Optical waveguide chip

Abstract

The optical waveguide chip (100) includes a substrate (1), a waveguide (10) formed on the substrate (1), and an exit end face (101) for emitting light guided by the waveguide (10) to the outside. In a plan view, the waveguide (10) has a first part (11) extending perpendicularly to the exit end face (101), a second part (12) extending obliquely to the exit end face (101) on the side of the first part (11) toward the exit end face (101), and a bent part (13) that connects the first part (11) and the second part (12) in a curve in a plan view. The waveguide (10) includes a core layer (3) and a semiconductor layer (4) formed on the side of the core layer (3) inside the curve formed by the bent part (13), and extending along at least one of the bent part (13) and the second part (12). The refractive index of the semiconductor layer (4) is lower than that of the core layer (3).

Description

Technical Field

[0001] This disclosure relates to an optical waveguide chip.

Background Art

[0002] In an optical waveguide chip, the reflected return light that is reflected at the emission end face of the chip and returns inside the chip has an adverse effect on the operating characteristics. As a countermeasure against the reflected return light, a structure is known in which a window region, which is a bulk semiconductor region, is provided between the end face of the waveguide and the emission end face of the chip. Since the end-face reflected light reflected at the emission end face of the chip is diffused in the window region, recombination of the end-face reflected light with the waveguide can be suppressed.

[0003] On the other hand, in the structure with the window region, there is a problem that the upper end of the emitted light emitted from the waveguide reaches the upper surface of the chip and causes a defect in the beam shape. The defect in the beam shape reduces the fiber coupling efficiency.

[0004] In Patent Document 1, a technique is disclosed in which the width of the core layer of the waveguide is narrowed toward the emission end face of the chip without providing a window region. Since the effective refractive index of the guided light decreases due to the narrowing, it is possible to easily cause the reflected return light recombined with the waveguide to leak out of the core layer. At the same time, the mode field diameter of the guided light can be adjusted so that the upper end of the guided light does not reach the upper surface of the chip.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Incidentally, in optical waveguide chips, there is a demand to increase the beam size of the emitted light in the thickness direction of the waveguide in order to improve the coupling efficiency with the optical fiber. To achieve this, it is necessary to reduce the thickness of the core layer.

[0007] However, in the technology of Patent Document 1, if the thickness of the core layer is reduced, the optical confinement effect weakens, and eventually the upper end of the guided light reaches the top surface of the chip, resulting in a defect in the beam shape. To maintain the optical confinement effect, it is necessary to widen the core layer to compensate for the reduction in thickness, but in the technology of Patent Document 1, the reduction of reflected light is achieved by narrowing the width of the core layer, so the structure is such that the thickness of the core layer cannot be reduced in practice. In other words, in the technology of Patent Document 1, there is a trade-off between reducing the thickness of the core layer and reducing reflected light.

[0008] To solve the above-mentioned problems, this disclosure aims to provide an optical waveguide chip that can reduce reflected light even when the thickness of the waveguide core layer is reduced. [Means for solving the problem]

[0009] The first aspect of this disclosure is, It comprises a substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the side of the first part closer to the exit end face, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide is, The core layer, Inside the curve formed by the aforementioned bend only The semiconductor layer is formed on the side of the core layer and extends along at least one of the bent portion and the second portion, Preferably, the optical waveguide chip has a refractive index lower than that of the core layer. Furthermore, the second aspect is, It comprises a substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the side of the first part closer to the exit end face, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide is, The core layer, The semiconductor layer is formed on the side of the core layer within the curve formed by the bent portion and extends along at least one of the bent portion and the second portion. The refractive index of the semiconductor layer is lower than that of the core layer. The substrate further comprises an embedding layer for embedding the waveguide, The semiconductor layer is formed between the core layer and the embedding layer inside the curve formed by the bend, Preferably, the optical waveguide chip has a refractive index of the semiconductor layer that is lower than the refractive index of the core layer and higher than the refractive index of the embedding layer. Furthermore, the third aspect is, It comprises a substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the side of the first part closer to the exit end face, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide is, The core layer, The semiconductor layer is formed on the side of the core layer within the curve formed by the bent portion and extends along at least one of the bent portion and the second portion. Preferably, the optical waveguide chip has a semiconductor layer whose refractive index is lower than that of the core layer and gradually decreases as it moves away from the core layer.

[0012] The 4 aspect has a substrate, a waveguide formed on the substrate, and an output end face for outputting externally the light guided by the waveguide, the waveguide in a plan view, has a first part extending perpendicular to the output end face, a second part extending obliquely with respect to the output end face on the output end face side of the first part, and a bending part connecting the first part and the second part with a curve in a plan view, is formed on the substrate separated from the waveguide only inside the curve formed by the bending part of the waveguide, and includes one or more guiding paths extending in a direction in which the angle formed by the waveguide and the output end face in a plan view is inverted about an axis perpendicular to the output end face, further has an embedding layer for embedding the waveguide and the guiding paths on the substrate, the guiding path is a semiconductor layer having a refractive index higher than that of the substrate and the embedding layer, It is preferable that the optical waveguide chip is terminated within the aforementioned embedding layer. [Effects of the Invention]

[0013] In the first embodiment, a semiconductor layer is formed between the core layer and the embedding layer to smooth the change in refractive index from the core layer to the embedding layer, thereby reducing the reflectivity of reflections within the waveguide.

[0014] In the second embodiment, the number of reflections of reflected light within the waveguide is increased by providing a tapered section and an inverse tapered section in the waveguide. This reduces the amount of reflected light.

[0015] In the third embodiment, since the end-reflected light reflected from the exit end face of the optical waveguide chip is diffused within the window region, the recombination of reflected light back into the waveguide can be suppressed. Furthermore, since the window region is a waveguide layer wider than the waveguide, the refractive index of the core layer necessary to confine the exit light can be secured. This prevents the upper end of the guided light from reaching the upper surface of the optical waveguide chip.

[0016] In the fourth embodiment, a guide path is formed that extends in the same direction as the end-face reflected light reflected at the exit end face, thereby moving the end-face reflected light and the reflected back light recombined with the waveguide away from the waveguide. This reduces the reflected back light.

[0017] According to the first to fourth aspects of this disclosure, reflected light can be reduced even when the thickness of the waveguide core layer is reduced. [Brief explanation of the drawing]

[0018] [Figure 1] This is a plan view showing the two-dimensional refractive index distribution of an optical waveguide chip according to Embodiment 1. [Figure 2] This figure shows the propagation analysis results of waveguide light according to Embodiment 1. [Figure 3] This is a cross-sectional view showing an example of the configuration of an optical waveguide chip according to Embodiment 1. [Figure 4]This figure shows an example in which a waveguide and a semiconductor laser are integrated in an optical waveguide chip according to Embodiment 1. [Figure 5] This figure shows an example in which a waveguide and a semiconductor laser are integrated in an optical waveguide chip according to Embodiment 1. [Figure 6] This figure shows an example in which a waveguide and a semiconductor laser are integrated in an optical waveguide chip according to Embodiment 1. [Figure 7] This is a cross-sectional view of a waveguide according to a modified example of Embodiment 1. [Figure 8] This is a cross-sectional view showing an example of the configuration of an optical waveguide chip using a Si-based material, according to a modification of Embodiment 1. [Figure 9] This is a cross-sectional view showing an example of the configuration of an optical waveguide chip using a Si-based material, according to a modification of Embodiment 1. [Figure 10] This is a plan view showing the two-dimensional refractive index distribution of an optical waveguide chip according to Embodiment 2. [Figure 11] This figure illustrates the effects of Embodiment 2. [Figure 12] This figure illustrates the effects of Embodiment 2. [Figure 13] This figure illustrates a simulation for determining the shapes of the tapered and reverse tapered sections according to Embodiment 2. [Figure 14] This figure shows the results of the taper length optimization simulation according to Embodiment 2. [Figure 15] This figure shows the simulation results for optimizing the inverse taper length and narrowing width according to Embodiment 2. [Figure 16] This figure shows the simulation results for optimizing the shortest distance according to Embodiment 2. [Figure 17] This is a plan view showing the two-dimensional refractive index distribution of an optical waveguide chip according to Embodiment 3. [Figure 18] This is a cross-sectional view of the window region according to Embodiment 3. [Figure 19] This figure shows the simulation results for optimizing the length of the window region according to Embodiment 3. [Figure 20]This is a plan view showing the two-dimensional refractive index distribution of an optical waveguide chip according to Embodiment 4. [Figure 21] This figure shows the propagation analysis results of waveguide light according to Embodiment 4. [Figure 22] This is a cross-sectional view of the waveguide and guide path according to Embodiment 4. [Figure 23] This figure shows the simulation results of the intensity of the waveguide light, which is the basic mode according to Embodiment 4. [Figure 24] This figure shows the simulation results of the intensity of reflected light, which is the primary mode, according to Embodiment 4. [Modes for carrying out the invention]

[0019] Embodiments of this disclosure will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repetition in the description may be omitted. Furthermore, the numerical values ​​for semiconductor layer materials, refractive index, width, thickness, etc., in the following description are examples and may differ from actual values.

[0020] Embodiment 1 Figure 1 is a plan view showing the two-dimensional refractive index distribution of the optical waveguide chip 100 according to Embodiment 1. Here, a cross-section of the core layer 3 of the waveguide 10 is shown. Air 30 with a refractive index of 1 is shown outside the optical waveguide chip 100.

[0021] In the color bar indicating refractive index, colors displayed towards the top of the page indicate a higher refractive index, while colors displayed towards the bottom of the page indicate a lower refractive index.

[0022] The waveguide 10 is embedded in the embedding layer 20. The waveguide 10 is flared, and as an example, the width of the core layer 3 gradually increases from 1.5 μm to 2.5 μm in the direction toward the end of the waveguide 10.

[0023] The waveguide 10 is bent at an angle to the output end face 101 of the optical waveguide chip 100, and then terminated at the output end face 101.

[0024] More specifically, the waveguide 10 has a first section 11, a second section 12, and a bent section 13. The first section 11 extends perpendicularly to the exit end face 101 of the optical waveguide tip 100. The second section 12 extends obliquely to the exit end face 101 on the side of the first section 11 and includes the end of the waveguide 10. The bent section 13 connects one end of the first section 11 on the exit end face 101 side with the other end of the second section 12 that is not the end side, in a curved manner in a plan view.

[0025] The guided light 50 guided through the waveguide 10 is incident obliquely on the exit end face 101 and is emitted to the outside. In this case, most of the end-face reflected light 51 reflected at the exit end face 101 is diffused into the embedding layer 20 located inside the curve formed by the bent portion 13. Therefore, it is possible to suppress the recombination of the end-face reflected light 51 into the waveguide 10 as reflected back light 53 (not shown).

[0026] The waveguide 10 of this embodiment further includes a semiconductor layer 4 formed between the core layer 3 and the embedding layer 20 inside the curve formed by the bent portion 13, and extending along the bent portion 13 and the second portion 12. The refractive index of the semiconductor layer 4 is adjusted to be lower than the refractive index of the core layer 3 and higher than the refractive index of the embedding layer 20.

[0027] For example, at a wavelength of 1550 nm, the refractive index of the i-InGaAsP core layer 3 is 3.398. The refractive index of the i-InGaAsP semiconductor layer 4 is 3.290. The refractive index of the Fe-doped InP embedding layer 20 is 3.169.

[0028] Here, if the angle between the second part 12 of the waveguide 10 and the exit end face 101 is θ (not shown), the reflected light 53 arrives from the waveguide 10 from a direction of 2θ. That is, since the reflected light 53 arrives at an oblique angle to the waveguide 10, it travels backward through the waveguide 10 while being reflected within the waveguide 10. Specifically, the reflected light 53 travels backward through the core layer 3 while being alternately reflected at the interface between the core layer 3 and the semiconductor layer 4, which is inside the curve formed by the bent portion 13, and at the interface between the core layer 3 and the embedding layer 20, which is outside the curve formed by the bent portion 13. Generally, it is known that the reflectance of reflection within a waveguide is proportional to the square of the refractive index difference at the interface.

[0029] In this embodiment, by forming a semiconductor layer 4 between the core layer 3 and the embedding layer 20, the change in refractive index from the core layer 3 to the embedding layer 20 is smoothed, thereby reducing the reflectance of reflections within the waveguide. In other words, in this embodiment, reflected light 53 is more easily leaked into the embedding layer 20 inside the curve formed by the bend 13. This makes it possible to reduce the reflected light 53 that has been recombined into the waveguide 10.

[0030] Figure 2 shows the propagation analysis results of the waveguide light 50 according to Embodiment 1. The same cross-section as in Figure 1 is shown here. In the color bar indicating amplitude, colors displayed towards the top of the page indicate higher amplitude, and colors displayed towards the bottom of the page indicate lower amplitude. The FDTD (Finite-Difference Time-Domain method) was used for the propagation analysis of the waveguide light 50. As a result of the propagation analysis, a reduction effect of 3 dB to 5 dB in reflected light 53 was observed.

[0031] Figure 3 is a cross-sectional view showing an example of the configuration of the optical waveguide chip 100 according to Embodiment 1. Here, the direction perpendicular to the plane of the paper is considered the waveguide direction. This figure also corresponds to the A-A' section of Figure 1, that is, the cross-sectional view of the second part 12 of the waveguide 10.

[0032] A mesa-type waveguide 10 is formed on an n-type InP substrate 1. The waveguide 10 has a first cladding layer 2 which is part of the substrate 1, a core layer 3, a semiconductor layer 4, and a second cladding layer 5 which is part of the p-type contact layer 8.

[0033] The first cladding layer 2 of the n-type InP is formed by processing a portion of the upper surface of the substrate 1 into a mesa shape.

[0034] The i-InGaAsP core layer 3 is formed on the first cladding layer 2. The thickness of the core layer 3 is, for example, 140 nm.

[0035] A semiconductor layer 4 made of i-InGaAsP is formed on the side of the core layer 3 on top of the first cladding layer. The width of the semiconductor layer 4 is, for example, 1.5 μm. The semiconductor layer 4 has the same thickness as the core layer 3 and is in contact with the side of the core layer 3. The composition ratio of the semiconductor layer 4 is adjusted so that its refractive index is lower than that of the core layer 3 and higher than that of the filling layer 20.

[0036] A second cladding layer 5 of p-type InP is formed on the core layer 3 and the semiconductor layer 4.

[0037] The embedding layer 20 embeds the sides of the mesa-type waveguide 10 on the substrate 1. The embedding layer 20 is an Fe-doped InP layer. On top of the embedding layer 20, an n-type InP layer 7 further embeds the sides of the mesa-type waveguide 10.

[0038] A p-type InP contact layer 8 is formed on top of the second cladding layer 5 and the n-type InP layer 7.

[0039] Thus, in this embodiment, the semiconductor layer 4 is formed on the side of the core layer 3 between the first cladding layer 2 and the second cladding layer 5.

[0040] In this disclosure, the thickness of the core layer 3 is assumed to be 200 nm or less (for example, 140 nm). This thickness is necessary to reduce the refractive index difference between the core layer 3 and the active layer of the semiconductor laser 90 connected to the core layer 3.

[0041] In semiconductor lasers 90, the number of quantum well layers in the active layer and the thickness of the quantum well layers are sometimes reduced to ensure stable optical amplification. This means that the effective refractive index of the active layer decreases. Since the active layer and core layer 3 are butt-jointed, it is desirable to reduce the refractive index of core layer 3 as well to reduce the refractive index difference with the active layer, from the viewpoint of suppressing interfacial reflection. To reduce the refractive index of core layer 3, it is necessary to reduce the thickness of core layer 3. It is said that when the thickness of core layer 3 is 200 nm or less, the refractive index difference between core layer 3 and the active layer falls within the design tolerance.

[0042] Furthermore, by reducing the thickness of the core layer 3, the lens that couples the emitted light 52 to the optical fiber can be miniaturized and have a longer focal length. This is because reducing the thickness of the core layer 3 increases the beam size of the emitted light 52 in the thickness direction, and thus reduces the divergence angle of the emitted light 52. A longer focal length for the lens means that the robustness of the fiber coupling efficiency to lens misalignment is increased. In addition, by increasing the beam size in the thickness direction of the core layer 3, the imbalance between the beam size in the thickness direction and the beam size in the width direction is improved, and the aspect ratio can be brought closer to 1. This makes it possible to improve the fiber coupling efficiency.

[0043] When the thickness of the core layer 3 is reduced, the refractive index of the core layer 3 decreases, and the optical confinement effect weakens. In this embodiment, the refractive index and, consequently the optical confinement effect, which are reduced by widening the waveguide 10, can be compensated for. This prevents the upper end of the guided light 50 from reaching the upper surface of the optical waveguide chip 100.

[0044] On the other hand, from the viewpoint of reducing the reflectivity of reflections within the waveguide, increasing the width of the core layer 3 and raising the refractive index of the core layer 3 is disadvantageous. However, in this embodiment, the reflectivity of reflections within the waveguide is reduced by forming the semiconductor layer 4. For these reasons, in this embodiment, even if the thickness of the core layer 3 of the waveguide 10 is reduced, the reflected light 53 can be reduced.

[0045] Figures 4 to 6 show an example in which the waveguide 10 and semiconductor laser 90 are monolithically integrated in the optical waveguide chip 100 according to Embodiment 1.

[0046] In Figure 4, a semiconductor laser 90 and a waveguide 10 are integrated on a substrate 1. The semiconductor laser 90 has an active layer with a multiple quantum well structure and emits laser light. The waveguide 10 guides the laser light from the semiconductor laser 90 and generates emitted light 52 by emitting it from the output end face 101.

[0047] In Figure 5, an SOA91 (Semiconductor Optical Amplifier) ​​is placed between the semiconductor laser 90 and the waveguide 10, as explained in Figure 4, to amplify the light from the semiconductor laser 90. These are being further accumulated.

[0048] In Figure 6, there are multiple semiconductor lasers 90 as described in Figure 5, and a coupler 92 that bundles the laser light from the multiple semiconductor lasers 90 is integrated between the multiple semiconductor lasers 90 and the SOA 91. The coupler 92 is, for example, an MMI (Multi-mode interference waveguide). These include, or AWG (Arrayed Waveguide Grating), etc.

[0049] As described above, in this embodiment, the reflected light 53 that recombines with the waveguide 10 can be reduced. Therefore, it is possible to suppress the entry of reflected light 53 into upstream semiconductor elements such as the semiconductor laser 90 and SOA 91. This results in the output light 52 with a narrow linewidth.

[0050] As described above, in this embodiment, by forming a semiconductor layer 4 between the core layer 3 and the embedding layer 20, the change in refractive index from the core layer 3 to the embedding layer 20 is smoothed out, thereby reducing the reflectance of reflections within the waveguide. As a result, even when the thickness of the core layer 3 of the waveguide 10 is reduced, the reflected light 53 can be reduced.

[0051] <Variation 1 of Embodiment 1> Figure 7 is a cross-sectional view of a waveguide 10 according to a modified example of Embodiment 1. This figure corresponds to the A-A' section in Figure 1, i.e., the cross-sectional view of the second part 12 of the waveguide 10. Here, the semiconductor layer 4 is a three-layer structure consisting of a first layer 41, a second layer 42, and a third layer 43 stacked perpendicular to the substrate 1. The refractive index is highest in the first layer in contact with the core layer 3, and lowest in the third layer 43 in contact with the embedding layer 20. As an example, the refractive index of the first layer 41 is 3.35. The refractive index n of the second layer 42 is 3.3. The refractive index n of the third layer 43 is 3.2. The answer is 5. Thus, the refractive index of the semiconductor layer 4 may gradually decrease as it moves away from the side in contact with the core layer 3. This makes the change in refractive index from the core layer 3 to the embedding layer 20 smoother than in Embodiment 1, and further reduces the reflectance of reflections within the waveguide.

[0052] <Modification 2 of Embodiment 1> In Figure 1, the semiconductor layer 4 is shown to extend along the bent portion 13 and the second portion 12. However, the semiconductor layer 4 does not necessarily have to extend along both the bent portion 13 and the second portion 12; it is sufficient for it to extend along at least one of them. In this case as well, the same effects as in Embodiment 1 can be obtained.

[0053] <Modification 3 of Embodiment 1> Figure 1 illustrates that the waveguide 10 is flared. By making it flared, the beam size of the emitted light 52 in the width direction of the core layer 3 can be increased and the divergence angle can be reduced. However, the waveguide 10 does not necessarily have to be flared, and the width of the waveguide 10 may be uniform toward the emission end face 101. This is common to all embodiments.

[0054] <Modification 4 of Embodiment 1> Furthermore, the optical waveguide chip 100 can also be made of materials other than InGaAsP / InP, such as semiconductor materials like InGaAsN / GaAs or InGaAlAs / InGaAsP, or Si-based materials used in silicon photonics. This is true for all embodiments. Figures 8 and 9 show examples using Si-based materials.

[0055] Figure 8 is a cross-sectional view showing an example of the configuration of an optical waveguide chip 100 using a Si-based material, according to a modification of Embodiment 1. Here, the direction perpendicular to the plane of the paper is considered the waveguide direction. This figure also corresponds to the A-A' section of Figure 1, i.e., the cross-sectional view of the second part 12 of the waveguide 10.

[0056] A waveguide 10 is formed on a Si substrate 1. The waveguide 10 has a first cladding layer 2, a core layer 3, and a semiconductor layer 4.

[0057] A first cladding layer 2 of SiO2 is formed over the entire upper surface of the Si substrate 1.

[0058] On the first cladding layer 2, a Si core layer 3 is formed at the location where the waveguide 10 is to be formed.

[0059] A semiconductor layer 4 is formed on the side of the core layer 3 on top of the first cladding layer 2. The semiconductor layer 4 has the same thickness as the core layer 3 and is in contact with the side of the core layer 3. The semiconductor layer 4 is made of a material with a lower refractive index than the core layer 3, and is composed of, for example, SiN, SiO, SiON, or Si3N4.

[0060] Furthermore, air 30 is used in the second cladding layer 5 and the embedding layer 20.

[0061] Thus, the same effects as in Embodiment 1 can be obtained by using a Si-based material. Furthermore, by using a Si-based material, the second cladding layer 5 and the embedding layer 20 are not necessarily required. This is common to all embodiments.

[0062] Figure 9 shows the optical waveguide chip 100 from Figure 8, with a protective film 9 of SiO2 formed on the first cladding layer 2, the core layer 3, and the semiconductor layer 4.

[0063] Embodiment 2 In Embodiment 1, the reflected light 53 was reduced by forming a semiconductor layer 4. In this embodiment, the reflected light 53 is reduced by providing a tapered portion 121 and an inverse tapered portion 122 in the waveguide 10. The changes from Embodiment 1 will be described below.

[0064] Figure 10 is a plan view showing the two-dimensional refractive index distribution of the optical waveguide chip 100 according to Embodiment 2. Similar to Figure 1 of Embodiment 1, the cross-section of the core layer 3 of the waveguide 10 is shown here as well. The explanation of the color bar indicating the refractive index is the same as in Figure 1 of Embodiment 1 and is therefore omitted.

[0065] The waveguide 10 is embedded in the embedding layer 20. The waveguide 10 is bent at an angle to the exit end face 101 of the optical waveguide chip 100 and terminated at the exit end face 101.

[0066] Similar to Embodiment 1, the waveguide 10 has a first section 11, a second section 12, and a bent section 13. In this embodiment, the waveguide 10 has a tapered section 121 in the second section 12 whose width gradually narrows toward the exit end face 101, and an inverse tapered section 122 connected to the narrowest end of the tapered section 121, from which the width gradually widens toward the exit end face 101. That is, in this embodiment, the waveguide 10 is narrowed by the tapered section 121 and the inverse tapered section 122.

[0067] Figures 11 and 12 illustrate the effects of Embodiment 2. Figure 11 shows the guided light 50 and the equiphase surface 60 of the guided light 50. Figure 12 shows the reflected light 53.

[0068] As shown in Figure 11, the guided light 50 propagates parallel to the waveguide 10. Therefore, even in the portion of the waveguide 10 that is narrowed by the tapered portion 121 and the inverse tapered portion 122, the guided light 50 can pass through by expanding its modes. On the other hand, as shown in Figure 12, the reflected light 53 travels backward through the waveguide 10 while being reflected within the waveguide 10. The reflected light 53 leaks into the embedding layer 20 each time it is reflected within the waveguide, but in this embodiment, by narrowing the waveguide 10, the number of reflections within the waveguide at the inverse tapered portion 122 can be increased. As a result, the reflected light 53 can be reduced.

[0069] In this embodiment as well, the refractive index lost due to the reduced thickness of the core layer 3 can be compensated for by widening the width of the waveguide 10. This prevents the upper end of the guided light 50 from reaching the upper surface of the optical waveguide chip 100.

[0070] Figure 13 illustrates a simulation for determining the shapes of the tapered portion 121 and the reverse tapered portion 122 according to Embodiment 2. In the simulation, the width of the widest end of the tapered portion 121 was defined as W1, and the width of the widest end of the reverse tapered portion 122 was defined as W2, with W1 = W2 = 2.5 μm. The thickness t of the core layer 3 was set to t = 0.14 μm.

[0071] Under the above conditions, optimization simulations were performed for the taper length L1, which is the length of the waveguide 10 in the tapered section 121; the inverse taper length L2, which is the length of the waveguide 10 in the inverse taper section 122; and the narrowing width W0, which is the width of the narrowest end in the tapered section 121. Furthermore, optimization simulations were performed for the shortest distance L3 between the wide end of the inverse taper section 122 and the exit end face 101.

[0072] Figures 14 to 16 show the results of the optimization simulation.

[0073] Figure 14 shows the results of the optimization simulation of the taper length L1 according to Embodiment 2. The vertical axis represents the transmittance of the guided light 50, and the horizontal axis represents the taper length L1. In the simulation, the narrowing width W0 was set to 1.0 μm, and the transmittance of the guided light 50 was calculated for three different cases where the inverse taper length L2 was 5 μm, 10 μm, and 20 μm.

[0074] Figure 14 shows that in all cases where the reverse taper length L2 was 5 μm, 10 μm, or 20 μm, a decrease in transmittance was observed in the region where the taper length L1 was less than 20 μm. Therefore, it can be said that a taper length L1 of 20 μm or more is preferable. On the other hand, no significant difference in transmittance was observed between 5 and 20 μm for the reverse taper length L2.

[0075] Figure 15 shows the optimization simulation results for the inverse taper length L2 and narrowing width W0 according to Embodiment 2. The vertical axis represents the reduction rate (dB) of reflected light 53, and the horizontal axis represents the inverse taper length L2. In the simulation, the shortest distance L3 = 20 μm was set, and the reduction rate of reflected light 53 was calculated for four different cases where the narrowing width W0 was 0.1 μm, 0.5 μm, 1.0 μm, and 1.5 μm.

[0076] Figure 15 shows that in all cases of narrowing width W0, a reduction effect of 5 dB or more in reflected light 53 was observed in the range of inverse taper length L2 from around 0 to 100 μm. From this viewpoint, a narrowing width W0 of 1.5 μm or less is preferable. Furthermore, in particular, when the narrowing width W0 = 1.0 μm, a reduction effect of 10 dB or more in reflected light 53 was observed in the range of inverse taper length L2 from 5 to 60 μm, and a reduction effect of 20 dB or more was observed when the inverse taper length L2 was 25 to 30 μm. From this viewpoint, a narrowing width W0 of 1.0 μm and an inverse taper length L2 of 5 to 60 μm is particularly preferable.

[0077] Figure 16 shows the results of the optimization simulation of the shortest distance L3 according to Embodiment 2. The vertical axis represents the reduction rate (dB) of reflected light 53, and the horizontal axis represents the inverse taper length L2. In the simulation, the narrowing width W0 was set to 1.0 μm, and the reduction rate of reflected light 53 was calculated for four cases where the shortest distance L3 was 0, 10 μm, 20 μm, and 40 μm. For the shortest distance L3 of 0 and 10 μm, an extremely small region was observed where the reduction rate deteriorated drastically with respect to the inverse taper length L2. Therefore, it can be said that it is preferable for the shortest distance L3 to be 20 μm or more.

[0078] In summary, as shown in Figures 14 to 16, it is preferable that the constriction width W0 is 1.5 μm or less, the taper length L1 is 20 μm or more, the reverse taper length L2 is 5 to 60 μm, and the shortest distance L3 is 20 μm or more.

[0079] As described above, in this embodiment, by providing a tapered section 121 and an inverse tapered section 122 in the waveguide 10, the number of reflections of reflected light 53 within the waveguide is increased. This reduces the reflected light 53. Therefore, even when the thickness of the core layer 3 of the waveguide 10 is reduced, the reflected light 53 can be reduced.

[0080] Embodiment 3 In this embodiment, reflected light 53 is reduced by forming a window region 70 between the end of the waveguide 10 and the exit end face 101 of the optical waveguide tip 100. The following describes the changes from Embodiment 1.

[0081] Figure 17 is a plan view showing the two-dimensional refractive index distribution of the optical waveguide chip 100 according to Embodiment 3. Similar to Figure 1 of Embodiment 1, a cross-section of the core layer 3 of the waveguide 10 is shown here as well. The explanation of the color bar indicating the refractive index is the same as in Figure 1 of Embodiment 1 and is therefore omitted.

[0082] A flared waveguide 10 is embedded in the embedding layer 20. The waveguide 10 is bent at an angle to the exit end face 101 of the optical waveguide chip 100 and then terminated.

[0083] In the optical waveguide chip 100 of this embodiment, a window region 70 is formed between the end of the waveguide 10 and the exit end face 101. One of the end faces of the window region 70 forms the exit end face 101, and both sides of the window region 70 perpendicular to the exit end face 101 form both sides of the optical waveguide chip 100. By providing the window region 70, the end-face reflected light 51 reflected at the exit end face 101 of the optical waveguide chip 100 can be diffused into the window region 70, thereby suppressing the recombination of the end-face reflected light 51 into the waveguide.

[0084] The window region 70 in this embodiment has a first cladding layer 2 formed on the substrate 1, a core layer 3 formed on the first cladding layer 2, and a second cladding layer 5 formed on the core layer 3. In other words, the window region 70 is a waveguide layer that guides the emitted light 52 emitted from the end of the waveguide 10 to the emission end face 101. That is, the first cladding layer 2, the core layer 3, and the second cladding layer 5 each extend from the waveguide 10 to the window region 70.

[0085] If the window region 70 were composed of a uniform semiconductor layer, the emitted light 52 would not propagate through the window region 70 as a waveguide mode, causing the upper end of the emitted light 52 to reach the upper surface of the optical waveguide chip 100, resulting in a defect in the beam shape. In this embodiment, the window region 70 is a waveguide layer wider than the waveguide 10, so the refractive index of the core layer 3 necessary to confine the emitted light 52 can be secured. This prevents the upper end of the waveguide light 50 from reaching the upper surface of the optical waveguide chip 100.

[0086] Figure 18 is a cross-sectional view of the window region 70 according to Embodiment 3. This figure corresponds to the BB cross-section in Figure 17. The explanation of the color bar indicating the refractive index is the same as in Figure 1 of Embodiment 1 and is therefore omitted. The thicknesses of the first cladding layer 2 and the core layer 3 in the window region 70 are equal to the thicknesses of each layer in the waveguide 10. This prevents the occurrence of a step in the core layer 3 at the boundary between the waveguide 10 and the window region 70.

[0087] Figure 19 shows the optimization simulation results for the length L of the window region 70 according to Embodiment 3. The horizontal axis represents the length L (μm) of the window region 70, and the vertical axis represents the fiber coupling efficiency (dB) of the waveguide light 50. However, the length L of the window region 70 refers to the length from the end of the waveguide 10 to the exit end face 101 in the window region 70, as shown in Figure 17. The relative transmittance with respect to length L is shown when the fiber coupling efficiency at L=0um is set to 0dB. It is.

[0088] A decrease in fiber coupling efficiency was observed as the length L increased, but even at the maximum length L (L=100μm), the decrease in fiber coupling efficiency was only about -2dB. On the other hand, if the window region 70 were a uniform semiconductor layer, the effect of beam shape defects would be superimposed on the decrease in fiber coupling efficiency due to the presence of the window region 70, and the final decrease in fiber coupling efficiency is expected to be about -1 to -3dB. In this embodiment, sufficient fiber coupling efficiency can be ensured even if the length L of the window region 70 is 100μm. For this reason, it can be said that the length L of the window region 70 is preferably 100μm or less.

[0089] As described above, in this embodiment, a window region 70 made of a waveguide layer is formed between the end of the waveguide 10 and the exit end face 101 of the optical waveguide chip 100. The end-face reflected light 51 reflected at the exit end face 101 of the optical waveguide chip 100 is diffused into the window region 70, so that the reflected back light 53 does not recombine with the waveguide 10. Furthermore, since the window region 70 in this embodiment is a waveguide layer wider than the waveguide 10, the refractive index of the core layer 3 necessary to confine the exit light 52 can be secured. This prevents the upper end of the waveguide light 50 from reaching the upper surface of the optical waveguide chip 100. Therefore, even if the thickness of the core layer 3 of the waveguide 10 is reduced, the reflected back light 53 can be reduced.

[0090] <Modification 1 of Embodiment 3> Figure 17 illustrates that the two sides of the window region 70 perpendicular to the exit end face 101 form the two sides of the optical waveguide chip 100. The wider the window region 70, the better the diffusion effect of the end-face reflected light 51 can be improved. However, the width of the window region 70 only needs to be wider than the waveguide 10, and it does not necessarily have to extend to the sides of the optical waveguide chip 100. In this case as well, the diffusion effect of the end-face reflected light 51 can be obtained.

[0091] Embodiment 4 In this embodiment, reflected light 53 is reduced by forming a guide path 80 inside the curve formed by the bend 13 of the waveguide 10. The changes from Embodiment 1 will be described below.

[0092] Figure 20 is a plan view showing the two-dimensional refractive index distribution of the optical waveguide chip 100 according to Embodiment 4. Similar to Figure 1 of Embodiment 1, a cross-section of the core layer 3 of the waveguide 10 is shown here as well. The explanation of the color bar indicating the refractive index is the same as in Figure 1 of Embodiment 1 and is therefore omitted.

[0093] A flared waveguide 10 is embedded in the embedding layer 20. The waveguide 10 is bent at an angle to the exit end face 101 of the optical waveguide chip 100 and terminated at the exit end face 101.

[0094] Similar to Embodiment 1, the waveguide 10 has a first section 11, a second section 12, and a bent section 13. In the optical waveguide chip 100 of this embodiment, there are a plurality of guide paths 80 that are formed inside the curve formed by the bent section 13 of the waveguide 10, spaced apart from the waveguide 10, and extending in a direction in which the angle between the waveguide 10 and the exit end face 101 is reversed along an axis perpendicular to the exit end face 101.

[0095] The guide path 80 is formed on the substrate 1 and embedded by the embedding layer 20. The guide path 80 is a semiconductor layer having a refractive index higher than that of the embedding layer 20 and the substrate 1. For example, the guide path 80 is made of the same i-InGaAsP as the core layer 3 of the waveguide 10 and has the same refractive index as the core layer 3.

[0096] The direction in which the guide path 80 extends is the same as the direction of the end-face reflected light 51 reflected from the exit end face 101. Therefore, in this embodiment, the end-face reflected light 51 and the reflected back light 53 that has been recombined with the waveguide 10 can be taken into the guide path 80 and guided away from the waveguide 10. This reduces the reflected back light 53.

[0097] Furthermore, since the guide path 80 in this embodiment is formed at a distance from the waveguide 10, it has the advantage of not disrupting the beam shape of the guided light 50.

[0098] Furthermore, in this embodiment as well, by not narrowing the width of the waveguide 10, it is possible to prevent the beam shape from being lost on the upper surface of the optical waveguide chip 100 for the same reasons as in Embodiment 1.

[0099] Furthermore, if the guideway 80 were to penetrate the embedding layer 20, the difference in refractive index between the guideway 80 and the air 30 would be greater than the difference in refractive index between the guideway 80 and the embedding layer 20. Therefore, the reflection would be greater compared to the case where the guideway 80 is terminated within the embedding layer 20. For this reason, it is preferable that the guideway 80 is terminated within the embedding layer 20.

[0100] Figure 21 shows the propagation analysis results of the waveguided light 50 according to Embodiment 4. The same cross-section as in Figure 20 is shown here. In the color bar indicating amplitude, colors displayed towards the top of the page indicate higher amplitude, and colors displayed towards the bottom of the page indicate lower amplitude. The FDTD method was used for the propagation analysis. It is clear that the beam shape of the waveguided light 50 is not distorted. It is also clear that the end-face reflected light 51 reflected at the exit end face 101 and the reflected back light 53 in the waveguide 10 are guided along the guide path 80.

[0101] Figure 22 is a cross-sectional view of the waveguide 10 and guide path 80 according to Embodiment 4. This figure corresponds to the C-C' cross-section in Figure 20. The explanation of the color bar indicating the refractive index is the same as in Figure 1 of Embodiment 1 and is therefore omitted. The guide path 80 is formed on the substrate 1 and its sides are embedded by the embedding layer 20. The guide path 80 is, for example, a semiconductor layer with a width of 0.5 μm and a height of 0.5 μm.

[0102] Figures 23 and 24 show the results of a simulation of the intensity of the guided light 50 to determine the separation distance D between the waveguide 10 and the guide path 80. As shown in Figure 20, the separation distance D is the amount of displacement of the starting end of the guide path 80 from the waveguide 10, and refers to the amount of displacement in the direction parallel to the exit end face 101 in a plan view.

[0103] Figure 23 shows the intensity simulation results of the waveguide light 50, which is the basic mode according to Embodiment 4. Figure 24 shows the intensity simulation results of the reflected light 53, which is the first-order mode according to Embodiment 4. Figures 23 and 24 are cross-sectional views of the waveguide 10, with the direction perpendicular to the plane of the paper being the waveguide direction. In the color bar showing amplitude, colors displayed towards the top of the paper indicate higher light intensity, and colors displayed towards the bottom of the paper indicate lower light intensity.

[0104] As shown in Figure 23, the amount of leakage of the basic mode, the guided light 50, to the side of the core layer 3 was approximately 0.2 μm. From the viewpoint of preventing the guide path 80 from mistakenly taking in the guided light 50 and disrupting the beam shape of the guided light 50, it is preferable that the separation distance D be 0.2 μm or more.

[0105] Since the reflected light 53 arrives at the waveguide 10 at an oblique angle, it includes higher-order modes, such as the first-order mode. As shown in Figure 24, the leakage of the first-order reflected light 53 to the side of the core layer 3 was approximately 1.5 μm. Therefore, from the viewpoint of incorporating the reflected light 53 recombined with the waveguide 10 into the guide path 80, it is preferable that the separation distance D be within 1.5 μm.

[0106] To summarize Figures 23 and 24 above, the separation distance D is preferably between 0.2 μm and 1.5 μm.

[0107] As described above, in this embodiment, by forming a guide path 80 that extends in the same direction as the end-face reflected light 51 reflected at the exit end face 101, the end-face reflected light 51 and the reflected back light 53 that has recombined with the waveguide 10 are moved away from the waveguide 10. This reduces the reflected back light 53. Therefore, even if the thickness of the core layer 3 of the waveguide 10 is reduced, the reflected back light 53 can be reduced.

[0108] <Variation 1> Note that the guideway 80 does not necessarily have to be multiple; it may be just one. In this case as well, the same effects as in Embodiment 4 can be obtained.

[0109] This disclosure is not limited to the embodiments described above, and various modifications can be made during implementation without departing from its essence. Furthermore, each embodiment and its modifications may be combined as appropriate, and in that case, the combined effects can be obtained. [Explanation of symbols]

[0110] 1: Substrate, 2: First cladding layer, 3: Core layer, 4: Semiconductor layer, 5: Second cladding layer, 7: n-type InP layer, 8: Contact layer, 9: Protective film, 10: Waveguide, 11: First part, 12: Second part, 13: Bending part, 20: Embedding layer, 30: Air, 41: First layer, 42: Second layer, 43: Third layer, 50: Waveguide light, 51: End-face reflected light, 52: Emitted light, 53: Reflected light, 60: Equiphase plane, 70: Window region, 80: Guide path, 90: Semiconductor laser, 92: Coupler, 100: Optical waveguide chip, 101: Emitting end face, 121: Tapered part, 122: Reverse tapered part, D: Separation distance, L1: Tapered length, L2: Reverse tapered length, L3: Shortest distance, W0: Narrowing width, t: Thickness

Claims

1. It comprises a substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the exit end face side of the first part, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide is, The core layer, A semiconductor layer is formed on the side of the core layer only on the inside of the curve formed by the bent portion, and extends along at least one of the bent portion and the second portion. An optical waveguide chip in which the refractive index of the semiconductor layer is lower than that of the core layer.

2. It comprises a substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the exit end face side of the first part, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide is, The core layer, The semiconductor layer is formed on the side of the core layer within the curve formed by the bent portion and extends along at least one of the bent portion and the second portion. The substrate further comprises an embedding layer for embedding the waveguide, The semiconductor layer is formed between the core layer and the embedding layer inside the curve formed by the bend, An optical waveguide chip in which the refractive index of the semiconductor layer is lower than that of the core layer and higher than that of the embedding layer.

3. The waveguide further includes a first cladding layer formed on the substrate, The core layer is formed on the first cladding layer, The optical waveguide chip according to claim 1 or 2, wherein the semiconductor layer is formed on the side of the core layer on the first cladding layer.

4. It comprises a substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the exit end face side of the first part, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide is, The core layer, The semiconductor layer is formed on the side of the core layer within the curve formed by the bent portion and extends along at least one of the bent portion and the second portion. An optical waveguide chip in which the refractive index of the semiconductor layer is lower than that of the core layer and gradually decreases as it moves away from the core layer.

5. A substrate, a waveguide formed on the substrate, and an exit end face for emitting light guided by the waveguide to the outside, The waveguide is, In a plan view, it has a first part extending perpendicularly to the exit end face, a second part extending obliquely to the exit end face on the exit end face side of the first part, and a bent part that connects the first part and the second part by a curve in a plan view. The waveguide comprises one or more guide paths formed on the substrate, spaced apart from the waveguide, only on the inside of the curve formed by the bend of the waveguide, and extending in a direction in which the angle between the waveguide and the exit end face is reversed on an axis perpendicular to the exit end face in a plan view, The substrate further comprises an embedding layer for embedding the waveguide and the guide path, The aforementioned taxiway is The semiconductor layer has a higher refractive index than the substrate and the embedding layer. An optical waveguide chip terminated within the aforementioned embedding layer.

6. The optical waveguide chip according to claim 5, wherein the guide path is positioned offset from the waveguide by 0.2 μm to 1.5 μm in a direction parallel to the output end face when viewed in plan.

Images