Optical device, substrate-type optical waveguide element, and optical communication device

Abstract

To provide an optical device etc., that comprises an unnecessary optical waveguide capable of reducing the quantity of reflected light input to an input waveguide while satisfying single mode conditions.SOLUTION: An optical device 1 has an input waveguide 2 and an output waveguide 3. The optical device 1 has an input part 4A which is optically coupled to the input waveguide 2 and an output part 4B which is optically coupled to the output waveguide 3, and also has an interference region 4 which has a wider waveguide width than those of the input waveguide 2 and output waveguide 3. The optical device 1 further has an unnecessary optical waveguide 5 which is installed at the output part 4B in the interference region 4, and extends in parallel with the output waveguide 3. The unnecessary optical waveguide 5 is a single mode waveguide which has a rib part 5A and a slab part 5B thinner than the rib part, and guides only light in a basic mode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical device, a planar optical waveguide element, and an optical communication device. [Background technology]

[0002] In recent years, in order to realize the miniaturization of optical communication equipment, there has been active development of optical devices including planar optical waveguide elements. Planar optical waveguide elements include optical devices that have a substrate, a core formed on the substrate, and a cladding that covers the core, and that guide light while providing various functions.

[0003] To realize an optical integrated circuit in an optical communication device, optical devices using planar optical waveguide elements such as multi-mode interference (MMI) couplers have been proposed as optical multiplexing and branching structures (e.g., Non-Patent Document 1). The optical waveguides used in MMI couplers are often channel waveguides (rectangular waveguides) that have strong optical confinement and can realize small devices.

[0004] Fig. 16 is an explanatory diagram showing an example of the configuration of an MMI coupler 100. The MMI coupler 100 shown in Fig. 16 is, for example, a 1 × 4 coupler with one input and four outputs. The MMI coupler 100 has one input waveguide 102, four output waveguides 103 (103A, 103B, 103C, and 103D), and an interference region 104 that optically couples the one input waveguide 102 and the four output waveguides 103. The output waveguide 103 has a first output waveguide 103A, a second output waveguide 103B, a third output waveguide 103C, and a fourth output waveguide 103D. Interference region 104 is a multimode waveguide that guides light in fundamental and higher modes, and has input section 104A that is optically coupled to one input waveguide 102 and output section 104B that is optically coupled to four output waveguides 103. The operating principle of MMI coupler 100 is that light entering interference region 104 from input waveguide 102 is expanded into multiple guided modes in interference region 104, which becomes a multimode waveguide, and the optical power is distributed to four output waveguides 103 due to the optical self-imaging effect.

[0005] Fig. 17 is an explanatory diagram showing an example of the optical self-imaging effect of the MMI coupler 100 shown in Fig. 16. The light expanded into each waveguide mode in the interference region 104 is condensed into a spot shape due to the self-imaging effect after being guided a certain distance within the interference region 104, as shown in Fig. 17. Therefore, each output waveguide 103 optically coupled to the interference region 104 is disposed at a position within the output section 104B of the interference region 104 where four spots due to the self-imaging effect are formed.

[0006] The interference region 104 functions as a branching circuit that branches the light input from the input waveguide 102 into four output waveguides 103. In addition, when light is input from the four output waveguides 103, the interference region 104 can also function as a multiplexing circuit that multiplexes the input light and outputs the multiplexed light from one input waveguide 102.

[0007] In the interference region 104 in the conventional MMI coupler 100, light that cannot be optically coupled to the output waveguide 103 is reflected and emitted, and some of the reflected and emitted light is optically coupled to the input waveguide 102. As a result, the light input from the input waveguide 102 affects the reflected light, causing optical resonance and interference, which may prevent the optical device from operating normally. Therefore, this issue will be explained by applying it to a 2 × 1 MMI coupler.

[0008] Fig. 18 is an explanatory diagram showing an example of the configuration of a 2 × 1 MMI coupler 100A. The MMI coupler 100A shown in Fig. 18 has two input waveguides 102 (102A and 102B), one output waveguide 103, and an interference region 104 that optically couples the two input waveguides 102 and the one output waveguide 103. The input waveguide 102 has a first input waveguide 102A and a second input waveguide 102B.

[0009] When the MMI coupler 100A is used as a multiplexing circuit, in principle, half of the light input from each input waveguide 102 is optically coupled to the output waveguide 103. However, in the MMI coupler 100A, the remaining half of the light input power is reflected or emitted from the discontinuous portion 114A or discontinuous portion 114B of the output section 104B in the interference region 104, and the reflected light is repeatedly reflected within the interference region 104. As a result, part of the power is input to each input waveguide 102.

[0010] To address this issue, an MMI coupler is known that places unnecessary optical waveguides on both sides of the output waveguide 103 and optically couples the reflected light through the unnecessary optical waveguides, thereby reducing the amount of reflected light entering the input waveguide 102. Fig. 19 is an explanatory diagram showing an example of the configuration of a conventional MMI coupler 100B.

[0011] 19 includes two input waveguides 102 (102A, 102B), one output waveguide 103, an interference region 104 that optically couples the two input waveguides 102 and the one output waveguide 103, and two unnecessary optical waveguides 105. The interference region 104 includes an input section 104A that optically couples with the input waveguide 102, and an output section 104B that optically couples with the output waveguide 103. The output section 104B is optically coupled to two unnecessary optical waveguides 105 arranged on both sides of the output waveguide 103. The unnecessary optical waveguides 105 are channel waveguides that optically couple reflected light generated in the interference region 104.

[0012] In the MMI coupler 100B, unwanted optical waveguides 105, each made of a multimode waveguide or a single-mode waveguide, are arranged on both sides of the output waveguide 103 at the output section 104B of the interference region 104. As a result, the amount of reflected light input to the input waveguide 102 can be reduced by optically coupling the reflected light generated in the interference region 104 using the unwanted optical waveguides 105. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-233294 [Patent Document 2] International Publication No. 2018 / 078992 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-323135 [Non-patent literature]

[0014] [Non-Patent Document 1] JM Heaton, RM Jenkins, DR Wight, JT Parker, JCH Birbeck, and KP Hilton, “Novel 1-to-N way integrated optical beam splitters using symmetric mode mixing in GaAs / AIGaAs multimode waveguides,” Appl. Phys. Lett., vol. 61, no. 15, pp. 1754-1756, 1992. Summary of the Invention [Problem to be solved by the invention]

[0015] However, in the conventional MMI coupler 100B, when the unwanted optical waveguide 105 is configured as a multimode waveguide of a channel waveguide, higher-order modes are guided within the unwanted optical waveguide 105. However, in a multimode waveguide, the confinement of higher-order modes of light is weakened, and therefore the higher-order mode light guided within the unwanted optical waveguide 105 is more likely to be optically coupled to the output waveguide 103. As a result, the higher-order mode light optically coupled to the output waveguide 103 interferes with the light guided through the output waveguide 103, causing ripples in the wavelength region, which may prevent the optical device from operating normally.

[0016] Furthermore, in the conventional MMI coupler 100B, when the unwanted optical waveguide 105 is configured with a single-mode channel waveguide, the waveguide width of the unwanted optical waveguide 105 becomes narrow, increasing the proportion of discontinuous portions in the output section 104B where reflected light occurs within the interference region 104. As a result, the unwanted optical waveguide 105 cannot sufficiently reduce the amount of reflected light input to the input waveguide 102.

[0017] Here, the relationship between the waveguide width of the unwanted optical waveguide 105 of a conventional 2×1 MMI coupler 100B and the amount of reflected light will be described based on the results of a simulation calculated using the finite-difference time-domain method. Fig. 20 is an explanatory diagram showing an example of the relationship between the waveguide width of the unwanted optical waveguide 105 of a conventional MMI coupler 100B and the amount of reflected light. The amount of reflected light is calculated, for example, by 10*Log10 [P(reflection) / P(incidence)]. P(incidence) is the optical power (mW) incident from the input waveguide 102 of the MMI coupler 100B, and P(reflection) is the reflected optical power (mW) detected at the same input waveguide 102.

[0018] The 2 × 1 MMI coupler 100B to be simulated employs a channel optical waveguide with a core material of Si, a cladding material of SiO2, and a core thickness h of 0.22 μm as the unwanted optical waveguide 105. Light with an optical wavelength of 1550 nm is input from the input waveguide 102, and the amount of reflected light input to the input waveguide 102 is calculated when the waveguide width w of the unwanted optical waveguide 105 is changed.

[0019] 20 , as the waveguide width w of the unnecessary optical waveguide 105 increases, the amount of reflected light decreases. In other words, as the waveguide width w of the unnecessary optical waveguide 105 increases, the number of discontinuous portions in the output section 104B within the interference region 104 decreases, thereby reducing the amount of reflected light. Furthermore, to satisfy the single-mode condition, which makes it difficult for higher-order modes to propagate within the unnecessary optical waveguide 105, the waveguide width w of the unnecessary optical waveguide 105 needs to be 0.4 μm or less. However, it is difficult to sufficiently reduce the amount of reflected light input from the interference region 104 to the input waveguide 102. Therefore, the MMI coupler 100B requires an unnecessary optical waveguide 105 that can reduce the amount of reflected light input to the input waveguide 102 while satisfying the single-mode condition.

[0020] In one aspect, an object is to provide an optical device or the like having an unnecessary optical waveguide that can reduce the amount of reflected light input to an input waveguide while satisfying the single-mode condition. [Means for solving the problem]

[0021] An optical device of one embodiment has an input waveguide and an output waveguide. The optical device has an input section optically coupled to the input waveguide and an output section optically coupled to the output waveguide, and an interference region having a waveguide width wider than the waveguide widths of the input waveguide and the output waveguide. The optical device further has an unwanted optical waveguide provided at the output section within the interference region and running parallel to the output waveguide. The unwanted optical waveguide has a rib portion and a slab portion that is thinner than the thickness of the rib portion, and is a single-mode waveguide that guides only light in the fundamental mode. [Effects of the Invention]

[0022] According to one aspect, it is possible to provide an optical device or the like that includes an unnecessary optical waveguide that can reduce the amount of reflected light that enters an input waveguide while satisfying the single-mode condition. [Brief explanation of the drawings]

[0023] [Figure 1]FIG. 1 is an explanatory diagram illustrating an example of the configuration of an MMI coupler according to a first embodiment. [Figure 2] FIG. 2 is an explanatory diagram showing an example of a schematic cross section taken along line AA shown in FIG. [Figure 3] FIG. 3 is an explanatory diagram showing an example of the state of the waveguide mode of light that is guided in the unnecessary light waveguide. [Figure 4] FIG. 4 is an explanatory diagram showing an example of the relationship between the waveguide width of the unnecessary optical waveguide of the MMI coupler of the first embodiment and the amount of reflected light. [Figure 5] FIG. 5 is an explanatory diagram showing an example of the relationship between the reflected light intensity and the optical wavelength for the MMI coupler of the first embodiment and a conventional MMI coupler. [Figure 6A] FIG. 6A is an explanatory diagram showing an example of the configuration of a 4×1 MMI coupler that is a modified example of the first embodiment. [Figure 6B] FIG. 6B is an explanatory diagram showing an example of the configuration of a 4×2 MMI coupler that is a modified example of the first embodiment. [Figure 6C] FIG. 6C is an explanatory diagram showing an example of the configuration of a 4×3 MMI coupler that is a modified example of the first embodiment. [Figure 6D] FIG. 6D is an explanatory diagram showing an example of the configuration of a 3×1 MMI coupler that is a modified example of the first embodiment. [Figure 6E] FIG. 6E is an explanatory diagram showing an example of the configuration of a 2×2 MMI coupler that is a modified example of the first embodiment. [Figure 6F] FIG. 6F is an explanatory diagram showing an example of the configuration of a 3×3 MMI coupler that is a modified example of the first embodiment. [Figure 7] FIG. 7 is an explanatory diagram illustrating an example of the configuration of the MMI coupler according to the second embodiment. [Figure 8A] FIG. 8A is an explanatory diagram showing an example of the configuration of an MMI type coupler (1×1 unnecessary optical waveguide) which is a modified example of the second embodiment. [Figure 8B] FIG. 8B is an explanatory diagram showing an example of the configuration of an MMI type coupler (1×1 unnecessary optical waveguide) which is a modified example of the second embodiment. [Figure 8C]FIG. 8C is an explanatory diagram showing an example of the configuration of an MMI type coupler (2×1 unnecessary optical waveguide) which is a modified example of the second embodiment. [Figure 8D] FIG. 8D is an explanatory diagram showing an example of the configuration of an MMI type coupler (2×2 unnecessary optical waveguides) which is a modified example of the second embodiment. [Figure 9] FIG. 9 is an explanatory diagram showing an example of a schematic cross section of an unnecessary optical waveguide of the MMI coupler according to the third embodiment. [Figure 10] FIG. 10 is an explanatory diagram illustrating an example of the configuration of the MMI coupler according to the fourth embodiment. [Figure 11] FIG. 11 is an explanatory diagram illustrating an example of the configuration of the MMI coupler according to the fifth embodiment. [Figure 12] FIG. 12 is an explanatory diagram showing an example of the configuration of the MMI coupler according to the sixth embodiment. [Figure 13] FIG. 13 is an explanatory diagram showing an example of the configuration of an MMI coupler according to a modified example of the sixth embodiment. [Figure 14] FIG. 14 is an explanatory diagram showing an example of the configuration of an MMI coupler according to a modified example of the sixth embodiment. [Figure 15] FIG. 15 is an explanatory diagram showing an example of an optical communication device incorporating the MMI type coupler of this embodiment. [Figure 16] FIG. 16 is an explanatory diagram showing an example of the configuration of a conventional MMI coupler. [Figure 17] FIG. 17 is an explanatory diagram showing an example of the optical self-imaging effect of the MMI coupler shown in FIG. [Figure 18] FIG. 18 is an explanatory diagram showing an example of the configuration of a conventional 2×1 MMI coupler. [Figure 19] FIG. 19 is an explanatory diagram showing an example of the configuration of a conventional MMI coupler. [Figure 20] FIG. 20 is an explanatory diagram showing an example of the relationship between the waveguide width of the unnecessary optical waveguide and the amount of reflected light in a conventional MMI coupler. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, examples of optical devices and the like disclosed in the present application will be described in detail with reference to the drawings. Note that the disclosed technology is not limited to these examples. Furthermore, the examples shown below may be combined as appropriate within the scope of not causing any contradiction. [Example]

[0025] Fig. 1 is an explanatory diagram showing an example of the configuration of an MMI coupler 1 according to a first embodiment. The MMI coupler 1 shown in Fig. 1 is a substrate-type optical waveguide element such as a 2x1 MMI coupler with two inputs and one output. The MMI coupler 1 has two input waveguides 2 (2A, 2B), one output waveguide 3, an interference region 4 that optically couples the two input waveguides 2 and the one output waveguide 3, and two unnecessary optical waveguides 5. The input waveguide 2 has a first input waveguide 2A and a second input waveguide 2B.

[0026] The input waveguide 2 is a rib-type waveguide having a rib portion and a slab portion whose thickness is thinner than the core thickness of the rib portion. The output waveguide 3 is also a rib-type waveguide having a rib portion and a slab portion whose thickness is thinner than the core thickness of the rib portion. Furthermore, the interference region 4 is also a rib-type waveguide having a rib portion and a slab portion whose thickness is thinner than the core thickness of the rib portion. For convenience of explanation, the input waveguide 2, output waveguide 3, and interference region 4 are exemplified as rib-type waveguides, but are not limited to this and may be, for example, a channel-type waveguide or a ridge-type waveguide, and can be modified as appropriate.

[0027] The interference region 4 has an input section 4A that is optically coupled to two input waveguides 2 and an output section 4B that is optically coupled to one output waveguide 3, and is a waveguide through which light input from the two input waveguides 2 is output from one output waveguide 3. The interference region 4 is a multimode waveguide whose waveguide width is wider than the waveguide widths of the input waveguides 2 and the output waveguide 3, and which guides light in the fundamental mode and higher modes.

[0028] In addition, the output section 4B in the interference region 4 has an output waveguide 3 disposed at a spot position where the light input from the two input waveguides 2 is focused. Furthermore, the output section 4B in the interference region 4 has two unnecessary optical waveguides 5 disposed on both sides of the output waveguide 3.

[0029] Fig. 2 is an explanatory diagram showing an example of a schematic cross section taken along line AA shown in Fig. 1. The schematic cross section taken along line AA shown in Fig. 2 is a schematic cross section of two unnecessary optical waveguides 5 and one output waveguide 3 that are optically coupled to the output section 4B in the interference region 4. Each unnecessary optical waveguide 5 has an asymmetric rib-type waveguide structure having a rib section 5A and a slab section 5B that is thinner than the core thickness of the rib section 5A. The waveguide width Wrib of the rib section 5A is, for example, 0.76 µm, and the waveguide width Wslab of the slab section 5B is, for example, 5.0 µm.

[0030] Each unnecessary optical waveguide 5 has its rib portion 5A disposed on the output waveguide 3 side, and the gap between the rib portion 5A and the output waveguide 3 is set to, for example, 0.2 μm, and is optically coupled to the output portion 4B in the interference region 4. Note that if the gap size is made smaller, the amount of reflected light input from the interference region 4 to the input waveguide 2 can be reduced. Each unnecessary optical waveguide 5 is a single-mode waveguide that guides only light in the fundamental mode. The thickness of the slab portion 5B in the unnecessary optical waveguide 5 is made thicker than the thicknesses of the input waveguide 2, output waveguide 3, and slab portions in the interference region 4.

[0031] FIG. 3 is an explanatory diagram showing an example of the state of the waveguide mode of light guided in the unnecessary optical waveguide 5. The unnecessary optical waveguide 5 includes a slab portion 5B, which weakens the light confinement of the rib portion 5A. Therefore, even if the waveguide width of the rib portion 5A is the same as that of a channel waveguide, higher-order mode light is less likely to be guided, and only fundamental mode light is guided. Therefore, even if the waveguide width of the rib portion 5A of the unnecessary optical waveguide 5 is increased, the single-mode condition, in which only fundamental mode light is guided, is satisfied. By satisfying the single-mode condition, the effect of higher-order mode light guided in the unnecessary optical waveguide 5 on the output waveguide 3 can be suppressed. As a result, the unnecessary optical waveguide 5 satisfies the single-mode condition while sufficiently reducing the amount of reflected light entering the input waveguide 2 from the interference region 4.

[0032] 4 is an explanatory diagram showing an example of the results of comparing the amount of reflected light depending on the waveguide width in the unnecessary optical waveguide 5 (105) of the MMI coupler 1 of Example 1 and the conventional MMI coupler 100B. In the MMI coupler 1 of Example 1, the unnecessary optical waveguide 5 is an asymmetric rib waveguide, whereas in the conventional MMI coupler 100B, the unnecessary optical waveguide 105 is a channel waveguide. In the conventional MMI coupler 100B, since the unnecessary optical waveguide 105 is a channel waveguide, the amount of reflected light input to the input waveguide 2 cannot be sufficiently reduced even though the waveguide width of the unnecessary optical waveguide 105 is 0.4 μm or less and satisfies the single-mode condition. In contrast, the unnecessary optical waveguide 5 of the MMI coupler 1 of Example 1 is an asymmetric rib-type waveguide, and therefore can sufficiently reduce the amount of reflected light input to the input waveguide 2 while satisfying the single-mode condition up to a waveguide width of 0.8 μm or less.

[0033] 5 is an explanatory diagram showing an example of the comparison results of the reflected light amount according to the optical wavelength in the unnecessary optical waveguide 5 (105) of the MMI coupler 1 of Example 1 and the conventional MMI coupler 100B. The wavelength of the light used is in the range of 1525 nm to 1570 nm, which includes the C band. The reflected light amount of the MMI coupler 1 of Example 1 is improved by about 13 dB compared to the reflected light amount of the conventional MMI coupler 100B.

[0034] In the MMI coupler 1 of Example 1, an unwanted optical waveguide 5 having an asymmetric rib-type waveguide structure is arranged in parallel with the output waveguide 3 at the output section 4B in the interference region 4. As a result, the unwanted optical waveguide 5 can sufficiently reduce the amount of reflected light input from the interference region 4 to the input waveguide 2 while satisfying the single-mode condition.

[0035] Although the MMI coupler 1 of Example 1 is exemplified as a 2×1 MMI coupler, the present invention is not limited to this and can be modified as appropriate. Modified examples of the MMI coupler 1 of Example 1 are shown in FIGS. 6A to 6F. FIG. 6A is an explanatory diagram showing an example of the configuration of a 4×1 MMI coupler 1, which is a modified example of Example 1. The MMI coupler 1 shown in FIG. 6A has four input waveguides 2 (2A, 2B, 2C, and 2D), one output waveguide 3, and an interference region 4 that optically couples the four input waveguides 2 and the one output waveguide 3. In an output section 4B within the interference region 4, unnecessary optical waveguides 5 are arranged on both sides of the output waveguide 3.

[0036] 6B is an explanatory diagram showing an example of the configuration of a 4×2 MMI coupler 1, which is a modified example of Example 1. The MMI coupler 1 shown in FIG. 6B has four input waveguides 2 (2A, 2B, 2C, and 2D), two output waveguides 3 (3A and 3B), and an interference region 4 that optically couples the four input waveguides 2 and the two output waveguides 3. In an output section 4B within the interference region 4, unnecessary optical waveguides 5 are arranged on both sides of the two output waveguides 3.

[0037] 6C is an explanatory diagram showing an example of the configuration of a 4×3 MMI coupler 1, which is a modified example of Example 1. The MMI coupler 1 shown in FIG. 6C has four input waveguides 2 (2A, 2B, 2C, 2D), three output waveguides 3 (3A, 3B, 3C), and an interference region 4 that optically couples the four input waveguides 2 and the three output waveguides 3. In an output section 4B within the interference region 4, unnecessary optical waveguides 5 are arranged on both sides of the three output waveguides 3.

[0038] 6D is an explanatory diagram showing an example of the configuration of a 3×1 MMI coupler 1 which is a modified example of Example 1. The MMI coupler 1 shown in FIG. 6D has three input waveguides 2 (2A, 2B, 2C), one output waveguide 3, and an interference region 4 which optically couples the three input waveguides 2 and the one output waveguide 3. In an output section 4B within the interference region 4, unnecessary optical waveguides 5 are arranged on both sides of the one output waveguide 3.

[0039] 6E is an explanatory diagram showing an example of the configuration of a 2×2 MMI coupler 1 which is a modified example of Example 1. The MMI coupler 1 shown in FIG. 6E has two input waveguides 2 (2A, 2B), two output waveguides 3 (3A, 3B), and an interference region 4 which optically couples the two input waveguides 2 and the two output waveguides 3. In an output section 4B within the interference region 4, unnecessary optical waveguides 5 are arranged on both sides of the two output waveguides 3.

[0040] 6F is an explanatory diagram showing an example of the configuration of a 3×3 MMI coupler 1 which is a modified example of Example 1. The MMI coupler 1 shown in FIG. 6F has three input waveguides 2 (2A, 2B, 2C), three output waveguides 3 (3A, 3B, 3C), and an interference region 4 which optically couples the three input waveguides 2 and the three output waveguides 3. In an output section 4B within the interference region 4, unnecessary optical waveguides 5 are arranged on both sides of the three output waveguides 3.

[0041] 6A to 6F, an unwanted optical waveguide 5 having an asymmetric rib-type waveguide structure is arranged in parallel with the output waveguide 3 at the output section 4B in the interference region 4. As a result, the unwanted optical waveguide 5 can sufficiently reduce the amount of reflected light input from the interference region 4 to the input waveguide 2 while satisfying the single-mode condition.

[0042] The MMI coupler 1 is an MMI coupler that can be applied to M×N input waveguides 2 and N output waveguides 3 .

[0043] In the MMI coupler 1 of Example 1, the unnecessary optical waveguides 5 are arranged on both sides of the output waveguide 3 in the output section 4B in the interference region 4, but they may be arranged on one side of the output waveguide 3, and this can be modified as appropriate. In addition, the MMI coupler 1 of Example 1 is a unidirectional coupler in which light is guided from the input waveguide 2 to the output waveguide 3. However, the present invention is also applicable to a bidirectional coupler in which light is guided in both directions between the input waveguide 2 and the output waveguide 3, and an embodiment thereof will be described below as Example 2. Note that the same components as those in the MMI coupler 1 of Example 1 are denoted by the same reference numerals, and descriptions of the overlapping components and operations will be omitted. [Example]

[0044] FIG. 7 is an explanatory diagram showing an example of the configuration of an MMI coupler 1A according to a second embodiment. The MMI coupler 1A shown in FIG. 7 is a coupler that guides light bidirectionally. The MMI coupler 1A has two input waveguides 2 (2A, 2B), one output waveguide 3, an interference region 4, and two unnecessary optical waveguides 5 arranged in an output section 4B within the interference region 4. The two input waveguides 2 are a first input waveguide 2A and a second input waveguide 2B. In the MMI coupler 1A, one unnecessary optical waveguide 6 is arranged near the first input waveguide 2A arranged in the input section 4A within the interference region 4.

[0045] The unwanted light waveguide 6 arranged in the input section 4A in the interference region 4 has an asymmetric rib-type waveguide structure having a rib section 6A and a slab section 6B that is thinner than the core thickness of the rib section 6A. The unwanted light waveguide 6 is a single-mode waveguide that guides only light in the fundamental mode.

[0046] In the MMI coupler 1A of the second embodiment, an unwanted optical waveguide 5 having an asymmetric rib waveguide structure is arranged in the output section 4B within the interference region 4, and an unwanted optical waveguide 6 having an asymmetric rib waveguide structure is arranged in the input section 4A within the interference region 4. As a result, even in the case of a bidirectional MMI coupler 1A, the unwanted optical waveguides 5 and 6 can sufficiently reduce the amount of reflected light input from the interference region 4 to the input waveguide 2 and the output waveguide 3 while satisfying the single-mode condition.

[0047] In the MMI coupler 1A of the second embodiment, a single unwanted optical waveguide 6 is disposed near the first input waveguide 2A with respect to the input section 4A in the interference region 4. However, the single unwanted optical waveguide 6 may be disposed near the second input waveguide 2B instead of the first input waveguide 2A, and this can be modified as appropriate. Furthermore, the unwanted optical waveguide 6 may be disposed at the input section 4A outside the first input waveguide 2A and outside the second input waveguide 2B, and this can be modified as appropriate.

[0048] In the MMI coupler 1A of the second embodiment, the unnecessary optical waveguides 6 and 5 are arranged at the input section 4A and the output section 4B in the interference region 4, but the present invention is not limited to this and can be modified as appropriate. Modified examples of the MMI coupler 1A of the second embodiment are shown in Figs. 8A to 8D.

[0049] 8A is an explanatory diagram showing an example of the configuration of an MMI coupler (1×1 unnecessary optical waveguide) 1A which is a modified example of Example 2. The MMI coupler 1A shown in FIG. 8A has two input waveguides 2 (2A, 2B), two output waveguides 3 (3A, 3B), and an interference region 4 which optically couples the two input waveguides 2 and the two output waveguides 3. The two input waveguides 2 are a first input waveguide 2A and a second input waveguide 2B, and the two output waveguides 3 are a first output waveguide 3A and a second output waveguide 3B.

[0050] In the MMI coupler 1A shown in FIG. 8A, an unnecessary optical waveguide 6 is arranged at an input section 4A in the interference region 4 so as to run parallel to the outside of a first input waveguide 2A, and an unnecessary optical waveguide 5 is arranged at an output section 4B in the interference region 4 so as to run parallel to the outside of a first output waveguide 3A.

[0051] Fig. 8B is an explanatory diagram showing an example of the configuration of an MMI coupler (1 × 1 unnecessary optical waveguide) 1A which is a modified example of the second embodiment. Note that the same components as those of the MMI coupler 1A shown in Fig. 8A are denoted by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The MMI coupler 1A shown in Fig. 8B has an unnecessary optical waveguide 6 arranged at an input section 4A in the interference region 4 so as to run parallel to and outside the first input waveguide 2A, and an unnecessary optical waveguide 5 arranged at an output section 4B in the interference region 4 so as to run parallel to and outside the second output waveguide 3B.

[0052] 8C is an explanatory diagram showing an example of the configuration of an MMI coupler (2×1 unnecessary optical waveguide) 1A that is a modified example of the second embodiment. The same components as those of the MMI coupler 1A shown in FIG. 8A are denoted by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The MMI coupler 1A shown in FIG. 8C has an unnecessary optical waveguide 6 disposed at an input section 4A in the interference region 4 so as to run parallel to the outside of a first input waveguide 2A, and an unnecessary optical waveguide 6 disposed at an outside of a second input waveguide 2B. Furthermore, the MMI coupler 1A has an unnecessary optical waveguide 5 disposed at an output section 4B in the interference region 4 so as to run parallel to the outside of a second output waveguide 3B.

[0053] 8D is an explanatory diagram showing an example of the configuration of an MMI coupler (2×2 unnecessary optical waveguides) 1A that is a modified example of the second embodiment. The same components as those of the MMI coupler 1A shown in FIG. 8A are denoted by the same reference numerals, and redundant descriptions of the configuration and operation will be omitted. The MMI coupler 1A shown in FIG. 8D has an unnecessary optical waveguide 6 arranged in an input section 4A within the interference region 4 so as to run parallel to the outside of the first input waveguide 2A, and an unnecessary optical waveguide 6 arranged in parallel to the outside of the second input waveguide 2B. Furthermore, the MMI coupler 1A has an unnecessary optical waveguide 5 arranged in an output section 4B within the interference region 4 so as to run parallel to the outside of the first output waveguide 3A, and an unnecessary optical waveguide 5 arranged in parallel to the outside of the second output waveguide 3B.

[0054] 8A to 8D, an unwanted optical waveguide 5 having an asymmetric rib waveguide structure is arranged at an output section 4B in the interference region 4, and an unwanted optical waveguide 6 having an asymmetric rib waveguide structure is arranged at an input section 4A in the interference region 4. As a result, even in the case of a bidirectional MMI coupler 1A, the unwanted optical waveguides 5 and 6 can sufficiently reduce the amount of reflected light input from the interference region 4 to the input waveguide 2 and output waveguide 3 while satisfying the single-mode condition.

[0055] Although the unnecessary optical waveguides 5 and 6 in the MMI coupler 1 (1A) are configured as asymmetric rib-type waveguides in the above example, the present invention is not limited to this, and an embodiment thereof will be described below as Example 3. [Example]

[0056] FIG. 9 is an explanatory diagram showing an example of a schematic cross-sectional portion of the unnecessary optical waveguide 5 (6) of Example 3. The unnecessary optical waveguide 5 (6) shown in FIG. 9 is an asymmetric rib-type waveguide having a rib portion 5A (6A), a first slab portion 5B1 (6B1) formed on one side of the rib portion 5A (6A), and a second slab portion 5B2 (6B2) formed on the other side of the rib portion 5A (6A). The unnecessary optical waveguide 5 (6) is covered with a clad 11. The first slab portion 5B1 (6B1) is configured to be thinner than the second slab portion 5B2 (6B2). The thickness of the second slab portion 5B2 (6B2) of the unnecessary optical waveguide 5 (6) is thicker than the thickness Hs of the input waveguide 2, the output waveguide 3, and the slab portion in the interference region 4.

[0057] In the asymmetric rib-type waveguide of the unnecessary optical waveguide 5 (6) of Example 3, the thickness of the first slab portion 5B1 (6B1) is thinner than the thickness Hs of the second slab portion 5B2 (6B2) on the output waveguide 3 side. Therefore, compared to when the first slab portion and the second slab portion have the same thickness Hs, the optical confinement in the unnecessary optical waveguide 5 (6) is weaker, and the single-mode condition can be satisfied even with a wider waveguide width. That is, in the unnecessary optical waveguide 5 (6) of the MMI coupler 1 of Example 3, the amount of reflected light of light input from the interference region 4 to the input waveguide 2 (output waveguide 3) can be sufficiently reduced compared to the unnecessary optical waveguide 5 (6) of the MMI coupler 1 (1A) of Examples 1 and 2, which is formed of a rib waveguide.

[0058] In the MMI coupler 1 of Example 1, the unnecessary optical waveguides 5 arranged on both sides of the output section 4B are configured as straight waveguides, but the present invention is not limited to this and can be modified as appropriate. This embodiment will be described below as Example 4. [Example]

[0059] FIG. 10 is an explanatory diagram showing an example of the configuration of an MMI coupler 1C according to a fourth embodiment. Components identical to those of the MMI coupler 1 according to the first embodiment are designated by the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The MMI coupler 1C shown in FIG. 10 includes two input waveguides 2 (2A, 2B), one output waveguide 3, and an interference region 4 that optically couples the two input waveguides 2 and the one output waveguide 3. The MMI coupler 1C further includes unwanted optical waveguides 50 disposed in an output section 4B within the interference region 4 and running parallel to both sides of the output waveguide 3. The unwanted optical waveguides 50 are asymmetric rib-type waveguides with a bent structure. The unwanted optical waveguides 50 have an asymmetric rib-type waveguide structure including a rib portion 50A and a slab portion 50B having a thickness thinner than the core thickness of the rib portion 50A. The unnecessary optical waveguide 50 is a single mode waveguide that guides only light in the fundamental mode. The rib portion 50A of each unnecessary optical waveguide is a waveguide with a bent structure that moves away from the output waveguide 3 in order.

[0060] In the MMI coupler 1C of the fourth embodiment, the unwanted optical waveguides 50 running parallel to both sides of the output waveguide 3 have a bent structure, so that the influence of the unwanted radiation modes inside the unwanted optical waveguides 50 that radiate within the unwanted optical waveguides 50 on the output waveguide 3 can be gradually reduced. [Example]

[0061] Fig. 11 is an explanatory diagram showing an example of the configuration of an MMI coupler 1D according to a fifth embodiment. Note that the same components as those of the MMI coupler 1 according to the first embodiment are denoted by the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The MMI coupler 1D shown in Fig. 11 has two input waveguides 2 (2A, 2B), one output waveguide 3, and an interference region 4 that optically couples the two input waveguides 2 and the one output waveguide 3. Furthermore, the MMI coupler 1D has unnecessary optical waveguides 51 that are arranged in an output section 4B within the interference region 4 and run in parallel on both sides of the output waveguide 3.

[0062] The unnecessary optical waveguide 51 is an asymmetric rib-type waveguide having a rib portion 51A and a slab portion 51B formed on one side of the rib portion 51A. Furthermore, the unnecessary optical waveguide 51 has a doped region 51C in which part or the entire region of the slab portion 51B is doped. The doping is N-doped or P-doped. The doped region 51C is a region that optically couples the unnecessary radiation mode radiating within the unnecessary optical waveguide 51.

[0063] In the MMI coupler 1D of the fifth embodiment, a part or the entire region of the slab portion 51B in the unwanted optical waveguide 51 running in parallel on both sides of the output waveguide 3 is doped, and therefore, the influence of the unwanted radiation mode in the unwanted optical waveguide 51 that radiates within the unwanted optical waveguide 51 on the output waveguide 3 can be reduced. [Example]

[0064] FIG. 12 is an explanatory diagram showing an example of the configuration of an MMI coupler 1E according to a sixth embodiment. The same components as those in the MMI coupler 1 according to the first embodiment are denoted by the same reference numerals, and redundant description of the configuration and operation will be omitted. The MMI coupler 1E shown in FIG. 12 includes four input waveguides 2 (2A, 2B, 2C, and 2D), one output waveguide 3, and an interference region 4 that optically couples the four input waveguides 2 and the one output waveguide 3. Furthermore, the MMI coupler 1E includes unnecessary optical waveguides 5 that are disposed in an output section 4B within the interference region 4 and run parallel to both sides of the output waveguide 3. The four input waveguides 2 include a first input waveguide 2A, a second input waveguide 2B, a third input waveguide 2C, and a fourth input waveguide 2D.

[0065] The unnecessary optical waveguide 5 is an asymmetric rib-type waveguide having a rib portion 5A and a slab portion 5B formed on one side of the rib portion 5A. The unnecessary optical waveguide 5 is optically coupled to an optical terminal portion 7 having a reverse tapered structure in which the waveguide width narrows along the light propagation direction.

[0066] The optical termination section 7 has a rib section 7A whose waveguide width tapers along the light traveling direction and which optically couples with a rib section 5A in the unnecessary optical waveguide 5, and a slab section 7B whose waveguide width tapers along the light traveling direction and which optically couples with a slab section 5B in the unnecessary optical waveguide 5. The optical termination section 7 with an inverse tapered structure terminates the light that is guided in the unnecessary optical waveguide 5, and therefore can suppress reflection of light within the unnecessary optical waveguide 5.

[0067] In the MMI coupler 1E of the sixth embodiment, the unwanted optical waveguides 5 running parallel to both sides of the output waveguide 3 are optically coupled to the optical termination sections 7 having an inverse tapered structure, so that the light from the unwanted optical waveguides 5 is terminated, thereby preventing the occurrence of re-reflection of light within the unwanted optical waveguides 5.

[0068] Although the MMI coupler 1E of the sixth embodiment has been described as an example in which the optical terminal end 7 having an inverse tapered structure is optically coupled to the unnecessary optical waveguide 5, the optical terminal end 7 is not limited to an inverse tapered structure and can be modified as appropriate. Modified examples of the MMI coupler 1E of the sixth embodiment are shown in Figs. 13 and 14.

[0069] Fig. 13 is an explanatory diagram showing an example of the configuration of an MMI coupler 1E that is a modified example of the sixth embodiment. Note that the same components as those in the MMI coupler 1E of the sixth embodiment are denoted by the same reference numerals, and descriptions of the overlapping configurations and operations will be omitted. The optical terminal unit 7 that is optically coupled to the unwanted optical waveguide 5 in the MMI coupler 1E shown in Fig. 13 has a photodetector 7C that converts light from the unwanted optical waveguide 5 into current, instead of an inverted tapered structure. The photodetector 7C converts light that is guided through the unwanted optical waveguide 5 into current, and therefore can suppress reflection of light within the unwanted optical waveguide 5.

[0070] In the MMI coupler 1E shown in FIG. 13, the photodetector 7C is optically coupled to the unwanted optical waveguides 5 running parallel to both sides of the output waveguide 3, and therefore the light from the unwanted optical waveguides 5 is converted into current, thereby preventing the occurrence of further optical reflection within the unwanted optical waveguides 5.

[0071] Fig. 14 is an explanatory diagram showing an example of the configuration of an MMI coupler 1E that is a modified example of the sixth embodiment. Note that the same components as those in the MMI coupler 1E of the sixth embodiment are denoted by the same reference numerals, and explanations of the overlapping configurations and operations will be omitted. The optical terminal section 7 that is optically coupled to the unwanted optical waveguide 5 in the MMI coupler 1E shown in Fig. 13 is configured with a doped region 7D that converts light from the unwanted optical waveguide 5 into heat, instead of an inverted tapered structure. The doped region 7D converts light that is guided through the unwanted optical waveguide 5 into heat, and therefore can suppress reflection of light within the unwanted optical waveguide 5.

[0072] In the MMI coupler 1E shown in FIG. 14, the unnecessary optical waveguides 5 running parallel to both sides of the output waveguide 3 are optically coupled to the doping regions 7D, and therefore, the light from the unnecessary optical waveguides 5 is converted into heat, thereby preventing the occurrence of re-reflection of light within the unnecessary optical waveguides 5.

[0073] FIG. 15 is an explanatory diagram showing an example of an optical communication device 80 incorporating the MMI coupler 1 of this embodiment. The optical communication device 80 shown in FIG. 15 is connected to an output optical fiber and an input optical fiber. The optical communication device 80 includes a DSP (Digital Signal Processor) 81, a light source 82, an optical transmitter 83, and an optical receiver 84. The DSP 81 is an electrical component that performs digital signal processing. For example, the DSP 81 performs processing such as encoding transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the optical transmitter 83. The DSP 81 also obtains an electrical signal including reception data from the optical receiver 84 and performs processing such as decoding of the obtained electrical signal to obtain the reception data.

[0074] The light source 82 includes, for example, a laser diode or the like, and generates light of a predetermined wavelength and supplies it to the optical transmitter 83 and the optical receiver 84. The optical transmitter 83 is an optical device that modulates the light supplied from the light source 82 with an electrical signal output from the DSP 81 and outputs the obtained transmission light to an optical fiber. The optical transmitter 83 generates transmission light by modulating the light supplied from the light source 82 with an electrical signal input to the optical modulator as the light propagates through the waveguide.

[0075] The optical receiver 84 receives an optical signal from the optical fiber and demodulates the received light using light supplied from the light source 82. The optical receiver 84 then converts the demodulated received light into an electrical signal and outputs the converted electrical signal to the DSP 81. The optical transmitter 83 and the optical receiver 84 each incorporate an MMI coupler 1 that guides light.

[0076] In the MMI coupler 1 in the optical communication device 80, the unwanted optical waveguide 5 having an asymmetric rib-type waveguide structure is arranged in the output section 4B in the interference region 4. As a result, the unwanted optical waveguide 5 can sufficiently reduce the amount of reflected light input from the interference region 2 to the input waveguide 2 while satisfying the single-mode condition.

[0077] For ease of explanation, each waveguide in the MMI coupler 1 may be a PLC in which the core and cladding are formed of SiO2, an InP waveguide, a GaAs waveguide, or a SiN (Silicon Nitride) waveguide, and other suitable alternatives are possible. Alternatively, a Si waveguide in which the core is Si or Si3N4, the lower cladding is SiO2, and the upper cladding is SiO2, air, or SiN, and other suitable alternatives are also possible. Si and SiN waveguides have a large relative refractive index difference, which provides strong optical confinement, enabling the realization of a low-loss curved waveguide even with a small R, thereby enabling the miniaturization of planar optical waveguide elements. [Explanation of symbols]

[0078] 1, 1A, 1C, 1D, 1E MMI type coupler 2. Input waveguide 3 Output waveguide 4 Interference area 4A input section 4B Output section 5 Unnecessary optical waveguide 5A Rib section 5B Slab section 6 Unnecessary optical waveguide 6A Rib section 6B Slab section 7 Optical termination 51C doping region 80 Optical communication equipment 82 Light source 83 Optical Transmitter 84 Optical receiver

Claims

1. an input waveguide; an output waveguide; and an interference region having an input portion optically coupled to the input waveguide and an output portion optically coupled to the output waveguide, the interference region having a waveguide width greater than the waveguide widths of the input waveguide and the output waveguide; an unnecessary optical waveguide provided at the output section within the interference region and running in parallel with the output waveguide; The unnecessary optical waveguide is A rib portion; a slab portion formed on one side surface of the rib portion and having a thickness smaller than that of the rib portion; The slab portion is not formed on the other side surface of the rib portion, and the waveguide is an asymmetric rib-type waveguide. an optical device, characterized in that the other side surface of the rib portion is disposed on a side surface of the output waveguide, forming a single mode waveguide that guides only a fundamental mode;

2. further comprising another unnecessary optical waveguide provided at the input section within the interference region and running in parallel with the input waveguide; The other unnecessary optical waveguide is A rib portion; a slab portion that is thinner than the thickness of the rib portion; 2. The optical device according to claim 1, which is a single-mode waveguide that guides only light in a fundamental mode.

3. The input waveguide, the output waveguide, and the interference region are a rib-type waveguide having a first rib portion and a first slab portion that is thinner than the thickness of the first rib portion; The slab portion of the unnecessary optical waveguide is 3. The optical device according to claim 1, wherein the thickness of the first slab portion is larger than the thickness of the second slab portion.

4. The slab portion in the unnecessary optical waveguide is a first slab portion formed on one side surface of the rib portion; 3. The optical device according to claim 1, further comprising: a second slab portion formed on the other side surface of the rib portion, the second slab portion being thicker than the first slab portion.

5. The unnecessary optical waveguide is 5. The optical device according to claim 1, wherein the optical device has a bent waveguide structure in which the bent waveguides are successively separated from the output waveguide.

6. The slab portion in the unnecessary optical waveguide is 6. The optical device according to claim 1, wherein a doping region is provided in a part or the entire region of the slab portion.

7. 7. The optical device according to claim 1, further comprising an optical termination section that optically couples with the unnecessary optical waveguide so as to terminate the light that is guided through the unnecessary optical waveguide.

8. an input waveguide; an output waveguide; and an interference region having an input portion optically coupled to the input waveguide and an output portion optically coupled to the output waveguide, the interference region having a waveguide width greater than the waveguide widths of the input waveguide and the output waveguide; an unnecessary optical waveguide provided at the output section in the interference region and running in parallel with the output waveguide, The unnecessary optical waveguide is A rib portion; a slab portion formed on one side surface of the rib portion and having a thickness smaller than that of the rib portion; The slab portion is not formed on the other side surface of the rib portion, and the waveguide is an asymmetric rib-type waveguide. a substrate-type optical waveguide element, characterized in that the other side surface of the rib portion is disposed on a side surface of the output waveguide, forming a single-mode waveguide that guides only light of a fundamental mode;

9. A light source and an optical transmitter that optically modulates light from a light source using a transmission signal and transmits the transmission light; an optical receiver that receives a reception signal from received light using light from the light source; an optical communication device having a substrate-type optical waveguide element that guides the light within the optical transmitter and the optical receiver, The planar optical waveguide element comprises: an input waveguide; an output waveguide; and an interference region having an input portion optically coupled to the input waveguide and an output portion optically coupled to the output waveguide, the interference region having a waveguide width greater than the waveguide widths of the input waveguide and the output waveguide; an unnecessary optical waveguide provided at the output section in the interference region and running in parallel with the output waveguide, The unnecessary optical waveguide is A rib portion; a slab portion formed on one side surface of the rib portion and having a thickness smaller than that of the rib portion; The slab portion is not formed on the other side surface of the rib portion, and the waveguide is an asymmetric rib-type waveguide. an optical communication device, characterized in that the other side surface of the rib portion is disposed on a side surface of the output waveguide, forming a single mode waveguide that guides only light of a fundamental mode;

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