Optical waveguide element

Abstract

To provide an optical waveguide element capable of suitably exhibiting a slow-light effect. [Solution] The optical waveguide element 1 comprises a substrate 2 having a photonic crystal structure, and a core 4 provided on a portion of the surface 20 of the substrate 2 in the X-axis direction along the surface 20, extending along the surface 20 and in the Z-axis direction. The core 4 receives incident light at one end 41 in the Z-axis direction and emits the light from the other end 42 in the Z-axis direction. The substrate 2 includes a basic layer 25 and a plurality of columnar regions 26 distributed two-dimensionally within the basic layer 25, which are made of photonic crystal slabs. The size of the columnar regions 26 when viewed from the Y-axis direction intersecting the surface 20 changes in the Z-axis direction.

Description

Technical Field

[0001] The present invention relates to an optical waveguide device.

Background Art

[0002] Patent Document 1 describes a dispersion compensation element. This dispersion compensation element includes a substrate, a cladding layer laminated on the substrate, a photonic crystal layer laminated on the cladding layer, and a core layer laminated on the photonic crystal layer. The photonic crystal layer has a photonic crystal region formed by forming a plurality of holes in a planar shape in a base material made of silicon and filling the holes with a silicon oxide film as a first substance having a dielectric constant different from that of the base material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the dispersion compensation element according to Patent Document 1, the photonic crystal layer is for imparting wavelength dispersion variation having dispersion characteristics including the absolute value of the variation amount of wavelength dispersion and either a positive or negative sign to the light propagating in the dispersion compensation element.

[0005] By the way, currently, an SC (Super Continuum) light source generated by irradiating a femtosecond laser into an optical waveguide medium and utilizing the nonlinear optical effect in the optical waveguide medium is attracting attention as a light source expected to have a wide range of applications such as optical element measurement, spectroscopic analysis, OCT (Optical Coherence Tomography), and sensing.

[0006] Typically, optical fibers, particularly PCFs (Photonic Crystal Fibers), have been used as the optical waveguide medium. However, due to their manufacturability, PCFs have a uniform waveguide cross-sectional shape along their longitudinal length, making it difficult to control the flexible nonlinear optical properties, wavelength dispersion properties, and group refractive index properties over the longitudinal length necessary for efficient SC light generation.

[0007] Meanwhile, research is progressing on SC light sources using PICs (Photonic Integrated Circuits) as the optical waveguide medium. By using PICs, the shape of the waveguide in both the longitudinal and cross-sectional directions can be freely controlled, allowing for precise control of nonlinear optical effects, wavelength dispersion, and group refractive index. Therefore, these SC light sources are expected to be more efficient than PCF types.

[0008] To efficiently generate SC light, one could consider increasing the average power of the femtosecond laser used as the seed light, or increasing the interaction length between the medium and the light. However, in the case of PICs, it is difficult to increase the interaction length between the medium and the light due to wafer size limitations and relatively high propagation losses. In contrast, it is considered effective to introduce a photonic crystal structure that has a slow-light effect, slowing the propagation speed of light by more than an order of magnitude, thereby effectively increasing the interaction length between the medium and the light.

[0009] Therefore, the present invention aims to provide an optical waveguide element capable of suitably exhibiting the slow-light effect. [Means for solving the problem]

[0010] The optical waveguide element according to the present invention is [1] "an optical waveguide element comprising a substrate having a photonic crystal structure, a core provided on a part of the surface in a first direction along the surface of the substrate and extending along the surface and in a second direction intersecting the first direction, wherein the core receives incident light at one end in the second direction and emits the light from the other end in the second direction, and the substrate includes a basic layer and a plurality of columnar regions made of photonic crystal slabs distributed two-dimensionally along the surface within the basic layer, wherein the size of the columnar regions when viewed from a third direction intersecting the surface changes in the second direction."

[0011] In this optical waveguide element, a core is provided on the surface of the substrate. The substrate includes a basic layer and a plurality of columnar regions made of photonic crystal slabs, which are distributed two-dimensionally along the surface within the basic layer. Therefore, light incident from one end in the extension direction (second direction) of the core can interact with the photonic crystal structure of the substrate as it exits from the other end in the extension direction of the core. In particular, in this optical waveguide element, the size of the columnar regions as viewed from a third direction intersecting the surface of the substrate changes in the extension direction of the core. By changing the size of the columnar regions in this way, the group refractive index can be improved, for example, at wavelengths near the stopband wavelength. As a result, this optical waveguide element makes it possible to suitably exhibit the slow light effect.

[0012] The optical waveguide element according to the present invention may also be [2] "the optical waveguide element according to [1] above, wherein the size of the columnar region when viewed from the third direction changes so that it increases from one side to the other side in the second direction." In this case, the group refractive index can be improved more preferably at wavelengths near the stopband wavelength.

[0013] The optical waveguide element according to the present invention may also be [3] "the optical waveguide element according to [1] or [2] above, comprising an intermediate layer having a refractive index between the refractive index of the core and the equivalent refractive index of the substrate, and interposed at least between the core and the surface." In this case, polarization mixing of the lowest-order TE mode and the first-order higher-order TM mode can be suppressed, thereby suppressing characteristic degradation due to polarization mixing.

[0014] The optical waveguide element according to the present invention may also be [4] "an optical waveguide element according to any one of [1] to [3] above, comprising a cladding layer provided around the core." In this case, optical loss is suppressed.

[0015] The optical waveguide element according to the present invention may also be [5] "the optical waveguide element according to any one of [1] to [4] above, wherein one end of the substrate in the second direction is a region with a uniform refractive index." In this case, it becomes possible to suitably couple incident light to the core.

[0016] The optical waveguide element according to the present invention may also be [6] "the optical waveguide element according to any one of [1] to [5] above, wherein the width of the core in the first direction is 1 to 5 times the distance between the centers of adjacent columnar regions when viewed from the third direction." In this case, the interaction with the photonic crystal structure can be enhanced with respect to the light propagating through the core while suppressing the risk of multimode propagation.

[0017] The optical waveguide element according to the present invention may also be [7] "the optical waveguide element according to any of [1] to [6] above, wherein the columnar region is circular when viewed from the third direction, and the radius of the columnar region when viewed from the third direction is 0.01 times or more and 0.499 times or less the distance between the centers of adjacent columnar regions when viewed from the third direction." In this case, columnar regions can be suitably arranged so that adjacent columnar regions do not touch each other.

[0018] The optical waveguide device according to the present invention may be "[8] the optical waveguide device according to any one of [1] to [7] above, wherein the height of the core in the third direction is 0.1 μm or more and 5.0 μm or less". In this case, while using a high refractive index core, the risk of multimode propagation can be suppressed.

[0019] The optical waveguide device according to the present invention may be "[9] the optical waveguide device according to any one of [1] to [8] above, wherein the height of the columnar region in the third direction is 0.05 μm or more and 0.22 μm or less". In this case, improvement in manufacturability and ensuring of characteristics can be achieved.

[0020] The optical waveguide device according to the present invention may be "

[10] the optical waveguide device according to any one of [1] to [9] above, wherein the distance between the centers of the adjacent columnar regions when viewed from the third direction is 0.1 μm or more and 2.0 μm or less". In this case, for example, a range from 0.35 μm to 7 μm can be covered as the operating wavelength.

[0021] The optical waveguide device according to the present invention may be "

[11] the optical waveguide device according to [3] above, wherein the thickness of the intermediate layer in the third direction is 0.05 μm or more and 0.2 μm or less". In this way, by setting the thickness of the intermediate layer to 0.05 μm or more, the influence of film thickness fluctuation can be relatively reduced, and an increase in loss due to light scattering caused by roughness can be suppressed. Further, by setting the thickness of the intermediate layer to 0.2 μm or less, it is possible to suppress the effective refractive indices of the TE fundamental mode and the TM mode from approaching and the mode separation from becoming insufficient.

Advantages of the Invention

[0022] According to the present invention, it is possible to provide an optical waveguide device that can suitably exhibit the slow light effect.

Brief Description of the Drawings

[0023] [Figure 1] FIG. 1 is a perspective view showing an optical waveguide device according to the present embodiment. [Figure 2]FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III of FIG. 1 (FIG. 3(a)), and a plan view showing an enlarged part of the optical waveguide element shown in FIG. 1 (FIG. 3(b)). [Figure 4] FIG. 4 is a plan view showing an enlarged part of the optical waveguide element shown in FIG. 1. [Figure 5] FIG. 5 is a graph showing the relationship between wavelength and transmittance in the optical waveguide element according to the first embodiment (FIG. 5(a)), and a graph showing the relationship between wavelength and group refractive index (FIG. 5(b)). [Figure 6] FIG. 6 is a graph showing group refractive index and dispersion when the wavelength of light incident on the optical waveguide element is changed from 1500 nm to 2400 nm. [Figure 7] FIG. 7 is a diagram showing the light distribution in the YZ cross-section of the optical waveguide elements according to the second embodiment and the comparative example. [Figure 8] FIG. 8 is a graph showing the relationship between wavelength and transmittance in the optical waveguide element according to the third embodiment (FIG. 8(a)), and a graph showing the relationship between wavelength and group refractive index (FIG. 8(b)). [Figure 9] FIG. 9 is a graph showing the relationship between wavelength and transmittance in the optical waveguide element according to the reference example (FIG. 9(a)) and a graph showing the relationship between wavelength and group refractive index (FIG. 9(b)). [Figure 10] FIG. 10 is a graph showing the relationship between wavelength and transmittance in the optical waveguide element according to another reference example (FIG. 10(a)) and a graph showing the relationship between wavelength and group refractive index (FIG. 10(b)). [Figure 11] FIG. 11 is a schematic diagram showing an SC light source device including the optical waveguide element shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0024] An embodiment will be described in detail below with reference to the drawings. In each drawing, the same or corresponding parts will be denoted by the same reference numerals, and redundant explanations may be omitted. In addition, each drawing may show a Cartesian coordinate system defined by the X, Y, and Z axes.

[0025] Figure 1 is a perspective view showing an optical waveguide element according to this embodiment. Figure 2 is a cross-sectional view along line II-II in Figure 1. Figure 3 is a cross-sectional view along line III-III in Figure 1 (Figure 3(a)), and a plan view showing an enlarged portion of the optical waveguide element shown in Figure 1 (Figure 3(b)). In Figures 2 and 3, the cladding layer shown in Figure 1 is omitted.

[0026] The optical waveguide element 1 shown in Figures 1-3 can be used in an SC (Super Continuum) light source device 100, for example, as shown in Figure 11, which emits SC (Super Continuum) light L by taking in seed light, which is a femtosecond laser output from a light source 50 (for example, via an optical fiber 51), and utilizing the nonlinear optical effect in the optical waveguide medium. The SC light source device 100 is expected to have a wide range of applications, such as optical element measurement, spectroscopic analysis, OCT (Optical Coherence Tomography), and sensing.

[0027] As shown in Figures 1-3, the optical waveguide element 1 comprises a substrate 2, an intermediate layer 3, a core 4, and a cladding layer 5. The substrate 2 has a surface 20. The surface 20 is oriented along the X-axis and Z-axis directions. When viewed from the Z-axis direction, the surface 20 has a first portion 20a, a second portion 20b (part of the surface), and a third portion 20c arranged sequentially in the X-axis direction. The substrate 2 also consists of an end 21 on one side in the Z-axis direction (negative Z-axis direction), an end 23 on the other side in the Z-axis direction (positive Z-axis direction), and a main body portion 22 between the end 21 and the end 23.

[0028] The core 4 is provided on a portion (second portion 20b) of the surface 20 of the substrate 2 in the X-axis direction (first direction) along the surface 20. The core 4 extends in an elongated shape along the surface 20 and in the Z-axis direction (second direction) which intersects the X-axis direction. The shape of the core 4 in the cross-section (XY plane) intersecting the direction of extension is rectangular. The core 4 includes an end 41 (end face) on one side in the Z-axis direction and an end 42 (end face) on the other side in the Z-axis direction. The core 4 receives incident light at the end 41 and emits the light from the end 42.

[0029] The intermediate layer 3 is provided on the surface 20 such that it is interposed at least between the core 4 and the surface 20 of the substrate 2. In other words, the core 4 is provided on the surface 20 via the intermediate layer 3. In this embodiment, the intermediate layer 3 extends outward from the region between the core 4 and the surface 20. That is, the intermediate layer 3 extends from the second portion 20b to the first portion 20a and the third portion 20c of the surface 20. As an example, the intermediate layer 3 is provided over the entire surface 20.

[0030] The cladding layer 5 is provided around the core 4. In this embodiment, the cladding layer 5 is provided on the first portion 20a, the third portion 20c of the surface 20 of the substrate 2, and on the core 4. An intermediate layer 3 is interposed between the cladding layer 5 and the first portion 20a and the third portion 20c. The cladding layer 5 fills the surface of the core 4 on the intermediate layer 3 side (substrate 2 side) and the surfaces other than both end faces in the Z direction.

[0031] The substrate 2 has a photonic crystal structure. More specifically, the substrate 2 includes a base layer 25 and a plurality of columnar regions 26 distributed two-dimensionally within the base layer 25, which are made up of photonic crystal slabs. The columnar regions 26 are regions with different refractive indices, having a different refractive index than the base layer 25. In this embodiment, the columnar regions 26 are not provided at the ends 21 and 23, but only at the main body 22. In other words, in this embodiment, the ends 21 and 23 are regions with a uniform refractive index (regions that do not include regions with different refractive indices).

[0032] In the illustrated example, the columnar regions 26 are aligned in the ΓK direction with respect to the Z direction, but they may also be aligned in the ΓM direction. Furthermore, in this embodiment, the columnar regions 26 are arranged in a triangular lattice when viewed from the Y-axis direction (third direction) intersecting the surface 20 of the substrate 2, but they may also be arranged in a square lattice or other lattice shapes to which topological photonics are applied.

[0033] The columnar region 26 penetrates the substrate 2 along the Y-axis direction (third direction) intersecting the surface 20, from the surface 20 to the back surface opposite the surface 20. In this embodiment, the columnar region 26 is circular in shape when viewed from the Y-axis direction (i.e., cylindrical) and has a radius r and a distance a between centers. The distance a is the distance between the centers c of adjacent columnar regions 26. As an example, the distance a is 0.1 μm or more and 2.0 μm or less, and the radius r is 0.01 times the distance a or more and 0.499 times the distance a. The height hphc of the columnar region 26 in the Y-axis direction is equivalent to the thickness of the substrate 2 in the Y-axis direction, for example, 0.05 μm or more and 0.22 μm or less. The base layer 25 contains, for example, silicon (Si), and the columnar region 26 contains, for example, silicon oxide (SiO2).

[0034] Core 4 has a height hcore in the Y-axis direction and a width wcore in the X-axis direction. For example, the height hcore is between 0.1 μm and 5.0 μm, and the width wcore is between 1 and 5 times the distance a. Core 4 includes, for example, a high refractive index medium with a large nonlinear refractive index n2, such as silicon nitride (Si3N4), silicon (Si), germanium (Ge), or chalcogenide. In this embodiment, core 4 is made of silicon nitride (Si3N4).

[0035] The intermediate layer 3 has a refractive index between the refractive index of the core 4 and the equivalent refractive index of the substrate 2. The intermediate layer 3 also has a thickness hmid in the Y-axis direction. For example, the thickness hmid is between 0.05 μm and 0.2 μm. For example, the intermediate layer 3 includes a silicon oxide (SiO2) or BCB layer (consisting of a silicon oxide (SiO2) or BCB layer). The intermediate layer 3 may be a single-layer structure made of a single material or a multilayer structure made of multiple materials. The cladding layer 5 includes, for example, silicon oxide (SiO2) (consisting of silicon oxide (SiO2)).

[0036] In this optical waveguide element 1, the size (radius r in this case) of the columnar region 26 as viewed from the Y direction changes in the Z-axis direction. More specifically, as shown in Figure 4, the radius r of the columnar region 26 is radius r1 near one end 21 in the Z-axis direction (see Figure 4(a)), while it is larger than radius r1, r2, near the other end 23 in the Z-axis direction. In other words, in this embodiment, the radius r of the columnar region 26 increases from one side to the other in the Z-axis direction. As an example, the change in radius r is monotonically increasing in the Z-axis direction. In this embodiment, the distance a is constant regardless of the position in the Z-axis direction. [First Embodiment]

[0037] In the first embodiment, the distance a (lattice constant) is set to 0.46 μm, the normalized radius of the columnar region 26 (i.e., radius r / distance a) is set to 0.200 (=radius r1 / distance a) at one end in the Z-axis direction (end of the main body 22), and to 0.470 (=radius r2 / distance a) at the other end in the Z-axis direction (end of the main body 22). The length L22 of the main body 22 in the Z-axis direction is set to 250 μm, and the rate of change of radius r is set to +500 nm / mm. The height hphc of the columnar region 26 is set to 0.10 μm, the height hcore of the core 4 is set to 0.4 μm, and the width wcore of the core 4 is set to 2.2a.

[0038] Figure 5 shows a graph (Figure 5(a)) illustrating the relationship between wavelength and transmittance in an optical waveguide element according to the first embodiment, and a graph (Figure 5(b)) illustrating the relationship between wavelength and group refractive index. As shown in Figure 5(a), in the first embodiment, wavelengths longer than approximately 1.68 μm (stop band wavelength λs) are in the transmission band, and as shown in Figure 5(b), a phenomenon in which the group refractive index increases at wavelengths near the transmission band close to the stop band wavelength λs was numerically confirmed. [Second Example]

[0039] Figure 6 is a graph showing the group refractive index and dispersion when the wavelength of light incident on the optical waveguide element is varied from 1500 nm to 2400 nm. In Figure 6, the solid line shows the dispersion and the dashed line shows the group refractive index. Figure 6(a) is a graph for a second embodiment of the optical waveguide element 1 according to this embodiment, and Figure 6(b) is a graph for a comparative example in which the optical waveguide element 1 does not have an intermediate layer 3. Figure 7 is a diagram showing the distribution of light in the YZ cross-section of the optical waveguide element according to the second embodiment and the comparative example. Figure 7(a) shows the basic mode (TE polarization) of the embodiment with the intermediate layer 3, Figure 7(b) shows the basic mode (TE polarization) of the comparative example without the intermediate layer 3, and Figure 7(c) shows the higher-order mode (TM polarization) of the embodiment with the intermediate layer 3.

[0040] In the second embodiment and comparative example, the width wcore is 800 nm, the height hcore is 250 nm, the thickness hmid is 15 nm, and the height hphc is 150 nm. As shown in Figures 6 and 7, even a ribbed waveguide with a Si core without an intermediate layer 3 (optical waveguide element according to the comparative example) exhibits almost the same wavelength dispersion characteristics as the optical waveguide element according to the second embodiment. Furthermore, as shown in the example values ​​above, by setting the rib width (width wcore of core 4) to be wider, the anomalous dispersion (positive wavelength dispersion value) is extended to the long wavelength region around 2400 nm.

[0041] However, in the optical waveguide element according to the comparative example without the intermediate layer 3, the effective refractive index of the higher-order modes of TM polarization becomes almost equal to the effective refractive index of the fundamental mode of TE polarization, resulting in polarization mixing and a deterioration of nonlinear optical characteristics. In contrast, when the intermediate layer 3 is present (i.e., in the optical waveguide element according to the second embodiment), the effective refractive index of the higher-order modes of TM polarization becomes significantly smaller compared to the comparative example, and the characteristic deterioration due to polarization mixing can be eliminated.

[0042] Although the intermediate layer 3 may generate slot modes with TM polarization, in the configuration of the optical waveguide element 1, the lowest-order mode with TM polarization does not become a slot mode and exhibits a mode distribution similar to the fundamental mode of TE polarization. By introducing a photonic crystal into the substrate 2, the normal dispersion characteristics due to the first branch of the photonic band are added to the anomalous dispersion characteristics that would be present in a waveguide with an intermediate layer 3 without a photonic crystal. As a result, the anomalous dispersion and the normal dispersion cancel each other out, making it possible to generate flat zero dispersion over a wide wavelength band.

[0043] Furthermore, it is believed that by adjusting the properties of the photonic crystal (i.e., the columnar region 26) and structural parameters such as the period (e.g., distance a) and the diameter of the circular hole (e.g., radius r), it is possible to generate the flat anomalous dispersion required for soliton propagation. Moreover, in the presence of the photonic crystal, the higher-order modes of TE polarization become the second branch or higher-order branch modes of the photonic band, and are therefore emitted from the waveguide as radiated modes, allowing the waveguide to function stably as a single-mode waveguide. Thus, it is understood that by having an intermediate layer 3 and a photonic crystal (i.e., the columnar region 26), an optical waveguide element with high nonlinear optical coefficients and stable polarization characteristics is provided. [Third Embodiment]

[0044] In the third embodiment, the distance a (lattice constant) is set to 0.46 μm, the normalized radius of the columnar region 26 (i.e., radius r / distance a) is set to 0.270 (=radius r1 / distance a) at one end in the Z-axis direction (end of the main body 22), and to 0.295 (=radius r2 / distance a) at the other end in the Z-axis direction (end of the main body 22). The length L22 of the main body 22 in the Z-axis direction is set to 60 μm, and the rate of change of radius r is set to +191.66 nm / mm. The height hphc of the columnar region 26 is set to 0.10 μm, the height hcore of the core 4 is set to 0.4 μm, and the width wcore of the core 4 is set to 2.2a.

[0045] Figure 8 shows a graph (Figure 8(a)) illustrating the relationship between wavelength and transmittance in an optical waveguide element according to the third embodiment, and a graph (Figure 8(b)) illustrating the relationship between wavelength and group refractive index. In Figures 8(a) and (b), the solid line shows the case where the radius r of the columnar region 26 is constant, and the dashed line shows the case where the radius r of the columnar region 26 is changed as described above. As shown in Figure 8, in the first embodiment, the transmission band is longer wavelengths from around 1.68 μm, and as shown in Figure 8(b), it was confirmed that the wavelength band in which the group refractive index is high is expanded by changing the radius r of the columnar region 26. [Reference example]

[0046] Figure 9 shows a graph (Figure 9(a)) illustrating the relationship between wavelength and transmittance in an optical waveguide element according to a reference example, and a graph (Figure 9(b)) illustrating the relationship between wavelength and group refractive index. In the reference example, the radius r of the columnar region 26 is assumed to be constant. In Figures 9(a) and (b), the solid line shows the case where the width wcore of core 4 is 1 times the distance a (lattice constant), the dashed line shows the case where the width wcore of core 4 is 2 times the distance a (lattice constant), and the dashed-dotted line shows the case where the width wcore of core 4 is 3 times the distance a (lattice constant).

[0047] As shown in Figure 9(a), it was confirmed that increasing the width wcore by 1x, 2x, and 3x the distance a shifts the transmission bandwidth (stopband wavelength) to the longer wavelength side. Furthermore, as shown in Figure 9(b), increasing the width wcore by 1x, 2x, and 3x the distance a changes the wavelength dependence of the group refractive index. This is thought to be due to the slow-light effect being emphasized in proportion to the number of periods of the columnar regions 26 present within the width wcore of core 4. It is also thought that the slow-light wavelength bandwidth may be expanded by appropriately changing the distance a (lattice constant) of the columnar regions 26 overlapping the width wcore of core 4, or the normalization radius (radius r / distance a) of the columnar regions 26. Note that the width wcore of core 4 may also be changed in the longitudinal direction of core 4.

[0048] Figure 10 shows graphs illustrating the relationship between wavelength and transmittance in an optical waveguide element related to another reference example (Figure 10(a)) and the relationship between wavelength and group refractive index (Figure 10(b)). In Figures 10(a) and (b), the solid line shows the case where the radius r of the columnar region 26 is constant, and the dashed line shows the case where the radius r of the columnar region 26 is changed as described above. On the other hand, in the example of Figure 10, the height hphc of the columnar region 26 is made larger than 0.22 μm. In this case, it is confirmed that not only is the effect of changing the radius r of the columnar region 26 weakened, but the slow-light effect itself due to the photonic crystal slab (columnar region 26) is also weakened. Therefore, it is considered that there is an optimal value for the height hphc of the columnar region 26.

[0049] As described above, in the optical waveguide element 1 according to this embodiment, a core 4 is provided on the surface 20 of the substrate 2. The substrate 2 includes a basic layer 25 and a plurality of columnar regions 26 made of photonic crystal slabs, which are distributed two-dimensionally along the surface 20 within the basic layer 25. Therefore, light incident from one end in the extending direction (Z-axis direction) of the core 4 can interact with the photonic crystal structure of the substrate 2 as it is emitted from the other end in the extending direction of the core. In particular, in the optical waveguide element 1, the size of the columnar regions 26 when viewed from the Y-axis direction intersecting the surface 20 of the substrate 2 changes in the extending direction of the core 4. By changing the size of the columnar regions 26 in this way, the group refractive index can be improved, for example, at wavelengths near the stopband wavelength λs. As a result, the optical waveguide element 1 can suitably exhibit the slow light effect.

[0050] Furthermore, in the optical waveguide element 1 according to this embodiment, the size of the columnar region 26 when viewed from the Y-axis direction changes so that it increases from one side to the other in the Z-axis direction. Therefore, it is possible to more preferably improve the group refractive index at wavelengths near the stopband wavelength λs.

[0051] Furthermore, the optical waveguide element 1 according to this embodiment has a refractive index between the refractive index of the core 4 and the equivalent refractive index of the substrate 2, and includes an intermediate layer 3 provided on the surface 20 so as to be interposed between the core 4 and the surface 20. Therefore, polarization mixing of the lowest-order TE mode and the first-order higher-order TM mode can be suppressed, thereby suppressing characteristic degradation due to polarization mixing.

[0052] Furthermore, the optical waveguide element 1 according to this embodiment includes a cladding layer 5 provided around the core 4. In this case, optical loss is suppressed.

[0053] Furthermore, in the optical waveguide element 1 according to this embodiment, one end 21 of the substrate 2 in the Z-axis direction is a region with a uniform refractive index. Therefore, it is possible to suitably couple incident light to the end 41 of the core 4.

[0054] Furthermore, in the optical waveguide element 1 according to this embodiment, the width wcore of the core 4 in the X-axis direction is between 1 and 5 times the distance a between the centers of adjacent columnar regions 26 when viewed from the Y-axis direction. Therefore, the interaction with the photonic crystal structure can be enhanced with respect to the light propagating through the core 4 while suppressing the risk of multimode propagation.

[0055] Furthermore, in the optical waveguide element 1 according to this embodiment, the columnar region 26 is circular when viewed from the Y-axis direction, and the radius r of the columnar region 26 when viewed from the Y-axis direction is 0.01 times or more and 0.499 times or less the distance a between the centers of adjacent columnar regions 26 when viewed from the Y-axis direction. Therefore, columnar regions 26 can be suitably arranged so that adjacent columnar regions 26 do not touch each other.

[0056] Furthermore, in the optical waveguide element 1 according to this embodiment, the height of the core 4 in the Y-axis direction is 0.1 μm or more and 5.0 μm or less. Therefore, it is possible to suppress the risk of multimode propagation while using a high refractive index core.

[0057] Furthermore, in the optical waveguide element 1 according to this embodiment, the height of the columnar region 26 in the Y-axis direction is 0.05 μm or more and 0.22 μm or less. Therefore, it is possible to improve manufacturability and ensure characteristics.

[0058] Furthermore, in the optical waveguide element 1 according to this embodiment, the distance a between the centers of adjacent columnar regions when viewed from the Y-axis direction is 0.1 μm or more and 2.0 μm or less. Therefore, for example, it is possible to cover an operating wavelength range of 0.35 μm or more and 7 μm or less.

[0059] Furthermore, in the optical waveguide element 1 according to this embodiment, the thickness of the intermediate layer 3 in the Y-axis direction is 0.05 μm or more and 0.2 μm or less. By setting the thickness of the intermediate layer 3 to 0.05 μm or more, the effect of film thickness fluctuations is relatively reduced, and it is possible to suppress large losses caused by light scattering due to roughness. In addition, by setting the thickness of the intermediate layer 3 to 0.2 μm or less, it is possible to suppress the close proximity of the effective refractive indices of the TE fundamental mode and the TM mode, which would result in insufficient mode separation.

[0060] The above embodiments illustrate one aspect of the present invention. Therefore, the present invention is not limited to the above embodiments and can be modified as needed.

[0061] For example, in the above embodiment, an example was described in which the intermediate layer 3 is provided over the entire surface 20, but the intermediate layer 3 only needs to be interposed between the core 4 and the surface 20, and may be provided on only a part of the surface 20. That is, another part of the surface 20 may be exposed from the intermediate layer 3.

[0062] Furthermore, in the above embodiment, a columnar region 26 was provided only on the main body portion 22 of the substrate 2, while the end portion 21 and end portion 23 were described as having a uniform refractive index. However, a columnar region 26 (region with different refractive indices) may also be provided on at least one of the end portion 21 and end portion 23.

[0063] Furthermore, in the above embodiment, an example was described in which the size of the columnar region 26, as viewed from the Y-axis direction, changes so that it increases from one side to the other in the Z-axis direction. However, the size of the columnar region 26, as viewed from the Y-axis direction, may also change so that it decreases from one side to the other in the Z-axis direction. Moreover, the manner in which the size of the columnar region 26 changes is not limited to monotonically increasing or decreasing, but is arbitrary. [Explanation of Symbols]

[0064] 1...Optical waveguide element, 2...Substrate, 3...Intermediate layer, 4...Core, 5...Cladding layer, 20...Surface, 21,23,41,42...Edges, 25...Basic layer, 26...Columnar region.

Claims

1. A substrate having a photonic crystal structure, A core provided on a portion of the surface in a first direction along the surface of the substrate, and extending along the surface and in a second direction intersecting the first direction, Equipped with, The core receives incident light at one end in the second direction and emits the light from the other end in the second direction. The substrate comprises a base layer, a plurality of columnar regions distributed two-dimensionally along the surface within the base layer and consisting of photonic crystal slabs, and The size of the columnar region, when viewed from a third direction intersecting the surface, changes in the second direction. Optical waveguide element.

2. The size of the columnar region when viewed from the third direction changes so that it increases from one side to the other side in the second direction. The optical waveguide element according to claim 1.

3. The intermediate layer is provided on the surface having a refractive index between the refractive index of the core and the equivalent refractive index of the substrate, and is interposed at least between the core and the surface. The optical waveguide element according to claim 1.

4. The core is provided with a cladding layer around it. The optical waveguide element according to claim 1.

5. One end of the substrate in the second direction is defined as a region with a uniform refractive index. The optical waveguide element according to claim 1.

6. The width of the core in the first direction is one to five times the distance between the centers of adjacent columnar regions when viewed from the third direction. The optical waveguide element according to claim 1.

7. The columnar region is circular when viewed from the third direction, The radius of the columnar region as viewed from the third direction is 0.01 times or more and 0.499 times or less the distance between the centers of adjacent columnar regions as viewed from the third direction. The optical waveguide element according to claim 1.

8. The height of the core in the third direction is 0.1 μm or more and 5.0 μm or less. The optical waveguide element according to claim 1.

9. The height of the columnar region in the third direction is 0.05 μm or more and 0.22 μm or less. The optical waveguide element according to claim 1.

10. The distance between the centers of adjacent columnar regions when viewed from the third direction is 0.1 μm or more and 2.0 μm or less. An optical waveguide element according to any one of claims 1 to 9.

11. The thickness of the intermediate layer in the third direction is 0.05 μm or more and 0.2 μm or less. The optical waveguide element according to claim 3.

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