Semiconductor optical amplifier

Abstract

To provide a semiconductor optical amplifier capable of obtaining high output by a simple constitution.SOLUTION: A semiconductor optical amplifier comprises an optical guide layer 50 through which light propagates, and a plurality of active layers 40 that are higher than average refraction index than the optical guide layer 50 and which amplify the light. The active layers 40 are stacked on at least the top surface side and undersurface side of the optical guide layer 50. Since the active layers 40 are stacked on the top surface side and undersurface side of the optical guide layer 50, high output can be obtained even in a simple constitution in which the active layers 40 are constituted in the same manner from an incident end to an emission end.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a semiconductor optical amplifier (SOA). [Background technology]

[0002] In optical transmitter / receiver devices such as LiDAR equipped with an optical IC (integrated circuit) for optical transmission and reception, the necessary optical output can be secured by placing an SOA after a laser light source that generates seed light, which is the signal light, and amplifying the output light of the laser light source. LiDAR is an abbreviation for Light Detection And Ranging.

[0003] Assume that such an optical transceiver is mounted on a vehicle for use in an advanced driving assistance system, an autonomous driving system, etc. In this case, taking into consideration the environmental temperature and the detection requirements in free space to achieve the above-mentioned applications, the SOA is required to have a higher optical output in a wide wavelength band over a wide temperature range, typically from -40°C to 85°C or higher, compared to an optical transceiver for communication. Furthermore, considering various vehicle requirements, it is desirable to operate the SOA at the desired performance without mounting a thermoelectric cooler (TEC), or to exert the performance of the SOA while reducing the operating power as much as possible even if a TEC is used. However, in an SOA designed for communication, the output is usually saturated before such a high output is obtained, even if strict temperature control is performed by a TEC.

[0004] In order to improve the output saturation limit and obtain high output, for example, in the SOA described in Patent Document 1, two active layers made of different materials and thicknesses are placed on the input and output sides of the optical signal, and the two active layers are butt-jointed to improve the optical output. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5505226 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the technique disclosed in Patent Document 1 has a complex structure in which two active layers made of different materials and with different thicknesses are arranged in the same layer, which increases the manufacturing cost of the SOA.

[0007] In view of the above, an object of the present disclosure is to provide an SOA that can obtain high output with a simple configuration. [Means for solving the problem]

[0008] In order to achieve the above object, according to one aspect of the present disclosure, a semiconductor optical amplifier comprises an optical guide layer (50, 51, 52) through which light propagates, and a plurality of active layers (40, 41, 42, 43) which have an average refractive index higher than that of the optical guide layer and amplify the light, and the active layers are stacked on at least the upper and lower sides of the optical guide layer.

[0009] In this way, by stacking the active layer on the upper and lower sides of the optical guide layer, it is possible to obtain high output even with a simple configuration in which the active layer has the same configuration from the input end to the output end.

[0010] The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and specific components described in the embodiments described below. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a cross-sectional view of the SOA according to the first embodiment. [Diagram 2] FIG. 1 is a cross-sectional view of Comparative Example 1. [Diagram 3] FIG. 11 is a cross-sectional view of Comparative Example 2. [Figure 4] FIG. 1 is a diagram showing the relationship between the output and amplification factor of an SOA. [Diagram 5] FIG. 1 is a diagram showing the saturated output of an SOA. [Figure 6] FIG. 11 is a cross-sectional view of an SOA according to a second embodiment. [Figure 7] FIG. 11 is a cross-sectional view of an SOA according to a third embodiment. [Figure 8] FIG. 13 is a cross-sectional view of an SOA according to a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other will be denoted by the same reference numerals.

[0013] (First embodiment) A first embodiment will be described. The SOA1 of this embodiment shown in FIG. 1 is used in an optical transceiver such as a LiDAR. In this embodiment, the SOA1 is used in an optical transceiver, but the SOA1 may be used for other purposes. The optical transceiver includes the SOA1, a laser light source, a transmitter, a receiver, a light receiving unit, and a signal processing unit. The SOA1, the laser light source, the transmitter, the receiver, and the light receiving unit are formed in an optical IC.

[0014] The SOA1 amplifies and outputs the input light. The SOA1 may be a B-SOA (Booster-SOA). Details of the SOA1 will be described later. The SOA1 is disposed after the laser light source, and is connected to the laser light source by a waveguide on the optical IC. The SOA1 is also connected to the transmitting section by a waveguide on the optical IC.

[0015] The laser light source outputs seed light as a signal light. The output light of the laser light source is amplified by the SOA 1 and then input to the transmission section.

[0016] The transmitting unit transmits an optical signal for distance measurement to the outside. The receiving unit receives an optical signal incident from the outside. The optical signal transmitted by the transmitting unit is reflected by an external object and received by the receiving unit. The optical signal received by the receiving unit is input to the light receiving unit.

[0017] The light receiving section is composed of a photodetector etc. The light receiving section is connected to the signal processing section by electrical wiring, and the output signal of the light receiving section is input to the signal processing section.

[0018] The signal processing unit calculates the position of the object that reflected the transmission signal, etc., based on the output signal from the light receiving unit.

[0019] The following describes in detail the SOA 1. As shown in FIG.

[0020] The substrate 10 is made of, for example, n-type InP (indium phosphide). An electrode (not shown) is formed on the lower surface of the substrate 10 for applying a voltage to the SOA 1. A cladding layer 20 is formed on the upper surface side of the substrate 10.

[0021] The cladding layer 20 is for confining the input light from the laser light source within the active layer 40 and the optical guide layer 50, and is formed on the upper and lower sides of the active layer 40 and the optical guide layer 50. Specifically, the cladding layer 20 is composed of a cladding layer 21 laminated on the upper surface of the substrate 10, and a cladding layer 22 laminated on the upper surface of a spacer layer 32, which will be described later. The cladding layer 21 is, for example, n + The cladding layer 22 is made of, for example, p + The device is made of InP.

[0022] The spacer layer 30 is for adjusting the distance between the cladding layer 20 and the active layer 40 and the optical guide layer 50, and is formed between the cladding layer 20 and the active layer 40 and the optical guide layer 50. Specifically, the spacer layer 30 is composed of a spacer layer 31 formed between the cladding layer 21 and the active layer 41 described below, and a spacer layer 32 formed between the cladding layer 22 and the active layer 42 described below. The spacer layer 31 is, for example, n - The spacer layer 32 is made of, for example, p - The spacer layer 31 is made of, for example, n-type InP. - The spacer layer 32 is made of, for example, p - The spacer layer 31 and the spacer layer 32 may be made of InGaAsP. The thickness of the spacer layer 31 and the thickness of the spacer layer 32 may be the same, or may have a suitable relationship of magnitude.

[0023] The active layer 40 amplifies light incident on the SOA 1, and is formed on the upper and lower surfaces of the optical guide layer 50. Specifically, the active layer 40 is composed of an active layer 41 formed between the spacer layer 31 and the optical guide layer 50, and an active layer 42 formed between the spacer layer 32 and the optical guide layer 50.

[0024] The active layer 40 is configured to include, for example, a bulk structure, a single quantum well (SQW) structure, and a multiple quantum well (MQW) structure. The active layer 40 is configured to include, for example, a single quantum dot (SQD) structure, and a multiple quantum dot (MQD) structure. The active layer 40 may be configured to include only one of the bulk structure, the SQW structure, the MQW structure, the SQD structure, and the MQD structure, or may be configured to include multiple of the bulk structure, the SQW structure, the MQW structure, the SQD structure, and the MQD structure.

[0025] The MQW structure is, for example, a structure in which quantum well layers made of InGaAsP and barrier layers made of n-type InP are alternately stacked, while the MQD structure is, for example, a structure in which quantum dot layers made of InAs (indium arsenide) and barrier layers made of GaAs (gallium arsenide) are alternately stacked.

[0026] The active layers 41 and 42 have the same configuration from the incident end to the output end. For example, the active layers 41 and 42 are made of the same material from the incident end to the output end and have the same thickness from the incident end to the output end. In this embodiment, the active layers 41 and 42 have the same configuration. For example, each of the active layers 41 and 42 is made of three MQW layers.

[0027] The optical guide layer 50 is a layer through which light incident on the SOA 1 propagates. The optical guide layer 50 is made of, for example, InGaAsP without any impurities added, specifically, undoped or unintentionally doped (UID) InGaAsP. The optical guide layer 50 has a higher refractive index than the cladding layer 20. The optical guide layer 50 may have a higher refractive index than the spacer layer 30, or may have a refractive index equal to that of the spacer layer 30. For example, when the spacer layers 31 and 32 are made of InGaAsP, the spacer layers 31 and 32 may have a refractive index equal to that of the optical guide layer 50. In this case, the spacer layers 31 and 32 not only adjust the distance between the cladding layers 21 and 22 and the active layers 41 and 42 and the optical guide layer 50, but also have a function of propagating a part of the light incident on the SOA 1 through the spacer layers 31 and 32. Furthermore, by making the film thicknesses of the spacer layers 31 and 32 different from each other, the refractive index distribution in the stacked structure of the spacer layer 30, the active layer 40, and the optical guide layer 50 may be made asymmetric with respect to the center plane of the optical guide layer 50. This center plane is a plane that passes through the center of the thickness direction, i.e., the stacking direction, of the optical guide layer 50 and is perpendicular to this thickness direction. The active layer 40 has an average refractive index higher than that of the optical guide layer 50.

[0028] The contact layer 60 is, for example, p ++The SOA 1 is made of InGaAs of the type shown in FIG. 1. An electrode (not shown) for applying a voltage to the SOA 1 is formed on the upper surface of the contact layer 60. By applying a voltage to the SOA 1 via the electrodes formed on the lower surface of the substrate 10 and the upper surface of the contact layer 60, the light propagating through the optical guide layer 50 is amplified by the active layer 40.

[0029] In this manner, the SOA1 of this embodiment is configured by stacking the cladding layer 21, the spacer layer 31, the active layer 41, the optical guide layer 50, the active layer 42, the spacer layer 32, the cladding layer 22, and the contact layer 60 in this order on the substrate 10.

[0030] In an SOA, carriers are consumed as light propagates, leading to saturation of the output. In response to this, the saturated output can be improved by distributing the light intensity in a low-loss waveguide region to reduce the optical confinement factor. The optical confinement factor is affected by the thickness of the waveguide.

[0031] In a configuration in which an active layer is laminated on one side of an optical guide layer, the optical intensity is distributed in the active layer, which has a higher refractive index than the optical guide layer. However, for example, by making the optical guide layer extremely thick, the optical intensity can be distributed in the optical guide layer, and the optical confinement factor can be reduced. If the thickness of the waveguide region consisting of the active layer and the optical guide layer can be kept relatively thin while distributing the optical intensity in the low-loss optical guide layer, an improvement in the saturation output can be expected. However, if the optical guide layer is made thicker, the power conversion efficiency decreases due to an increase in series resistance.

[0032] In contrast to this, in this embodiment, active layers 41 and 42 having a higher refractive index than the optical guide layer 50 are disposed on the upper and lower sides of the optical guide layer 50. In this configuration, even if the optical guide layer 50 is thin, it is possible to distribute a region with high optical intensity near the center of the optical guide layer 50 by adjusting the balance of refractive indexes at the cross section of the path through which the light propagates. This makes it possible to reduce the optical confinement coefficient and improve the saturation output.

[0033] Comparative Example 1 will be described. In the SOA101 of Comparative Example 1 shown in Fig. 2, the optical guide layer 50 is composed of an optical guide layer 51 arranged on the lower surface side of the active layer 40, and an optical guide layer 52 arranged on the upper surface side of the active layer 40. That is, the SOA101 is configured such that the cladding layer 21, the spacer layer 31, the optical guide layer 51, the active layer 40, the optical guide layer 52, the spacer layer 32, the cladding layer 22, and the contact layer 60 are laminated in this order on the substrate 10.

[0034] Comparative Example 2 will now be described. In the SOA 102 of Comparative Example 2 shown in Fig. 3, an active layer 40 is disposed on the upper surface side of an optical guide layer 50. That is, the SOA 102 has a configuration in which a cladding layer 21, a spacer layer 31, an optical guide layer 50, an active layer 40, a spacer layer 32, a cladding layer 22, and a contact layer 60 are laminated in this order on a substrate 10.

[0035] The results of simulations performed for Comparative Examples 1 and 2, this embodiment, and the second to fourth embodiments described later are shown in Figs. 4 and 5. Note that, in this embodiment, the spacer layer 31 is - The spacer layer 32 is made of p - A simulation was performed using a structure of the active layer 40 made of InP. Data A and B shown in Figs. 4 and 5 are simulation results of Comparative Examples 1 and 2, respectively, and data C and D are simulation results of this embodiment. In the data A, the thicknesses of the optical guide layers 51 and 52 are 0.3 μm, and in the data B, the thickness of the optical guide layer 50 is 0.6 μm. In the data C, the thickness of the optical guide layer 50 is 0.6 μm, and in the data D, the thickness of the optical guide layer 50 is 0.9 μm. In the data A and B, the active layer 40 is configured to include six MQW layers. In the data C and D, the active layers 41 and 42 are configured to include three MQW layers.

[0036] 4, the SOA1 of this embodiment has a higher amplification factor, i.e., a higher gain, in a high output region than the SOAs 101 and 102. Also, as shown in FIG. 5, the SOA1 of this embodiment has a higher saturated output than the SOAs 101 and 102.

[0037] As described above, in this embodiment, the active layer 40 having a refractive index larger than that of the optical guide layer 50 is laminated on the upper and lower surfaces of the optical guide layer 50. With this, even in a simple configuration in which the active layer 40 has the same configuration from the input end to the output end, the optical confinement coefficient is reduced, thereby improving the saturated output and obtaining a high output.

[0038] In addition, in order to obtain optical coupling with the rear stage, the intensity distribution of the output light of the SOA is required to be easy to propagate to the rear waveguide. In this embodiment, the mode of the guided light is the same as that of Comparative Examples 1 and 2, so that the decrease in coupling efficiency with the waveguide can be suppressed.

[0039] Moreover, in the SOA1 of this embodiment, high output can be obtained even if the thickness of the optical guide layer 50 is the same as in Comparative Examples 1 and 2. Therefore, it is possible to suppress an increase in series resistance due to an increase in the film thickness of the optical guide layer 50, and suppress a decrease in power conversion efficiency. This makes it possible to reduce heat generation, and therefore suppress a decrease in performance in the vehicle temperature range due to heat generation of the SOA1 itself. Furthermore, even when a TEC is used, its operating power can be reduced. Thus, the SOA1 of this embodiment is suitable as an SOA for vehicle use.

[0040] Second embodiment The second embodiment will be described. This embodiment is different from the first embodiment in that the laminated structure of the active layer 40 and the optical guide layer 50 is changed, and other aspects are the same as those of the first embodiment, so only the differences from the first embodiment will be described.

[0041] 6, in this embodiment, the stacked structure of the active layer 40 and the light guide layer 50 is asymmetric with respect to a center plane S1 in the thickness direction. That is, the stacked structure of the active layer 40 and the light guide layer 50 has different structures above and below the center plane S1. The center plane S1 is a plane that passes through the center of the thickness direction, i.e., the stacking direction, of the stacked structure of the active layer 40 and the light guide layer 50 and is perpendicular to this thickness direction.

[0042] Specifically, the spacer layer 30 includes a spacer layer 33 in addition to the spacer layers 31 and 32, and the spacer layer 33 is laminated between the optical guide layer 50 and the active layer 42. As a result, the active layer 42 laminated on the upper surface side of the optical guide layer 50 and the active layer 41 laminated on the lower surface side of the optical guide layer 50 have different optical distances to the center in the thickness direction of the optical guide layer 50. The spacer layer 33 is made of, for example, InP to which no impurities are added, specifically, undoped or unintentionally doped.

[0043] Moreover, the active layer 41 and the active layer 42 differ in the number of MQW layers that constitute them. For example, the active layer 41 is configured to include two MQW layers, and the active layer 42 is configured to include four MQW layers.

[0044] With this configuration, the light intensity distribution becomes as shown in Fig. 6. That is, in the laminated structure of the active layer 41, the light guide layer 50, the spacer layer 33, and the active layer 42, the peak of the light intensity is located below the center plane S1, i.e., on the side closer to the spacer layer 31, and the light intensity is greater below the center plane S1 than above. The light intensity is greater in the active layer 41 than in the active layer 42.

[0045] By varying the light intensity in this way, below the central plane S1, the light is greatly amplified near the input end, and the amplification rate saturates near the output end. On the other hand, above the central plane S1, the amplification rate is small near the input end, but increases near the output end, making a large contribution to the output of the SOA1. This makes it possible to efficiently amplify light above and below the central plane S1, improving the optical output.

[0046] Data E shown in Figures 4 and 5 are simulation results for this embodiment. In data E, the active layers 41 and 42 are configured to include two and four MQW layers, respectively, and the film thickness of the optical guide layer 50 is 0.6 μm. As shown in Figures 4 and 5, this embodiment also has a higher amplification factor and saturation output in the high output region than the SOAs 101 and 102.

[0047] This embodiment has the same configuration and operation as the first embodiment, and therefore can obtain the same effects as the first embodiment.

[0048] Furthermore, according to the above embodiment, the following effects can be obtained.

[0049] (1) The laminated structure of the active layers 41, 42 and the optical guide layer 50 is asymmetric with respect to the center plane S1. This allows the gain and the optical confinement factor to be adjusted, and the optical output of the entire SOA1 to be improved by ensuring the gain near the input end of the SOA1 and improving the saturated output near the output end.

[0050] Third embodiment A third embodiment will be described. This embodiment is different from the first embodiment in that the configurations of the active layer 40 and the optical guide layer 50 are changed, and other aspects are the same as those of the first embodiment, so only the differences from the first embodiment will be described.

[0051] The light guide layer 50 of this embodiment is composed of a plurality of layers. Specifically, as shown in FIG. 7, the light guide layer 50 includes a light guide layer 51 arranged on the upper surface side of the active layer 41, and a light guide layer 52 arranged on the upper surface side of the light guide layer 51. The active layer 40 includes an active layer 43 in addition to the active layers 41 and 42, and the active layer 43 is arranged between the light guide layer 51 and the light guide layer 52. That is, the active layer 40 and the light guide layer 50 have a structure in which the active layer 41, the light guide layer 51, the active layer 43, the light guide layer 52, and the active layer 42 are stacked in this order on the upper surface of the spacer layer 31. The light guide layers 51 and 52 correspond to the first light guide layer and the second light guide layer, respectively.

[0052] The optical guide layers 51 and 52 are made of, for example, undoped or unintentionally doped InGaAsP. The active layers 41, 42, and 43 each include, for example, two MQW layers.

[0053] The light guide layer 50 may be composed of three or more light guide layers. In this case, among the multiple light guide layers constituting the light guide layer 50, the light guide layer located at the bottom corresponds to the first light guide layer, and the light guide layer located at the top corresponds to the second light guide layer.

[0054] In this way, even in a configuration in which the optical guide layer 50 is divided by the active layer 40, the optical confinement factor can be reduced by adjusting the refractive index.

[0055] The data F shown in Figures 4 and 5 are the simulation results of this embodiment. In the data F, the active layers 41, 42, and 43 each include two MQW layers, and the film thickness of the optical guide layers 51 and 52 is 0.3 μm. As shown in Figures 4 and 5, this embodiment also has a higher amplification factor and saturation output in the high output region than the SOAs 101 and 102.

[0056] This embodiment has the same configuration and operation as the first embodiment, and therefore can obtain the same effects as the first embodiment.

[0057] (Fourth embodiment) The fourth embodiment will be described. This embodiment is the same as the third embodiment except for changing the position of the active layer 43, and therefore only the differences from the third embodiment will be described.

[0058] In the second embodiment, the stacked structure of the active layer 40 and the light guide layer 50 is asymmetrical depending on the number of layers of the active layers 41, 42 and the distance from the active layers 41, 42 to the center plane of the light guide layer 50. However, in the third embodiment, the stacked structure of the active layer 40 and the light guide layer 50 may be asymmetrical with respect to the center plane S1 by shifting the position of the active layer 43 in the thickness direction from the center plane S1. For example, as shown in FIG. 8, the active layer 43 may be disposed above the center plane S1.

[0059] The data G shown in Figures 4 and 5 are the simulation results of this embodiment. In the data G, the active layers 41, 42, and 43 each include two MQW layers, and the film thicknesses of the optical guide layers 51 and 52 are 0.4 μm and 0.2 μm, respectively. As shown in Figures 4 and 5, this embodiment also has a higher amplification factor and saturation output in the high output region than the SOAs 101 and 102.

[0060] This embodiment has the same configuration and operation as the first to third embodiments, and therefore can obtain the same effects as the first to third embodiments.

[0061] Fifth embodiment A fifth embodiment will be described. This embodiment is different from the first embodiment in that the configuration of the active layer 40 is changed, and other aspects are the same as those of the first embodiment, so only the differences from the first embodiment will be described.

[0062] In the second embodiment, the stacked structure of the active layer 40 and the optical guide layer 50 is asymmetrical depending on the number of layers in the active layers 41, 42 and the distance from the active layers 41, 42 to the central plane of the optical guide layer 50. However, the stacked structure of the active layer 40 and the optical guide layer 50 may be asymmetrical with respect to the central plane S1 by making the active layers 41, 42 have different structures. For example, the active layer 41 may be configured to include three layers of MQDs, and the active layer 42 may be configured to include three layers of MQWs.

[0063] Comparing SQW and MQW with SQD and MQD, for the same number of layers, SQW and MQW have a higher gain than SQD and MQD, and SQD and MQD have a smaller optical confinement factor than SQW and MQW, resulting in a higher saturated output. Therefore, by combining MQW and MQD as described above, it is possible to ensure gain near the input end of SOA1 by using MQW, and to improve the saturated output by using MQD near the output end. This makes it possible to improve the output of SOA1.

[0064] This embodiment has the same configuration and operation as the first and second embodiments, and therefore can obtain the same effects as the first and second embodiments.

[0065] (Other embodiments) The present disclosure is not limited to the above-described embodiments, and can be modified as appropriate. The above-described embodiments are not unrelated to each other, and can be combined as appropriate, except when the combination is clearly impossible. In the above-described embodiments, it goes without saying that the elements constituting the embodiments are not necessarily essential, except when they are specifically stated to be essential or when they are clearly considered to be essential in principle. In the above-described embodiments, when the numbers, values, amounts, ranges, etc. of the components of the embodiments are mentioned, they are not limited to the specific numbers, except when they are specifically stated to be essential or when they are clearly limited to a specific number in principle. In the above-described embodiments, when the shapes, positional relationships, etc. of the components are mentioned, they are not limited to the shapes, positional relationships, etc., except when they are specifically stated to be essential or when they are clearly limited to a specific shape, positional relationship, etc. in principle.

[0066] In the second to fourth embodiments, the structures of the active layers 41, 42 may be different from each other as in the fifth embodiment. In the second embodiment, the stacked structure of the active layer 40 and the light guide layer 50 may be asymmetric with respect to the center plane S1 depending on only one of the number of layers of the active layers 41, 42 and the distance of the active layers 41, 42 to the center plane of the light guide layer 50.

[0067] In order to distribute light in the center of the light guide layer 50, the refractive index of the light guide layer 50 may be changed depending on the position in the thickness direction. The refractive index of the light guide layer 50 can be adjusted by the composition and ratio of the materials of the light guide layer 50. For example, the refractive index of the center part in the thickness direction may be higher than that of the surface layer part on the upper surface side and the surface layer part on the lower surface side of the light guide layer 50. The refractive index of the light guide layer 50 may be changed discontinuously, i.e., stepwise, or may be changed continuously.

[0068] (In view of the present disclosure) [First viewpoint] 1. A semiconductor optical amplifier, comprising: a light guide layer (50, 51, 52) through which light propagates; a plurality of active layers (40, 41, 42, 43) having an average refractive index higher than that of the optical guide layer and amplifying light; The active layer is laminated at least on the upper and lower surfaces of the optical guide layer. [Second viewpoint] The semiconductor optical amplifier according to a first aspect, wherein a laminated structure of the optical guide layer and the plurality of active layers is asymmetric with respect to a center plane in a thickness direction. [Third Perspective] The semiconductor optical amplifier according to the first or second aspect, wherein the active layer stacked on the upper surface side of the optical guiding layer and the active layer stacked on the lower surface side of the optical guiding layer have different distances to a center plane in a thickness direction of the optical guiding layer. [Fourth viewpoint] The semiconductor optical amplifier according to any one of the first to third aspects, wherein the active layer stacked on the upper surface side of the optical guiding layer and the active layer stacked on the lower surface side of the optical guiding layer have different numbers of layers. [Fifth viewpoint] The semiconductor optical amplifier according to any one of the first to fourth aspects, wherein the active layer stacked on the upper surface side of the optical guiding layer and the active layer stacked on the lower surface side of the optical guiding layer have different structures. [Sixth viewpoint] The semiconductor optical amplifier according to any one of the first to fifth aspects, wherein the active layer includes any one of a bulk structure, a single quantum well structure, a multiple quantum well structure, a single quantum dot structure, and a multiple quantum dot structure. [Seventh viewpoint] A plurality of the light guide layers are provided, The light guide layer located at the bottom of the plurality of light guide layers is designated as a first light guide layer (51); The uppermost light guide layer among the plurality of light guide layers is designated as a second light guide layer (52), The semiconductor optical amplifier according to any one of the first to sixth aspects, wherein the active layer is laminated not only on a lower surface side of the first optical guiding layer and an upper surface side of the second optical guiding layer, but also between the first optical guiding layer and the second optical guiding layer. [Eighth viewpoint] The semiconductor optical amplifier according to any one of the first to seventh aspects, wherein the optical guide layer has a refractive index that changes depending on a position in a thickness direction. [Ninth viewpoint] The semiconductor optical amplifier according to any one of the first to eighth aspects, wherein the refractive index of the optical guide layer changes stepwise. [10th viewpoint] The semiconductor optical amplifier according to any one of the first to ninth aspects, wherein the optical guide layer has a higher refractive index at a central portion in a thickness direction than at a surface portion on an upper surface side and a surface portion on a lower surface side. [Eleventh viewpoint] The semiconductor optical amplifier according to any one of the first to tenth aspects, wherein the refractive index of the optical guide layer changes continuously. [Explanation of symbols]

[0069] 40, 41, 42, 43 active layer 50, 51, 52 Light guide layer

Claims

1. 1. A semiconductor optical amplifier, comprising: a light guide layer (50, 51, 52) through which light propagates; a plurality of active layers (40, 41, 42, 43) having an average refractive index higher than that of the optical guide layer and amplifying light; The active layer is laminated at least on the upper and lower surfaces of the optical guide layer.

2. 2. The semiconductor optical amplifier according to claim 1, wherein a laminated structure of said optical guide layer and said plurality of active layers is asymmetric with respect to a center plane in a thickness direction.

3. 3. The semiconductor optical amplifier according to claim 2, wherein the active layer stacked on the upper surface side of the optical guiding layer and the active layer stacked on the lower surface side of the optical guiding layer have different distances to a center plane in a thickness direction of the optical guiding layer.

4. 3. The semiconductor optical amplifier according to claim 2, wherein the number of layers of the active layer laminated on the upper surface side of the optical guide layer is different from the number of layers of the active layer laminated on the lower surface side of the optical guide layer.

5. 3. The semiconductor optical amplifier according to claim 2, wherein the active layer laminated on the upper surface side of the optical guide layer and the active layer laminated on the lower surface side of the optical guide layer have different structures.

6. The semiconductor optical amplifier according to claim 5 , wherein the active layer includes any one of a bulk structure, a single quantum well structure, a multiple quantum well structure, a single quantum dot structure, and a multiple quantum dot structure.

7. A plurality of the light guide layers are provided, The light guide layer located at the bottom of the plurality of light guide layers is designated as a first light guide layer (51); The uppermost light guide layer among the plurality of light guide layers is designated as a second light guide layer (52); 7. The semiconductor optical amplifier according to claim 1, wherein the active layer is laminated on a lower surface side of the first optical guiding layer, an upper surface side of the second optical guiding layer, and also between the first optical guiding layer and the second optical guiding layer.

8. 3. The semiconductor optical amplifier according to claim 1, wherein the refractive index of the optical guide layer varies with the position in the thickness direction.

9. 9. The semiconductor optical amplifier according to claim 8, wherein the refractive index of the optical guide layer changes stepwise.

10. 9. The semiconductor optical amplifier according to claim 8, wherein the optical guide layer has a higher refractive index in a central portion in a thickness direction than in a surface portion on an upper surface side and a surface portion on a lower surface side.

11. 9. The semiconductor optical amplifier according to claim 8, wherein the refractive index of the optical guide layer changes continuously.

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