Optical device, substrate-type optical waveguide element, optical communication device, and waveguide inter-transfer method

Abstract

The present disclosure provides an optical device, a substrate-type optical waveguide element, an optical communication apparatus, and a waveguide inter-transferring method. The optical device includes a transfer unit in which a first waveguide and a second waveguide are disposed in an overlapping manner such that a magnitude relationship between an effective refractive index between a vertical mode propagating the first waveguide and a vertical mode propagating the second waveguide is reversed at a position of an input end and a position of an output end. The transfer unit allows the second waveguide as a single-mode waveguide at the input end and allows the second waveguide as a multi-mode waveguide at the output end, through which TM0 light in a maximum vertical mode and light in a high-order mode propagate. The optical device includes a removal unit that allows the second waveguide as a single-mode waveguide through which TM0 light propagates by removing light in a high-order mode from light received from the transfer unit.

Description

TECHNICAL FIELD

[0001] Embodiments disclosed herein relate to an optical device, a substrate-type optical waveguide element, an optical communication apparatus, and an inter-waveguide transition method. BACKGROUND

[0002] In recent years, with an increase in communication capacity, there is an increasing demand for optical fiber communication, which requires a small optical device that converts an electrical signal into an optical signal. Therefore, in recent years, development of a substrate-type optical waveguide element (hereinafter, simply referred to as an optical device) typified by silicon photonics has been actively studied. In the optical device, two or more waveguides made of different materials can be integrally mounted on the same chip.

[0003] The optical components that constitute the optical device each have different characteristics depending on, for example, the refractive index of the material, so that the characteristics of the optical device can be improved by using waveguides made of materials suitable for each optical component. Therefore, the optical device configured with waveguides made of different materials has an inter-waveguide transition structure in which light propagates through indirect transitions between different waveguides.

[0004] Moreover, in the optical device, in order to increase the communication capacity, a polarization transmission technique is used, which transmits different signals by using two orthogonal polarization components. The two orthogonal polarization components include a TE-like mode (hereinafter, simply referred to as a TE mode) in which an electric field component parallel to the substrate is a main component and a TM-like mode (hereinafter, simply referred to as a TM mode or a vertical mode) in which an electric field component perpendicular to the substrate is a main component.

[0005] In the TM mode, the confinement of light to the core is weaker than in the TE mode, resulting in an increase in light loss in the indirect transition between different waveguides. Therefore, in the inter-waveguide transition structure, it is necessary to suppress the light loss in the TM mode caused by the indirect transition between different waveguides.

[0006] Figure 9 FIG. 1 is a diagram illustrating an example of a conventional inter-waveguide transition structure 200. In the inter-waveguide transition structure 200, a waveguide made of Si 201 and a waveguide made of Si3N4 202 are arranged in this order from the substrate. Figure 9 In the illustrated inter-waveguide transition structure 200, the waveguide made of Si 201 and the waveguide made of Si3N4 202 are covered with a cladding layer. In the inter-waveguide transition structure 200, light in a TE0 mode having the largest effective refractive index among TE modes is propagated through indirect transitions between the waveguide made of Si 201 and the waveguide made of Si3N4 202.

[0007] In the inter-waveguide migration structure 200, at a position in the middle of the Si waveguide 201, the SiN waveguide 202 is provided above the Si waveguide 201, the waveguide width of the Si waveguide 201 is gradually reduced along the direction of travel of light, and at the same time, the waveguide width of the SiN waveguide 202 is gradually increased. Then, the inter-waveguide migration structure has the following structure: the Si waveguide 201 is finally terminated and only the SiN waveguide 202 remains. Then, in the inter-waveguide migration structure 200, the electric field E localized in the Si waveguide 201 at the position between the Si waveguide 201 and the SiN waveguide 202 gradually and adiabatically migrates to the SiN waveguide 202.

[0008] Patent Literature 1: Japanese Laid-Open Patent Publication No. 2016-180865;

[0009] Patent Literature 2: Japanese Laid-Open Patent Publication No. 2015-191110;

[0010] Patent Literature 3: International Publication Pamphlet No. WO 2008 / 114624;

[0011] Patent Literature 4: U.S. Patent No. 7397995;

[0012] Non-Patent Literature 1: Wesley D. Sacher, et al., "Monolithically Integrated Multilayer Silicon Nitride-on-Silicon Waveguide Platforms for 3-D Photonic Circuits and Devices," Proceedings of the IEEE, Vol. 106, No. 12, December 2018.

[0013] However, the conventional inter-waveguide migration structure 200 includes a first cross-sectional portion X1 in which the SiN waveguide 202 having a small waveguide width is discontinuously generated upstream of the Si waveguide 201. Also, in the conventional inter-waveguide migration structure 200, a second cross-sectional portion X2 is generated downstream of the SiN waveguide 202 in which the Si waveguide 201 having a narrow waveguide width gradually disappears. In other words, the conventional inter-waveguide migration structure 200 includes two discontinuous cross-sectional portions. In the inter-waveguide migration structure 200, the migration light is scattered by the discontinuous cross-sectional portions X1 and X2, and light loss occurs.

[0014] Further, for example, in the case where the wavelength of light is 1.55 μm, the material refractive index of Si is about 3.5, and the material refractive index of Si3N4 is about 2.0, so that the material refractive index of Si is larger. In general, a core having a higher material refractive index is capable of confining an electric field in the core. Therefore, in the second cross-sectional portion X2 of the Si waveguide 201 having a narrow waveguide width and terminating at a position downstream of the SiN waveguide 202, the loss due to discontinuity increases at the position because the electric field E is more confined in the core of the terminated Si waveguide 201. As a result, the light loss affected in the second cross-sectional portion X2 where the Si waveguide 201 terminates is significant due to the increase due to discontinuity. The effect of the light loss in the second cross-sectional portion X2 where the Si waveguide 201 terminates is produced not only in the case of indirect transfer between the Si waveguide 201 and the SiN waveguide 202, but also in the case of a waveguide-to-waveguide transfer structure using waveguides having a magnitude relationship between material refractive indices. In addition, the increase in light loss due to scattering of light in a discontinuous cross-sectional portion is more significant for light in the TM0 mode than for light in the TE0 mode.

[0015] In the conventional waveguide-to-waveguide transfer structure 200, light in the TM0 mode propagating through the Si waveguide 201 is adiabatically transferred to light in the TM0 mode propagating through the SiN waveguide 202. In order to transfer light in the TM0 mode, it is necessary to arrange the Si waveguide 201 and the SiN waveguide 202 such that the effective refractive index of the TM0 mode propagating through the Si waveguide 201 gradually matches the effective refractive index of the TM0 mode propagating through the SiN waveguide 202. Then, the TM0 mode propagating through the Si waveguide 201 and the TM0 mode propagating through the SiN waveguide 202 interact with each other, so that the electric field distribution in the TM0 mode is distributed in the Si waveguide 201 and the SiN waveguide 202.

[0016] In the input portion 200A and the output portion 200B included in the waveguide-to-waveguide transfer structure 200, it is preferable to effectively establish a connection with an externally isolated waveguide. Further, the externally isolated waveguide mentioned herein is, for example, a separate waveguide of the Si waveguide 201 which does not include the SiN waveguide 202 above the Si waveguide 201, or a separate waveguide of the SiN waveguide 202 which does not include the Si waveguide 201 below the SiN waveguide 202.

[0017] However, in the input section 200A and the output section 200B, the following interaction is not preferable in which the electric field is distributed in both the Si waveguide 201 and the SiN waveguide 202. In the input section 200A and the output section 200B, if the electric field is distributed in both the waveguides, when the input section 200A and the output section 200B are connected to an external isolated waveguide, light loss, mode conversion, reflection, and the like are caused due to a mismatch of mode distribution, for example.

[0018] Therefore, an object of an aspect of embodiments of the present application is to provide an optical device or the like capable of suppressing light loss due to indirect migration between different waveguides. SUMMARY

[0019] According to an aspect of embodiments, there is provided an optical device in which light in a vertical mode propagates through the optical device by indirect migration between a first waveguide having a first material refractive index and a second waveguide having a second material refractive index lower than the first material refractive index, the optical device including a migration unit and a removal unit. The optical device overlaps the first waveguide and the second waveguide in a state in which the first waveguide and the second waveguide are separated from each other, to reverse a magnitude relationship between an effective refractive index of the vertical mode propagating through the first waveguide and an effective refractive index of the vertical mode propagating through the second waveguide at a position of an input section of the migration unit from a position of an output section of the migration unit. At the input section, the migration unit allows the second waveguide as a single-mode waveguide, to propagate TM0 light of the vertical mode having a maximum effective refractive index among the vertical modes through the single-mode waveguide. And at the output section, the migration unit allows the second waveguide as a multi-mode waveguide, to propagate TM0 light of the vertical mode having the maximum effective refractive index and light of a high-order mode among the vertical modes through the multi-mode waveguide. The removal unit propagates TM0 light of the vertical mode having the maximum effective refractive index among the vertical modes through the single-mode waveguide by removing light of the high-order mode from light in the vertical mode received from the migration unit, to allow the second waveguide as the single-mode waveguide. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a drawing illustrating an example of a substrate-type optical waveguide element according to the present embodiment;

[0021] Figure 2A is a drawing schematically illustrating an example of a cross-sectional portion taken along Figure 1 the illustrated line A-A;

[0022] Figure 2Bis a diagram schematically illustrating a cross-sectional portion taken along Figure 1 is an example of a diagram of a cross-sectional portion taken along

[0023] Figure 2C is a diagram schematically illustrating a cross-sectional portion taken along Figure 1 is an example of a diagram of a cross-sectional portion taken along

[0024] Figure 2D is a diagram schematically illustrating a cross-sectional portion taken along Figure 1 is an example of a diagram of a cross-sectional portion taken along

[0025] Figure 2E is a diagram schematically illustrating a cross-sectional portion taken along Figure 1 is an example of a diagram of a cross-sectional portion taken along

[0026] Figure 2F is a diagram schematically illustrating a cross-sectional portion taken along Figure 1 is an example of a diagram of a cross-sectional portion taken along

[0027] Figure 3A is an example of a diagram illustrating a relationship between effective refractive indexes of respective modes of a SiN waveguide in an output portion according to a conventional waveguide intermigration structure (core thickness 0.4 μm);

[0028] Figure 3B is an example of a diagram illustrating a relationship between effective refractive indexes of respective modes of a SiN waveguide in an output portion according to a migration structure (core thickness 0.3 μm);

[0029] Figure 4 is an example of a diagram illustrating a calculation result of respective effective refractive indexes of a TE0 mode and a TM0 mode propagating through respective cross-sectional positions in a migration unit;

[0030] Figure 5 is an example of a diagram illustrating an optical communication device according to the present embodiment in which a substrate-type optical waveguide element is built-in;

[0031] Figure 6 is an example of a diagram illustrating a substrate-type optical waveguide element according to a comparative example;

[0032] Figure 7A is a diagram schematically illustrating a cross-sectional portion taken along Figure 6 is an example of a diagram of a cross-sectional portion taken along

[0033] Figure 7B is a diagram schematically illustrating a cross-sectional portion taken along Figure 6 is an example of a diagram of a cross-sectional portion taken along

[0034] Figure 7Cis a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 6 is a diagram schematically illustrating an example of a cross-sectional portion taken along

[0035] Figure 7D is a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 6 is a diagram schematically illustrating an example of a cross-sectional portion taken along

[0036] Figure 7E is a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 6 is a diagram schematically illustrating an example of a cross-sectional portion taken along

[0037] Figure 8 is a diagram schematically illustrating an example of a cross-sectional portion taken along

[0038] Figure 9 is a diagram schematically illustrating an example of a conventional waveguide intermigration structure. DETAILED DESCRIPTION

[0039] Figure 6 is a diagram schematically illustrating an example of a substrate-type optical waveguide element 100 according to a comparative example. Also, for the convenience of description, in Figure 6 is not illustrated. Figure 6 The illustrated substrate-type optical waveguide element 100 includes a Si waveguide 101, a SiN waveguide 102, and a cladding layer 103 covering the Si waveguide 101 and the SiN waveguide 102. Also, the substrate-type optical waveguide element 100 includes a migration unit 104 that optically couples the Si waveguide 101 and the SiN waveguide 102 based on indirect migration. The Si waveguide 101 is made of, for example, Si, and the material refractive index of Si at an optical wavelength of 1.55 μm is 3.48. The SiN waveguide 102 is made of, for example, Si3N4 (hereinafter, simply referred to as SiN), and the material refractive index of SiN at an optical wavelength of 1.55 μm is 1.99. The cladding layer 103 is made of, for example, SiO2, and the material refractive index of SiO2 at an optical wavelength of 1.55 μm is 1.44.

[0040] The Si waveguide 101 includes a first straight waveguide 110 and a first tapered waveguide 120 optically coupled to the first straight waveguide 110. The first tapered waveguide 120 has a tapered structure in which the waveguide width gradually decreases from an output port of the first straight waveguide 110 toward an input port of the SiN waveguide 102 according to the traveling direction of light. The waveguide width of the first straight waveguide 110 is defined to be, for example, 0.48 μm. Also, the waveguide width of an input portion 120A of the first tapered waveguide 120 is defined to be, for example, 0.48 μm, and the waveguide width of an output portion 120B of the first tapered waveguide 120 is defined to be, for example, 0.09 μm. The thickness of each core of the first straight waveguide 110 and the first tapered waveguide 120 is defined to be, for example, 0.22 μm.

[0041] The SiN waveguide 102 includes a second tapered waveguide 130 and a second straight waveguide 140 optically coupled to the second tapered waveguide 130. The second tapered waveguide 130 has a tapered structure in which the waveguide width gradually increases from the output port of the first straight waveguide 110 toward the input port of the second straight waveguide 140. Also, the waveguide width of an input portion 130A of the second tapered waveguide 130 is defined to be, for example, 0.25 μm, and the waveguide width of an output portion 130B of the second tapered waveguide 130 is defined to be, for example, W SiN . The waveguide width of the second straight waveguide 140 is defined to be, for example, W SiN . The thickness of each core of the second straight waveguide 140 and the second tapered waveguide 130 is defined to be, for example, 0.3 μm.

[0042] The substrate-type optical waveguide element 100 includes a migration unit 104 configured by overlapping a portion of the second tapered waveguide 130 over the first tapered waveguide 120 in a state in which the first tapered waveguide 120 and the second tapered waveguide 130 are separated. Also, the interval between the first tapered waveguide 120 and the second tapered waveguide 130 is defined to be, for example, 0.3 μm.

[0043] The migration unit 104 includes an input portion 104A, an output portion 104B, and an intermediate portion 104C between the input portion 104A and the output portion 104B. Figure 7A is a diagram schematically illustrating a cross-sectional portion taken along Figure 6 the line A-A in Figure 7A is a diagram schematically illustrating a cross-sectional portion taken along Figure 6 the line A-A in

[0044] Figure 7B is an example of a diagram schematically illustrating a cross-sectional portion taken along Figure 6 of the illustrated line B-B. Figure 7B is an example of a diagram schematically illustrating a cross-sectional portion taken along Figure 6 of the illustrated line B-B is a schematic cross-sectional portion of the input portion 104A of the migration unit 104, and has the following structure: the waveguide width of the input portion 120A of the first tapered waveguide 120 is larger than the waveguide width of the input portion 130A of the second tapered waveguide 130. The waveguide width of the input portion 120A of the first tapered waveguide 120 is defined to be, for example, 0.48 μm, and the waveguide width of the input portion 130A of the second tapered waveguide 130 is defined to be, for example, 0.25 μm. The thickness of the core of the first tapered waveguide 120 is defined to be, for example, 0.22 μm, and the thickness of the core of the second tapered waveguide 130 is defined to be, for example, 0.3 μm. The spacing between the first tapered waveguide 120 and the second tapered waveguide 130 is defined to be, for example, 0.3 μm.

[0045] Figure 7C is an example of a diagram schematically illustrating a cross-sectional portion taken along Figure 6 of the illustrated line C-C. Figure 7C is an example of a diagram schematically illustrating a cross-sectional portion taken along Figure 6 of the illustrated line C-C is a schematic cross-sectional portion of the intermediate portion 104C of the migration unit 104, and has the following structure: the waveguide width of the first tapered waveguide 120 is smaller than the waveguide width of the second tapered waveguide 130. The thickness of the core of the first tapered waveguide 120 is defined to be, for example, 0.22 μm, and the thickness of the core of the second tapered waveguide 130 is defined to be, for example, 0.3 μm. The spacing between the first tapered waveguide 120 and the second tapered waveguide 130 is defined to be, for example, 0.3 μm.

[0046] Figure 7D is an example of a diagram schematically illustrating a cross-sectional portion taken along Figure 6 of the illustrated line D-D. Figure 7D is an example of a diagram schematically illustrating a cross-sectional portion taken along Figure 6 of the illustrated line D-D is a schematic cross-sectional portion of the output portion 104B of the migration unit 104, and has the following structure: the waveguide width of the output portion 130B of the second tapered waveguide 130 is larger than the waveguide width of the output portion 120B of the first tapered waveguide 120. The waveguide width of the output portion 120B of the first tapered waveguide 120 is defined to be, for example, 0.09 μm, and the waveguide width of the output portion 130B of the second tapered waveguide 130 is defined to be W SiNThe thickness of the core of the first taper waveguide 120 is defined to be, for example, 0.22 μm, and the thickness of the core of the second taper waveguide 130 is defined to be, for example, 0.3 μm. The spacing between the first taper waveguide 120 and the second taper waveguide 130 is defined to be, for example, 0.3 μm.

[0047] Figure 7E is a diagram schematically illustrating a cross-sectional portion taken along Figure 6 the line E-E illustrated. Figure 7E is a diagram schematically illustrating a cross-sectional portion taken along Figure 6 The cross-sectional portion in the line E-E in the second straight waveguide 140 included in the SiN waveguide 102 is a schematic cross-sectional portion. Also, the waveguide width of the second straight waveguide 140 is defined to be W SiN , and the thickness of the core is defined to be, for example, 0.3 nm.

[0048] The input portion 104A of the migration unit 104 includes a first cross-sectional portion in which the light received from the first straight waveguide 110 is discontinuous at a portion where the input portion 120A of the first taper waveguide 120 and the input portion 130A of the second taper waveguide 130 overlap each other. The waveguide width of the input portion 120A is different from the waveguide width of the input portion 130A, so that a discontinuous portion of the light is constituted between the first taper waveguide 120 and the second taper waveguide 130.

[0049] The output portion 104B of the migration unit 104 includes a second cross-sectional portion in which the light output to the second straight waveguide 140 is discontinuous at a portion where the output portion 120B of the first taper waveguide 120 and the output portion 130B of the second taper waveguide 130 overlap each other. The waveguide width of the output portion 120B is different from the waveguide width of the output portion 130B, so that a discontinuous portion of the light is constituted between the first taper waveguide 120 and the second taper waveguide 130.

[0050] The input portion 104A of the migration unit 104 has a structure in which the waveguide width of the first taper waveguide 120 is larger and the waveguide width of the second taper waveguide 130 is smaller, and, conversely, the output portion 104B has a structure in which the waveguide width of the first taper waveguide 120 is smaller and the waveguide width of the second taper waveguide 130 is larger. That is, the structure is constituted so that the waveguide width of the first taper waveguide 120 gradually decreases from the input portion 120A toward the output portion 120B, and the waveguide width of the second taper waveguide 130 gradually increases from the input portion 130A toward the output portion 130B. In general, as the waveguide width of the waveguide increases, the light confinement to the core becomes stronger, so that the effective refractive index increases due to the influence of the material refractive index of the core.

[0051] The input portion 104A and the output portion 104B of the migration unit 104 each have a structure in which there is a large difference between the effective refractive index of the TM0 mode propagating through the first tapered waveguide 120 and the effective refractive index of the TM0 mode propagating through the second tapered waveguide 130. As a result, in the input portion 104A and the output portion 104B of the migration unit 104, it is possible to reduce the interaction in which the electric field is distributed in both the Si waveguide 101 and the SiN waveguide 102. For example, the output portion 104B of the migration unit 104, which corresponds to the cross-sectional portion in which the Si waveguide 101 having a small waveguide width is terminated downstream of the SiN waveguide 102, is a portion in which the electric field E is confined in the core of the SiN waveguide 102, so that it is possible to suppress the loss due to discontinuity.

[0052] Further, the intermediate portion 104C of the migration unit 104 has a structure in which the effective refractive index of the TM0 mode propagating through the first tapered waveguide 120 and the effective refractive index of the TM0 mode propagating through the second tapered waveguide 130 are close to and identical to each other, and are matched. As a result, the interaction in which the electric field is distributed in both the Si waveguide 101 and the SiN waveguide 102 is intensified.

[0053] In the migration unit 104 according to the comparative example, by applying a large difference between the effective refractive index of the TM0 mode propagating through the Si waveguide 101 and the effective refractive index of the TM0 mode propagating through the SiN waveguide 102, it is possible to suppress the optical loss due to light scattering at the discontinuous cross-sectional portion.

[0054] However, it is difficult to provide a difference in the effective refractive index of the TM0 mode propagating through different waveguides due to the following verification results. Figure 8 is a graph that is an example illustrating the calculation results of the effective refractive indices of the TE0 mode and the TM0 mode propagating through each cross-sectional position in the migration unit 104.

[0055] Figure 8 is a graph that graphically illustrates the calculation results of the effective refractive indices of the TE0 mode and the TM0 mode propagating through each cross-sectional position in the Si waveguide 101 and the SiN waveguide 102 included in the migration unit 104. The first cross-sectional position "0" corresponds to the input portion 104A of the migration unit 104, and is a cross-sectional portion taken along the line B-B illustrated in FIG. 6. Figure 7B The second cross-sectional position "1" corresponds to the output portion 104B of the migration unit 104, and is a cross-sectional portion taken along the line D-D illustrated in FIG. 6. The third cross-sectional position "0.5" corresponds to the intermediate portion 104C included in the migration unit 104, and is a cross-sectional portion taken along the line C-C illustrated in FIG. 6. Figure 7D The second cross-sectional position "1" corresponds to the output portion 104B of the migration unit 104, and is a cross-sectional portion taken along the line D-D illustrated in FIG. 6. The third cross-sectional position "0.5" corresponds to the intermediate portion 104C included in the migration unit 104, and is a cross-sectional portion taken along the line C-C illustrated in FIG. 6. Figure 7CA cross-sectional portion taken along the line C-C is illustrated. The wavelength of light passing through the Si waveguide 101 and the SiN waveguide 102 is defined as 1.55 μm. The effective refractive index is calculated using the finite element method. It is assumed in this calculation that the effective refractive index is calculated in a case where the Si waveguide 101 and the SiN waveguide 102 are isolated. Also, "isolation" means that waveguides other than the target waveguide are separated to infinity.

[0056] It is assumed that the maximum waveguide width W SiN of 1.0 μm satisfies the single mode condition (as Figure 3B illustrated later. In other words, it is assumed that in the SiN waveguide 102, the TE1 mode as a high order mode in the TE mode and the TM1 mode as a high order mode in the TM mode do not propagate.

[0057] At the second cross-sectional position "1" where the Si waveguide 101 terminates, as Figure 8 illustrated, the difference between the effective refractive index of the TE0 mode propagating through the Si waveguide 101 and the effective refractive index of the TE0 mode propagating through the SiN waveguide 102 is 0.126. In contrast to this, the difference between the effective refractive index of the TM0 mode propagating through the Si waveguide 101 and the effective refractive index of the TM0 mode propagating through the SiN waveguide 102 is 0.056. Therefore, it is difficult for the TM0 mode to cause a difference in the effective refractive index between the Si waveguide 101 and the SiN waveguide 102.

[0058] Also, in the input portion 104A and the output portion 104B of the migration unit 104, in order to reduce the influence of the discontinuity of the waveguide, it is necessary to increase the waveguide width in the case of the TM0 mode compared to the case of the TE0 mode. In particular, this is more noticeable in a case where the discontinuous cross-sectional portion is the Si waveguide 101 having a high refractive index. Therefore, when light propagates by indirect migration while propagating from the Si waveguide 101 to the SiN waveguide 102, it is necessary to increase the waveguide width of the SiN waveguide 102 located upstream of the discontinuous cross-sectional portion of the Si waveguide 101. However, if the waveguide width of the SiN waveguide 102 is excessively increased, the condition of the single mode waveguide is not satisfied, and a high order mode thus propagates. As a result, if a high order mode propagates, a high order mode is generated due to the influence of a rough sidewall generated inside the waveguide at the time of manufacture, and an unnecessary disturbance or the like is generated due to the generation of the high order mode, all of which lead to degradation of the characteristics of the optical signal.

[0059] Therefore, embodiments of the substrate type optical waveguide element 1 that solve the above-described situation will be described in detail below with reference to the drawings. Also, the present application is not limited to these embodiments. In addition, the embodiments described below can also be used in any appropriate combination, as long as the embodiments do not conflict with each other.

[0060] Embodiments

[0061] Figure 1 is a drawing illustrating an example of a substrate optical waveguide element 1 according to the present embodiment. Figure 1 The illustrated substrate optical waveguide element 1 includes a silicon (Si) waveguide 2, a silicon nitride (SiN) waveguide 3, and a cladding layer 4 covering the Si waveguide 2 and the SiN waveguide 3. The substrate optical waveguide element 1 allows light to migrate between the Si waveguide 2 and the SiN waveguide 3 in a manner of adiabatic indirect migration. The Si waveguide 2 is a first waveguide made of, for example, Si. The material refractive index of Si is 3.48 at an optical wavelength of 1.55 μm. The material refractive index of Si is a first material refractive index. The SiN waveguide 3 is a second waveguide made of, for example, Si3N4 (hereinafter referred to as SiN). The material refractive index of SiN is 1.99 at an optical wavelength of 1.55 μm. The material refractive index of SiN is a second material refractive index that is smaller than the first material refractive index. The cladding layer 4 is a layer made of, for example, SiO2. The material refractive index of SiO2 is 1.44 at an optical wavelength of 1.55 μm.

[0062] The Si waveguide 2 includes a first straight waveguide 10 and a first tapered waveguide 20 optically coupled to the first straight waveguide 10. The first tapered waveguide 20 has a tapered structure in which a waveguide width gradually decreases from an output portion of the first straight waveguide 10 toward an input portion of a second straight waveguide 40 included in the SiN waveguide 3. The waveguide width of the first straight waveguide 10 is defined to be, for example, 0.48 μm. Also, the waveguide width of an input portion 20A of the first tapered waveguide 20 is defined to be, for example, 0.48 μm, and the waveguide width of an output portion 20B of the first tapered waveguide 20 is defined to be, for example, 0.09 μm. The thickness of a core of each of the first straight waveguide 10 and the first tapered waveguide 20 is defined to be, for example, 0.22 μm.

[0063] The SiN waveguide 3 includes a second tapered waveguide 30, a second straight waveguide 40 optically coupled to the second tapered waveguide 30, a third tapered waveguide 50 optically coupled to the second straight waveguide 40, and a third straight waveguide 60 optically coupled to the third tapered waveguide 50. The second tapered waveguide 30 has a tapered structure in which a waveguide width gradually increases from the output portion of the first straight waveguide 10 toward the input portion of the second straight waveguide 40. Also, the waveguide width of an input portion 30A of the second tapered waveguide 30 is defined to be, for example, 0.25 μm, and the waveguide width of an output portion 30B of the second tapered waveguide 30 is defined to be, for example, 1.8 μm. The waveguide width of the second straight waveguide 40 is defined to be, for example, 1.8 μm.

[0064] The third tapered waveguide 50 has a tapered structure in which the waveguide width gradually decreases from the output portion of the second straight waveguide 40 toward the input portion of the third straight waveguide 60. Further, the waveguide width of the input portion 50A of the third tapered waveguide 50 is defined to be, for example, 1.8 μm, and the waveguide width of the output portion 50B of the third tapered waveguide 50 is defined to be, for example, 1 μm. The waveguide width of the third straight waveguide 60 is defined to be, for example, 1 μm. The thickness of the core of each of the second tapered waveguide 30, the second straight waveguide 40, the third tapered waveguide 50, and the third straight waveguide 60 is defined to be, for example, 0.3 μm.

[0065] The substrate-type optical waveguide element 1 includes a migration unit 5 and a removal unit 6. The migration unit 5 includes the first tapered waveguide 20 included in the Si waveguide 2 and the second tapered waveguide 30 included in the SiN waveguide 3. In the migration unit 5, by disposing a portion of the second tapered waveguide 30 over the first tapered waveguide 20 in a state of being separated between the first tapered waveguide 20 and the second tapered waveguide 30 in an overlapping manner, a TM0 mode propagating through the first tapered waveguide 20 is migrated into a TM0 mode propagating through the second tapered waveguide 30. At a position of an input end of the migration unit 5, the second tapered waveguide 30 is used as a single-mode waveguide through which light in a TM0 mode having the largest effective refractive index among TM modes propagates. At a position of an output end of the migration unit 5, the second tapered waveguide 30 is used as a multi-mode waveguide through which light in a TM0 mode and light in high-order modes included in TM modes propagates. Further, a gap between the first tapered waveguide 20 and the second tapered waveguide 30 is defined to be, for example, 0.3 μm.

[0066] In the migration unit 5, the waveguide width of the second tapered waveguide 30 included in the SiN waveguide 3 is larger than the waveguide width of the first tapered waveguide 20 included in the Si waveguide 2, so that a region in which an optical field of the SiN waveguide 3 is confined is relatively larger than that of the Si waveguide 2. As a result, it is possible to reduce an optical scattering effect caused by discontinuity at the output portion 5B corresponding to the second cross-sectional portion at which the first tapered waveguide 20 terminates. However, in the multi-mode waveguide, due to the influence of random discontinuity of the waveguide caused by a rough side wall generated inside the waveguide at the time of manufacture, and the like, the TM0 mode and the high-order modes included in the TM modes interfere with each other, and thus, a moire is generated in a wavelength spectral region, so that the characteristics of the optical device are deteriorated. Therefore, the structure is configured so that the high-order modes included in the TM modes are removed in the removal unit 6.

[0067] The removing unit 6 allows the third tapered waveguide 50 to function as a single-mode waveguide through which only light in the TM0 mode propagates by removing light in high-order modes from the light in the TM0 mode received from the migration unit 5. The removing unit 6 includes the second straight waveguide 40 and the third tapered waveguide 50. The second straight waveguide 40 is a multi-mode waveguide through which high-order TM modes higher than the TM1 mode propagate. The third tapered waveguide 50 is a single-mode waveguide through which only the TM0 mode propagates by removing only light in high-order TM modes received from the second straight waveguide 40. The third tapered waveguide 50 can avoid interference of high-order modes while suppressing light loss in the TM0 mode by removing high-order TM modes from the multi-mode received from the second straight waveguide 40.

[0068] The migration unit 5 includes an input portion 5A, an output portion 5B, and an intermediate portion 5C. Also, the waveguide length between the input portion 5A and the output portion 5B of the migration unit 5 is defined to be, for example, 80 μm. At the input portion 5A of the migration unit 5, the second tapered waveguide 30 is set to be a single-mode waveguide through which only the TM0 mode in the TM modes having the largest effective refractive index propagates. Also, at the output portion 5B of the migration unit 5, the second tapered waveguide 30 is set to be a multi-mode waveguide through which light in the TM0 mode and light in high-order modes in the TM modes propagate. The removing unit 6 includes an input portion 6A and an output portion 6B. Also, the waveguide length of the second straight waveguide 40 included in the removing unit 6 is defined to be, for example, 2 μm. In addition, the waveguide length of the third tapered waveguide 50 included in the removing unit 6 is defined to be, for example, 20 μm.

[0069] Figure 2A is a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 1 the line A-A illustrated in Figure 2A is a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 1 the line A-A illustrated in

[0070] Figure 2B is a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 1 the line B-B illustrated in Figure 2B is a diagram schematically illustrating an example of a cross-sectional portion taken along Figure 1The cross-sectional portion taken along the line B-B in FIG. 5 is a schematic cross-sectional portion of the input portion 5A of the migration unit 5, and has the following structure: the waveguide width of the input portion 20A of the first tapered waveguide 20 is larger than the waveguide width of the input portion 30A of the second tapered waveguide 30. The waveguide width of the input portion 20A of the first tapered waveguide 20 is defined to be, for example, 0.48 μm, and the waveguide width of the input portion 30A of the second tapered waveguide 30 is defined to be, for example, 0.25 μm. The thickness of the core of the first tapered waveguide 20 is defined to be, for example, 0.22 μm, and the thickness of the core of the second tapered waveguide 30 is defined to be, for example, 0.3 μm. The spacing between the first tapered waveguide 20 and the second tapered waveguide 30 is defined to be, for example, 0.3 μm.

[0071] Figure 2C is a view schematically illustrating an example of a cross-sectional portion taken along the line C-C illustrated in FIG. 6. Figure 1 is a view schematically illustrating an example of a cross-sectional portion taken along the line C-C illustrated in FIG. 6. Figure 2C is a view schematically illustrating an example of a cross-sectional portion taken along the line C-C illustrated in FIG. 6. Figure 1 The cross-sectional portion taken along the line C-C in FIG. 6 is a schematic cross-sectional portion of the middle portion 5C of the migration unit 5, and has the following structure: the waveguide width of the first tapered waveguide 20 is smaller than the waveguide width of the second tapered waveguide 30. The thickness of the core of the first tapered waveguide 20 is defined to be, for example, 0.22 μm, and the thickness of the core of the second tapered waveguide 30 is defined to be, for example, 0.3 μm. The spacing between the first tapered waveguide 20 and the second tapered waveguide 30 is defined to be, for example, 0.3 μm.

[0072] Figure 2D is a view schematically illustrating an example of a cross-sectional portion taken along the line D-D illustrated in FIG. 7. Figure 1 is a view schematically illustrating an example of a cross-sectional portion taken along the line D-D illustrated in FIG. 7. Figure 2D The cross-sectional portion taken along the line D-D in FIG. 7 is a schematic cross-sectional portion of the output portion 5B of the migration unit 5, and has the following structure: the waveguide width of the output portion 30B of the second tapered waveguide 30 is larger than the waveguide width of the output portion 20B of the first tapered waveguide 20. The waveguide width of the output portion 20B of the first tapered waveguide 20 is defined to be, for example, 0.09 μm, and the waveguide width of the output portion 130B of the second tapered waveguide 30 is defined to be, for example, 1.8 μm. The thickness of the core of the first tapered waveguide 20 is defined to be, for example, 0.22 μm, and the thickness of the core of the second tapered waveguide 30 is defined to be, for example, 0.3 μm. The spacing between the first tapered waveguide 20 and the second tapered waveguide 30 is defined to be, for example, 0.3 μm.

[0073] Figure 2E is a view schematically illustrating an example of a cross-sectional portion taken along the line E-E illustrated in FIG. 8. Figure 1 is a view schematically illustrating an example of a cross-sectional portion taken along the line E-E illustrated in FIG. 8. Figure 2EThe cross-sectional portion schematically illustrated in FIG. 6 along the line E-E is a schematic cross-sectional portion of the second straight waveguide 40 included in the unit 6. Also, the waveguide width of the second straight waveguide 40 is defined to be, for example, 1.8 μm, and the thickness of the core is defined to be, for example, 0.3 μm.

[0074] Figure 2F is a schematic cross-sectional portion of the third tapered waveguide 50 corresponding to the output portion 6B of the unit 6, and has the following structure: the waveguide width of the output portion 50B of the third tapered waveguide 50 is smaller than the waveguide width of the second straight waveguide 40. The waveguide width of the output portion 50B of the third tapered waveguide 50 is the same as the waveguide width of the third straight waveguide 60. The waveguide width of the output portion 50B of the third tapered waveguide 50 is defined to be, for example, 1 μm, and the waveguide width of the third straight waveguide 60 is defined to be, for example, 1 μm. The thickness of the core of each of the third tapered waveguide 50 and the third straight waveguide 60 is defined to be, for example, 0.3 μm. Figure 1 Figure 2F The cross-sectional portion schematically illustrated in FIG. 6 along the line E-E is a schematic cross-sectional portion of the second straight waveguide 40 included in the unit 6. Also, the waveguide width of the second straight waveguide 40 is defined to be, for example, 1.8 μm, and the thickness of the core is defined to be, for example, 0.3 μm.

[0075] The input portion 5A of the migration unit 5 includes a first cross-sectional portion in which the light from the first straight waveguide 10 becomes discontinuous at a position where the input portion 20A of the first tapered waveguide 20 and the input portion 30A of the second tapered waveguide 30 overlap each other. The waveguide width of the input portion 20A is different from the waveguide width of the input portion 30A so that a discontinuous portion of the signal light is constituted between the first tapered waveguide 20 and the second tapered waveguide 30.

[0076] The output portion 5B of the migration unit 5 includes a second cross-sectional portion in which the light to the second straight waveguide 40 does not become discontinuous at a position where the output portion 20B of the first tapered waveguide 20 and the output portion 30B of the second tapered waveguide 30 overlap each other. The waveguide width of the output portion 20B is different from the waveguide width of the output portion 30B so that a discontinuous portion of the light is constituted between the first tapered waveguide 20 and the second tapered waveguide 30.

[0077] At the input portion 5A of the migration unit 5, the waveguide width of the first tapered waveguide 20 is large and the waveguide width of the second tapered waveguide 30 is small, and conversely, at the output portion 5B, the waveguide width of the first tapered waveguide 20 is small and the waveguide width of the second tapered waveguide 30 is large. In other words, the structure is constituted so that the waveguide width of the first tapered waveguide 20 gradually decreases from the input portion 20A toward the output portion 20B, and the waveguide width of the second tapered waveguide 30 gradually increases from the input portion 30A toward the output portion 30B.

[0078] ​The structure of each of the input portion 5A and the output portion 5B of the migration unit 5 is configured so that a large difference is provided between the effective refractive index of the TM0 mode propagating through the first tapered waveguide 20 and the effective refractive index of the TM0 mode propagating through the second tapered waveguide 30. At the input portion 5A of the migration unit 5, the effective refractive index of the TM0 mode propagating through the first tapered waveguide 20 is higher than the effective refractive index of the TM0 mode propagating through the second tapered waveguide 30. In other words, the interaction in which the electric field is distributed in both the Si waveguide 2 and the SiN waveguide 3 can be reduced. Also, at the output portion 5B of the migration unit 5, the effective refractive index of the TM0 mode propagating through the second tapered waveguide 30 is higher than the effective refractive index of the TM0 mode propagating through the first tapered waveguide 20. In other words, the interaction in which the electric field is distributed in both the Si waveguide 2 and the SiN waveguide 3 can be reduced. As a result, at the input portion 5A and the output portion 5B of the migration unit 5, the interaction in which the electric field is distributed in both the Si waveguide 2 and the SiN waveguide 3 can be reduced.

[0079] Also, at the intermediate portion 5C of the migration unit 5, the structure is configured so that the effective refractive index of the TM0 mode propagating through the first tapered waveguide 20 and the effective refractive index of the TM0 mode propagating through the second tapered waveguide 30 are close and identical. As a result, the interaction in which the electric field is distributed in both the Si waveguide 2 and the SiN waveguide 3 is intensified.

[0080] At the input portion 5A, the effective refractive index of the TM0 mode propagating through the first tapered waveguide 20 obtained in the case where the first tapered waveguide 20 is set as an isolated waveguide is greater than the effective refractive index of the TM0 mode propagating through the second tapered waveguide 30 obtained in the case where the second tapered waveguide 30 is set as an isolated waveguide. At the output portion 5B, the effective refractive index of the TM0 mode propagating through the second tapered waveguide 30 obtained in the case where the second tapered waveguide 30 is set as an isolated waveguide is greater than the effective refractive index of the TM0 mode propagating through the first tapered waveguide 20 obtained in the case where the first tapered waveguide 20 is set as an isolated waveguide. In other words, at the input portion 5A and the output portion 5B of the migration unit 5, the structure is configured so that the magnitude relationship of the effective refractive index of the propagating TM0 mode is reversed, that is, a large difference is produced between the effective refractive indices of the TM0 modes, so that the light loss caused by light scattering at the discontinuous cross-sectional portions can be suppressed.

[0081] The migration unit 5 has been configured so that the waveguide width of the SiN waveguide 3 (the second tapered waveguide 30) is increased to, for example, 1.8 μm to suppress the influence caused by the discontinuous portion on the TM0 mode as much as possible so that the influence caused by the discontinuous portion on the TM0 mode can be suppressed. However, the migration unit 5 has been configured so that the waveguide width of the SiN waveguide 3 (the second tapered waveguide 30) is increased, and thus a high-order mode included in the propagating TM mode.

[0082] Therefore, the removal unit 6 provided in the rear stage of the migration unit 5 removes the high-order mode from the TM mode received from the migration unit 5 and propagates only the TM0 mode so that the interference of the high-order mode can be avoided.

[0083] Figure 3A is a graph illustrating an example of a relationship between the effective refractive indexes of the respective modes of the waveguide width (core thickness 0.4 μm) of the SiN waveguide 202 exhibited at the output portion 200B according to the conventional inter-waveguide migration structure 200. Also, it is assumed that the effective refractive indexes are calculated by using the finite element method.

[0084] First, in the conventional inter-waveguide migration structure 200, the thickness of the core of the SiN waveguide 202 at 1.55 μm of the C band is defined to be 0.4 μm, and the waveguide width of the portion of the SiN waveguide 202 in which the Si waveguide 201 is terminated is defined to be 0.9 μm. The waveguide width of the SiN waveguide 202 included in the conventional inter-waveguide migration structure 200 is 0.9 μm; thus, when referring to Figure 3A , it is found that the SiN waveguide 202 is a waveguide that propagates the TM0 mode and the TE0 mode and does not propagate a high-order mode higher than the TM1 mode and the TE1 mode. That is, the SiN waveguide 202 included in the conventional inter-waveguide migration structure 200 is a single-mode waveguide through which, for example, the TM0 mode propagates.

[0085] Figure 3B is a graph illustrating an example of a relationship between the effective refractive indexes of the respective modes of the waveguide width (core thickness 0.3 μm) of the SiN waveguide 3 at the output portion 5B according to the migration unit 5. Also, it is assumed that the effective refractive indexes are calculated by using the finite element method.

[0086] First, in the migration unit 104 according to the comparative example, the thickness of the core of the SiN waveguide 102 at 1.55 μm of the C band is defined to be 0.3 μm, and the waveguide width of the portion of the SiN waveguide 102 in which the Si waveguide 101 is terminated is defined to be 1.0 μm. The waveguide width of the SiN waveguide 102 of the migration unit 104 according to the comparative example is 1.0 μm; thus, when referring to Figure 3BAt this time, it is found that the SiN waveguide 102 is a waveguide that propagates TM0 mode and TE0 mode and does not propagate higher-order modes higher than TM1 mode and TE1 mode. That is, the SiN waveguide 102 included in the migration unit 104 according to the comparative example is a single-mode waveguide through which, for example, TM0 mode propagates.

[0087] In contrast to this, in the second tapered waveguide 30 corresponding to the output portion 5B of the migration unit 5 according to the present embodiment, the waveguide width of the second tapered waveguide 30 in which the first tapered waveguide 20 terminates is, for example, 1.8 μm. Therefore, when referring to Figure 3B , the output portion 5B of the migration unit 5 is a multi-mode waveguide through which TM0 mode, TE0 mode, and higher-order modes higher than TM1 mode and TE1 mode propagate. In contrast to this, the SiN waveguide 202 included in the conventional inter-waveguide migration structure 200 is a single-mode waveguide. Therefore, the SiN waveguide 3 corresponding to the output portion 5B of the migration unit 5 according to the present embodiment is a multi-mode waveguide, so that it can be said that the conventional SiN waveguide 202 is different from the SiN waveguide 3 according to the present embodiment.

[0088] Further, the waveguide width of the output portion 50B of the third tapered waveguide 50 included in the removal unit 6 according to the present embodiment is 1 μm; therefore, when referring to Figure 3B , the third tapered waveguide 50 is a single-mode waveguide that does not propagate higher-order modes corresponding to TM1 mode. Therefore, the third tapered waveguide 50 removes higher-order modes from the plurality of modes and propagates TM0 mode.

[0089] Figure 4 is a drawing that is an example of a calculation result of effective refractive indexes of TE0 mode and TM0 mode propagating at each cross-sectional position in the migration unit 5. The first cross-sectional position "0" corresponds to a cross-sectional portion of the input portion 5A of the migration unit 5, and is a cross-sectional portion taken along the line B-B illustrated in Figure 2B . The second cross-sectional position "1" corresponds to a cross-sectional portion of the output portion 5B of the migration unit 5, and is a cross-sectional portion taken along the line D-D illustrated in Figure 2D . The third cross-sectional position "0.5" corresponds to a cross-sectional portion of the intermediate portion 5C of the migration unit 5, and is a cross-sectional portion taken along the line C-C illustrated in Figure 2C . The wavelength of light propagating through the Si waveguide 2 and the SiN waveguide 3 is defined to be 1.55 μm. The effective refractive indexes are calculated using the finite element method. Further, at the time of calculation, the effective refractive indexes in a case where each waveguide is isolated are calculated.

[0090] At the output portion 5B of the migration unit 5 at which the Si waveguide 2 terminates and which corresponds to the second cross-sectional position "1", when referring to Figure 4at the time when the TM0 mode propagates through the Si waveguide 2 and the TM0 mode propagates through the SiN waveguide 3 is 0.087. Also, as Figure 8 As illustrated, the difference between the effective refractive indexes of the output portion 104B of the migration unit 104 according to the comparative example is 0.056. That is, the difference between the effective refractive indexes at the output portion 5B of the migration unit 5 according to the present embodiment is larger than the difference between the effective refractive indexes at the output portion 104B of the migration unit 104 according to the comparative example. As a result, in the migration unit 5, by increasing the difference between the effective refractive indexes, it is possible to reliably suppress the light loss caused by light scattering at the intermittent cross-sectional portion.

[0091] Also, the electric field component in the vertical direction of the TM0 mode at the output portion 104B of the migration unit 104 according to the comparative example is compared with the electric field component in the vertical direction of the TM0 mode at the output portion 5B of the migration unit 5 according to the present embodiment. The electric field component in the vertical direction is calculated by using the finite element method. Also, the core thickness of the SiN waveguide 102 included in the migration unit 104 according to the comparative example is defined as 0.3 μm, and the waveguide width of the SiN waveguide 102 at the output portion 104B is defined as 1.0 μm. The core thickness of the SiN waveguide 3 included in the migration unit 5 according to the present embodiment is defined as 0.3 μm, and the waveguide width of the SiN waveguide 3 at the output portion 5B is defined as 1.8 μm.

[0092] The proportion of the power localized in the Si waveguide 101 at the output portion 104B of the migration unit 104 according to the comparative example is 1.0%, and the proportion of the power localized in the Si waveguide 2 at the output portion 5B of the migration unit 5 according to the present embodiment is 0.7%. That is, at the output portion 5B of the migration unit 5 according to the present embodiment, a larger amount of optical power is localized in the SiN waveguide 3 than at the output portion 5B of the migration unit 104 according to the comparative example. As a result, at the output portion 5B of the migration unit 5, even if the Si waveguide is intermittently removed, a large portion of the power is localized in the SiN waveguide 3, so that it is possible to connect to the second straight waveguide 40 with low loss.

[0093] Further, by using the finite-difference time-domain method, the transition loss obtained when TM0 mode is input to the substrate optical waveguide element 100 according to the comparative example and the substrate optical waveguide element 1 according to the present embodiment was calculated. The transition loss can be calculated based on -10 log10 (power of TM0 output / power of TM0 input). The transition loss in the substrate optical waveguide element 100 according to the comparative example was 0.051 dB, while the transition loss in the substrate optical waveguide element 1 according to the present embodiment was 0.013 dB. Further, the transition loss in the transition unit 5 included in the substrate optical waveguide element 1 was 0.012 dB, and the transition loss in the removal unit 6 was 0.001 dB.

[0094] As a result, the transition unit 5 functions as a multimode waveguide, so that the transition loss is greatly improved. For example, in an optical path of an optical communication device in which the substrate optical waveguide element 1 (100) is built-in, a case in which a plurality of substrate optical waveguide elements 1 (100) (for example, 10 substrate optical waveguide elements 1 (100)) are built-in is assumed. The transition loss in the communication device in which the substrate optical waveguide element 100 according to the comparative example is built-in was 0.51 dB. In contrast to this, the transition loss in the communication device in which the substrate optical waveguide element 1 according to the present embodiment is built-in was 0.13 dB. As a result, with the communication device in which the substrate optical waveguide element 1 according to the present embodiment is built-in, the transition loss can be reduced by 0.38 dB compared to the case in which the communication device in which the substrate optical waveguide element 100 according to the comparative example is built-in.

[0095] Further, in the substrate optical waveguide elements 1, 100, the Si waveguide 2, 101 is formed by performing photolithography and etching on Si that is uniformly formed on the surface of a wafer. Then, for example, when the SiN waveguide 3, 102 is separately formed above the Si waveguide 2, 101, the SiN waveguide 3, 102 is formed on the Si waveguide 2, 101 by performing photolithography using a mask. However, there can be a case in which the positional relationship between the Si waveguide 2, 101 and the SiN waveguide 3, 102 is shifted due to mask shift.

[0096] If mask shift occurs between the Si waveguide 2, 101 and the SiN waveguide 3, 102, the distribution of the electric field at the output portion 104B of the transition unit 104 included in the substrate optical waveguide element 100 according to the comparative example changes, so that the transition loss increases. In contrast to this, at the output portion 5B of the transition unit 5 included in the substrate optical waveguide element 1 according to the present embodiment, the waveguide width of the SiN waveguide 3 (1.8 μm) is larger than the waveguide width according to the comparative example, so that the electric field can be concentrated in the SiN waveguide 3 by reducing the influence of the shift caused by mask shift.

[0097] To verify this effect, a case where the SiN waveguide 3 is displaced by 0.12 μm in a direction perpendicular to the direction of travel of light with respect to the Si waveguide 2 due to mask shift is assumed. Then, the transfer loss that occurs when the TM0 mode is input to the input portion 5A of the transfer unit 5 is calculated by using the finite-difference time-domain method. The transfer loss can be calculated based on -10 log10 (power of TM0 output / power of TM0 input).

[0098] The transfer loss of the transfer unit 104 included in the substrate-type optical waveguide element 100 according to the comparative example is 0.106 dB, while the transfer loss in the substrate-type optical waveguide element 1 according to the present embodiment is 0.015 dB. The transfer loss in the transfer unit 5 included in the substrate-type optical waveguide element 1 is 0.014 dB, and the transfer loss in the removal unit 6 is 0.001 dB. As a result, the SiN waveguide 3 included in the transfer unit 5 according to the present embodiment is a multimode waveguide having a wide waveguide width, so that the transfer loss can be greatly improved even if mask shift occurs, compared to the comparative example.

[0099] Moreover, in the case where mask shift occurs, asymmetry is generated in the waveguide. At this time, the polarization plane is tilted so that polarization conversion between the TE0 mode and the TM0 mode occurs in the discontinuous portion. If polarization conversion occurs when the amount of transmission is increased by adding different signals to the TE0 mode and the TM0 mode, crosstalk occurs between the polarizations, and thus, the quality of the signal deteriorates (bit error increases). Moreover, if waveguide transfer is performed in two portions, wavelength ripples occur when the polarization before conversion is returned after one polarization conversion has been performed due to interference with the electric field that has not undergone polarization conversion.

[0100] However, with the substrate-type optical waveguide element 1 according to the present embodiment, at the output portion 5B of the transfer unit 5, which is a discontinuous portion, the electric field can be confined in the SiN waveguide 3, so that the effect caused by displacement of the relative position with the Si waveguide 2 can be reduced. To verify this effect, a case where the SiN waveguide 3 is displaced by 0.12 μm in a direction perpendicular to the direction of travel of light with respect to the Si waveguide 2 due to mask shift is assumed. Then, the transmittance of the TE0 mode obtained when the TM0 mode is input to the transfer unit 5 is calculated by using the finite-difference time-domain method. The transmittance can be calculated based on 10 log10 (power of TE0 output / power of TM0 input).

[0101] The transmittance of the migration unit 104 of the substrate-type optical waveguide element 100 according to the comparative example was -18.93 dB, while the transmittance of the substrate-type optical waveguide element 1 according to the present embodiment was -39.47 dB. As a result, the substrate-type optical waveguide element 1 according to the present embodiment can greatly suppress the effect of polarization conversion compared to the comparative example.

[0102] Furthermore, in the present embodiment, the electric field can be confined to the SiN waveguide 3 in the discontinuous portion, so that the reflection occurring in the discontinuous Si portion can also be reduced.

[0103] With the migration unit 5 according to the present embodiment, the case where the difference in the effective refractive index is ensured by changing the waveguide width has been described as an example, but due to the influence of the boundary condition of the electric field, the change in the effective refractive index when the waveguide width is changed is smaller in the TM0 mode than in the TE0 mode. In the TE0 mode, the electric field is discontinuously held on the waveguide side wall corresponding to the boundary between the Si waveguide 2 and the cladding 4 and the boundary between the SiN waveguide 3 and the cladding 4, so that if the waveguide width is changed, the effect of the discontinuous electric field distribution is strongly exerted. In contrast to this, in the TM0 mode, although there is a discontinuous electric field distribution in the thickness direction of the waveguide, there is a continuous electric field distribution in the width direction of the waveguide, so that the effect of the discontinuous electric field distribution is smaller in the width direction. Therefore, compared to the case of the TE0 mode, it is difficult to ensure the difference in the effective refractive index by changing the waveguide width for the TM0 mode. Therefore, in the present embodiment, in order to ensure that there is a sufficient difference in the effective refractive index even for the TM0 mode, the waveguide width is increased to the extent of functioning as a multimode waveguide related to the TM. Furthermore, as a result of removing the influence of the unit 6, it is possible to avoid the influence exerted on the TM0 mode that occurs in the multimode waveguide.

[0104] Furthermore, in the substrate-type optical waveguide element 1 according to the present embodiment, generally, the Si waveguide 2 and the SiN waveguide 3 are formed by performing photolithography and etching on a core material film formed uniformly on the surface of a wafer. Therefore, by performing this process by adjusting only the width of the waveguide, it is possible to simplify the process of forming the waveguide.

[0105] Examples of the second waveguide as the migration destination of the migration unit 5 and the first waveguide as the migration source can include a planar lightwave circuit (PLC) in which both the core and the cladding 4 are made of SiO2, an InP waveguide, and a GaAs waveguide. The core can be Si or Si3N4, the lower cladding can be SiO2, and the upper cladding can be SiO2, air, or the like. Also, this can be applicable when the material refractive index of the waveguide provided at the migration destination is higher than the material refractive index of the waveguide provided at the migration source. For example, in the case of the PLC, it is also applicable to change the material refractive indices at the migration source and the migration destination by changing the amount of doping in the glass waveguide.

[0106] In the case of the PLC, the material refractive index can be changed by changing the amount of doping in the core. In the case of the SiN waveguide 3 and the Si waveguide 2, the difference in the relative refractive index is large, so that the light is strongly confined, and as a result, even if the radius R is small, a curved waveguide with low loss can be realized, and thus the size of the substrate-type optical waveguide element 1 can be reduced.

[0107] The structure of each of the Si waveguide 2 and the SiN waveguide 3 can be a rib waveguide, a ridge waveguide, a channel waveguide, and appropriate modifications are possible. If the structure of each of the Si waveguide 2 and the SiN waveguide 3 is a rib waveguide, light also leaks to the plate portion, the influence of the rough sidewall of the core is small, and light loss can be suppressed. If the structure of each of the Si waveguide 2 and the SiN waveguide 3 is a channel waveguide, the confinement of light is strong, so that the waveguide can be sharply bent, and thus the size of the substrate-type optical waveguide element 1 can be reduced. The cladding 4 can be made of any material as long as the material refractive index is smaller than the refractive index of the core, and appropriate modifications are possible.

[0108] The case in which the substrate-type optical waveguide element 1 according to the present embodiment is a silicon optical waveguide in which the Si waveguide 2 made of Si as a material and the cladding 4 made of SiO2 as a material are constituted has been described as an example. However, a PLC in which the material of each of the Si waveguide 2 and the cladding 4 is SiO2, an InP waveguide, and a GaAs waveguide are also applicable.

[0109] Figure 5 is a drawing illustrating an example of an optical communication device 80 in which the substrate-type optical waveguide element 1 according to the present embodiment is built in. The optical communication device 80 will be described below. Figure 5The illustrated optical communication device 80 is connected to an optical fiber provided on an output side and an optical fiber provided on an input side. The optical communication device 80 includes a digital signal processor (DSP) 81, a light source 82, a light transmitter 83, and a light receiver 84. The DSP 81 is an electric component that performs digital signal processing. The DSP 81 performs processing such as encoding of transmission data and the like, generates an electric signal including the transmission data, and outputs the generated electric signal to the light transmitter 83. Also, the DSP 81 acquires an electric signal including reception data from the light receiver 84, and obtains the reception data by performing, for example, decoding processing of the acquired electric signal.

[0110] The light source 82, for example, includes a laser diode or the like, generates light having a predetermined wavelength, and supplies the generated light to the light transmitter 83 and the light receiver 84. The light transmitter 83 is an optical device that modulates light supplied from the light source 82 by using an electric signal output from the DSP 81, and outputs the obtained transmission light to an optical fiber. The light transmitter 83 modulates the light supplied from the light source 82 with an electric signal input to the light modulator while the light propagates in a waveguide, thereby generating transmission light.

[0111] The light receiver 84 receives an optical signal from an optical fiber, and demodulates reception light by using light supplied from the light source 82. Then, the light receiver 84 converts the demodulated reception light into an electric signal, and outputs the converted electric signal to the DSP 81. In each of the light transmitter 83 and the light receiver 84, a substrate type optical waveguide element 1 that functions as a waveguide through which light propagates is built in.

[0112] In the migration unit 5 of the substrate type optical waveguide element 1 included in the optical communication device 80, the structure is configured so that the waveguide width of the SiN waveguide 3 (the second tapered waveguide 30) is increased to, for example, 1.8 μm, so that it is possible to suppress the influence caused by the discontinuous portion with respect to the TM0 mode.

[0113] Also, in the removal unit 6 included in the substrate type optical waveguide element 1 included in the optical communication device 80, a high-order mode is removed from the TM mode received from the migration unit 5, and only the TM0 mode is propagated, so that it is possible to avoid the interference of the high-order mode.

[0114] Also, for the convenience of description, the case where the removal unit 6 according to the present embodiment includes the second straight waveguide 40 and the third tapered waveguide 50 has been described as an example. However, the removal unit 6 can include only the third tapered waveguide 50, and in this case, it is possible to include the second tapered waveguide 30 included in the output portion 5B of the migration unit 5 and the third tapered waveguide 50 on which light is coupled, and appropriate modification is possible.

[0115] According to an aspect of an embodiment, an optical device or the like capable of suppressing light loss caused by indirect migration between different waveguides can be provided.

Claims

1. An optical device in which light in TM mode propagates through the optical device via indirect migration between a first waveguide disposed on a first layer and having a first material refractive index and a second waveguide disposed on a second layer different from the first layer and having a second material refractive index lower than the first material, the first waveguide comprising a first tapered waveguide in which the waveguide width gradually decreases according to the direction of light propagation. The second waveguide includes: A second tapered waveguide, in which optical coupling to the first tapered waveguide is achieved based on the indirect migration, and the waveguide width gradually increases according to the direction of light travel. A third tapered waveguide is optically coupled to the second tapered waveguide, and the waveguide width gradually decreases according to the direction of light travel. The optical device includes: A migration unit comprising an input portion of a second tapered waveguide having a single-mode waveguide through which TM0 light with maximum effective refractive index propagates, and an output portion of the second tapered waveguide having a multimode waveguide through which the TM0 light and light in higher-order TM modes propagate, the input portions of the first and second tapered waveguides overlapping, with a first space between them, and the output portions of the first and second tapered waveguides overlapping, with a second space between them, such that the magnitude relationship between the effective refractive index of the TM mode propagating through the first waveguide and the effective refractive index of the TM mode propagating through the second waveguide is reversed at the input portions of the first and second tapered waveguides and at the output portions of the first and second tapered waveguides; and The removal unit removes light in the higher-order TM mode from the light in the TM mode received from the second tapered waveguide through the third tapered waveguide and allows the TM0 light from the second tapered waveguide to propagate through the third tapered waveguide, and includes the third tapered waveguide as the single-mode waveguide through which the TM0 light propagates.

2. The optical device according to claim 1, wherein, Light in the TM mode and the TE mode orthogonal to the TM mode propagates through the optical device via indirect migration between the first waveguide and the second waveguide.

3. The optical device according to claim 1, wherein, In the migration unit, the magnitude relationship between the effective refractive index of the TM mode propagating through the first tapered waveguide when the first tapered waveguide is an isolated waveguide and the effective refractive index of the TM mode propagating through the second tapered waveguide when the second tapered waveguide is an isolated waveguide is reversed at the input portion of the first tapered waveguide and the input portion of the second tapered waveguide, and at the output portion of the first tapered waveguide and the output portion of the second tapered waveguide.

4. The optical device according to claim 3, wherein, The migration unit has the following structure: the waveguide width of the second tapered waveguide located at the output portion of the migration unit is greater than the waveguide width of the first tapered waveguide located at the input portion of the migration unit.

5. The optical device according to any one of claims 1 to 4, wherein, The first waveguide, covered by the cladding on the substrate, is made of a material including silicon (Si). The second waveguide, covered by the cladding on the substrate, is made of a material including silicon nitride (SiN), and The cladding is made of a material including SiO2.

6. The optical device according to any one of claims 1 to 4, wherein, The first waveguide and the second waveguide are ribbed waveguides.

7. A substrate-type optical waveguide element, wherein light in TM mode propagates through the substrate-type optical waveguide element via indirect migration between a first waveguide disposed on a first layer and having a first material refractive index, and a second waveguide disposed on a second layer different from the first layer and having a second material refractive index lower than the first material. The first waveguide includes a first tapered waveguide, wherein the waveguide width gradually decreases according to the direction of light propagation. The second waveguide includes: A second tapered waveguide, in which optical coupling to the first tapered waveguide is achieved based on the indirect migration, and the waveguide width gradually increases according to the direction of light travel. A third tapered waveguide is optically coupled to the second tapered waveguide, and the waveguide width gradually decreases according to the direction of light travel. The substrate-type optical waveguide element includes: A migration unit comprising an input portion of a second tapered waveguide having a single-mode waveguide through which TM0 light with maximum effective refractive index propagates, and an output portion of the second tapered waveguide having a multimode waveguide through which the TM0 light and light in higher-order TM modes propagate, the input portions of the first and second tapered waveguides overlapping, with a first space between them, and the output portions of the first and second tapered waveguides overlapping, with a second space between them, such that the magnitude relationship between the effective refractive index of the TM mode propagating through the first waveguide and the effective refractive index of the TM mode propagating through the second waveguide is reversed at the input portions of the first and second tapered waveguides and at the output portions of the first and second tapered waveguides; and The removal unit removes light in the higher-order TM mode from the light in the TM mode received from the second tapered waveguide through the third tapered waveguide and allows the TM0 light from the second tapered waveguide to propagate through the third tapered waveguide, and includes the third tapered waveguide as the single-mode waveguide through which the TM0 light propagates.

8. An optical communication device, the optical communication device comprising: light source; An optical transmitter that uses a transmission signal to perform optical modulation on light received from the light source and transmits the transmitted light; An optical receiver that uses light received from the light source to receive a received signal from the received light; as well as A substrate-type optical waveguide element, which functions as a waveguide for allowing light to propagate through the optical transmitter and the optical receiver, wherein... Light in TM mode propagates through the substrate-type optical waveguide element via indirect migration between a first waveguide disposed on a first layer and having a first material refractive index, and a second waveguide disposed on a second layer different from the first layer and having a second material refractive index lower than the first material. The first waveguide includes a first tapered waveguide in which the waveguide width gradually decreases according to the direction of light propagation. The second waveguide includes: A second tapered waveguide, in which optical coupling to the first tapered waveguide is achieved based on the indirect migration, and the waveguide width gradually increases according to the direction of light travel. A third tapered waveguide is optically coupled to the second tapered waveguide, and the waveguide width gradually decreases according to the direction of light travel. The substrate-type optical waveguide element includes: A migration unit comprising an input portion of a second tapered waveguide having a single-mode waveguide through which TM0 light with maximum effective refractive index propagates, and an output portion of the second tapered waveguide having a multimode waveguide through which the TM0 light and light in higher-order TM modes propagate, the input portions of the first and second tapered waveguides overlapping, with a first space between them, and the output portions of the first and second tapered waveguides overlapping, with a second space between them, such that the magnitude relationship between the effective refractive index of the TM mode propagating through the first waveguide and the effective refractive index of the TM mode propagating through the second waveguide is reversed at the input portions of the first and second tapered waveguides and at the output portions of the first and second tapered waveguides; and The removal unit removes light in the higher-order TM mode from the light in the TM mode received from the second tapered waveguide through the third tapered waveguide and allows the TM0 light from the second tapered waveguide to propagate through the third tapered waveguide, and includes the third tapered waveguide as the single-mode waveguide through which the TM0 light propagates.

9. A waveguide migration method, said waveguide migration method being used to enable light in TM mode to propagate via indirect migration between a first waveguide disposed on a first layer and having a first material refractive index and a second waveguide disposed on a second layer different from the first layer and having a second material refractive index lower than the first material, the first waveguide comprising a first tapered waveguide, wherein the waveguide width gradually decreases according to the direction of light travel. The second waveguide includes: A second tapered waveguide, in which optical coupling to the first tapered waveguide is achieved based on the indirect migration, and the waveguide width gradually increases according to the direction of light travel. A third tapered waveguide is optically coupled to the second tapered waveguide, and the waveguide width gradually decreases according to the direction of light travel. The waveguide migration method is used to perform the following processes: The input portions of the first tapered waveguide and the second tapered waveguide overlap, with a first space between them. The output portions of the first tapered waveguide and the second tapered waveguide overlap, with a second space between them. The input portion of the second tapered waveguide serves as a single-mode waveguide through which TM0 light with the maximum effective refractive index propagates. The output portion of the second tapered waveguide serves as a multimode waveguide through which TM0 light and light in higher-order TM modes propagate, such that the relationship between the effective refractive index of the TM mode propagating through the first waveguide and the effective refractive index of the TM mode propagating through the second waveguide is reversed at the input portions of the first and second tapered waveguides and at the output portions of the first and second tapered waveguides. The light in the higher-order TM mode is removed from the light in the TM mode received from the second tapered waveguide through the third tapered waveguide, the third tapered waveguide serving as the single-mode waveguide through which the TM0 light propagates; as well as The TM0 light from the second tapered waveguide is propagated through the third tapered waveguide.

Images