractive index of the cladding 12 on the opposite side of the curved portion 11b of the core 11 from the center of curvature o decreases continuously with increasing distance from the center of curvature o, but the present invention is not limited to this. An example in which the refractive index of the cladding 12 on the opposite side of the curved portion 11b of the core 11 from the center of curvature o decreases stepwise with increasing distance from the center of curvature o may also be employed.

[0038] 3D illustrates an example in which the refractive index of the core 11 decreases continuously with increasing distance from the center of curvature o, but the present invention is not limited to this. An example in which the refractive index of the core 11 decreases stepwise with increasing distance from the center of curvature o may also be adopted.

[0039] 2B, the refractive index profile n'(x) in the curved portion 11b of the core 11 also satisfies the above-mentioned condition A because the maximum value n0out of the refractive index of the cladding 12 on the side opposite to the center of curvature o with respect 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 side of the center of curvature o. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

[0040] 2B, in addition to the above-mentioned conditions B and C, the refractive index distribution n′(x) in the curved portion 11b of the core 11 satisfies the condition (hereinafter also referred to as “condition D”) that the refractive index of the cladding 12 on the side of the curvature center o with respect to the curved portion 11b of the core 11 gradually increases toward the curvature center o. This makes it possible to further suppress mismatching of the 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 to the curvature center o, the core 11, and the cladding 12 on the side of the curvature center o.

[0041] (Application Example) 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] FIG. 4 is a perspective view of an optical device 100 including the optical waveguide 1 as an edge coupler 120 .

[0043] The optical device 100 includes a silicon waveguide 110 and a multicore fiber 130 in addition to the edge coupler 120. 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 includes 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, and the lower cladding layer 112 and the upper cladding layer are made of silica. The refractive index of silica constituting the lower cladding layer 112 and the upper cladding layer is lower than the refractive index of silicon constituting each of the plurality of cores 113. This achieves light confinement in each of the plurality of cores 113.

[0045] The multicore fiber 130 includes a plurality of cores 131 and a clad 132 surrounding each of the plurality of cores 131. The cores 131 and the clad 132 are made of silica glass. A dopant is added to the cores 131 and the clad 132 of the multicore fiber 130 so that the refractive index of the clad 132 is lower than the refractive index of the cores 131. This achieves light confinement in each of the plurality of cores 131.

[0046] The edge coupler 120 includes a plurality of cores 121 and a clad 122 surrounding each of the plurality of cores 121. The cores 121 and the clad 122 are an example of the optical waveguide 1 described above, and are made of polyvinyl alcohol doped with titanium oxide. The cores 121 and the clad 122 of the edge coupler 120 are irradiated with laser light so that the refractive index of the clad 122 is lower than the refractive index of the cores 121. This achieves confinement of light in each of the plurality of cores 121.

[0047] One end face of the core 121 of the edge coupler 120 is connected to the core 113 of the silicon waveguide 110, and the other end face is connected to the core 131 of the multicore fiber 130. This achieves 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 2 × 3 matrix. Therefore, in the edge coupler 120, the cores 121 meander three-dimensionally inside the clad 122. When the edge coupler 120 is manufactured using three-dimensional optical shaping technology, it is also easy to make the cores 121 meander three-dimensionally inside the clad 122.

[0049] It should be noted that the size of the core 121 may or may not be constant in the edge coupler 120. As an example of the latter, when 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 (for example, reduced in a tapered shape) at the end on the silicon waveguide 110 side. 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 on the silicon waveguide 110 side, and is matched to the size of the core 131 of the multicore fiber 130 in the remaining portion.

[0051] Configuration 2: The size of the core 121 of the edge coupler 120 is expanded (for example, expanded in an inverse tapered shape) at the end on the multicore fiber 130 side. 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 can be matched with the size of the core 113 of the silicon waveguide 110 in the remaining part.

[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 multi-core fiber 130 can be kept small.

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

[0054] 5A is a side view of a connectorized optical fiber MF to which a connector housing CH is attached via a ferrule F, and FIG. 5B is a cross-sectional view of the connectorized optical fiber MF. Here, the connector included in the connectorized optical fiber MF is configured with a connector housing CH that accommodates one end face including the core of the optical waveguide 1 so that it can be connected to another optical waveguide. For example, the optical waveguide 1 included in the connectorized optical fiber MF may have one core, and in this case, the optical fiber included in the connectorized optical fiber MF may be a single-core fiber. Furthermore, for example, the optical waveguide 1 included in the connectorized optical fiber MF may have multiple cores, and in this case, the optical fiber included in the connectorized optical fiber MF may be a multi-core optical fiber or a bundle optical fiber. FIG. 5B illustrates, as an example, multiple cores a1 to a4 included in the connectorized optical fiber MF.

[0055] For example, the silicon waveguide 110, the edge coupler 120, and the multi-core fiber 130 included in the above-described optical device 100 correspond to another optical waveguide to which the connectorized optical fiber MF is connected, the optical waveguide 1 included in the connectorized optical fiber MF, and the optical fiber included in the connectorized optical fiber MF, respectively. That is, the optical waveguide 1 included in the connectorized optical fiber MF optically couples the other optical waveguide with the optical fiber included in the connectorized optical fiber MF, in the same way that, for example, the edge coupler 120 optically couples the silicon waveguide 110 with the multi-core fiber 130. However, the other optical waveguide and the connectorized optical fiber MF are detachably connected by the above-described connector.

[0056] This allows coupling between an optical waveguide having multiple cores and a multi-core optical fiber or an optical fiber bundle.

[0057] For example, the shortest distance between the cores included in one end face of the optical waveguide 1 included in the connectorized optical fiber MF may be shorter than the shortest distance between the cores included in the other end face of the optical waveguide 1. This allows coupling between the multicore fiber and the silicon waveguide to be achieved for a silicon waveguide whose shortest distance between cores is shorter than that of the multicore fiber.

[0058] Furthermore, for example, the shortest distance between each core included in one end face of the optical waveguide 1 provided in the connectorized optical fiber MF may be longer than the shortest distance between each core included in the other end face of the optical waveguide 1. This allows coupling between a multi-core fiber and a single-core fiber to be achieved for a plurality of single-core fibers whose shortest inter-core distances are longer than that of a multi-core fiber.

[0059] Furthermore, for example, the diameter of the core included in one end face of the optical waveguide 1 included in the connectorized optical fiber MF may be smaller than the diameter of the core included in the other end face of the optical waveguide 1. This allows coupling between the multicore fiber and the silicon waveguide to be achieved for a silicon waveguide having a core diameter smaller than that of the multicore fiber.

[0060] Furthermore, for example, a lens may be provided on one end face of the optical waveguide 1 included in the connectorized optical fiber MF. That is, the lens is interposed between the optical waveguide 1 and another optical waveguide to which the connectorized optical fiber MF is connected. Here, the lens may be formed, for example, by the ImpFab method described below. This makes it possible to realize collimated light that does not diffuse in the gap between the end faces where the optical waveguide included in the connectorized optical fiber and the other optical waveguide are connected. Furthermore, conversion of the mode field diameter is realized in the gap between the end faces.

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

[0062] For example, the optical waveguides according to the above-described embodiments can be fabricated by a stereolithography method, in which a desired shape pattern or refractive index distribution is formed in a swollen gel by exposing the gel to light. One example of such a stereolithography method is the Implosion Fabrication (ImpFab) method (see, for example, References 1 and 2 below). The ImpFab method uses a high-magnification, high-NA objective lens to expose a gel (e.g., a hydrogel) and then dry-shrinks the gel, thereby enabling stereolithography of micrometer- or nanometer-scale patterns. The ImpFab method applies the principles of single-photon absorption or multiphoton absorption to perform stereolithography. Therefore, in addition to stereolithography in the in-plane direction parallel to the main surface of the gel, stereolithography can also be performed in the thickness direction perpendicular to the main surface of the gel.

[0063] Here, the dried gel obtained by drying and shrinking the swollen gel after exposure as described above is not particularly limited as long as it can swell and shrink in a solvent. For example, it may contain poly(meth)acrylic acid, polyvinyl alcohol, polyethylene glycol, etc., or at least one of polymethacrylic acid and polyacrylic acid. The dried gel does not need to be composed of a single monomer unit but can be a copolymer of multiple types of monomers, such as polyacrylic acid or a copolymer of acrylic acid and acrylamide. The dried gel may also contain a crosslinking agent, such as N,N'-methylenebisacrylamide, which may be crosslinked. The dried gel may also be a block polymer or block copolymer in which these materials are linearly polymerized. In this case, the network structure may be formed by chemical or physical crosslinking using a crosslinking agent.

[0064] (Reference 1) US Patent Application Publication No. 2017 / 0081489 (Reference 2) Daniel Oran et. al., Science 362, 1281-1285 (2018) 14 December 2018 [Summary] The optical waveguide according to aspect 1 of the present invention is an optical waveguide comprising a core including a curved portion, and a cladding surrounding the core and having a refractive index lower than that of the core, wherein the refractive index of the cladding on the opposite side of the curved portion from the center of curvature of the curved portion is lower than the refractive index of the cladding on the center of curvature of the curved portion.

[0065] The above configuration can suppress mismatching of the refractive index distribution that may occur at the boundary between the straight section and the curved section, and can also suppress coupling of light guided through the core with a radiation mode that leaks to the side opposite to the center of curvature, thereby suppressing bending loss.

[0066] An optical waveguide according to a second aspect of the present invention is the optical waveguide according to the first aspect, wherein the refractive index of the cladding on the opposite side of the curved portion from the center of curvature gradually decreases with increasing distance from the center of curvature.

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

[0068] An optical waveguide according to a third aspect of the present invention is the optical waveguide according to either one of the first or second aspects, wherein the refractive index of the core gradually decreases with increasing distance from the center of curvature.

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

[0070] An optical waveguide according to a fourth aspect of the present invention is the optical waveguide according to any one of the first to third aspects, in which the refractive index of the cladding on the side of the center of curvature of the curved portion gradually increases as the distance to the center of curvature approaches.

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

[0072] An optical waveguide according to Aspect 5 of the present invention is the optical waveguide according to any one of Aspects 1 to 4, wherein the core includes a straight portion, and a 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 formula (1): Here, n(x) is the refractive index distribution on a line segment that is 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.

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

[0074] An optical waveguide according to a sixth aspect of the present invention is the optical waveguide according to the fifth aspect, in which 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 formula (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 formula (3): According to the above configuration, it is possible to eliminate mismatching of the refractive index distribution that may occur at the boundary between the straight portion and the curved portion.

[0075] An optical waveguide according to a seventh aspect of the present invention is the optical waveguide according to any one of the first to sixth aspects, in which the core meanders three-dimensionally within the cladding.

[0076] According to the above configuration, an optical waveguide with small bending loss can be realized even though the core meanders three-dimensionally within the cladding.

[0077] An optical waveguide according to an eighth aspect of the present invention is the optical waveguide according to any one of the first to seventh aspects, which is a coupler interposed between a first optical waveguide and a second optical waveguide, for optically coupling a core of the first optical waveguide and a core of the second optical waveguide.

[0078] According to the above configuration, the first optical waveguide and the second optical waveguide can be coupled via an optical waveguide with small bending loss.

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

[0080] According to the above configuration, the connection loss with the first optical waveguide or the connection loss with the second optical waveguide can be kept small.

[0081] A connectorized optical fiber according to aspect 10 of the present invention comprises an optical waveguide according to any one of aspects 1 to 9, a connector that accommodates one end face of the optical waveguide that includes the core so that it can be connected to another optical waveguide, and an optical fiber that is optically connected to the other end face of the optical waveguide that includes the core.

[0082] According to the above configuration, it is possible to couple another optical waveguide to a connectorized optical fiber via an optical waveguide with small bending loss.

[0083] An eleventh aspect of the present invention is the connectorized optical fiber according to the tenth aspect, in which the optical waveguide has a plurality of cores, and the optical fiber is a multi-core optical fiber or a bundle optical fiber.

[0084] According to the above configuration, an optical waveguide having a plurality of cores can be coupled to a multi-core optical fiber or an optical fiber bundle.

[0085] A connectorized optical fiber according to aspect 12 of the present invention is a connectorized optical fiber according to aspect 10, in which the shortest distance between each core included in one end face of the optical waveguide is shorter than the shortest distance between each core included in the other end face of the optical waveguide.

[0086] According to the above configuration, it is possible to couple a multi-core fiber to a silicon waveguide in which the shortest distance between cores is shorter than that of a multi-core fiber.

[0087] A connectorized optical fiber according to aspect 13 of the present invention is a connectorized optical fiber according to aspect 10, in which 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.

[0088] According to the above configuration, it is possible to couple a multi-core fiber with a single-core fiber for a plurality of single-core fibers whose shortest inter-core distance is greater than that of the multi-core fiber.

[0089] A connectorized optical fiber according to aspect 14 of the present invention is a connectorized optical fiber according to aspect 10, in which 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] According to the above configuration, it is possible to couple a multicore fiber to a silicon waveguide having a core diameter smaller than that of the multicore fiber.

[0091] A connectorized optical fiber according to a fifteenth aspect of the present invention is the connectorized optical fiber according to the tenth aspect, in which a lens is provided on one end face of the optical waveguide.

[0092] According to the above configuration, it is possible to realize collimated light without diffusing light in the gap between the end faces where the optical waveguide included in the connectorized optical fiber and another optical waveguide are connected, and it is also possible to convert the mode field diameter in the gap between the end faces.

[0093] An optical waveguide according to a sixteenth aspect of the present invention is the optical waveguide according to any one of the first to ninth aspects, which is made of a dried gel obtained by exposing a swollen gel to light and then drying and shrinking it.

[0094] According to the above configuration, a micrometer-scale pattern or a nanometer-scale pattern can be optically formed.

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

[0096] According to the above configuration, a micrometer-scale pattern or a nanometer-scale pattern can be optically fabricated more suitably.

[0097] REFERENCE SIGNS LIST 1 Optical waveguide 11 Core 11a Straight portion 11b Curved portion 11c Central axis 12 Cladding

Claims

1. An optical waveguide comprising: a core including a curved portion; and a cladding surrounding the core and having a refractive index lower than that of the core, wherein the refractive index of the cladding on the side opposite the center of curvature of the curved portion is lower than the refractive index of the cladding on the center of curvature of the curved portion.

2. The optical waveguide according to claim 1, wherein the refractive index of the cladding on the opposite side of the curved portion from the center of curvature gradually decreases with increasing distance from the center of curvature.

3. An optical waveguide according to claim 1 or 2, wherein the refractive index of the core gradually decreases with increasing distance from the center of curvature.

4. An optical waveguide according to any one of claims 1 to 3, wherein the refractive index of the cladding on the curvature center side of the curved portion gradually increases with increasing distance from the curvature center.

5. An optical waveguide according to any one of claims 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 expressed by the following formula (1): Here, n(x) is the refractive index distribution on a line segment that is 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. The optical waveguide according to claim 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 formula (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 formula (3).

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

8. The optical waveguide according to any one of claims 1 to 7, which is a coupler interposed between a first optical waveguide and a second optical waveguide, for optically coupling a core of the first optical waveguide and a core of the second optical waveguide.

9. The optical waveguide according to claim 8, wherein an end of the core of the optical waveguide on the side of the first optical waveguide is reduced to match with the core of the first optical waveguide, or an end of the core of the optical waveguide on the side of the second optical waveguide is expanded to match with the core of the second optical waveguide.

10. A connectorized optical fiber comprising: an optical waveguide according to any one of claims 1 to 9; a connector that accommodates one end face of the optical waveguide that includes the core so that it can be connected to another optical waveguide; and an optical fiber that is optically connected to the other end face of the optical waveguide that includes the core.

11. The connectorized optical fiber according to claim 10, wherein the optical waveguide has a plurality of cores, and the optical fiber is a multi-core optical fiber or an optical fiber bundle.

12. The connectorized optical fiber according to claim 11, wherein the shortest distance between the cores included in one end face of the optical waveguide is shorter than the shortest distance between the cores included in the other end face of the optical waveguide.

13. The connectorized optical fiber according to claim 11, wherein the shortest distance between the cores included in one end face of the optical waveguide is greater than the shortest distance between the cores included in the other end face of the optical waveguide.

14. The connectorized optical fiber according to claim 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.

15. The connectorized optical fiber according to claim 10, wherein a lens is provided on one end face of the optical waveguide.

16. An optical waveguide according to any one of claims 1 to 9, which is made of a dried gel obtained by exposing a swollen gel to light and then drying and shrinking it.

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

Images