Optical integrated laser device

Abstract

To provide an optical integrated laser element capable of reducing the occurrence of a mode hop while increasing the intensity of an output laser beam.SOLUTION: The optical integrated laser element includes a distributed feedback laser part, a semiconductor optical amplification part, and a high reflection film. The distributed feedback laser portion includes a diffraction grating and a first optical waveguide. The first optical waveguide is provided along the diffraction grating and has a gain. The semiconductor optical amplifier unit is provided on the substrate, is provided on a first side of the distributed-feedback laser unit, is optically connected to the first optical waveguide, has a gain, and includes a second optical waveguide having a width wider than a width of the first optical waveguide. The high reflection film is provided on an end surface on a second side opposite to the first side of the distributed feedback laser portion. The distributed feedback laser portion and the semiconductor optical amplification portion are integrated on the substrate. The length of the first optical waveguide in the optical waveguide direction is 1000 μm or less, and the length of the second optical waveguide in the optical waveguide direction is 1.5 times or more and 7 times or less the length of the first optical waveguide in the optical waveguide direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to integrated optical laser devices. [Background technology]

[0002] BACKGROUND ART Optical integrated laser elements are known in which a distributed feedback (DFB) laser section and a semiconductor optical amplifier section are integrated. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent Application Publication No. 2012 / 243074 Summary of the Invention [Problem to be solved by the invention]

[0004] In an optical integrated laser element in which a distributed feedback laser section and a semiconductor optical amplifier section are integrated, a highly reflective film is provided on one of the two end faces of the distributed feedback laser section, the end face opposite the semiconductor optical amplifier section. This increases the intensity of the laser light output from the distributed feedback laser section, and as a result, the intensity of the light output from the optical integrated laser element. However, when a highly reflective film is provided, mode hopping is more likely to occur in the distributed feedback laser section. An object of the present disclosure is to provide an optical integrated laser element that can reduce the occurrence of mode hopping while increasing the intensity of the output laser light. [Means for solving the problem]

[0005] An optical integrated laser device according to an embodiment of the present disclosure includes a substrate, a distributed feedback laser section, a semiconductor optical amplifier section, and a high-reflection film. The distributed feedback laser section has a diffraction grating and a first optical waveguide. The diffraction grating is provided on the substrate. The first optical waveguide is provided along the diffraction grating and has a gain. The semiconductor optical amplifier section is provided on the substrate, is provided on a first side of the distributed feedback laser section, is optically connected to the first optical waveguide, has a gain, and has a second optical waveguide having a width wider than that of the first optical waveguide. The high-reflection film is provided on an end facet on a second side opposite to the first side of the distributed feedback laser section. The distributed feedback laser section and the semiconductor optical amplifier section are monolithically integrated on the substrate. The length of the second optical waveguide in the optical waveguiding direction is 1.5 to 7 times the length of the first optical waveguide in the optical waveguiding direction. [Effects of the Invention]

[0006] According to the present disclosure, it is possible to provide an optical integrated laser element that can increase the intensity of output laser light while reducing the occurrence of mode hopping. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a plan view showing an integrated optical laser device according to a first embodiment of the present disclosure. [Figure 2] Fig. 2(a) is a cross-sectional view taken along line II-II in Fig. 1. Fig. 2(b) is a cross-sectional view taken along line III-III in Fig. 1. [Figure 3] Fig. 3(a) is a cross-sectional view taken along line VI-VI in Fig. 1. Fig. 3(b) is a cross-sectional view taken along line VV in Fig. 1. [Figure 4] 4(a) and 4(b) are cross-sectional views showing steps in a method for manufacturing an integrated optical laser device. [Figure 5] 5(a) and 5(b) are cross-sectional views showing steps in a method for manufacturing an integrated optical laser device. [Figure 6] 6(a) and 6(b) are cross-sectional views showing steps in a method for manufacturing an integrated optical laser device. [Figure 7]7(a) and 7(b) are cross-sectional views showing steps in a method for manufacturing an integrated optical laser device. [Figure 8] 8(a) and 8(b) are cross-sectional views showing steps in a method for manufacturing an integrated optical laser device. [Figure 9] FIG. 9 is a schematic plan view of an integrated optical laser device. [Figure 10] FIG. 10 is a graph showing the relationship between the wavelength of the laser light and the normalized threshold gain and light intensity. [Figure 11] FIG. 11 is a diagram showing how the graph shown in FIG. 10 fluctuates. [Figure 12] FIG. 12 is a diagram showing how the graph shown in FIG. 10 fluctuates. [Figure 13] FIG. 13 is a plan view showing an optical integrated laser device according to a modification of the above embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Description of the embodiments of the present disclosure] First, the details of the embodiments of the present disclosure will be listed and described. [1] An optical integrated laser device according to one embodiment of the present disclosure includes an InP substrate, a distributed feedback laser section, a semiconductor optical amplifier section, and a high-reflection film. The distributed feedback laser section has a diffraction grating and a first optical waveguide. The diffraction grating is provided on the InP substrate. The first optical waveguide is provided along the diffraction grating, has gain, and includes a single-mode active layer. The semiconductor optical amplifier section is provided on the InP substrate, is provided on a first side of the distributed feedback laser section, is optically connected to the first optical waveguide, has gain, and has a second optical waveguide having a width wider than that of the first optical waveguide. The high-reflection film is provided on an end facet on a second side opposite the first side of the distributed feedback laser section. The reflectivity of the high-reflection film is 70% or more. The distributed feedback laser section and the semiconductor optical amplifier section are integrated on the InP substrate. The length of the first optical waveguide in the optical waveguide direction is 1000 μm or less, and the length of the second optical waveguide in the optical waveguide direction is 1.5 to 7 times the length of the first optical waveguide in the optical waveguide direction.

[0009] In the optical integrated laser device described above in [1], a high-reflection film is provided on the end face of the second side, opposite to the first side, of the distributed feedback laser section. This allows for sufficient enhancement of the intensity of the laser light output from the optical integrated laser device. In addition, the length of the second optical waveguide in the optical waveguide direction is 1.5 times or more the length of the first optical waveguide in the optical waveguide direction. Mode hopping in the distributed feedback laser section is more likely to occur as the emission intensity of the distributed feedback laser section increases. Therefore, in order to reduce the occurrence of mode hopping, it is effective to shorten the length of the distributed feedback laser section in the optical waveguide direction to reduce the emission intensity of the distributed feedback laser section. Furthermore, in order to sufficiently enhance the intensity of the laser light output from the optical integrated laser device, it is effective to increase the length of the semiconductor optical amplifier in the optical waveguide direction to increase the gain of the semiconductor optical amplifier. By making the length of the second optical waveguide in the optical waveguide direction 1.5 times or more the length of the first optical waveguide in the optical waveguide direction, it is possible to sufficiently enhance the intensity of the output laser light while reducing the occurrence of mode hopping. Furthermore, by making the length of the second optical waveguide in the optical waveguide direction 7 times or less the length of the first optical waveguide in the optical waveguide direction, the first optical waveguide does not become too short, and the side mode suppression ratio (SMSR) can be maintained at an appropriate value.

[0010] [2] In the optical integrated laser device of [1], the length of the second optical waveguide in the optical waveguide direction may be 2.0 times or more the length of the first optical waveguide in the optical waveguide direction, which can further increase the intensity of the output laser light and further reduce the occurrence of mode hopping.

[0011] [3] In the optical integrated laser device of [1] or [2] above, the width of the second optical waveguide may be 1.0 to 6.5 times the width of the first optical waveguide. In this case, the gain of the semiconductor optical amplifier is increased. Therefore, the intensity of the output laser light can be further increased.

[0012] [4] In the optical integrated laser device of [1] or [2] above, the optical guiding direction of the second optical waveguide may be inclined with respect to the optical guiding direction of the first optical waveguide. In this case, the intensity of the laser light can be further increased by lengthening the second optical waveguide while shortening the overall length of the optical integrated laser device.

[0013] [5] In the integrated optical laser device of [4] above, the planar shape of the InP substrate may be a rectangle having a second long side and a first long side extending parallel to each other, and the optical guiding direction of the first optical waveguide may extend parallel to the first long side and the second long side. The second optical waveguide may be inclined so as to approach the first long side from the connection point with the first optical waveguide. In this case, the second optical waveguide can be lengthened to further increase the intensity of the laser light while avoiding an increase in the width of the integrated optical laser device. [Details of the embodiments of the present disclosure]

[0014] Specific examples of the present disclosure will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the following description, the same elements in the description of the drawings will be given the same reference numerals, and duplicate explanations will be omitted. [First embodiment]

[0015] 1 is a plan view showing an integrated optical laser device 10 according to a first embodiment of the present disclosure. The integrated optical laser device 10 of this embodiment includes a substrate 12, a distributed feedback laser section 20, a semiconductor optical amplifier section 30, a high-reflection film 41, and an anti-reflection film 42. The distributed feedback laser section 20 and the semiconductor optical amplifier section 30 are monolithically integrated on the substrate 12. In the integrated optical laser device 10, the distributed feedback laser section 20 functions as a DFB laser, and the semiconductor optical amplifier section 30 functions as an SOA.

[0016] The substrate 12 is, for example, a semiconductor substrate. In one example, the substrate 12 is an n-type indium phosphide (InP) substrate. The planar shape of the substrate 12 is rectangular. The substrate 12 has a first long side 121 and a second long side 122 that extend parallel to each other, and a first short side 123 and a second short side 124 that extend parallel to each other and are perpendicular to the first long side 121 and the second long side 122. In one example, the length of the substrate 12 in the direction along the first long side 121 and the second long side 122 is 2000 μm or more and 4000 μm or less. One example is 2500 μm. The width of the substrate 12 in the direction along the first short side 123 and the second short side 124 is, for example, 400 μm.

[0017] The distributed feedback laser section 20 has a first optical waveguide 21 provided on the substrate 12. The first optical waveguide 21 has a linear optical axis 22. The optical guiding direction of the first optical waveguide 21 along the optical axis 22 extends parallel to the first long side 121 and the second long side 122. The first optical waveguide 21 is closer to the second long side 122 than to the first long side 121. The length La of the first optical waveguide 21 in the optical guiding direction (the direction along the first long side 121 and the second long side 122) is preferably 200 μm or more to ensure stability of laser oscillation. In preferred embodiments, the length La may be 200 μm or more, 400 μm or more, or 500 μm or more. The upper limit is 1000 μm or less, and may be 800 μm or less or 650 μm or less as necessary.

[0018] The semiconductor optical amplifier 30 is provided on the first side of the distributed feedback laser section 20 in the direction along the first long side 121 and the second long side 122. The semiconductor optical amplifier 30 has a second optical waveguide 31 provided on the substrate 12. The second optical waveguide 31 has a linear optical axis 32. The second optical waveguide 31 is optically connected to the first optical waveguide 21 and has gain. The second optical waveguide 31 has a tapered portion 311 and a parallel portion 312. The tapered portion 311 is provided between the first optical waveguide 21 and the parallel portion 312 and connects the parallel portion 312 to the first optical waveguide 21. The parallel portion 312 has a constant width Wb along the optical waveguiding direction. The width Wb is wider than the width Wa of the first optical waveguide 21. The width of the tapered portion 311 changes continuously from a width Wa at the first end connected to the first optical waveguide 21 to a width Wb at the second end connected to the parallel portion 312 .

[0019] The optical waveguide direction of the second optical waveguide 31 along the optical axis 32 is inclined with respect to the optical waveguide direction of the first optical waveguide 21. In this embodiment, the second optical waveguide 31 is inclined from the connection point with the first optical waveguide 21 toward the first long side 121. As described above, when the length of the long side of the substrate 12 is 4000 μm or less, the length Lb of the second optical waveguide 31 in the optical waveguide direction is 1.5 times or more, 2.0 times or more, or 2.5 times or more the length La of the first optical waveguide 21 in the optical waveguide direction. The length Lb of the second optical waveguide 31 in the optical waveguide direction is 3.0 times or less, 3.5 times or less, or 7 times or less the length La of the first optical waveguide 21 in the optical waveguide direction. In one example, when the length of the substrate 12 is 2500 μm, the length Lb is 1500 μm or more when the length La is 1000 μm or less. In another example, when the length La is 650 μm or less, the length Lb is 1350 μm or more.

[0020] The width Wb of the second optical waveguide 31 is 1.0 to 6.5 times the width Wa of the first optical waveguide 21. For example, the width Wa of the first optical waveguide 21 is the width that constitutes a single-mode active layer. For example, Wa is 1.5 μm to 3.0 μm. The width Wb of the second optical waveguide 31 is 1.5 μm to 10.0 μm. For example, when the length of the substrate 12 is 2000 μm, the widths of the first optical waveguide 21 and the second optical waveguide 31 are 2.0 μm and 5.0 μm, respectively, and when the length of the substrate 12 is 2500 μm, the widths of the first optical waveguide 21 and the second optical waveguide 31 are 2.0 μm and 5.5 μm, respectively.

[0021] The high-reflection film 41 is provided on the end facet of the second side, which is opposite to the first side (semiconductor optical amplifier 30 side) of the distributed feedback laser section 20. The reflectance of the high-reflection film 41 is preferably 70% or more, and more preferably 90% or more. The material of the high-reflection film 41 has a structure in which, for example, aluminum oxide (Al2O3), tantalum dioxide (Ta2O3) layers, and titanium oxynitride (TiON) layers as high-refractive-index films are alternately stacked with silicon oxide (SiO2) or titanium oxide (TiO2) films as low-refractive-index films, and these are configured with an optically designed thickness.

[0022] The antireflection coating 42 is provided on the end face of the semiconductor optical amplifier 30 opposite to the end face on the distributed feedback laser section 20 side. The antireflection coating 42 includes a structure in which, for example, aluminum oxide (Al2O3) or tantalum dioxide (Ta2O3) layers as high refractive index films and silicon oxide (SiO2) or titanium oxide (TiO2) films as low refractive index films are alternately stacked.

[0023] Fig. 2(a) is a cross-sectional view taken along line II-II in Fig. 1, showing a cross-section of the distributed feedback laser section 20 along the optical waveguiding direction. Fig. 2(b) is a cross-sectional view taken along line III-III in Fig. 1, showing a cross-section of the distributed feedback laser section 20 perpendicular to the optical waveguiding direction. Fig. 3(a) is a cross-sectional view taken along line VI-VI in Fig. 1, showing a cross-section of the semiconductor optical amplifier section 30 along the optical waveguiding direction. Fig. 3(b) is a cross-sectional view taken along line VV in Fig. 1, showing a cross-section of the semiconductor optical amplifier section 30 perpendicular to the optical waveguiding direction.

[0024] As shown in these figures, an integrated optical laser device 10 has a semiconductor layer 13 provided on a primary surface 12a of a substrate 12. The semiconductor layer 13 has a buffer layer 14, a diffraction grating layer 15, an n-type cladding layer 16, an active layer 17, and a p-type cladding layer 18.

[0025] The buffer layer 14 is provided on the substrate 12. The buffer layer 14 is, for example, an n-type InP layer having a thickness of approximately 500 nm. The grating layer 15 is provided on the buffer layer 14. The grating layer 15 is, for example, an n-type gallium indium arsenide phosphide (GaInAsP) layer having a thickness of approximately 50 nm. As shown in FIG. 2(a), in the distributed feedback laser section 20, a diffraction grating with a constant period is formed in the grating layer 15. On the other hand, as shown in FIG. 3(a), in the semiconductor optical amplifier section 30, no diffraction grating is formed in the grating layer 15, and the entire upper surface of the buffer layer 14 is covered with the grating layer 15. The n-type cladding layer 16 is provided on the grating layer 15 and the buffer layer 14. The n-type cladding layer 16 covers the grating layer 15. The n-type cladding layer 16 is, for example, an n-type InP layer having a thickness of approximately 500 nm. The active layer 17 is provided on the n-type cladding layer 16. The active layer 17 has a quantum well layer and two barrier layers sandwiching the quantum well layer. The quantum well layer is, for example, a GaInAsP layer or an aluminum gallium indium arsenide (AlGaInAs) layer with a thickness of about 80 nm. The barrier layers are, for example, GaInAsP layers or AlGaInAs layers with a thickness of about 30 nm. The active layer 17 is a single-mode active layer having cross-sectional dimensions that enable single-mode waveguiding. The p-type cladding layer 18 is provided on the active layer 17. The p-type cladding layer 18 is, for example, a p-type InP layer with a thickness of about 200 nm.

[0026] As shown in FIG. 2(b), mesas that become the first optical waveguide 21 and the second optical waveguide 31 are formed in the semiconductor layer 13. The mesas are formed so as to expose a portion of the buffer layer 14. The height of the mesas is, for example, approximately 1000 nm. The mesa that becomes the first optical waveguide 21 has a first surface 21a and a second surface 21b that are perpendicular to the major surface 12a. The distance between the first surface 21a and the second surface 21b defines the width Wa described above. The mesa that becomes the second optical waveguide 31 has a third surface 31a and a fourth surface 31b that are perpendicular to the major surface 12a. The distance between the third surface 31a and the fourth surface 31b defines the width Wb described above.

[0027] The semiconductor layer 60 is provided on the side of the mesa so as to embed the mesa. The semiconductor layer 60 has a p-type block layer 61 and an n-type block layer 62. The semiconductor layer 60 is in contact with the first face 21a, the second face 21b, the third face 31a, and the fourth face 31b. At least a portion of the upper surface of the p-type cladding layer 18 is exposed from the semiconductor layer 60.

[0028] The p-type blocking layer 61 is provided on the buffer layer 14. The p-type blocking layer 61 contacts the first surface 21a, the second surface 21b, the third surface 31a, and the fourth surface 31b. The p-type blocking layer 61 contacts each side surface of the buffer layer 14, the diffraction grating layer 15, the n-type cladding layer 16, the active layer 17, and the p-type cladding layer 18. The p-type blocking layer 61 is a p-type InP layer. The thickness of the thickest part of the p-type blocking layer 61 is, for example, not less than 1000 nm and not more than 1500 nm. The n-type blocking layer 62 is provided on the p-type blocking layer 61. The n-type blocking layer 62 is an n-type InP layer. The thickness of the thickest part of the n-type blocking layer 62 is, for example, not less than 300 nm and not more than 500 nm.

[0029] The optical integrated laser device 10 further includes a p-type semiconductor layer 63 , a contact layer 64 , an electrode 51 , an electrode 52 , a wiring 53 , and an insulating film 71 .

[0030] The p-type semiconductor layer 63 is provided on the p-type cladding layer 18 and the n-type block layer 62. The p-type semiconductor layer 63 is, for example, a p-type InP layer. The thickness of the thickest part of the p-type semiconductor layer 63 is, for example, not less than 2500 nm and not more than 3500 nm. The p-type semiconductor layer 63 can function as part of the p-type cladding layer 18.

[0031] The contact layer 64 is provided on the p-type semiconductor layer 63. The contact layer 64 has a p-type GaInAsP layer and a p-type indium gallium arsenide (InGaAs) layer. The GaInAsP layer is provided on the p-type semiconductor layer 63. The GaInAsP layer has a thickness of, for example, about 200 nm. The InGaAs layer is provided on the GaInAsP layer. The InGaAs layer has a thickness of, for example, about 300 nm. The band gap of the contact layer 64 is smaller than the band gap of the p-type semiconductor layer 63. The electrode 51 is provided on the contact layer 64. The electrode 51 is provided so as to overlap with the mesa in a planar view. That is, the outline of the mesa is inside the outline of the electrode 51 in a planar view.

[0032] Two trenches 65 are formed in a stack including the substrate 12, the buffer layer 14, the semiconductor layer 60, the p-type semiconductor layer 63, and the contact layer 64. The two trenches 65 are formed so as to sandwich a mesa therebetween. The two trenches 65 extend along the mesa.

[0033] The insulating film 71 covers the upper surface of the contact layer 64, the upper and side surfaces of the electrode 51, and the inner wall and bottom surfaces of the trench 65. The insulating film 71 is, for example, a silicon oxide (SiO2) film, a silicon oxynitride (SiON) film, or a silicon nitride (SiN) film. An opening 71a is formed in the insulating film 71 to expose a portion of the upper surface of the electrode 51.

[0034] The wiring 53 is provided on the insulating film 71. The wiring 53 is also provided inside the trench 65. The wiring 53 contacts the electrode 51 at the opening 71a. The wiring 53 is, for example, a gold (Au) wiring. The electrode 52 is provided on the back surface 12b of the substrate 12. The electrode 52 contacts the substrate 12. The contact layer 64, the electrode 51, and the wiring 53 are insulated and separated between the distributed feedback laser section 20 and the semiconductor optical amplifier section 30, allowing voltages to be applied to the distributed feedback laser section 20 and the semiconductor optical amplifier section 30 independently of each other.

[0035] Next, a method for manufacturing the integrated optical laser device 10 according to the first embodiment will be described. Figures 4(a) to 8(b) are cross-sectional views showing the method for manufacturing the integrated optical laser device 10, and show cross sections perpendicular to the optical waveguide direction.

[0036] First, as shown in FIG. 4(a), a substrate 12 having a main surface 12a and a back surface 12b is prepared, and a buffer layer 14 is formed on the main surface 12a. Next, a diffraction grating layer 15 is formed on the buffer layer 14. The diffraction grating layer 15 may be formed to be wider than its final dimensions. In the portion included in the distributed feedback laser section 20, a diffraction grating is formed in the diffraction grating layer 15 (see FIG. 2(a)), and in the portion included in the semiconductor optical amplifier section 30, the entire top surface of the buffer layer 14 is covered with the diffraction grating layer 15.

[0037] Next, as shown in FIG. 4(b), an n-type cladding layer 16 is formed on the diffraction grating layer 15 and the buffer layer 14. The n-type cladding layer 16 covers the diffraction grating layer 15. Next, an active layer 17 is formed on the n-type cladding layer 16, and a p-type cladding layer 18 is formed on the active layer 17. Next, as shown in FIG. 5(a), a mask 91 is formed on the p-type cladding layer 18. The mask 91 is formed on an area that will become a mesa. The mask 91 is, for example, a SiO2 film. 5(b), the mask 91 is used as an etching mask to dry-etch the p-type cladding layer 18, the active layer 17, the n-type cladding layer 16, the diffraction grating layer 15, and part of the buffer layer 14. As a result, mesas are formed that will become the first optical waveguide 21 and the second optical waveguide 31. The dry etching is reactive ion etching (RIE) using, for example, silicon tetrachloride (SiCl4).

[0038] 6(a), a mask 91 is used as a selective growth mask to form a p-type block layer 61 on the buffer layer 14 exposed from the mesa, and an n-type block layer 62 is formed on the p-type block layer 61. As a result, the mesa is buried with a semiconductor layer 60 including the p-type block layer 61 and the n-type block layer 62.

[0039] Next, as shown in FIG. 6(b), the mask 91 is removed. The mask 91 can be removed using, for example, hydrofluoric acid (HF). Next, a p-type semiconductor layer 63 is formed on the p-type cladding layer 18 and the n-type block layer 62, and a contact layer 64 is formed on the p-type semiconductor layer 63. The p-type semiconductor layer 63 is integrated with the p-type cladding layer 18. Next, as shown in FIG. 7(a), an electrode 51 is formed on the contact layer 64.

[0040] 7(b), two trenches 65 are formed in a stack including the substrate 12, buffer layer 14, semiconductor layer 60, p-type semiconductor layer 63, and contact layer 64. These trenches 65 are formed by dry etching using, for example, an etching mask (not shown). The dry etching is, for example, RIE using SiCl4. Next, an insulating film 71 is formed to cover the upper surface of the contact layer 64, the upper and side surfaces of the electrode 51, and the inner wall and bottom surfaces of the trenches 65, and an opening 71a is formed in the insulating film 71 to expose a portion of the upper surface of the electrode 51.

[0041] Next, as shown in FIG. 8(a), wiring 53 is formed on the insulating film 71. Wiring 53 is also formed inside trench 65. Wiring 53 contacts electrode 51. Next, as shown in FIG. 8(b), the substrate 12 is polished from the rear surface 12b. Next, an electrode 52 that contacts the substrate 12 is formed on the rear surface 12b. Through the above steps, the distributed feedback laser section 20 and the semiconductor optical amplifier section 30 are formed. Next, a high-reflection film 41 that covers the end face of the distributed feedback laser section 20 and an anti-reflection film 42 that covers the end face of the semiconductor optical amplifier section 30 are formed. Through the above steps, the optical integrated laser device 10 according to this embodiment is fabricated.

[0042] In the optical integrated laser device 10, when a voltage is applied between the electrodes 51 and 52 in the distributed feedback laser section 20, light is generated in the active layer 17, and the light resonates in the first optical waveguide 21 to become laser light. At this time, the central wavelength of the laser light is determined by the action of the diffraction grating. The laser light is amplified by the semiconductor optical amplifier section 30 and output to the outside of the optical integrated laser device 10 through the antireflection film 42. Note that because the optical axis 32 of the semiconductor optical amplifier section 30 is inclined with respect to the optical axis 22 of the distributed feedback laser section 20, the traveling direction of the laser light in the semiconductor optical amplifier section 30 is different from the traveling direction of the laser light in the distributed feedback laser section 20. However, because the semiconductor layer 60 is provided, the laser light is confined within the mesa, and leakage of the laser light to the outside of the mesa is reduced.

[0043] The effects obtained by the optical integrated laser element 10 according to the present embodiment described above will be described. In the optical integrated laser element 10 according to the present embodiment, a high-reflection film 41 is provided on the end face on the second side, which is opposite to the first side, of the distributed feedback laser section 20. This makes it possible to sufficiently increase the intensity of the laser light output from the optical integrated laser element 10.

[0044] However, providing the high-reflection film 41 (the reflectivity of the high-reflection film 41 here is 70% or more) causes the following problem. Part (a) of FIG. 9 is a schematic plan view of the optical integrated laser device 10, and part (b) of FIG. 9 is a partially enlarged view thereof. The end face of the distributed feedback laser section 20 is formed by dicing. Because the dicing accuracy is low, the position of the high-reflection film 41 of the distributed feedback laser section 20 in the optical waveguide direction varies within a range of about ±10 μm. On the other hand, one period of the diffraction grating is about 200 nm. Therefore, the distance L between the end of the diffraction grating and the high-reflection film 41 is determined randomly. As a result, the time required for the laser light to reflect off the high-reflection film 41 and return is random, and the resulting slight time difference significantly affects the oscillation mode of the laser light.

[0045] Part (a) of Fig. 10 is a graph showing the relationship between the wavelength of the laser light and the normalized threshold gain (αL). Part (b) of Fig. 10 is a graph showing the relationship between the wavelength of the laser light and the light intensity. B is the Bragg wavelength determined by the period of the diffraction grating, and D is the stop band width. As shown in part (a) of Figure 10, the normalized threshold gain varies depending on the wavelength of the laser light. Multiple oscillation modes P1 exist discretely on the variation curve E1. The peak wavelength of the laser light is determined depending on which of the multiple oscillation modes P1 the laser light oscillates in. Part (b) of Figure 10 shows, as an example, the case where oscillation occurs in the oscillation mode P1 with the smallest normalized threshold gain. There is a difference in normalized threshold gain (oscillation mode gain difference) C between each oscillation mode P1 and the adjacent oscillation mode P1.

[0046] 11 and 12 are diagrams showing how the graph shown in FIG. 10 fluctuates. When the emission intensity inside the distributed feedback laser section 20, the current flowing through the distributed feedback laser section 20, or the temperature of the distributed feedback laser section 20 changes, the wavelength and threshold gain of each oscillation mode change, as shown in part (a) of FIG. 11. Point P2 in the diagram indicates the oscillation mode before the change, and point P3 indicates the oscillation mode after the change. The relationship between the wavelength and light intensity of the laser light also changes in accordance with the change in the oscillation mode, as shown in part (b) of FIG. 11. Curve G1 in the diagram indicates the relationship before the change, and curve G2 indicates the relationship after the change.

[0047] However, when the change in the emission intensity, current, or temperature of the distributed feedback laser section 20 becomes large, the change in the wavelength and threshold gain of each oscillation mode also becomes large, as shown in part (a) of FIG. 12. As a result, a phenomenon occurs in which the oscillation mode instantaneously changes to an adjacent oscillation mode. This phenomenon is called mode hopping. Due to this mode hopping, the peak wavelength of the laser light also instantaneously changes to the wavelength corresponding to the adjacent oscillation mode, as shown in part (b) of FIG. 12. Therefore, if mode hopping occurs frequently, the peak wavelength of the laser light output from the optical integrated laser device 10 becomes unstable.

[0048] Mode hopping in the distributed feedback laser section 20 is more likely to occur as the emission intensity of the distributed feedback laser section 20 increases. This is because, as shown in part (a) of FIG. 11 , the fluctuation in the threshold gain of the oscillation mode becomes more pronounced as the fluctuation in the emission intensity increases. As the length La increases, the emission intensity of the distributed feedback laser section 20 increases, resulting in larger oscillation mode gain fluctuations and a tendency to transition to a different oscillation mode. Therefore, in order to reduce the occurrence of mode hopping and stabilize the peak wavelength of the laser light, it is effective to reduce the oscillation mode gain fluctuation by shortening the length La of the distributed feedback laser section 20 in the optical waveguide direction. Setting the length La of the first optical waveguide 21, which constitutes a single-mode active layer lattice-matched to the InP substrate, in the optical waveguide direction to 1000 μm or less can reduce the occurrence of mode hopping. Reducing the occurrence of mode hopping can stabilize the peak wavelength of the laser light. However, since the emission intensity of the distributed feedback laser section 20 depends on the length La, shortening the length La also reduces the emission intensity of the distributed feedback laser section 20.

[0049] Therefore, in order to sufficiently increase the intensity of the laser light output from the optical integrated laser device 10, the length Lb of the second optical waveguide 31 in the optical waveguide direction is increased to increase the gain of the semiconductor optical amplifier 30. In this embodiment, by making the length Lb of the second optical waveguide 31 in the optical waveguide direction at least 1.5 times the length La of the first optical waveguide 21 in the optical waveguide direction, it is possible to sufficiently increase the intensity of the output laser light while reducing the occurrence of mode hopping. That is, to address the problem of mode hopping being more likely to occur when a high-reflection film 41 is provided on the second-side end facet, the length La of the optical waveguide (first optical waveguide 21) in the optical waveguide direction of the distributed feedback laser formed by a single-mode active layer lattice-matched to an InP substrate is set to 1000 μm or less to suppress mode hopping, and the length Lb of the second optical waveguide 31 in the optical waveguide direction at least 1.5 times the length La of the first optical waveguide 21 in the optical waveguide direction is set to ensure the gain of the semiconductor optical amplifier 30 and compensate for insufficient optical output.

[0050] Furthermore, as in this embodiment, the length Lb of the second optical waveguide 31 in the optical waveguide direction may be seven times or less the length La of the first optical waveguide 21 in the optical waveguide direction. This prevents the first optical waveguide 21 from becoming too short, and makes it possible to maintain an appropriate side mode suppression ratio (SMSR).

[0051] The length Lb of the second optical waveguide 31 in the optical waveguide direction may be 2.0 times or more the length La of the first optical waveguide 21 in the optical waveguide direction. In this case, the intensity of the output laser light can be further increased while further reducing the occurrence of mode hopping.

[0052] As in this embodiment, the width of the second optical waveguide 31 may be 1.0 to 6.5 times the width of the first optical waveguide 21. In this case, the gain of the semiconductor optical amplifier 30 becomes larger. Therefore, the intensity of the output laser light can be further increased.

[0053] As in this embodiment, the optical waveguide direction of the second optical waveguide 31 may be inclined with respect to the optical waveguide direction of the first optical waveguide 21. In this case, the intensity of the laser light can be further increased by lengthening the second optical waveguide 31 while shortening the overall length of the optical integrated laser device 10.

[0054] As in this embodiment, the planar shape of the substrate 12 is a rectangle having second long sides 122 and first long sides 121 extending parallel to each other, the optical guiding direction of the first optical waveguide 21 extends parallel to the second long side 122 and the first long side 121, the first optical waveguide 21 is closer to the second long side 122 than to the first long side 121, and the second optical waveguide 31 may be inclined so as to approach the first long side 121 from the connection point with the first optical waveguide 21. In this case, the second optical waveguide 31 can be lengthened to further increase the intensity of the laser light while avoiding an increase in the width of the optical integrated laser device 10.

[0055] As in this embodiment, the length La of the first optical waveguide 21 in the optical waveguide direction may be 1000 μm or less. In this case, the occurrence of mode hopping can be effectively reduced. Alternatively, the length La of the first optical waveguide 21 in the optical waveguide direction may be 300 μm or more. In this case, the SMSR can be maintained at an appropriate size. [Variations]

[0056] 13 is a plan view showing an optical integrated laser element 11 according to a modification of the above embodiment. The optical integrated laser element 11 differs from the above embodiment in that the width Wb of the second optical waveguide 31 is the same as the width Wa of the first optical waveguide 21, but is identical to the above embodiment in other respects. Even with this modification, the same effects as the above embodiment can be obtained.

[0057] The optical integrated laser device according to the present disclosure is not limited to the above-described embodiment, and various other modifications are possible. For example, although the above-described embodiment shows an example in which the optical waveguiding direction of the second optical waveguide is inclined with respect to the optical waveguiding direction of the first optical waveguide, the optical waveguiding direction of the second optical waveguide may coincide with the optical waveguiding direction of the first optical waveguide. [Explanation of symbols]

[0058] 10, 11... Optical integrated laser element 12... Circuit board 12a…main surface 12b...back side 13...Semiconductor layer 14...Buffer layer 15...diffraction grating layer 16...n-type cladding layer 17…Active layer 18...p-type cladding layer 20...Distributed feedback laser section 21...First optical waveguide 21a…Side 1 21b…Second side 22...Optical axis 30...Semiconductor optical amplifier 31...Second optical waveguide 31a...Third side 31b...Side 4 32...Optical axis 41...High reflective film 42...Anti-reflection film 51,52...electrode 53...Wiring 60...Semiconductor layer 61...p-type block layer 62...n-type blocking layer 63...p-type semiconductor layer 64...Contact layer 65...Trench 71...insulating film 71a...Opening 91...Mask 121...first long side 122...Second long side 123...First short side 124...Second short side 311...Tapered section 312...Parallel section C...Oscillation mode gain difference G1,G2…Curve L…Distance La, Lb...length P1...Oscillation mode P2,P3…points Wa, Wb...Width

Claims

1. an InP substrate; a distributed feedback laser section having a first optical waveguide including a diffraction grating provided on the InP substrate and a single-mode active layer having a gain provided along the diffraction grating; a semiconductor optical amplifier section provided on the InP substrate, the semiconductor optical amplifier section having a second optical waveguide provided on a first side of the distributed feedback laser section, the second optical waveguide being optically connected to the first optical waveguide, the second optical waveguide having a gain and including a width wider than that of the first optical waveguide; a highly reflective film having a reflectance of 70% or more, the highly reflective film being provided on an end surface of the distributed feedback laser portion on a second side opposite to the first side; Equipped with the distributed feedback laser section and the semiconductor optical amplifier section are integrated on the InP substrate, an optical integrated laser element, wherein the length of the first optical waveguide in the optical waveguide direction is 1000 μm or less, and the length of the second optical waveguide in the optical waveguide direction is 1.5 to 7 times the length of the first optical waveguide in the optical waveguide direction.

2. 2. The optical integrated laser device according to claim 1, wherein the length of said second optical waveguide in said optical waveguiding direction is at least 2.0 times the length of said first optical waveguide in said optical waveguiding direction.

3. 3. The optical integrated laser device according to claim 1, wherein the width of said second optical waveguide is 1.0 to 6.5 times the width of said first optical waveguide.

4. 3. The optical integrated laser device according to claim 1, wherein the optical guiding direction of the second optical waveguide is inclined with respect to the optical guiding direction of the first optical waveguide.

5. the InP substrate has a planar shape that is a rectangle having a first long side and a second long side that extend parallel to each other; the optical waveguide direction of the first optical waveguide extends parallel to the second long side and the first long side, 5. The optical integrated laser device according to claim 4, wherein said second optical waveguide is inclined so as to approach said first long side from the connection point with said first optical waveguide.

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