Semiconductor optical element

Abstract

A laser unit (2) generates laser light. Core layers (4, 9, 18) through which the laser light passes have a butt joint part. A window part (5) is formed on the output side of the core layers (4, 9, 18). The butt joint part, or the interface between the core layers (4, 9, 18) and the window part (5), is a periodic structure (23) in plan view.

Description

semiconductor optical device

[0001] The present disclosure relates to semiconductor optical devices.

[0002] Semiconductor optical integrated devices can become unstable due to reflected back light. In particular, devices with many butt-joints, such as modulator-equipped EMLs, have many reflection points, making them prone to mode instability. Mode instability causes a discontinuity in optical output called a kink, which can lead to device failure. As a countermeasure against light back light, it has been proposed to tilt the butt-joints (see, for example, Patent Document 1).

[0003] Japanese Patent Application Publication No. 2011-124445

[0004] However, if the butt joint is tilted, the beam split component increases, and the scattered component in a direction deviating from the waveguide direction increases. Therefore, although the reflected component can be suppressed, the light loss increases and the optical output decreases.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its object is to obtain a semiconductor optical device that can achieve both mode stabilization and high output power.

[0006] The semiconductor optical device according to the present disclosure has a laser portion that generates laser light, a butt-joint portion, a core layer through which the laser light passes, and a window portion formed on the output side of the core layer, and is characterized in that the butt-joint portion or the interface between the core layer and the window portion has a periodic structure in plan view.

[0007] In the present disclosure, the butt joint of the core layer, which can be a reflection point, or the interface between the core layer and the window portion, has a periodic structure in plan view. The periodic structure can suppress the occurrence of reflected back light. Furthermore, forming a periodic structure does not increase the amount of scattered light outside the waveguide, so the optical output does not decrease. As a result, it is possible to achieve both mode stabilization and high output.

[0008] 1 is a cross-sectional view showing a semiconductor optical device according to a first embodiment; FIG. 2 is a cross-sectional view showing a laser portion; FIG. 3 is a cross-sectional view showing an optical modulator; FIG. 4 is a cross-sectional view showing a transparent waveguide; FIG. 5 is a cross-sectional view showing a window portion; FIG. 6 is a plan view showing a core layer of a semiconductor optical device according to a first embodiment; FIG. 7 is a plan view showing a core layer of a semiconductor optical device according to a first embodiment; FIG. 8 is a plan view showing a core layer of a semiconductor optical device according to a second embodiment; FIG. 9 is a plan view showing a core layer of a semiconductor optical device according to a second embodiment; FIG. 10 is a plan view showing a modified example of the core layer of a semiconductor optical device according to a second embodiment; FIG. 11 is a plan view showing a modified example of the core layer of a semiconductor optical device according to a third embodiment; FIG. 12 is a plan view showing a core layer of a semiconductor optical device according to a third embodiment;

[0009] A semiconductor optical device according to an 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.

[0010] First Embodiment. Figure 1 is a cross-sectional view showing a semiconductor optical device according to a first embodiment. This semiconductor optical device is a semiconductor laser with an electroabsorption modulator (DFB laser). A laser section 2, an optical modulator 3, and a transparent waveguide 4 are formed on a semiconductor substrate 1. A window section 5 is formed on the output side of the transparent waveguide 4. A low-reflectivity film 6 is formed on the front end face from which the laser light is emitted. A high-reflectivity film 7, which has a higher reflectivity for laser light than the low-reflectivity film 6, is formed on the rear end face opposite the front end face. The low-reflectivity film 6 and the high-reflectivity film 7 have a structure in which Si layers and SiO layers are alternately stacked, and the reflectivity is adjusted by changing the thickness of each layer.

[0011] A laser unit 2 generates laser light. An optical modulator 3 modulates the laser light. The modulated laser light is guided through a transparent waveguide 4 and emitted to a fiber outside the device via a window 5 and a low-reflectance film 6.

[0012] 2 is a cross-sectional view of a laser portion. A first-conductivity-type lower cladding layer 8, an undoped active layer 9, an undoped diffraction grating layer 10, and a second-conductivity-type upper cladding layer 11 are formed in this order on a first-conductivity-type semiconductor substrate 1. Current blocking layers 12 are formed on both sides of the ridge structure of the active layer 9, diffraction grating layer 10, and upper cladding layer 11. A second-conductivity-type upper cladding layer 13 and a second-conductivity-type contact layer 14 are formed in this order on the upper cladding layer 11 and current blocking layer 12. One of the first and second conductivity types is n-type, and the other is p-type. An electrode 16 is connected to the contact layer 14 through an opening in an insulating film 15. An electrode 17 is formed on the underside of the semiconductor substrate 1.

[0013] The semiconductor substrate 1, the lower cladding layer 8, and the upper cladding layers 11 and 13 are made of InP. The active layer 9 and the diffraction grating layer 10 are made of AlGaInAs or InGaAsP. The current blocking layer 12 is a pnpn or npnp thyristor structure or a semi-insulating layer made of InP. The contact layer 14 is made of InGaAs. The impurity concentration of the contact layer 14 is 10 １９ cm －３ It is a stand.

[0014] 3 is a cross-sectional view showing an optical modulator. A lower cladding layer 8, an undoped modulator layer 18, and an upper cladding layer 19 are formed in this order on a semiconductor substrate 1. Current blocking layers 12 are formed on both sides of the ridge structure of the modulator layer 18 and upper cladding layer 19. An upper cladding layer 13 and a contact layer 14 are formed in this order on the upper cladding layer 19 and current blocking layer 12. An electrode 20 is connected to the contact layer 14.

[0015] Fig. 4 is a cross-sectional view showing a transparent waveguide. A lower cladding layer 8, an undoped transparent waveguide layer 21, and an upper cladding layer 22 are formed in this order on a semiconductor substrate 1. Current blocking layers 12 are formed on both sides of the ridge structure of the transparent waveguide layer 21 and the upper cladding layer 22. An upper cladding layer 13 and an insulating film 15 are formed in this order on the upper cladding layer 22 and the current blocking layer 12. Fig. 5 is a cross-sectional view showing a window portion. The material and structure of the window portion 5 are the same as those of the current blocking layer 12.

[0016] The active layer 9, modulator layer 18, and transparent waveguide layer 21 are core layers through which laser light passes. The modulator layer 18 and transparent waveguide layer 21 are made of AlGaInAs or InGaAsP. The active layer 9, modulator layer 18, and transparent waveguide layer 21 may be made of the same compound, but in that case, they have different compositions. The bonding interfaces between the active layer 9, modulator layer 18, and transparent waveguide layer 21 form butt joints where different materials are bonded.

[0017] 6 to 8 are plan views showing the core layer of the semiconductor optical device according to the first embodiment. As shown in Fig. 6, the butt-joint portion between the modulator layer 18 of the optical modulator 3 and the transparent waveguide layer 21 of the transparent waveguide 4 forms a periodic structure 23 in a plan view. As shown in Fig. 7, the interface between the transparent waveguide layer 21 of the transparent waveguide 4 and the window portion 5 forms a periodic structure 23 in a plan view. As shown in Fig. 8, the butt-joint portion between the active layer 9 of the laser portion 2 and the modulator layer 18 of the optical modulator 3 forms a periodic structure 23 in a plan view. Note that if the optical modulator 3 is omitted, the butt-joint portion between the active layer 9 of the laser portion 2 and the transparent waveguide layer 21 of the transparent waveguide 4 may also have a periodic structure.

[0018] The periodic structure 23 has a plurality of periodically arranged rectangular patterns. For example, if the width of the waveguide is 1.6 μm, a rectangular pattern with a 0.2 μm pitch will form eight rectangular patterns. The light passing through the convex and concave portions of the periodic structure 23 has different optical path lengths, resulting in a phase difference. The presence of such a subwavelength periodic structure 23 is equivalent to having a layer with an intermediate refractive index interposed between two layers, reducing the reflectance of the laser light.

[0019] If the pitch of the periodic structure 23 is smaller than the oscillation wavelength of the laser light generated by the laser unit 2, an anti-reflection effect can be obtained. Specifically, if the oscillation wavelength of the laser light is λ and the refractive index of the core layer is n, the pitch Λ is set as Λ = λ / n / 2. For example, in the case of a 1.3 μm long-wave laser, the refractive index of InP is about 3.2, so the pitch of the periodic structure 23 is set to about 0.2 μm. Since the oscillation wavelength of the laser light generated by the laser unit 2 for communications is 1.27 μm to 1.58 μm, it is preferable that the pitch of the periodic structure 23 be 20 nm to 600 nm. The reason for setting the pitch of the periodic structure to 20 nm or more is to ensure processing accuracy.

[0020] As described above, in this embodiment, the butt joint of the core layer, which can be a reflection point, or the interface between the core layer and the window portion 5, has the periodic structure 23 in plan view. The periodic structure 23 can suppress the occurrence of reflected back light. Furthermore, even if the periodic structure 23 is formed, the amount of scattered light outside the waveguide does not increase, so the optical output does not decrease. As a result, it is possible to achieve both mode stabilization and high output. Furthermore, it is possible to suppress loss components at the window portion 5 and improve the coupling efficiency between the semiconductor optical element and the optical fiber.

[0021] To stabilize the operation of semiconductor devices for optical communications, it is necessary to obtain single longitudinal mode oscillation. To obtain a single wavelength mode, a uniform diffraction grating structure requires the adoption of an asymmetric end facet reflectivity structure. Therefore, a high-reflectivity film 7 is formed on the rear end facet. Forming a high-reflectivity film 7 on the rear end facet can further improve the optical output from the front end facet. However, as the optical output from the front end facet increases, the reflected back light also increases, making the operation unstable. Therefore, a structure using a periodic structure 23 to prevent reflected back light is particularly effective. The configuration of this embodiment can also be applied to a phase-shift diffraction grating structure. In this case, the high-reflectivity film 7 is not formed, and both end faces are non-reflective to obtain stable single wavelength characteristics.

[0022] 9 and 10 are plan views showing the core layer of a semiconductor optical device according to embodiment 2. As shown in Fig. 9, the periodic structure 23 at the butt-joint portion between the modulator layer 18 and the transparent waveguide layer 21 has a plurality of periodically arranged trapezoidal patterns. As shown in Fig. 10, the periodic structure 23 at the interface between the transparent waveguide layer 21 and the window portion 5 also has a plurality of periodically arranged trapezoidal patterns. Note that the butt-joint portion between the active layer 9 and the modulator layer 18 may also have a periodic structure having a plurality of periodically arranged trapezoidal patterns.

[0023] 11 and 12 are plan views showing modified examples of the core layer of the semiconductor optical device according to embodiment 2. As shown in Fig. 11, the periodic structure 23 at the butt-joint portion between the modulator layer 18 and the transparent waveguide layer 21 has a plurality of periodically arranged triangular patterns. As shown in Fig. 12, the periodic structure 23 at the interface between the transparent waveguide layer 21 and the window portion 5 has a plurality of periodically arranged triangular patterns. Note that the butt-joint portion between the active layer 9 and the modulator layer 18 may also have a periodic structure 23 having a plurality of periodically arranged triangular patterns.

[0024] These trapezoidal and triangular patterns can be created by changing the insulating film mask used in pattern formation. By forming the periodic structure 23 in the same manner as in the first embodiment, it is possible to achieve both mode stabilization and high output.

[0025] Since the refractive index of the core layer is higher than that of the cladding layer, most of the light is confined within the core layer. Therefore, even if light is scattered at the inclined portions of the trapezoidal or triangular pattern of the periodic structure 23, scattering outside the core layer is unlikely to occur, resulting in no light loss. Furthermore, when light travels from the transparent waveguide layer 21, which has a high refractive index, to the window portion 5, which has a low refractive index, the distribution of light is broadened by diffraction, but the component scattered outside the transparent waveguide layer 21 at the inclined portions is minute compared to the scattered component. Therefore, the inclined portions of the trapezoidal or triangular pattern of the periodic structure 23 have almost no effect on the optical output.

[0026] 13 and 14 are plan views showing a core layer of a semiconductor optical device according to a third embodiment. As shown in FIG. 13, a periodic structure 23 is formed at the butt-joint portion between the modulator layer 18 and the transparent waveguide layer 21, as in the first embodiment. As shown in FIG. 14, a periodic structure 23 is formed at the interface between the transparent waveguide layer 21 and the window portion 5, as in the first embodiment. However, in this embodiment, the transparent waveguide layer 21 has a waveguide that is tapered in plan view in order to adjust the waveform of the emitted light or to convert the mode. Even in a core layer having such a tapered waveguide, by forming a periodic structure 23, as in the first embodiment, it is possible to achieve both mode stabilization and high output.

[0027] 2 laser portion, 3 optical modulator, 4 transparent waveguide (core layer), 5 window portion, 6 low reflectivity film, 7 high reflectivity film, 9 active layer (core layer), 18 modulator layer (core layer), 21 transparent waveguide layer, 23 periodic structure

Claims

1. A semiconductor optical device comprising: a laser section that generates laser light; a core layer having a butt-joint section and through which the laser light passes; and a window section formed on the output side of the core layer, wherein the butt-joint section or the interface between the core layer and the window section has a periodic structure in plan view.

2. The semiconductor optical device according to claim 1, wherein the pitch of said periodic structure is smaller than the oscillation wavelength of said laser light.

3. The semiconductor optical device according to claim 2, wherein the pitch of the periodic structure is 20 nm to 600 nm.

4. A semiconductor optical device according to any one of claims 1 to 3, further comprising: an optical modulator that modulates the laser light; and a transparent waveguide that guides the modulated laser light, wherein the butt-joint portion is a junction between a modulator layer of the optical modulator and a transparent waveguide layer of the transparent waveguide.

5. A semiconductor optical device according to any one of claims 1 to 3, further comprising an optical modulator that modulates the laser light, wherein the butt-joint portion is a junction between an active layer of the laser portion and a modulator layer of the optical modulator.

6. A semiconductor optical device according to any one of claims 1 to 3, further comprising a transparent waveguide for guiding the laser light, wherein the butt-joint portion is a junction between an active layer of the laser portion and a transparent waveguide layer of the transparent waveguide.

7. A semiconductor optical device according to any one of claims 1 to 6, wherein the periodic structure has a plurality of periodically arranged rectangular patterns.

8. A semiconductor optical device according to any one of claims 1 to 6, wherein the periodic structure has a pattern of a plurality of trapezoids or triangles arranged periodically.

9. A semiconductor optical device according to any one of claims 1 to 8, wherein the core layer has a waveguide that is tapered in plan view.

10. A semiconductor optical device according to any one of claims 1 to 9, further comprising: a low-reflectivity film formed on the front end face from which the laser light is emitted; and a high-reflectivity film formed on the rear end face opposite the front end face, the high-reflectivity film having a higher reflectivity for the laser light than the low-reflectivity film.

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