JP5326456B2 - Optical waveguide device - Google Patents

Description

  The present invention relates to an optical waveguide device including a buried waveguide and a high mesa waveguide.

  With the advancement of optoelectronic technology, multifunctional optical elements are demanded. From the viewpoint of energy saving and space saving, further integration of semiconductor optical devices is required. An embedded waveguide is suitable for a semiconductor laser device as a light emitting element in order to efficiently inject current into the active layer. Further, a high-mesa waveguide with a small bending loss of the waveguide is suitable for a waveguide as an optical circuit to which the light emitting element and the optical modulator are connected in order to make the optical circuit as small as possible.

  The guided light of the buried waveguide is different from the guided light of the high mesa waveguide. For this reason, when both are connected, optical coupling efficiency falls. The components that are not coupled are radiated out of the waveguide as stray light. Depending on the waveguide structure, stray light may recombine with the waveguide and the device characteristics may deteriorate.

  Further, since the equivalent refractive index of the waveguide is different between the buried waveguide and the high mesa waveguide, reflection due to impedance mismatching occurs. The element characteristics may be deteriorated due to the reflected light returned to the waveguide on the incident side.

  By interposing a spot size conversion waveguide in the connection portion between the buried waveguide and the high mesa waveguide, reflection at the connection portion can be suppressed (Patent Document 1). A tapered clad made of a semiconductor is disposed on the side surface of the spot size conversion waveguide. The width of the tapered cladding gradually decreases from the buried waveguide toward the high mesa waveguide. A medium having a low refractive index such as polyimide is filled outside the tapered clad.

JP 2002-311267 A

  An interface between the tapered clad and a medium such as polyimide on the outer side acts as a reflecting surface. The light reflected from the interface is radiated out of the waveguide, thereby reducing the coupling efficiency. Further, when the reflected light recombines with the waveguide, the device characteristics may be deteriorated.

An optical waveguide device for solving the above problems is as follows.
An embedded waveguide formed on a substrate and embedded on both sides of the core with a first medium;
A high-mesa waveguide formed on the substrate and having a second medium having a refractive index smaller than that of the first medium on both sides of the core;
A connection waveguide formed on the substrate and optically connecting the buried waveguide and the high mesa waveguide; and the connection waveguide is at least one side of the core of the connection waveguide disposed, seen including a plurality of fins arranged in the guiding direction,
The thickness of the gap between the fins adjacent to each other changes so as to increase from the buried waveguide toward the high mesa waveguide.

  Due to the plurality of fins, the equivalent refractive index of the connection waveguide gradually changes. Thereby, the reflection in the connection location of a buried waveguide and a high mesa waveguide can be suppressed.

  Examples 1 to 11 will be described with reference to FIGS.

  FIG. 1 is a perspective view of the optical waveguide device according to the first embodiment. On the surface of the substrate 20 made of n-type InP, a buried waveguide region 50A and a high mesa waveguide region 52A are defined with a certain distance therebetween. A connection waveguide region 51A is defined between the two. A mesa 41 is formed on the substrate 20. The mesa 41 extends from the buried waveguide region 50A to the high mesa waveguide region 52A across the connection waveguide region 51A. The width of the mesa 41 is, for example, 1.3 μm.

  In the embedded waveguide region 50A, the mesa 41 has a stacked structure in which the surface layer portion of the substrate 20, the core layer 21, the upper cladding layer 22, and the contact layer 23 are stacked in this order. The core layer 21 has a multiple quantum well structure formed of a GaInAsP mixed crystal semiconductor. The thickness is, for example, 200 nm. The upper cladding layer 22 is made of p-type InP and has a thickness of, for example, 2.5 μm. The contact layer 23 is made of p-type GaInAs and has a thickness of 0.3 μm, for example.

  Current blocking layers 45 are embedded on both sides of the mesa 41 in the embedded waveguide region 50A. The current blocking layer 45 is made of semi-insulating InP doped with Fe. The height of the upper surface is substantially equal to the height of the upper surface of the contact layer 23. The core layer 21, the substrate 20, the upper cladding layer 22, and the current blocking layer 45 constitute a buried waveguide 50. Since the current blocking layer 45 made of a compound semiconductor material having an equivalent refractive index smaller than that of the core layer 21 is disposed on both sides of the core layer 21, the guided light of the embedded waveguide 50 is confined in the lateral direction.

  An upper electrode 70 is formed on the contact layer 23 in the buried waveguide region 50A. The upper electrode 70 has a three-layer structure of Au / Zn / Au. A back electrode 71 is formed on the back surface of the substrate 20. The back electrode 71 has a two-layer structure of AuGe / Au. The buried waveguide 50 functions as a 1.3 μm band semiconductor laser element.

  The mesa 41 of the connection waveguide region 51A and the high mesa waveguide region 52A has a stacked structure in which the surface layer portion of the substrate 20, the core layer 31, and the upper cladding layer 32 are stacked in this order. The core layer 31 has a bulk structure made of a GaInAsP mixed crystal semiconductor and has a thickness of, for example, 200 nm. The upper cladding layer 32 is made of n-type or semi-insulating InP, and its thickness is, for example, 2.8 μm. The substrate 20, the core layer 31, and the upper clad layer 32 in the high mesa waveguide region 52A constitute the high mesa waveguide 52.

  A plurality of thin plate-like fins 45a arranged in the waveguide direction are arranged on both sides of the mesa 41 in the connection waveguide region 51A. The substrate 20, the core layer 31, the upper clad layer 32, and the fin 45 a constitute the connection waveguide 51. The connection waveguide 51 optically connects the buried waveguide 50 and the high mesa waveguide 52.

  Each of the fins 45 a is arranged in a posture perpendicular to the longitudinal direction of the mesa 41 and having one end surface in contact with the side surface of the mesa 41. The thickness D1 of each fin 45a is, for example, 20 nm. The thickness D2 of the gap between the fins 45a adjacent to each other changes from 20 nm to 160 nm so as to increase in steps of 5 nm as it approaches the high mesa waveguide 52 from the buried waveguide 50. That is, there are 29 fins 45a arranged on one side of the mesa 41. The length L of each fin 45a extending in the lateral direction from the mesa 41 is sufficiently larger than the spread of the beam cross section of the guided light. For example, the length L is set to 5 μm or more.

  The fins 45a are formed of a material having a smaller refractive index than that of the core layer 31. From the viewpoint of simplifying the manufacturing process, the fins 45a are preferably formed of the same material as the current blocking layer 45.

  The gaps between both sides of the mesa 41 in the high mesa waveguide region 52A and the fins 45a in the connection waveguide region 51A are filled with the atmosphere. In addition to the atmosphere, a medium having a refractive index smaller than the refractive index of the current blocking layer 45 embedded on both sides of the mesa 41 in the embedded waveguide region 50A may be disposed. Examples of such a medium include resins such as polyimide and benzocyclobutene (BCB). If a medium having a refractive index smaller than the refractive index of the current blocking layer 45, such as air or resin, is disposed, the high mesa waveguide 52 can obtain a higher confinement effect in the lateral direction than the buried waveguide 50. Can do.

  Next, with reference to FIGS. 2 to 5, a method of manufacturing the optical waveguide device according to the first embodiment will be described.

  Processes until the mesa 41 shown in FIG. 2 is formed will be described. A core layer 21, an upper cladding layer 22, and a contact layer 23 having a multiple quantum well structure are formed on a substrate 20 made of n-type InP. These layers are formed, for example, by metal organic chemical vapor deposition (MOVPE). Trimethylindium can be used as the In material, trimethylgallium as the Ga material, phosphine as the P material, and arsine as the As material.

  The buried waveguide region 50A is covered with a mask pattern such as silicon oxide, and the contact layer 23, the upper cladding layer 22 and the core layer 21 in the connection waveguide region 51A and the high mesa waveguide region 52A are removed by etching. For etching these layers, reactive ion etching (RIE) using a methane-based or chlorine-based gas can be used.

  Next, a bulk type core layer 31 and an upper cladding layer 32 are selectively grown on the substrate 20 exposed by etching. MOVPE can be used to form these layers. The upper surface of the core layer 31 has substantially the same height as the upper surface of the core layer 21, and the upper surface of the upper cladding layer 32 has substantially the same height as the upper surface of the contact layer 23. After the upper cladding layer 32 is formed, the mask pattern used as an etching mask and a selective growth mask is removed. As a result, the contact layer 23 is exposed in the buried waveguide region 50A.

  A first mask pattern 40 made of silicon oxide is formed on the contact layer 23 and the upper cladding layer 32. The first mask pattern 40 has a belt-like planar pattern that extends from the embedded waveguide region 50A to the high mesa waveguide region 52A across the connection waveguide region 51A. The width of the first mask pattern 40 is, for example, 1.3 μm.

  Using the first mask pattern 40 as an etching mask, the surface layer of the substrate 20 is etched. Thereby, the mesa 41 is formed.

  As shown in FIG. 3, on both sides of the mesa 41, a semi-insulating current blocking layer 45 made of Fe-doped InP is selectively grown by MOVPE. The top surface of the current blocking layer 45 is almost the same height as the bottom surface of the first mask pattern 40. After the formation of the current blocking layer 45, the first mask pattern 40 is removed.

  As shown in FIG. 4, a second mask pattern 48 made of silicon oxide is formed on the mesa 41 and the current blocking layer 45. The second mask pattern 48 covers the entire embedded waveguide region 50A, the upper surface of the mesa 41 in the high mesa waveguide region 52A, the upper surface of the mesa 41 in the connection waveguide region 51A, and the fins 45a. For the formation of the second mask pattern 48, for example, a lithography technique using electron beam exposure can be applied.

  As shown in FIG. 5, the current blocking layer 45 is etched using the second mask pattern 48 as an etching mask, and the etching is stopped when reaching the surface of the substrate 20. For this etching, RIE or the like is used. After the etching, the second mask pattern 48 is removed. Thereafter, the upper electrode 70 and the back electrode 71 shown in FIG. 1 are formed.

  FIG. 6 shows an equivalent refractive index distribution in the longitudinal direction of the mesa 41. The transmission refractive index of the optical waveguide device according to Example 1 is indicated by a solid line p. The equivalent refractive index of the buried waveguide 50 is higher than the equivalent refractive index of the high mesa waveguide 52.

  Since the distance between the centers of the fins 45a adjacent to each other is shorter than the guide wavelength of the waveguide light having a wavelength of 1.3 μm, the fins 45a do not act as a reflecting surface or a diffraction grating. The guided light feels a local average refractive index generated by the fin 45a and the gap portion therebetween. Since the thickness of the gap between the fins 45a changes so as to increase from the embedded waveguide 50 toward the high mesa waveguide 52, the connection from the embedded waveguide 50 toward the high mesa waveguide 52 is increased. The equivalent refractive index of the waveguide 51 is lowered. At the interface between the connection waveguide 51 and the high mesa waveguide 52, a step having a small refractive index due to one fin 45a occurs. This step is sufficiently smaller than the difference between the equivalent refractive index of the embedded waveguide 50 and the equivalent refractive index of the high mesa waveguide 52.

  As shown in FIG. 6, the equivalent refractive index gradually changes in the connection waveguide 51. By arranging the connection waveguide 51 between the buried waveguide 50 and the high mesa waveguide 52, it is possible to suppress the generation of a step having a large equivalent refractive index. Thereby, it is possible to suppress the reflection of the guided light at the connection portion between the embedded waveguide 50 and the high mesa waveguide 52.

  If the connecting waveguide 51 is too short, a sufficient effect of relaxing the discontinuity of the equivalent refractive index cannot be obtained. In order to obtain a sufficient effect of alleviating the discontinuity of the equivalent refractive index, it is preferable that the length of the connection waveguide 51 is 50 times or more the guide wavelength of the guided light. Further, if the connection waveguide 51 is made too long, the size of the device will be increased. It is sufficient that the length of the connecting waveguide 51 is 250 times the guide wavelength of the guided light. Here, when single-waveguide light propagates, the length of the connection waveguide 51 may be set based on the center wavelength of the spectrum of the guided light. In addition, when the waveguide light of a plurality of wavelength-multiplexed channels propagates, the length of the connection waveguide 51 may be set with reference to the center wavelength of the spectrum of the waveguide light of the longest wavelength.

  Further, the fin 45a has a size including the beam cross section of the guided light in the lateral direction. In order to change the beam diameter of the guided light, if a tapered cladding whose width gradually changes is arranged on the side of the core of the connection waveguide 51, the outer side surface of the tapered cladding acts as a reflecting surface. End up. In the first embodiment, since the fin 45a includes the beam cross section, there is no boundary surface acting as a reflection surface on the side of the waveguide. For this reason, generation | occurrence | production of the stray light resulting from unexpected reflection can be suppressed.

  In the first embodiment, the n-type semiconductor material is used for the substrate 20, but a semi-insulating semiconductor material may be used. When a semi-insulating substrate is used, an electrode cannot be formed on the back surface of the substrate. Therefore, it is preferable to adopt a coplanar type for the electrode structure. The coplanar electrode is used in an optical waveguide device according to Example 6 described later. Further, a p-type semiconductor material may be used for the substrate 20, and an n-type semiconductor material may be used for the upper clad layer of the buried waveguide 50.

  By using an AlGaInAs mixed crystal semiconductor material for the core layers 21 and 31, it is also possible to form a 1.55 μm band waveguide. In this case, the distance between the centers of the fins 45a may be shorter than the guide wavelength of the guided light having a wavelength of 1.55 μm. Thus, the distance between the centers of the fins 45a is preferably adjusted based on the guide wavelength of the guided light.

  The core layer 21 may have a bulk structure instead of the multiple quantum well structure. Instead of the InP substrate, a composite substrate in which an InP epitaxial structure is bonded to a silicon substrate on which a driver circuit or the like is formed may be used as the substrate 20. Instead of the semi-insulating current blocking layer 45, a current blocking layer having a pnpn thyristor structure may be used.

  When the high mesa waveguide 52 functions as an active element, the conductivity type of the upper cladding layer 32 may be a p-type opposite to the conductivity type of the substrate 20.

  The buried waveguide 50 does not need to function as a semiconductor laser element, and may function as an optical modulator, or may function as a simple optical circuit instead of an active element. When not functioning as an active element, it is not necessary to use a compound semiconductor material for the substrate, the core layer, the cladding layer, and the like. An organic substance or a silicon-based material may be used for the substrate, the core layer, the clad layer, and the like.

  FIG. 7 is a perspective view of the optical waveguide device according to the second embodiment in the middle of manufacturing. The configuration shown in FIG. 7 corresponds to the configuration shown in FIG. In the first embodiment, the depth of the gap between the fins 45a is uniform, but in the second embodiment, the depth of the thin gap is made shallower than the depth of the thick gap. This is because the microloading effect at the time of etching appears greatly.

  Since the thickness of the gap between the fins 45a changes so as to become thicker as it approaches the high mesa waveguide 52, the gap becomes deeper as it approaches the high mesa waveguide 52 from the embedded waveguide 50.

  FIG. 6 shows a transmission refractive index distribution of the optical waveguide structure according to the second embodiment with a broken line q. In the region close to the buried waveguide 50, portions of the substrate side of the current block layer 45 having a higher refractive index than the atmosphere remain on both sides of the mesa 41, so that the equivalent refractive index of the connection waveguide 51 is It is higher than the equivalent refractive index of one connection waveguide 51. As the high mesa waveguide 52 is approached, the equivalent refractive index approaches the equivalent refractive index of the first embodiment.

  Also in the second embodiment, the equivalent refractive index of the connection waveguide 51 changes gently. Further, the fin 45a is sufficiently larger than the beam cross section of the guided light in the lateral direction. For this reason, the same effect as Example 1 is acquired.

  FIG. 8 is a perspective view of the optical waveguide device according to the third embodiment in the middle of manufacturing. FIG. 8 corresponds to the state after removing the second mask pattern 48 shown in FIG. 5 of the first embodiment. In Example 1, the side edge of the second mask pattern coincided with the edge of the mesa 41 in the connection waveguide region 51A and the high mesa waveguide region 52A. For this reason, in the connection waveguide region 51A, the side surfaces of the core layer 31 and the upper cladding layer 32 are exposed in the gaps between the fins 45a. In the high mesa waveguide region 52A, the entire side surfaces of the core layer 31 and the upper cladding layer 32 were exposed.

  In Example 3, a pattern corresponding to the second mask pattern 48 shown in FIG. 5 extends slightly outside the edge of the mesa 41. Therefore, after the current blocking layer 45 is etched, the side surface coating film 60 made of the same semi-insulating InP as the current blocking layer 45 remains on the side surface of the mesa 41.

  The side surface coating film 60 plays a role of protecting the side surfaces of the core layer 31 and the upper clad layer 32. Thereby, the density of the interface state on the side surface can be reduced.

  If the side surface coating film 60 is too thick, the difference between the high mesa waveguide 52 and the buried waveguide 50 is eliminated. In general, the beam cross section of the guided light in the embedded waveguide 50 extends to the outside by about 2 μm from the side surface of the mesa 41. When the thickness of the side surface coating film 60 is set to 1/10 or less of the dimension of the beam cross-section extending from the mesa 41 to the side, it is considered that the sufficient function of lateral confinement of the high mesa waveguide 52 is maintained. . Therefore, the thickness of the side surface coating film 60 is preferably 0.2 μm or less. The lower limit value of the preferable range of the thickness of the side surface coating film 60 is not particularly limited. In principle, a thickness of one monolayer is sufficient.

  The side coating film 60 may be disposed in the connection waveguide region 51A, and the side coating film 60 may not be disposed in the high mesa waveguide region 52A.

  FIG. 9 is a perspective view of the optical waveguide device according to the fourth embodiment in the middle of manufacturing. In the third embodiment, the thickness of the side surface coating film 60 is substantially constant with respect to the propagation direction of the guided light, but in the fourth embodiment, the side surface coating film 60 in the connection waveguide region 51A is embedded in the buried waveguide. The thickness decreases from 50 toward the high mesa waveguide 52. Other configurations are the same as those of the third embodiment.

  In the fourth embodiment, the same effect as in the third embodiment can be obtained.

  FIG. 10 is a perspective view of the optical waveguide device according to the fifth embodiment in the middle of manufacturing. In Example 1, the boundary between the core layer 21 on the buried waveguide 50 side and the core layer 31 on the high mesa waveguide 52 side illustrated in FIG. 2 is the same as the boundary between the buried waveguide 50 and the connection waveguide 51. I did it. In the fifth embodiment, the boundary 65 between the core layer 21 and the core layer 31 slightly enters the connection waveguide 51 from the boundary 66 between the buried waveguide 50 and the connection waveguide 51.

  FIG. 11 is a perspective view of an optical waveguide device according to a modification of the fifth embodiment in the middle of manufacturing. In this modification, the boundary 65 between the core layer 21 and the core layer 31 slightly enters the embedded waveguide 50 from the boundary 66 between the embedded waveguide 50 and the connection waveguide 51.

  Thus, even if the boundary 65 between the core layer 21 and the core layer 31 does not exactly match the boundary 66 between the buried waveguide 50 and the connection waveguide 51, the same effect as in the first embodiment can be obtained. . At the time of alignment when forming the second mask pattern 48 shown in FIG. 4, the boundary 66 between the embedded waveguide 50 and the connection waveguide 51 is guided from the boundary 65 between the core layer 21 and the core layer 31. Even if the displacement is in the direction, there is no problem in the operation of the optical waveguide device as long as the displacement is 5 μm or less.

  With reference to FIG.12 and FIG.13, the manufacturing method of the optical waveguide device by Example 6 is demonstrated. The basic manufacturing process is the same as the manufacturing process of the optical waveguide device according to the first embodiment. Hereinafter, the description will be given focusing on differences from the manufacturing method of the first embodiment.

  Semi-insulating InP is used for the substrate 20 shown in FIG. A lower clad layer 25 made of n-type InP is formed on the substrate 20. The thickness of the lower cladding layer 25 is 0.5 μm, for example. Subsequent steps are the same as those in the first embodiment. When the current blocking layer 45 is etched using the second mask pattern 48 as an etching mask, the etching is stopped at the interface between the lower cladding layer 25 and the substrate 20.

  The thickness D1 of each fin 45a is 80 nm. The gap thickness D2 between the fins 45a changes from 80 nm to 160 nm so as to increase in steps of 5 nm as it approaches the high-mesa waveguide 52 from the buried waveguide 50. The distance between the centers of the fins 45a adjacent to each other is in the range of 160 nm to 240 nm. Since this dimension is substantially equal to the guide wavelength of the guided light having a wavelength of 1.3 μm, a plurality of fins 45a function as a diffraction grating. However, since the period changes slowly, not only the guided light with a specific wavelength is strongly diffracted, but the guided light with a wavelength of 1100 nm to 1500 nm has a weak diffraction of about several tens to several tens of%. receive. After the current blocking layer 45 is etched, the second mask pattern 48 is removed.

  As shown in FIG. 13, a recess reaching the lower cladding layer 25 is formed in the current blocking layer 45 on the side of the core layer 21 of the buried waveguide 50. A lower electrode 72 is formed on the bottom surface of the recess. An upper electrode 70 is formed on the contact layer 23. In Example 6, since a semi-insulating substrate is used, a coplanar electrode structure is adopted as shown in FIG.

  Also in the sixth embodiment, the thickness of the gap between the fins 45a changes so as to increase gradually as the embedded waveguide 50 approaches the high mesa waveguide 52. The rate change can be moderated. Thereby, the increase in the reflection resulting from the discontinuity of the refractive index can be suppressed.

  Consider the case where guided light travels from the high-mesa waveguide 52 toward the buried waveguide 50. The guided light is diffracted by a diffraction grating composed of a plurality of fins 45a. This diffracted light travels to the high mesa waveguide 52 side. Since there are no buried layers (current blocking layers) made of a semiconductor material on both sides of the core layer 31 of the high mesa waveguide 52, the diffracted light hardly recombines with the high mesa waveguide 52.

  In the sixth embodiment, a semi-insulating semiconductor material is used for the substrate 20, but an n-type semiconductor material may be used as in the first embodiment. In this case, a back electrode may be formed on the back surface of the substrate 20 instead of the lower electrode 72.

  When the optical waveguide device according to the sixth embodiment is applied to 1.55 μm band guided light, the fins 45a are arranged so that the distance between the centers of the adjacent fins 45a varies within a range of 190 nm to 290 nm. do it.

  FIG. 14 is a perspective view of the optical waveguide device according to Example 7 in the middle of manufacturing. FIG. 14 corresponds to the state after the second mask pattern 48 of Example 6 shown in FIG. 12 is removed. The planar shape of the second mask pattern 48 is the same as that shown in FIG.

  In the second embodiment, the fins 45 a are arranged in a posture orthogonal to the longitudinal direction of the mesa 41. In the seventh embodiment, the fins 45 a are arranged in a posture inclined with respect to the surface of the substrate 20. The inclination direction and the inclination angle are the same for all the fins 45a. As an example, the surface of the fin 45 a facing the high mesa waveguide 52 side is inclined so as to face the upper side of the substrate 20. The inclination angle is, for example, 75 °. Such a structure can be formed by tilting and loading the substrate into the RIE apparatus when the current blocking layer 45 is etched.

  In Example 7, the same effect as in Example 6 can be obtained. Further, when the guided light from the high mesa waveguide 52 toward the embedded waveguide 50 is diffracted by the plurality of fins 45a, the diffracted light is directed obliquely upward with respect to the substrate surface. Therefore, compared to the sixth embodiment, the diffracted light is less likely to recombine with the high mesa waveguide 52.

  The guided light from the embedded waveguide 50 toward the high mesa waveguide 52 is diffracted obliquely downward with respect to the substrate surface. For this reason, it becomes difficult for the diffracted light to recombine with the buried waveguide 50 as compared with the sixth embodiment.

  FIG. 15 is a perspective view of the optical waveguide device according to Example 8 in the middle of manufacturing. The basic manufacturing process is the same as in the first embodiment. FIG. 15 corresponds to a state after the second mask pattern 48 shown in FIG. 5 is removed. In Example 1, the thickness D1 of the fin 45a was made constant, and the thickness D2 of the gap was changed. In the eighth embodiment, the thickness D2 of the gap is made constant, and the thickness D1 of the fin 45a is changed.

  For example, the gap thickness D2 is 40 nm. The thickness D1 of the fin 45a changes from 140 nm to 40 nm so as to gradually decrease in steps of 5 nm as it approaches the high-mesa waveguide 52 from the buried waveguide 50.

  The distance between the centers of the fins 45a adjacent to each other is shorter than the guide wavelength of the 1.55 μm band guided light. For this reason, the guided light in the 1.55 μm band feels a local average refractive index based on the fins 45a and the gaps as in the case of the first embodiment. Thereby, the reflection resulting from the discontinuity with a large equivalent refractive index can be suppressed.

  FIG. 16 is a perspective view of the optical waveguide device according to Example 9 in the middle of manufacturing. When the optical waveguide device according to the ninth embodiment is compared with the optical waveguide device according to the eighth embodiment, the thickness D1 of the fin 45a and the thickness D2 of the gap portion are different. Furthermore, in Example 9, the core layer 21 has a multiple quantum well structure formed of an AlGaInAs-based mixed crystal semiconductor. Other configurations are the same as those of the optical waveguide device according to the eighth embodiment.

  In the ninth embodiment, the distance D2 is 80 nm, and the thickness D1 is gradually reduced from 200 nm to 80 nm in steps of 5 nm from the buried waveguide 50 toward the high mesa waveguide 52. Since the distance between the centers of the fins 45a is close to the guide wavelength of 1.55 μm band guided light, the plurality of fins 45a function as a diffraction grating. With this diffraction grating, light having a wavelength of 1400 nm to 1700 nm is subjected to weak diffraction of about 10% to several tens of%.

  Also in the ninth embodiment, similarly to the eighth embodiment, it is possible to suppress the reflection caused by the discontinuity of the equivalent refractive index between the buried waveguide 50 and the high mesa waveguide 52. As in the sixth embodiment, the guided light from the high mesa waveguide 52 toward the buried waveguide 50 is diffracted, but this diffracted light hardly recombines with the high mesa waveguide.

  FIG. 17 is a perspective view of the optical waveguide device according to Example 10 in the middle of manufacturing. In the tenth embodiment, the substrate 20 is inclined with respect to a virtual plane perpendicular to the surface of the substrate 20 and parallel to the waveguide direction (virtual plane parallel to the side surface of the mesa 41). Other configurations are the same as those of the optical waveguide device according to the ninth embodiment. Such a structure is formed by inclining a branch portion corresponding to the fin 45 a in the second mask pattern 48 shown in FIG. 4 from the longitudinal direction of the mesa 41.

  The normal vector of the surface of the fin 45 a facing the high mesa waveguide 52 is inclined so as to face away from the mesa 41. The angle formed between the longitudinal direction of the mesa 41 and the normal vector is, for example, 30 ° (the inclination angle with respect to the virtual plane is 60 °).

  A portion of the guided light from the high mesa waveguide 52 toward the embedded waveguide 50 is diffracted by the plurality of fins 45a. The traveling direction of the diffracted light faces the high mesa waveguide 52 side and moves away from the core layer 31 as it travels. For this reason, it becomes difficult to re-couple to the high mesa waveguide 52.

  When used for the purpose of propagating guided light from the embedded waveguide 50 to the high mesa waveguide 52, the normal vector of the surface of the fin 45a facing the embedded waveguide 50 is the mesa 41 (or the core layer 21). The inclination direction of the fins 45a may be set so as to face away from the head. In this case, when part of the guided light from the embedded waveguide 50 toward the high mesa waveguide 52 is diffracted, the diffracted light propagates away from the mesa 41. For this reason, recombination of the diffracted light into the buried waveguide 50 can be suppressed.

  FIG. 18 is a perspective view of the optical waveguide device according to Example 11 in the middle of manufacturing. FIG. 18 corresponds to a state after the second mask pattern 48 shown in FIG. In the eleventh embodiment, both the thickness D1 of the fin 45a and the thickness D2 of the gap change. As the thickness D1 approaches the high mesa waveguide 52 from the buried waveguide 50, the thickness D1 changes in a stepwise manner from 180 nm to 20 nm in steps of 5 nm, and the gap thickness D2 increases from 20 nm to 180 nm. It changes so as to increase gradually in increments of 5 nm. The distance D3 between the centers of the fins 45a adjacent to each other in the thickness direction is constant at 200 nm.

  The equivalent refractive index along the waveguide direction changes gently as in the first embodiment. For this reason, the reflection resulting from the discontinuity with a large equivalent refractive index can be suppressed. In the eleventh embodiment, the plurality of fins 45a function as a diffraction grating for guided light having an in-tube wavelength of 1310 nm. When guided light having a wavelength of 1310 nm travels from the high mesa waveguide 52 to the buried waveguide 50, the guided light is effectively diffracted in the direction opposite to the incident direction by the diffraction grating composed of the fins 45a. Since no buried layer of a semiconductor material or the like is disposed on the side of the core 31 of the high mesa waveguide 52, the diffracted light hardly recombines with the high mesa waveguide 52.

  The period of the diffraction grating composed of the plurality of fins 45 a may be configured to gradually increase from the embedded waveguide 50 toward the high mesa waveguide 52.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

1 is a perspective view of an optical waveguide device according to Example 1. FIG. FIG. 3 is a perspective view of the optical waveguide device according to Example 1 in the middle of manufacturing. FIG. 3 is a perspective view of the optical waveguide device according to Example 1 in the middle of manufacturing. FIG. 3 is a perspective view of the optical waveguide device according to Example 1 in the middle of manufacturing. FIG. 3 is a perspective view of the optical waveguide device according to Example 1 in the middle of manufacturing. It is a graph which shows the example of distribution of the equivalent refractive index regarding a waveguide direction. FIG. 10 is a perspective view of an optical waveguide device according to Example 2 in the middle of manufacture. FIG. 10 is a perspective view of an optical waveguide device according to Example 3 in the middle of manufacture. FIG. 10 is a perspective view of an optical waveguide device according to Example 4 in the middle of manufacturing. FIG. 10 is a perspective view of an optical waveguide device according to Example 5 in the middle of manufacturing. FIG. 16 is a perspective view of an optical waveguide device according to a modification of Example 5 in the middle of manufacturing. FIG. 10 is a perspective view of an optical waveguide device according to Example 6 in the middle of manufacture. FIG. 10 is a perspective view of an optical waveguide device according to Example 6. FIG. 10 is a perspective view of an optical waveguide device according to Example 7 in the middle of manufacture. FIG. 16 is a perspective view of an optical waveguide device according to Example 8 in the middle of manufacture. FIG. 10 is a perspective view of an optical waveguide device according to Example 9 in the middle of manufacture. FIG. 16 is a perspective view of an optical waveguide device according to Example 10 in the middle of manufacture. FIG. 16 is a perspective view of an optical waveguide device according to Example 11 in the middle of manufacture.

Explanation of symbols

20 substrate 21 core layer 22 upper clad layer 23 contact layer 25 lower clad layer 31 core layer 32 upper clad layer 40 first mask pattern 41 mesa 45 current blocking layer 45a fin 48 second mask pattern 50 buried waveguide 50A buried Embedded waveguide region 51 Connection waveguide 51A Connection waveguide region 52 High-mesa waveguide 52A High-mesa waveguide region 60 Side coating film 65 Core layer boundary 66 Boundary between buried waveguide and connection waveguide 70 Upper electrode 71 Back electrode 72 Lower electrode

Claims (5)

An embedded waveguide formed on a substrate and embedded on both sides of the core with a first medium;
A high-mesa waveguide formed on the substrate and having a second medium having a refractive index smaller than that of the first medium on both sides of the core;
A connection waveguide formed on the substrate and optically connecting the buried waveguide and the high mesa waveguide; and the connection waveguide is at least one side of the core of the connection waveguide disposed, seen including a plurality of fins arranged in the guiding direction,
An optical waveguide device in which the thickness of a gap between the fins adjacent to each other changes so as to increase from the buried waveguide toward the high mesa waveguide . 2. The optical waveguide device according to claim 1, wherein the thickness of the fin changes so as to become thinner from the embedded waveguide toward the high mesa waveguide. An embedded waveguide formed on a substrate and embedded on both sides of the core with a first medium;
A high-mesa waveguide formed on the substrate and having a second medium having a refractive index smaller than that of the first medium on both sides of the core;
A connection waveguide formed on the substrate and optically connecting the buried waveguide and the high mesa waveguide; and the connection waveguide is at least one side of the core of the connection waveguide disposed, seen including a plurality of fins arranged in the guiding direction,
An optical waveguide device in which the thickness of the fin changes so as to become thinner from the buried waveguide toward the high mesa waveguide . The fins, optical waveguide device according to any one of claims 1 to 3 is formed in the same medium and the first medium. The connecting waveguide of the equivalent refractive index, the optical waveguide device according to Umakomishirube waveguide in any one of the high mesa claim is reduced toward the waveguide 1 to 4.

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