Optical waveguide structure and semiconductor optical amplifier

Abstract

An optical waveguide structure (100) is provided with: a first waveguide (10) having a first core layer (11) extending in a second direction and a first cladding layer (12) surrounding the first core layer; and a second waveguide (20) having a second core layer (21) which is adjacent to the first core layer in the second direction, is optically connected to the first core layer, and extends in the second direction, and a second cladding layer (22) which sandwiches the second core layer in the first direction, an end surface (12a) of the first cladding layer in the second direction constituting a first interface with a site having a different refractive index from that of the first cladding layer. The first interface has a first reflective surface reflecting a portion (Lc) of the transmitted light, the first reflective surface reflecting a reflected light (Lr) on the first reflective surface for the portion of the light in a second direction or a direction inclined relative to an opposite direction of the second direction such that the reflected light (Lr) is not recoupled with the first waveguide or the second waveguide.

Description

Technical Field

[0001] The present invention relates to an optical waveguide structure and a semiconductor optical amplifier. Background Art

[0002] Conventionally, a semiconductor optical amplifier having an optical waveguide structure including a buried waveguide and a high mesa waveguide has been known (for example, Patent Document 1).

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2021-27314 Summary of the Invention

[0006] -Problems to be Solved by the Invention-

[0007] As a result of in-depth research by the inventors, it has been found that in such an optical waveguide structure, when an interface with different refractive indexes is generated at the end face of the cladding of the buried waveguide on the high mesa waveguide side, the light reflected at the interface sometimes re-couples with the buried waveguide. When the reflected light is input to a semiconductor optical amplifier including the optical waveguide structure or optically connected to the optical waveguide structure, there may be problems such as fluctuations in the gain spectrum.

[0008] Therefore, one of the problems of the present invention is to obtain an improved new optical waveguide structure and a semiconductor optical amplifier that can suppress the re-coupling of the reflected light at the interface of the cladding with the waveguide.

[0009] -Means for Solving the Problems-

[0010] The optical waveguide structure of the present invention includes, for example, a first waveguide and a second waveguide. The first waveguide has a stacked structure laminated in a first direction, and includes: a first core layer extending at least at an end portion in a second direction intersecting the first direction along the second direction; and a first cladding surrounding the first core layer. The second waveguide has a stacked structure laminated in the first direction, and includes: a second core layer adjacent to the first core layer in the second direction, optically connected to the first core layer, and extending at least at an end portion in a direction opposite to the second direction along the second direction; and a second cladding sandwiching the second core layer in the first direction. The optical waveguide structure transmits light in the second direction or a direction opposite to the second direction. An end face of the first cladding in the second direction constitutes a first interface with a portion having a refractive index different from that of the first cladding. The optical waveguide structure is characterized in that the first interface has a first reflecting surface that reflects a part of the transmitted light, and the first reflecting surface reflects the reflected light of the part of the light in a direction inclined with respect to the second direction or a direction opposite to the second direction, so that the reflected light is not recoupled with the first waveguide or the second waveguide.

[0011] In the optical waveguide structure, it may also be that the first reflecting surface is inclined with respect to the second direction or a direction opposite to the second direction so that it approaches the central axis of the first core layer or the second core layer as it goes in the second direction.

[0012] In the optical waveguide structure, it may also be that the first interface is a boundary with a cavity.

[0013] In the optical waveguide structure, it may also be that the first interface is a boundary with a substance accommodated in an opening provided in the optical waveguide structure.

[0014] In the optical waveguide structure, it may also be that an end portion of the second core layer in a direction opposite to the second direction has a tapered portion, and a width of the tapered portion in a third direction intersecting the first direction and the second direction gradually narrows as it goes in the second direction.

[0015] In the optical waveguide structure, it may also be that the end face has an extended portion located between the first reflecting surface and the second core layer and extending so as to continue from a first side face in the third direction or a direction opposite to the third direction of the tapered portion.

[0016] In the optical waveguide structure, it is also possible that an end portion of the first core layer in the second direction is connected to the first interface to form a second interface with a portion having a refractive index different from that of the first core layer, the second interface having a second reflecting surface that reflects a part of the transmitted light, and the second reflecting surface reflects the reflected light of the part of the light at the second reflecting surface in a direction inclined with respect to the second direction or the opposite direction of the second direction so that the reflected light is not recoupled to the first waveguide or the second waveguide.

[0017] In the optical waveguide structure, it is also possible that the first reflecting surface is provided between a position closer to the optical axis side than the outer edge of the light beam of the light guided in the optical waveguide structure and a position closer to the side opposite to the optical axis than the outer edge.

[0018] In the optical waveguide structure, it is also possible that the second waveguide is a high mesa waveguide.

[0019] In the optical waveguide structure, it is also possible that the second waveguide is a double-clad waveguide.

[0020] The semiconductor optical amplifier of the present invention includes, for example, the optical waveguide structure.

[0021] -Advantages of the Invention-

[0022] According to the present invention, an improved new optical waveguide structure and a semiconductor optical amplifier can be obtained. Description of the Drawings

[0023] Figure 1 is an exemplary and schematic top view of the optical waveguide structure of the first embodiment.

[0024] Figure 2 is Figure 1 a cross-sectional view taken along line II-II of

[0025] Figure 3 is Figure 1 a cross-sectional view taken along line III-III of

[0026] Figure 4 is Figure 1 a cross-sectional view taken along line IV-IV of

[0027] Figure 5 is Figure 1 a cross-sectional view taken along line V-V of

[0028] Figure 6 is an exemplary and schematic top view of a part of the optical waveguide structure of a modification of the first embodiment.

[0029] Figure 7 is an exemplary and schematic top view of the optical waveguide structure of the second embodiment.

[0030] Figure 8 is Figure 7 the VIII-VIII cross-sectional view.

[0031] Figure 9 is an exemplary and schematic top view of the optical waveguide structure of the third embodiment.

[0032] Figure 10 is an exemplary and schematic top view of the optical waveguide structure of the fourth embodiment.

[0033] Figure 11 is an exemplary and schematic top view of a semiconductor optical amplifier having the optical waveguide structure of the fourth embodiment.

[0034] Figure 12 is Figure 11 the XII-XII cross-sectional view. Detailed Embodiments

[0035] Hereinafter, exemplary embodiments and modified examples of the present invention will be disclosed. The structures, operations, and results (effects) brought about by the structures shown below are examples. The present invention can also be implemented by structures other than those disclosed in the following embodiments and modified examples. In addition, according to the present invention, at least one of various effects (including derived effects) obtained by the structure can be achieved.

[0036] The following multiple embodiments and modified examples have the same structure. Therefore, based on the structures of the respective embodiments and modified examples, the same operations and effects based on the same structure can be obtained. In addition, hereinafter, the same reference numerals may be assigned to these same structures, and repeated descriptions may be omitted.

[0037] In this specification, ordinal numbers are given for the convenience of distinguishing directions, parts, etc., and do not represent priority, order, nor limit the number.

[0038] In each figure, the X direction is indicated by an arrow X, the Y direction is indicated by an arrow Y, and the Z direction is indicated by an arrow Z. The X direction, Y direction, and Z direction intersect and are orthogonal to each other. In addition, the X direction is referred to as the extending direction, the Y direction is referred to as the width direction, and the Z direction is referred to as the height direction or the stacking direction. In this specification, a top view means observing in the direction opposite to the Z direction.

[0039] In addition, each figure is a schematic diagram for illustrative purposes, and the scales and ratios in each figure and the actual object are not necessarily the same.

[0040] [First Embodiment]

[0041] [Basic Structure]

[0042] Figure 1 is a top view of the optical waveguide structure 100A (100) of the first embodiment.

[0043] As Figure 1 shown, the optical waveguide structure 100A (100) includes a first waveguide 10A (10) and a second waveguide 20A (20). The first waveguide 10 is a buried waveguide, and the second waveguide 20 is a high mesa waveguide.

[0044] Figure 2 is Figure 1 a cross-sectional view taken along line II-II. As Figure 2 shown, the first waveguide 10 has a stacked structure in which layers are stacked in the Z direction on a substrate 30. The substrate 30 is made of, for example, n-InP. The first waveguide 10 can be fabricated by a known manufacturing method. In addition, the first waveguide 10 has a core layer 11 and a cladding layer 12 that surrounds the core layer 11. The core layer 11 is made of, for example, InGaAsP. In addition, the cladding layer 12 has a p-cladding layer 12P and an n-cladding layer 12N. The p-cladding layer 12P is made of, for example, p-InP, and the n-cladding layer 12N is made of, for example, n-InP. The Z direction is an example of a first direction, the core layer 11 is an example of a first core layer, and the cladding layer 12 is an example of a first cladding layer.

[0045] In addition, the core layer 11 extends in the X direction within the range shown in Figure 1 i.e., at least the end portions of the first waveguide 10 in the X direction, with a substantially constant width in the Y direction and a substantially constant height in the Z direction within the cladding layer 12. The X direction is an example of a second direction.

[0046] Figure 3 is Figure 1 a cross-sectional view taken along line III-III. As Figure 3 shown, the second waveguide 20 also has a stacked structure in which layers are stacked in the Z direction on the substrate 30. The second waveguide 20 can also be fabricated by a known manufacturing method. In addition, the second waveguide 20 has a core layer 21 and a cladding layer 22 that sandwiches the core layer 21 in the Z direction. The core layer 21 is made of, for example, InGaAsP. In addition, the cladding layer 22 has a p-cladding layer 22P and an n-cladding layer 22N. The p-cladding layer 22P is made of, for example, p-InP, and the n-cladding layer 22N is made of, for example, n-InP. The core layer 21 is an example of a second core layer, and the cladding layer 22 is an example of a second cladding layer.

[0047] As Figure 1 , Figure 3As shown in FIG. 1 , the second waveguide 20 is provided with two recesses 101 that are recessed in opposite directions from the end face 100a in the Z direction, so that the second waveguide 20 protrudes in the Z direction relative to the bottom face 101a of the recess 101. In addition, in the present embodiment, the recess 101 is a cavity, but a certain substance may be filled in the recess 101. In this case, the refractive index of the substance is at least different from the refractive index of the core layers 11 and 21 and also different from the refractive index of the cladding layers 12 and 22. The recess 101 is an example of a cavity and also an example of an opening.

[0048] In addition, the core layer 21 and the cladding layer 22 are Figure 1 The range shown, that is, at least the end portion of the second waveguide 20 in the direction opposite to the X direction, extends in the X direction with a substantially constant width in the Y direction and a substantially constant height in the Z direction.

[0049] Figure 4 yes Figure 1 IV-IV cross-sectional view. Figure 4 As shown, the n-cladding 22N of the second waveguide 20 is adjacent to the n-cladding 12N of the first waveguide 10 in the X direction, the core 21 of the second waveguide 20 is adjacent to the core 11 of the first waveguide 10 in the X direction, and the p-cladding 22P of the second waveguide 20 is adjacent to the p-cladding 12P of the first waveguide 10 in the X direction. In the Z direction, the position of the core 11 is the same as the position of the core 21, and the height of the core 11 is the same as the height of the core 21. In addition, as shown in FIG. Figure 1 As shown, in the Y direction, the position of the core layer 11 is the same as that of the core layer 21, and the width of the core layer 11 is the same as that of the core layer 21. With such a structure, the first waveguide 10 and the second waveguide 20 are optically connected.

[0050] Figure 5 yes Figure 1 VV section view of Figure 5 as well as Figure 3 As shown, the end surface 12 a of the cladding 12 of the first waveguide 10 , which is located in the Y direction and in the opposite direction to the Y direction relative to the core 11 , faces the recess 101 .

[0051] Here, Figure 3 The region within the dotted ellipse is the propagation region of the light L. The light L propagating in the first waveguide 10 having the above-mentioned structure is generally as follows: Figure 3 As shown, the light L is transmitted in a wider range than the core layer 11. As an example, the outer edge of the light L can be defined as a region whose intensity is 1 / e of the maximum intensity of the center portion of the light L. 2The position. In addition, the refractive index of the cladding 12 is different from the refractive index of the gas (such as air) in the recess 101. Therefore, the part of the light L transmitted in the X direction in the first waveguide 10 and transmitted in the cladding 12 is reflected at the end face 12a. That is, the end face 12a constitutes an interface (first interface) with different refractive indices. The part of the end face 12a that reflects the part transmitted in the cladding 12 is an example of the first reflection surface.

[0052] In this case, assuming that the end face 12a extends in the Y direction in plan view ( Figure 1 ), the reflected light at the end face 12a is in the opposite direction of the X direction in the cladding 12 and may be recoupled with the cladding 12. When the reflected light recoupled with the cladding 12 is input to a semiconductor optical amplifier including the optical waveguide structure 100 or optically connected to the optical waveguide structure 100, there may be adverse conditions such as fluctuations in the gain spectrum.

[0053] For this reason, in the present embodiment, as Figure 1 shown, in plan view, the end face 12a is configured to be inclined with respect to the Y direction, and the part Lc of the light L transmitted by the cladding 12 is reflected at the end face 12a in a direction inclined with respect to the opposite direction of the X direction. Figure 1 Among them, Lr is the reflected light of the light L with respect to the part Lc transmitted by the cladding 12. According to the present embodiment, with such a structure, it is possible to suppress the reflected light Lr at the end face 12a of the part Lc of the light L transmitted by the cladding 12 from being recoupled with the first waveguide 10, and further suppress adverse phenomena caused by such recoupling.

[0054] In addition, for the light traveling from the second waveguide 20 to the first waveguide 10 in the opposite direction of the X direction, the action and effect obtained from the end face 12a can also be obtained.

[0055] In addition, it has been found that in this structure, the absolute value of the acute angle θ between the normal direction Dn of the region (first reflection surface) of the end face 12a that reflects the part Lc of the light L transmitted by the cladding 12 and the central axis Ax of the core layers 11 and 21 is preferably 15° or more and 35° or less in plan view.

[0056] Furthermore, as Figure 1 shown, in the present embodiment, the region (first reflection surface) of the end face 12a that reflects the part Lc of the light L transmitted by the cladding 12 is preferably inclined so as to approach the central axis Ax of the core layers 11 and 21 as it goes in the X direction. This is because, assuming the opposite is true, that is, as Figure 6In the modified example, when the end face 12a is inclined so as to move away from the central axis Ax of the core layers 11 and 21 as it goes in the X direction, in the vicinity of the connection portion with the second waveguide 20, there may be a portion P on the end face 12a where the normal direction Dn faces the opposite direction of the X direction and the reflected light Lr faces the opposite direction of the X direction.

[0057] In addition, as Figure 3 shown, in the end face 12a, the portion of the light L transmitted by the cladding 12, i.e., the portion Lc (refer to Figure 1 ), the first reflection surface that reflects the portion Lc is provided between the position on the optical axis Ax1 side closer to the optical axis Ax1 than the outer edge of the light L (the position of the double-dashed line in Figure 3 ) and the position on the opposite side of the optical axis Ax1. Thus, in the end face 12a, the portion Lc of the light L transmitted by the cladding 12 can be more reliably reflected in a direction where it does not recouple with the light L.

[0058] [Second Embodiment]

[0059] Figure 7 is a top view of the optical waveguide structure 100B (100) of the second embodiment, Figure 8 is Figure 7 the VIII-VIII cross-sectional view. In the present embodiment, similar to the above-described first embodiment, the cladding 12 of the first waveguide 10B (10) has an end face 12a. Therefore, according to the present embodiment, the same effects as those of the above-described first embodiment can also be obtained.

[0060] However, in the present embodiment, as Figure 7 shown, in the first waveguide 10B (10), the core layer 11 has a straight portion 11s with a substantially constant width in the Y direction and a widened portion 11w with a width gradually increasing in the Y direction as it goes in the X direction. In addition, the second waveguide 20B (20) has a tapered portion 20t with a width gradually decreasing in the Y direction as it goes in the X direction and a straight portion 20s with a substantially constant width in the Y direction. The tapered portion 20t constitutes a spot size converter. The Y direction is an example of the third direction.

[0061] The side surface 20t1 of the tapered portion 20t is inclined with respect to the X direction so as to approach the central axis Ax of the core layer 11 or the core layer 21 as it goes in the X direction. The side surface 20t1 is the side surface in the Y direction or the opposite direction of the Y direction of the tapered portion 20t and is an example of the first side surface.

[0062] The end portion of the cladding 12 of the first waveguide 10B in the X direction has a side surface 12b located between the end surface 12a and the side surface 20t1. The side surface 12b is adjacent to the side surface 20t1 of the tapered portion 20t in the opposite direction in the X direction and extends so as to continue from the side surface 20t1 and is inclined with respect to the X direction. The side surface 12b is an example of an extended portion.

[0063] In this case, the portion of the light L that hits the end surface 12a passes through the end surface 12a and is reflected in a direction where it does not recouple with the light L, similarly to the first embodiment. In addition, the portion of the light L that is closer to the inside than the side surface 12b is enclosed inside the side surface 12b and is coupled to the core layer 21.

[0064] In this way, the structure in which the cladding 12 has the end surface 12a can also be applied to the optical waveguide structure 100B having a spot size converter between the first waveguide 10 and the second waveguide 20.

[0065] [Third Embodiment]

[0066] Figure 9 It is a top view of the optical waveguide structure 100C (100) of the third embodiment. In this embodiment, similarly to the first embodiment above, the cladding 12 of the first waveguide 10C (10) has an end surface 12a. Therefore, according to this embodiment, the same effect as the first embodiment above can also be obtained.

[0067] However, in this embodiment, as Figure 9 shown, the second waveguide 20C (20) is a double-clad waveguide. That is, in the second waveguide 20C, the cladding 22 has a portion that sandwiches the core layer 21 in the Z direction and a portion 22s that sandwiches the core layer 21C (21) in the Y direction, and surrounds the core layer 21. The cladding 22 functions as the inner cladding of the double cladding. In addition, the recess 101 is located on the side opposite to the core layer 21 with respect to the portion 22s, and the recess 101 functions as the outer cladding of the double cladding.

[0068] In addition, in this embodiment, the width of the core layer 21C (21) in the Y direction gradually decreases as it goes in the X direction, constituting a spot size converter.

[0069] In this way, the structure in which the cladding 12 has the end surface 12a can also be applied to the optical waveguide structure 100C in which the second waveguide 20C (20) is a double-clad waveguide.

[0070] [Fourth Embodiment]

[0071] Figure 10This is a top view of the optical waveguide structure 100D (100) of the fourth embodiment. In this embodiment, similar to the first embodiment described above, the cladding 12 of the first waveguide 10D (10) has an end face 12a. Therefore, according to this embodiment, the same effects as those of the first embodiment can also be obtained.

[0072] However, in this embodiment, as Figure 10 shown, the width of the second waveguide 20D (20) in the Y direction is narrower than the width of the core layer 11 of the first waveguide 10D (10) in the Y direction, and a height difference is formed between the core layer 11 and the core layer 21. In this height difference, the core layer 11 of the first waveguide 10D (10) has an end face 11a connected to the end face 12a. The end face 11a forms an interface between the core layer 11 and the medium in the recess 101 where the refractive index is different from that of the core layer 11. The end face 11a is an example of the second interface.

[0073] Similar to the end face 12a, the end face 11a is configured to be inclined with respect to the Y direction in a top view, and the portion Lc of the light L transmitted by the cladding 12 is reflected in a direction inclined in the opposite direction with respect to the X direction at the end face 11a. According to this embodiment, with such a structure, it is possible to suppress the re-coupling of the reflected light at the end face 11a with the first waveguide 10 and reduce the transmission characteristics of the light L. The end face 11a is an example of the second reflection surface.

[0074] [Fifth Embodiment]

[0075] Figure 11 This is a top view of the optical waveguide structure 100E (100) of the fifth embodiment. In this embodiment, two first waveguides 10B (10) similar to those of the second embodiment described above and two second waveguides 20B (20) optically connected to the first waveguides 10B (10) are provided. Therefore, according to this embodiment, the same effects as those of the second embodiment can be obtained, and further the same effects as those of the first embodiment can be obtained.

[0076] In addition, the optical waveguide structure 100E (100) is configured as a semiconductor optical amplifier. That is, in this embodiment, the two second waveguides 20B are optically connected via a U-shaped waveguide 20E. The waveguide 20E is a high mesa waveguide having the same cross-sectional structure as the second waveguide 20B and is a passive waveguide.

[0077] Moreover, the optical waveguide structure 100E (100) includes a waveguide 10E that functions as an optical amplification unit on the side opposite to the first waveguide 10B (10) and the second waveguide 20B (20). Figure 12 This is Figure 11 the XII-XII cross-sectional view. As Figure 12As shown, the waveguide 10E is a buried waveguide. The core layer 11E is an active layer and is made of, for example, InGaAsP. A contact layer 13 is provided on the side opposite to the cladding layer 12P (12) with respect to the core layer 11E, and an electrode 40P is provided on the side opposite to the contact layer 13 with respect to the core layer 11E. The contact layer 13 is made of P-type InGaAsP. The electrode 40P is a P-side electrode and has a structure including Au and AuZn, for example. On the other hand, an electrode 40N is provided on the side opposite to the substrate 30 with respect to the core layer 11E. The electrode 40N is an N-side electrode and has a structure including AuGe, Ni, Au, etc., for example.

[0078] In the waveguide 10E, an optical amplification effect can be obtained by flowing an excitation current between the electrodes 40P and 40N.

[0079] In this way, the structure in which the cladding layer 12 has the end face 12a can also be applied to the optical waveguide structure 100E (100) including the waveguide 10E (10) that functions as an optical amplification unit. In addition, in the present embodiment, the structure in which the optical waveguide structure 100E that functions as a semiconductor optical amplifier includes the first waveguide 10B and the second waveguide 20B of the second embodiment is illustrated, but it is not limited thereto. The optical waveguide structure 100 that functions as a semiconductor optical amplifier may include the first waveguide 10 of other embodiments or the second waveguide 20 of other embodiments.

[0080] As described above, embodiments of the present invention have been illustrated, but the above embodiments are examples and are not intended to limit the scope of the invention. The above embodiments can be implemented in various other ways, and various omissions, substitutions, combinations, and changes can be made without departing from the gist of the invention. In addition, specifications such as each structure and shape (structure, type, direction, type, size, length, width, thickness, height, quantity, arrangement, position, material, etc.) can be appropriately changed and implemented.

[0081] For example, the optical waveguide structure of the present embodiment can also be applied to other optical devices such as a wavelength variable laser.

[0082] Industrial Applicability

[0083] The present invention can be used for optical waveguide structures and semiconductor optical amplifiers.

[0084] -Description of Reference Numerals-

[0085] 10, 10A to 10D... First waveguide

[0086] 10E... Waveguide (amplification unit)

[0087] 11... Core layer (first core layer)

[0088] 11E... Core layer (active layer)

[0089] 11a... End face (second interface, second reflecting surface)

[0090] 11s... Straight portion

[0091] 11w... Widening portion

[0092] 12... Cladding (first cladding)

[0093] 12N... n-cladding

[0094] 12P... p-cladding

[0095] 12a... End face (first interface, first reflecting surface)

[0096] 12b... Side surface (extended portion)

[0097] 20, 20A~20D... Second waveguide

[0098] 20s... Straight portion

[0099] 20t... Tapered portion

[0100] 20t1... Side surface (extended portion)

[0101] 21, 21C... Core layer (second core layer)

[0102] 22... Cladding (second cladding)

[0103] 22N... n-cladding

[0104] 22P... p-cladding

[0105] 22s... Portion

[0106] 30... Substrate

[0107] 40P... Electrode (P-side electrode)

[0108] 40N... Electrode (N-side electrode)

[0109] 100, 100A~100D... Optical waveguide structure

[0110] 100E... Optical waveguide structure (semiconductor optical amplifier)

[0111] 100a... End face

[0112] 101... Recess (cavity)

[0113] 101a... Bottom surface

[0114] Ax... Central axis

[0115] Ax1... Optical axis

[0116] Dn…Normal direction

[0117] L…Light

[0118] Lc…(The part transmitted in the cladding)

[0119] Lr…Reflected light

[0120] P…Part

[0121] X…Direction (second direction)

[0122] Y…Direction (third direction)

[0123] Z…Direction (first direction)

[0124] θ…Angle.

Claims

1. An optical waveguide structure includes a first waveguide and a second waveguide, The first waveguide has a stacked structure stacked in a first direction and includes: a first core layer extending at least along a second direction at an end in the second direction intersecting the first direction; and a first cladding layer that surrounds the first core layer; and The second waveguide has a stacked structure stacked in the first direction and includes: a second core layer that is adjacent to the first core layer in the second direction, is optically connected to the first core layer, and extends along the second direction at least at ends in opposite directions of the second direction; and a second cladding layer that sandwiches the second core layer in the first direction, The optical waveguide structure transmits light in the second direction or the direction opposite to the second direction, An end face of the first cladding layer in the second direction constitutes a first interface with a portion having a refractive index different from that of the first cladding layer, The optical waveguide structure is characterized in that The first interface has a first reflecting surface that reflects a part of the transmitted light, and the first reflecting surface reflects the reflected light of a part of the light on the first reflecting surface in a direction inclined with respect to the second direction or the direction opposite to the second direction, so that the reflected light is not recoupled with the first waveguide or the second waveguide.

2. The optical waveguide structure according to claim 1, wherein The first reflecting surface is inclined with respect to the second direction or the direction opposite to the second direction so that it approaches the central axis of the first core layer or the second core layer as it goes in the second direction.

3. The optical waveguide structure according to claim 1 or 2, wherein The first interface is a boundary with a cavity.

4. The optical waveguide structure according to claim 1 or 2, wherein The first interface is a boundary with a substance accommodated in an opening provided in the optical waveguide structure.

5. The optical waveguide structure according to claim 1 or 2, wherein An end portion of the second core layer in the direction opposite to the second direction has a tapered portion, and the width of the tapered portion in a third direction intersecting the first direction and the second direction gradually narrows as it goes in the second direction.

6. The optical waveguide structure according to claim 5, wherein The end face has an extended portion that is located between the first reflecting surface and the second core layer and extends so as to continue from a first side surface in the third direction or the direction opposite to the third direction of the tapered portion.

7. The optical waveguide structure according to claim 1 or 2, wherein An end portion of the first core layer in the second direction is connected to the first interface and constitutes a second interface with a portion having a refractive index different from that of the first core layer, The second interface has a second reflecting surface that reflects a part of the transmitted light, and the second reflecting surface reflects the reflected light of a part of the light on the second reflecting surface in a direction inclined with respect to the second direction or the direction opposite to the second direction, so that the reflected light is not recoupled with the first waveguide or the second waveguide.

8. The optical waveguide structure according to claim 1 or 2, wherein The first reflecting surface is provided between a position closer to the optical axis side than the outer edge of the light beam of the light guided in the optical waveguide structure and a position closer to the side opposite to the optical axis than the outer edge.

9. The optical waveguide structure according to claim 1 or 2, wherein, the second waveguide is a high mesa waveguide.

10. The optical waveguide structure according to claim 1 or 2, wherein, the second waveguide is a double-clad waveguide.

11. A semiconductor optical amplifier, characterized in that, it has the optical waveguide structure according to claim 1 or 2.

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