Planar lightwave circuit

Abstract

A planar lightwave circuit disclosed herein comprises: a substrate (1); a cladding layer (2) provided on the substrate (1); a core (3) that is embedded in the cladding layer (2) and constitutes a waveguide; and a recessed region (210) in which a plurality of recess sections (5) formed in the cladding layer (2) are two-dimensionally disposed in the in-plane direction of the cladding layer (2).

Description

Planar light wave circuit

[0001] The present disclosure relates to planar lightwave circuits.

[0002] Planar lightwave circuits (PLCs) using optical waveguides fabricated on a substrate can integrate various interferometers, such as optical filters, optical multiplexers / demultiplexers, and optical switches, on a chip, and are realized using a variety of materials. For example, planar lightwave circuits constructed with silica-based optical waveguides are widely used in the optical communications field because of their low optical loss, stability, and / or reliability.

[0003] Japanese Patent Application Publication No. 2010-32921 Publication No. 9-5548

[0004] Known planar lightwave circuits have room for improvement in suppressing or reducing crosstalk due to stray light propagating through cladding layers.

[0005] An object of the present disclosure is to provide a planar lightwave circuit that can improve the effect of suppressing or reducing crosstalk caused by light (e.g., stray light) propagating through a cladding layer.

[0006] A planar lightwave circuit according to one aspect of the present disclosure comprises a substrate, a cladding layer provided on top of the substrate, a core embedded in the cladding layer to form a waveguide, and a recessed region in which a plurality of recessed portions formed in the cladding layer are arranged two-dimensionally in an in-plane direction of the cladding layer.

[0007] A planar lightwave circuit according to an aspect of the present disclosure can improve the effect of suppressing or reducing crosstalk caused by light (e.g., stray light) propagating through a cladding layer.

[0008] 1 is a top view schematically showing an optical circuit configured using planar lightwave circuit technology for fabricating silica-based optical waveguides on a silicon substrate. It is a partial cross-sectional view partially showing a cross section along the cutting line X-X' in FIG. 1. It is a top view schematically showing an optical circuit that is a non-limiting example of a planar lightwave circuit including a light-shielding groove group according to an embodiment. It is a partially enlarged view for explaining details of each light-shielding groove group exemplified in FIG. 3. It is a partial cross-sectional view partially showing a cross section along the cutting line X-X' shown in FIG. 4. It is a partial cross-sectional view partially showing a cross section along the cutting line Z-Z' shown in FIG. 4. Each of (a) to (h) is a top view schematically showing an example of an arrangement pattern of deep grooves in a light-shielding groove group. It is a diagram showing a detailed example of how stray light propagates across multiple deep grooves in a light-shielding groove group according to an embodiment. It is a graph showing an example of a position change in the y-axis direction as stray light propagating in the z-axis direction travels through a light-shielding groove group according to an embodiment. It is a cross-sectional view schematically showing an example in which a light-receiving element is mounted on an end face of an optical circuit according to an embodiment. 7A and 7B are diagrams showing an example of an arrangement pattern corresponding to a configuration in which the diameters of the deep grooves are made different for some or all of the multiple divided regions in the example of the arrangement pattern of deep grooves shown in FIG. 7D. FIG. 7B is a cross-sectional view showing an example of a finished shape of a groove near the processing limit of an embodiment. FIG. 7C is a top view showing an example of a test circuit produced to confirm the effect of a light-shielding groove group according to an example. FIG. 7D is a graph showing an example of the results of evaluating the transmittance spectrum versus wavelength of a test circuit according to an example.

[0009] Hereinafter, embodiments and examples of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and repeated explanations may be omitted. When materials and numerical values ​​are described in the following description, they are merely examples, and the present disclosure may be implemented by additionally or alternatively using other materials and numerical values.

[0010] [Outline] FIG. 1 is a top view that schematically shows an optical circuit 100 configured using planar lightwave circuit technology for fabricating a silica-based optical waveguide on a silicon substrate.

[0011] As will be described later with reference to Fig. 2, the optical circuit 100 includes a substrate 1, a cladding layer 2 laminated on top of the substrate 1, and a core 3 embedded inside the cladding layer 2 and functioning as a waveguide. Note that the core 3 may be referred to as a waveguide core 3 or a waveguide 3 below. The cladding layer 2 may be abbreviated as a cladding 2.

[0012] The optical circuit 100 includes, for example, optical interferometer circuits 110a and 110b that function as optical filters, optical interferometer circuits 120a and 120b that function as optical switches or attenuators, and optical branching circuits 130a and 130b that are cascade-connected by a waveguide 3. Furthermore, on the chip end face of the optical circuit 100, for example, input optical fibers 101a and 101b, output optical fibers 102a and 102b, and light-receiving elements 103a and 103b are mounted.

[0013] As described above, two sets of circuits or elements are integrated or implemented in the optical circuit 100, and these components are distinguished by adding "a" or "b" to the end of the reference numeral; however, when there is no need to distinguish between them, the final symbol is omitted.

[0014] Both the optical interferometer circuit 110 and the optical interferometer circuit 120 are Mach-Zehnder interferometers configured by cascading two optical couplers with two waveguides 3. In the optical interferometer circuit 110, for example, the waveguide lengths (in other words, the optical path length difference) of the two cascaded waveguides 3 differ by ΔL. Due to a known interference phenomenon, light of a specific wavelength having a repetition period determined by this optical path length difference ΔL passes through the optical interferometer circuit 110, and light of other wavelengths is blocked.

[0015] In the optical interferometer circuit 120, a thermo-optic phase shifter is illustratively provided in one of the two cascaded waveguides 3. Fig. 2 is a partial cross-sectional view partially showing a cross section taken along the cutting line X-X' in Fig. 1, illustrating a cross section focusing on one optical interferometer circuit 120a. Note that the cross section focusing on the other optical interferometer circuit 120b may also be understood to be equivalent to the cross section focusing on the optical interferometer circuit 120a.

[0016] 2, in the optical interferometer circuit 120, a thin-film heater 4 is provided on top of a cladding 2 laminated on a substrate 1. When current is applied to the thin-film heater 4, the temperature of the waveguide core 3 located below the thin-film heater 4 changes locally, and the equivalent refractive index of the waveguide core 3 changes due to the thermo-optic effect in response to this temperature change.

[0017] The phase of the light propagating through the waveguide core 3 shifts in response to the change in the equivalent refractive index, and the interference conditions of the optical interferometer circuit 120 change, resulting in a change in the transmittance of the optical interferometer circuit 120. The optical branching circuit 130 branches the light into two using a circuit called a Y-branch waveguide, which has a structure in which one waveguide is split into two.

[0018] Light input from an input optical fiber 101 to the optical circuit 100 is filtered in an optical interferometer circuit 110 so that light of wavelength components different from a specific wavelength is blocked, and the light of the specific wavelength is guided to a subsequent optical interferometer circuit 120. The light transmitted through the optical interferometer circuit 110 is attenuated in the optical interferometer circuit 120 and then branched into two beams by an optical branching circuit 130. One of the branched beams is output to an output optical fiber 102, and the other beam is guided to a light-receiving element 103.

[0019] Here, in the optical circuit 100, undesired light called "stray light" may be generated, which propagates within the cladding without propagating through the waveguide 3. For example, in the optical circuit 100, leakage light occurs at the connection between the input optical fiber 101 and the optical circuit 100 due to a mismatch in propagation mode or misalignment of the mounting axis, and this leakage light becomes stray light.

[0020] In the optical interferometer circuit 110, light having a wavelength different from the specific wavelength described above is output from the unconnected port 112 and becomes stray light. Similarly, in the optical interferometer circuit 120, attenuated light is output from the unconnected port 122 and becomes stray light. Furthermore, in the optical branching circuit 130, leakage light occurs from the portion corresponding to the "fork" of the Y-branching waveguide due to, for example, deviation in shape caused by imperfections in manufacturing, and this leakage light becomes stray light.

[0021] When the stray light described above enters the waveguide 3 or the like from some or all of the multiple locations listed below, it causes crosstalk with the light propagating through the waveguide 3, resulting in deterioration of the characteristics (or performance) of the optical circuit 100. Note that, for convenience, the stray light "entering" the waveguide 3 or the like may hereinafter be expressed as the stray light "jumping into" the waveguide 3 or the like.

[0022] (Positions where stray light can enter) Unconnected port 111 of the optical interferometer circuit 110 Unconnected port 121 of the optical interferometer circuit 120 Portion where the waveguide 3 gradually becomes thicker due to the Y branching of the optical branching circuit 130 Connection portion between the optical circuit 100 and the output optical fiber 102 Light receiving surface of the light receiving element 103

[0023] The locations where losses may occur in the optical circuit 100 are locations where stray light is generated and also locations where stray light may enter. Several methods have been considered for reducing or eliminating this stray light. For example, Patent Document 1 describes a method using total reflection grooves.

[0024] This method involves providing a total reflection groove in the cladding that crosses the direction in which stray light travels at an angle, and totally reflecting the stray light at the boundary between the air and the cladding, thereby preventing the stray light from traveling to the point where it jumps in. For example, a total reflection groove is used that has a triangular shape with one side at an interface where the angle between the traveling direction of the stray light and the normal to the interface is equal to or greater than 43°, which is the critical angle.

[0025] On the other hand, Patent Document 2 describes a method using a light-blocking material. This method involves forming grooves in the cladding and filling the grooves with a light-absorbing material as a light-blocking material to block stray light. The light-absorbing material is a material obtained by mixing light-absorbing fine particles into a resin base material, and the light-absorbing material is filled into the grooves by applying this material.

[0026] However, the method using total reflection grooves described in Patent Document 1 merely changes the direction of travel of stray light within the circuit surface, and does not change the conditions under which stray light propagates through the cladding, for example, the conditions under which stray light is refraction in a direction perpendicular to the circuit surface. When the circuit scale is small, it is possible to arrange total reflection grooves so that no propagation path in the cladding connecting the point where stray light is generated and the point where stray light enters is created. On the other hand, when the circuit scale increases, the number of points where stray light is generated and the number of points where stray light enters increase, and it may become difficult to arrange total reflection grooves taking into account all combinations of the points where stray light is generated and the points where stray light enters.

[0027] 1, for example, if a total reflection groove is disposed between the unconnected port 112a and the output optical fiber 102 to prevent stray light emitted from the unconnected port 112a from entering the unconnected port 121a, the stray light can be prevented from entering the unconnected port 121a. On the other hand, there is a possibility that stray light emitted from the unconnected port 112a will enter other locations, such as the unconnected port 121b located next to the unconnected port 121a or the optical branching circuit 130b at the subsequent stage. Similarly, for example, if a total reflection groove is disposed between the unconnected port 122 and the output optical fiber 102 to prevent stray light from entering the output optical fiber 102 from the unconnected port 122, there is a possibility that the stray light will enter the light-receiving element 103 or the like located at the subsequent stage of the unconnected port 122.

[0028] Furthermore, the light receiving element 103 mounted on the end face of the optical circuit 100 has a light receiving surface that is larger than the core size of the waveguide 3, and can receive light incident on the end face of the optical circuit 100 from any angle, making crosstalk more likely to occur.

[0029] Therefore, the method described in Patent Document 1 is not practical because it is difficult to arrange the total reflection grooves taking into account all the paths of incoming light in circuits with large circuit scales and high integration densities. Furthermore, the total reflection grooves reflect all the stray light that has spread and propagated in the cladding, so the boundary surface that satisfies the critical angle must be set to a certain size, which may hinder improvements in the integration density of optical circuits.

[0030] On the other hand, the method using the light-shielding material described in Patent Document 2 adds a step of filling the light-absorbing material, which increases the number of manufacturing steps and can lead to increased costs. Furthermore, the positional accuracy of the application of the light-absorbing material is not sufficient, so a sufficient gap must be provided between the application area of ​​the light-absorbing material and other areas such as the chip edge or the insulating groove in the optical circuit. Furthermore, gases such as siloxane can be generated from the resin used in the light-absorbing material over the long term, which can be a factor in reducing the reliability of the optical circuit.

[0031] Therefore, the following describes non-limiting embodiments and examples of planar lightwave circuits that can improve the effect of suppressing or reducing crosstalk caused by light (e.g., stray light) propagating through the cladding layer and avoid increased costs or reduced reliability. Note that "stray light" may be understood as a non-limiting example of "unwanted light that is not desired to propagate" or "light that is desired to be attenuated" in the cladding layer.

[0032] In the embodiments and examples described below, an example of a planar lightwave circuit using a silica-based waveguide 3 is shown, but in the present disclosure, the material of the waveguide 3 is not limited to silica. For example, instead of the silica-based waveguide 3, waveguides of other materials such as polymer-based waveguides, silicon (Si) waveguides, and indium phosphide (InP)-based waveguides may be applied to the planar lightwave circuit according to the present disclosure.

[0033] [Embodiment] Fig. 3 is a top view schematically showing an optical circuit 200, which is a non-limiting example of a planar lightwave circuit having a light-shielding groove group according to an embodiment. The optical circuit 200 illustrated in Fig. 3 corresponds to a configuration in which light-shielding groove groups 210-1, 210-2, and 210-3 are additionally provided in the configuration illustrated in Fig. 1. Note that the other components of the optical circuit 200 may be understood to be equivalent or similar to the components illustrated in Fig. 1.

[0034] For example, the light-shielding groove group 210-1 is provided between the connection portion of the input optical fiber 101 and the optical interferometer circuit 110, and the light-shielding groove group 210-2 is provided between the optical interferometer circuit 110 and the optical interferometer circuit 120. The light-shielding groove group 210-3 is provided between the connection portion of the optical interferometer circuit 120 and the optical branching circuit 130 and the output optical fiber 102. In other words, the optical circuit 200 is individually provided with the light-shielding groove groups 210-1, 210-2, and 210-3 corresponding to the positions between the points where stray light is generated and the points where stray light enters.

[0035] Fig. 4 is a partially enlarged view for explaining the details of each light-shielding groove group 210-k (k is an integer of 1 or more) illustrated in Fig. 3. Note that while Fig. 3 shows the waveguide 3 crossing the light-shielding groove group 210-3 as a curved waveguide 3 including a curved portion, for ease of explanation, Fig. 4 shows an example in which the waveguide 3 crossing each light-shielding groove group 210-k is a straight waveguide 3. Furthermore, hereinafter, when it is not necessary to distinguish between the individual light-shielding groove groups 210-k, they may be abbreviated as "light-shielding groove group 210."

[0036] 5 and 6 are partial cross-sectional views showing the cut surfaces taken along the cut lines XX' and ZZ' shown in Fig. 4, respectively. In each of Fig. 3 to Fig. 6, the z-axis corresponds to the input / output direction of the waveguide 3, the x-axis corresponds to the in-plane width direction of the substrate 1, and the y-axis corresponds to the direction perpendicular to the substrate 1 (in other words, the thickness direction of the substrate 1). The directions indicated by the x-axis, y-axis, and z-axis are the same in the drawings used in the following description.

[0037] As shown in Figures 4 to 6, each of the light-shielding groove groups 210 exemplarily has a configuration in which a plurality of deep grooves 5 are two-dimensionally arranged, each deep groove 5 being formed by partially removing the cladding 2 around the waveguide core 3 at a plurality of locations. Also, as shown in Figures 5 and 6, the angle formed between the normal direction to the substrate surface and the normal direction to the sidewall surfaces of the deep grooves 5 is an angle other than 90°. The normal direction to the substrate surface is, for example, the direction from the back surface of the substrate 1 to the front surface. The normal direction to the sidewall surfaces of the deep grooves 5 is, for example, the direction from the inside to the outside of the cladding 2.

[0038] For example, as shown in FIG. 5, when the angle formed by the normal direction of the substrate surface and the side wall surface of deep groove 5 is represented as θ, the angle formed by the two normal directions described above is represented as "90°-θ", and therefore, "an angle different from 90°" means that θ≠0°.

[0039] In the present disclosure, the term "deep groove" may be interchangeably read as other terms such as "groove," "groove portion," "hole," "hole portion," "cavity," "cavity portion," "recess," or "cladding removal portion." The terms "cavity," "cavity portion," "recess," or "cladding removal portion" may be understood as a generic term that includes "groove," "groove portion," "hole," and "recess portion." Furthermore, in the light-shielding groove group 210, the portion of the cladding 2 that remains without being removed may be referred to for convenience by terms such as "remaining cladding," "protruding cladding," "convex cladding," or "mesh cladding."

[0040] A "light-shielding groove group" composed of multiple "deep grooves" may be referred to as a "groove group," or, focusing on the functional structure described below, may be interchangeably read as other terms such as a "stray light suppression structure," a "stray light reduction structure," or a "light refraction control structure." The term "structure" may be interchangeably read as other terms such as "configuration," "means," or "part." The term "part" may be interchangeably read as a "region," for example.

[0041] 5 and 6 show an example in which the deep grooves 5 are formed so that their size (e.g., diameter) in a top view decreases toward the substrate 1. In this example, the angle between the two normal lines is smaller than 90° (i.e., θ>0°). The deep grooves 5 may be formed, for example, by being excavated from the upper surface of the cladding 2 to a depth that reaches the upper surface of the substrate 1. Alternatively, the deep grooves 5 may be formed to a shallower depth, for example, reaching at least the height of the waveguide core 3 where stray light is generated. The position corresponding to the height of the waveguide core 3 is, for example, a position corresponding to the distance in the y-axis direction relative to the substrate 1 in FIG. 5, and may be, for example, a y position corresponding to the lower surface of the waveguide core 3.

[0042] As will be described later, it is desirable to arrange the deep grooves 5 as densely as possible, and therefore Fig. 4 shows an example in which the deep grooves 5 are arranged in a hexagonal packing (which may be read as a "hexagonal lattice"). In addition, in the optical circuit 200, the main traveling direction of stray light is the z-axis direction in Figs. 4 to 6. The arrangement axis of the hexagonal packing is arranged in a direction different from the z-axis direction, which is the main traveling direction of stray light, as shown by the dotted line in Fig. 4, for example.

[0043] With such an arrangement of the array axes, for example, stray light propagating in the z-axis direction propagates by frequently crossing the plurality of deep grooves 5 in each of the plurality of light-shielding groove groups 210. As the number of deep grooves 5 crossed by stray light propagating in the z-axis direction increases, the stray light is more easily scattered in the in-plane direction of the substrate 1, as will be described later, and the number of times the stray light is refracted in the direction approaching the substrate 1 increases, making it easier to expel the stray light from the cladding 2.

[0044] The pattern shape of the deep grooves 5 is not particularly limited, but for example, by forming the deep grooves 5 in a circular shape in plan view, which is easy to process and form by densely packing small patterns, it becomes easy to arrange the deep grooves 5 as densely as possible. Alternatively, the deep grooves 5 may be formed in a shape other than a circle in plan view, such as a polygon such as a square or a hexagon, or an ellipse.

[0045] When the pattern shape of the deep grooves 5 is circular, there is also the effect of making it easier to attenuate stray light by scattering in the in-plane direction (xz plane direction) of the substrate 1. For example, as shown in Figure 4, if the diameter of the deep grooves 5 is represented as D and the arrangement interval between the deep grooves 5 is represented as P, the condition under which stray light propagating in the z-axis direction crosses the deep grooves 5 is represented by the following equation 1.

[0046] Furthermore, by defining the relationship between D and P as expressed by the following formula 2, for example, it is possible to leave as much of the cladding 2 between the deep grooves 5 as possible in any direction within the substrate surface (xz plane). Therefore, for example, it is possible to easily maintain the structural strength of the light-shielding groove group 210 at a certain level.

[0047] From the above, it is desirable that the diameter D of the deep groove 5 be set to a value that satisfies the formulas 1 and 2, that is, a value that satisfies the following formula 2a.

[0048] Furthermore, when D and P satisfy the relationship expressed by the following formula 3, stray light not only in a direction intersecting the arrangement axis of the deep grooves 5 but also in a direction along the arrangement axis will cross the deep grooves 5. Therefore, having D and P satisfy the relationship of formula 3 is useful when stray light can be generated in any direction including the direction along the arrangement axis of the deep grooves 5.

[0049] By providing the light-shielding groove group 210 that satisfies Equations 1 to 3 (in other words, Equations 2a and 3), stray light propagates across the many deep grooves 5, and is scattered in the in-plane direction of the substrate 1. Furthermore, by setting θ to an angle other than 0°, the stray light is refracted multiple times in the direction perpendicular to the substrate 1 and is expelled to the outside of the cladding 2. For example, at an angle of θ>0°, as shown in FIG. 6 , the stray light is refracted in a direction approaching the substrate 1 each time it crosses multiple deep grooves 5.

[0050] If the refractive index of the substrate 1 is higher than that of the cladding 2, the stray light propagates with greater refraction in the substrate 1. For example, in the 1.55 micrometer (μm) wavelength band, this is the case because the refractive index of the silica-based waveguide 3 is about 1.4 and the refractive index of the silicon substrate is about 3.5.

[0051] When a doped wafer known as a low-resistivity wafer is used as the silicon substrate, significant light absorption loss occurs even in the 1.55 μm wavelength band, so stray light can be effectively attenuated. Furthermore, when θ is an angle other than 0°, the reflected stray light can also be deflected in a direction perpendicular to the substrate 1. In this way, the multiple deep grooves 5 can effectively scatter not only refracted stray light but also reflected stray light outside the cladding 2, thereby effectively attenuating stray light.

[0052] When manufacturing the optical circuit 200 of this embodiment, a step of forming the light-shielding groove group 210 is added compared to the optical circuit 100. For example, a step of forming the deep grooves 5 by removing a part of the cladding 2 using a microfabrication technique such as photolithography is added.

[0053] 1, in many cases, a heat insulating groove is formed around the thermo-optic phase shifter to increase the efficiency of local heating by the thin film heater 4. The process of forming this heat insulating groove can be shared with the process of forming the deep grooves 5 of the light-shielding groove group 210, so by forming the deep grooves 5 of the light-shielding groove group 210 at the same time as forming the heat insulating groove, an increase in the number of processes can be avoided.

[0054] [Examples of various arrangement patterns of deep grooves constituting the light-shielding groove group] Various patterns are possible for the arrangement of the deep grooves 5 in the light-shielding groove group 210. Figure 7 is a top view schematically showing examples of arrangement patterns of the deep grooves in the light-shielding groove group 210, and eight arrangement patterns (a) to (h) are shown as non-limiting examples.

[0055] 7A shows an example of a basic arrangement pattern, in which the deep grooves 5 are arranged in a checkerboard pattern along both the x-axis and z-axis directions. When the traveling direction of stray light deviates from the z-axis direction, such a checkerboard arrangement pattern can eliminate or suppress the stray light. When the traveling direction of stray light is along the z-direction, there are gaps between the deep grooves 5 that are visible to the stray light, so that a light-blocking means for stray light traveling through the gaps between the deep grooves 5 may be applied individually. Note that a "visible gap" for stray light means, for example, the presence of a cladding 2 through which the stray light can propagate in the traveling direction without crossing the deep grooves 5.

[0056] 7(b) shows an example of an arrangement pattern in which the deep grooves 5 are arranged in a grid pattern but tilted diagonally. Fig. 7(c) shows an example of an arrangement pattern in which the deep grooves 5 are arranged with a half-pitch offset between odd-numbered and even-numbered columns. In both of the arrangement pattern examples shown in Fig. 7(b) and Fig. 7(c), by using a diameter D that is smaller than the arrangement interval P of the deep grooves 5 and is sufficiently large in relation to the arrangement interval P (in other words, not too small relative to the arrangement interval P), it is possible to eliminate (or reduce) gaps that are visible to stray light propagating along the z-axis direction.

[0057] 7(d) shows an example of an arrangement pattern in which the deep grooves 5 are arranged by hexagonal packing, similar to the arrangement pattern shown in FIG. 4 . The arrangement pattern shown in FIG. 7(d) differs from the arrangement pattern shown in FIG. 7(c) in that the arrangement spacing of the deep grooves 5 in the z-axis direction is different. For example, if the arrangement spacing in the x-axis direction is represented as P, the arrangement spacing in the z-axis direction in the arrangement pattern shown in FIG. 7(c) is P, whereas in the arrangement pattern shown in FIG. 7(d), it is P·(√3) / 2, which is an isotropic arrangement spacing. This arrangement allows the density of the deep grooves 5 to be increased not only in the x-axis direction but also in the z-axis direction within the substrate plane (xz plane).

[0058] 7( e) shows an example of an arrangement pattern in which the deep grooves 5 are arranged randomly instead of regularly arranging the deep grooves 5. This arrangement pattern example corresponds to an arrangement pattern in which the arrangement positions of the deep grooves 5 in the light-shielding groove group 210 are randomly shifted from the periodic positions. By arranging the deep grooves 5 randomly in this way, it is possible to eliminate (or reduce) gaps that allow stray light to see between the deep grooves 5 in all directions at locations in the optical circuit 200 where stray light may enter.

[0059] In other words, this random arrangement pattern example is a probabilistic arrangement pattern with respect to the removal or suppression of stray light. Because it is a probabilistic arrangement pattern, the more the number of deep grooves 5 to be arranged, the more the probability of gaps remaining that are visible to stray light can be reduced. Therefore, in this arrangement pattern, for example, the arrangement area size of the deep grooves 5 may be expanded within the range of the substrate size that allows the arrangement of the light-shielding groove group 210 on the substrate plane (xz plane), thereby increasing the number of deep grooves 5 to be arranged as much as possible.

[0060] 7(f) corresponds to an arrangement pattern in which, for example, in the hexagonal packing arrangement pattern example shown in Fig. 7(d), the waveguides 3 passing through the light-shielding groove group 210 are waveguides including curved waveguides instead of straight waveguides. In the hexagonal packing arrangement pattern example shown in Fig. 7(d), as described above, by using a diameter D appropriate for the arrangement interval P of the deep grooves 5 for the deep grooves 5, it is possible to eliminate (or reduce) gaps through which stray light can pass.

[0061] On the other hand, gaps that can be seen through by stray light propagating near the waveguide 3 are likely to remain. Therefore, by applying a waveguide including a curved waveguide to the waveguide 3 that passes through the light-shielding groove group 210, as in the arrangement pattern exemplified in Fig. 7(f), it is possible to eliminate (or reduce) gaps that can be seen through by stray light propagating near the waveguide 3.

[0062] Fig. 7(g) shows an example of an arrangement pattern in which the waveguide 3 passing through the light-shielding groove group 210 is a waveguide including a curved waveguide, similar to the example of the arrangement pattern shown in Fig. 7(f). The example of the arrangement pattern shown in Fig. 7(f) corresponds to an arrangement pattern in which, for example, deep grooves 5 are arranged regularly and uniformly, but deep grooves 5 are not provided in the portions of the light-shielding groove group 210 through which the waveguide 3 passes. The portions of the pattern of the waveguide 3 where deep grooves 5 are not provided may be determined, for example, by a predetermined algorithm used in pattern design.

[0063] In contrast, in the example of the arrangement pattern shown in FIG. 7( g), instead of providing deep grooves 5 of the same basic shape (circular in FIG. 7( f)) around the waveguide 3, deep grooves 5 of a shape obtained by removing the pattern of the waveguide 3 from the basic shape are provided. This arrangement pattern is determined, for example, by performing pattern calculation based on a ridge pattern in which a predetermined width (in other words, a margin) is added to the width of the waveguide 3 for the deep grooves 5 in the portion of the light-shielding groove group 210 through which the waveguide 3 passes. In this example, since it is sufficient to perform calculation processing on the pattern drawing, there is an advantage that pattern design is easy. The light-shielding groove group 210-3 illustrated in FIG. 3 corresponds to the arrangement pattern of this example.

[0064] Fig. 7(h) shows an example of an arrangement pattern in which a plurality of deep grooves 5 each having an elongated shape in the x-axis direction (which may be conveniently referred to as "vertical stripes") are arranged in the z-axis direction while avoiding the waveguides 3. Note that "D" added in Fig. 7(h) represents the width of the deep grooves 5 each having an elongated shape in the x-axis direction, and "P'" represents the repetition pitch of the deep grooves 5 in the z-axis direction, and these will be used in the explanation of an example of propagation of stray light, which will be described later with reference to Fig. 8.

[0065] In the example arrangement pattern shown in Figure 7(h), there is no (or the gaps are minimized) through which stray light traveling in directions other than the x-axis direction can see. Therefore, when the stray light propagates in a direction generally along the z-axis direction, the stray light can be effectively attenuated. Note that in this arrangement pattern, the cladding 2 that remains without being removed forms an elongated ridge, which increases flexibility, for example, along the x-axis, compared to the example arrangement patterns shown in Figures 7(a) to 7(g).

[0066] [Amount of Refraction by Light-Shielding Groove Group] Fig. 8 is a diagram showing a detailed example of how stray light propagates across the plurality of deep grooves 5 in the light-shielding groove group 210, and corresponds to a partially enlarged view of the cross section shown in Fig. 6. Hereinafter, with reference to Fig. 8, we will explain how stray light propagating through the cladding 2 is refracted by the deep grooves 5 in the yz plane and travels toward the substrate 1.

[0067] For simplicity, the arrangement pattern of the deep grooves 5 is assumed to be a uniform pattern in the x-axis direction, as shown in FIG. 7( h). The deviation angle of the sidewalls of the deep grooves 5 from the 90° angle is represented by θ, the width of the pattern of the deep grooves 5 is represented by D, and the repetition pitch of the deep grooves 5 in the z-axis direction is represented by P'. Here, when θ≠0°, the values ​​of D and P' strictly vary depending on the position in the y-axis direction, but for the sake of simplicity of calculation, they are assumed to be constant values. Also, it is assumed that stray light travels toward the positive side of the z-axis direction in the yz plane.

[0068] Stray light travels through the (i-1)th (i is an integer of 1 or more) cladding 2 along the z axis with a wavelength of φ i-1 At the boundary surface with the i-th deep groove 5, the position in the y-axis direction is y i-1 Assume that the angle of incidence ξ at the interface between the cladding 2 and the deep trench 5 is i (=φ i-1 +θ) and the exit angle ζ i The relationship between the above is expressed by the following equation 4 from Snell's law:

[0069] Here, n clad represents the refractive index of the cladding 2, and the refractive index of the deep groove 5 is the refractive index of air (=1). Therefore, the angle of travel φ' of the stray light in the i-th deep groove 5 is i (=ζ i −θ) is expressed by the following equation 5:

[0070] Similarly, the angle of incidence ζ' at the boundary surface between the i-th deep trench 5 and the i-th cladding 2 is i (=θ-φ' i ) and the exit angle ξ' i The relationship between the angle of propagation of the stray light in the i-th cladding 2 and the angle of propagation of the stray light in the i-th cladding 2 is expressed by the following equation 6. i (=ζ i -θ), the following equation 7 is derived based on the relationship expressed by the following equation 6.

[0071] The change in the y direction of the stray light in the i-th deep groove 5 is "y' i -y i-1" is "-D tanφ i ", and the change amount "y i -y' i-1 " is "-(P'-D)・tanφ i Therefore, the position y in the y-axis direction on the boundary surface between the ith cladding 2 and the (i-1)th deep trench 5 is expressed by i can be expressed by the following Equation 8.

[0072] The position in the y-axis direction of the stray light incident on the light-shielding groove group 210, in other words, the initial position y on the y axis, corresponds to the height position of the center of the waveguide core 3 relative to the substrate 1, and is, as a non-limiting example, about 25 μm. Also, the initial angle φ is, for example, zero. In the case of a silica-based waveguide 3, n clad ≒1.445.

[0073] By using these parameters and Equations 5, 7, and 8, the position y of the stray light in the y-axis direction can be calculated. i Therefore, the position y of the stray light in the y-axis direction can be calculated. i Let i be the smallest i such that is less than or equal to zero. min When the length of the light-shielding groove group 210 in the direction along the propagation direction of the stray light is expressed as i min If it is longer than P′, it can be said that the stray light can be removed from the cladding 2 by the light-shielding groove group 210 .

[0074] 9 is a graph showing an example of the position change in the y-axis direction of stray light propagating in the z-axis direction as it travels through the light-shielding groove group 210. Fig. 9(a) illustrates a case where θ = 1° and three different combinations of the width D and repetition pitch P' of the deep grooves 5 are used.

[0075] For example, in Figure 9(a), the diamond plot (◆) represents (P', D) = (60 μm, 30 μm), the triangular plot (▲) represents (P', D) = (120 μm, 60 μm), and the square plot (■) represents (P', D) = (69.3 μm, 30 μm).

[0076] 9(a), instead of repeating the arrangement of deep grooves 5 in the z-axis direction, a case in which the cladding 2 extends without being removed beyond the position where one deep groove is arranged in the z-axis direction is shown by a cross plot (x). In the comparative example, the deep groove provided alone has a width D in the x-axis direction of 50 μm.

[0077] In either case, as the stray light propagates in the z-axis direction, it is refracted toward the substrate 1. Here, the position of the stray light in the y-axis direction is y = 0, in other words, the propagation distance in the z-axis direction until the stray light reaches the substrate 1 is about 2300 μm for a single groove (plotted with a cross).

[0078] In contrast, in the case where the light-shielding groove group 210 is provided (three types of ◆ plots, ▲ plots, and ■ plots), the propagation distance in the z-axis direction at which the y-axis position of the stray light becomes y0 = 0 is approximately 450 to 650 μm, which is significantly shorter than 2300 μm.

[0079] In addition, as can be seen from a comparison between the ◆ plot and the ▲ plot, for example, the smaller the repetition pitch P' and the width D are and the denser the arrangement of the deep grooves 5, the shorter the propagation distance in the z-axis direction of stray light until it reaches the substrate 1. In other words, the length in the z-axis direction of the region where the light-shielding groove group 210 is arranged can be shortened.

[0080] In this way, in the optical circuit 200, by arranging the light-shielding groove group 210 in which a plurality of deep grooves 5 are repeated at least in the direction along the propagation direction of the stray light, the stray light can be effectively expelled to the outside of the cladding 2 (for example, the substrate 1) and attenuated. Furthermore, the higher the arrangement density of the deep grooves 5, the more effective it is at attenuating the stray light.

[0081] 9A also shows, for example, the angle φ with respect to the z-axis when the stray light reaches the substrate 1. It can be understood that the shorter the propagation distance of the stray light in the z-axis direction until it reaches the substrate 1, the larger the angle φ tends to be. The larger the angle φ with respect to the z-axis when the stray light reaches the substrate 1 from inside the cladding 2, the more easily reflection at the boundary surface between the cladding 2 and the substrate 1 is suppressed, and therefore, for example, the efficiency of expelling stray light from the cladding 2 to the substrate 1 can be increased.

[0082] As an example, consider the case of the hexagonal packing arrangement pattern example shown in Fig. 4. Stray light that has crossed the first row of deep grooves 5 passes through the cladding 2 remaining between the second row of deep grooves 5 and crosses the third row of deep grooves 5. Therefore, the effective repetition pitch P' of the deep grooves 5 can be expressed by the following equation 9.

[0083] In the examples described below, as a non-limiting example, P = 45 μm and D = 30 μm. In this case, P' = 69.3 μm. In FIG. 9( a), the square plot (■) corresponds to an example of a calculation result using these parameters. It can be seen that stray light reaches the substrate 1 when the propagation distance in the z-axis direction is approximately 500 μm.

[0084] 9(b) shows an example in which the positional change of stray light in the y-axis direction is calculated for each of four cases in which the angle θ is changed to θ = 0.5°, θ = 1°, θ = 5°, and θ = 20° when P' = 69.3 μm and D = 30 μm. The sidewalls of deep trench 5 are generally processed to be perpendicular to substrate 1 (θ = 0°), but by adjusting the process conditions of steps such as photolithography or etching, it is possible to process them to be inclined to substrate 1 (θ ≠ 0°).

[0085] Here, the greater the inclination angle θ of the sidewall of the deep groove 5 with respect to the substrate 1, the shorter the propagation distance in the z-axis direction of stray light until it reaches the substrate 1, but even when θ = 0.5°, which is the smallest of the four angles θ mentioned above, the propagation distance in the z-axis direction is about 720 μm. Therefore, compared to the single groove indicated by the cross plot in Figure 9(a), it is possible to significantly shorten the propagation distance in the z-axis direction of stray light.

[0086] 9(b), as in FIG. 9(a), the angle φ with respect to the z-axis when the stray light reaches the substrate 1 is also shown. When the height (or thickness) of the cladding 2 in the y-axis direction is 40 μm, in order to configure the deep trench 5 so that it almost reaches the substrate 1, if D=30 μm on the upper surface of the cladding 2, the maximum value of the inclination angle θ is approximately 20.6°.

[0087] In the explanation so far, the distance over which stray light reaches the substrate 1 has been used as a guideline for the length over which stray light can be removed, in other words, the length of the region in which the light-shielding groove group 210 is arranged. However, depending on the configuration of the optical circuit 200, it may not be an essential condition that stray light reaches the substrate 1. This point will be explained below.

[0088] 10 is a cross-sectional view schematically illustrating an example in which a light-receiving element 6 is mounted on an end face of an optical circuit (e.g., optical circuit 200). The light-receiving element 6 is exemplarily sealed in a space formed by a package 1001 and a package window 1002, and is positioned at a position where it can receive output light from the waveguide core 3 and is mounted on the chip end face of the optical circuit.

[0089] When focusing on the reduction of crosstalk in the light-receiving element 6, it is sufficient that the signal light propagating through the waveguide core 3 and emitted from the chip end face is received by the light-receiving element 6, and that stray light does not enter the light-receiving surface of the light-receiving element 6. Therefore, even if the stray light propagating through the cladding 2 does not reach the substrate 1, it is sufficient that the y position of the stray light is lower than the position corresponding to the lower end of the light-receiving surface of the light-receiving element 6.

[0090] 10, a case can also be envisioned in which the size of the light-receiving surface of the light-receiving element 6 overlaps with the end face of the substrate 1 in the y-axis direction, as in the case of the light-receiving element 6a indicated by the dotted line. In this case, the y position of stray light propagating through the substrate 1 may not fall below the position on the chip end face that corresponds to the lower end of the light-receiving surface of the light-receiving element 6a.

[0091] For example, when an undoped wafer known as a high-resistivity wafer is used for the substrate 1, the absorption loss is not so large when light in the 1.55 μm wavelength band propagates through the substrate 1. Therefore, in such a case, parameters such as the size and repetition pitch of the light-shielding groove group 210 may be determined so that the y position of stray light propagating through the substrate 1 is lower than the position corresponding to the lower end of the light-receiving surface of the light-receiving element 6 a on the chip end face.

[0092] As described above, when focusing on different specific traveling directions of stray light, parameters such as the size of the light-shielding groove group 210 and the repetition pitch of the deep grooves 5 may be determined according to the respective conditions. If the number of locations where stray light is generated and enters increases as the circuit scale of the optical circuit 200 increases, the above-mentioned parameters may be determined, for example, by approximately using the distance that stray light in a specific traveling direction takes to reach the substrate 1 as a guide for stray light removal. This makes it possible to deal with cases where, for example, many traveling directions of stray light are to be considered for parameter determination.

[0093] [Light-shielding groove group using deep grooves of different sizes in part or all of multiple regions] In the optical circuit 200, for example, the arrangement area of ​​one light-shielding groove group 210 may be divided into multiple regions, and deep grooves 5 of different sizes may be applied to part or all of the multiple divided regions (which may also be referred to as "divided regions").

[0094] For example, Fig. 11 is a diagram showing an example of an arrangement pattern corresponding to a configuration in which the diameter D of the deep grooves 5 is made different for some or all of the multiple partitioned regions in the arrangement pattern example of the deep grooves 5 shown in Fig. 7(d). Note that Fig. 11 illustrates an arrangement pattern based on the hexagonal packing arrangement shown in Fig. 7(d).

[0095] 11(a), for example, among the plurality of (e.g., five) regions 311 to 315, the diameters D of the deep grooves 53 and 52 are different between the region 313 and the regions 312 and 314. In addition, the diameters D of the deep grooves 52 and 51 are different between the regions 312 and 314 and the regions 311 and 315. The diameters D of the deep grooves between the regions 312 and 314 and between the regions 311 and 315 may be the same or different.

[0096] The deep grooves 53 in the region 313 may be understood to have the same configuration as the deep grooves 5 shown in Fig. 7(d), but the diameter D of the deep grooves 53 corresponds to, for example, the diameter D of the maximum size that can almost completely excavate the cladding 2. The diameter D of the deep grooves 52 in the regions 312 and 314 is, for example, smaller than the diameter D of the deep grooves 53 in the region 313. Furthermore, the diameter of the deep grooves 51 in the regions 311 and 315 is, for example, even smaller than the diameter of the deep grooves 52 in the regions 312 and 314. Thus, the provision of a plurality of deep grooves 51 to 53 of different sizes differs from the arrangement pattern example of the light-shielding groove group 210 shown in Fig. 7.

[0097] Here, if the size of the groove to be machined is smaller than the machining limit, it is difficult to completely dig the groove. Also, near the machining limit, the shape of the portion corresponding to the bottom of the groove may be relatively rounded. Figure 12 is a cross-sectional view schematically showing an example of the finished shape of a groove near the machining limit of the embodiment.

[0098] 12, the deep groove 53 is exemplified as a finish in which the cladding 2 is dug to the depth that is just enough to reach the upper surface of the substrate 1. On the other hand, the deep grooves 52 and 51 are exemplified as finishes in which the cladding 2 is shallower than the deep groove 53, since it is difficult to dig the cladding 2 to the depth that is just enough to reach the upper surface of the substrate 1.

[0099] Furthermore, since the deep grooves 51 to 53 are all rounded near the bottom of the groove, the angle of each sidewall relative to the substrate 1 tends to deviate from 90° (for example, the deviation angle θ tends to increase). Therefore, the stray light passing near the bottom of this groove can be deflected more toward the substrate 1 than the stray light passing through other portions.

[0100] Therefore, stray light propagating through the cladding 2 can be more effectively expelled to the outside of the cladding 2. Furthermore, since the deep grooves 53, 52, and 51 have bottoms at different positions in the y-axis direction, multiple stray light beams at different y positions can be shared and deflected in a direction approaching the substrate 1.

[0101] The arrangement pattern 310 illustrated in Figure 11(a) is an example of an "even pitch" arrangement pattern, in which deep grooves 51-53 of different sizes are allocated and arranged so that the repeat pitch is even among different partitioned regions 311-315. However, such clear allocation by region is not essential, and for example, deep grooves 51-53 of different sizes may be arranged randomly. For example, the sizes of deep grooves 51-53 may be selected randomly in each of partitioned regions 311-315.

[0102] 11(b) is a diagram showing an example of an arrangement pattern 320 in which the diameters of the deep grooves are individually varied for multiple regions and the arrangement intervals of the deep grooves are varied according to the diameters. In FIG. 11(b), it may be understood that the diameter D of each of the deep grooves 51 to 53 is the same as that of the arrangement pattern 310 shown in FIG.

[0103] However, the ratio of the diameter (D) to the arrangement interval (P) is constant for each of the region 323 where the deep grooves 53 are arranged, the regions 322 and 324 where the deep grooves 52 are arranged, and the regions 321 and 325 where the deep grooves 51 are arranged. In other words, the arrangement pattern 320 illustrated in Figure 11(b) is an example of an arrangement pattern of "uniform aperture ratio type." By using this arrangement pattern 320, it is possible to configure the light-shielding groove group 210 that satisfies the condition shown in Equation 3 above for each of the regions 321 to 325.

[0104] As described above, the optical circuit 200 according to the embodiment has a light-shielding groove group 210 configured by two-dimensionally arranging a plurality of deep grooves 5 formed by removing the cladding 2 around the waveguide core 3. The light-shielding groove group 210 can attenuate stray light propagating through the cladding 2 by scattering it through refraction or reflection.

[0105] For example, by setting the angle of the sidewall of the deep groove 5 with respect to the substrate 1 to an angle other than 90° (in other words, the aforementioned angle θ is other than 0°), the traveling direction of the stray light is deflected in the y-axis direction, thereby increasing the reliability of the stray light being scattered outside the cladding 2 and attenuated. Therefore, for example, compared to when the stray light is deflected within the circuit plane (in other words, within the xz plane), the effect of reducing stray light in the optical circuit 200 can be significantly improved because the stray light is deflected in the y-axis direction.

[0106] 7(a) to 7(g) or 11, a light-shielding groove group 210 configured by two-dimensionally arranging deep grooves 5 causes refraction or reflection in the x-axis direction in addition to refraction in the y-axis direction, thereby further attenuating stray light propagating in the z-axis direction. Furthermore, when the groove structure is miniaturized, the arrangement density of the walls of the cladding 2 can be increased, thereby relatively increasing the mechanical strength of the optical circuit 200.

[0107] Furthermore, the denser the arrangement of the deep grooves, the more effectively light can be scattered over a short distance. Furthermore, by arranging the light-shielding groove group 210 in the curved waveguide portion, stray light propagating through the cladding 2 around the waveguide core 3 can be reliably attenuated.

[0108] Therefore, even if the circuit scale and integration density of the optical circuit 200 increase, it is not necessary to consider in detail all of the stray light entry paths and the direction of total reflection. For example, stray light can be effectively attenuated by designing the circuit according to certain rules, such as providing a light-shielding groove group 210 of a predetermined area size between the optical interferometer circuits 110 and 120. This reduces the difficulty of designing the optical circuit 200, and eliminates or minimizes restrictions on the placement of the interferometer, the connection positions of the input / output optical fibers, and the position of the light-receiving element.

[0109] In the above-described embodiment, for example, filling by application of a light absorbing material is not required, and therefore concerns about increased costs or reliability can be avoided. However, a method of filling all or part of the deep grooves 5 constituting the light-shielding groove group 210 with a light absorbing material by application may also be applied in the present disclosure. In this case, in addition to the structural stray light attenuation effect of the deep grooves 5, the material stray light attenuation effect of the light absorbing material can be obtained.

[0110] 7(a) to 7(g), a light-shielding material may be filled in a light-shielding groove group 210 configured by two-dimensionally arranging deep grooves 5. In this case, the mechanical strength of the optical circuit 200 can be increased compared to a configuration in which the cladding that is not removed but remains is limited to the periphery of the waveguide core 3, as described in Patent Document 2, for example.

[0111] [Supplementary Note on the Embodiment] In the optical circuit 200 illustrated in FIG. 3 based on the configuration of FIG. 1, the circuits such as the optical interferometer circuits 110 and 120 and the optical branching circuit 130 do not need to be provided in two sets each, and for example, one set or three or more sets may be provided.

[0112] In addition, in the optical circuit 200 shown in FIG. 3, the light-shielding groove groups 210 are arranged in three locations on the substrate 1 according to the number of locations where stray light enters, but in the optical circuit 200, the number of light-shielding groove groups 210 arranged may be one or two, or four or more.

[0113] Furthermore, in the above-described embodiment, an example has been described in which the light to be attenuated by the light-shielding groove group 210 is stray light, but the light-shielding groove group 210 may also be used to attenuate light other than stray light.

[0114] 13 is a top view schematically showing an example of a test circuit 500 fabricated to confirm the effect of the light-shielding groove group according to the embodiment. A silica-based cladding 2 is fabricated on a silicon substrate. A light-shielding groove group 550 is fabricated above the center of the cladding 2 in plan view. Note that, in the test circuit 500, a waveguide (core) 3 has not yet been fabricated, since the purpose is to confirm the effect of reducing stray light propagating through the cladding 2.

[0115] The arrangement pattern of the deep grooves constituting the light-shielding groove group 550 is a hexagonal packing arrangement as illustrated in FIG. 4, and the shape of the deep grooves in plan view is circular. The diameter D of the deep grooves is illustratively 30 μm. The arrangement interval P of the deep grooves is illustratively 45 μm. The height of the cladding 2 in the y-axis direction is illustratively about 40 μm. The deviation angle (θ) of the angle of the sidewall of the deep groove relative to the substrate 1 from 90° is approximately 1°. The length of the region in the z-axis direction in which the light-shielding groove group 550 is provided is approximately 1 mm.

[0116] Input optical fibers 511 and 521 and output optical fibers 512 and 522 are attached to the chip end face of this test circuit 500 at a height (y-axis direction) of about 25 μm from the substrate 1. The input optical fiber 511 and the output optical fiber 512 are attached, for example, at a position where their end faces face each other (x position in the x-axis direction), and a light-shielding groove group 550 is arranged between these input optical fiber 511 and output optical fiber 512.

[0117] On the other hand, the input optical fiber 521 and the output optical fiber 522 are attached so that their end faces face each other at an x ​​position different from the attachment positions (x positions) of the input optical fiber 511 and the output optical fiber 512. Unlike between the input optical fiber 511 and the output optical fiber 512, no light-shielding groove group 550 is arranged in the clad 2 between the input optical fiber 521 and the output optical fiber 522.

[0118] 14 is a graph showing an example of the results of evaluating the transmittance spectrum versus wavelength of the test circuit 500 according to the example. As shown in FIG. 14, it can be seen that the average light transmittance in the optical path where the light-shielding groove group 550 is arranged is lower than the average light transmittance in the optical path where the light-shielding groove group 550 is not arranged, regardless of the light polarization mode (TE mode or TM mode).

[0119] For example, when a light source in the 1.55 μm band was used, the average light transmittance of the optical path from the input optical fiber 511 to the output optical fiber 512 via the light-shielding groove group 550 was evaluated and found to be approximately −53 dB. On the other hand, the average light transmittance of the optical path from the input optical fiber 521 to the output optical fiber 522 without passing through the light-shielding groove group 550 was found to be approximately −21 dB. Therefore, it was confirmed that the presence of the light-shielding groove group 550 can reduce stray light by approximately 32 dB.

[0120] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0121] The present disclosure is useful, for example, in optical communication technology.

[0122] REFERENCE SIGNS LIST 1 substrate 2 cladding layer 3 waveguide (core) 4 thin film heater 5, 51 to 53 deep groove 6, 6a light receiving element 100, 200 optical circuit 101a, 101b, 511, 521 input optical fiber 102a, 102b, 512, 522 output optical fiber 103a, 103b light receiving element 110a, 110b, 120a, 120b optical interferometer circuit 111a, 111b, 112a, 112b, 121a, 121b, 122a, 122b unconnected port 130a, 130b optical branching circuit 210-1, 210-2, 210-3, 550 light-shielding groove group 311 to 315, 321 to 325 area 500 test circuit 1001 Package 1002 Package window

Claims

1. A planar lightwave circuit comprising: a substrate; a cladding layer provided on top of the substrate; a core embedded in the cladding layer to form a waveguide; and a recessed region in which a plurality of recesses are formed in the cladding layer and arranged two-dimensionally in the in-plane direction of the cladding layer.

2. The planar lightwave circuit according to claim 1, wherein said recess is an air space.

3. The planar lightwave circuit according to claim 2, wherein an angle between a normal to a sidewall surface of said recess and a normal to a surface of said substrate is different from 90°.

4. A planar lightwave circuit as described in claim 3, wherein a normal direction to a side wall surface of the recess is a direction from the inside to the outside of the cladding layer, a normal direction to a substrate surface of the substrate is a direction from the substrate to the cladding layer, and an angle formed between the normal direction to the side wall surface and the normal direction to the substrate surface is smaller than 90°.

5. The planar lightwave circuit according to claim 1, wherein the recess has a circular shape in a plan view.

6. The planar lightwave circuit according to claim 1 or 2, wherein the arrangement of the plurality of recesses in the recess region is a hexagonal packing arrangement.

7. The planar lightwave circuit according to claim 2, wherein the shape of the recess is circular in a plan view, the arrangement of the plurality of recesses in the recess region is a hexagonal packing arrangement, and when a diameter of the circle is represented by D and an arrangement interval of the recesses is represented by P, the diameter D and the arrangement interval P satisfy the following formula 1:

8. A planar lightwave circuit according to claim 7, wherein the diameter D and the arrangement interval P satisfy the following formula 2.

9. The normal direction of the sidewall surface of the dent is a direction from the inside to the outside of the cladding layer, the normal direction of the substrate surface of the substrate is a direction from the substrate to the cladding layer, and the angle between the normal direction of the sidewall surface and the normal direction of the substrate surface is smaller than 90°, the shape of the dent is circular in a plan view, and the arrangement of the dents in the dent region is a hexagonal packing arrangement, the angle between the normal direction of the sidewall surface and the normal direction of the substrate surface is expressed as 90°-θ using a deviation angle θ (≠0°) from the angle, and the position of the central axis of the core relative to the substrate is an initial position y0, and the refractive index of the cladding layer is n clad , the diameter of the circle is represented by D, the arrangement interval of the dents is represented by P, and the repetition pitch of the dents is represented by P' = (√3) · P, the initial angle of the travel angle of the stray light in the cladding layer of the i-th dent region (i is an integer of 1 or more) with respect to the substrate surface is φ0 = 0°, and the position of the stray light with respect to the substrate is represented by y i When the length of the recessed region in the direction along the propagation direction of the stray light is expressed by the following formulas 3 to 5, the length of the recessed region in the direction along the propagation direction of the stray light is expressed by the position y i Let i be the smallest i such that is less than or equal to zero. min When expressed as i min The planar lightwave circuit of claim 2, wherein the length is longer than P'.

10. A planar lightwave circuit as claimed in claim 2, wherein some or all of the plurality of recesses constituting the recess region have different sizes in either or both of a plan view and a cross-sectional view with respect to the substrate.

11. A planar lightwave circuit as claimed in claim 10, wherein the recessed area is divided into a plurality of regions, and the sizes of the recessed portions and the spacing between the recessed portions are different in some or all of the plurality of divided regions.

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