semiconductor optical amplifier

Abstract

To provide a technology capable of achieving both of miniaturization and lower loss in a semiconductor optical amplifier having a configuration where incidence and emission are performed on one end face by inverting an emission direction of light from an incident direction.SOLUTION: A semiconductor optical amplifier (1) includes: a multimode interference waveguide (2) having a first end face (21) being an end face at one side in a first direction (X), and a second end face (22) being an end face at another side; and a pair of input / output waveguides (3 and 4) which are aligned in a second direction (Y) orthogonal to the first direction while being arranged adjacent to the multimode interference waveguide with respect to the first direction. The first end face is optically coupled with the pair of input / output waveguides. The second end face is subjected to high reflection processing. Each of the pair of input / output waveguides is formed with a width, being a size in the second direction, which is smaller than a with of the multimode interference waveguide. The pair of input / output waveguides respectively include input / output end faces (33 and 43) subjected to low reflection processing in a side opposite to the side optically coupled to the first end face.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present invention relates to a semiconductor optical amplifier. [Background technology]

[0002] When integrating a semiconductor optical amplifier into an optical integrated circuit, etc., alignment of the end faces is required. In this regard, Patent Document 1 discloses an optical integrated circuit that integrates a planar lightwave circuit with a semiconductor element fixed on a silicon substrate.

[0003] In the optical integrated circuit disclosed in Patent Document 1, the planar lightwave circuit includes a PLC substrate and two linear optical waveguides formed on the PLC substrate. PLC stands for Planar Lightwave Circuit. Each of the two optical waveguides extends from one end face of the planar lightwave circuit to the other end face. That is, one end of the optical waveguide contacts one end face of the PLC substrate, and the other end contacts the other end face of the PLC substrate. The PLC substrate is, for example, a silicon substrate. The semiconductor element includes a semiconductor substrate and a semiconductor optical amplifier formed on the semiconductor substrate. An input semiconductor waveguide and an output semiconductor waveguide are formed on the semiconductor substrate, respectively, on the input and output sides of the semiconductor optical amplifier. The output semiconductor waveguide has a folded portion on the semiconductor substrate, which reverses the direction of light propagation and contacts the end face of the semiconductor element on the same side as the input semiconductor waveguide. The planar lightwave circuit and the semiconductor element are fixed at a single contact surface. That is, the other end face of the planar lightwave circuit and the end face of the semiconductor element are fixed. With this configuration, the alignment work is simple, good coupling efficiency is easily obtained, and it is possible to realize a compact optical integrated circuit and optical integrated circuit module in which the planar lightwave circuit and the semiconductor element are integrated. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-250019 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, in the semiconductor optical amplifier provided in the optical integrated circuit disclosed in Patent Document 1, the waveguide is formed in a U-shape to reverse the traveling direction of light. In such a configuration, in order to suppress loss in the U-shaped portion, it is necessary to make the radius of curvature large, on the order of submillimeters. This makes it difficult to reduce the chip size.

[0006] The present invention has been made in view of the circumstances exemplified above, and provides a technology that can achieve both miniaturization and low loss in a semiconductor optical amplifier configured to input and output light at one end facet by reversing the output direction of light from the input direction, for example. [Means for solving the problem]

[0007] The semiconductor optical amplifier (1) according to claim 1 comprises: a multi-mode interference waveguide (2) having a first end face (21) which is an end face on one side in a first direction (X) and a second end face (22) which is an end face on the other side; a pair of input / output waveguides (3, 4) arranged adjacent to the multi-mode interference waveguide in the first direction and arranged in a second direction (Y) perpendicular to the first direction; a first electrode (81) disposed adjacent to the multi-mode interference waveguide in a third direction (Z) perpendicular to the first direction and perpendicular to the second direction; a second electrode (82) disposed adjacent to one (3) of the pair of input / output waveguides in the third direction; a third electrode (83) disposed adjacent to the other (4) of the pair of input / output waveguides different from the one of the pair of input / output waveguides in the third direction; Equipped with the first end face is optically coupled to the pair of input and output waveguides; the second end face is highly reflective; each of the pair of input and output waveguides is formed so that the width, which is the dimension in the second direction, is narrower than that of the multi-mode interference waveguide, and has an input and output end face (33, 43) that is subjected to low reflection processing on the side opposite to the side optically coupled to the first end face; At least one of the second electrode and the third electrode is separated from the first electrode. do.

[0008] In addition, in each section of the application documents, each element may be assigned a reference symbol in parentheses. In this case, the reference symbol merely indicates an example of the correspondence between the element and the specific configuration described in the embodiment described below. Therefore, the present invention is not limited in any way by the description of the reference symbol. [Brief explanation of the drawings]

[0009] [Figure 1A] 1 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a first embodiment of the present invention. [Figure 1B] 1B is an enlarged cross-sectional view of a portion of the semiconductor optical amplifier shown in FIG. 1A. [Figure 2A] 1B is a schematic diagram for explaining the operation principle of the multi-mode interference waveguide shown in FIG. 1A. FIG. [Figure 2B] 1B is a schematic diagram for explaining the operation principle of the multi-mode interference waveguide shown in FIG. 1A. FIG. [Figure 2C] 1B is a schematic diagram for explaining the operation principle of the multi-mode interference waveguide shown in FIG. 1A. FIG. [Figure 3] FIG. 4 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a second embodiment of the present invention. [Figure 4] FIG. 10 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a third embodiment of the present invention. [Figure 5] FIG. 10 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a fourth embodiment of the present invention. [Figure 6]FIG. 10 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a fifth embodiment of the present invention. [Figure 7] 7 is an enlarged cross-sectional view of a portion of the semiconductor optical amplifier shown in FIG. 6. FIG. [Figure 8] FIG. 10 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a sixth embodiment of the present invention. [Figure 9] FIG. 11 is a plan view showing a schematic configuration of a semiconductor optical amplifier according to a seventh embodiment of the present invention. [Figure 10] 10 is an enlarged cross-sectional view of a portion of the semiconductor optical amplifier shown in FIG. 9. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] (Embodiment) Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that various modifications applicable to one embodiment may be hindered from being understood if they are introduced in the middle of a series of explanations relating to the embodiment. Therefore, the modifications will not be introduced in the middle of a series of explanations relating to the embodiment, but will be explained together after the series of explanations.

[0011] (First embodiment: configuration) The configuration of a semiconductor optical amplifier 1 according to the first embodiment will be described with reference to Figures 1A and 1B. For ease of description, an XYZ three-dimensional coordinate system is set as shown in Figures 1A and 1B. The X-axis direction in the figures corresponds to the "first direction," the Y-axis direction in the figures corresponds to the "second direction," and the Z-axis direction in the figures corresponds to the "third direction." The first direction and the second direction are orthogonal to each other, and the third direction is orthogonal to both the first direction and the second direction.

[0012] 1A, the semiconductor optical amplifier 1 is formed in a substantially rectangular shape in a plan view with its longitudinal direction in the X-axis direction and a rectangular parallelepiped shape with its thickness direction in the Z-axis direction. The semiconductor optical amplifier 1 according to this embodiment has a so-called B-SOA configuration and is configured to amplify input light and output the generated output light. B-SOA stands for Booster Semiconductor Optical Amplifier. In this embodiment, the semiconductor optical amplifier 1 is a so-called "reflective SOA" and has an input / output end face 1a on one side (i.e., the positive X-axis side) in the X-axis direction in the figure and a reflecting end face 1b on the other side. That is, the semiconductor optical amplifier 1 is configured to be optically connected to another optical integrated circuit (not shown) at the input / output end face 1a. The semiconductor optical amplifier 1 is configured to input signal light (i.e., input light) from another optical integrated circuit and output signal light (i.e., output light) to the other optical integrated circuit at the input / output end face 1a, and to reverse the traveling direction of the light at the reflecting end face 1b.

[0013] Specifically, the semiconductor optical amplifier 1 includes a multi-mode interference waveguide 2, an input waveguide 3, an output waveguide 4, an embedding layer 5, a low-reflection film 6, a high-reflection film 7, and an electrode 8. Each component constituting the semiconductor optical amplifier 1 according to this embodiment will be described below in order.

[0014] The multi-mode interference waveguide 2 has a first end facet 21, which is an end face on one side in the X-axis direction (i.e., the X-axis positive direction side), and a second end facet 22, which is an end face on the other side. The multi-mode interference waveguide 2 is configured as a "1×1 MMI" type waveguide that inputs and outputs signal light at the first end facet 21. "1×1 MMI" is an abbreviation for 1 input-1 output MMI. MMI is an abbreviation for Multi-Mode Interference. In other words, the multi-mode interference waveguide 2 is optically coupled to the input waveguide 3 and the output waveguide 4 at the first end facet 21.

[0015] The input waveguide 3 and output waveguide 4, which form a pair of input and output waveguides, are disposed adjacent to the multimode interference waveguide 2 in the X-axis direction in the figure, and are arranged in the Y-axis direction perpendicular to the X-axis direction in the figure. One input waveguide 3 and one output waveguide 4 are provided. The input waveguide 3 and the output waveguide 4 are formed so that their widths, which are the dimensions in the Y-axis direction, are narrower than those of the multimode interference waveguide 2. In this embodiment, the input waveguide 3 and the output waveguide 4 are configured as single-mode waveguides. The input waveguide 3 and the output waveguide 4 are disposed at a predetermined interval in the Y-axis direction. An embedded layer 5 is provided around the combined body of the multimode interference waveguide 2, the input waveguide 3, and the output waveguide 4.

[0016] The input waveguide 3 has an input-side connection portion 31 and an input-side inclined portion 32. The input-side connection portion 31 extends in the positive direction of the X axis in the figure from the point where it is optically coupled with the multi-mode interference waveguide 2. The input-side inclined portion 32 is inclined from the end of the input-side connection portion 31 in the positive direction of the X axis in the figure toward the negative direction of the Y axis in the figure. The input waveguide 3 has an input end face 33, which is one of a pair of input / output end faces. The input end face 33 is the end face of the input waveguide 3 opposite the side optically coupled to the first end face 21, i.e., the end face of the input-side inclined portion 32, and is provided at the input / output end face 1a of the semiconductor optical amplifier 1. In this embodiment, the input waveguide 3 is configured so that the extension direction of the input-side inclined portion 32 from the input end face 33 (i.e., the incident direction D1 shown in FIG. 1A) is inclined with respect to the input end face 33. The incident direction D1 is the direction in which signal light passes through the input end face 33.

[0017] The output waveguide 4 has an output-side connection portion 41 and an output-side inclined portion 42. The output-side connection portion 41 extends in the positive direction of the X-axis in the figure from the point where it is optically coupled with the multi-mode interference waveguide 2. That is, the output-side connection portion 41 is provided parallel to the input-side connection portion 31. The output-side inclined portion 42 is provided so as to incline from the end of the output-side connection portion 41 in the positive direction of the X-axis in the figure toward the negative direction of the Y-axis in the figure. The output waveguide 4 has an output end face 43, which is the other of a pair of input / output end faces. The output end face 43 is the end face of the output waveguide 4 opposite the side optically coupled to the first end face 21, i.e., the end face of the output-side inclined portion 42, and is provided at the input / output end face 1 a of the semiconductor optical amplifier 1. In this embodiment, the output waveguide 4 is provided so that the extension direction of the output-side inclined portion 42 from the output end face 43 (i.e., the opposite direction to the output direction D2 shown in FIG. 1A) is inclined with respect to the output end face 43. The output direction D2 is the direction in which signal light passes through the output end face 43.

[0018] The semiconductor optical amplifier 1 has an input / output end face 1a that is treated for low reflection, and a reflecting end face 1b that is treated for high reflection. That is, the second end face 22 of the multi-mode interference waveguide 2 is treated for high reflection. The input end face 33 of the input waveguide 3 and the output end face 43 of the output waveguide 4 are also treated for low reflection. Specifically, the input end face 33 of the input waveguide 3 and the output end face 43 of the output waveguide 4 are treated for low reflection by forming a low-reflection film 6 on the input / output end face 1a. On the other hand, the second end face 22 of the multi-mode interference waveguide 2 is treated for high reflection by forming a high-reflection film 7 on the reflecting end face 1b.

[0019] The electrodes 8 are provided on both sides of the semiconductor optical amplifier 1 in the thickness direction. Specifically, the semiconductor optical amplifier 1 has a first electrode 81, a second electrode 82, and a third electrode 83. The first electrode 81 is disposed adjacent to the multi-mode interference waveguide 2 in the thickness direction of the semiconductor optical amplifier 1. The second electrode 82 is disposed adjacent to the input waveguide 3 in the thickness direction of the semiconductor optical amplifier 1. The third electrode 83 is disposed adjacent to the output waveguide 4 in the thickness direction of the semiconductor optical amplifier 1. In this embodiment, the first electrode 81, the second electrode 82, and the third electrode 83 are integrally formed. That is, the first electrode 81, the second electrode 82, and the third electrode 83 are provided so that the same voltage is applied to them. The "electrodes" in the electrode 8, the first electrode 81, the second electrode 82, and the third electrode 83 may also be referred to as an "electrode pair."

[0020] 1A , the electrode pair 8 includes a lower electrode 84 and an upper electrode 85 arranged in the thickness direction, and a semiconductor layer 90 is provided between the lower electrode 84 and the upper electrode 85. In this embodiment, the lower electrodes 84 of the first electrode 81, the second electrode 82, and the third electrode 83 are integrally formed. Similarly, the upper electrodes 85 of the first electrode 81, the second electrode 82, and the third electrode 83 are integrally formed.

[0021] The semiconductor layer 90 includes a base layer 91, an undercladding layer 92, an active layer 93, an overcladding layer 94, and a contact layer 95. The base layer 91, the undercladding layer 92, the active layer 93, the overcladding layer 94, and the contact layer 95 are stacked in this order.

[0022] The base layer 91 is a layer of a semiconductor substrate and is formed of a semiconductor material such as GaAs. A lower electrode 84 is provided on the bottom surface of the base layer 91. The upper surface of the base layer 91 is bonded to an undercladding layer 92. The undercladding layer 92 is an n-type cladding layer and is formed of n-type AlGaAs or the like. The upper surface of the undercladding layer 92 is bonded to an active layer 93. That is, the active layer 93 is sandwiched between the undercladding layer 92 and the overcladding layer 94. The active layer 93 is formed of GaAs or the like. The overcladding layer 94 is a p-type cladding layer and is formed of p-type AlGaAs or the like. The upper surface of the overcladding layer 94 is bonded to a contact layer 95. The contact layer 95 is a layer for establishing electrical continuity between the upper electrode 85 and the semiconductor layer 90 and is formed of a semiconductor material such as GaAs. The upper electrode 85 is provided on the upper surface of the contact layer 95.

[0023] As described above, when signal light is input from an adjacent optical integrated circuit (not shown) on the positive side of the X-axis, the semiconductor optical amplifier 1 according to this embodiment is configured to amplify the signal light and output it to the optical integrated circuit. That is, the semiconductor optical amplifier 1 is configured such that the signal light input from the input end face 33 to the input waveguide 3 is amplified by applying current to the electrode 8, and the traveling direction of the signal light is reversed in the multi-mode interference waveguide 2, and the signal light is output from the output end face 43 via the output waveguide 4.

[0024] (First embodiment: effect) Below, an overview of the operation of the semiconductor optical amplifier 1 according to this embodiment will be described together with the effects achieved by this configuration.

[0025] Signal light input to the input waveguide 3 from the input end face 33 enters the multimode interference waveguide 2 at the first end face 21, is reflected at the second end face 22, propagates from the first end face 21 to the output waveguide 4, and is output from the output end face 43. During this time, the signal light is amplified by applying current to the electrode 8. Here, using FIGS. 2A to 2C, an outline of the operating principle of the multimode interference waveguide 2 shown in FIG. 1A will be explained. First, FIG. 2A shows a non-reflective, i.e., transmissive, 2×2 MMI waveguide, which is the prerequisite configuration for the multimode interference waveguide 2 shown in FIG. 1A. Ports 1 to 4, which are input and output ports of the 2×2 MMI waveguide, indicate the connection points between the 2×2 MMI waveguide and the input and output waveguides. When light is input through Port 1, the guided light forms two images at the midpoint of the 2×2 MMI waveguide, indicated by the dashed line in the figure, and the optical output is uniform. Here, there are two types of 2 × 2 MMI waveguides: paired type and general type. In the paired type, the L π / 2, and 3L for General type π / 2, where L π is called the beat length, and L π =π / (β0-β1), where β0 and β1 are the propagation constants of the fundamental mode and the first-order mode, respectively. When propagating further through the 2x2 MMI waveguide, all the light is imaged at a position symmetrical in the Y direction with respect to Port1, and is output to Port4. The X-direction position is L π , 3L for General type π In Fig. 2A, the propagation length of light from when it enters the 2x2 MMI waveguide from Port 1 until it is focused to be output at Port 4 (i.e., the length of the diagonal arrow shown by the solid line in the figure) is called the "coupling length." In the 2x2 MMI waveguide shown in Fig. 2A, where light incident from Port 1 is output at Port 4, the structure cut at the middle of the waveguide length is the multimode interference waveguide 2 shown in Fig. 1A. In the paired type, the waveguide length is L π / 2, 3L for General type π / 2. To explain its operation, Figure 2B shows the propagation of light incident on Port 1 until it reaches the cut end face, and Figure 2C shows the propagation of light reflected from that end face until it reaches the output port, i.e., Port 2. First, in Figure 2B, light incident on Port 1 propagates and forms two images at the end face with a reflector. After being reflected back by the reflector and propagating further, all of the light forms an image at Port 2 and is output, as shown in Figure 2C. This Port 2 corresponds to Port 4 in Figure 2A. The reflector is positioned to prevent light reaching this end face from being emitted to the left side of the figure. Because the reflectivity can be increased to approximately 99% by using a dielectric multilayer film, for example, the loss at this reflector is negligibly small. Using multimode interference waveguide 2, a reflective 1 × 1 MMI waveguide with this configuration, the spacing between the input and output ports can be reduced to approximately several tens of micrometers. While the waveguide width of a single-mode waveguide SOA is about 2 μm, that of an MMI is only several tens of μm, which means that the operating voltage can be reduced in inverse proportion to the waveguide width. Furthermore, the waveguide area can be halved compared to a structure using a transmission MMI.

[0026] As described above, the semiconductor optical amplifier 1 according to this embodiment uses a multi-mode interference waveguide 2 to reverse the light propagation direction, rather than a U-shaped waveguide as in the configuration disclosed in Patent Document 1. In this regard, when using a U-shaped waveguide as in the configuration disclosed in Patent Document 1, a large curvature radius of approximately submillimeter order is required to suppress loss in the U-shaped portion. This makes it difficult to reduce the chip size. In contrast, the semiconductor optical amplifier 1 according to this embodiment uses the multi-mode interference waveguide 2, which, as described above, enables miniaturization while maintaining low loss. That is, according to this embodiment, it is possible to achieve both miniaturization and low loss in a reflective SOA whose end facets can be easily and accurately aligned.

[0027] Furthermore, in this embodiment, in addition to a first electrode 81 corresponding to the multi-mode interference waveguide 2 that mainly exhibits an amplifying effect, the electrodes 8 include a second electrode 82 corresponding to the input waveguide 3 and a third electrode 83 corresponding to the output waveguide 4. With this configuration, signal attenuation in the input waveguide 3 and the output waveguide 4 can be suppressed as much as possible.

[0028] Furthermore, in this embodiment, the input waveguide 3 is provided so that the extension direction of the input waveguide 3, which is the direction in which the input waveguide 3 extends from the input end face 33, is inclined with respect to the input end face 33. Similarly, the output waveguide 4 is provided so that the extension direction of the output waveguide 4, which is the direction in which the output waveguide 4 extends from the output end face 43, is inclined with respect to the output end face 43. In the case of a straight waveguide structure in which the waveguide is not inclined, oscillation due to end face reflection is likely to occur. In contrast, with this configuration, by using an oblique waveguide structure in which the waveguide is inclined to reduce the end face reflectivity, internal end face reflection is suppressed, and therefore oscillation due to end face reflection can be effectively suppressed.

[0029] (Other embodiments) Other embodiments will be described below. Note that in the following description of the other embodiments, differences from the first embodiment will be mainly described. Furthermore, identical or equivalent parts between the first embodiment and the other embodiments are denoted by the same reference numerals. Therefore, in the following description of the other embodiments, the description of the first embodiment can be appropriately applied to components denoted by the same reference numerals as those in the first embodiment, unless there is a technical contradiction or a special additional explanation.

[0030] At least one of the second electrode 82 and the third electrode 83 may be separated from the first electrode 81. That is, the upper electrode 85 of at least one of the second electrode 82 and the third electrode 83 and the upper electrode 85 of the first electrode 81 may be provided separately. In this case, the lower electrodes 84 of the first electrode 81, the second electrode 82, and the third electrode 83 may be integrally formed. Alternatively, the lower electrode 84 of at least one of the second electrode 82 and the third electrode 83 and the lower electrode 84 of the first electrode 81 may be provided separately.

[0031] FIG. 3 shows a schematic configuration of a semiconductor optical amplifier 1 according to a second embodiment. As shown in FIG. 3, in this embodiment, the third electrode 83 is separated from the first electrode 81. Specifically, the upper electrode 85 of the third electrode 83 and the upper electrode 85 of the first electrode 81 are separated so that they are not electrically connected to each other and can have different potentials. Meanwhile, the second electrode 82 is integrally formed with the first electrode 81. Specifically, the upper electrode 85 of the second electrode 82 and the upper electrode 85 of the first electrode 81 are integrated so that they have the same potential. The first electrode 81, the second electrode 82, and the lower electrode 84 of the third electrode 83 may be integrally formed. This configuration allows different amounts of current to be applied to the integrated first electrode 81 and second electrode 82 and the separated third electrode 83. Specifically, for example, the SOA operation of the all-electrode-integrated configuration shown in FIG. 1A and the separated electrode configuration shown in FIG. 3 can be compared as follows. For convenience, the figure is divided into upper and lower regions from the center position in the Y-axis direction, i.e., the second electrode 82 side and the third electrode 83 side. The former is referred to as the "incident side" and the latter as the "exit side." Also, as a hypothetical example, the total applied power is assumed to be 1 W, the loss in the MMI is assumed to be 50%, and the luminous efficiency is assumed to be 10%. In this regard, with the all-electrode integrated configuration shown in FIG. 1A, half of 1 W, or 500 mW, is consumed on the incident side, resulting in an emitted light output of 50 mW. Due to losses, the light output drops to 25 mW. Meanwhile, half of 1 W, or 500 mW, is also consumed on the exit side, resulting in an emitted light output of 50 mW. The final output is then 25 mW + 50 mW = 75 mW. In contrast, with the electrode separation configuration shown in Figure 3, for example, if a total applied power of 1W is 100mW on the input side and 900mW on the output side, the input side consumes 100mW and emits 10mW, reducing the amount of light to 5mW due to losses, and the output side consumes 900mW and emits 90mW, resulting in a final output of 5mW + 90mW = 95mW. In other words, even with the same total applied power, a larger output, i.e., higher efficiency, can be obtained by setting the applied voltage distribution between the input and output sides so that it is higher on the output side, taking into account losses in the MMI.In this way, with this configuration, by applying a larger power, i.e., a higher current, to the output waveguide 4 side, it is possible to perform signal amplification in the semiconductor optical amplifier 1 while minimizing the influence of loss in the multi-mode interference waveguide 2.

[0032] FIG. 4 shows a schematic configuration of a semiconductor optical amplifier 1 according to a third embodiment. As shown in FIG. 4, in this embodiment, the second electrode 82 and the third electrode 83 are separated from the first electrode 81. That is, the upper electrode 85 of the first electrode 81 is separated from the upper electrodes 85 of the second electrode 82 and the third electrode 83. The lower electrodes 84 of the first electrode 81, the second electrode 82, and the third electrode 83 may be integrally formed. This configuration enables the multimode interference waveguide 2 to be driven by independent electrodes. Here, manufacturing errors may occur in the shape or dimensions of the multimode interference waveguide 2. Such manufacturing errors may cause the imaging position at the port on the output waveguide 4 side to deviate from the predetermined design position, resulting in increased loss. Meanwhile, the coupling length in the multimode interference waveguide 2 varies depending on the refractive index, which in turn varies depending on the carrier density, which in turn varies with the current. Therefore, changing the applied current changes the carrier density, which in turn changes the refractive index, which in turn changes the coupling length. Therefore, with this configuration, it is possible to minimize loss by adjusting the coupling length in the multi-mode interference waveguide 2 with the current applied to the first electrode 81. Furthermore, by separating the second electrode 82 and the third electrode 83 from each other, the same effects as those of the second embodiment can be achieved.

[0033] Fig. 5 shows a schematic configuration of a semiconductor optical amplifier 1 according to a fourth embodiment. Fig. 6 shows a schematic configuration of a semiconductor optical amplifier 1 according to a fifth embodiment. Note that in Figs. 5 and 6, the electrode 8 shown in Fig. 1A, 3, or 4 is omitted.

[0034] If the distance between the input waveguide 3 and the output waveguide 4 is small, there is a risk of loss occurring due to signal light propagating through the input waveguide 3 being coupled to the output waveguide 4, or signal light propagating through the output waveguide 4 being coupled to the input waveguide 3.

[0035] 5 , in the fourth embodiment, a pair of input and output waveguides, that is, an input waveguide 3 and an output waveguide 4, are provided at both ends of a multimode interference waveguide 2 in the Y-axis direction in the figure. That is, the end of the multimode interference waveguide 2 on the positive side of the Y-axis substantially coincides with the end of the input waveguide 3 on the positive side of the Y-axis. Similarly, the end of the multimode interference waveguide 2 on the negative side of the Y-axis substantially coincides with the end of the output waveguide 4 on the negative side of the Y-axis. With this configuration, it is possible to maximize the distance between the input waveguide 3 and the output waveguide 4 within the width of the multimode interference waveguide 2, thereby effectively suppressing the occurrence of the above-mentioned loss.

[0036] On the other hand, as shown in FIG. 6 , in the fifth embodiment, a low-refractive-index portion 96 is provided between the input waveguide 3 and the output waveguide 4, which are a pair of input / output waveguides. Specifically, the low-refractive-index portion 96 may be formed by a high-mesa structure, as shown in FIG. 7 . The “high-mesa structure” is a structure in which the base layer 91, which is a substrate layer, is removed by etching or the like to a position lower than the waveguide layer in a non-waveguide region (i.e., a region where no waveguide is provided in the in-plane direction perpendicular to the thickness direction). In other words, the input waveguide 3 and the output waveguide 4 may be formed as a so-called “high-mesa waveguide.” This configuration makes it possible to effectively suppress the occurrence of the above-mentioned loss without changing the distance between the input waveguide 3 and the output waveguide 4.

[0037] (Variation) The present invention is not limited to the above-described embodiment. Therefore, the above-described embodiment can be modified as appropriate. Representative modifications will be described below. In the following description of the modifications, differences from the above-described embodiment will be mainly described. Furthermore, the same reference numerals are used for parts that are identical or equivalent to each other in the above-described embodiment and the modifications. Therefore, in the following description of the modifications, the description of the above-described embodiment can be used as appropriate for components that have the same reference numerals as the above-described embodiment, unless there is a technical contradiction or special additional explanation.

[0038] The present invention is not limited to the specific device configurations shown in the above embodiments. That is, Fig. 1A and other figures are simplified schematic diagrams for easily explaining the overview of the semiconductor optical amplifier 1 according to the present invention. Therefore, the configuration of the semiconductor optical amplifier 1 actually manufactured and sold does not necessarily match the exemplary configuration shown in Fig. 1A and other figures. Furthermore, the configuration of the semiconductor optical amplifier 1 actually manufactured and sold can be changed as appropriate from the exemplary configuration shown in Fig. 1A and other figures.

[0039] For example, there are no particular limitations on the specific structure of each part of the semiconductor optical amplifier 1 or the materials constituting these parts. Specifically, in the configuration shown in FIG. 1A , the first electrode 81, the second electrode 82, and the third electrode 83 may be integrally formed by a substantially rectangular single electrode that covers the entire semiconductor optical amplifier 1 in a plan view. That is, the first electrode 81 corresponds to a portion of the single electrode that corresponds to the multi-mode interference waveguide 2. The second electrode 82 corresponds to a portion of the single electrode that corresponds to the input waveguide 3. Similarly, the third electrode 83 corresponds to a portion of the single electrode that corresponds to the output waveguide 4. In this case, it is difficult or impossible to distinguish the first electrode 81, the second electrode 82, and the third electrode 83 from each other in appearance.

[0040] The planar shape of the multimode interference waveguide 2 (i.e., the shape shown in FIG. 1A, etc.) can be set based on the propagation constant in the propagation mode, etc. Specifically, as described above, the multimode interference waveguide 2 can be formed so as to satisfy the following formula. In the following formula, the length L is the dimension of the multimode interference waveguide 2 in the X-axis direction in FIG. 1A, etc. That is, L=L π / 2.

number

[0041] By changing the propagation constant of the waveguide between the input side and the output side, optical coupling can be suppressed, thereby reducing loss. Furthermore, the wider the waveguide width of an SOA, the higher its efficiency at high output power. Therefore, as shown in FIG. 8, the output waveguide 4 can be formed with a wider width in the second direction, i.e., the Y-axis direction in the figure, than the input waveguide 3. In particular, applying the electrode separation configuration shown in FIG. 3 to the waveguide configuration shown in FIG. 8 enables even higher efficiency.

[0042] 9 and 10, the multi-mode interference waveguide 2 can be formed as a so-called high mesa waveguide, in which high mesa structures 97 are provided on both sides in the second direction, i.e., the Y-axis direction in the drawings. This strengthens the confinement of light, and the value of β0-β1 increases. As a result, the beat length L π This reduces the value of L in the above formula, that is, the length dimension of the multi-mode interference waveguide 2. Therefore, the multi-mode interference waveguide 2 can be further miniaturized.

[0043] It goes without saying that the elements constituting the above-described embodiments are not necessarily essential unless expressly stated as essential or clearly considered essential in principle. Furthermore, when numerical values ​​such as the number, amount, range, etc. of components are mentioned, the present invention is not limited to those specific numerical values ​​unless expressly stated as essential or clearly limited to specific numerical values ​​in principle. Similarly, when the shape, direction, positional relationship, etc. of components are mentioned, the present invention is not limited to those shapes, directions, positional relationship, etc. unless expressly stated as essential or clearly limited to specific shapes, directions, positional relationship, etc. in principle.

[0044] The modified examples are not limited to the above examples. For example, other than those exemplified above, multiple embodiments may be combined with each other as long as there is no technical contradiction. Similarly, multiple modified examples may be combined with each other as long as there is no technical contradiction. [Explanation of symbols]

[0045] 1. Semiconductor Optical Amplifier 2. Multimode interference waveguide 21 First end surface 22 Second end face 3 Input waveguide (input / output waveguide) 33 Input end face 4 Output waveguide (input / output waveguide) 43 Output end face (input / output end face) 6 Low reflective coating 7 High reflective film (input / output end face)

Claims

1. A semiconductor optical amplifier (1), a multi-mode interference waveguide (2) having a first end face (21) which is an end face on one side in a first direction (X) and a second end face (22) which is an end face on the other side; a pair of input / output waveguides (3, 4) arranged adjacent to the multi-mode interference waveguide in the first direction and arranged in a second direction (Y) perpendicular to the first direction; a first electrode (81) disposed adjacent to the multi-mode interference waveguide in a third direction (Z) perpendicular to the first direction and perpendicular to the second direction; a second electrode (82) disposed adjacent to one of the pair of input / output waveguides (3) in the third direction; a third electrode (83) disposed adjacent to the other (4) of the pair of input / output waveguides different from the one of the pair of input / output waveguides in the third direction; Equipped with the first end face is optically coupled to the pair of input and output waveguides; the second end face is highly reflective; each of the pair of input and output waveguides is formed so that the width, which is the dimension in the second direction, is narrower than that of the multi-mode interference waveguide, and has an input and output end face (33, 43) that is subjected to low reflection processing on the side opposite to the side optically coupled to the first end face; At least one of the second electrode and the third electrode is separated from the first electrode. Semiconductor optical amplifier.

2. The second end face is highly reflective by forming a highly reflective film (7).

2. The semiconductor optical amplifier according to claim 1.

3. the input / output waveguides are provided such that an extension direction (D1, D2) of the input / output waveguides from the input / output end faces is inclined with respect to the input / output end faces; 3. The semiconductor optical amplifier according to claim 1.

4. the second electrode and the third electrode are separated from the first electrode; 4. The semiconductor optical amplifier according to claim 1.

5. The second electrode and the third electrode are separated from each other.

5. The semiconductor optical amplifier according to claim 4.

6. the pair of input and output waveguides are provided at both ends of the multi-mode interference waveguide in the second direction; 6. The semiconductor optical amplifier according to claim 1.

7. The optical waveguide further includes a low refractive index portion (96) provided between one and the other of the pair of input and output waveguides.

7. The semiconductor optical amplifier according to claim 1.

8. The low refractive index portion is formed by a high mesa structure.

8. The semiconductor optical amplifier according to claim 7.

9. each of the pair of input and output waveguides is a single-mode waveguide; 9. The semiconductor optical amplifier according to claim 1.

10. an output waveguide, which is one of the pair of input and output waveguides, is formed to have a width, which is a dimension in the second direction, wider than that of the other input waveguide; 10. The semiconductor optical amplifier according to claim 1.

11. The multimode interference waveguide is formed as a high mesa waveguide having high mesa structures (97) on both sides in the second direction. The semiconductor optical amplifier according to any one of claims 1 to 10.

12. The propagation constant in the fundamental mode of the multimode interference waveguide is β0, The propagation constant in the higher mode of the multimode interference waveguide is β1, L is the length of the multi-mode interference waveguide in the first direction; In this case, The multimode interference waveguide comprises: [Equation 1] It was formed to satisfy The semiconductor optical amplifier according to any one of claims 1 to 11.

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