Optical waveguide

Abstract

Provided is an optical waveguide (200) of an embedded type, which is formed on a substrate (201) and includes lower cladding (202), a core (203), and upper cladding (204). The optical waveguide (200) has a loop shape including a circular arc shape, and comprises a groove (205) that is formed on one of the sides of a side surface thereof with respect to the light propagation direction of the core (203) and that extends in a parallel direction to the core (203). The groove (205) may be formed so as to extend to the entire periphery of the loop shape including the circular arc shape. The radius of curvature of the circular arc shape may be a minimum bending radius determined by a specific refractive index difference.　Through the present disclosure, it is possible to provide an optical waveguide for realizing a waveguide waveplate which has leeway in tolerance and also improves yield.

Description

Optical waveguide

[0001] The present disclosure relates to optical waveguides, and more particularly to embedded optical waveguides formed on a substrate.

[0002] With the expansion of data communication networks such as the Internet, further increase in capacity is desired for optical communication networks. In order to respond to such expansion of network demand, various optical devices have been put into practical use, and examples thereof include variable optical attenuators and front ends for coherent communication. For these optical devices, silica-based optical waveguide technology is the main constituent technology due to their high integration and manufacturability.

[0003] Polarization is an important indicator of light waves, and the wave plate function that rotates polarized light inside a silica-based optical waveguide is an important functional element. Previously, Patent Document 1 discloses a waveguide wave plate having a configuration in which a groove extending in a direction parallel to the core is provided on one of the side surfaces with respect to the light propagation direction of the core of the optical waveguide. FIG. 1 is a perspective view schematically showing the structure of an optical waveguide 100 having a conventional polarization rotator disclosed in Patent Document 1.

[0004] As shown in FIG. 1, the optical waveguide 100 is an embedded optical waveguide including a lower clad 102 formed on a substrate 101, a core 103, and an upper clad 104. In the optical waveguide 100, a groove 105 is formed on one of the side surfaces with respect to the light propagation direction (the Z direction in FIG. 1) and extends in a direction parallel to the core 103 (the Z direction in FIG. 1). In this example, the groove 105 is formed with a depth reaching directly above the substrate 101, but the depth of the groove 105 may be shallower than that. Here, the description is given on the assumption that the optical waveguide is a silica-based optical waveguide using quartz glass, but materials applicable to the optical waveguide 100 may also be, for example, silicon semiconductors, ferroelectrics, SiOx, SiON, SiN, etc.

[0005] In the optical waveguide 100, the formation of grooves 105 partially releases internal stresses in the X and Y directions remaining within each glass layer constituting the optical waveguide (lower cladding 102, core 103, upper cladding 104). However, in the lower cladding 102, the substrate 101 is constrained near the corner where the interface between the groove 105 and the side wall intersects with the substrate 101, resulting in locally high stress (stress concentration). Since the region where this stress concentration occurs is located obliquely in the XY plane from the perspective of the core 103, the stress distribution becomes asymmetric with respect to the light propagation direction (Z direction). In this way, by forming grooves 105 and controlling the internal stress of the optical waveguide, and inducing stress oblique to the horizontal direction of the substrate 101, the principal axis of the refractive index ellipse of the optical waveguide mode can be tilted with respect to the substrate coordinate system (XY plane) via the photoelastic effect.

[0006] In this case, the inclination angle (rotation angle) of the principal axis of the refractive index ellipse depends on the distance between the optical waveguide and the groove. Therefore, by controlling the distance between the optical waveguide (core) 103 and the groove 105, a waveplate with an arbitrary optical axis angle can be formed within the optical waveguide substrate.

[0007] Furthermore, the retardation caused by this waveplate depends on the length of the groove formed along the core; therefore, by controlling the length of the groove 105, it is possible to set the desired retardation.

[0008] In silica-based optical waveguides, grooves 105 are formed by an etching process, but the groove formation position is determined by a photolithography process. In this process, the alignment of the photomask is often done manually by visual inspection using alignment marks, which can lead to errors in the groove formation position during the manufacturing process. For example, in the photolithography process for manufacturing silica-based optical waveguides, errors of about 2 μm can typically occur when using a projection exposure machine.

[0009] In the optical waveguide 100 shown in Figure 1, if the formation position of the groove 105 shifts and the distance between the optical waveguide (core) 103 and the groove 105 changes, the optical axis angle (the inclination of the principal axis of the refractive index ellipse of the optical waveguide) changes significantly. Figure 2 is a graph showing the relationship between the rotation angle of the principal axis of the refractive index ellipse and the distance between the optical waveguide (core) 103 and the groove 105. In this specification, the distance between the optical waveguide and the groove 105 is defined as the distance between the center of the optical waveguide core 103 and the side wall of the groove 105 that is closer to the core.

[0010] For example, when configuring a waveplate function in which the optical axis, which is the principal axis of the refractive index ellipse, is tilted by 45° with respect to the horizontal direction of the substrate 101, the groove distance should be set to 45.9 μm. In this case, if the groove distance fluctuates by ±1 μm due to, for example, a shift in the groove formation position, the tilt of the optical axis will change from 42.7° to 47.3°. Thus, in conventional technology, errors in the groove formation position during the manufacturing process have a significant impact on the variation in the optical axis angle. For this reason, conventional optical waveguides have strict tolerances and could cause problems in terms of manufacturing yield. This disclosure aims to resolve these problems in the prior art.

[0011] International Publication No. 2024 / 042588 (A1)

[0012] The purpose of this disclosure is to provide an optical waveguide with a structure that reduces the impact of groove distance errors occurring in the manufacturing process on the optical waveplate function.

[0013] To achieve this objective, one embodiment of the present disclosure is an embedded optical waveguide formed on a substrate, comprising a lower cladding, a core, and an upper cladding, wherein the optical waveguide has a loop shape including an arc shape and comprises a groove formed on one side of the core with respect to the direction of light propagation and extending parallel to the core.

[0014] According to this embodiment, even if the groove formation position is shifted and an error occurs in the groove distance, the fluctuation of the optical axis can be canceled out, and the effect can be suppressed. As a result, there is more tolerance compared to the conventional technology, and the yield is also improved.

[0015] Figure 1 is a schematic perspective view showing the structure of a conventional optical waveguide having a polarization rotor. Figure 2 is a diagram showing the relationship between the groove distance (the distance between the center of the core 103 and the side wall of the groove 105 closer to the core) and the optical axis angle, which is the angle of the principal axis of the refractive index ellipse with respect to the horizontal plane of the substrate 101. Figure 3 is a schematic perspective view showing the structure of an optical waveguide according to one embodiment of the present disclosure. Figure 4 is a schematic top view showing the shape of an optical waveguide according to another embodiment of the present disclosure. Figure 5 shows two cases in an optical waveguide: one where the groove formation position is in an ideal configuration, and another where a misalignment occurs in the groove formation position due to manufacturing errors. Figure 5(a) is a schematic top view showing the case where the groove formation position of an optical waveguide according to one embodiment of the present disclosure is in an ideal configuration. Figure 5(b) is a schematic top view showing the case where a misalignment occurs in the groove formation position of an optical waveguide according to one embodiment of the present disclosure. Figure 5(c) is a schematic top view showing the case where the groove formation position of a conventional optical waveguide is in an ideal configuration. Figure 5(d) is a schematic top view showing the case where a misalignment occurs in the groove formation position of a conventional optical waveguide. Figure 6 is a diagram illustrating the effect of a misalignment in the groove formation position. Figure 6(a) shows the relationship between the trajectory of the optical waveguide core and the trajectory of the groove when the exposure position of the photolithography in groove formation is shifted by (X, Z) = (2 μm, 2 μm) in an optical waveguide according to one embodiment of the present disclosure. Figure 6(b) is a graph plotting the change in groove distance in the direction of light propagation in the optical waveguide under the conditions shown in Figure 6(a). Figure 7 shows the change in groove distance when the same misalignment occurs in the groove in an optical waveguide according to one embodiment of the present disclosure and a conventional optical waveguide. Figure 7(a) is for the optical waveguide according to one embodiment of the present disclosure, and Figure 7(b) is for the conventional optical waveguide. Figure 8 is a schematic diagram of the optical system configuration assumed to explain the effects of the optical waveguide according to the embodiment of the present disclosure. Figure 9 shows the results of calculating the extinction spectrum output from the optical system 800. Figure 9(a) shows the optical waveguide according to the embodiment of this disclosure, both when it is designed and when the groove formation position is shifted. Figure 9(b) shows the optical waveguide according to the prior art, both when it is designed and when the groove formation position is shifted.

[0016] Embodiments of this disclosure will be described in detail below with reference to the drawings. The following description is illustrative, and embodiments with modified configurations are possible without departing from the gist of this disclosure. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. Numerical values ​​in the following description are illustrative, and other numerical values ​​may be used in carrying out this disclosure without departing from the gist of this disclosure.

[0017] Figure 3 is a perspective view showing a schematic configuration of an optical waveguide according to the embodiment of this disclosure.

[0018] The optical waveguide 200 according to this embodiment is an embedded optical waveguide including a lower cladding 202, a core 203, and an upper cladding 204 formed on a substrate 201. The optical waveguide according to this embodiment is laid out in a loop shape including an arc shape, and a groove 205 is formed on one side of the core 203 with respect to the direction of light propagation, i.e., on the outer circumference side, so as to be parallel to the core. The arc shape may be not only a part of a pure circle, but also a clothoid curve or a Bézier curve, etc., in which the curvature gradually changes.

[0019] Figure 3 shows one embodiment of this design in which the loop shape of the optical waveguide 200 is an arc shape. As shown in Figures 3 and 5(a), in this embodiment, the core 203 is laid out in a loop shape consisting of an arc shape configured in a C shape that connects the input waveguide and the output waveguide that constitute the input / output section 206. In addition, a groove 205 is formed on the outer circumference of the side surface of the core 203 with respect to the direction of light propagation.

[0020] In one embodiment of this design, an example is shown in which the loop shape of the optical waveguide is arc-shaped. However, as shown in Figure 4, the loop shape may be a quadrilateral with four arc-shaped corners. The optical waveguide 300 in this design is an embedded optical waveguide, similar to the optical waveguide 200, and includes a lower cladding, a core, and an upper cladding formed on a substrate. The optical waveguide 300 is laid out in a loop shape (approximately C-shaped) that includes arc shapes, and a groove 305 is formed on one side of the core 303 with respect to the direction of light propagation, i.e., on the outer circumference. The quadrilateral shape of the optical waveguide 300 may also be rectangular.

[0021] In this embodiment, the case in which grooves 205 and 305 are formed on the outer circumference of the loop-shaped portion including the arc shape of the optical waveguide (core 203, core 303) is illustrated and described as an example. However, the grooves 205 and 305 may also be formed on the other side of the core 203 and 303 with respect to the direction of light propagation, i.e., on the inner circumference. The following description uses optical waveguide 200 as an example, but the same applies to optical waveguide 300.

[0022] In this embodiment, the groove 205 is formed to a depth just above the substrate 201, but the depth of the groove 205 may be shallower. The groove 205 is formed by etching via a photolithography process after the upper cladding 204 has been formed, similar to the prior art. The formation of this groove 205 partially releases the internal stress of each glass layer (lower cladding 202, core 203, upper cladding 204), and similarly, the angle of the principal axis of the refractive index ellipse given by the photoelastic effect can be rotated (tilted) with respect to the horizontal direction of the substrate 201.

[0023] In this case, the inclination angle (rotation angle) of the principal axis of the refractive index ellipse depends on the distance between the optical waveguide and the groove. Therefore, by controlling the distance between the core 203 and the groove 205, a waveplate with any optical axis angle can be formed. Furthermore, the retardation of this waveplate depends on the length of the groove 205 formed along the core 203; therefore, by controlling the length of the groove 205, it is possible to set the desired retardation.

[0024] The retardation can be determined by the following equation (1), using the birefringence B and the length L of the groove 205. The birefringence B can be determined by the difference between the minor axis and the major axis of the refractive index ellipse.

[0025] Retardation = B × L ... (Equation 1)

[0026] In Figure 3 and the following explanation, the groove 205 is depicted as being formed around the entire circumference of the loop-shaped portion. However, it is also possible that the groove is formed on only a portion of the loop shape, corresponding to a length L that allows the desired retardation to be achieved according to the birefringence B corresponding to the groove distance W.

[0027] Figure 5 shows the case where the groove formation position in the optical waveguide is in an ideal arrangement state, and the case where a positional shift occurs in the groove formation position due to errors in the manufacturing process. Figures 5(a) and 5(b) are schematic top views showing only the core 203 and groove 205 of the optical waveguide 200 shown as one embodiment of the present disclosure shown in Figure 3.

[0028] Figure 5(a) shows the case where the groove formation position is in an ideal configuration. That is, the arc-shaped optical waveguide (core 203) and the arc centers of the grooves are formed to coincide so that the groove distance remains constant in the direction of light propagation. However, if an exposure position shift occurs during photolithography for groove formation, the arc centers of the optical waveguide (core 203) and the groove 205 do not coincide, as shown in Figure 5(b). As a result, the groove distance W fluctuates along the direction of light propagation. In Figure 5(b), the groove 205 is shown to have been formed shifted from its original formation position.

[0029] In this case, the shapes of the optical waveguide (core) and groove are defined by the photomask, and therefore only have the potential errors that can occur in the photomask; thus, they can be assumed to be formed as designed. Consequently, the radii of curvature of the respective arc shapes of the optical waveguide and groove are as designed.

[0030] Figures 5(c) and 5(d) are schematic top views showing only the core 103 and groove 105 of the conventional optical waveguide 100 shown in Figure 1. Figure 5(c) shows the case where the groove formation position is in an ideal arrangement state, while Figure 5(d) shows the case where a shift in the exposure position occurs during photography for groove formation, resulting in a shift in the groove formation position.

[0031] Figure 6(a) plots the relationship between the trajectory 601 of the optical waveguide (core) and the trajectory 602 of the groove when the exposure position of the photolithography in groove formation is shifted by (X, Z) = (2 μm, 2 μm), as an example. In Figure 6(a), for ease of viewing and explanation, the radius of curvature of the optical waveguide (core 203) is set to 100 μm, the radius of curvature of the groove 205 is set to 110 μm, and the width between the core and groove is assumed to be negligible, i.e., the ideal groove distance W is set to 10 μm. When actually constructing a waveguide waveplate, the radius of curvature of the optical waveguide (core) is selected to match the specific refractive index difference of the optical waveguide, and the radius of curvature of the groove that constitutes the groove distance W is selected to match the optical axis rotation angle to be achieved.

[0032] Figure 6(b) is a graph plotting the change in groove distance in the direction of light propagation in the optical waveguide under the conditions shown in Figure 6(a). The horizontal axis of Figure 6(b) represents the position (distance) in the direction of light propagation along the arc from the starting point 603 to the ending point 604 of the groove. While the ideal groove distance W is 10 μm, it can be seen that in the conditions shown in Figure 6(a), the groove distance changes oscillatorily relative to 10 μm. The same is true for the optical waveguide 300 shown in Figure 4.

[0033] For comparison, Figure 7(b) shows the change in groove distance in the longitudinal direction (direction of light propagation) of the optical waveguide when the exposure position for groove formation is similarly shifted by (X, Z) = (2 μm, 2 μm) in the conventional optical waveguide 100. Figure 7(a) is the same graph as Figure 6(b). According to Figure 7(b), in the conventional linear optical waveguide 100, unlike the optical waveguide 200 of this embodiment, the groove distance always has a constant error with respect to the direction of light propagation in the optical waveguide.

[0034] Next, the effects of the optical waveguide of this embodiment will be explained using the waveplate function, which changes the polarization mode of the optical waveguide, as an example.

[0035] Generally, the orthogonal polarization modes in an optical waveguide are the TE (Transverse Electric) mode, where the electric field oscillates horizontally to the substrate, and the TM (Transverse Magnetic) mode, where it oscillates perpendicularly to the substrate.

[0036] In optical circuits, a function to convert one polarization component to the other is often required. To realize such a function, a waveplate function having a principal axis of a refractive index ellipse tilted at 45° with respect to the electric field oscillation direction of TE or TM can be configured within the optical waveguide. In this case, the retardation of the waveplate function is set to correspond to half a wavelength with respect to the wavelength of the optical signal. Below, in order to explain the operation and effects of this embodiment, a schematic optical system using this waveplate function is assumed.

[0037] The optical system 800 in Figure 8 is an open nicol optical system for verifying the function of converting TE linear polarization to TM linear polarization. In this optical system, TE linear polarization from 801 passes through the waveguide waveplate 802, is polarization-controlled, and then output to 803. The light from 803 is then transmitted to 805 only with TE polarization by a TE transmission analyzer 804.

[0038] Therefore, if the waveguide waveplate 802 is configured as designed, the waveguide waveplate converts all TE linear polarization to TM linear polarization, and no light is output from the TE transmission analyzer 804 to 805. In other words, if the waveguide waveplate 802 is not configured as designed and is imperfect, the TE linear polarization component remains in 803, and this remaining TE linear polarization component passes through the TE transmission analyzer 804 and is output to 805. It should be noted that the imperfection of the waveguide waveplate includes wavelength dependence, which is unavoidable in principle.

[0039] Figure 9 shows the results of calculating the extinction spectrum, which is the spectrum of light output from the waveguide waveplate 803 to 805, using the optical system 800 to evaluate the effect of groove misalignment.

[0040] Figure 9(a) is a graph showing the calculated extinction spectra of the optical system 800 in a waveguide waveplate configured with the optical waveguide 200 according to the present disclosure, for cases where the groove is formed at the designed position and when the groove formation position is shifted. For the calculation, the radius of curvature of the arc shape of the optical waveguide (core 203) was set to 1000 μm, and the radius of curvature and length of the groove 205 were set to be λ / 2 waveplate for a light wave of 1.45 μm.

[0041] Figure 9(b) is a graph showing the extinction spectra of an optical system 800 in a waveguide waveplate constructed using a conventional optical waveguide 100, for two cases: when the groove is formed at the designed position and when the groove formation position is shifted. For the calculation, the groove distance and the length of the groove 105 were set to the condition that it becomes a λ / 2 waveplate for a light wave of 1.45 μm. The amount of positional shift of the groove formation position was set to (X, Z) = (2 μm, 2 μm) in both cases. In the graph, the dotted line represents the extinction spectrum when the groove is formed at the designed position, and the solid line represents the extinction spectrum when the groove formation position is shifted.

[0042] In conventional optical waveguides, when a straight groove is formed in a straight optical waveguide, the spectrum changes significantly if the groove formation position is shifted. In contrast, in the optical waveguide of this embodiment, when a loop-shaped groove including an arc is formed in an optical waveguide that includes an arc shape, and the groove is set to correspond to the target groove distance, the change in the spectrum is very small even if the groove formation position is shifted. Thus, the configuration according to the embodiment of this disclosure enables the realization of a waveguide waveplate with high manufacturing tolerance.

[0043] In the above, as an embodiment of the present disclosure, a waveplate waveguide that improves manufacturing tolerance can be formed by an optical waveguide having a loop-shaped groove including an arc shape formed on an optical waveguide including an arc shape.

[0044] The reason for this action and effect can be explained as follows. For example, in FIG. 5, when the formation position of the groove shifts to the positive side in the X direction (upward on the paper surface), the groove distance on the upper side of the arc increases. In contrast, the groove distance on the lower side of the arc decreases. In this way, fluctuation in retardation can be suppressed by mutually canceling the influence caused by the change in groove distance. In contrast, in the conventional optical waveguide 100, as shown in FIG. 7(b), the change in groove distance is a constant amount in one direction, so the influence is not canceled.

[0045] According to the optical waveguide according to the embodiment of the present disclosure, fluctuation in retardation can be suppressed by mutually canceling the influence caused by the change in groove distance due to the displacement of the groove formation position. On the other hand, in the conventional art, since the groove distance does not change due to displacement in the longitudinal direction (Z direction) of the optical waveguide core, the displacement of the groove in the Z direction does not affect retardation.

[0046] Therefore, in the embodiment described below, the object is to reduce as much as possible the influence caused by the displacement of the groove formation position in the Z direction.

[0047] In the configuration of the waveguide wave plate, if the loop shape including the arc shape of the optical waveguide is formed into a C-shape that is as close to a circle as possible, and the groove is configured to surround the optical waveguide as much as possible in the up, down, left and right directions (a shape close to a circle), in FIG. 5(a), the influence due to the positional displacement of the groove in the Z-axis direction can also be offset.

[0048] At this time, as shown in FIG. 5(a), when the optical waveguide 200 is laid out in an arc shape, by making the portion 206 where the input / output paths to the arc-shaped portion are formed as small as possible, the optical waveguide can be laid out in an arc shape that is close to a circle.

[0049] Based on this concept, as still another aspect according to the embodiment of the present disclosure, it is desirable to design the portion 206 where the input / output paths of the optical waveguide core 203 in FIG. 5(a) are formed as small as possible, and to extend the groove 205 over the entire circumference of the arc-shaped portion other than the input / output path portion.

[0050] As described above, in an optical waveguide in which a groove is formed on one side of the core, the optical axis angle is determined by the groove distance, and as a result, the birefringence B of the optical waveguide is determined. Then, from this determined birefringence B, the groove length L required to achieve the design value retardation is determined.

[0051] Therefore, ideally, the radius of curvature of the arc shape of the optical waveguide should be such that the length L of the groove, when extended over the entire circumference of the arc-shaped portion of the optical waveguide, is the length required to achieve the design value retardation.

[0052] In practice, it is undesirable for the radius of curvature of the arc shape to be smaller than the minimum bending radius determined by the difference in specific refractive index. Therefore, it is desirable to reduce the radius of curvature to approximately the minimum bending radius determined by the difference in specific refractive index, so that the groove surrounds the entire circumference of the arc shape of the optical waveguide.

[0053] Based on this idea, in one embodiment of the present disclosure, it is desirable to set the radius of curvature of the optical waveguide (core 203) to the smallest bending radius determined by the difference in specific refractive index, and to ensure that the groove 205 surrounds the arc-shaped portion of the optical waveguide as much as possible. This makes it possible to further cancel out the effects of positional displacement in the left-right direction (Z-axis direction) of the groove formation position.

[0054] In the above description, the optical waveguide according to the embodiment of this disclosure was described as a silica-based optical waveguide using silica glass. However, the material applied to the optical waveguide according to the embodiment of this disclosure may also be, for example, a silicon semiconductor, a ferroelectric material, SiOx, SiON, SiN, etc.

[0055] The above example shows forming a groove on the outside of a loop-shaped optical waveguide, but it is possible to obtain a similar effect by forming a groove on the inside of the optical waveguide loop. However, it should be noted that when light waves propagate through a loop-shaped optical waveguide, the center of gravity of the modes is biased outward, so in order to effectively take advantage of the stress caused by the groove, it is preferable to form the groove on the outside of the loop.

[0056] This disclosure makes it possible to provide an optical waveguide that realizes a waveguide waveplate with increased tolerance and improved yield.

[0057] 100, 200, 300... Optical waveguide 101, 201... Substrate 102, 202... Lower cladding 103, 203, 303... Optical waveguide core 104, 204... Upper cladding 105, 205, 305... Grooves 206, 306... Input / output path section 800... Optical system 802... Waveguide waveplate 804... TE transmission analyzer

Claims

1. An embedded optical waveguide formed on a substrate, comprising a lower cladding, a core, and an upper cladding, wherein the optical waveguide has a loop shape including an arc shape, and comprises a groove formed on one side of the core with respect to the direction of light propagation, extending in a direction parallel to the core.

2. The optical waveguide according to the present invention, characterized in that the groove is formed to extend around the entire circumference of the loop shape including the arc shape.

3. The optical waveguide according to claim 1, characterized in that the radius of curvature of the arc shape is the minimum bending radius determined by the difference in relative refractive index.

4. The optical waveguide according to claim 1, wherein the refractive index distribution in a plane perpendicular to the direction of light propagation is a distribution in which the principal axis of the refractive index ellipse has a rotation angle with respect to the vertical direction of the substrate plane.

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