Spot size converter and manufacturing method thereof

Abstract

A spot size converter and a manufacturing method thereof. The spot size converter includes: a first waveguide including a first core layer, the first waveguide propagating light; and a second waveguide including a second core layer and arranged on the first waveguide, the second waveguide propagating light. The first waveguide and the second waveguide extend in a waveguide direction. A first region and a second region are continuously arranged along the waveguide direction. In the first region, the second waveguide has a tapered shape in a cross section, which becomes narrower as it moves away from the first waveguide. An angle between a side surface of the second waveguide and a bottom surface of the second waveguide is 60° or less.

Description

Technical Field

[0001] The invention relates to a spot size converter and a method for manufacturing the same. Background Art

[0002] Japanese Patent Application Laid-Open No. 2002-519842 discloses a spot size converter for converting the spot size of light. Summary of the invention

[0003] A spot size converter has, for example, two waveguides through which light propagates. Thus, light is transferred between the waveguides. However, the coupling between the waveguides is weak, and the conversion efficiency of light may be reduced. Therefore, an object of the present disclosure is to provide a spot size converter capable of improving conversion efficiency and a method for manufacturing the same.

[0004] The spot size converter according to the present disclosure includes: a first waveguide, the first waveguide includes a first core layer, the first waveguide propagates light; and a second waveguide, the second waveguide is arranged above the first waveguide and includes a second core layer, the second waveguide propagates light. The first waveguide and the second waveguide extend in the waveguide direction. A first region and a second region are continuously arranged along the waveguide direction. In the first region, the second waveguide has a tapered shape in a cross section, which becomes narrower as it moves away from the first waveguide. The angle between the side surface of the second waveguide and the bottom surface of the second waveguide is 60° or less.

[0005] The method for manufacturing a spot size converter according to the present disclosure includes the following steps: forming a first waveguide and a second waveguide in this order; and etching the second waveguide to form a tapered shape in a cross section, which becomes narrower as it moves away from the first waveguide. The angle between the side surface of the second waveguide and the bottom surface of the second waveguide is 60° or less. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1A is a cross-sectional view illustrating a spot size converter according to the first embodiment.

[0007] Figure 1B is a cross-sectional view illustrating a spot size converter according to the first embodiment.

[0008] Figure 1C is a cross-sectional view illustrating a spot size converter according to the first embodiment.

[0009] Figure 2A is a cross-sectional view illustrating a spot size converter.

[0010] Figure 2B is a cross-sectional view illustrating a spot size converter.

[0011] Figure 2Cis a cross-sectional view illustrating a spot size converter.

[0012] Figure 3A is a cross-sectional view illustrating a spot size converter.

[0013] Figure 3B is a cross-sectional view illustrating a spot size converter.

[0014] Figure 4 It is a graph showing the calculation results of conversion efficiency.

[0015] Figure 5A is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0016] Figure 5B is a plan view illustrating a method of manufacturing a spot size converter.

[0017] Figure 5C is a plan view illustrating a method of manufacturing a spot size converter.

[0018] Fig. 6A is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0019] Figure 6B is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0020] Fig. 7A is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0021] Figure 7B is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0022] Figure 8 is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0023] Fig. 9A is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0024] Fig. 9B is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0025] Fig. 9C is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0026] Fig. 10A is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0027] Fig. 10B is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0028] Fig. 10Cis a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0029] Fig.11A is a cross-sectional view illustrating a method of manufacturing a spot size converter.

[0030] Fig. 11B is a plan view illustrating a method of manufacturing a spot size converter.

[0031] Fig. 11C is a plan view illustrating a method of manufacturing a spot size converter.

[0032] Fig.12 is a cross-sectional view illustrating a spot size converter according to a second embodiment. DETAILED DESCRIPTION

[0033] First, the contents of the embodiments of the present disclosure will be described by enumeration.

[0034] An embodiment of the present disclosure is (1) a spot size converter, comprising: a first waveguide including a first core layer, the first waveguide propagating light; and a second waveguide including a second core layer and disposed on the first waveguide, the second waveguide propagating light. The first waveguide and the second waveguide extend in a waveguide direction. A first region and a second region are continuously arranged along the waveguide direction. In the first region, the second waveguide has a conical shape in a cross section, which becomes narrower as it moves away from the first waveguide. An angle between a side surface of the second waveguide and a bottom surface of the second waveguide is 60° or less. Therefore, the conversion efficiency of light between the first core layer and the second core layer is improved.

[0035] (2) The second waveguide may include a first cladding layer disposed between the first core layer and the second core layer. The second core layer and the first cladding layer are included in a tapered shape in a cross section. Compared with a case where the first cladding layer is included in a rectangular portion of the second waveguide, an effective refractive index of the first cladding layer is increased. Therefore, conversion efficiency is improved.

[0036] (3) The second waveguide may include a second cladding layer disposed on the second core layer, and the second cladding layer in the first region may be thinner than the second cladding layer in the second region, or the second cladding layer may not necessarily be disposed in the first region. Since the effective refractive index of the second cladding layer is reduced, the conversion efficiency is improved.

[0037] (4) The first waveguide may have a tapered portion in which the width of the first waveguide decreases from the first region to the second region. The second waveguide may have a tapered portion in which the width of the second waveguide decreases from the second region to the first region. The conversion efficiency of light between the waveguides is improved. In addition, light is cut off due to the decrease in the width of the first waveguide.

[0038] (5) The first region may have a light input / output surface on the side opposite to the second region. Light input from the input / output surface may be transferred from the first waveguide to the second waveguide with high conversion efficiency. In addition, light propagating through the second waveguide is transferred to the first waveguide with high conversion efficiency, and high-intensity light may be output from the input / output surface.

[0039] (6) An embodiment of the present disclosure is a method for manufacturing a spot size converter, comprising the steps of: forming a first waveguide having a first core layer and a second waveguide having a second core layer in this order; and etching the second waveguide to form a tapered shape in a cross section, the tapered shape becoming narrower as it moves away from the first waveguide. An angle between a side surface of the second waveguide and a bottom surface of the second waveguide is 60 degrees or less. Therefore, the conversion efficiency of light between the first waveguide and the second waveguide is improved.

[0040] (7) The second waveguide may include a cladding layer disposed on the second core layer. The cladding layer may include an etch stop layer. The step of etching the second waveguide may include the steps of wet etching the cladding layer until the etch stop layer is exposed to form a mesa in the cladding layer, and dry etching the mesa and the second core layer, the mesa including the cladding layer. The cross-sectional shape of the second core layer may be tapered by dry etching to transfer the shape of the mesa including the cladding layer to the second core layer.

[0041] (8) The mesa of the cladding layer may extend in the <0-11> direction. By extending along the <0-11> direction, the mesa of the cladding layer is made tapered after the wet etching process. By transferring the shape of the mesa to the second core layer through dry etching, the cross-sectional shape of the second core layer can be made tapered.

[0042] Specific examples of the spot size converter and the method for manufacturing the same according to the present disclosure will be described below with reference to the accompanying drawings. It should be noted that the present invention is not limited to these examples but is indicated by the claims and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0043] First embodiment

[0044] (Spot Size Converter) Figures 1A to 3B 1 is a cross-sectional view illustrating a spot size converter 100 according to the first embodiment. In the drawings, the X-axis direction (waveguide direction) is the direction along which light propagates, the Y-axis direction is the direction of the width of the waveguide, and the Z-axis direction is the direction along which semiconductor layers are stacked. These axes are orthogonal to each other. The dotted lines in each figure are imaginary lines showing the boundaries of the lower waveguide 30 (first waveguide) and the upper waveguide 32 (second waveguide). In order to avoid contaminating the drawings, hatching is omitted in the cross-sectional views.

[0045] like Figures 1A to 3BAs shown, the spot size converter 100 has a lower waveguide 30 (first waveguide) and an upper waveguide 32 (second waveguide). The lower waveguide 30 and the upper waveguide 32 extend in the X-axis direction, are optically coupled to each other, and are stacked along the Z-axis direction.

[0046] The lower waveguide 30 includes a core layer 12 (first core layer). The upper waveguide 32 includes a core layer 16 (second core layer) and a cladding layer 26 (second cladding layer) stacked in sequence. The cladding layer 26 includes a cladding layer 20, an etching stop layer 22, and a cladding layer 24 laminated in sequence on the core layer 16. The cladding layer 14 (first cladding layer) is arranged between the core layer 12 and the core layer 16. The lower part of the cladding layer 14 is included in the lower waveguide 30, and the upper part of the cladding layer 14 is included in the upper waveguide 32. The core layer 16 is in contact with the cladding layers 14 and 20. The core layer 12 is in contact with the substrate 10 and the cladding layer 14. The core layer 12 and the core layer 16 are optically coupled to each other, and light transition occurs between them.

[0047] like Figure 1A As shown, the spot size converter 100 has a transition region 36 (first region) and a connecting portion 38 (second region) along the X-axis direction. The transition region 36 and the connecting portion 38 are provided on the substrate 10 and are continuously arranged along the X-axis direction. The length L1 of the transition region 36 is, for example, 800 μm, and the length L2 of the connecting portion 38 is, for example, 100 μm.

[0048] Figures 1B to 3B Describes the Figure 1A The cross section of line A1-A7. Figure 1C and Figure 2A As shown, the upper portion of the core layer 12 is included in the mesa of the lower waveguide 30. Figures 2B to 3B As shown, in the transition region 36, all of the core layer 12 forms a mesa, and a portion of the substrate 10 is included in the mesa. Figure 2B In the embodiment, a portion of the side surface of the mesa from the substrate 10 to the lower portion of the core layer 12 is inclined inwardly relative to the Z-axis direction. Figures 2C to 3B , the side surface of the lower waveguide 30 extends in the Z-axis direction.

[0049] like Figure 1A and Figure 1C As shown, at a portion of line A2, the cladding layer 24 of the cladding layer 26 is removed to form an opening 25 and expose the etching stop layer 22. Figures 2A to 3A In the cross section, the upper side of the coating layer 24 is partially removed, and the coating layer 24 is thinner than Figure 1B The narrowest in cross section.

[0050] like Figures 1B to 3B As shown, the upper waveguide 32 has a terrace. Figure 1BAs shown, the upper waveguide 32 has a mesa having a rectangular cross section. The upper waveguide 32 extends in the Z-axis direction. The width W1 of the mesa of the upper waveguide 32 is, for example, 1.5 μm, and the height H1 is, for example, 2.8 μm.

[0051] like Figures 2B to 3B As shown, the cross section of the upper waveguide 32 in the YZ plane in the transition region 36 has a tapered shape, and its width becomes narrower from the core layer 12 upward. The cross section of the upper waveguide 32 in the transition region 36 is a trapezoid or a triangle. That is, the upper surface of the cladding layer 14, the cladding layer 26, and the core layer 16 are tapered. Figure 2B and Figure 2C , the upper waveguide 32 has a trapezoidal cross-sectional shape, and the side surface of the mesa from the core layer 16 to the cladding layer 26 is inclined inwardly.

[0052] exist Figure 3A and Figure 3B In FIG. 1 , the upper waveguide 32 has a triangular cross-sectional shape. That is, the side surfaces of the cladding layer 14, the core layer 16, and the cladding layer 26 form a triangle in cross section. Figure 3A In FIG. 1 , the cladding layer 24 includes the vertices of the triangle. Figure 3B In the embodiment, the cladding layers 20, 24 and the etch stop layer 22 are removed. Thus, the core layer 16 is exposed. Figure 3B , the core layer 16 includes the vertices of a triangle. In the triangle included in the cross section of the upper waveguide 32, the base angle θ of the triangle is, for example, 54°, and may be 60° or less. The triangle is an isosceles triangle. The side surface of the upper waveguide 32 may be a straight inclined surface, or may be a curved surface that is convex upward or convex downward. When the side surface has a straight inclined surface, the base angle θ is defined as the angle formed by the bottom surface and the side surface of the upper waveguide 32. When the side surface is curved, the base angle θ is the angle at which the bottom surface intersects an imaginary line connecting the top vertex and the bottom vertex of the triangle in the cross section. Figure 3B The width W2 of the mesa portion of the lower waveguide 30 is shown to be, for example, 3.5 μm. The width W3 of the core layer 16 of the upper waveguide 32 is smaller than the width W2 and is, for example, 0.8 μm. The height H2 from the bottom surface of the core layer 16 to the exposed surface of the cladding layer 14 is, for example, 0.1 μm.

[0053] From the connecting portion 38 to the transition region 36 (from Figure 1C to Figure 3B The width of the upper waveguide 32 decreases from the transition region 36 to the connecting portion 38 (from Figure 3B to Figure 1C ), the width of the closer portion of the upper waveguide 32 of the lower waveguide 30 decreases.

[0054] The substrate 10 includes, for example, a compound semiconductor wafer formed of indium phosphide (InP) doped with iron (Fe) and a buffer layer formed of undoped InP. The thickness of the substrate 10 is, for example, 600 μm. The core layer 12 includes, for example, a plurality of InP layers and indium gallium arsenide phosphide (InGaAsP) layers stacked alternately. The refractive index of the core layer 12 is adjusted by laminating an InP layer with a low refractive index and an InGaAsP layer with a high refractive index. The refractive index of the core layer 12 is an average refractive index determined by the thickness and refractive index of the InGaAsP layer and the thickness of the InP layer. The refractive index of the InGaAsP layer is adjusted by composition (i.e., band gap wavelength). The thickness of the InGaAsP layer is, for example, 50 nm to 200 nm. The thickness of the InP layer included in the core layer 12 is, for example, 100 nm to 800 nm. The number of layers is, for example, 6 to 10 layers.

[0055] The cladding layer 14 is formed of n-type InP doped with silicon (Si), and has a thickness of, for example, 500 nm. The core layer 16 has a multiple quantum well (MQW) structure obtained by laminating, for example, an aluminum indium arsenide (AlInAs) layer and an aluminum gallium indium arsenide (AlGaInAs) layer. The core layer 16 is also used as an active layer in an optical modulator (not shown) monolithically integrated into the spot size converter 100. The thickness of the core layer 16 is, for example, 500 nm. The refractive index of the core layer 16 is higher than the refractive index of the InGaAsP layer. The cladding layer 20 is formed of, for example, undoped InP, and has a thickness of 200 nm. The etching stop layer 22 is formed of, for example, InGaAsP doped with zinc (Zn), and has a thickness of 10 nm to 20 nm. The cladding layer 24 is formed of, for example, p-type InP doped with Zn, and has a thickness of 1000 nm. An insulating film, a resin, or the like may be provided on the upper waveguide 32 , and the upper waveguide 32 is not exposed to the air.

[0056] The connection portion 38 is, for example, a portion connected to an optical modulator (not shown). Figure 1A In the +X side of the transition region 36, the end face 31 is an input / output surface where light is incident or emitted. The wavelength of the light is, for example, 1.55 μm. The light incident from the end face 31 propagates through the lower waveguide 30, transitions from the core layer 12 of the lower waveguide 30 to the core layer 16 of the upper waveguide 32 in the transition region 36, propagates in the core layer 16, and then enters the optical modulator. Figure 1A , an optical modulator (not shown) is connected to the -X side of the connection portion 38. Light propagating from the optical modulator propagates toward the transition region 36 through the connection portion 38, is transferred from the core layer 16 of the upper waveguide 32 to the core layer 12 of the lower waveguide 30, and is emitted from the end face 31.

[0057] The refractive index of the cladding layers 14 and 26 is lower than the refractive index of the core layers 12 and 16. Therefore, light can be confined in the core layers 12 and 16. The refractive index of the core layer 12 is less than the refractive index of the core layer 16. The refractive index is adjusted so that light is transferred between the core layers 12 and 16. For example, in the transition region 36, the effective refractive index of the lower waveguide 30 and the effective refractive index of the upper waveguide 32 are comparable, preferably equal. In order to reduce the effective refractive index of the upper waveguide 32, the upper waveguide 32 can be made thinner and narrower than the lower waveguide 30. However, when the cladding layer 14 is made as narrow as the core layer 16, the effective refractive index at the cladding layer 14 is excessively reduced. Therefore, there is a possibility that the conversion efficiency of light between the core layers is reduced. In addition, the mechanical strength of the upper waveguide 32 is reduced by narrowing the upper waveguide 32, resulting in the possibility of cracking.

[0058] In the first embodiment, the cross-sectional shape of the upper waveguide 32 is tapered upward, and the cladding layer 14 has a bottom surface wider than a rectangular cross section of the same effective refractive index. Therefore, the effective refractive index of the cladding layer 14 is maintained equal to the effective refractive index of the lower waveguide 30 and the upper waveguide 32, and the conversion efficiency between the core layers is improved. In addition, compared with the case of a rectangular cross section, the mechanical strength is also increased.

[0059] Figure 4 1 is a graph showing the calculation results of the conversion efficiency. The horizontal axis represents the width W3 of the base of the triangle of the core layer 16 at the end face 31. The structure of the end face 31 is different from that along the Figure 1A The cross section of line A7 in is the same. Figure 4 The vertical axis represents the conversion efficiency of light between the core layers when light incident on the lower waveguide 30 is transmitted to the upper waveguide 32. The broken line is an example in which the bottom angle θ of the upper waveguide 32 is 90°, that is, the upper waveguide 32 is rectangular. The dotted line and the solid line correspond to the upper waveguide 32 having a tapered shape. The dotted line is an example in which the bottom angle θ is 60°, and the solid line is an example in which the bottom angle θ is 54°. In the calculation, the length L1 of the transition region 36 is set to 800μm, and the width of the lower waveguide 30 is set to 3.5μm. As it moves away from the end face 31, the width of the bottom surface of the upper waveguide 32 in the transition region 36 increases by 0.32μm on one side (each +Y side, -Y side).

[0060] In the example where θ is 90°, the conversion efficiency is about 40% at a width of 0.5 μm to 0.7 μm, and when the width becomes 0.8 μm or more, the conversion efficiency decreases sharply. In the example where θ is 60° and in the example where θ is 54°, a high conversion efficiency of about 60% is maintained within a range of a width of 0.8 μm or more. That is, when the upper waveguide 32 is tapered and the effective refractive index of the cladding layer 14 is maintained equal to the waveguide, the conversion efficiency increases as the bottom width increases.

[0061] (Manufacturing method) Figure 5A and FIG. 6A to FIG. 11A is a cross-sectional view illustrating a method of manufacturing the spot size converter 100 . Figure 5B , Figure 5C , Fig. 11B and Fig. 11C 1 is a plan view illustrating a method of manufacturing the spot size converter 100 .

[0062] like Figure 5A and Figure 5B As shown, the core layer 12, the cladding layer 14, the core layer 16, the cladding layer 20, the etching stop layer 22 and the cladding layer 24 are epitaxially grown in this order on the substrate 10 by, for example, metal organic chemical vapor deposition (MOCVD). In addition, a contact layer (not shown) is grown. The contact layer is removed from the portion that will become the spot size converter and remains in the portion that will become the optical modulator.

[0063] like Figure 5C As shown, the cladding layer 24 is etched. An insulating film having a thickness of 400 nm, for example, a silicon nitride (SiN) film is formed on the cladding layer 24. A resist mask is formed on the insulating film by photolithography. Reactive ion etching (RIE) using carbon tetrafluoride (CF4) gas or the like is performed to transfer the pattern of the resist mask to the insulating film. The resist mask is removed by oxygen (O2) ashing. The cladding layer 24 is etched by wet etching using hydrochloric acid. The wet etching stops at the etching stop layer 22. The insulating film is stripped using buffered hydrofluoric acid (BHF) or the like. As Figure 5C As shown, the cladding layer 24 has a tapered portion in a plan view, the width of which increases from the +X side to the -X side. After the wet etching step, the side view from the Y-axis direction is similar to Figure 5A same.

[0064] Figure 5A The lines B1 to B7 are located with Figure 1A The lines A1 to A7 are at the same position. Respectively, Fig. 6A illustrates a cross section along line B1 to B3 after a wet etching step, Figure 6B illustrates a cross section along line B4, Fig. 7A illustrates a cross section along line B5, Figure 7B illustrates a cross section along line B6, and Figure 8 A cross section along line B7 is shown. FIG. 6B to FIG. 8 As shown in FIG. 1 , the cross-sectional shape of the upper waveguide 32 becomes a tapered shape by wet etching. Figure 5C As shown, the mesa of the upper waveguide 32 extends in the <0-11> direction. The cross-sectional shape is FIG. 6B to FIG. 7B is trapezoidal in shape and Figure 8Specifically, the cladding layer 24 of the upper waveguide 32 has a tapered shape in cross section.

[0065] like Fig. 9A As shown, after wet etching, the cladding layer 24, the etching stop layer 22, the cladding layer 20, the core layer 16 and the cladding layer 14 are etched by using a dry etching method to transfer the tapered shape of the cladding layer 24 to these layers. Specifically, an insulating film such as a SiN film having a thickness of 400nm is formed on the cladding layer 24, and a SiN mask is formed from the SiN film by photolithography of a resist. The SiN film is etched with CF4 gas, and the resist is removed by O2 ashing. Using a gas containing hydrogen iodide (HI) and silicon tetrachloride (SiCl4), dry etching of the cladding layer 24 to the cladding layer 14 is performed by RIE. The SiN mask is removed by BHF.

[0066] Fig. 9A The lines C1 to C7 are located with Figure 1A The cross section along lines C1 and C2 is the same as Fig. 6A same. Fig. 9B illustrates a cross section along line C3, Fig. 9C illustrates a cross section along line C4, Fig. 10A illustrates a cross section along line C5, Fig. 10B illustrates a cross section along line C6, and Fig. 10C A cross section along line C7 is illustrated. FIG. 9B to FIG. 10B , the cladding layer 24 is thinned by dry etching, and Fig. 10C In the embodiment of the present invention, the coating layer 24 is removed.

[0067] like FIG. 9C to FIG. 10C As shown, the cladding layer 14, the core layer 16 and the cladding layer 26 have a tapered shape in cross section. Fig. 9C and Fig. 10A , the cross-sectional shape includes a trapezoid. Fig. 10B and Fig. 10C The cross-sectional shapes in the include triangles. The widths of these layers (the widths of the mesas) vary from FIG. 9C to FIG. 10C Decrease (from -X side to +X side).

[0068] like Fig.11A and Fig. 11B As shown, a portion of the cladding layer 24 is removed by wet etching to form the opening 25. Specifically, an insulating film such as a SiN film having a thickness of 400 nm is formed on the upper waveguide 32, and a resist pattern is formed on the SiN film by photolithography. The SiN film is etched by RIE using CF4 gas or the like, and the resist pattern is transferred to the SiN film. The resist pattern is removed by O2 ashing. By using a SiN mask, the cladding layer 24 is wet etched with an etchant containing hydrochloric acid. As shown in FIG. Fig. 11B As shown, the shape of the opening 25 on the XY plane is, for example, a parallelogram. Fig.11A As shown, the wall surface of the cladding layer 24 is inclined from the Z axis, and the etching stop layer 22 is exposed from the opening 25. The SiN mask is stripped by BHF.

[0069] like Fig. 11C As shown, by dry etching the lower waveguide 30, as indicated by the broken line, the lower waveguide 30 is tapered from the +X side to the -X side. Specifically, an insulating film is formed to cover the surface of the semiconductor layer including the upper waveguide 32, the etching stop layer 22, and the cladding layers 14 and 24. A resist pattern is formed by photolithography. The resist pattern has Fig. 11C The pattern of the disconnected line shown in FIG. The pattern of the resist pattern is transferred to the insulating film by RIE, and then the semiconductor layer is dry-etched using a gas including HI and SiCl4. Figures 1A to 3B As shown, a mesa is formed in the lower waveguide 30. Thereafter, forming an insulating film (not shown) to cover the waveguide and dividing the substrate into chips are performed to obtain a device having the spot size converter 100.

[0070] According to the first embodiment, the cross-sectional shape of the upper waveguide 32 is a tapered shape that becomes narrower as it is away from the lower waveguide 30 in the Z-axis direction. Figure 3A and Figure 3B The bottom angle θ is shown to be 60 degrees or less, for example, 54 degrees. Therefore, the effective refractive index of the core layer 16 is adjusted, and the conversion efficiency of light between the core layer 12 and the core layer 16 can be improved.

[0071] The cladding layer 14 disposed between the core layer 12 and the core layer 16 has a tapered shape like the core layer 16. Since the bottom surface of the tapered cladding layer 14 increases compared to a rectangular shape giving the same effective refractive index to the upper waveguide 32, the effective refractive index at the cladding layer 14 also increases. Therefore, the conversion efficiency of light between the core layers is improved. In order to further increase the effective refractive index of the cladding layer 14, the bottom angle θ is set to 60° or less.

[0072] By making the cross-sectional shape of the core layer 16 triangular or trapezoidal, the strength is improved compared to the case of a rectangle, and the possibility of cracking can be suppressed. In particular, when the bottom angle θ is 60 degrees or less, the bottom surface becomes wide and the strength is improved.

[0073] The upper waveguide 32 includes a core layer 16 and a cladding layer 26 on the core layer 16. Figure 1A and Figures 2B to 3A As shown, the coating 26 in the transition region 36 is thinner than the coating in the connecting portion 38. Figure 3BAs shown, a portion of the cover layer 26 is removed. Therefore, the effective refractive index of the cladding layer 26 in the transition region 36 is lower than the effective refractive index of the connecting portion 38. Therefore, the conversion efficiency between the core layers is improved. Since the effective refractive index can be reduced by thinning the cladding layer 26, it is not necessary to make the width of the upper waveguide 32 extremely narrow. For example, Figure 1B The width W1 shown is 1.5 μm, Figure 3B The width W3 shown is 0.8 μm. Therefore, the strength of the upper waveguide 32 is improved, and damage is suppressed.

[0074] like Fig. 11C As shown, the lower waveguide 30 has a tapered portion in which the width decreases from the transition region 36 to the connection portion 38, and the upper waveguide 32 has a tapered portion in which the width decreases from the connection portion 38 to the transition portion. Therefore, the effective refractive indexes of the lower waveguide 30 and the upper waveguide 32 are adjusted, thereby improving the conversion efficiency of light between the waveguides in the transition region 36. For example, when the effective refractive index of the upper waveguide 32 decreases in the transition region 36, the light in the upper waveguide 32 is easily transferred to the lower waveguide 30. In addition, by the lower waveguide 30 having the tapered portion, the light propagating through the lower waveguide 30 is cut off when it approaches the connection portion 38.

[0075] The end face 31 is an input / output surface of light, and light is transferred from the lower waveguide 30 to the upper waveguide 32 in the transition region 36. The connection portion 38 is connected to an optical modulator, for example. Light incident from the end face 31 is transferred from the lower waveguide 30 to the upper waveguide 32 with high conversion efficiency, and then it can be incident on the optical modulator. Light emitted from the optical modulator can be transferred from the upper waveguide 32 to the lower waveguide 30 with high conversion efficiency, and it can be emitted from the end face 31.

[0076] like FIG. 6B to FIG. 8 As shown, since the wet etching is stopped by the etch stop layer 22, a mesa having a tapered cross-sectional shape is formed in the cladding layer 24. In addition, the tapered shape of the cladding layer 24 can be transferred to the core layer 16 and the cladding layer 14 by dry etching.

[0077] The mesa of the cladding layer 24 after wet etching extends in the <0-11> direction. When the InP layer (cladding layer 24) disposed on an InP substrate whose main surface is a (100) plane is wet etched with a mixed etchant of hydrochloric acid and water (mixing ratio of hydrochloric acid and water: 2:3), due to the etching selectivity depending on the crystal direction, the (111) plane or its equivalent plane appears in the side surface of the mesa. The angle between the (111) plane and the (100) plane is about 55°. As an example of obtaining another angle, wet etching using a mixture of hydrochloric acid and phosphoric acid (mixing ratio of 1:5) causes the mesa to have a (211) plane or an equivalent plane on the side surface. In this example, the angle of the side surface of the mesa is about 35°. By dry etching the cladding layer 26, the core layer 16 and the cladding layer 14 together with the cladding layer 24, as FIG. 9C to FIG. 10C As shown, the cross-sectional shape of the upper waveguide 32 may be tapered.

[0078] Second embodiment

[0079] In the second embodiment, the shape of the upper waveguide 32 in the transition region 36 is different from that in the first embodiment. Fig.12 is a cross-sectional view illustrating a spot size converter according to a second embodiment, and illustrates a configuration corresponding to Figure 1A A cross-sectional view of line A7 in FIG. The cross-sectional shape of the core layer 16 is triangular. The cladding layer 14 is rectangular and covers the surface of the core layer 12. The core layer 16 of thickness T1 is retained on the surface of the cladding layer 14. The thickness T1 is, for example, 0.1 μm, and the width of the cladding layer (the base of the triangle) W4 is, for example, 0.8 μm. The other configurations are the same as those of the first embodiment. According to the second embodiment, the width of the cladding layer 14 is large, and the cross-sectional shape of the core layer 16 is triangular. Therefore, the effective refractive index of the upper waveguide 32 is increased. The conversion efficiency can be improved.

[0080] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

Claims

1. A spot size converter, comprising: a first waveguide comprising a first core layer, the first waveguide propagating light; as well as a second waveguide including a second core layer and disposed on the first waveguide, the second waveguide propagating light, in, The first waveguide and the second waveguide extend in a waveguide direction, A first region and a second region are continuously arranged along the waveguide direction, In the first region, the second waveguide has a tapered shape in cross section, the tapered shape becoming narrower as it moves away from the first waveguide, An angle between a side surface of the second waveguide and a bottom surface of the second waveguide is 60° or less, and The first waveguide and the second waveguide form a mesa extending to the first region and the second region, and a cross-sectional shape of the mesa in the second region is a rectangle.

2. The spot size converter according to claim 1, wherein: The second waveguide includes a first cladding layer disposed between the first core layer and the second core layer, and The second core layer and the first cladding layer are included in the tapered shape in the cross section.

3. The spot size converter according to claim 1 or 2, wherein: The second waveguide includes a second cladding layer disposed on the second core layer, and The second cladding layer in the first region is thinner than the second cladding layer in the second region, or the second cladding layer is not provided in the first region.

4. The spot size converter according to claim 3, wherein: The second cladding layer is disposed in one region of the first region and is not disposed in another region of the first region, The one zone of the first region is closer to the second region than the other zone of the first region.

5. The spot size converter according to claim 1, wherein: The first waveguide has a tapered portion whose width decreases from the first region to the second region, and The second waveguide has a tapered portion whose width decreases from the second region to the first region.

6. The spot size converter according to claim 1, wherein: The first region has an input / output surface on a side opposite to the second region.

7. The spot size converter according to claim 1, wherein: A cross section of the second waveguide in the first region is trapezoidal or triangular.

8. The spot size converter according to claim 1, wherein: The cross section of the second waveguide is trapezoidal in one region of the first area, and the cross section of the second waveguide is triangular in another region of the first area, The one zone of the first region is closer to the second region than the other zone of the first region.

9. The spot size converter according to claim 1, wherein: The refractive index of the first core layer is lower than the refractive index of the second core layer.

10. The spot size converter according to claim 1, wherein: A width of the first waveguide in the first region is greater than a width of the second waveguide in the second region.

11. A method for manufacturing a spot size converter, comprising the following steps: forming a first waveguide having a first core layer and a second waveguide having a second core layer in this order; as well as etching the second waveguide to form a tapered shape in cross-section that becomes narrower as it moves away from the first waveguide, in, An angle between a side surface of the second waveguide and a bottom surface of the second waveguide is 60 degrees or less, and The etching also forms mesas of the first and second waveguides extending to the first and second regions, the cross-sectional shape of the mesas in the second region being rectangular.

12. The method for manufacturing a spot size converter according to claim 11, wherein: The second waveguide includes the second core layer and a cladding layer disposed on the second core layer, The cladding layer includes an etch stop layer, The step of etching the second waveguide comprises the following sub-steps: wet etching the cladding layer until the etch stop layer is exposed to form a mesa in the cladding layer; and The cladding layer and the second core layer having the mesa are dry-etched.

13. The method for manufacturing a spot size converter according to claim 12, wherein: The mesas of the cladding layer extend in a <0-11> direction.

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