WO2025173311A1 - Optical waveguide - Google Patents

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

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

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

Japanese Patent Publication No. 2004-29613

In optical waveguides, bending loss occurs when the core contains a curved section. There are two reasons why bending loss occurs:

First reason: In the straight section, the axis of symmetry of the waveguide mode is located on the central axis of the core, whereas in the curved section, the axis of symmetry of the waveguide mode is located outside the central axis of the core (farther from the center of curvature of the curved section). This mismatch in the waveguide mode causes loss at the boundary between the straight and curved sections.

Second reason: The smaller the bending radius of the curved section, the stronger the mode coupling between the guided mode and the radiation mode. Therefore, when the bending radius is small, this mode coupling causes losses in the curved section.

One known method for suppressing bending loss is to shift the curved portion of the core inward (toward the center of curvature of the curved portion), as shown in Figure 6(a), thereby reducing the mismatch of the waveguide modes mentioned above. 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 loss due to mode coupling to the radiation mode mentioned above.

Another known method for suppressing bending loss is to shape the curved portion of the core into a clothoid curve, as shown in Figure 6(b), thereby reducing the mismatch of the waveguide modes mentioned above. However, while this technique can reduce loss at the boundary between the straight and curved portions, loss occurs throughout the 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 curved portion, the curved portion becomes longer, resulting in greater waveguide loss in the curved portion. Furthermore, this technique cannot suppress loss due to mode coupling to the radiation mode mentioned above.

One aspect of the present invention was made in consideration of the above problems, and its purpose is to minimize bending loss in an optical waveguide whose core includes a curved portion.

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 lower refractive index than 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.

According to the present invention, bending loss can be minimized in optical waveguides whose cores include curved sections.

1A and 1B are a plan view and a cross-sectional view showing the configuration of an optical waveguide according to an embodiment of the present invention. 2 is a graph showing a refractive index distribution in the optical waveguide shown in FIG. 1 . 2 is a graph showing a modified example of the refractive index profile in the optical waveguide shown in FIG. 1 . 2 is a perspective view showing an optical device including the optical waveguide shown in FIG. 1 as an edge coupler. FIG. 2A is a side view of a connectorized optical fiber including the optical waveguide shown in FIG. 1, and FIG. 2B is a cross-sectional view of the connectorized optical fiber. FIG. 10 is a plan view showing the configuration of an optical waveguide according to a reference embodiment.

(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.

As shown in Figure 1, the optical waveguide 1 has a core 11 and a cladding 12.

The core 11 is a region with a higher refractive index than 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 rectangular, circular, or elliptical, for example. 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.

The cladding 12 is a region with a lower refractive index than the core 11, and serves the function of confining 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 extension direction of the core 11.

In this embodiment, the material of the core 11 and cladding 12 is a polymer whose refractive index can be controlled by laser irradiation, more specifically, polyvinyl alcohol with added titanium oxide. 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.

The refractive index of titanium oxide-doped polyvinyl alcohol can be increased by irradiating it with a femtosecond laser having an energy density of 2 J/ ^mm2 and 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.

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

Conversely, the refractive index of polyvinyl alcohol doped with titanium oxide can be reduced by irradiating it with a femtosecond laser having an energy density of 20 J/ ^mm2 and a wavelength of 515 nm (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.

In other words, in areas irradiated with a 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 that area and, as a result, reducing the refractive index of that area. In this case, the amount of reduction in refractive index can be adjusted by changing the power and/or irradiation time of the high-energy density laser that is irradiated.

(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.

Note that 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."

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.

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.

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 side opposite the center of curvature o (outside) is lower than the refractive index of the cladding 12 on the side opposite the center of curvature o (inside) with respect to the curved portion 11b of the core 11. This suppresses mismatching of the refractive index distributions that can occur at the boundary between the straight portion 11a and the curved portion 11b, and also suppresses light guided through the core 11 from coupling to a radiation mode leaking to the side opposite 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 minimized.

Figure 3 shows an example of the refractive index distribution n'(x) in the curved portion 11b that satisfies the above condition A.

In the refractive index distribution n'(x) of the curved portion 11b shown in Figure 3(a), the refractive index n0out (constant value) of the cladding 12 on the opposite side of 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, so the above condition A is satisfied. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

In the refractive index distribution n'(x) of the curved portion 11b shown in Figure 3(b), the maximum refractive index n0out of the cladding 12 on the side opposite the center of curvature o of 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, so the above condition A is satisfied. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

Furthermore, the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(b) satisfies the condition (hereinafter also referred to as "Condition B") that the refractive index of the cladding 12 on the opposite side of 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 of the center of curvature o.

Note that Figure 3(b) 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 this is not limiting. It is also possible to adopt 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.

In the refractive index distribution n'(x) of the curved portion 11b shown in Figure 3(c), the refractive index n0out (constant value) of the cladding 12 on the side opposite the center of curvature o of 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, so condition A above is satisfied. Therefore, the loss (i.e., bending loss) caused by the inclusion of the curved portion 11b in the core 11 can be kept small.

Furthermore, the refractive index distribution n'(x) in the curved portion 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 with increasing distance from the center of curvature o. Therefore, mismatches in the refractive index distribution that can occur at the boundary between the straight portion 11a and the curved portion 11b can be further suppressed in the core 11.

Note that while (c) in Figure 3 illustrates an example in which the refractive index of the core 11 decreases continuously with increasing distance from the center of curvature o, this is not limiting. It is also possible to adopt an example in which the refractive index of the core 11 decreases stepwise with increasing distance from the center of curvature o.

In the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(d), the maximum refractive index n0out of the cladding 12 on the side opposite the center of curvature o of 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, 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.

Furthermore, the refractive index distribution n'(x) in the curved portion 11b shown in Figure 3(d) satisfies the above 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 core 11 on the opposite side from the center of curvature o.

Note that (d) in Figure 3 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 this is not limiting. It is also possible to adopt 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.

Furthermore, while (d) in Figure 3 illustrates an example in which the refractive index of the core 11 decreases continuously with increasing distance from the center of curvature o, this is not limiting. It is also possible to adopt an example in which the refractive index of the core 11 decreases stepwise with increasing distance from the center of curvature o.

Note that the refractive index distribution n'(x) in the curved portion 11b shown in Figure 2(b) also satisfies the above condition A because the maximum value n0out of the refractive index of the cladding 12 on the side opposite the center of curvature o of 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.

Furthermore, the refractive index distribution n'(x) in the curved portion 11b shown in Figure 2(b) satisfies, in addition to the above conditions B and C, the condition (hereinafter also referred to as "condition D") that the refractive index of the cladding 12 on the side of the center of curvature o relative to the curved portion 11b of the core 11 gradually increases as it approaches the center of curvature o. This makes it possible to further suppress mismatches in 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 of the center of curvature o, the core 11, and the cladding 12 on the side of the center of curvature o.

(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.

Figure 4 is a perspective view of an optical device 100 that includes an optical waveguide 1 as an edge coupler 120.

In addition to the edge coupler 120, the optical device 100 also includes 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.

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 cores 113 are made of silicon, while the lower cladding layer 112 and upper cladding layer are made of silica. The refractive index of the silica that makes up the lower cladding layer 112 and upper cladding layer is lower than the refractive index of the silicon that makes up each of the plurality of cores 113. This achieves light confinement in each of the plurality of cores 113.

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

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

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.

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 snake three-dimensionally inside the clad 122. When manufacturing the edge coupler 120 using three-dimensional optical fabrication technology, it is also easy to make the cores 121 snake three-dimensionally inside the clad 122.

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.

Configuration 1: The size of the core 121 of the edge coupler 120 is reduced (e.g., tapered) 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.

Configuration 2: The size of the core 121 of the edge coupler 120 is expanded at the end on the multicore fiber 130 side (for example, expanded in an inverted tapered shape). In other words, the size of the core 121 of the edge coupler 120 can be matched to the size of the core 131 of the multicore fiber 130 at the end on the multicore fiber 130 side, and can be matched to the size of the core 113 of the silicon waveguide 110 in the remaining part.

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.

(Other application examples)
For example, the 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.

In Figure 5, (a) is a side view of a connectorized optical fiber MF to which a connector housing CH is attached via a ferrule F, and (b) is a cross-sectional view of the connectorized optical fiber MF. Here, the connector included in the connectorized optical fiber MF is composed of 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, in which case the optical fiber included in the connectorized optical fiber MF may be a single-core fiber. Also, for example, the optical waveguide 1 included in the connectorized optical fiber MF may have multiple cores, in which case the optical fiber included in the connectorized optical fiber MF may be a multi-core optical fiber or a bundle optical fiber. Figure 5(b) shows, as an example, multiple cores a1 to a4 included in the connectorized optical fiber MF.

For example, the silicon waveguide 110, edge coupler 120, and multi-core fiber 130 included in the optical device 100 described above correspond to another optical waveguide connected to the connectorized optical fiber MF, the optical waveguide 1 included in the connectorized optical fiber MF, and the optical fiber included in the connectorized optical fiber MF, respectively. In other words, 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 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 connector described above.

This enables coupling between an optical waveguide having multiple cores and a multi-core optical fiber or bundle optical fiber.

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

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 greater 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 multiple single-core fibers whose shortest inter-core distances are greater than those of a multi-core fiber.

Furthermore, for example, the diameter of the core included in one end face of the optical waveguide 1 provided 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 whose core diameter is smaller than that of the multicore fiber.

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, using the ImpFab method described below. This makes it possible to achieve 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, a conversion of the mode field diameter is achieved in the gap between the end faces.

(Additional 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.

For example, the optical waveguides according to each of the above-described embodiments can be fabricated by a stereolithography method in which a desired shape pattern or refractive index distribution is fabricated in a gel by exposing a swollen gel to light. An example of such a stereolithography method is the Implosion Fabrication (hereinafter abbreviated as ImpFab) method (see, for example, References 1 and 2 below). With the ImpFab method, a gel (e.g., a hydrogel) is exposed to light using a high-magnification, high-NA objective lens, and then the gel is dried and shrunk, allowing micrometer- or nanometer-scale patterns to be stereolithographed. With the ImpFab method, stereolithography is performed by applying the principles of single-photon absorption or multi-photon absorption. Therefore, in addition to stereolithography in the in-plane direction parallel to the main surface of the gel, stereolithography in the thickness direction perpendicular to the main surface of the gel can also be performed.

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 have to be composed of a single monomer unit but can be a copolymer of multiple types of monomers. For example, it can be composed of a material containing polyacrylic acid or a copolymer of acrylic acid and acrylamide. The dried gel can also contain a crosslinking agent. For example, a copolymer of acrylic acid and acrylamide can be crosslinked with a crosslinking agent such as N,N'-methylenebisacrylamide. The dried gel can also be a block polymer or block copolymer in which these materials are polymerized in a linear chain. In this case, a network structure can be formed by chemical crosslinking or physical crosslinking with a crosslinking agent.

(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〕
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.

The above configuration can suppress mismatches in the refractive index distribution that can occur at the boundary between the straight and curved sections, and can also prevent light guided through the core from coupling with radiation modes that leak to the side opposite the center of curvature. This can therefore suppress bending loss.

The optical waveguide of Aspect 2 of the present invention is the optical waveguide of Aspect 1, in which the refractive index of the cladding on the side opposite the center of curvature of the curved portion gradually decreases with increasing distance from the center of curvature.

With the above configuration, the mismatch in refractive index distribution that can occur at the boundary between the straight section and the curved section can be further suppressed in the cladding on the opposite side of the center of curvature of the curved section.

The optical waveguide according to aspect 3 of the present invention is the optical waveguide according to either aspect 1 or 2, in which the refractive index of the core gradually decreases with increasing distance from the center of curvature.

The above configuration further suppresses mismatches in the refractive index distribution that can occur at the boundaries between straight and curved sections in the core.

The optical waveguide according to aspect 4 of the present invention is an optical waveguide according to any one of aspects 1 to 3, 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.

With the above configuration, the mismatch in refractive index distribution that can occur at the boundary between the straight section and the curved section can be further suppressed in the cladding on the side closer to the center of curvature than the curved section.

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.

The above configuration eliminates mismatches in the refractive index distribution that can occur at the boundaries between straight and curved sections.

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.

The optical waveguide of aspect 7 of the present invention is the optical waveguide of any one of aspects 1 to 6, in which the core meanders three-dimensionally within the cladding.

The above configuration makes it possible to realize an optical waveguide with low bending loss, even though the core meanders three-dimensionally within the cladding.

An optical waveguide according to an eighth aspect of the present invention 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.
The optical waveguide according to any one of Aspects 1 to 7.

The above configuration makes it possible to couple the first optical waveguide and the second optical waveguide via an optical waveguide with low bending loss.

The optical waveguide of Aspect 9 of the present invention is the optical waveguide of Aspect 8, in which the end of the core of the optical waveguide facing the first optical waveguide is reduced to match the core of the first optical waveguide, or the end of the core of the optical waveguide facing the second optical waveguide is expanded to match the core of the second optical waveguide.

The above configuration makes it possible to minimize connection loss with the first optical waveguide or the second optical waveguide.

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.

The above configuration makes it possible to couple another optical waveguide to a connectorized optical fiber via an optical waveguide with low bending loss.

A connectorized optical fiber according to aspect 11 of the present invention is the connectorized optical fiber according to aspect 10, in which the optical waveguide has multiple cores and the optical fiber is a multi-core optical fiber or a bundled optical fiber.

The above configuration makes it possible to couple an optical waveguide having multiple cores to a multi-core optical fiber or bundle optical fiber.

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.

The above configuration makes it possible to couple a multicore fiber to a silicon waveguide in which the shortest distance between cores in the silicon waveguide is shorter than that of the multicore fiber.

A connectorized optical fiber according to aspect 13 of the present invention is the 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.

The above configuration makes it possible to couple a multi-core fiber with a single-core fiber for multiple single-core fibers whose shortest inter-core distance is greater than that of the multi-core fiber.

A connectorized optical fiber according to aspect 14 of the present invention is the 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.

The above configuration makes it possible to couple a multicore fiber to a silicon waveguide when the silicon waveguide has a core diameter smaller than that of the multicore fiber.

A connectorized optical fiber according to aspect 15 of the present invention is the connectorized optical fiber according to aspect 10, in which a lens is provided on one end face of the optical waveguide.

The above configuration prevents light from diffusing in the gap between the end faces where the optical waveguide in the connectorized optical fiber connects to another optical waveguide, achieving collimated light. It also makes it possible to convert the mode field diameter in the gap between the end faces.

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

The above configuration makes it possible to photo-fabricate micrometer-scale or nanometer-scale patterns.

The optical waveguide of Aspect 17 of the present invention is the optical waveguide of Aspect 16, in which the dry gel contains poly(meth)acrylic acid, or at least one of polymethacrylic acid and polyacrylic acid.

The above configuration makes it possible to more effectively photo-fabricate micrometer-scale or nanometer-scale patterns.

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

Claims (17)

a core including a curved portion;
a cladding surrounding the core and having a refractive index lower than that of the core;
a 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 a refractive index of the cladding on the center of curvature of the curved portion;
optical waveguide. 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;
2. The optical waveguide according to claim 1. The refractive index of the core gradually decreases with increasing distance from the center of curvature.
3. The optical waveguide according to claim 1. the refractive index of the cladding on the curvature center side of the curved portion gradually increases toward the curvature center;
The optical waveguide according to any one of claims 1 to 3. the core includes a straight portion,
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):
The optical waveguide according to any one of claims 1 to 4.
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. The refractive index distribution n(x) on the line segment orthogonal to the central axis of the core in the linear portion is expressed by the following formula (2):
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):
6. The optical waveguide according to claim 5.
The core is three-dimensionally meandering within the cladding.
The optical waveguide according to any one of claims 1 to 6. 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;
The optical waveguide according to any one of claims 1 to 7. 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;
9. The optical waveguide according to claim 8. An optical waveguide according to any one of claims 1 to 9;
a connector that accommodates one end face of the optical waveguide including the core so that the end face can be connected to another optical waveguide;
an optical fiber optically connected to the other end face of the optical waveguide including the core;
Optical fiber with connector. The optical waveguide has a plurality of cores, and the optical fiber is a multi-core optical fiber or a bundle optical fiber.
The connectorized optical fiber of claim 10. 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;
12. The connectorized optical fiber of claim 11. 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;
12. The connectorized optical fiber of claim 11. 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 connectorized optical fiber of claim 10. a lens is provided on one end face of the optical waveguide;
The connectorized optical fiber of claim 10. The optical waveguide is made of a dried gel obtained by exposing a swollen gel to light and then drying and shrinking it.
The optical waveguide according to any one of claims 1 to 9. The dry gel contains poly(meth)acrylic acid, or at least one of polymethacrylic acid and polyacrylic acid.
17. The optical waveguide of claim 16.