Optical semiconductor devices

Abstract

The optical semiconductor element (500) of the present disclosure comprises: a first waveguide (10) formed on a semiconductor substrate (20) and allowing propagation of light in a higher-order mode; a tapered waveguide (12, 82) formed on the semiconductor substrate (20), one end of which is connected to the other end of the first waveguide (10) and the waveguide width decreases toward the other end; and a second waveguide (11) formed on the semiconductor substrate (20), one end of which is connected to the tapered waveguide and has a waveguide width narrower than the waveguide width of the first waveguide (10). The tapered waveguide (12, 82) has a tapered shape in which fundamental mode light incident from the other end of the first waveguide (10) propagates to excite higher-order mode light in the tapered waveguide (12, 82), and an imaging position exists where the intensity of the propagating light is imaged at one point due to interference between the fundamental mode light and the higher-order mode light in the tapered waveguide (12, 82), and the tapered waveguide (12, 82) and the second waveguide (11) are connected at the imaging position.

Description

[Technical field]

[0001] The present disclosure relates to an optical semiconductor device. [Background technology]

[0002] As one of the devices for optical communication, an optical semiconductor element that generates and outputs laser light using a semiconductor is known. For example, an optical semiconductor element that integrates a laser section, an electroabsorption (EA) optical modulator, and a semiconductor optical amplifier (SOA) is known (for example, Patent Document 1).

[0003] In such conventional optical semiconductor elements, the laser section generates laser light in response to an injected current. The EA modulator has a modulation function using the effect of the optical absorption spectrum of the semiconductor layer changing in response to an applied voltage, and generates modulated laser light by modulating the laser light generated by the laser section. The semiconductor optical amplifier has a function of increasing the light intensity by generating stimulated emission in response to the injected current, and generates amplified laser light by amplifying the modulated laser light modulated by the EA modulator.

[0004] However, with the spread of optical communication technology, further increases in the optical output of optical semiconductor elements are expected. As an SOA suitable for high output, an element structure has been reported in which a tapered SOA that increases the waveguide width along the light propagation direction and expands the spot size is integrated, and a long spot size converter is further integrated to effectively couple the output light to an optical fiber (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2019-160840 A [Patent Document 2] Japanese Patent Application Publication No. 08-195525 Summary of the Invention [Problem to be solved by the invention]

[0006] By providing an SOA at the rear stage of an optical semiconductor element, it is possible to increase the optical output. It is known that under operating conditions where the light is amplified and the photon density at the active layer position of the SOA increases, the carriers injected with the current are consumed and depleted by stimulated emission, resulting in gain saturation. Therefore, in the semiconductor optical integrated element described in Patent Document 1, when an SOA with a uniform waveguide width is used, the amplification of the optical intensity becomes saturated, limiting the optical output beyond the saturation intensity.

[0007] The semiconductor laser described in Patent Document 2 has an element structure in which the waveguide width is increased in the light propagation direction and a tapered SOA that expands the spot size is integrated to enable higher optical output. While the optical intensity is amplified, the increase in photon density is suppressed by the horizontal expansion of the spot size, thereby reducing gain saturation and achieving higher optical output.

[0008] The waveguide mode shape of output light from a tapered SOA that expands the spot size has a smaller circularity, which is the ratio of the vertical size to the horizontal size of the waveguide mode, and this leads to poor coupling to an optical fiber. Patent Document 2 discloses an element structure that integrates a tapered spot size converter that reduces the spot size after the SOA to make the waveguide mode shape closer to a perfect circle and improve coupling to an optical fiber. However, in order to convert the spot shape so that the horizontally expanded waveguide mode shape approaches a perfect circle, a long conversion element is required, which causes a problem of increasing the size of the optical semiconductor element.

[0009] The present disclosure has been made to solve the problems described above, and has an object to provide an optical semiconductor element that can convert guided light having a guided mode shape with a large horizontal spot size and low circularity on a semiconductor substrate, into a guided mode shape with high circularity that can be easily coupled to an optical fiber. [Means for solving the problem]

[0010] The optical semiconductor element according to the present disclosure comprises: a first waveguide formed on a semiconductor substrate and allowing propagation of light in a higher order mode; a tapered waveguide formed on the semiconductor substrate, one end of which is connected to the other end of the first waveguide and the width of the waveguide decreases toward the other end; a waveguide formed on the semiconductor substrate, one end of which is connected to the tapered waveguide and has a waveguide width narrower than the waveguide width of the first waveguide; It is a transparent waveguide that is transparent to the wavelength of the propagating light. A second waveguide; and a gain taper type waveguide formed on the semiconductor substrate, the gain taper type waveguide having a gain waveguide core layer with optical gain and a shape in which the waveguide width increases along a light propagation direction; Equipped with The tapered waveguide is a transparent tapered waveguide that is transparent to the wavelength of the propagating light; a tapered shape in which fundamental mode light incident on the other end of the first waveguide propagates to excite higher-order mode light within the tapered waveguide; the gain tapered waveguide, the first waveguide, and the transparent tapered waveguide are arranged in this order along a light propagation direction; The present invention is characterized in that an imaging position exists where the intensity of the propagating light is imaged at one point due to interference between the fundamental mode light and the higher-order mode light in the tapered waveguide, and the tapered waveguide and the second waveguide are connected at the imaging position. Effect of the Invention

[0011] The optical semiconductor element according to the present disclosure has the effect of obtaining an optical semiconductor element that converts guided light having a guided mode shape with a large horizontal spot size and small circularity on a semiconductor substrate into a guided mode shape with high circularity that can be easily coupled to an optical fiber. [Brief description of the drawings]

[0012] [Figure 1] 1A and 1B are top views illustrating an optical semiconductor element according to a first embodiment. [Diagram 2] 1 is a cross-sectional view showing an optical semiconductor element according to a first embodiment. [Diagram 3]3 is a diagram showing a light intensity distribution of propagating light in the optical semiconductor element according to the first embodiment. FIG. [Figure 4] 13 is a diagram showing a light intensity distribution of propagating light in an optical semiconductor element according to a modification of the first embodiment. FIG. [Diagram 5] FIG. 5A is a top view showing an optical semiconductor element according to the first embodiment, and FIGS. 5B and 5C are top views showing optical semiconductor elements according to other variations of the first embodiment. [Figure 6] FIG. 11 is a top view showing an optical semiconductor element according to a second embodiment. [Figure 7] FIG. 11 is a cross-sectional view showing an optical semiconductor element according to a second embodiment. [Figure 8] 8A and 8B are top views showing an optical semiconductor element according to a first modification of the second embodiment. [Figure 9] 9A and 9B are top views showing an optical semiconductor element according to Modification 2 of Embodiment 2. FIG. [Figure 10] FIG. 10A is a top view showing an optical semiconductor element according to the second embodiment, and FIGS. 10B, 10C, and 10D are top views showing an optical semiconductor element according to a third modification of the second embodiment. [Figure 11] 13 is a top view showing an optical semiconductor element according to a fourth modification of the second embodiment. FIG. [Figure 12] FIG. 11 is a top view showing an optical semiconductor element according to a third embodiment. [Figure 13] FIG. 11 is a cross-sectional view showing an optical semiconductor element according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An optical semiconductor device according to each embodiment will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0014] <Embodiment 1> 1A and 1B are top views showing optical semiconductor elements 500 and 500a according to embodiment 1. In order to make the figures easier to understand, the scale of the light propagation direction of the waveguide and the direction perpendicular to the light propagation direction are compressed to the light propagation direction.

[0015] Optical semiconductor device 500 according to the first embodiment includes a first waveguide 10, a transparent tapered waveguide 12, a second waveguide 11, and a lateral cladding region 13.

[0016] As shown in FIG. 1A, a first waveguide 10 having a waveguide width W1, a tapered waveguide (hereinafter referred to as a transparent tapered waveguide 12) having a waveguide width W1 at one end on the first waveguide 10 side, a waveguide width W2 at the other end, and a waveguide length L1 in the light propagation direction, which is transparent at the wavelength of the propagating light (hereinafter referred to as the wavelength used), that is, does not absorb the propagating light, and a second waveguide 11 having a waveguide width W3 are connected in order from the left side of FIG. 1A, that is, along the light propagation direction. The second waveguide 11 is also transparent at the wavelength used, like the transparent tapered waveguide 12. That is, it can be said that the second waveguide 11 and the transparent tapered waveguide 12 are transparent waveguides at the wavelength used. The reason for using a transparent waveguide is that it is transparent at the focusing position where the light intensity in the waveguide increases, i.e., the rear part of the transparent tapered waveguide 12 and the second waveguide 11, and therefore it is possible to suppress the occurrence of deterioration of the signal light due to gain saturation caused by excessive photon density. This makes it possible to miniaturize the mode conversion element, i.e., the transparent tapered waveguide 12, which converts the laser light in the first waveguide 10, which has low circularity and is difficult to couple to an optical fiber, into the laser light in the second waveguide 11, which has high circularity and has an improved coupling rate to an optical fiber, while suppressing deterioration of the signal light.

[0017] The tapered shape of the transparent tapered waveguide 12 has a relationship of waveguide width W1>waveguide width W2. That is, the transparent tapered waveguide 12 has a shape in which the waveguide width decreases along the light propagation direction from the waveguide width W1 at one end to the waveguide width W2 at the other end. In addition, it is preferable that the ratio L1 / W1 of the waveguide length L1 of the transparent tapered waveguide 12 to the waveguide width W1 at one end of the transparent tapered waveguide 12, that is, the first waveguide 10 side, is 10 or less. This is because such a configuration can reduce the size of the mode conversion element, i.e., the transparent tapered waveguide 12, which converts the laser light in the first waveguide 10, which has low circularity and is difficult to couple to an optical fiber, into the laser light in the second waveguide 11, which has high circularity and has an improved coupling rate to an optical fiber.

[0018] In the optical semiconductor device 500, a lateral cladding region 13 is provided in the region other than where the first waveguide 10, the transparent tapered waveguide 12, and the second waveguide 11 are formed.

[0019] Examples of size combinations of the waveguide widths W1, W2, and W3 of the first waveguide 10, the transparent tapered waveguide 12, and the second waveguide 11 include a combination of W1=10.0 μm, W2=3.5 μm, W3=2.0 μm, and L1=80 μm, or W1=7.0 μm, W2=2.8 μm, W3=2.0 μm, and L1=50 μm, when the wavelength used is 1.3 μm. The tapered shape of the transparent tapered waveguide 12 is trapezoidal when viewed from above.

[0020] FIG. 1B is a top view of an optical semiconductor device 500a in which the waveguide, which is a component of the optical semiconductor device 500 shown in FIG. 1A, is used as a gain waveguide. That is, the optical semiconductor device 500a includes a first gain waveguide 80, a second gain waveguide 81, a gain tapered waveguide 82, and a lateral cladding region 13. In the following, the transparent tapered waveguide 12 and the gain tapered waveguide 82 may be collectively referred to simply as a tapered waveguide. In addition, although the following description will be given with respect to FIG. 1A, the optical semiconductor device 500a shown in FIG. 1B has the effect of amplifying the intensity of the propagating light, thereby enabling the optical semiconductor device to have a high output, in addition to the effect of the optical semiconductor device 500 shown in FIG. 1A described later. The use of the gain tapered waveguide 82 makes it possible to increase the optical intensity within the gain tapered waveguide 82 and the output optical intensity in the second gain waveguide 81 under optical intensity conditions in which nonlinear phenomena such as gain saturation, which is likely to occur under high optical intensity conditions and is a cause of optical signal degradation, only results in minor signal degradation.

[0021] <Cross-sectional structure of optical semiconductor element according to first embodiment> Fig. 2 is a cross-sectional view showing an optical semiconductor element 500 according to the first embodiment, showing a cross section taken along line AA in Fig. 1A. The optical semiconductor element 500 is made up of a lower cladding layer 21 and a transparent waveguide core layer 22 formed on a semiconductor substrate 20, and includes a transparent waveguide mesa portion 22a having a convex cross section, lateral cladding layers 23 formed on both side surfaces of the transparent waveguide mesa portion 22a, and an upper cladding layer 24 formed on the upper surface of the transparent waveguide mesa portion 22a and the upper surface of the lateral cladding layer 23.

[0022] <Method of Manufacturing Optical Semiconductor Device According to First Embodiment> A lower cladding layer 21 and a transparent waveguide core layer 22 are sequentially laminated on a semiconductor substrate 20 by epitaxial crystal growth or the like. An etching mask having a desired shape is formed by photolithography and etching, and the lateral cladding layer 23 is etched away by dry etching or the like until the entire transparent waveguide core layer 22 and the middle of the lower cladding layer 21 are reached, forming a convex transparent waveguide mesa portion 22a. Thereafter, the lateral cladding layer 23 is grown by buried crystal growth so as to bury both side surfaces of the transparent waveguide mesa portion 22a. After the buried crystal growth, an upper cladding layer 24 is laminated by epitaxial crystal growth or the like, thereby obtaining the cross-sectional structure shown in FIG. 2.

[0023] A material for the transparent waveguide core layer 22 is selected so that the refractive index of the transparent waveguide core layer 22 is higher than the refractive index of each of the lower cladding layer 21, the lateral cladding layer 23, and the upper cladding layer 24. As such a material, the semiconductor substrate 20, the lower cladding layer 21, the lateral cladding layer 23, and the upper cladding layer 24 are each made of InP, and the transparent waveguide core layer 22 is made of InGaAsP or InAlGaAs, which is a composition transparent to light of the wavelength used. Each of these layers can be epitaxially grown on the semiconductor substrate using known crystal growth techniques.

[0024] 1A also exhibit a similar cross-sectional structure, except for the different waveguide widths of the transparent waveguide core layer 22. The thickness of the upper cladding layer 24 is preferably, for example, 2.0 μm, and the thickness of the transparent waveguide core layer 22 is preferably, for example, 0.4 μm.

[0025] 3 shows the results of calculations using a commercially available waveguide simulator of the light intensity distribution of propagating light when fundamental mode light is incident from the first waveguide 10 side for the optical semiconductor device 500 according to the first embodiment shown in the top view of FIG. 1A. The light intensity is displayed in shading, with darker shading indicating higher light intensity. The fundamental mode light incident from one end of the first waveguide 10 enters the transparent tapered waveguide 12 from the other end of the first waveguide 10 and propagates while narrowing the waveguide width.

[0026] The propagating light at the position of the DD line in Fig. 3 is very similar to the propagating light in the first waveguide 10. As the propagating light propagates in the transparent tapered waveguide 12, the fundamental mode light is converted to higher mode light, and the shape of the light intensity of the entire propagating light becomes complex due to interference between the fundamental mode light and the higher mode light. The propagating light at the position of the EE line in Fig. 3 exhibits a light intensity shape split into two peaks, with the light intensity at the center of the optical semiconductor element 500 no longer being maximum, due to interference between guided mode lights of different orders.

[0027] 3, the phases of the guided mode lights of different orders become aligned and are imaged as light with a single-peaked light intensity shape having a smaller beam size than the propagating light in the first waveguide 10. By connecting to the second waveguide 11 that matches the size of the beam imaged at this imaging position, the propagating light in the first waveguide 10 can be coupled to the second waveguide 11 with low loss. That is, it can be said that the transparent tapered waveguide 12 constituting the optical semiconductor device 500 according to the first embodiment operates as a one-output, one-input MMI (Multimode Interferometer) capable of mode size conversion. In addition, if the transparent tapered waveguide 12 is significantly lengthened, the fundamental mode light propagates adiabatically in an optical sense, and therefore conversion to higher-order mode light does not occur, and the intensity distribution of the propagating light remains single-peaked, and does not become the intensity distribution of the propagating light as shown in Figure 3.

[0028] 3, the loss in optical intensity of the guided light moving from first waveguide 10 to second waveguide 11 is a low value of 0.06 dB. The device structure of optical semiconductor device 500 according to the first embodiment makes it possible to provide an optical semiconductor device that converts guided light having a waveguide mode shape with a large horizontal spot size and low circularity on semiconductor substrate 20 into a waveguide mode shape with high circularity that is easy to couple to an optical fiber.

[0029] That is, in the optical semiconductor element 500 according to the first embodiment, the transparent tapered waveguide 12 has a tapered shape in which the fundamental mode light incident from the other end of the first waveguide 10 propagates to excite high-order mode light in the transparent tapered waveguide 12, and further, there is an imaging position where the intensity of the propagating light is imaged at one point due to interference between the fundamental mode light and the high-order mode light in the transparent tapered waveguide 12, and the transparent tapered waveguide 12 and the second waveguide 11 are connected at this imaging position, so that it is possible to provide an optical semiconductor element that converts guided light having a guided mode shape with a large horizontal spot size and low circularity into a guided mode shape with high circularity that is easy to couple to an optical fiber. Note that the above-mentioned imaging position is preferably a position where an image is first formed when the light incident from the first waveguide 10 propagates. Furthermore, the connection of the transparent tapered waveguide 12 and the second waveguide 11 at the imaging position means that the two are connected within a range of ±5 times the optical wavelength, which is a range in which good light-gathering properties are maintained with the imaging position as the center.

[0030] The feature of the element structure of optical semiconductor element 500 according to the first embodiment, that is, there is an imaging position where the intensity of propagating light is imaged at one point due to interference between fundamental mode light and higher-order mode light in transparent tapered waveguide 12, and transparent tapered waveguide 12 and second waveguide 11 are connected at this imaging position, can be realized, for example, by using the above-mentioned waveguide simulator to repeat calculations while changing each parameter, such as the waveguide width W1 of first waveguide 10, the waveguide width W2 of transparent tapered waveguide 12 on the second waveguide 11 side, the waveguide length L1 of transparent tapered waveguide 12, and the waveguide width W3 of second waveguide 11.

[0031] <Modification of the first embodiment> 4 shows the results of calculating the light intensity distribution of the propagating light when fundamental mode light is incident from the first waveguide 10 side in the optical semiconductor element according to the modified example of the first embodiment, using a commercially available waveguide simulator. The size combination of the optical semiconductor element in FIG. 4 is W1=10.0 μm, W2=2.8 μm, W3=2.0 μm, and L1=150 μm at a used wavelength of 1.3 μm. The light incidence situation is the same as in FIG. 3.

[0032] The light intensity profile of the propagating light at the position of line GG in Fig. 4 is similar to the fundamental mode of the first waveguide 10, as is the light intensity profile of the propagating light at the position of line DD in Fig. 3. The light intensity profiles of line HH and line II are qualitatively the same as the light intensity profiles of line EE and line FF in Fig. 3, respectively, that is, a bimodal profile and a profile focused at one point in the center of the waveguide. As the light propagates further, the light intensity profile becomes bimodal again at line JJ, and a second focused profile at line KK. Fig. 4 shows a device structure in which this focused light is coupled to a second waveguide 11 with a waveguide width that matches the width of the waveguide.

[0033] The size of the imaged light at the position of the II line is smaller than that at the position of the KK line, and the beam size reduction rate at the position of the II line is inferior. In addition, in the second and subsequent imaging positions, the closer the imaging position is to the tip of the tapered shape, the closer the intervals between the positions where the imaging positions are repeatedly generated become, so that when coupling to the second waveguide 11, which has a uniform waveguide width, the coupling rate becomes more sensitive to variations in device structure. In other words, comparing the device structures of Figures 3 and 4, the earlier the imaging position appears as in Figure 3, the greater the tolerance of optical coupling to variations in device structure, and therefore it is more suitable as an optical semiconductor device.

[0034] <Other Modifications of the First Embodiment> 5B and 5C are top views of optical semiconductor elements 510a and 510b according to other modifications of embodiment 1. Fig. 5A is an enlarged view of the top view of optical semiconductor element 500 according to embodiment 1 shown in Fig. 1A, showing the vicinity of a connection portion between transparent tapered waveguide 12 and second waveguide 11. The element structures of optical semiconductor element 510a shown in Fig. 5B and optical semiconductor element 510b shown in Fig. 5C are modifications obtained by modifying the element structure of optical semiconductor element 500 shown in Fig. 5A.

[0035] In the element structure shown in Fig. 5B and Fig. 5C which is an optical semiconductor element according to another modification of the first embodiment, a connecting transparent tapered waveguide 14 is provided between the transparent tapered waveguide 12 and the second waveguide 11 to connect them. The connecting transparent tapered waveguide 14 has a waveguide width W4 at the connection portion with the transparent tapered waveguide 12, which is set to a waveguide width equal to or smaller than the waveguide width W2 of the transparent tapered waveguide 12. At the connection portion with the second waveguide 11, the waveguide width of the connecting transparent tapered waveguide 14 is set to the same waveguide width as the waveguide width W3 of the second waveguide 11. The connecting transparent tapered waveguide 14 has a waveguide length L2 in the light propagation direction.

[0036] A structure in which a tapered waveguide is provided at the connection point between the MMI and the input / output waveguide is desirable because it has the effect of mitigating the phenomenon in which loss at the time of connection changes sensitively to variations in element structure and reducing the reflectance of backlight generated at the junction interface.

[0037] There is no particular upper limit to the waveguide length L2 in the light propagation direction of the connecting transparent tapered waveguide 14 as long as reconversion to a higher mode does not occur in the connecting transparent tapered waveguide 14, but it is preferable that it is short in terms of miniaturization of the element. For example, even if the waveguide length L2 is set to 10 times or less the wavelength used in order to miniaturize the optical semiconductor element, the effect of improving the tolerance of the element structure variation and reducing the return light reflectance can be obtained. Also, an element structure in which W4 = W2, such as the element structure of the optical semiconductor element 510b shown in Figure 5C, can achieve the same effect as the element structure shown in Figure 5B.

[0038] In the first embodiment, the transparent tapered waveguide 12 has a trapezoidal shape in top view. However, even if both sides of the waveguide (the hypotenuses of the trapezoid) are not linear but are curved and symmetrical with respect to the center of the optical semiconductor element, as long as the waveguide width is tapered so as to monotonically decrease from the first waveguide 10 side along the light propagation direction, an optical semiconductor element can be obtained that is small, has low loss, and can be easily coupled to an optical fiber with a highly circular waveguide mode shape, as in the case of a linear tapered shape.

[0039] <Advantages of the First Embodiment> As described above, the optical semiconductor element according to the first embodiment has a configuration in which an imaging position exists where the intensity of the propagating light is imaged at one point due to interference between the fundamental mode light and higher-order mode light in the tapered waveguide, and the transparent tapered waveguide and the second waveguide are connected at this imaging position, thereby providing an optical semiconductor element that can convert guided light having a guided mode shape with a large spot size in the horizontal direction of the semiconductor substrate and small circularity into a guided mode shape with high circularity that can be easily coupled to an optical fiber.

[0040] <Embodiment 2> In optical semiconductor device 550 according to the second embodiment, in addition to the device structure described in the first embodiment, a gain waveguide section 100 is provided so as to be connected to a first waveguide 10, as shown in Fig. 6. Fig. 6 is a top view of optical semiconductor device 550, similar to that shown in Figs. 1A and 1B.

[0041] Optical semiconductor element 550 according to the second embodiment includes, from the incident direction of signal light, gain waveguide entrance portion 16, gain tapered waveguide 15, first waveguide 10, transparent tapered waveguide 12, and second waveguide 11. Gain waveguide entrance portion 16 and gain tapered waveguide 15 configure gain waveguide portion 100. Optical semiconductor element 550 has low-reflection coating film 17 provided on its output end face and low-reflection coating film 18 provided on its input end face.

[0042] 1A, that is, the optical semiconductor device 500 according to the first embodiment. In the gain waveguide section 100, a gain waveguide entrance section 16 and a gain tapered waveguide 15 are connected in this order from the incident direction of the signal light. The gain tapered waveguide 15 is connected to the first waveguide 10 at the end side opposite to the gain waveguide entrance section 16.

[0043] When signal light enters the optical semiconductor element 550 from the gain waveguide entrance portion 16 side and exits from the second waveguide 11 side, the optical intensity of the signal light is amplified in the gain waveguide portion 100. By providing the low-reflection coating film 17 on the exit end face of the optical semiconductor element 550 and the low-reflection coating film 18 on the entrance end face, it is possible to prevent behavior such as wavelength ripple in the gain spectrum caused by interference of reflected return light from both end faces, and therefore the optical semiconductor element 550 can operate as an optical amplifying element with reduced wavelength dependency.

[0044] <Method of Manufacturing Optical Semiconductor Device According to Second Preferred Embodiment> 7 is a cross-sectional view of an optical semiconductor device 550 according to the second embodiment taken along the line CC in FIG. 6. A gain waveguide core layer 25 is laminated on a semiconductor substrate 20 and a lower cladding layer 21 which are common to the device structure of the first embodiment shown in FIG. 2. In order to form a gain waveguide mesa portion 25a, the entire gain waveguide core layer 25 and a part of the lower cladding layer 21 are removed by etching, and the lateral cladding layer 23 is recrystallized. After the recrystallization, an upper cladding layer 24 is laminated to form the cross-sectional structure shown in FIG. 7. The etching and the recrystallization of the lateral cladding layer 23 can be performed in the same manner as in the case of forming the device structure of the first embodiment shown in FIG. 2.

[0045] <Structural Features of the Optical Semiconductor Device According to the Second Embodiment> A material is selected so that the refractive index of the gain waveguide core layer 25 is higher than the refractive indexes of the lower cladding layer 21, the lateral cladding layer 23, and the upper cladding layer 24. The gain waveguide core layer 25 has an optical gain when the device is operated. As an example of the material, the semiconductor substrate 20, the lower cladding layer 21, the lateral cladding layer 23, and the upper cladding layer 24 are each made of InP, and the gain waveguide core layer 25 is made of a multiple quantum well layer of InGaAsP or InAlGaAs, which has a composition that has an optical gain for light of a wavelength used when a forward current is injected. Each of these layers can be epitaxially grown on the semiconductor substrate using a known crystal growth technique.

[0046] 6. Cross sections other than those along the CC line in FIG. 6 also exhibit the same cross-sectional structure, except that the waveguide width of the gain waveguide core layer 25 is different.

[0047] For current injection, for example, n-type doped n-InP is used for the semiconductor substrate 20 and lower cladding layer 21, p-type doped p-InP is used for the upper cladding layer 24, while the gain waveguide core layer 25 is intentionally set to be undoped. The lateral cladding layer 23 uses known semi-insulating Fe-doped InP used for current confinement, or known InP with a pn junction that is made highly resistive by creating a reverse bias state at the interface between the p-InP and n-InP.

[0048] As electrodes for applying a forward voltage, a lower electrode 30 is provided on the back surface of the semiconductor substrate 20, and a contact layer 31 and an upper electrode 32 are provided on the upper cladding layer 24. An example of the thickness of each layer is 0.5 μm for the lower cladding layer 21, 0.2 μm for the gain waveguide core layer 25, and 2.0 μm for the upper cladding layer 24.

[0049] In addition, in order to avoid an increase in propagation loss due to a mismatch in the waveguide mode shape at the interface with the first waveguide 10, it is preferable to adjust the specifications of the gain waveguide core layer 25 and the transparent waveguide core layer 22 so that the mode size in the thickness direction of the semiconductor substrate 20 does not diverge between the transparent waveguide and the gain waveguide at the interface with the first waveguide 10.

[0050] The optical semiconductor device 550 according to the second embodiment has the above-mentioned device structure, so that the current injected from the upper electrode 32 efficiently reaches the gain waveguide core layer 25, and an optical gain for light of the wavelength used can be generated.

[0051] As described in the first embodiment, the transparent tapered waveguide 12 is adjusted so as to couple the fundamental mode light at the connection portion on the first waveguide 10 side to the second waveguide 11 with low loss. Therefore, in order to couple the fundamental mode signal light incident on the gain waveguide section 100 to the second waveguide 11 with as low a loss as possible, it is necessary that the optical intensity of the fundamental mode light among the signal lights amplified at the connection portion between the gain tapered waveguide 15 and the first waveguide 10 is dominant compared to the optical intensity of the higher mode light that can be excited in the gain tapered waveguide 15.

[0052] In other words, it is necessary that the signal light is guided adiabatically in the optical sense in the gain tapered waveguide 15 and the spot size is enlarged without exciting any higher-order modes. For this purpose, the shape of the gain tapered waveguide 15 needs to be elongated with a taper angle equal to or smaller than the upper limit for adiabatically guiding the light. For example, one example that satisfies the above-mentioned conditions is that the waveguide width W1 on the first waveguide 10 side is 10 μm and the waveguide length L3 of the gain tapered waveguide 15 is 400 μm or more.

[0053] The upper limit of the taper angle for adiabatically guiding the light varies depending on the structure of the gain tapered waveguide 15, i.e., the thickness of the gain waveguide core layer 25 and the difference in refractive index with the cladding layer. The thinner the gain waveguide core layer 25, the smaller the change in effective refractive index of the guided mode relative to the change in the waveguide width, making it more difficult for the fundamental mode to be converted to a higher-order mode. In other words, for the same waveguide width W1, it becomes possible to use a gain tapered waveguide 15 having a shorter waveguide length and a larger taper angle.

[0054] Furthermore, as the composition of the gain waveguide core layer 25 is made shorter in wavelength, the refractive index difference with the cladding layer becomes smaller, and the change in the effective refractive index of the waveguide mode relative to the change in the waveguide width becomes smaller. As a result, it becomes possible to use a gain tapered waveguide 15 having a tapered shape with a shorter waveguide length and a larger taper angle, as in the above case.

[0055] In addition, in the optical semiconductor device 550 according to the second embodiment, the waveguide width of the gain tapered waveguide 15 can be made wider than that of a semiconductor optical amplifier having a known constant-width waveguide. This reduces the photon density in the waveguide during optical amplification, thereby suppressing the gain saturation phenomenon, thereby enabling an increase in the total output power of the gain tapered waveguide 15.

[0056] <First Modification of the Second Embodiment> 8A and 8B are top views of an optical semiconductor element 560 and an optical semiconductor element 560a according to the first modification of the second embodiment. The optical semiconductor element 560 shown in FIG. 8A includes, from the incident direction of the signal light, an oblique gain waveguide 43, a curved gain waveguide 42, a gain waveguide entrance portion 16, a gain tapered waveguide 15, a first waveguide 10, a transparent tapered waveguide 12, a second waveguide 11, a curved waveguide 40, an oblique waveguide 41, and a lateral cladding region 13. A low-reflection coating film 17 is provided on the output side end face of the optical semiconductor element 560, and a low-reflection coating film 18 is provided on the incident side end face. The oblique gain waveguide 43, the curved gain waveguide 42, the gain waveguide entrance portion 16, and the gain tapered waveguide 15 constitute a gain waveguide portion with oblique incidence 101.

[0057] The device structure of the optical semiconductor device 560 shown in Fig. 8A has a configuration in which the signal light emitted from the second waveguide 11 is guided to the emission side end face provided with the low-reflection coating film 17 via the curved waveguide 40 and the oblique waveguide 41 in the device structure of the optical semiconductor device 550 according to the second embodiment shown in Fig. 6. The oblique waveguide 41 has an inclination angle θ 1 The angle θ is inclined with respect to the light-emitting end surface. 1 An example of the inclination angle θ is 7 degrees. 1 By setting the angle to 7 degrees or more, the return light reflectance from the output end face can be made even lower than when the waveguide is perpendicular, so that the element can function as an optical amplifier with reduced wavelength dependency.

[0058] Similarly, a curved gain waveguide 42 and an oblique gain waveguide 43 are provided for the gain waveguide entrance portion 16 to form a gain waveguide portion 101 with oblique entrance, and the wave is guided to the entrance end face provided with a low-reflection coating film 18. The oblique gain waveguide 43 has an inclination angle θ 2 The angle is inclined by θ 2 An example of the inclination angle θ is 7 degrees. 2 By making the angle 7 degrees or more, the return light reflectance from the incident end face can be made even lower than when the waveguide is perpendicular, so that the element can function as an optical amplifier with reduced wavelength dependency.

[0059] In the element structure of optical semiconductor element 560a shown in Fig. 8B, a spot-size conversion waveguide 44 having a tapered shape is provided instead of the diagonal waveguide 41 in Fig. 8A. Also, a spot-size conversion gain waveguide 45 having a tapered shape is provided instead of the diagonal gain waveguide 43 in Fig. 8A, thereby forming a gain waveguide with spot-size conversion 102. By using such an element structure, it is possible to reduce the coupling loss with the optical fiber when inputting and outputting signal light using the optical fiber, thereby realizing an optical amplifying element with a larger gain.

[0060] <Modification 2 of the second embodiment> 9A and 9B are top views of an optical semiconductor element 570 and an optical semiconductor element 570a according to Modification 2 of Embodiment 2. Compared to optical semiconductor element 560 shown in Fig. 8A, optical semiconductor element 570 shown in Fig. 9A has an element structure in which curved gain waveguide 42 and angled gain waveguide 43 are replaced with curved waveguide 40 and angled waveguide 41, which are transparent waveguides.

[0061] Compared to the optical semiconductor device 570 shown in Fig. 9A, the optical semiconductor device 570a shown in Fig. 9B has an element structure in which the diagonal waveguide 41 is replaced with a spot-size conversion waveguide 44. Even with this element structure, as in the case of the element structure shown in Fig. 8A, it is possible to reduce the coupling loss with the optical fiber when inputting and outputting signal light using the optical fiber, thereby realizing an optical amplifier with a large gain.

[0062] <Modification 3 of the Second Embodiment> Fig. 10A is an enlarged view showing the vicinity of first waveguide 10 in the element structure of optical semiconductor element 550 according to embodiment 2 shown in Fig. 6. Figs. 10B, 10C, and 10D are top views showing optical semiconductor element 580a, optical semiconductor element 580b, and optical semiconductor element 580c, respectively, which are modification 3 of embodiment 2.

[0063] 10B to 10D, the waveguide core layer in a partial region of the first waveguide 10 that contacts the gain taper type waveguide 15 is made into a gain waveguide core layer 25. In any case, the element structure is such that the interface between the gain waveguide core layer 25 and the transparent waveguide core layer 22 is inclined with respect to the light propagation direction of the waveguide.

[0064] By using each of the element structures shown in FIGS. 10B to 10D, back reflection occurring at the interface between the gain waveguide core layer 25 and the transparent waveguide core layer 22 is reduced compared to the element structure shown in FIG. 10A, so that it is possible to configure an ideal optical amplifier.

[0065] <Fourth Modification of the Second Preferred Embodiment> FIG. 11 is a top view of an optical semiconductor device 590 according to the fourth modification of the second embodiment, in which, in the device structure shown in FIG. 9A , a part of the side of the gain waveguide entrance portion 16 not in contact with the gain taper waveguide 15 is made into a transparent waveguide core layer 22, and the interface between the gain waveguide core layer 25 and the transparent waveguide core layer 22 is inclined with respect to the light propagation direction of the waveguide.

[0066] By using the element structure shown in Fig. 11, back reflection occurring at the interface between the gain waveguide core layer 25 and the transparent waveguide core layer 22 is reduced compared to the element structure shown in Fig. 9A, making it possible to configure an ideal optical amplifier. Since an oblique interface can be provided at the gain waveguide entrance part 16 not only in the element structure shown in Fig. 9A but also in the element structure shown in Fig. 9B, it becomes possible to configure an ideal optical amplifier by similarly reducing back reflection.

[0067] <Advantages of the second embodiment> As described above, the optical semiconductor element according to the second embodiment has an advantage that, by appropriately connecting a gain waveguide to the element structure of the optical semiconductor element according to the first embodiment, it is possible to convert the amplified signal light into a waveguide mode shape with high circularity that is small, has low loss, and is easy to couple to an optical fiber.

[0068] <Embodiment 3> 12 is a top view of an optical semiconductor device 600 according to the third embodiment. The optical semiconductor device 600 according to the third embodiment includes, from the incident direction of the signal light, a laser light source section 50, a laser light modulator connection section 51, an optical modulator section 52, an optical modulator gain waveguide connection section 53, a gain waveguide entrance section 16, a gain tapered waveguide 15, a first waveguide 10, a transparent tapered waveguide 12, a transparent tapered waveguide for connection 14, a second waveguide 11, a curved waveguide 40, a spot size conversion waveguide 44, and a lateral cladding region 13. The laser light source section 50, the laser light modulator connection section 51, the optical modulator section 52, and the optical modulator gain waveguide connection section 53 constitute a modulated signal light generating section 200. A high-reflection coating film 19 is provided on the output end face of the optical semiconductor element 600, and a low-reflection coating film 18 is provided on the input end face.

[0069] The optical semiconductor device 600 according to the third embodiment is characterized in that, compared to the optical semiconductor device 550 according to the second embodiment, a modulated signal light generating section 200 is provided so as to be connected to a gain waveguide entrance section 16, as shown in Fig. 12. In the optical semiconductor device 600 shown in Fig. 12, as the modulated signal light generating section 200, a laser light source section 50 which is a distributed feedback semiconductor laser (DFB laser) and an optical modulator section 52 which is an electroabsorption optical modulator are connected via a laser optical modulator connection section 51 which is a transparent waveguide, and the optical modulator section 52 is connected to the gain waveguide entrance section 16 via an optical modulator gain waveguide connection section 53 which is a transparent waveguide.

[0070] The modulated signal light generating section 200 may have another configuration, for example, a modulated signal light generating section such as a direct modulation type semiconductor laser, a semiconductor laser with a Mach-Zehnder optical modulator, or a semiconductor laser with a microring optical modulator.

[0071] Fig. 13 is a cross-sectional view in a direction parallel to the substrate thickness direction in a cross section (non-planar) along the center of the straight and curved waveguides with respect to the device structure of the optical semiconductor device 600 according to the third embodiment shown in Fig. 12. The cross section from the laser light source unit 50 to the curved waveguide 40 is a cross section in a direction perpendicular to the substrate end face, and the cross section from the curved waveguide 40 to the spot size conversion waveguide 44 is a cross section in a direction inclined with respect to the substrate end face.

[0072] In the optical semiconductor element 600 according to the third embodiment, as layers through which signal light is guided, a laser active layer 60, a transparent waveguide core layer 61, an optical modulator active layer 62, a transparent waveguide core layer 63, a gain waveguide core layer 25, and a transparent waveguide core layer 22 are provided in the direction in which the signal light is incident.

[0073] The semiconductor substrate 20, the lower cladding layer 21, and the upper cladding layer 24 are the same as the optical semiconductor device 550 according to the second embodiment. A lower electrode 30 is provided on the back surface of the semiconductor substrate 20. Furthermore, a contact layer 35 and an upper electrode 36 for injecting current into the laser light source section 50 are provided. A laser active layer 60 that is not intentionally doped with a dopant is laminated on the lower cladding layer 21, and a diffraction grating 70 for outputting light of a wavelength used as a distributed feedback semiconductor laser is provided above and adjacent to the laser active layer 60. The diffraction grating 70 may have a structure in which a known phase shift portion with a different diffraction grating interval is provided inside, which is known to have an effect of stabilizing the single mode oscillation of a semiconductor laser. A high reflection coating film 19 is provided on the incident end surface on the laser light source section 50 side, so that the output light to the gain tapered waveguide 15 side is increased.

[0074] The transparent waveguide core layer 61 and the transparent waveguide core layer 63 are formed together with the transparent waveguide core layer 22, with the same composition and thickness. The optical modulator active layer 62 is composed of a multiple quantum well layer adjusted so that the oscillation wavelength of the laser light source section 50 is in the wavelength region near the absorption edge, and the quantum confined Stark effect occurs when a reverse bias is applied, increasing the optical absorption coefficient in the wavelength region near the absorption edge. A contact layer 33 and an upper electrode 34 are provided to apply a reverse bias to the optical modulator active layer 62.

[0075] The laser optical modulator connection part 51 and the optical modulator gain waveguide connection part 53 are provided with a necessary waveguide length so that the electrical isolation between the upper electrode 36, the upper electrode 34, and the upper electrode 32 does not cause a low resistance. As an example of such a waveguide length, for example, the laser optical modulator connection part 51 and the optical modulator gain waveguide connection part 53 each have a waveguide length of 50 μm.

[0076] In the optical semiconductor device 600 according to the third embodiment, an independent constant forward voltage is applied between the upper electrode 36 and the upper electrode 32 to drive the laser light source section 50 and the gain waveguide section 100. An appropriate reverse bias voltage and a modulated electric signal in which a high-frequency voltage signal encoded for optical communication is superimposed are applied to the upper electrode 34. The optical absorption coefficient of the optical modulator section 52 changes in response to the applied modulated electric signal, generating modulated signal light.

[0077] The modulated signal light enters the gain waveguide section 100, where the optical intensity is amplified in a state where the fundamental mode is dominant, and reaches the first waveguide 10. Thereafter, while the modulated signal light propagates through the transparent tapered waveguide 12, it is converted into a guided mode shape with high circularity that is small, has low loss, and is easy to couple to an optical fiber, reaches the second waveguide 11, and is finally emitted from the spot size conversion waveguide 44.

[0078] <Advantages of the Third Embodiment> As described above, according to the optical semiconductor element of the third embodiment, a modulated signal light generating unit is further provided, which has the effect of increasing the output power of the modulated optical signal by the optical amplifier and enabling the output light to be shaped into a good beam.

[0079] Although the present disclosure describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations.

[0080] Therefore, countless modifications not exemplified are assumed within the scope of the technology disclosed in the present specification, including, for example, modifying, adding, or omitting at least one component, and further, extracting at least one component and combining it with a component of another embodiment. [Explanation of symbols]

[0081] 10 first waveguide, 11 second waveguide, 12 transparent tapered waveguide, 13 lateral cladding region, 14 connecting transparent tapered waveguide, 15, 82 gain tapered waveguide, 16 gain waveguide entrance portion, 17, 18 low reflection coating film, 19 high reflection coating film, 20 semiconductor substrate, 21 lower cladding layer, 22 transparent waveguide core layer, 22a transparent waveguide mesa portion, 23 lateral cladding layer, 24 upper cladding layer, 25 gain waveguide core layer, 25a gain waveguide mesa portion, 30, 32 lower electrode, 31, 33, 35 contact layer, 32, 34, 36 upper electrode, 40 bent waveguide, 41 oblique waveguide, 42 bent gain waveguide, 43 oblique gain waveguide, 44 spot size conversion waveguide, 45 Spot size conversion gain waveguide, 50 laser light source section, 51 laser optical modulator connection section, 52 optical modulator section, 53 optical modulator gain waveguide connection section, 60 laser active layer, 61, 63 transparent waveguide core layer, 62 optical modulator active layer, 70 diffraction grating, 80 first gain waveguide, 81 second gain waveguide, 100 gain waveguide section, 101 gain waveguide section with oblique incidence, 102 gain waveguide with spot size conversion, 200 modulated signal light generation section, 500, 500a, 510a, 510b, 550, 560, 560a, 570, 570a, 580a, 580b, 580c, 590, 600 optical semiconductor element

Claims

1. a first waveguide formed on a semiconductor substrate and allowing propagation of light in a higher order mode; a tapered waveguide formed on the semiconductor substrate, one end of which is connected to the other end of the first waveguide and the width of the waveguide decreases toward the other end; a second waveguide formed on the semiconductor substrate, one end of which is connected to the tapered waveguide, the second waveguide having a waveguide width narrower than a waveguide width of the first waveguide, and being a transparent waveguide that is transparent to the wavelength of propagating light; a gain taper type waveguide formed on the semiconductor substrate, the gain taper type waveguide having a gain waveguide core layer with optical gain and a shape in which the waveguide width increases along a light propagation direction; Equipped with the tapered waveguide is a transparent tapered waveguide that is transparent to the wavelength of the propagating light, and has a tapered shape in which a high-order mode light is excited in the tapered waveguide by the propagation of fundamental mode light incident from the other end of the first waveguide; the gain tapered waveguide, the first waveguide, and the transparent tapered waveguide are arranged in order along a propagation direction of light; an imaging position exists where the intensity of propagating light is imaged at one point due to interference between the fundamental mode light and the higher-order mode light in the tapered waveguide, and the tapered waveguide and the second waveguide are connected at the imaging position.

2. A first waveguide formed on a semiconductor substrate and allowing propagation of light in a higher order mode; a tapered waveguide formed on the semiconductor substrate, one end of which is connected to the other end of the first waveguide and the width of the waveguide decreases toward the other end; a second waveguide formed on the semiconductor substrate, one end of which is connected to the tapered waveguide and has a waveguide width narrower than a waveguide width of the first waveguide; a gain tapered waveguide formed on the semiconductor substrate, connected to one end of the first waveguide, having a gain waveguide core layer with optical gain, and presenting a tapered shape in which a waveguide width increases toward the first waveguide; an optical semiconductor element, characterized in that the tapered waveguide has a tapered shape in which fundamental mode light incident from the other end of the first waveguide propagates through the tapered waveguide to excite higher-order mode light, an imaging position is present where an intensity of propagating light is imaged at one point due to interference between the fundamental mode light and the higher-order mode light in the tapered waveguide, and the tapered waveguide and the second waveguide are connected at the imaging position.

3. 3. The optical semiconductor element according to claim 1, wherein the imaging position is a position where an image is first formed when light incident from the first waveguide propagates.

4. 3. The optical semiconductor element according to claim 1, wherein at a connection portion where the other end of the tapered waveguide and one end of the second waveguide are connected, a waveguide width at the other end of the tapered waveguide is larger than a waveguide width at one end of the second waveguide.

5. 3. The optical semiconductor element according to claim 1, wherein L / W, which is a ratio of the waveguide length L to the waveguide width W, is 10 or less, where L is a waveguide length in a light propagation direction of the tapered waveguide and W is a waveguide width of one end of the tapered waveguide at a connection portion between the other end of the first waveguide and the one end of the tapered waveguide.

6. 3. The optical semiconductor element according to claim 2, wherein the tapered waveguide is a transparent tapered waveguide that is transparent to the wavelength of the propagating light, and the second waveguide is a transparent waveguide that is transparent to the wavelength of the propagating light.

7. 3. The optical semiconductor element according to claim 1, wherein the gain taper waveguide has a tapered shape that suppresses generation of higher-order modes when light incident from a side of the gain taper waveguide where the waveguide width is smaller propagates within the gain taper waveguide.

8. A modulated signal light generating unit is formed on the semiconductor substrate and modulates the propagating light.

7. The optical semiconductor device according to claim 2, wherein an input side of the gain tapered waveguide is connected to an output side of the modulated signal light generating portion.

9. a laser light source unit that generates a laser light in the modulated signal light generating unit, and an optical modulator unit that optically modulates the laser light generated by the laser light source unit and outputs a modulated laser light; 9. The optical semiconductor device according to claim 8, wherein the modulated signal light generating section includes the laser light source section, the optical modulator section, and the gain tapered waveguide arranged in this order along a light propagation direction.

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