Optical devices and optical communication equipment

Abstract

To provide an optical device and the like capable of suppressing coupling loss with an optical fiber.SOLUTION: An optical device comprises two first waveguides arranged side by side on a substrate, and one second waveguide arranged on the substrate so as to be side by side with and away from the first waveguides. The first waveguide includes a first tapered waveguide and a second tapered waveguide. The second waveguide includes a third tapered waveguide disposed side by side with the first waveguide, and a third waveguide. The waveguide width of the first tapered waveguide is wider as it is closer to the second tapered waveguide, the waveguide width of the second tapered waveguide is narrower as it is farther away from the first tapered waveguide, and the waveguide width of the third tapered waveguide is wider as it is closer to the third waveguide. The first waveguide has a structure constituted such that a first gap between the two first waveguides at the starting point of the first tapered waveguide is made wider than a second gap between the two first waveguides at a connection portion between the first tapered waveguide and the second tapered waveguide.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an optical device and an optical communication apparatus.

Background Art

[0002] In recent years, with the increase in communication capacity, the demand for optical fiber communication has been increasing, and thus small optical devices for converting electrical signals into optical signals are being used. Therefore, in recent years, the development of ultra-small substrate-type optical waveguide elements (simply referred to as optical devices) typified by silicon photonics has been actively carried out. In an optical device, it is possible to integrate two or more waveguides made of different materials on the same chip.

[0003] Since the optical components constituting the optical device have different characteristics obtained depending on the material refractive index, for example, by using a waveguide of an appropriate material for each optical component, the characteristics of the optical device can be improved. Therefore, in an optical device using waveguides of different materials, there is a structure in which light indirectly transitions between different waveguides.

[0004] FIG. 14 is an explanatory diagram showing an example of a conventional optical device 200. The optical device 200 shown in FIG. 14 is a substrate-type optical waveguide element that optically couples with the core FC of an optical fiber. The optical device 200 has, for example, a waveguide (simply referred to as a SiN (Silicon Nitride) waveguide) 201 covered with a SiO2 cladding 211 and, for example, a waveguide (simply referred to as a Si waveguide) 202 covered with the cladding 211. The optical device 200 has an adiabatic conversion section 203 in which light indirectly transitions between the Si waveguide 202 and the SiN waveguide 201.

[0005] The SiN waveguide 201 is a linear waveguide having a constant waveguide width from a starting point X201 to an ending point X202. The starting point X201 of the SiN waveguide 201 starts from the chip end face D1 of the optical device 200 that optically couples with the core FC of the optical fiber.

[0006] The Si waveguide 202 comprises a tapered waveguide 202A and a straight waveguide 202B. The tapered waveguide 202A is a waveguide having a tapered structure in which the waveguide width gradually widens from the starting point Y201 to the ending point Y202. The straight waveguide 202B is a waveguide in which the waveguide width is constant from the starting point Y202 to the ending point Y203. The Si waveguide 202 is constructed by optically coupling the ending point Y202 of the tapered waveguide 202A with the starting point Y202 of the straight waveguide 202B. The ending point Y203 of the straight waveguide 202B of the Si waveguide 202 is the tip end face D2 facing the tip end face D1 of the optical device 200.

[0007] At the starting point, the adiabatic conversion section 203 is in a state where the midpoint X203 of the SiN waveguide 201 and the starting point Y201 of the tapered waveguide 202A within the Si waveguide 202 are separated. At the ending point, the adiabatic conversion section 203 is in a state where the ending point X202 of the SiN waveguide 201 and the ending point Y202 of the tapered waveguide 202A within the Si waveguide 202 are separated. The adiabatic conversion section 203 has a structure in which the SiN waveguide 201 is superimposed on the Si waveguide 202 via the cladding 211.

[0008] Figure 15A is an explanatory diagram showing an example of a approximate cross-sectional portion of line AA shown in Figure 14. The approximate cross-sectional portion shown in Figure 15A is a cross-sectional area of ​​an optical device 200 in which a SiN waveguide 201 is arranged. The optical device 200 has a Si substrate 212, a cladding 211 stacked on the Si substrate 212, and a SiN waveguide 201 arranged within the cladding 211.

[0009] Figure 15B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 14. The substantially cross-sectional portion shown in Figure 15B is a cross-sectional area of ​​the optical device 200 in which the adiabatic conversion section 203 is located. The optical device 200 has a Si substrate 212, a cladding 211 stacked on the Si substrate 212, a SiN waveguide 201 arranged within the cladding 211, and a tapered waveguide 202A arranged within the cladding 211 beneath the SiN waveguide 201.

[0010] Figure 15C is an explanatory diagram showing an example of a approximate cross-sectional portion of the CC line shown in Figure 14. The approximate cross-sectional portion shown in Figure 15C is a cross-sectional area of ​​the optical device 200 in which the straight waveguide 202B of the Si waveguide 202 is arranged. The optical device 200 has a Si substrate 212, a cladding 211, and a straight waveguide 202B arranged within the cladding 211.

[0011] Then, in the adiabatic conversion section 203, light gradually transitions adiabatically between the tapered waveguide 202A and the SiN waveguide 201 within the Si waveguide 202.

[0012] In the conventional adiabatic conversion section 203 within the optical device 200, the waveguide width of the Si waveguide 202 changes in a tapered manner, and since the SiN waveguide 201 has a lower refractive index than the Si waveguide 202, the optical mode field is enlarged to approach that of the optical fiber mode field. As a result, coupling loss with the optical fiber can be reduced. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2014-191301 [Patent Document 2] U.S. Patent Application Publication No. 2019 / 0154919 [Patent Document 3] U.S. Patent No. 10429582 [Patent Document 4] Japanese Patent Publication No. 2011-22464 [Overview of the project] [Problems that the invention aims to solve]

[0014] However, in conventional optical devices 200, the mode field of the SiN waveguide 201 is smaller than that of the optical fiber. Therefore, in optical devices 200, coupling loss occurs between the SiN waveguide 201 and the optical fiber due to a mismatch in their mode fields.

[0015] One aspect of this is the objective to provide optical devices that can suppress coupling loss with optical fibers. [Means for solving the problem]

[0016] One embodiment of the optical device comprises two first waveguides arranged in parallel on a substrate, and one second waveguide arranged on the substrate parallel to and spaced apart from the first waveguides. The first waveguide has a first tapered waveguide and a second tapered waveguide connected to the first tapered waveguide. The second waveguide has a third tapered waveguide running parallel to the first waveguide and a third waveguide connected to the opposite side of the third tapered waveguide from the side where the first tapered waveguide is provided. The first tapered waveguide is structured so that its waveguide width gradually widens as it approaches the second tapered waveguide. The second tapered waveguide is structured so that its waveguide width gradually narrows as it moves away from the first tapered waveguide. The third tapered waveguide is structured so that its waveguide width gradually widens as it approaches the third waveguide. The first waveguide is structured such that the first distance between the two first waveguides at the starting point of the first tapered waveguide is wider than the second distance between the two first waveguides at the connection point between the first tapered waveguide and the second tapered waveguide. [Effects of the Invention]

[0017] From one perspective, coupling loss between the first waveguide and the optical fiber that optically couples can be suppressed. [Brief explanation of the drawing]

[0018] [Figure 1] Figure 1 is an explanatory diagram showing an example of an optical device in Example 1. [Figure 2A] Figure 2A is an explanatory diagram showing an example of a approximate cross-sectional portion of line AA shown in Figure 1. [Figure 2B] Figure 2B is an explanatory diagram showing an example of a approximate cross-sectional portion of the BB line shown in Figure 1. [Figure 2C] Figure 2C is an explanatory diagram showing an example of a schematic cross-sectional portion of the C-C line shown in FIG. 1. [Figure 3] Figure 3 is an explanatory diagram showing an example of the optical device of Example 2. [Figure 4A] Figure 4A is an explanatory diagram showing an example of a schematic cross-sectional portion of the A-A line shown in FIG. 3. [Figure 4B] Figure 4B is an explanatory diagram showing an example of a schematic cross-sectional portion of the B-B line shown in FIG. 3. [Figure 4C] Figure 4C is an explanatory diagram showing an example of a schematic cross-sectional portion of the C-C line shown in FIG. 3. [Figure 5] Figure 5 is an explanatory diagram showing an example of the optical device of Example 3. [Figure 6A] Figure 6A is an explanatory diagram showing an example of a schematic cross-sectional portion of the A-A line shown in FIG. 5. [Figure 6B] Figure 6B is an explanatory diagram showing an example of a schematic cross-sectional portion of the B-B line shown in FIG. 5. [Figure 6C] Figure 6C is an explanatory diagram showing an example of a schematic cross-sectional portion of the C-C line shown in FIG. 5. [Figure 7] Figure 7 is an explanatory diagram showing an example of an optical communication device incorporating an optical device. [Figure 8] Figure 8 is an explanatory diagram showing an example of the optical device of Comparative Example 1. [Figure 9A] Figure 9A is an explanatory diagram showing an example of a schematic cross-sectional portion of the A-A line shown in FIG. 8. [Figure 9B] Figure 9B is an explanatory diagram showing an example of a schematic cross-sectional portion of the B-B line shown in FIG. 8. [Figure 9C] Figure 9C is an explanatory diagram showing an example of a schematic cross-sectional portion of the C-C line shown in FIG. 8. [Figure 10] Figure 10 is an explanatory diagram showing an example of the optical device of Comparative Example 2. [Figure 11A] Figure 11A is an explanatory diagram showing an example of a schematic cross-sectional portion of the A-A line shown in FIG. 10. [Figure 11B] Figure 11B is an explanatory diagram showing an example of a schematic cross-sectional portion of the B-B line shown in FIG. 10. [Figure 11C] Figure 11C is an explanatory diagram showing an example of a approximate cross-sectional portion of the CC line shown in Figure 10. [Figure 12] Figure 12 is an explanatory diagram showing an example of an optical device in Comparative Example 3. [Figure 13A] Figure 13A is an explanatory diagram showing an example of a approximate cross-sectional portion of line AA shown in Figure 12. [Figure 13B] Figure 13B is an explanatory diagram showing an example of a approximate cross-sectional portion of the BB line shown in Figure 12. [Figure 13C] Figure 13C is an explanatory diagram showing an example of a approximate cross-sectional portion of the CC line shown in Figure 12. [Figure 14] Figure 14 is an explanatory diagram showing an example of a conventional optical device. [Figure 15A] Figure 15A is an explanatory diagram showing an example of a approximate cross-sectional portion of line AA shown in Figure 14. [Figure 15B] Figure 15B is an explanatory diagram showing an example of a approximate cross-sectional portion of the BB line shown in Figure 14. [Figure 15C] Figure 15C is an explanatory diagram showing an example of a approximate cross-sectional portion of the CC line shown in Figure 14. [Modes for carrying out the invention]

[0019] <Comparative Example 1> Figure 8 is an explanatory diagram showing an example of the optical device 100 of Comparative Example 1. The optical device 100 shown in Figure 8 is a substrate-type optical waveguide element that optically couples with the core FC of an optical fiber. The optical device 100 has a SiN waveguide 101, a Si waveguide 102, and a cladding 111 that covers the Si waveguide 102 and the SiN waveguide 101. Furthermore, the optical device 100 has an adiabatic conversion section 103 that optically couples the Si waveguide 102 and the SiN waveguide 101 by indirect transition. The SiN waveguide 101 is formed of, for example, Si3N4 (hereinafter simply referred to as SiN), and the refractive index of the SiN material is 1.99 when the optical wavelength is 1.55 μm. The Si waveguide 102 is formed of, for example, Si, and the refractive index of the Si material is 3.48 when the optical wavelength is 1.55 μm. Cladding 111 is formed from, for example, SiO2, and the refractive index of SiO2 is 1.44 when the light wavelength is 1.55 μm.

[0020] The SiN waveguide 101 comprises two first straight waveguides 101A and a first tapered waveguide 101B that optically couples with the two first straight waveguides 101A. The first straight waveguides 101A are waveguides in which the waveguide width is constant from the starting point X101 to the ending point X102. The first tapered waveguide 101B is a waveguide having a tapered structure in which the waveguide width gradually narrows from the ending point X102 to the ending point X103 of the first straight waveguide 101A. The waveguide width at the starting point X102 of the first tapered waveguide 101B is wider than the waveguide width at the ending point X103 of the first tapered waveguide 101B. The core thickness of the first straight waveguide 101A and the first tapered waveguide 101B are the same. The starting point X101 of the SiN waveguide 101 is the tip end face D1 of the optical device 100 that optically couples with the optical fiber core FC.

[0021] The Si waveguide 102 includes a second tapered waveguide 102A and a second straight waveguide 102B that is optically coupled to the second tapered waveguide 102A. The second tapered waveguide 102A is a waveguide having a tapered structure in which the waveguide width gradually widens from the starting point Y101 to the ending point Y102. The second straight waveguide 102B is a waveguide with a constant waveguide width from the starting point Y102 to the ending point Y103. The core thickness of the second straight waveguide 102B and the second tapered waveguide 102A are the same. The ending point Y103 of the second straight waveguide 102B of the Si waveguide 102 is the tip end face D2 facing the tip end face D1 of the optical device 100.

[0022] The adiabatic conversion section 103 is constructed by placing the second tapered waveguide 102A above the first tapered waveguide 101B, with the two waveguides separated vertically. The distance between the first tapered waveguide 101B and the second tapered waveguide 102A is kept constant.

[0023] The adiabatic conversion section 103 has a starting point X102 (Y101), an ending point X103 (Y102), and an intermediate section between the starting point and the ending point. Figure 9A is an explanatory diagram showing an example of a substantially cross-sectional portion of line AA shown in Figure 8. The substantially cross-sectional portion of line AA shown in Figure 9A is a cross-sectional area of ​​the optical device 100 in which two first straight waveguides 101A of the SiN waveguide 101 are arranged. The optical device 100 has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, and two first straight waveguides 101A arranged within the cladding 111.

[0024] Figure 9B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 8. The substantially cross-sectional portion of the BB line shown in Figure 9B is a cross-sectional area of ​​the optical device 100 in which the adiabatic conversion section 103 is located. The optical device 100 has a Si substrate 112, a cladding 111 laminated on the Si substrate 112, a first tapered waveguide 101B located within the cladding 111, and a second tapered waveguide 102A located within the cladding 111. At the starting point of the adiabatic conversion section 103, the waveguide width of the first tapered waveguide 101B is wider than that of the second tapered waveguide 102A. The distance between the first tapered waveguide 101B and the second tapered waveguide 102A is the same. At the end of the adiabatic conversion section 103, the waveguide width of the first tapered waveguide 101B is made narrower compared to the waveguide width of the second tapered waveguide 102A.

[0025] Figure 9C is an explanatory diagram showing an example of a substantially cross-sectional portion of the CC line shown in Figure 8. The substantially cross-sectional portion of the CC line shown in Figure 9C is a cross-sectional area of ​​the optical device 100 in which the second straight waveguide 102B within the Si waveguide 102 is located. The optical device 100 has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, and the second straight waveguide 102B within the Si waveguide 102 located within the cladding 111.

[0026] The adiabatic conversion section 103 has a structure where the waveguide width of the first tapered waveguide 101B is wide and the waveguide width of the second tapered waveguide 102A is narrow at the starting point, and at the ending point, the waveguide width of the first tapered waveguide 101B is narrow and the waveguide width of the second tapered waveguide 102A is wide. In other words, the waveguide width of the first tapered waveguide 101B gradually narrows from the starting point X102 to the ending point X103, and the waveguide width of the second tapered waveguide 102A gradually widens from the starting point Y101 to the ending point Y102. Generally, the wider the waveguide width of a waveguide, the stronger the optical confinement in the core, and therefore the effective refractive index increases due to the influence of the refractive index of the core material.

[0027] In the optical device 100 of Comparative Example 1, the SiN waveguide 101 that optically couples with the optical fiber core FC is configured as two separate first straight waveguides 101A. As a result, compared to the conventional optical device 200, the mode field of the SiN waveguide 101 can be brought closer to the mode field of the optical fiber, thereby suppressing coupling loss with the optical fiber.

[0028] Furthermore, in the adiabatic conversion section 103 of the optical device 100, two first straight waveguides 101A are optically coupled to one first tapered waveguide 101B, and light is adiabatically transitioned from the first tapered waveguide 101B to the second tapered waveguide 102A of the Si waveguide.

[0029] However, at the discontinuity in the SiN waveguide 101 where one first tapered waveguide 101B and two first straight waveguides 101A are optically coupled, the optical mode field changes abruptly, resulting in optical radiation loss and reflection loss. Therefore, the optical device 100A of Comparative Example 2 is conceivable to address this situation.

[0030] <Comparative Example 2> Figure 10 is an explanatory diagram showing an example of the optical device 100A of Comparative Example 2. The optical device 100A shown in Figure 10 is a substrate-type optical waveguide element that optically couples with the core FC of an optical fiber. The optical device 100A has a SiN waveguide 101, a Si waveguide 102, and a cladding 111 that covers the Si waveguide 102 and the SiN waveguide 101. Furthermore, the optical device 100A has an adiabatic conversion section 103A that optically couples the Si waveguide 102 and the SiN waveguide 101 by indirect transition.

[0031] The SiN waveguide 101 has two straight waveguides 101C, the waveguide width of which is constant from the starting point X101 to the ending point X102A. The starting point X101 of the SiN waveguide 101 is the tip end face D1 of the optical device 100 that optically couples with the optical fiber core FC.

[0032] The Si waveguide 102 includes a second tapered waveguide 102A and a second straight waveguide 102B that is optically coupled to the second tapered waveguide 102A. The second tapered waveguide 102A is a waveguide having a tapered structure in which the waveguide width gradually widens from the starting point Y101 to the ending point Y102. The second straight waveguide 102B is a waveguide with a constant waveguide width from the starting point Y102 to the ending point Y103. The core thickness of the second straight waveguide 102B and the second tapered waveguide 102A are the same. The ending point Y103 of the second straight waveguide 102B of the Si waveguide 102 is the tip end face D2 facing the tip end face D1 of the optical device 100.

[0033] The adiabatic conversion section 103A is constructed by placing a second tapered waveguide 102A between two straight waveguides 101C, with a gap between a portion of the straight waveguide 101C and the second tapered waveguide 102A, and positioning the second tapered waveguide 102A below the straight waveguide 101C. The gap between the straight waveguide 101C and the second tapered waveguide 102A is kept constant. In the adiabatic conversion section 103A, the second tapered waveguide 102A is placed between the two straight waveguides 101C without optical coupling. Even if the SiN waveguide 101 is not directly above the Si waveguide 102, the mode field straddles the two straight waveguides 101C, so light will transition adiabatically from the SiN waveguide 101 to the Si waveguide 102.

[0034] The adiabatic conversion section 103A has a starting point X102 (Y101), an ending point X103 (Y102), and an intermediate section between the starting point and the ending point. Figure 11A is an explanatory diagram showing an example of a substantially cross-sectional portion of line AA shown in Figure 10. The substantially cross-sectional portion of line AA shown in Figure 11A is a cross-sectional area of ​​the optical device 100A in which two straight waveguides 101C are arranged within the SiN waveguide 101. The optical device 100A has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, and two straight waveguides 101C arranged within the SiN waveguide 101 in the cladding 111.

[0035] Figure 11B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 10. The substantially cross-sectional portion of the BB line shown in Figure 11B is a cross-sectional area of ​​the optical device 100A in which the adiabatic conversion section 103A is located. The optical device 100A has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, two straight waveguides 101C arranged within the cladding 111, and a second tapered waveguide 102A arranged within the cladding 111. The adiabatic conversion section 103A has a structure in which the second tapered waveguide 102A runs parallel between the two straight waveguides 101C. The distance between the straight waveguides 101C and the second tapered waveguide 102A is kept constant.

[0036] Figure 11C is an explanatory diagram showing an example of a substantially cross-sectional portion of the CC line shown in Figure 10. The substantially cross-sectional portion of the CC line shown in Figure 11C is a cross-sectional area of ​​the optical device 100A in which the second straight waveguide 102B within the Si waveguide 102 is located. The optical device 100A has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, and a second straight waveguide 102B located within the cladding 111.

[0037] In the adiabatic conversion section 103A of Comparative Example 2, a second tapered waveguide 102A is placed between the two straight waveguides 101C without optical coupling, so that light is adiabatically transitioned from the straight waveguide 101C to the second tapered waveguide 102A. As a result, there are no discontinuities within the SiN waveguide 101, so the occurrence of radiation loss and reflection loss of light can be suppressed.

[0038] However, in the adiabatic conversion section 103A, the confinement of light in the two straight waveguides 101C is weak, resulting in a large radiation loss at the starting point Y101 of the second tapered waveguide 102A within the Si waveguide 102. As a result, it is necessary to ensure a certain length for the adiabatic conversion section 103A, which is arranged in parallel with the Si waveguide 102 and the SiN waveguide 101 at a distance from each other, thus increasing the size of the component. Therefore, to address this situation, the optical device 100B of Comparative Example 3 can be considered.

[0039] <Comparative Example 3> Figure 12 is an explanatory diagram showing an example of the optical device 100B of Comparative Example 3. The optical device 100B shown in Figure 12 is a substrate-type optical waveguide element that optically couples with the core FC of an optical fiber. The optical device 100B has a SiN waveguide 101, a Si waveguide 102, and a cladding 111 that covers the Si waveguide 102 and the SiN waveguide 101. Furthermore, the optical device 100B has an adiabatic conversion section 103B that optically couples the Si waveguide 102 and the SiN waveguide 101 by indirect transition.

[0040] The SiN waveguide 101 has two third tapered waveguides 101D and two fourth tapered waveguides 101E. The third tapered waveguide 101D is a waveguide with a structure that gradually widens from the starting point X101 to the ending point X102. The fourth tapered waveguide 101E is a waveguide with a structure that gradually narrows from the starting point X102 to the ending point X103. Optical coupling is performed between the third tapered waveguide 101D and the fourth tapered waveguide 101E by optical coupling the ending point X102 of the third tapered waveguide 101D and the starting point X102 of the fourth tapered waveguide 101E. The core thickness of the third tapered waveguide 101D and the fourth tapered waveguide 101E are the same. The starting point X101 of the SiN waveguide 101 is the tip end face D1 of the optical device 100 that optically couples with the optical fiber core FC.

[0041] The Si waveguide 102 includes a second tapered waveguide 102A and a second straight waveguide 102B that is optically coupled to the second tapered waveguide 102A. The second tapered waveguide 102A is a waveguide having a tapered structure in which the waveguide width gradually widens from the starting point Y101 to the ending point Y102. The second straight waveguide 102B is a waveguide with a constant waveguide width from the starting point Y102 to the ending point Y103. The core thickness of the second straight waveguide 102B and the second tapered waveguide 102A are the same. The ending point Y103 of the second straight waveguide 102B of the Si waveguide 102 is the tip end face D2 facing the tip end face D1 of the optical device 100.

[0042] The adiabatic conversion section 103B is constructed by placing the second tapered waveguide 102A below the fourth tapered waveguide 101E, with the second tapered waveguide 102A spaced apart between the two fourth tapered waveguides 101E. The distance between the fourth tapered waveguide 101E and the second tapered waveguide 102A is kept constant. In the adiabatic conversion section 103B, the second tapered waveguide 102A is placed between the two fourth tapered waveguides 101E. Even if the SiN waveguide 101 is not directly above the Si waveguide 102, the mode field straddles the two fourth tapered waveguides 101E, so light will transition adiabatically from the SiN waveguide 101 to the Si waveguide 102.

[0043] The adiabatic conversion section 103B has a starting point X102 (Y101), an ending point X103 (Y102), and an intermediate section between the starting point and the ending point. Figure 13A is an explanatory diagram showing an example of a substantially cross-sectional portion of line AA shown in Figure 12. The substantially cross-sectional portion of line AA shown in Figure 13A is a cross-sectional portion of the optical device 100B in which two third tapered waveguides 101D are arranged within the SiN waveguide 101. The optical device 100B has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, and two third tapered waveguides 101D arranged within the cladding 111.

[0044] Figure 13B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 12. The substantially cross-sectional portion of the BB line shown in Figure 13B is a cross-sectional area of ​​the optical device 100B in which the adiabatic conversion section 103B is located. The optical device 100B has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, a fourth tapered waveguide 101E located within the cladding 111, and a second tapered waveguide 102A located within the cladding 111. The adiabatic conversion section 103B has a structure in which the second tapered waveguide 102A runs parallel between two fourth tapered waveguides 101E. The distance between the fourth tapered waveguide 101E and the second tapered waveguide 102A is kept constant.

[0045] Figure 13C is an explanatory diagram showing an example of a approximate cross-sectional portion of the CC line shown in Figure 12. The approximate cross-sectional portion of the CC line shown in Figure 13C is a cross-sectional area of ​​the optical device 100B in which the second straight waveguide 102B within the Si waveguide 102 is located. The optical device 100B has a Si substrate 112, a cladding 111 stacked on the Si substrate 112, and a second straight waveguide 102B located within the cladding 111.

[0046] In the optical device 100B of Comparative Example 3, the waveguide widths of the two third tapered waveguides 101D within the SiN waveguide 101 are gradually widened, thereby strengthening the confinement of light. As a result, the radiation loss at the tip of the Si waveguide 3 at the starting point of the adiabatic conversion section 4 is reduced, allowing the length of the adiabatic conversion section 103B to be reduced. However, if the confinement of the SiN waveguide 101 remains strong, the conversion efficiency of the adiabatic conversion section 103B will vary depending on the wavelength and polarization, thus increasing the dependence of the conversion efficiency on wavelength and polarization.

[0047] Therefore, in the adiabatic conversion section 103B of the optical device 100B, the waveguide widths of the two fourth tapered waveguides 101E within the SiN waveguide 101 are gradually narrowed, thereby reducing the dependence of the conversion efficiency on wavelength and polarization.

[0048] However, in the optical device 100B of Comparative Example 3, the waveguide width of the two third tapered waveguides 101D within the SiN waveguide 101 is gradually narrowed, resulting in a structure where the distance between the starting points of the two third tapered waveguides 101D that optically couple with the optical fiber core FC is narrowed. As a result, the mode field of the optical device 100B at the chip end face D1 is small, the mode field of the optical fiber core FC is large, and the coupling efficiency of the optical device 100B with the optical fiber core FC is poor.

[0049] Therefore, an embodiment of the optical device 1 that resolves this situation will be described in detail with reference to the drawings. However, the present invention is not limited to this embodiment. Furthermore, the embodiments shown below may be combined as appropriate, as long as they do not cause inconsistencies. [Examples]

[0050] Figure 1 is an explanatory diagram showing an example of the optical device 1 of Embodiment 1. The optical device 1 shown in Figure 1 is an optical chip that incorporates a substrate-type optical waveguide element that optically couples with the core FC of an optical fiber. The optical device 1 has a SiN (Silicon Nitride) waveguide 2, a Si (Silicon) waveguide 3, and a cladding 11 that covers the Si waveguide 3 and the SiN waveguide 2. The optical device 1 has an adiabatic conversion unit 4 that transitions light between the Si waveguide 3 and the SiN waveguide 2 by an adiabatic indirect transition.

[0051] SiN waveguide 2 is a first waveguide formed from, for example, Si3N4 (hereinafter simply referred to as SiN). The refractive index of SiN is 1.99 when the wavelength of light is 1.55 μm. Si waveguide 3 is a second waveguide formed from, for example, Si. The refractive index of Si is 3.48 when the wavelength of light is 1.55 μm. The refractive index of Si is the second refractive index. The refractive index of SiN is smaller than that of Si. Cladding 11 is a layer formed from, for example, SiO2. The refractive index of SiO2 is 1.44 when the wavelength of light is 1.55 μm.

[0052] The SiN waveguide 2 comprises two first tapered waveguides 2A and two second tapered waveguides 2B that are optically coupled to the two first tapered waveguides 2A. The first tapered waveguides 2A are waveguides having a tapered structure in which the waveguide width gradually widens from the starting point X1 to the ending point X2. In other words, the first tapered waveguides 2A have a structure in which the waveguide width gradually widens as they move toward the second tapered waveguides. The second tapered waveguides 2B are waveguides having a tapered structure in which the waveguide width gradually narrows from the starting point X2 to the ending point X3. In other words, the second tapered waveguides 2B have a structure in which the waveguide width gradually narrows as they move toward the first tapered waveguides 2A. Optical coupling is performed between the first tapered waveguide 2A and the second tapered waveguide 2B by optically coupling the endpoint X2 of the first tapered waveguide 2A and the starting point X2 of the second tapered waveguide 2B. The core thickness of the first tapered waveguide 2A and the second tapered waveguide 2B are the same. The starting point X1 of the SiN waveguide 2 is the tip end face D1 of the optical device 1 that optically couples with the core FC of the optical fiber.

[0053] The line connecting the core center of the first tapered waveguide 2A at the starting point X1 and the core center of the first tapered waveguide 2A at the ending point X2 is defined as the first centerline CL1. Note that the ending point X2 of the first tapered waveguide 2A and the starting point X2 of the second tapered waveguide 2B are the same. The line connecting the core center of the second tapered waveguide 2B at the starting point X2 and the core center of the second tapered waveguide 2B at the ending point X3 is defined as the second centerline CL2.

[0054] The first interval L1 is defined as the distance between the core center of the starting point X1 of one first tapered waveguide 2A and the core center of the starting point X1 of the other first tapered waveguide 2A. In other words, the first interval L1 is the distance between the two first waveguides 2 at the starting point X1 of the first tapered waveguide 2A. The second interval L2 is defined as the distance between the core center of the ending point X2 of one first tapered waveguide 2A and the core center of the ending point X2 of the other first tapered waveguide 2A. In other words, the second interval L2 is the distance between the two first waveguides 2 at the connection point between the first tapered waveguide 2A and the second tapered waveguide 2B. The third interval L3 is defined as the distance between the core center of the ending point X3 of one second tapered waveguide 2B and the core center of the ending point X3 of the other second tapered waveguide 2B. Furthermore, the relationships between the first interval L1, the second interval L2, and the third interval L3 are L1>L2, L1>L3, and L2=L3. In other words, the first waveguide 2 has a structure in which the first interval L1 is wider than the second interval L2.

[0055] Optical device 1 widens the portion of the SiN waveguide 2 at the tip end face D1 that optically couples with the optical fiber core FC, and sets the distance between the two first tapered waveguides 2A to a first spacing L1 in order to bring it closer to the mode field of the optical fiber core FC. As a result, the mode field of optical device 1 at the tip end face D1 approaches the mode field of the optical fiber core FC, improving the coupling efficiency of optical device 1 with the optical fiber core FC.

[0056] The Si waveguide 3 comprises a third tapered waveguide 3A and a straight waveguide 3B that optically couples with the third tapered waveguide 3A. The third tapered waveguide 3A is a waveguide having a tapered structure in which the waveguide width gradually widens from the starting point Y1 to the ending point Y2. In other words, the third tapered waveguide 3A has a structure in which the waveguide width gradually widens as it approaches the straight waveguide 3B, which is the third waveguide. The straight waveguide 3B is a waveguide in which the waveguide width is constant from the starting point Y2 to the ending point Y3. In other words, the straight waveguide 3B is a waveguide that connects to the side of the third tapered waveguide 3A opposite to the side in which the first tapered waveguide 2A is provided. The core thickness of the third tapered waveguide 3A and the straight waveguide 3B are the same. The endpoint Y3 of the straight waveguide 3B of the Si waveguide 3 is the chip end face D2 of the optical device 1, which is opposite to the chip end face D1.

[0057] The adiabatic conversion unit 4 has two second tapered waveguides 2B within the SiN waveguide 2 and a third tapered waveguide 3A within the Si waveguide 3. The adiabatic conversion unit 4 is constructed by arranging the third tapered waveguide 3A between the two second tapered waveguides 2B, and arranging the third tapered waveguide 3A parallel to the second tapered waveguides 2B at a distance from each other. The distance between the second tapered waveguides 2B and the third tapered waveguide 3A is kept constant. In the adiabatic conversion unit 4, the third tapered waveguide 3A is arranged between the two second tapered waveguides 2B. Even if the SiN waveguide 2 is not directly above the Si waveguide 3, the mode field straddles the two second tapered waveguides 2B, so light will transition adiabatically from the SiN waveguide 2 to the Si waveguide 3.

[0058] The adiabatic conversion section 4 has a starting point X2 (Y1), an ending point X3 (Y2), and an intermediate section between the starting point and the ending point. Figure 2A is an explanatory diagram showing an example of a substantially cross-sectional portion of line AA shown in Figure 1. The substantially cross-sectional portion of line AA shown in Figure 2A is a cross-sectional area of ​​the optical device 1 in which two first tapered waveguides 2A are arranged within the SiN waveguide 2. The optical device 1 has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, and two first tapered waveguides 2A arranged within the cladding 11.

[0059] Figure 2B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 1. The substantially cross-sectional portion of the BB line shown in Figure 2B is a cross-sectional area of ​​the optical device 1 in which the adiabatic conversion section 4 is located. The optical device 1 has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, two second tapered waveguides 2B arranged within the cladding 11, and a third tapered waveguide 3A arranged within the cladding 11. The adiabatic conversion section 4 has a structure in which the third tapered waveguide 3A runs parallel below between the two second tapered waveguides 2B. The distance between the second tapered waveguides 2B and the third tapered waveguide 3A is kept constant.

[0060] Figure 2C is an explanatory diagram showing an example of a substantially cross-sectional portion of the CC line shown in Figure 1. The substantially cross-sectional portion of the CC line shown in Figure 2C is a cross-sectional area of ​​the optical device 1 in which a straight waveguide 3B is arranged within the Si waveguide 3. The optical device 1 has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, and a straight waveguide 3B arranged within the cladding 11.

[0061] The starting point of the adiabatic conversion section 4 is the location where the starting point X2 of the second tapered waveguide 2B and the starting point Y1 of the third tapered waveguide 3A are positioned. The waveguide width at the starting point X2 of the second tapered waveguide 2B is wider than the waveguide width at the starting point Y1 of the third tapered waveguide 3A. The adiabatic conversion section 4 has a structure in which the third tapered waveguide 3A runs parallel to the two second tapered waveguides 2B below.

[0062] The endpoint of the adiabatic conversion section 4 is the location where the endpoint X3 of the second tapered waveguide 2B and the endpoint Y2 of the third tapered waveguide 3A are located. The waveguide width at the endpoint X3 of the second tapered waveguide 2B is narrower than the waveguide width at the endpoint Y3 of the third tapered waveguide 3A.

[0063] In the adiabatic conversion section 4 of the optical device 1 of Example 1, a third tapered waveguide 3A is placed between two second tapered waveguides 2B, so that light is adiabatically transitioned between the second tapered waveguides 2B and the third tapered waveguide 3A. As a result, there are no discontinuities within the SiN waveguide 2, so the occurrence of radiation loss and reflection loss of light can be suppressed.

[0064] In optical device 1, the waveguide widths of the two first tapered waveguides 2A within the SiN waveguide 2 are gradually widened, thereby strengthening light confinement. As a result, the radiation loss at the tip of the Si waveguide 3 at the starting point of the adiabatic conversion section 4 is reduced, allowing the length of the adiabatic conversion section 4 to be shortened.

[0065] Furthermore, in the optical device 1, the waveguide widths of the two second tapered waveguides 2B within the SiN waveguide 2 are gradually narrowed, which reduces the effective refractive index of the SiN waveguide 2. This reduces the dependence of the conversion efficiency on wavelength and polarization, while suppressing a decrease in conversion efficiency.

[0066] Furthermore, the optical device 1 has a structure in which the spacing between the two first tapered waveguides 2A gradually narrows from the starting point X1 to the ending point X2. In the optical device 1, the portion of the SiN waveguide 2 at the tip end face D1 that optically couples with the optical fiber core FC is widened, and the first spacing L1 is wider than the second spacing L2. As a result, the mode field of the optical device 1 at the tip end face D1 approaches the mode field of the optical fiber core FC, thereby improving the coupling efficiency of the optical device 1 with the optical fiber core FC.

[0067] In the optical device 1 of Example 1, the relationship between the second interval L2 and the third interval L3 is L2 = L3, so the mode field of the adiabatic conversion unit 4 is wide and cannot be brought close to the mode field of the straight waveguide 3B in the Si waveguide 3 that optically couples with the adiabatic conversion unit 4. As a result, coupling loss occurs due to a mismatch in the mode fields between the adiabatic conversion unit 4 and the Si waveguide 3. Therefore, an embodiment that addresses this situation will be described below as Example 2. [Examples]

[0068] Figure 3 is an explanatory diagram showing an example of the optical device 1A of Example 2. Note that components identical to those of optical device 1 of Example 1 are denoted by the same reference numerals, and explanations of their overlapping components and operations are omitted. The difference between optical device 1A of Example 2 and optical device 1 of Example 1 is that the second spacing L2 at the starting point X2(Y1) of the adiabatic conversion section 4A is narrower than the third spacing L3 at the ending point X3(Y2) of the adiabatic conversion section 4A.

[0069] The SiN waveguide 2 comprises two first tapered waveguides 2A and two second tapered waveguides 2C. The second tapered waveguides 2C are waveguides having a tapered structure in which the waveguide width gradually narrows from the starting point X2 to the ending point X3. The line connecting the core center of the first tapered waveguide 2A at the starting point X1 and the core center of the first tapered waveguide 2A at the ending point X2 is defined as the first centerline CL1. The line connecting the core center of the second tapered waveguide 2C at the starting point X2 and the core center of the second tapered waveguide 2C at the ending point X3 is defined as the third centerline CL3.

[0070] Let the distance between the core center of endpoint X2 of one first tapered waveguide 2A and the core center of endpoint X2 of the other first tapered waveguide 2A be the second interval L2A. Let the distance between the core center of endpoint X3 of one second tapered waveguide 2C and the core center of endpoint X3 of the other second tapered waveguide 2C be the third interval L3A. The relationships between the first interval L1, the second interval L2A, and the third interval L3A are L1>L2A, L1>L3A, and L2A>L3A.

[0071] Optical device 1A widens the portion of the SiN waveguide 2 at the tip end face D1 that optically couples with the optical fiber core FC, and sets the distance between the two first tapered waveguides 2A to a first spacing L1 in order to bring it closer to the mode field of the optical fiber core FC. As a result, the mode field of optical device 1 at the tip end face D1 approaches the mode field of the optical fiber core FC, improving the coupling efficiency of optical device 1 with the optical fiber core FC.

[0072] The adiabatic conversion section 4A is configured with two second tapered waveguides 2C such that the third spacing L3A is narrower than the second spacing L2A. As a result, the mode field of the adiabatic conversion section 4A approaches the mode field of the straight waveguide 3B within the Si waveguide 3, thereby suppressing coupling losses between the adiabatic conversion section 4A and the Si waveguide 3.

[0073] The adiabatic conversion section 4A has two second tapered waveguides 2C within the SiN waveguide 2 and a third tapered waveguide 3A within the Si waveguide 3. The adiabatic conversion section 4A is constructed by arranging the third tapered waveguide 3A between the two second tapered waveguides 2C, and arranging the third tapered waveguide 3A parallel to the second tapered waveguides 2C at a distance from each other. The distance between the second tapered waveguides 2C and the third tapered waveguide 3A is the same.

[0074] The adiabatic conversion section 4A has a starting point X2 (Y1), an ending point X3 (Y2), and an intermediate section between the starting point and the ending point. Figure 4A is an explanatory diagram showing an example of a substantially cross-sectional portion of line AA shown in Figure 3. The substantially cross-sectional portion of line AA shown in Figure 4A is a cross-sectional portion of the optical device 1A in which two first tapered waveguides 2A are arranged within the SiN waveguide 2. The optical device 1A has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, and two first tapered waveguides 2A arranged within the cladding 11.

[0075] Figure 4B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 3. The substantially cross-sectional portion of the BB line shown in Figure 4B is a cross-sectional area of ​​the optical device 1A in which the adiabatic conversion section 4A is located. The optical device 1A has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, two second tapered waveguides 2C arranged within the cladding 11, and a third tapered waveguide 3A arranged within the cladding 11. The adiabatic conversion section 4A has a structure in which the third tapered waveguide 3A runs parallel below between the two second tapered waveguides 2C. The distance between the second tapered waveguides 2C and the third tapered waveguide 3A is kept constant.

[0076] Figure 4C is an explanatory diagram showing an example of a substantially cross-sectional portion of the CC line shown in Figure 3. The substantially cross-sectional portion of the CC line shown in Figure 4C is a cross-sectional area of ​​the optical device 1A in which a straight waveguide 3B is arranged within the Si waveguide 3. The optical device 1A has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, and a straight waveguide 3B arranged within the cladding 11.

[0077] The starting point of the adiabatic conversion section 4A is the location where the starting point X2 of the second tapered waveguide 2C and the starting point Y1 of the third tapered waveguide 3A are located. The waveguide width at the starting point X2 of the second tapered waveguide 2C is wider than the waveguide width at the starting point Y1 of the third tapered waveguide 3A. The adiabatic conversion section 4A has a structure in which the third tapered waveguide 3A runs parallel to the two second tapered waveguides 2C below. The ending point of the adiabatic conversion section 4A is the location where the ending point X3 of the second tapered waveguide 2C and the ending point Y2 of the third tapered waveguide 3A are located.

[0078] In the optical device 1A of Example 2, the adiabatic conversion section 4A is configured such that the distance between the two second tapered waveguides 2C is gradually narrowed, so that the third distance L3A at the endpoints of the two second tapered waveguides 2C is narrower than the second distance L2A at the starting points of the two second tapered waveguides 2C. As a result, while suppressing a decrease in the conversion efficiency in the adiabatic conversion section 4A, the mode field of the adiabatic conversion section 4A becomes closer to the mode field of the straight waveguide 3B, thereby improving the coupling loss with the Si waveguide 3.

[0079] In the adiabatic conversion section 4A, the effective refractive index is controlled by tapering the SiN waveguide 2, and the spacing between the second tapered waveguides 2C is gradually narrowed, thereby controlling the mode field of light propagating through the SiN waveguide 2. The effective refractive index and mode field of the light guiding through the SiN waveguide 2 are brought closer to the effective refractive index and mode field of the light propagating through the Si waveguide 3. As a result, the length of the adiabatic conversion section 4A can be shortened while reducing the coupling loss between the SiN waveguide 2 and the Si waveguide 3.

[0080] In the adiabatic conversion section 4A of the optical device 1A in Example 2, the endpoint X3 of the second tapered waveguide 2C within the SiN waveguide 2 terminates while still close to the Si waveguide 3. As a result, the SiN waveguide 2 is interrupted at the endpoint of the adiabatic conversion section 4A, causing a rapid change in the refractive index distribution of light. Consequently, light scattering loss occurs due to the change in cross-sectional shape at the endpoint of the adiabatic conversion section 4A. Therefore, an embodiment that addresses this situation will be described below as Example 3. [Examples]

[0081] Figure 5 is an explanatory diagram showing an example of the optical device 1B of Example 3. Note that components identical to those of the optical device 1A of Example 2 are denoted by the same reference numerals, and explanations of their overlapping components and operations are omitted. The difference between the optical device 1B of Example 3 and the optical device 1A of Example 2 is that the end of the second tapered waveguide 2D within the SiN waveguide 2 at the endpoint X3(Y2) of the adiabatic conversion section 4B gradually moves away from the Si waveguide 3.

[0082] The SiN waveguide 2 comprises two first tapered waveguides 2A, two second tapered waveguides 2D, and two curved waveguides 2E. The second tapered waveguides 2D are waveguides having a tapered structure in which the waveguide width gradually narrows from the starting point X2 to the ending point X3. The curved waveguides 2E are waveguides that curve from the starting point X3 to the ending point X4, gradually moving away from the Si waveguide 3.

[0083] The line connecting the core center of the first tapered waveguide 2A at the starting point X1 and the core center of the first tapered waveguide 2A at the ending point X2 is defined as the first centerline CL1. The line connecting the core center of the second tapered waveguide 2C at the starting point X2 and the core center of the second tapered waveguide 2C at the ending point X3 is defined as the third centerline CL3.

[0084] Let the distance between the core center of endpoint X2 of one first tapered waveguide 2A and the core center of endpoint X2 of the other first tapered waveguide 2A be the second interval L2A. Let the distance between the core center of endpoint X3 of one second tapered waveguide 2C and the core center of endpoint X3 of the other second tapered waveguide 2C be the third interval L3A. The relationships between the first interval L1, the second interval L2A, and the third interval L3A are L1>L2A, L1>L3A, and L2A>L3A.

[0085] Optical device 1B widens the portion of the SiN waveguide 2 at the tip end face D1 that optically couples with the optical fiber core FC, and sets the distance between the two first tapered waveguides 2A to a first spacing L1 in order to bring it closer to the mode field of the optical fiber core FC. As a result, the mode field of optical device 1 at the tip end face D1 approaches the mode field of the optical fiber core FC, improving the coupling efficiency of optical device 1 with the optical fiber core FC.

[0086] The adiabatic conversion section 4B is configured with two second tapered waveguides 2D such that the third spacing L3A is narrower than the second spacing L2A. As a result, the mode field of the adiabatic conversion section 4B approaches the mode field of the straight waveguide 3B within the Si waveguide 3, thereby suppressing coupling losses between the adiabatic conversion section 4B and the Si waveguide 3.

[0087] The adiabatic conversion section 4B has two second tapered waveguides 2D within the SiN waveguide 2 and a third tapered waveguide 3A within the Si waveguide 3. The adiabatic conversion section 4B is constructed by arranging the third tapered waveguide 3A between the two second tapered waveguides 2D, and arranging the third tapered waveguide 3A parallel to the second tapered waveguides 2D at a distance from each other. The distance between the second tapered waveguides 2D and the third tapered waveguide 3A is the same.

[0088] Furthermore, the curved waveguide 2E, which optically couples with the endpoint X3 of each second tapered waveguide 2D, causes the end of the SiN waveguide 2 to gradually move away from the straight waveguide 3B within the Si waveguide 3.

[0089] The adiabatic conversion section 4B has a starting point X2 (Y1), an ending point X3 (Y2), and an intermediate section between the starting point and the ending point. Figure 6A is an explanatory diagram showing an example of a substantially cross-sectional portion of line AA shown in Figure 5. The substantially cross-sectional portion of line AA shown in Figure 6A is a cross-sectional portion of the optical device 1B in which two first tapered waveguides 2A are arranged within the SiN waveguide 2. The optical device 1B has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, and two first tapered waveguides 2A arranged within the cladding 11.

[0090] Figure 6B is an explanatory diagram showing an example of a substantially cross-sectional portion of the BB line shown in Figure 5. The substantially cross-sectional portion of the BB line shown in Figure 6B is a cross-sectional area of ​​the optical device 1B in which the adiabatic conversion section 4B is located. The optical device 1B has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, two second tapered waveguides 2D arranged within the cladding 11, and a third tapered waveguide 3A arranged within the cladding 11. The adiabatic conversion section 4B has a structure in which the third tapered waveguide 3A runs parallel to the two second tapered waveguides 2D below. The distance between the second tapered waveguides 2D and the third tapered waveguide 3A is kept constant.

[0091] Figure 6C is an explanatory diagram showing an example of a substantially cross-sectional portion of the CC line shown in Figure 5. The substantially cross-sectional portion of the CC line shown in Figure 6C is a cross-sectional area of ​​the optical device 1B in which the straight waveguide 3B within the Si waveguide 3 is arranged. The optical device 1B has a Si substrate 12, a cladding 11 stacked on the Si substrate 12, and a straight waveguide 3B arranged within the cladding 11.

[0092] The starting point of the adiabatic conversion section 4B is where the starting point X2 of the second tapered waveguide 2D and the starting point Y1 of the third tapered waveguide 3A are located. The waveguide width at the starting point X2 of the second tapered waveguide 2D is wider than the waveguide width at the starting point Y1 of the third tapered waveguide 3A. The adiabatic conversion section 4B has a structure in which the third tapered waveguide 3A runs parallel to the two second tapered waveguides 2C below. The ending point of the adiabatic conversion section 4B is where the ending point X3 of the second tapered waveguide 2D and the ending point Y2 of the third tapered waveguide 3A are located.

[0093] In the optical device 1B of Example 3, a curved waveguide 2E is optically coupled to the end point X3 of the second tapered waveguide 2D within the adiabatic conversion section 4B, so as to gradually move away from the straight waveguide 3B of the Si waveguide 3. As a result, since the end of the SiN waveguide 2 gradually moves away from the Si waveguide 3 at the end point of the adiabatic conversion section 4B, the refractive index distribution of light changes slowly, thus suppressing light scattering loss.

[0094] In this embodiment, the second tapered waveguide 2B and the third tapered waveguide 3A of the adiabatic conversion section 4 (4A, 4B) may be a PLC (Planar Lightwave Circuit) with both the core and cladding made of SiO2, or an InP waveguide or a GaAs waveguide. The core may be Si or Si3N4, the lower cladding may be SiO2, and the upper cladding may be SiO2 or air, etc. This method is applicable when the refractive index of the destination waveguide material is higher than the refractive index of the source waveguide material. For example, in the case of a PLC, this method can be applied by changing the refractive index of the source and destination materials by changing the doping amount of the glass waveguide.

[0095] For illustrative purposes, SiN waveguide 2 was used as the first waveguide, Si waveguide 3 as the second waveguide, and SiO2 as the cladding. However, the refractive index of the cladding material should be smaller than that of the first waveguide material, and the refractive index of the first waveguide material should be smaller than that of the second waveguide material. Thus, the materials of the first waveguide, second waveguide, and cladding can be changed as appropriate.

[0096] In the case of PLC, the refractive index of the material can be changed by changing the amount of doping to the core. In the case of SiN waveguide 2 and Si waveguide 3, the large difference in specific refractive index results in strong light confinement, which enables the realization of a low-loss bent waveguide even with a small radius, thereby allowing for miniaturization of the optical device 1.

[0097] The structure of SiN waveguide 2 and Si waveguide 3 can be a rib waveguide, a ridge waveguide, or a channel waveguide, and can be changed as appropriate. When SiN waveguide 2 and Si waveguide 3 are rib waveguides, light seeps into the slab portion, reducing the effect of roughness on the core sidewalls and suppressing optical loss. When SiN waveguide 2 and Si waveguide 3 are channel waveguides, strong optical confinement allows for steep bending of the waveguide, enabling miniaturization of the optical device 1. The cladding 11 can be any material as long as its refractive index is lower than that of the core, and can be changed as appropriate.

[0098] The optical device 1 (1A, 1B) of this embodiment exemplifies a silicon optical waveguide in which the Si waveguide 3 is made of Si and the cladding 11 is made of SiO2. However, it is also applicable to PLC, InP waveguides, and GaAs waveguides in which the Si waveguide 3 and cladding 11 are made of SiO2.

[0099] Figure 7 is an explanatory diagram showing an example of an optical communication device 50 incorporating the optical devices 1 (1A, 1B) of this embodiment. The optical communication device 50 shown in Figure 7 is connected to an output optical fiber and an input optical fiber. The optical communication device 50 includes a DSP (Digital Signal Processor) 51, a light source 52, an optical transmitter 53, and an optical receiver 54. The DSP 51 is an electrical component that performs digital signal processing. For example, the DSP 51 performs processing such as encoding the transmission data, generates an electrical signal containing the transmission data, and outputs the generated electrical signal to the optical transmitter 53. The DSP 51 also acquires an electrical signal containing the received data from the optical receiver 54, performs processing such as decoding the acquired electrical signal, and obtains the received data.

[0100] The light source 52, for example, is equipped with a laser diode and generates light of a predetermined wavelength, which it supplies to the optical transmitter 53 and the optical receiver 54. The optical transmitter 53 modulates the light supplied from the light source 52 using an electrical signal output from the DSP 51, and outputs the resulting transmitted light to the optical fiber. The optical transmitter 53 generates transmitted light by modulating the light supplied from the light source 52 with an electrical signal input to the optical modulator as the light propagates through the waveguide.

[0101] The optical receiver 54 receives an optical signal from the optical fiber and demodulates the received light using the light supplied from the light source 52. The optical receiver 54 then converts the demodulated received light into an electrical signal and outputs the converted electrical signal to the DSP 51. The optical transmitter 53 and the optical receiver 54 incorporate optical devices 1 (1A, 1B), which are substrate-type optical waveguide elements that guide the light.

[0102] In the adiabatic conversion section 4(4A, 4B) within the optical device 1(1A, 1B) in the optical communication device 50, the mode field of the optical device 1 at the chip end face D1 approaches the mode field of the optical fiber core FC, thereby improving the coupling efficiency of the optical device 1 with the optical fiber core FC.

[0103] For the sake of explanation, the optical communication device 50 is shown as an example in which it incorporates an optical transmitter 53 and an optical receiver 54. However, the optical communication device 50 may incorporate only one of the optical transmitter 53 or the optical receiver 54. For example, the optical device 1 may be applied to an optical communication device 50 that incorporates an optical transmitter 53, or to an optical communication device 50 that incorporates an optical receiver 54, and can be modified as appropriate. [Explanation of symbols]

[0104] 1, 1A, 1B Optical Devices 2 SiN waveguide 2A First tapered waveguide 2B, 2C, 2D Second Tapered Waveguide 3 Si waveguide 3A Third Tapered Waveguide 4, 4A, 4B Adiabatic conversion section 11 Clad 50 Optical communication equipment 51 DSP 52 Light source 53 Optical Transmitter 54 Optical receiver

Claims

1. The system comprises two first waveguides arranged in parallel on a substrate, and one second waveguide arranged on the substrate in a state parallel to and spaced apart from the first waveguides. The first waveguide is, It comprises a first tapered waveguide and a second tapered waveguide connected to the first tapered waveguide. The second waveguide is, The waveguide includes a third tapered waveguide running parallel to the first waveguide, and a third waveguide connected to the opposite side of the third tapered waveguide from the side where the first tapered waveguide is provided. The first tapered waveguide is As you move towards the second tapered waveguide, the waveguide width gradually widens. The second tapered waveguide described above is As you move away from the first tapered waveguide, the waveguide width gradually narrows. The third tapered waveguide is, As you move towards the third waveguide, the waveguide width gradually widens. The two first waveguides mentioned above are A structure in which the distance between the core centerline connecting the start and end points of one first tapered waveguide and the core centerline connecting the start and end points of the other first tapered waveguide gradually narrows from the start point to the end point, A structure in which the first distance between the starting point of one first tapered waveguide and the starting point of the other first tapered waveguide is wider than the second distance between the ending point of one first tapered waveguide and the ending point of the other first tapered waveguide at the connection point between the first tapered waveguide and the second tapered waveguide, An optical device characterized by having the following features.

2. The first waveguide is, The optical device according to claim 1, characterized in that the third distance between the two second tapered waveguides at the endpoint of the second tapered waveguide is narrower than the second distance.

3. The first waveguide is, The optical device according to claim 1, characterized in that it has a curved waveguide that optically couples with the endpoint of the second tapered waveguide and gradually separates from the third waveguide within the second waveguide.

4. The first waveguide, which is clad on the substrate, Formed from a material containing SiN (Silicon Nitride), The second waveguide, which is clad on the substrate, Formed from a material containing Si (Silicon), The aforementioned cladding is SiO ２ The optical device according to claim 1, characterized in that it is formed from a material containing the following:

5. The optical device according to claim 1, characterized in that when the first waveguide and the second waveguide are covered with cladding on the substrate, the refractive index of the cladding material is smaller than the refractive index of the first waveguide material, and the refractive index of the first waveguide material is smaller than the refractive index of the second waveguide material.

6. The first waveguide and the second waveguide are, The optical device according to claim 1, characterized in that it is a rib waveguide.

7. Light source and An optical transmitter that uses a transmission signal to optically modulate light from the light source and transmit the transmitted light, An optical communication device having an optical device that guides the light within the optical transmitter, The optical device is The system comprises two first waveguides arranged in parallel on a substrate, and one second waveguide arranged on the substrate in a state parallel to and spaced apart from the first waveguides. The first waveguide is, It comprises a first tapered waveguide and a second tapered waveguide connected to the first tapered waveguide. The second waveguide is, The waveguide includes a third tapered waveguide running parallel to the first waveguide, and a third waveguide connected to the opposite side of the third tapered waveguide from the side where the first tapered waveguide is provided. The first tapered waveguide is As you move towards the second tapered waveguide, the waveguide width gradually widens. The second tapered waveguide described above is As you move away from the first tapered waveguide, the waveguide width gradually narrows. The third tapered waveguide is, As you move towards the third waveguide, the waveguide width gradually widens. The two first waveguides mentioned above are A structure in which the distance between the core centerline connecting the start and end points of one first tapered waveguide and the core centerline connecting the start and end points of the other first tapered waveguide gradually narrows from the start point to the end point, A structure in which the first distance between the starting point of one first tapered waveguide and the starting point of the other first tapered waveguide is wider than the second distance between the ending point of one first tapered waveguide and the ending point of the other first tapered waveguide at the connection point between the first tapered waveguide and the second tapered waveguide, An optical communication device characterized by having the following features.

8. Light source and A light receiver that uses light from the aforementioned light source to receive a received signal from the received light, An optical communication device having an optical device that guides the light within the optical receiver, The optical device is The system comprises two first waveguides arranged in parallel on a substrate, and one second waveguide arranged on the substrate in a state parallel to and spaced apart from the first waveguides. The first waveguide is, It comprises a first tapered waveguide and a second tapered waveguide connected to the first tapered waveguide. The second waveguide is, The waveguide includes a third tapered waveguide running parallel to the first waveguide, and a third waveguide connected to the opposite side of the third tapered waveguide from the side where the first tapered waveguide is provided. The first tapered waveguide is As you move towards the second tapered waveguide, the waveguide width gradually widens. The second tapered waveguide described above is As you move away from the first tapered waveguide, the waveguide width gradually narrows. The third tapered waveguide is, As you move towards the third waveguide, the waveguide width gradually widens. The two first waveguides mentioned above are A structure in which the distance between the core centerline connecting the start and end points of one first tapered waveguide and the core centerline connecting the start and end points of the other first tapered waveguide gradually narrows from the start point to the end point, A structure in which the first distance between the starting point of one first tapered waveguide and the starting point of the other first tapered waveguide is wider than the second distance between the ending point of one first tapered waveguide and the ending point of the other first tapered waveguide at the connection point between the first tapered waveguide and the second tapered waveguide, An optical communication device characterized by having the following features.