Optical circuit and manufacturing method of optical circuit

Abstract

To provide an optical circuit in which optical waveguides made of different kinds of materials are coupled efficiently and a manufacturing method of the same.SOLUTION: An optical circuit has a board, a first optical waveguide disposed on the board, and a second optical waveguide that is disposed on the board and is optically coupled to the first optical waveguide. The first optical waveguide has a first clad, a second clad, and a first core disposed between the first clad and the second clad, and the second optical waveguide has a third clad, a fourth clad, and a second core disposed between the third clad and the fourth clad. The first clad is disposed closer to the board than the second clad. The third clad is disposed closer to the board than the fourth clad. The material of the first clad differs from the material of the third clad.SELECTED DRAWING: Figure 2B

Description

Technical Field

[0001] The present disclosure relates to an optical circuit and a method for manufacturing an optical circuit.

Background Art

[0002] An optical circuit in which a semiconductor optical element and an optical waveguide are integrated on a substrate is in high demand in the fields of optical communication and optical sensors. Semiconductor optical elements such as laser light sources and light receiving elements are often formed of compound semiconductors. On the other hand, optical waveguides are formed by silicon photonics technology. It is necessary to efficiently couple materials suitable for each of the semiconductor optical element and the optical waveguide.

[0003] A configuration has been proposed in which a stacked structure including an active optical waveguide and a stacked structure including a passive optical waveguide are joined on a common cladding (see, for example, Patent Document 1). When coupling optical waveguide regions formed of different materials on a common cladding, it is necessary to select a low refractive index material that can serve as a common cladding for each of the different materials forming the core.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Currently, it is difficult to efficiently optically couple optical waveguides formed of different materials using different low refractive index materials. One aspect of the present disclosure provides an optical circuit that efficiently couples optical waveguides of different materials and a method for manufacturing the same.

Means for Solving the Problems

[0006] In one embodiment, the optical circuit includes a substrate, and a first optical waveguide disposed on the substrate; a second optical waveguide disposed on the substrate and optically coupled to the first optical waveguide; comprising: The first optical waveguide has a first cladding, a second cladding, and a first core disposed between the first cladding and the second cladding. The second optical waveguide has a third cladding, a fourth cladding, and a second core disposed between the third cladding and the fourth cladding. The first cladding is disposed closer to the substrate than the second cladding. The third cladding is disposed closer to the substrate than the fourth cladding. The material of the first cladding is different from the material of the third cladding.

[0007] In another embodiment, a method for manufacturing an optical circuit includes: preparing a first laminate having a first substrate, a first cladding, a first core, and a first protective layer in this order; preparing a second laminate having a second substrate, a third cladding containing a material different from the first cladding, a second core, and a second protective layer in this order; butting the first laminate and the second laminate so that the first core and the second core face each other, and fixing the first protective layer and the second protective layer to a temporary substrate; bonding the first substrate and the second substrate to a third substrate; removing the temporary substrate after bonding the first substrate and the second substrate to the third substrate; processing the first laminate to obtain a first optical waveguide including at least the first cladding and the first core, and processing the second laminate to obtain a second optical waveguide including at least the third cladding and the second core and optically coupled to the first optical waveguide. including:

Advantages of the Invention

[0008] An optical circuit in which optical waveguides made of different materials are efficiently coupled to each other and a method for manufacturing the same are realized.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2A

Figure 2B

Figure 3

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6A

Figure 6B

Figure 6C

Figure 6D

Figure 6E

Figure 6F

Figure 7A

Figure 7B

Figure 8

Figure 9A

Figure 9B

Figure 9C

Figure 9D

Figure 9E

Figure 10

Figure 11

Figure 12

Figure 13A

Figure 13B

Figure 14

Figure 15

Figure 16

Figure 17A

Figure 17B

Figure 18

Figure 19

Embodiments for Carrying Out the Invention

[0010] Before describing the configuration and manufacturing method of the optical circuit according to the embodiment, the technical problems of the conventional configuration using a common cladding will be described with reference to FIG. 1. FIG. 1 schematically shows the bonding of laminates using a common cladding. On a substrate 1011 having an SiO2 layer 1012, the laminates 500 and 600 are arranged such that their cleavage surfaces face each other, pressurized at 1 MPa in the stacking direction, and heated and bonded at 300 °C in a predetermined gas atmosphere. The laminate 500 is processed in a later step to form a waveguide structure of a laser die. The laminate 600 is processed in a later step to form a waveguide structure of a passive die. In this structure, the SiO2 layer 1012 functions as a common cladding for both of the two waveguide cores, and further, since it is required to be a material that enables bonding, the degree of freedom in material selection is low.

[0011] In the embodiment, the first laminate and the second laminate formed of different materials are transferred from a temporary substrate to a common substrate, and the first laminate and the second laminate are processed into optical waveguides through a mask formed in a batch, thereby highly efficiently coupling the first optical waveguide and the second optical waveguide made of different materials.

[0012] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. The following description is for embodying the technical idea of the present disclosure, and unless otherwise specified, the present disclosure is not limited to the following description. In each drawing, members having the same function may be denoted by the same reference numerals. For the sake of explanation of the gist or ease of understanding, the embodiments may be shown separately for convenience, but partial substitution or combination of the configurations shown in different embodiments or examples is possible. In the embodiments described later, mainly the matters different from the previously described embodiments will be described, and the overlapping descriptions of the matters common to the previously described embodiments may be omitted. The sizes and positional relationships of the members shown in each drawing may be exaggerated for clarity of explanation.

[0013] <First Embodiment> FIG. 2A is a schematic top view of the optical circuit 100 of the first embodiment, and FIG. 2B is a cross-sectional view taken along line IIB-IIB of FIG. 2A. In the coordinate system of FIGS. 2A and 2B, the stacking direction perpendicular to the substrate 11 is the Z direction, the light guiding direction is the Y direction, and the direction orthogonal to the Y direction and the Z direction is the X direction. The optical circuit 100 includes a substrate 11, a first optical waveguide 20 disposed on the substrate 11, and a second optical waveguide 30 disposed on the substrate 11 and optically coupled to the first optical waveguide 20. The first optical waveguide 20 has a first cladding 22, a second cladding 24, and a first core 23 disposed between the first cladding 22 and the second cladding 24. The second optical waveguide 30 has a third cladding 32, a fourth cladding 34, and a second core 33 disposed between the third cladding 32 and the fourth cladding 34. The first cladding 22 is disposed closer to the substrate 11 than the second cladding 24, the third cladding 32 is disposed closer to the substrate 11 than the fourth cladding 34, and the materials of the first cladding 22 and the third cladding 32 are different. Since the materials of the first cladding 22 and the third cladding 32 of the optical circuit of the first embodiment are different, an appropriate material can be selected as the second core 33 of the second optical waveguide 30. Here, "the materials are different" means, for example, nitride semiconductors such as Al x Ga 1-x N (0 ≦ x ≦ 1), and oxides or fluorides such as SiO2, Al2O3, and MgF2, which have different material systems. On the other hand, when only the mixed crystal ratio x is different, such as Al x Ga 1-x N (0 < x < 1), it shall be treated as "the materials are the same". However, when the mixed crystal ratio x is 0 or 1, that is, GaN and AlN shall be regarded as different materials. The same applies to other material systems.

[0014] The first optical waveguide 20 includes a straight waveguide 123 at the end on the side of the second optical waveguide 30, and the second optical waveguide 30 includes a straight waveguide 133 at the end on the side of the first optical waveguide 20. The first optical waveguide 20 and the second optical waveguide 30 face each other in the optical axis direction along the IIB-IIB line. There is a second core 33 on the extension line of the first optical axis AX1 of the first optical waveguide 20, or there is a first core 23 on the extension line of the second optical axis AX2 of the second optical waveguide 30. As shown in FIGS. 2A and 2B, it is preferable from the viewpoint of coupling efficiency that the first optical axis AX1 and the second optical axis AX2 are coaxial, but it is not limited to this example. The first optical axis AX1 and the second optical axis AX2 may have an angular deviation within the range where optical coupling is possible, or an axial deviation in the X direction, Y direction, or Z direction. Further, as will be described later, the first optical waveguide 20 and the second optical waveguide 30 may be optically coupled via a third waveguide that is oblique or curved in the XY plane.

[0015] The first core 23 of the first optical waveguide 20 includes an active layer, the first cladding 22 includes, for example, an n-side nitride semiconductor, and the second cladding 24 includes a p-side nitride semiconductor. The second cladding 24 may include a transparent conductive film. The transparent conductive film may be ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide), etc. The second core 33 of the second optical waveguide 30 includes an oxide or a nitride. The third cladding 32 and the fourth cladding 34 are oxides or nitrides or fluorides. The third cladding 32 and the fourth cladding 34 may include an amorphous material. At least a part of at least the second core 33 of the second optical waveguide 30 may include an amorphous material. By making at least a part of the second core 33 an amorphous material, the refractive index difference between the second core 33 and the third cladding 32 and the fourth cladding 34 can be increased, and the optical confinement in the second optical waveguide 30 can be increased. The fourth cladding 34 may be an air layer instead of an oxide or a nitride or a fluoride.

[0016] The active layer of the first core 23 is formed of, for example, quantum wells. The light generated in the active layer by carrier injection is amplified by reciprocating in the Y direction in the straight waveguide 123. The refractive index of the first core 23 is higher than the refractive indices of the first cladding 22 and the second cladding 24, and the light is confined between the first cladding 22 and the second cladding 24 in the stacking direction (Z direction) perpendicular to the substrate 11. The first core 23 may include a first optical guiding layer between the quantum wells and the first cladding 22. Also, the first core 23 may include a second optical guiding layer between the quantum wells and the second cladding 24. The straight waveguide 123 is formed, for example, as a ridge waveguide to achieve light confinement in the X direction.

[0017] In the straight waveguide 133 of the second optical waveguide 30, the light is confined in the second core 33 by the third cladding 32 and the fourth cladding 34. In the X direction parallel to the substrate 11, the light is confined by the media on both sides in the width direction of the straight waveguide 133. The light confinement in the width direction of the straight waveguide 133 may be defined by the processed shape of the second optical waveguide 30, or the second core 33 may be etched into a thin wire shape and surrounded by a low refractive index medium. A first substrate 21 may be disposed between the first cladding 22 and the substrate 11. A second substrate 31 may be disposed between the third cladding 32 and the substrate 11. Depending on the material of the first substrate 21, the first substrate 21 may function as a cladding for the first core 23 together with the first cladding 22. Depending on the material of the second substrate 31, the second substrate 31 may function as a cladding for the second core 33 together with the third cladding 32.

[0018] From the viewpoint of realizing high-efficiency optical coupling, the difference between the width of the first core 23 in the direction perpendicular to the stacking direction of the first optical waveguide 20 in a cross-section perpendicular to the first optical axis AX1 and the width of the second core 33 in the direction perpendicular to the stacking direction of the second optical waveguide 30 in a cross-section perpendicular to the second optical axis AX2 is desirably 3 μm or less. This difference between the width of the first core 23 and the width of the second core 33 may be 2 μm or less, 1 μm or less, 0.5 μm or less, or 0.1 μm or less. Also, this difference between the width of the first core 23 and the width of the second core 33 may be substantially 0. This is realized by dry etching using a mask with a constant ridge width in the manufacturing method described later. Here, the "width of the core" refers to the core width up to a position about 10 μm away from the end faces of the first optical waveguide 20 and the second optical waveguide 30 facing each other. As described above, positional displacement in the X direction, Y direction, or Z direction is allowed within the range where optical coupling can be obtained between the first optical axis AX1 and the second optical axis AX2. The end faces of the first optical waveguide 20 and the second optical waveguide 30 facing each other may be cleavage planes or etched surfaces. For example, a surface formed by reactive ion etching may be used as the facing surface. In FIG. 2A, the first optical waveguide 20 is depicted as having a straight waveguide 123 and the second optical waveguide 30 as having a straight waveguide 133, but the first optical waveguide 20 and the second optical waveguide 30 may include not only straight waveguides but also oblique or curved optical waveguides.

[0019] The optical circuit 100 can be fabricated at low cost as described later. By using a core material and a cladding material suitable for each of the first optical waveguide 20 and the second optical waveguide 30, the first optical waveguide 20 and the second optical waveguide 30 can be optically coupled with high efficiency and low cost and integrated on the substrate 11. The coupling efficiency can be, for example, 50% or more and 99% or less, 75% or more and 99% or less, or 90% or more and 99% or less. This is because the height and width of the first core 23 and the second core 33 can be adjusted so that the first optical waveguide 20 and the second optical waveguide 30 can be optically coupled. The first optical waveguide 20 can be applied as an active waveguide to a laser light source, a light receiving element, a modulation element, etc. The second optical waveguide 30 can be used as a passive waveguide. The optical circuit 100 is applicable to various combinations of semiconductor optical elements and optical waveguides.

[0020] FIG. 3 is a top view showing an application example of the optical circuit 100. The optical circuit 100 is applicable to an optical gyro sensor. The first optical waveguide 20 is used as a laser light source of the optical gyro sensor. When applying the first optical waveguide 20 as a laser light source, III-V compound semiconductors such as GaN, GaAs, and InP can be used. These compound semiconductor materials may be binary, ternary, or quaternary. Among these, GaN having a refractive index of about 2.4 is suitable because the refractive index difference from the second optical waveguide 30 is small. When using a GaN-based compound semiconductor, nitride semiconductors such as AlGaN, AlInN, and AlGaInN are used as the first cladding 22 and the second cladding 24. An n-type impurity such as Si or Ge may be added to the first cladding 22 to form an n-type nitride semiconductor, and a p-type impurity such as Be or Mg may be added to the second cladding 24 to form a p-type nitride semiconductor. The second cladding 24 may include a transparent conductive film in addition to the p-type nitride semiconductor. Further, the first optical waveguide 20 may be provided at the output ports P1 and P2 of the optical circuit 100 and function as a light receiving element.

[0021] The second optical waveguide 30 includes a waveguide 131 including a straight line and a curve, and a resonator 15 optically coupled to the waveguide 131. The second core 33 constituting the waveguide 131 of the second optical waveguide 30 may include an oxide or a nitride. As the oxide or nitride, for example, SiN, SiON, Ta2O5, Nb2О5, TiO2, HfO2, AlN, LiNbO3, etc. and their mixed materials can be used. The third cladding 32 and the fourth cladding 34 may include an oxide or a fluoride. As the oxide or fluoride, for example, SiO2, MgF2, CaF2, Al2O3, etc. and their mixed materials can be used. The materials of the second core 33, the third cladding 32, and the fourth cladding 34 may include impurities to adjust the refractive index. The resonator 15 may be formed of the same material in the same process as the waveguide 131, or may be formed of a different material.

[0022] The waveguide 131 of the second optical waveguide 30 is optically coupled to the straight waveguide 123 of the first optical waveguide 20 used as a laser light source. The light emitted from the first optical waveguide 20 as a laser light source is coupled to the second optical waveguide 30 and branched by a directional coupler. As indicated by the white arrows in the figure, the light circulates in opposite directions through the waveguide 131. In the process of circulating in opposite directions through the waveguide 131, the light is coupled to the resonator 15 and circulates in opposite directions through the resonator 15. The light that circulates clockwise through the waveguide 131 circulates counterclockwise through the resonator 15, is coupled to the waveguide 131, and is output from the output port P1. The light that circulates counterclockwise through the waveguide 131 circulates clockwise through the resonator 15, is coupled to the waveguide 131, and is taken out from the output port P2.

[0023] When the optical circuit 100 receives rotation from the outside, the optical path length that the light circulating in opposite directions through the resonator 15 feels changes according to the rotation angle or angular velocity, and the resonance frequencies of the light circulating clockwise and counterclockwise through the resonator 15 change. As a result, the resonance frequencies of the resonator 15 output from the output ports P1 and P2 change. The angular velocity can be calculated from the difference in the resonance frequencies of the two output lights.

[0024] <Method for manufacturing an optical circuit> An example of a method for manufacturing the optical circuit 100 will be described with reference to FIGS. 4A to 9C. The method for manufacturing the optical circuit 100 includes preparing a first laminate 120 having a first substrate, a first cladding, a first core, a second cladding, and a first protective layer in this order; preparing a second laminate 130 having a second substrate, a third cladding containing a material different from that of the first cladding, a second core, and a second protective layer in this order; abutting the first laminate and the second laminate so that the first core and the second core face each other, and fixing the first protective layer and the second protective layer to a temporary substrate; bonding the first substrate and the second substrate to a third substrate; removing the temporary substrate after bonding the first substrate and the second substrate to the third substrate; processing the first laminate 120 to obtain a first optical waveguide 20 including the first cladding, the first core, and the second cladding; and processing the second laminate 130 to obtain a second optical waveguide 30 including at least the third cladding and the second core.

[0025] <Preparation of the First Stacked Body and the Second Stacked Body> FIG. 4A is a perspective view of a first stacked body 120 used for fabricating a first optical waveguide 20 of an optical circuit 100, and FIG. 4B is a side view of the first stacked body 120. FIG. 5A is a perspective view of a second stacked body 130 used for fabricating a second optical waveguide 30, and FIG. 5B is a side view of the second stacked body 130. The manufacturing method of the optical circuit 100 includes preparing the first stacked body 120 and preparing the second stacked body 130. The first stacked body 120 has a substrate 210, a first cladding 22, a first core 23, a second cladding 24, and a first protective layer 25 in this order. The second stacked body 130 has a second substrate 31, a third cladding 32 containing a material different from the first cladding 22, a second core 33, and a second protective layer 340 in this order.

[0026] Referring to FIGS. 4A and 4B, the first stacked body 120 is obtained by dicing into a predetermined size a structure in which a first cladding 22, a first core 23, a second cladding 24, and a first protective layer 25 are formed in this order on a substrate 210. As described above, the first stacked body 120 is formed including a compound semiconductor such as a GaN-based, InP-based, GaAs-based, etc. In this example, the first stacked body 120 is formed of a GaN-based material. However, the first protective layer 25 may be of another material system such as SiO2. On the substrate 210, the first cladding 22, the first core 23, the second cladding 24, and the first protective layer 25 are sequentially laminated by a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method. As the PVD method, a sputtering method or a molecular beam epitaxy (MBE) method may be used. By forming a film by the CVD method or the PVD method, the film thickness can be accurately controlled.

[0027] As the first cladding 22, for example, Al x Ga 1-x N (0 ≦ x ≦ 1) -containing semiconductor layer is formed, as the first core 23, for example, a quantum well containing InGaN is formed, and as the second cladding 24, for example, Al xGa 1-x A semiconductor layer containing N(0≦x≦1) is formed, but it is not limited to this example, and the material can be appropriately selected according to the wavelength. In the first cladding 22, Si is added as an n-type impurity at a concentration of 1×10 18 cm -3 or more and 5×10 19 cm ―3 or less. In the second cladding 24, Mg is added as a p-type impurity at a concentration of 1×10 16 cm -3 or more and 1×10 22 cm ―3 or less. In the region of the first core 23 that becomes the active layer, impurities may be added.

[0028] Referring to FIGS. 5A and 5B, the second laminate 130 is obtained by dicing into a predetermined size a film formed by sequentially depositing a third cladding 32, a second core 33, and a second protective layer 340 on a substrate 310. The third cladding 32, the second core 33, and the second protective layer 340 are sequentially laminated by CVD method or PVD method. The second laminate 130 is formed of, for example, a Si-based material. On a Si substrate, SiO2 is formed as the third cladding 32, undoped Nb2O5 is formed as the second core 33, and SiO2 is formed as the second protective layer 340 by sputtering or the like, but it is not limited to this example. The second core 33 may be formed as an amorphous layer by reactive sputtering or the like. By making at least the second core 33 of the second laminate 130 an amorphous layer, the second optical waveguide 30 is formed as an amorphous waveguide in a later process. The third cladding 32 may also be an amorphous layer. Using an amorphous material for the core and the cladding makes it easy to create a refractive index difference and can enhance light confinement. Also, by enhancing light confinement, the bending loss of the bent waveguide can be reduced. Also, in an amorphous waveguide, a low-loss thin-film waveguide is easily obtained. The second protective layer 340 may be the same material as or different from the fourth cladding 34 (see FIG. 2B). The second protective layer 340 can be used as a height adjustment layer for aligning the heights of the first core 23 of the first laminate 120 and the second core 33 of the second laminate 130.

[0029] Thus, the first laminate 120 is formed such that the first core 23 includes an active layer, the first cladding 22 includes an n-side nitride semiconductor, and the second cladding 24 includes a p-side nitride semiconductor. The second laminate 130 is formed such that the second core 33 includes an oxide or a nitride, the third cladding 32 includes a second oxide or a fluoride, and has a second protective layer 340 on the second core 33.

[0030] Note that, before fixing the second laminate 130 to the temporary substrate, the second laminate 130 may be annealed at a temperature lower than the heating temperature of the growth substrate when laminating the first laminate 120. Thereby, when the second optical waveguide 30 is formed, the transmittance with respect to the wavelength of the light propagating through the second optical waveguide 30 is improved. In this way, by preparing the first laminate 120 and the second laminate 130 individually, not only can they be integrated in the process described later, but also heat treatment suitable for each laminate can be performed in advance. Thereby, each laminate does not receive unnecessary thermal damage. Further, when the second laminate 130 is annealed at a predetermined temperature or higher, the structure of the second core 33 changes from amorphous to crystalline. Such a structural change is suppressed, and a desired optical waveguide can be formed.

[0031] The difference between the height h1 (see FIG. 4B) from the center in the height direction of the first core 23 of the first laminate 120 to the surface of the first protective layer and the height h2 (see FIG. 5B) from the center in the height direction of the second core 33 of the second laminate 130 to the surface of the second protective layer 340 is aligned with an accuracy of 500 nm or less, preferably 100 nm or less. By manufacturing each of the first laminate 120 and the second laminate 130 by a CVD method or a PVD method, the difference between h1 and h2 can be controlled to 500 nm or less, preferably about 100 nm. As a result, when the first laminate 120 and the second laminate 130 are integrated on the substrate 11 in a later process, the distance from the upper end of the second cladding 24 to the center of the first core 23 and the distance from the upper end of the second protective layer 340 to the center of the second core 33 are laminated so that the difference is 500 nm or less. By setting the difference between h1 and h2 to 500 nm or less, 100 nm or less, 50 nm or less, or 30 nm or less, optical coupling can be efficiently performed. Alignment of the optical axis in a plane parallel to the substrate is realized by forming an optical waveguide in a state where the first laminate 120 and the second laminate 130 are joined on a common substrate 11, as will be described later.

[0032] <Fixing the laminate to the temporary substrate> FIGS. 6A to 6F are manufacturing process diagrams of an optical circuit using the prepared first laminate 120 and second laminate 130. As shown in FIGS. 6A and 6B, the first laminate 120 and the second laminate 130 are abutted so that the first core 23 and the second core 33 face each other, and the first protective layer 25 and the second protective layer 340 are fixed to the temporary substrate 41.

[0033] In FIG. 6A, the end face of the first core 23 of the first laminate 120 and the end face of the second core 33 of the second laminate 130 are aligned and abutted against each other, and while abutting against each other, the first protective layer 25 of the first laminate 120 and the second protective layer 340 of the second laminate 130 are fixed to the temporary substrate 41. The temporary substrate 41 is preferably made of a material such as silicon that is easy to etch. Fixing to the temporary substrate 41 is performed by any bonding method such as atomic diffusion bonding, hydroxyl bonding, or surface activation bonding. As a result, as shown in FIG. 6B, the first laminate 120 and the second laminate 130 are arranged on the temporary substrate 41. At this stage, the heights of the first laminate 120 and the second laminate 130 do not have to be aligned.

[0034] In FIG. 6C, at least one of the substrate 210 and the substrate 310 is polished to obtain the first substrate 21 and the second substrate 31. With the first laminate 120 and the second laminate 130 fixed on the temporary substrate 41, by processing the first substrate 21 and the second substrate 31, the heights of the first laminate 120 and the second laminate 130 are made uniform on the temporary substrate 41. The heights of the first laminate 120 and the second laminate 130 may be made uniform by any method. For example, after reducing the thickness of the substrate 210 of the first laminate 120 to a certain extent by grinding, the surfaces of the substrate 210 and the substrate 310 may be polished and flattened. As another method, after depositing layers such as metal, semiconductor, insulator, etc. on the substrate 210 and the substrate 310, the surface may be flattened by polishing. In FIG. 6C, by grinding and polishing the substrate 210, the heights of the first substrate 21 and the second substrate 31 are made uniform on the polished surface. Prior to the transfer to the substrate 11, by making the heights of the first laminate 120 and the second laminate 130 match, the alignment of the optical axis can be maintained even after bonding to the substrate 11.

[0035] <Bonding to the substrate 11> In FIG. 6D, the substrate 11 is bonded to the surfaces of the first laminate 120 and the second laminate 130 on the side opposite to the fixing surface to the temporary substrate 41. The substrate 11 is bonded to the first substrate 21 and the second substrate 31 by an appropriate method. The bonding method may be, for example, surface activation bonding, atomic diffusion bonding, or hydroxyl bonding. These bondings are preferably low-temperature bondings, and from this perspective, surface activation bonding and atomic diffusion bonding are preferred. The temperature required for bonding may be, for example, 100 °C or lower, 50 °C or lower, or 40 °C or lower.

[0036] In FIG. 6E, the structure with the substrate 11 bonded is turned upside down, and in FIG. 6F, the temporary substrate 41 is removed. Thereby, the first laminate 120 and the second laminate 130 are arranged adjacent to each other on the substrate 11. The method for removing the temporary substrate 41 is not particularly limited. For example, after thinning the temporary substrate 41 by grinding, the remaining portion may be removed by etching.

[0037] Figs. 7A and 7B show the bonding state of the first laminate 120 and the second laminate 130 on the substrate 11. Fig. 7A is a top view of the structure on the substrate 11 after the temporary substrate 41 is removed, and Fig. 7B is a cross-sectional view taken along line VIIB-VIIB of Fig. 7A. Although waveguides are not yet formed in the first laminate 120 and the second laminate 130, the central axis of the first core 23 in the stacking direction (Z direction) is within the film thickness range of the second core 33. Alternatively, the central axis of the second core 33 in the stacking direction (Z direction) is within the film thickness range of the first core 23. This configuration is an example of a configuration that can improve the efficiency of optical coupling. However, the first core 23 and the second core 33 may be optically coupled via another optical waveguide therebetween. The second protective layer 340 may be used as the fourth cladding 34 in a later process, or may be removed once and a new fourth cladding 34 may be provided.

[0038] <Formation of Waveguide> Fig. 8 is a top view showing the formation of waveguides in the laminate. Fig. 9A is a cross-sectional view taken along line IXA-IXA of Fig. 8, Fig. 9B is a cross-sectional view taken along line IXB-IXB of Fig. 8, and Fig. 9C is a cross-sectional view taken along line IXC-IXC of Fig. 8. In Fig. 8, the first laminate 120 and the second laminate 130 (see Fig. 7) bonded to the substrate 11 are processed to form the first optical waveguide 20 and the second optical waveguide 30. The processing of the first laminate 120 and the second laminate 130 may be performed in either order. For layers that can be etched simultaneously depending on the material, they may be etched simultaneously in one step. For example, the first laminate 120 and the second laminate 130 may be processed by reactive ion etching through a mask formed with a certain width on the first laminate 120 and the second laminate 130 to obtain the first optical waveguide 20 and the second optical waveguide 30. The first optical waveguide 20 and the second optical waveguide 30 can be obtained by sequentially processing using an etching gas that provides a suitable selectivity ratio for each of the first laminate 120 and the second laminate 130.

[0039] By processing the first laminate 120, a first optical waveguide 20 including a first cladding 22, a first core 23, and a second cladding 24 is obtained. The first optical waveguide 20 has a straight waveguide 123 including an active layer at least at an end on the side of the second optical waveguide 30. By processing the second laminate 130, a second optical waveguide 30 including at least a third cladding 32 and a second core 33 is obtained. The fourth cladding 34 may be the same as or different from the second protective layer 340 as described above. The second protective layer 340 or the fourth cladding 34 does not necessarily need to be maintained until the end. The second optical waveguide 30 may include a waveguide 131 and a ring resonator 135. The waveguide 131 may be a straight waveguide or may include a bent waveguide. At least a part of the second core 33 of the second optical waveguide 30 may include an amorphous material. For example, at least one of the waveguide 131 and the ring resonator 135 may be a low-loss amorphous waveguide in which the second core 33 is formed of SiN, SiON, Ta2O5, Nb2O5, TiO2, HfO2, AlN, etc. or a mixed material thereof.

[0040] In the IXA-IXA cross-section of FIG. 9A, the extension line of the first optical axis AX1 of the first core 23 overlaps with the second core 33. Or the extension line of the second optical axis AX2 of the second core 33 overlaps with the first core 23. Alignment of the first optical axis AX1 and the second optical axis AX2 in the direction perpendicular to the substrate 11 is realized by adjusting the height with the second protective layer 340 when fixing the second laminate 130 to the temporary substrate 41. Alignment of the first optical axis AX1 and the second optical axis AX2 in the plane parallel to the substrate 11 is realized by processing the first laminate 120 and the second laminate 130 bonded on the substrate 11 sequentially or simultaneously to form waveguides. Thereby, high-efficiency optical coupling can be realized in a relatively simple process similar to the CMOS process. The coupling example in FIG. 9A is an example of high-efficiency optical coupling. However, as described above, the first optical axis AX1 and the second optical axis AX2 may be displaced within the range where optical coupling is possible in a cross-section orthogonal to the first optical axis AX1 or the second optical axis AX2. Note that the first optical waveguide 20 and the second optical waveguide 30 may be optically coupled via a third optical waveguide therebetween.

[0041] In the IXB-IXB cross-section of FIG. 9B, as an example, a step 221 is formed in the first cladding 22 of the first optical waveguide 20, and a ridge 26 is formed in the second cladding 24. The step 221 and the ridge 26 can be formed by dry etching. After the formation of the step 221 and the ridge 26, the entire stack of the first optical waveguide 20 is covered with an insulating film 27. A part of the insulating film 27 is removed to form an electrode 28 electrically connected to the second cladding 24 and an electrode 29 electrically connected to the first cladding 22, and the first optical waveguide 20 is completed. In this example, the second cladding 24 is formed to include a p-side nitride semiconductor, and a p-side contact layer may be provided between the second cladding 24 and the electrode 28. A straight waveguide 123 may be formed in the active layer included in the first core 23 sandwiched between the first cladding 22 and the second cladding 24. The active layer is formed of, for example, quantum wells, excited by carrier injection, and emits light in a desired wavelength band. For example, when the straight waveguide 123 of the first optical waveguide 20 is an end-face emission type laser element, light reciprocates and is amplified in a direction perpendicular to the paper surface in the straight waveguide 123 defined by the width of the ridge 26, output as laser light, and coupled to the second core 33 of the second optical waveguide 30.

[0042] In the IXC-IXC cross-section of FIG. 9C, the second optical waveguide 30 is a complete refractive index type waveguide. That is, a ridge including a third cladding 32, a second core 33, and a fourth cladding 34 is formed. Thereby, the second optical waveguide has strong light confinement and can reduce bending loss. At least a part of the third cladding 32, the second core 33, and the fourth cladding 34 includes an amorphous material. Thereby, the refractive index difference between the core and the cladding can be increased, and the light confinement becomes stronger. Therefore, the bending loss of a curved waveguide such as the ring resonator 135 in the second optical waveguide 30 is reduced. The optical waveguide 30 can be manufactured, for example, as follows. That is, as shown in FIG. 9D, through the mask 344, at least a part of the second protective layer 340 of the second laminate 130, at least a part of the second core 33, and at least a part of the third cladding 32 may be removed. Next, as shown in FIG. 9E, the second protective layer 340 and the mask 344 are removed. The second optical waveguide 30 is formed by laminating a fourth cladding 34 that covers the third cladding 32 and the second core 33 that remain without being removed. Thereby, a complete refractive index type waveguide with strong light confinement is formed. The material of the fourth cladding 34 may be the same as the material of the third cladding 32. The width of the waveguide 131 may be somewhat different from the width of the ridge 26 due to the relationship of the refractive index of the material. Instead of providing the fourth cladding 34, the second core 33 may be processed into a ridge shape to form the waveguide 131. In this case, the air layer functions as a cladding.

[0043] With the optical circuit 100 of the first embodiment, waveguides of different materials can be efficiently coupled to each other. The configuration of the optical circuit 100 can efficiently couple waveguides of active elements such as a light receiving element and a modulation element as well as a laser light source, and passive waveguides formed of different materials.

[0044] <Second Embodiment> FIG. 10 is a schematic diagram of the optical circuit 100A of the second embodiment. Similar to FIG. 2B, in the coordinate system of FIG. 10, the light guiding direction of light is the Y direction, the direction perpendicular to the substrate 11 or the stacking direction is the Z direction, and the direction orthogonal to the Y direction and the Z direction is the X direction. The optical circuit 100A has a third optical waveguide 40 disposed between the first optical waveguide 20 and the second optical waveguide 30 on the substrate 11. The third optical waveguide 40 has a fifth cladding 42, a sixth cladding 44, and a third core 43 disposed between the fifth cladding 42 and the sixth cladding 44. The third optical waveguide 40 includes an amorphous material, one end of the third optical waveguide 40 is optically coupled to the first core 23, and the other end of the third optical waveguide 40 is optically coupled to the second core 33. The third optical waveguide 40 may be amorphous for any of the fifth cladding 42, the sixth cladding 44, and the third core 43. The first optical waveguide 20 and the second optical waveguide 30 are optically coupled via the third optical waveguide 40. The cross-section of the third optical waveguide 40 in the ZX plane direction may be a perfect refractive index type waveguide similar to the second optical waveguide 30. That is, a ridge including the fifth cladding 42, the third core 43, and the sixth cladding 44 may be formed. The sixth cladding 44 may cover the side surface of the third core 43.

[0045] When the first laminate 120 and / or the second laminate 130 is formed by cleavage, depending on the crystal plane of the substrate, one or both of the opposing surfaces of the first optical waveguide 20 and the second optical waveguide 30 may be inclined from the plane perpendicular to the substrate 11. In this case, the distance between the first optical waveguide 20 and the second optical waveguide 30 is limited by the angle of the crystal plane. Therefore, an amorphous third optical waveguide 40 is disposed between the first optical waveguide 20 and the second optical waveguide 30 to maintain the coupling efficiency. As shown in FIG. 10, the third optical waveguide 40 is disposed between the first end face 201 of the first optical waveguide 20 and the second end face 301 of the second optical waveguide 30, and at least one of the first end face 201 and the second end face 301 is inclined with respect to the normal of the substrate 11. By filling the inclined space between the first end face 201 and the second end face 301 with an amorphous third optical waveguide, the loss between the first core 23 and the second core 33 can be reduced.

[0046] FIG. 11 is a cross-sectional view of a laminate fixed to a temporary substrate 41 in the manufacturing process of the optical circuit 100A. A previously prepared first laminate 120 and a second laminate 130 are abutted and fixed to the temporary substrate 41 (see FIGS. 6A and 6B). At this time, the first end face 201 of the first laminate 120 and the second end face 301 of the second laminate 130 are not in contact with each other, but are abutted with a gap therebetween. Thereby, it becomes easier to form a third laminate 140 between the first laminate 120 and the second laminate 130. At least one of the first end face 1201 including the first core 23 of the first laminate 120 and the second end face 1301 including the second core 33 of the second laminate 130 that are abutted with each other is inclined with respect to the normal of the temporary substrate 41, and a third laminate 140 is formed between the first end face 1201 of the first laminate 120 and the second end face 1301 of the second laminate 130.

[0047] The third laminate 140 is formed by growing a sixth cladding 44, a third core 43, and a fifth cladding 42 in this order between the first laminate 120 and the second laminate 130. By using a film forming method with high straightness such as a sputtering method, the gap between the first end face 1201 of the first laminate 120 and the second end face 1301 of the second laminate 130 can be filled. After forming the third laminate 140, the upper surfaces of the first laminate 120, the second laminate 130, and the third laminate 140 are planarized and bonded to the substrate 11. After removing the temporary substrate 41, the third laminate 140 is processed together with the first laminate 120 and the second laminate 130. Thereby, a third optical waveguide 40 including the fifth cladding 42, the third core 43, and the sixth cladding 44 is obtained. The thickness of the third core 43 in the Z direction may be 0.5 times or more and 2 times or less the thickness of the second core 33 in the Z direction. Also, the width of the third core 43 in the X direction may be 0.8 times or more and 1.2 times or less the width of the second core 33 in the X direction. The length of the third optical waveguide 40 in the optical axis direction may be 5 μm or more and 600 μm or less, 30 μm or more and 500 μm or less, or 100 μm or more and 400 μm or less. By setting it within such a range, the influence of the loss of the third optical waveguide 40 is reduced. Also, the third laminate 140 is easily formed by the sputtering method.

[0048] Returning to FIG. 10, in the Z direction perpendicular to the substrate 11, the position of the third optical axis AX3 of the third optical waveguide 40 is aligned within a range where optical coupling is possible with the first optical axis AX1 of the first optical waveguide 20 and the second optical axis AX2 of the second optical waveguide 30. In the XY plane parallel to the upper surface of the substrate 11, the third laminate 140 is etched through a mask formed with a certain width on the first laminate 120, the third laminate 140, and the second laminate 130, thereby aligning the first core 23, the third core 43, and the second core 33 in the X and Y directions. Thereby, high-efficiency optical coupling is realized. Note that the waveguide formed in the third core 43 of the third optical waveguide 40 does not necessarily have to be a straight waveguide. In the XY plane parallel to the substrate 11, the first optical axis AX1 and the second optical axis AX2 may be offset from each other in the X direction. In that case, by connecting the third optical axis AX3 to the first optical axis AX1 and the second optical axis AX2, optical coupling in the Y direction is realized in the XY plane.

[0049] Depending on the angle of the crystal plane of the first laminate 120 or the second laminate 130, at the stages of FIGS. 6A and 6B, when the first laminate 120 and the second laminate 130 are butted against each other, they may be fixed to the temporary substrate 41 with a thin plate sandwiched between the first laminate 120 and the second laminate 130, and after fixing to the temporary substrate 41, the thin plate may be removed to secure a gap. This gap may be filled in the order of the sixth cladding 44, the third core 43, and the fifth cladding 42 by a PVD method or a CVD method to obtain the third laminate 140 formed of an amorphous material. By disposing the third optical waveguide 40 made of an amorphous material between the first optical waveguide 20 and the second optical waveguide 30, the loss between the first core 23 and the second core 33 can be reduced.

[0050] The third laminate 140 and the third optical waveguide 40 may be formed not limited to the above example. For example, when the first end face 201 and the second end face 301 are parallel to each other, particularly when they are perpendicular to the upper surface of the substrate 11, the third laminate 140 may also be formed, and the third optical waveguide 40 may be provided by processing this. For example, if the material is adjusted so that the equivalent refractive index of the third optical waveguide 40 is between the equivalent refractive indices of the first optical waveguide 20 and the second optical waveguide 30, the loss of optical coupling is reduced. Also, if the difference between the equivalent refractive index of the first optical waveguide 20 and the equivalent refractive index of the second optical waveguide 30 is sufficiently small, the material of the third optical waveguide 40 may be the same as the material of the second optical waveguide 30.

[0051] <Third Embodiment> FIG. 12 is a schematic diagram of the optical circuit 100B of the third embodiment, FIG. 13A is a perspective view of the first laminate having a dielectric multilayer film, and FIG. 13B is a side view parallel to the YZ plane of the first laminate of FIG. 13A. In the optical circuit 100B, the first optical waveguide 20 has a dielectric multilayer film 50 formed at the end on the side of the second optical waveguide 30. The direction in which the dielectric multilayer film 50 is laminated is a direction parallel to the first optical axis AX1 of the first optical waveguide 20. As shown in FIGS. 13A and 13B, when manufacturing the optical circuit 100B, in the step of preparing the first laminate 120, a dielectric multilayer film 50 laminated in a direction different from the lamination direction of the first laminate 120 is formed at the end of the first laminate 120 facing the second laminate 130. The first core 23 and the second core 33 are butted against each other and fixed to the temporary substrate so that the dielectric multilayer film 50 is positioned between the first laminate 120 and the second laminate 130 (see FIG. 6B etc.). In FIGS. 13A and 13B, the lamination direction of the first laminate 120 is the Z direction, and the lamination direction of the dielectric multilayer film 50 is the Y direction in which light is guided.

[0052] The dielectric multilayer film 50 is formed by alternately laminating materials with different refractive indices, and suppresses reflection depending on the refractive index difference between the first core 23 and the second core 33. As the low refractive index material of the dielectric multilayer film 50, SiO2, MgF2, etc. may be used. As the high refractive index material, TiO2, Ta2O5, Al2O3, Hf02, etc. may be used. By providing the dielectric multilayer film 50 between the first optical waveguide 20 and the second optical waveguide 30, unnecessary reflection is suppressed, and the light generated in the first optical waveguide 20 is efficiently transmitted to the second optical waveguide 30.

[0053] FIG. 14 is a cross-sectional view of an optical circuit 100C which is a modified example of the optical circuit 100B. The optical circuit 100C combines the optical circuit 100A of the second embodiment and the optical circuit 100B of the third embodiment. The optical circuit 100C has a third optical waveguide 40 and a dielectric multilayer film 50 between the first optical waveguide 20 and the second optical waveguide 30. The dielectric multilayer film 50 is preferably provided on the side where the refractive index difference is large with respect to the third optical waveguide 40. Thereby, Fresnel reflection can be reduced. In the example of FIG. 14, when the third core 43 of the third optical waveguide 40 is formed of an amorphous material such as SiN, SiON, Ta2O5, Nb2O5, TiO2, HfO2, AlN, or LN, the dielectric multilayer film 50 may be provided between the third optical waveguide 40 and the first optical waveguide 20. This is because the refractive index difference between the third optical waveguide 40 and the second optical waveguide 30 is small, and reflection at the interface is reduced.

[0054] <Fourth Embodiment> FIG. 15 is a top view of an optical circuit 100D according to the fourth embodiment. The optical circuit 100D has a second optical waveguide 30D that optically couples with the first optical waveguide 20. The optical circuit 100D has a third optical waveguide 40 between the first optical waveguide 20 and the second optical waveguide 30D, and has a dielectric multilayer film 50 between the first optical waveguide 20 and the third optical waveguide 40. The second optical waveguide 30D has a ring resonator 135 that optically couples with a waveguide 131 through which the light guided from the first optical waveguide 20 passes, and a resonator is formed between the first optical waveguide 20 and the ring resonator 135. The optical circuit 100D is used as a laser light source with a resonator.

[0055] In this example, both end faces of the first optical waveguide 20 in the light guiding direction are mirror surfaces, and it is used as an end face emitting type semiconductor laser element. The dielectric multilayer film 50 may be used as one of the mirror surfaces. By injecting carriers, light is generated in the straight waveguide 123 including the active layer of the first optical waveguide 20, coupled to the waveguide 131 of the second optical waveguide 30, coupled to the ring resonator 135, and circulates counterclockwise. A part of the light coupled to the ring resonator 135 circulates clockwise around the ring resonator 135 due to backscattering. This backscattered light is coupled to the waveguide 131 and applies optical feedback to the first optical waveguide 20. The light selected in wavelength by the circulation of the ring resonator 135 is emitted from the output port Pout.

[0056] The ring resonator 135 is formed by etching as a passive waveguide together with the waveguide 131. By forming a resonator between the ring resonator 135 and the first optical waveguide 20, the selectivity of the oscillation wavelength can be enhanced, and the spectral linewidth of the light source can be narrowed. Since the end face of the second optical waveguide 30 becomes the output port Pout and the active layer of the straight waveguide 123 is not exposed at the end face, sudden death of the light source device can be reduced.

[0057] The third cladding 32, the second core 33, and the fourth cladding 34 of the second optical waveguide 30D may all contain an amorphous material. This makes it possible to select a material such that the refractive index difference between the core and the cladding becomes large, and optical confinement can be strengthened. Also, since the optical confinement is strengthened, the bending loss of a curved waveguide such as a ring resonator becomes small.

[0058] FIG. 16 is a top view of an optical circuit 100E which is a first modification of the fourth embodiment. The optical circuit 100E has a third optical waveguide 40 between a first optical waveguide 20 and a second optical waveguide 30E, and has a dielectric multilayer film 50 between the first optical waveguide 20 and the third optical waveguide 40. Similar to the optical circuit 100D of FIG. 15, the first optical waveguide 20 of FIG. 16 is used as an end-face emitting semiconductor laser element. The second optical waveguide 30E of the optical circuit 100E has an 8-shaped ring resonator 136 as a passive waveguide ring resonator. The 8-shaped ring resonator 136 can also be formed in the same process as the waveguide 131 by etching the second laminate 130. The optical circuit 100E is also used as a laser light source with a resonator. A resonator is formed between the first optical waveguide 20 and the 8-shaped ring resonator 136. The light that is enhanced and wavelength-selected between the ring resonator 136 and the first optical waveguide 20 is emitted from the output port Pout. Similar to the optical circuit 100D of FIG. 15, the selectivity of the oscillation wavelength can be enhanced and the spectral linewidth of the light source can be narrowed. Also, since the active layer is not exposed at the light output end of the straight waveguide 123, sudden death can be reduced. Further, when this laser light source is used as the light source of an optical gyro sensor, the frequency fluctuation of the light source due to rotation is reduced.

[0059] The third cladding 32, the second core 33, and the fourth cladding 34 of the second optical waveguide 30E may all contain an amorphous material. Thereby, it becomes possible to select a material such that the refractive index difference between the core and the cladding becomes large, and the light confinement can be strengthened. Also, since the light confinement is strengthened, the bending loss of a bent waveguide such as a ring resonator becomes small.

[0060] Note that the figure-eight ring resonator 136 can be rephrased as a ring resonator 136 having two closed regions surrounded by optical waveguides. The ring resonator 136 has two intersecting straight waveguides and two curved waveguides respectively connected to these straight waveguides. The smaller angle among the angles formed by the two intersecting straight waveguides may be 80 degrees or more and 90 degrees or less, or 89 degrees or more and 90 degrees or less. When the angle formed by the two straight waveguides is 90 degrees, light can circulate around the ring resonator 136 most efficiently. The areas of the two closed regions surrounded by the optical waveguides may be 0.9 times or more and 1 times or less, or 0.98 times or more and 1 times or less of each other. When the areas of the two closed regions surrounded by the optical waveguides are the same, the frequency variation of the light source due to rotation is canceled. This is because the Sagnac effect is proportional to the area of the closed region surrounded by the optical waveguide.

[0061] FIG. 17A is a top view of the optical circuit 100F according to the second modification of the fourth embodiment, and FIG. 17B is a cross-sectional view taken along line XVIIB-XVIIB of FIG. 17A. Both FIGS. 17A and 17B show the state before electrode formation. The optical circuit 100F has a first optical waveguide 20F and second optical waveguides 30F-1 and 30F-2 arranged on both sides in the optical axis direction of the first optical waveguide 20F. The first optical waveguide 20F has a first active layer 125-1 and a second active layer 125-2 provided in parallel. The first active layer 125-1 and the second active layer 125-2 are formed in the first core 23, and the direction along the first active layer 125-1 and the second active layer 125-2 is the optical axis direction of the first optical waveguide 20F.

[0062] The second optical waveguide 30F-1 has a first curved waveguide 131-1 arranged on one side of the first optical waveguide 20F. The second optical waveguide 30F-2 has a second curved waveguide 131-2 arranged on the other side of the first optical waveguide 20F. The first curved waveguide 131-1 is optically coupled to the first active layer 125-1 and the second active layer 125-2 on one side of the first optical waveguide 20F, and the second curved waveguide 131-2 is connected to the first active layer 125-1 and the second active layer 125-2 on the other side of the first optical waveguide 20F. A resonator is formed by the first optical waveguide 20F, the first curved waveguide 131-1, and the second curved waveguide 131-2.

[0063] As shown in FIG. 17B, the first bent waveguide 131-1 has a second core 33-1 sandwiched between a third cladding 32-1 and a fourth cladding 34-1. As shown in FIG. 17B, the second bent waveguide 131-2 has a second core 33-2 sandwiched between a third cladding 32-2 and a fourth cladding 34-2. The first bent waveguide 131-1 and the second bent waveguide 131-2 may be low-loss passive waveguides formed of an amorphous material. Thereby, it becomes possible to select a material such that the refractive index difference between the core and the cladding becomes large, and optical confinement can be enhanced. Also, since the optical confinement is enhanced, the bending loss of the bent waveguide becomes small. The light generated and amplified in the first active layer 125-1 and the second active layer 125-2 circulates through the ring formed by the first bent waveguide 131-1 and the second bent waveguide 131-2, and the oscillation wavelength is selected. Either one of the second optical waveguides 30F-1 and 30F-2 is provided with a waveguide 137 for extracting light and an output port Pout connected to the waveguide 137. Thereby, a ring resonator type laser light source is formed in the optical circuit 100F. Similar to the optical circuits 100D and 100E, the optical circuit 100F can enhance the selectivity of the oscillation wavelength and narrow the spectral line width of the light source. Also, since the first active layer 125-1 and the second active layer 125-2 are not exposed at the light emission end, sudden death can be suppressed.

[0064] FIG. 18 is a schematic diagram of an optical circuit 100G according to a third modification of the fourth embodiment. The optical circuit 100G has an optical reflection structure 138 formed by a waveguide of a second optical waveguide 30G, and a resonator is formed between the first optical waveguide 20 and the optical reflection structure 138. In the example of FIG. 18, the optical reflection structure 138 is a distributed Bragg reflector (DBR). The surface of the first optical waveguide 20 opposite to the surface facing the second optical waveguide 30G serves as the other reflection surface. Light reciprocates between this reflection surface and the optical reflection structure 138 and oscillates at a wavelength determined by the periodic structure of the DBR. The end of the optical reflection structure 138 opposite to the first optical waveguide 20 is connected to the output port Pout, and a laser beam with a narrow linewidth is emitted. The optical reflection structure 138 may be formed of an amorphous material. In this case, a low-loss DBR is formed. The optical circuit 100G can also enhance the selectivity of the oscillation wavelength and narrow the linewidth of the light source. Further, since the active layer is not exposed at the light output end, sudden death can be reduced.

[0065] FIG. 19 is a schematic diagram of an optical circuit 100H according to a fourth modification of the fourth embodiment. The optical circuit 100H has an optical reflection structure formed by a directional coupler CPL of a second optical waveguide 30H and a bent waveguide 139L. In this example, both ends of the bent waveguide 139L are connected to the directional coupler CPL. The surface of the first optical waveguide 20 opposite to the surface facing the second optical waveguide 30H serves as the other reflection surface. The optical circuit 100H forms a laser light source having the reflection surface of the first optical waveguide 20 and a resonator formed by the bent waveguide 139L. The bent waveguide 139L is connected to the output port Pout. The bent waveguide 139L may be formed of an amorphous material. In this case, a low-loss optical reflection structure is formed. The optical circuit 100H can obtain an arbitrary reflectivity by adjusting the distance for optical coupling and the interval between the opposing optical waveguides in the directional coupler CPL. Further, since the active layer is not exposed at the light output end of the straight waveguide 123, sudden death can be reduced.

[0066] The optical circuits 100D to 100E described with reference to FIGS. 15 to 19 can be used as light sources for an optical gyro sensor as shown in FIG. 3. The output port Pout can correspond to the output ports P1 and P2 in FIG. 3.

[0067] The above has been described based on specific configuration examples, but the present disclosure is not limited to the above-described configuration examples. Each embodiment can be combined with each other. For example, the configuration of FIG. 10 in which the third optical waveguide 40 is disposed between the first optical waveguide 20 and the second optical waveguide 30 may be applied to the optical circuits 100D to 100H of the fourth embodiment. The optical axis of the third optical waveguide 40 may be coaxial with one of the optical axes of the first optical waveguide 20 or the second optical waveguide 30 and may be displaced from the other optical axis to such an extent that the optical coupling efficiency is not reduced. The resonator of the optical circuit of the fourth embodiment may be formed of a ring waveguide with a grating. Even in this configuration, the oscillation wavelength can be specified by the structure of the ring waveguide with a grating. A heater or an electrode for voltage application may be provided in the resonator. In this case, wavelength sweeping becomes possible by changing the heating temperature or the applied voltage. In any configuration, optical waveguides formed of different materials are efficiently coupled to each other.

[0068] Embodiments of the present disclosure may include the following configurations. (Item 1) A substrate, A first optical waveguide disposed on the substrate, A second optical waveguide disposed on the substrate and optically coupled to the first optical waveguide, Comprising, The first optical waveguide has a first cladding, a second cladding, and a first core disposed between the first cladding and the second cladding, The second optical waveguide has a third cladding, a fourth cladding, and a second core disposed between the third cladding and the fourth cladding, The first cladding is disposed closer to the substrate than the second cladding, The third cladding is disposed closer to the substrate than the fourth cladding, The material of the first cladding is different from the material of the third cladding, Optical circuit. (Item 2) The first optical waveguide includes a first straight waveguide at an end on the side of the second optical waveguide, The second optical waveguide includes a second straight waveguide at an end on the side of the first optical waveguide. The optical circuit according to claim 1. (Item 3) The difference between the width of the first core in a direction perpendicular to the stacking direction of the first optical waveguide in a cross-section perpendicular to the first optical axis of the first optical waveguide and the width of the second core in a direction perpendicular to the stacking direction of the second optical waveguide in a cross-section perpendicular to the second optical axis of the second optical waveguide is 3 μm or less. The optical circuit according to claim 1 or 2. (Item 4) The second optical waveguide includes at least a part of the second core containing an amorphous material. The optical circuit according to any one of claims 1 to 3. (Item 5) The first core includes an active layer. The first cladding includes an n-side nitride semiconductor. The second cladding includes a p-side nitride semiconductor or a transparent conductive film. The second core includes an oxide or a nitride. The third cladding and the fourth cladding are oxides or fluorides. The optical circuit according to any one of claims 1 to 4. (Item 6) A third optical waveguide disposed between the first optical waveguide and the second optical waveguide on the substrate. further comprising The third optical waveguide has a fifth cladding, a sixth cladding, and a third core disposed between the fifth cladding and the sixth cladding. The third optical waveguide includes an amorphous material, one end of the third optical waveguide is optically coupled to the first core, and the other end of the third optical waveguide is optically coupled to the second core. The optical circuit according to any one of claims 1 to 5. (Item 7) The third optical waveguide is disposed between the first end face of the first optical waveguide and the second end face of the second optical waveguide, and at least one of the first end face including the first core and the second end face including the second core is inclined with respect to the normal of the substrate. The optical circuit according to claim 6. (Item 8) The first optical waveguide has a dielectric multilayer film formed at an end on the side of the second optical waveguide, and the direction in which the dielectric multilayer film is laminated is a direction parallel to the first optical axis of the first optical waveguide. The optical circuit according to any one of Items 1 to 7. (Item 9) The second optical waveguide further has a ring resonator to which the light guided from the first optical waveguide is coupled, and a resonator is formed between the first optical waveguide and the ring resonator. The optical circuit according to any one of Items 1 to 8. (Item 10) The first optical waveguide has a first active layer and a second active layer provided in parallel, the second optical waveguide is disposed on both sides in the optical axis direction of the first optical waveguide, and includes a first bent waveguide disposed on one side of the first optical waveguide and a second bent waveguide disposed on the other side of the first optical waveguide, the first bent waveguide is connected to the first active layer and the second active layer on the one side, the second bent waveguide is connected to the first active layer and the second active layer on the other side, and a resonator is formed by the first bent waveguide and the second bent waveguide. The optical circuit according to any one of Items 1 to 8. (Item 11) The second optical waveguide has an optical reflection structure formed of a waveguide, and a resonator is formed between the first optical waveguide and the optical reflection structure. The optical circuit according to any one of Items 1 to 8. (Item 12) Preparing a first laminate having a first substrate, a first cladding, a first core, a second cladding, and a first protective layer in this order, Preparing a second laminate having a second substrate, a third cladding containing a material different from that of the first cladding, a second core, and a second protective layer in this order, Butting the first laminate and the second laminate so that the first core and the second core face each other, and fixing the first protective layer and the second protective layer to a temporary substrate; Bonding the first substrate and the second substrate to a third substrate; Removing the temporary substrate after bonding the first substrate and the second substrate to the third substrate; Processing the first laminate to obtain a first optical waveguide including the first cladding, the first core, and the second cladding, and Processing the second laminate to obtain a second optical waveguide including at least the third cladding and the second core and optically coupled to the first optical waveguide; A method for manufacturing an optical circuit, including: (Item 13) Preparing the first laminate includes laminating the first cladding, the first core, the second cladding, and the first protective layer by chemical vapor deposition or physical vapor deposition; Preparing the second laminate includes laminating the third cladding, the second core, and the second protective layer by chemical vapor deposition or physical vapor deposition; The first laminate and the second laminate are laminated such that the difference between the distance from the upper end of the first protective layer to the center of the first core and the distance from the upper end of the second protective layer to the center of the second core is 500 nm or less; The method for manufacturing an optical circuit according to Item 12. (Item 14) Obtaining the second optical waveguide includes removing at least a part of the second protective layer, at least a part of the second core, and at least a part of the third cladding through a mask, and Laminating a fourth cladding covering the third cladding and the second core remaining without being removed; The method for manufacturing an optical circuit according to Item 12 or 13. (Item 15) After butting the first laminate and the second laminate on the temporary substrate, laminating a fifth cladding, a third core, and a sixth cladding in this order between the first laminate and the second laminate to form a third laminate, and After removing the temporary substrate, the third laminate is processed to form a third optical waveguide that includes the fifth cladding, the third core, and the sixth cladding, and that optically couples to both the first optical waveguide and the second optical waveguide. further comprising The method for manufacturing an optical circuit according to any one of items 12 to 14. (Item 16) At least one of the first end face including the first core of the first laminate and the second end face including the second core of the second laminate that are abutted against each other is inclined with respect to the normal line of the temporary substrate. Forming the third laminate between the first end face of the first laminate and the second end face of the second laminate. The method for manufacturing an optical circuit according to item 15. (Item 17) The preparation of the first laminate includes forming a dielectric multilayer film laminated in a direction different from the lamination direction of the first laminate on one side surface of the first laminate. The first core and the second core are opposed to each other and the first laminate and the second laminate are abutted against each other so that the dielectric multilayer film is positioned between the first laminate and the second laminate. The method for manufacturing an optical circuit according to any one of items 12 to 16. (Item 18) At least one of the first substrate and the second substrate is polished, and then the first substrate and the second substrate are bonded to the third substrate. The method for manufacturing an optical circuit according to any one of items 12 to 17.

Description of Signs

[0069] 100, 100A - 100H Optical circuit 11 Substrate 15 Resonator 20, 20F First optical waveguide 21 First substrate 22 First cladding 23 First core 24 Second cladding 25 First protective layer 26, 36 Ridge 30, 30D, 30E, 30F-1, 30F-2 Second optical waveguide 31 Second substrate 32, 32-1, 32-2 Third cladding 33, 33-1, 33-2 Second core 34, 34-1, 34-2 Fourth cladding 40 Third optical waveguide 41 Temporary substrate 42 Fifth cladding 43 Third core 44 Sixth cladding 50 Dielectric multilayer film 120 First laminate 123, 133 Straight optical waveguide 125-1 First active layer 125-2 Second active layer 130 Second laminate 131 Optical waveguide 131-1 First bent optical waveguide 132-2 Second bent optical waveguide 135, 136 Ring resonator 137 Optical waveguide 138 Optical reflection structure 139L Bent optical waveguide 140 Third laminate 200, 300 End 340 Second protective layer CPL Evanescent coupler

Claims

1. A substrate, a first optical waveguide disposed on the substrate, a second optical waveguide disposed on the substrate and optically coupled to the first optical waveguide, comprising: The first optical waveguide has a first cladding, a second cladding, and a first core disposed between the first cladding and the second cladding, The second optical waveguide has a third cladding, a fourth cladding, and a second core disposed between the third cladding and the fourth cladding, The first cladding is disposed closer to the substrate than the second cladding, The third cladding is disposed closer to the substrate than the fourth cladding, The material of the first cladding is different from the material of the third cladding, an optical circuit.

2. The first optical waveguide includes a first straight waveguide at an end portion on the side of the second optical waveguide, The second optical waveguide includes a second straight waveguide at an end portion on the side of the first optical waveguide, The optical circuit according to claim 1.

3. The difference between the width of the first core in a direction perpendicular to the stacking direction of the first optical waveguide in a cross section perpendicular to the first optical axis of the first optical waveguide and the width of the second core in a direction perpendicular to the stacking direction of the second optical waveguide in a cross section perpendicular to the second optical axis of the second optical waveguide is 3 μm or less, The optical circuit according to claim 1.

4. At least a part of the second core of the second optical waveguide contains an amorphous material, The optical circuit according to claim 1.

5. The first core includes an active layer, The first cladding includes an n-side nitride semiconductor, The second cladding includes a p-side nitride semiconductor or a transparent conductive film, The second core includes an oxide or a nitride, The third cladding and the fourth cladding are oxides or fluorides, The optical circuit according to claim 1.

6. A third optical waveguide disposed between the first optical waveguide and the second optical waveguide on the substrate, further comprising: The third optical waveguide has a fifth cladding, a sixth cladding, and a third core disposed between the fifth cladding and the sixth cladding, The third optical waveguide includes an amorphous material, one end of the third optical waveguide is optically coupled to the first core, and the other end of the third optical waveguide is optically coupled to the second core, The optical circuit according to claim 1.

7. The third optical waveguide is disposed between a first end face of the first optical waveguide and a second end face of the second optical waveguide, and at least one of the first end face including the first core and the second end face including the second core is inclined with respect to the normal of the substrate. The optical circuit according to claim 6.

8. The first optical waveguide has a dielectric multilayer film formed at an end portion on the side of the second optical waveguide. The direction in which the dielectric multilayer film is laminated is a direction parallel to the first optical axis of the first optical waveguide. The optical circuit according to claim 1.

9. The second optical waveguide further has a ring resonator to which light guided from the first optical waveguide is coupled. and A resonator is formed between the first optical waveguide and the ring resonator. The optical circuit according to claim 1.

10. The first optical waveguide has a first active layer and a second active layer provided in parallel. The second optical waveguide is disposed on both sides in the optical axis direction of the first optical waveguide, and has a first bent waveguide disposed on one side of the first optical waveguide and a second bent waveguide disposed on the other side of the first optical waveguide. The first bent waveguide is connected to the first active layer and the second active layer on the one side, the second bent waveguide is connected to the first active layer and the second active layer on the other side, and a resonator is formed by the first bent waveguide and the second bent waveguide. The optical circuit according to claim 1.

11. The second optical waveguide has an optical reflection structure formed by a waveguide. A resonator is formed between the first optical waveguide and the optical reflection structure. The optical circuit according to claim 1.

12. Preparing a first laminate having a first substrate, a first cladding, a first core, a second cladding, and a first protective layer in this order. Preparing a second laminate having a second substrate, a third cladding containing a material different from the first cladding, a second core, and a second protective layer in this order. Butting the first laminate and the second laminate so that the first core and the second core face each other, and fixing the first protective layer and the second protective layer to a temporary substrate. Bonding the first substrate and the second substrate to a third substrate. Removing the temporary substrate after bonding the first substrate and the second substrate to the third substrate. Processing the first laminate to obtain a first optical waveguide including the first cladding, the first core, and the second cladding, and Processing the second laminate to obtain a second optical waveguide including at least the third cladding and the second core and optically coupled to the first optical waveguide. A method for manufacturing an optical circuit, comprising:

13. Preparing the first laminate includes laminating the first cladding, the first core, the second cladding, and the first protective layer by chemical vapor deposition or physical vapor deposition. Preparing the second laminate includes laminating the third cladding, the second core, and the second protective layer by chemical vapor deposition or physical vapor deposition. The first laminate and the second laminate are laminated such that the difference between the distance from the upper end of the first protective layer to the center of the first core and the distance from the upper end of the second protective layer to the center of the second core is 500 nm or less. The method for manufacturing an optical circuit according to claim 12.

14. Obtaining the second optical waveguide includes removing, via a mask, at least a part of the second protective layer, at least a part of the second core, and at least a part of the third cladding, and laminating a fourth cladding covering the third cladding and the second core that remain without being removed. The method for manufacturing an optical circuit according to claim 12.

15. After butting the first laminate and the second laminate on the temporary substrate, laminating a fifth cladding, a third core, and a sixth cladding in sequence between the first laminate and the second laminate to form a third laminate, and after removing the temporary substrate, processing the third laminate to form a third optical waveguide including the fifth cladding, the third core, and the sixth cladding and optically coupled to both the first optical waveguide and the second optical waveguide. further comprising: The method for manufacturing an optical circuit according to claim 12.

16. At least one of the first end face including the first core of the first laminate and the second end face including the second core of the second laminate, which are abutted against each other, is inclined with respect to the normal of the temporary substrate, and forming the third laminate between the first end face of the first laminate and the second end face of the second laminate. The method for manufacturing an optical circuit according to claim 15.

17. Preparing the first laminate includes forming a dielectric multilayer film laminated on one side surface of the first laminate in a direction different from the lamination direction of the first laminate. The first core and the second core are opposed to each other so that the first laminate and the second laminate are butted against each other with the dielectric multilayer film positioned between the first laminate and the second laminate. The method of manufacturing an optical circuit according to claim 12.

18. After polishing at least one of the first substrate and the second substrate, the first substrate and the second substrate are bonded to the third substrate. The method of manufacturing an optical circuit according to claim 12.

Images