Semiconductor laser

Abstract

An active layer (103) is formed so as to extend to a second region (122) continuously from a first region (121) on a substrate (101), and a diffraction grating (105) is formed so as to extend from the first region (121) to the second region (122), elements of the grating being disposed at a uniform pitch. The equivalent refractive index of the second region (122) is different from the equivalent refractive index of the first region (121). By setting the active layer (103) formed in the second region (122) to a thickness different from that of the active layer (103) formed in the first region (121), the equivalent refractive index of the second region (122) is different from the equivalent refractive index of the first region (121).

Description

semiconductor laser

[0001] The present invention relates to a semiconductor laser.

[0002] In recent years, with the increase in communication capacity within data centers and other areas, wavelength division multiplexing (WDM) technology, which has traditionally been used for relatively long-distance communications, is now also required for short-distance communications within data centers and other areas. Since a huge number of optical transceivers are required for short-distance communications, the cost of each transceiver must be reduced. Furthermore, power consumption must be kept as low as possible. Therefore, there is a demand for inexpensive production of compact, low-power transceivers.

[0003] As a heterogeneous material integrated device that meets the above-mentioned demands for compact size and low power consumption, optical devices using compound semiconductors have been developed that form a buried heterostructure within a stacked structure of multiple thin semiconductor layers (Non-Patent Documents 1 to 6). A buried heterostructure is a structure in which a semiconductor (active layer) with a high refractive index and a small band gap is sandwiched vertically and horizontally between semiconductors with relatively low refractive index and a large band gap. This structure makes it possible to improve the optical confinement coefficient of the active layer, which greatly contributes to various performance aspects of semiconductor devices.

[0004] Devices with this type of buried heterostructure typically use a semiconductor multilayer structure approximately 200 nm to 500 nm thick, characterized by the fact that the volume of the semiconductor active layer is reduced to minimize power consumption. Furthermore, by forming a diffraction grating with an arbitrary pitch on top of the active layer, single-mode operation at the desired wavelength required for WDM can be easily achieved. The active layer also employs a multiple quantum well (MQW) structure, which provides excellent carrier coupling efficiency. To apply an electric field and inject current into the device, a lateral pin structure is employed, with the semiconductor layers on either side of the active layer being p-type and n-type, respectively.

[0005] Furthermore, from the perspective of reducing the cost of transceivers, it is desirable to use a photonic integrated circuit (PIC) in which multiple devices are fabricated and integrated on a single wafer, rather than fabricating each device individually and then integrating them, because this makes it possible to significantly reduce the device assembly cost.

[0006] Silicon photonics technology, which involves microfabrication of silicon (Si) to create thin-wire waveguides and passive devices, is widely used in the fabrication of optical integrated circuits. While silicon photonics applications have advantages such as the low cost of the material itself and the ability to utilize microfabrication techniques developed in the electronics field, silicon is an indirect transition material, and thus no highly efficient light-emitting devices have been realized to date. Therefore, heterogeneous integration with direct transition materials, which can achieve highly efficient light emission, is essential.

[0007] For these reasons, the technology of integrating thin-film compound semiconductor devices with the aforementioned buried heterostructure on silicon is considered promising. With this technology, it is possible to fabricate waveguides, modulators, arrayed waveguide gratings (AWGs), optical switches, photodetectors, and other devices fabricated using silicon photonics technology in the silicon layer below the active layer, making it possible to fabricate optical integrated circuits using silicon photonics integration.

[0008] International Publication No. 2021 / 199137

[0009] S. Matsuo et al., "Directly modulated buried heterostructure DFB laser on SiO2 / Si substrate fabricated by regrowth of InP using bonded active layer", Optics Express, vol. 22, no. 10, pp. 12139-12147, 2014.T. Fujii et al., "Epitaxial growth of InP to bury directly bonded thin active layer on SiO2 / Si substrate for fabricating distributed feedback lasers on silicon", IET Optoelectron, vol. 9, no. 4, iss. 151-157, 2015.T. Okamoto et al., "Optically Pumped Membrane BH-DFB Lasers for Low-Threshold and Single-Mode Operation", IEEE Journal of Selected Topics in Quantum Electronics, vol. 9, no. 5, pp. 1361-1366, 2003.S. Matsuo et al., "Room-temperature continuous-wave operation of lateral current injection wavelength-scale embedded active-region photonic-crystal laser", Optics Express, vol. 20, no. 4, pp. 3773-3780, 2012.K. Hasebe et al., "High-Speed Modulation of Lateral p-i-n Diode Structure Electro-Absorption Modulator Integrated With DFB Laser", Journal of Lightwave Technology, vol. 33, no.6, pp. 1235-1240, 2014.H. Nishi et al., "Membrane distributed-reflector laser integrated with SiOx-based spot-size converter on Si substrate", Optics Express, vol. 24, no. 16, pp. 18346-18352, 2016.

[0010] To realize WDM technology using the optical integrated circuits described above, a laser with excellent wavelength controllability and stable single-mode oscillation is required. A typical example of such a laser is a DFB laser that uses a diffraction grating with a λ / 4 phase shifter that inverts the phase by π.

[0011] 11A shows a thin-film DFB laser using a diffraction grating with a λ / 4 phase shift portion. This laser has a lower semiconductor layer 302, an active layer 303, and an upper semiconductor layer 304 stacked on an insulating layer 301, with a diffraction grating 305 formed in a region near the active layer 303 where the optical mode field exists. The diffraction grating 305 has a structure in which the refractive index changes periodically in the longitudinal direction of the resonator. By providing a phase shift portion 306 in a portion (typically the center) of the diffraction grating 305, stable single-mode oscillation can be obtained at the center wavelength of the stop band.

[0012] This structure is widely used due to its ease of design, but it has the characteristic that the light intensity distribution within the resonator becomes non-uniform, resulting in a stronger light intensity around the phase shift portion 306. When the product of the length (L) of the active layer 303 in the optical axis direction and the coupling coefficient (κ) of the diffraction grating 305 is large, i.e., when the structure is one in which the light is largely confined in the resonator in the optical axis direction, spatial hole burning occurs due to the non-uniformity of the light intensity, destabilizing the oscillation and impairing the single-mode characteristics. To avoid oscillation instability due to spatial hole burning, it is generally considered necessary to keep the product of κ and L to approximately 1.5 or less.

[0013] Another structure that can solve this problem is the DR (Distributed Reflector) laser shown in Fig. 11B. This laser comprises a DFB laser 321 in which a lower semiconductor layer 302, an active layer 303, and an upper semiconductor layer 304 are stacked on an insulating layer 301, and a diffraction grating 305 is formed on the active layer 303, and a DBR region 322. The DBR region 322 comprises a semiconductor core 303a formed in contact with the active layer 303, and a diffraction grating 305a formed thereon. In this structure, the diffraction grating 305 does not have a phase shift.

[0014] In the DR laser, a DBR region 322 having a high reflectivity for either the short wavelength side or the long wavelength side of the oscillation spectrum of a DFB laser 321 determined by a diffraction grating 305 formed on an active layer 303 is provided on the opposite side of the laser emission side as viewed from the active layer 303.

[0015] This configuration makes it possible to avoid the concentration of light intensity in the phase shift region when a phase shift region is provided, and it is possible to achieve both a high κL structure that strongly confines light within the resonator and single-mode performance. According to Non-Patent Document 6, good single-mode performance is obtained even when κL reaches 7.5.

[0016] However, in this structure, it is necessary to form the diffraction gratings 305 and 305a with different periods in the DFB laser 321 and the DBR region 322, which have different equivalent refractive indices, and to control the relative Bragg wavelengths. The required wavelength accuracy is about half the stop band width of the DFB laser 321. For example, when κ is set to 500 cm -1 When the wavelength is set to about 1310 nm, the width of the stop band at an oscillation wavelength of 1310 nm is approximately 10 nm, and the precision of the detuning amount required for the diffraction grating 305a is about 5 nm. If the wavelength is outside this range, neither of the oscillation modes at either end of the stop band of the DFB laser 321 will be selected by the DBR region 322, resulting in a significant increase in the threshold value and a decrease in optical output. Alternatively, the oscillation mode opposite to the intended one will be selected, resulting in mode hopping.

[0017] However, the connection between the active layer of the DFB laser and the InP core of the DBR region is generally fabricated separately using a crystal regrowth process, which creates uncertainty in the thickness of each layer, leading to uncertainty in the equivalent refractive index, making it difficult to precisely control the relative wavelength.

[0018] As described above, the conventional technology has a problem in that stable single mode characteristics cannot be obtained in a structure in which the product of the length in the optical axis direction and the coupling coefficient of the diffraction grating is large.

[0019] The present invention has been made to solve the above problems, and aims to provide stable single-mode characteristics in a structure in which the product of the length in the optical axis direction and the coupling coefficient of the diffraction grating is large.

[0020] The semiconductor laser according to the present invention comprises an active layer formed from a first region on a substrate to a second region continuous with the first region, a diffraction grating formed at a uniform pitch from the first region to the second region on the substrate, a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer, an n-type electrode connected to the n-type semiconductor layer, and a p-type electrode connected to the p-type semiconductor layer, and the second region has an equivalent refractive index different from that of the first region.

[0021] As described above, according to the present invention, an active layer is formed from the first region to the second region, a diffraction grating is formed at a uniform pitch from the first region to the second region, and the second region has an equivalent refractive index different from that of the first region. Therefore, stable single-mode characteristics can be obtained in a structure in which the product of the length in the optical axis direction and the coupling coefficient of the diffraction grating is large.

[0022] FIG. 1 is a cross-sectional view showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 2A is a cross-sectional view showing the state of a semiconductor laser in an intermediate step for explaining a method for manufacturing a semiconductor laser according to the first embodiment of the present invention. FIG. 2B is a cross-sectional view showing the state of a semiconductor laser in an intermediate step for explaining a method for manufacturing a semiconductor laser according to the first embodiment of the present invention. FIG. 2C is a cross-sectional view showing the state of a semiconductor laser in an intermediate step for explaining a method for manufacturing a semiconductor laser according to the first embodiment of the present invention. FIG. 2D is a cross-sectional view showing the state of a semiconductor laser in an intermediate step for explaining a method for manufacturing a semiconductor laser according to the first embodiment of the present invention. FIG. 2E is a cross-sectional view showing the state of a semiconductor laser in an intermediate step for explaining a method for manufacturing a semiconductor laser according to the first embodiment of the present invention. FIG. 2F is a cross-sectional view showing the state of a semiconductor laser in an intermediate step for explaining a method for manufacturing a semiconductor laser according to the first embodiment of the present invention. FIG. 3 is a cross-sectional view showing the configuration of a semiconductor laser according to a second embodiment of the present invention. FIG. 4 is a cross-sectional view showing a partial configuration of another semiconductor laser according to an embodiment of the present invention. FIG. 5 is a cross-sectional view showing a partial configuration of a semiconductor laser according to a third embodiment of the present invention. FIG. 6 is a cross-sectional view showing the configuration of a semiconductor laser according to a fourth embodiment of the present invention. Fig. 7 is a characteristic diagram showing the relationship between the total length of the first and second regions, the length of the second region being x μm, and the threshold gain of the entire semiconductor laser including the first and second regions, where the total length of the first and second regions is 80 μm. Fig. 8 is a characteristic diagram showing changes in the oscillation spectrum from the front and rear of the resonator for each value of x. Fig. 9 is a characteristic diagram showing the calculation results of the equivalent refractive index of the cross-sectional mode when the thickness of the active layer is changed. Fig. 10 is a characteristic diagram showing the calculation results of the equivalent refractive index when the width of the active layer is changed. Fig. 11A is a diagram showing the configuration of a conventional DFB laser. Fig. 11B is a diagram showing the configuration of a conventional DR laser.

[0023] A semiconductor laser according to an embodiment of the present invention will now be described.

[0024] First Embodiment First, a semiconductor laser according to a first embodiment of the present invention will be described with reference to Fig. 1. This semiconductor laser includes an active layer 103 formed on a substrate 101, a diffraction grating 105 formed on the active layer 103, an n-type semiconductor layer 106 and a p-type semiconductor layer 107 formed in contact with the active layer 103, an n-type electrode 108 connected to the n-type semiconductor layer 106, and a p-type electrode 109 connected to the p-type semiconductor layer 107. In this configuration, a current is injected into the active layer 103 in a direction parallel to the plane of the substrate 101.

[0025] A lower cladding layer 102 is formed on a substrate 101, and an active layer 103 is formed thereon. The active layer 103 is sandwiched between a first semiconductor layer 104a and a second semiconductor layer 104b in the vertical direction when viewed from the substrate 101. The stacked structure of the first semiconductor layer 104a, the active layer 103, and the second semiconductor layer 104b is sandwiched between an n-type semiconductor layer 106 and a p-type semiconductor layer 107. The n-type semiconductor layer 106 and the p-type semiconductor layer 107 are formed to sandwich the active layer 103 in a direction parallel to the plane of the substrate 101.

[0026] In this example, an active layer 103 is formed on and in contact with a first semiconductor layer 104a, and a second semiconductor layer 104b is formed on and in contact with the active layer 103. A buried heterostructure is formed by the first semiconductor layer 104a, the active layer 103, and the second semiconductor layer 104b. An n-type semiconductor layer 106 and a p-type semiconductor layer 107 are formed on and in contact with the side portions of the stacked structure of the first semiconductor layer 104a, the active layer 103, and the second semiconductor layer 104b.

[0027] The active layer 103 extends a predetermined length in the light emission (optical axis) direction, and in the resonator region 131 in this extending direction, a diffraction grating 105 is formed in a region that couples with the mode field of the active layer 103. In this example, the diffraction grating 105 is formed on the top surface of the second semiconductor layer 104b.

[0028] Additionally, the upper surfaces of the second semiconductor layer 104b, the n-type semiconductor layer 106, and the p-type semiconductor layer 107 between the n-type electrode 108 and the p-type electrode 109 are protected by a protective layer 110. The protective layer 110 on the second semiconductor layer 104b functions as an upper cladding layer.

[0029] The substrate 101 may be made of, for example, silicon, and the lower cladding layer 102 may be made of, for example, SiO2. The lower cladding layer 102 may be made of, for example, SiO2, which is silica with an adjusted Si and O composition. x The lower cladding layer 102 can be made of a material that is transparent to the communication wavelength band and has a lower refractive index than InP, such as SiC, Al2O3, or diamond. The protective layer 110 can be made of, for example, SiO2 or SiO x The protective layer 110 can be made of a material such as BCB that is transparent to the communication wavelength band and has a lower refractive index than InP. The protective layer 110 can also be omitted.

[0030] The active layer 103 can be made of a compound semiconductor such as InGaAsP, InGaAs, or InGaAlAs, and can have a multi-quantum well (MQW) structure in which the above-mentioned compound semiconductors are stacked with different compositions.

[0031] Furthermore, for example, the first semiconductor layer 104a and the second semiconductor layer 104b can be made of undoped InP (i-InP). Furthermore, the first semiconductor layer 104a and the second semiconductor layer 104b can be made of InGaAsP, InGaAlAs, or the like, which has a larger band gap than the active layer 103. With this configuration, light and carriers can be efficiently injected into the active layer 103.

[0032] The n-type semiconductor layer 106 contains 1×10 18 cm -3 The p-type semiconductor layer 107 is made of n-type InP (n-InP) doped to a degree of 1×10 18 cm -3 The n-type semiconductor layer 106 and the p-type semiconductor layer 107 may be made of p-type InP (p-InP) doped to a certain extent. In order to reduce electrical resistance, a contact layer made of a mixed crystal such as InGaAsP may be formed on the n-type semiconductor layer 106 and the p-type semiconductor layer 107, and an n-type electrode 108 and a p-type electrode 109 may be formed on the contact layer.

[0033] In the first embodiment, the active layer 103 is formed on the substrate 101 from a first region 121 to a second region 122 that is continuous with the first region 121. The diffraction grating 105 is formed at a uniform pitch from the first region 121 to the second region 122. The diffraction grating 105 is provided with a refractive index change at a uniform pitch in the waveguiding direction and without a phase shift from first region 121 to first region 121.

[0034] Furthermore, the equivalent refractive index of the second region 122 is made different from that of the first region 121. The first region 121 constitutes a distributed feedback (DFB) type semiconductor laser. In this example, the active layer 103 formed in the second region 122 has a different thickness from that of the active layer 103 formed in the first region 121, thereby making the equivalent refractive index of the second region 122 different from that of the first region 121.

[0035] In addition, by making the active layer 103 formed in the second region 122 have a different composition from the active layer 103 formed in the first region 121, the equivalent refractive index of the second region 122 can be made different from the equivalent refractive index of the first region 121. For example, In 1-x Ga x As y P 1-y By making the x and y of the first region 121 and the second region 122 different from each other, the equivalent refractive index of the second region 122 can be made different from the equivalent refractive index of the first region 121. 1-x-y Ga x Al y By making the x and y of the active layer 103 made of As different between the first region 121 and the second region 122, the equivalent refractive index of the second region 122 can be made different from the equivalent refractive index of the first region 121.

[0036] As described above, the cross-sectional equivalent refractive index is different between the first region 121 and the second region 122. Therefore, simply by providing the diffraction grating 105 having a uniform period without periodic variation, the resonator can be provided with regions having different Bragg wavelengths.

[0037] Here, the difference in the Bragg wavelength between the first region 121 and the second region 122, which is determined from the refractive index difference between the active layer 103 and the claddings above and below the active layer 103 and the period of the diffraction grating 105, is set to about half the stop band width of the first region 121 (second region 122). With this configuration, when the second region 122 is considered to be a distributed Bragg reflector (DBR) portion, the reflectance is not uniform for the two oscillation modes of the first region 121, and one of the modes becomes the oscillation mode in the first region 121. In this way, the semiconductor laser according to the first embodiment achieves single-mode operation based on a principle similar to that of a DR laser.

[0038] As described above, according to the first embodiment, a diffraction grating with a uniform pitch enables single-mode laser oscillation while suppressing spatial hole burning, and stable single-mode characteristics can be obtained in a structure in which the product of the length L in the optical axis direction and the coupling coefficient κ of the diffraction grating is large.

[0039] Next, a method for manufacturing a semiconductor laser according to the first embodiment will be described with reference to Figures 2A to 2F. In Figures 2A to 2F, (a) shows a cross section parallel to the optical axis (waveguiding) direction, and (b) shows a cross section taken along line aa' in (a).

[0040] First, a substrate (silicon substrate) 101 is prepared, which includes a lower cladding layer 102 made of silicon oxide. For example, the lower cladding layer 102 is formed by thermally oxidizing the main surface of the substrate 101. Next, as shown in FIG. 2A , a first semiconductor layer 104a is formed on the lower cladding layer 102 over the entire region, including the first region 121 and the second region 122.

[0041] For example, a sacrificial layer made of InGaAs and a first semiconductor layer 104a made of InP are sequentially epitaxially grown on a growth substrate made of InP (not shown). For example, each layer can be grown by well-known methods such as metalorganic vapor phase epitaxy or molecular beam epitaxy. Next, the top surface of this epitaxially grown growth substrate is directly bonded to the surface of the lower cladding layer 102 of the substrate 101 using a known wafer bonding technique, and then the growth substrate and the sacrificial layer are removed. As a result, the lower cladding layer 102 and the first semiconductor layer 104a are formed on the substrate 101, as shown in FIG. 2A .

[0042] Next, as shown in FIG. 2B , the active layer 103 and the second semiconductor layer 104b made of InP are sequentially formed on the first semiconductor layer 104a. For example, each layer can be grown sequentially by metalorganic vapor phase epitaxy or molecular beam epitaxy. For example, the active layer 103 and the second semiconductor layer 104b can be formed using selective growth masks extending in the optical axis direction and sandwiching the regions in which the active layer 103 and the second semiconductor layer 104b are to be formed. The selective growth masks can be made of an insulating material such as SiO . By making the width of the selective growth mask wider in the second region 122 than in the first region 121 in a plan view normal to the plane of the substrate 101, the active layer 103 and the second semiconductor layer 104b formed in the second region 122 can be made thicker than the active layer 103 and the second semiconductor layer 104b formed in the first region 121.

[0043] When the selective growth described above is used, the thickness of each selectively grown layer changes almost uniformly depending on the width of the selective growth mask. For example, if the active layer 103 having a thickness of 100 nm and the second semiconductor layer 104b having a thickness of 120 nm are grown sequentially in the first region 121, the thickness of the active layer 103 in the second region 122 becomes 1.1 times thicker, at 110 nm, and the thickness of the second semiconductor layer 104b also becomes approximately 1.1 times thicker, at 132 nm.

[0044] The total thickness of the entire semiconductor layer fabricated through the above steps is set to, for example, 150 to 350 nm, with reference to Patent Document 1. The difference in thickness between the first region 121 and the second region 122 can be precisely controlled to the order of several nm by devising the shape, width, and arrangement of the selective growth mask.

[0045] By appropriately setting the thickness of the first region 121 and the thickness of the second region 122, the difference in the Bragg wavelengths between the first region 121 and the second region 122, which is calculated from the refractive index difference and the period of the diffraction grating, can be set to approximately half the stop band width of the first region 121 (second region 122).

[0046] Next, as shown in FIG. 2C, a mask 131 made of an inorganic material such as SiO2 is formed by known lithography technology, and an etching process using the formed mask 131 is performed to form the active layer 103 and the second semiconductor layer 104b into a ridge shape with a predetermined width (800 nm) extending in the optical axis direction.

[0047] 2D, using mask 131 as a selective growth mask, compound semiconductor layer 131 made of, for example, InP is regrown on first semiconductor layer 104a exposed on both sides of ridge-shaped active layer 103. This growth can be performed by well-known methods such as metalorganic chemical vapor deposition and molecular beam epitaxy.

[0048] When InP is grown as described above, it is typically exposed to high temperatures of around 600°C. Under these conditions, a structure incorporating heterogeneous materials undergoes thermal strain due to the difference in thermal expansion coefficients between the materials as the temperature changes from room temperature. For example, if the substrate is made of silicon (thermal expansion coefficient 2.62), the difference in thermal expansion coefficient with InP (thermal expansion coefficient 4.6) and the thermal strain caused by the temperature change from room temperature to 600°C are generally unacceptable, resulting in numerous defects in the InP layer. For these reasons, it is common knowledge that crystal growth after direct bonding of InP and Si is difficult. However, as shown in Non-Patent Document 2, in a thin-film structure where the semiconductor thickness is limited to 430 nm or less, InP can elastically deform without generating dislocations, enabling high-quality crystal growth.

[0049] Next, after removing the mask 131, the compound semiconductor layer 131 on one side of the ridge-shaped active layer 103 and second semiconductor layer 104b is made n-type and the compound semiconductor layer 131 on the other side is made p-type by a technique such as selective doping. As a result, the n-type semiconductor layer 106 and the p-type semiconductor layer 107 are formed, as shown in Fig. 2E. Note that the n-type semiconductor layer 106 and the p-type semiconductor layer 107 can be formed by supplying a dopant during the above-mentioned regrowth process.

[0050] 2F, a diffraction grating 105 is formed on the upper surface of the second semiconductor layer 104b. The width of the diffraction grating 105 can be formed to extend over a portion of the n-type semiconductor layer 106 and a portion of the p-type semiconductor layer 107 on both sides of the second semiconductor layer 104b. For example, the diffraction grating 105 can be fabricated by providing a periodic refractive index change (unevenness) on the surface of the second semiconductor layer 104b (the n-type semiconductor layer 106 and the p-type semiconductor layer 107) by a technique such as selective etching using an inorganic material mask, a resist mask, or the like formed by lithography.

[0051] In the above description, unevenness is formed on the second semiconductor layer 104b, but a layer of a material that is transparent to the operating wavelength, such as SiO2 or SiN, can be separately formed on the upper surface of the second semiconductor layer 104b, and this layer can be processed to form the diffraction grating 105.

[0052] Next, an n-type electrode 108, a p-type electrode 109, and a protective layer 110 are formed to obtain the semiconductor laser shown in FIG. 1. Each electrode can be formed by, for example, the well-known lift-off method. The protective layer 110 can be formed from a coating film of benzocyclobutene (BCB). Alternatively, the protective layer 110 can be formed from SiO2, SiN, or the like deposited by p-CVD.

[0053] In the above description, the diffraction grating 105 is formed in the second semiconductor layer 104b above the active layer 103 when viewed from the substrate 101 side, but this is not limiting. The diffraction grating may be present in a region that couples with the mode field of the optical waveguide having the active layer 103 as its core. For example, a periodic structure may be formed in the active layer 103 that is formed in a ridge shape to form a diffraction grating.

[0054] Second Embodiment Next, a semiconductor laser according to a second embodiment of the present invention will be described with reference to Fig. 3. This semiconductor laser includes an active layer 103 formed on a substrate 101, a diffraction grating 105 formed on the active layer 103, an n-type semiconductor layer 106 and a p-type semiconductor layer 107 formed in contact with the active layer 103, an n-type electrode 108 connected to the n-type semiconductor layer 106, and a p-type electrode 109 connected to the p-type semiconductor layer 107.

[0055] A lower cladding layer 102 is formed on a substrate 101, and an active layer 103 is formed thereon. The active layer 103 is sandwiched between a first semiconductor layer 104a and a second semiconductor layer 104b in the vertical direction when viewed from the substrate 101. The stacked structure of the first semiconductor layer 104a, the active layer 103, and the second semiconductor layer 104b is sandwiched between an n-type semiconductor layer 106 and a p-type semiconductor layer 107. The n-type semiconductor layer 106 and the p-type semiconductor layer 107 are formed to sandwich the active layer 103 in a direction parallel to the plane of the substrate 101.

[0056] The active layer 103 is formed on the substrate 101 from a first region 121 to a second region 122 continuous with the first region 121. The equivalent refractive index of the second region 122 is different from that of the first region 121. The first region 121 constitutes a distributed feedback (DFB) semiconductor laser. For example, by making the active layer 103 formed in the second region 122 have a different thickness from that of the active layer 103 formed in the first region 121, the equivalent refractive index of the second region 122 can be made different from that of the first region 121. Furthermore, by making the active layer 103 formed in the second region 122 have a different composition from that of the active layer 103 formed in the first region 121, the equivalent refractive index of the second region 122 can be made different from that of the first region 121.

[0057] Additionally, the upper surfaces of the second semiconductor layer 104b, the n-type semiconductor layer 106, and the p-type semiconductor layer 107 between the n-type electrode 108 and the p-type electrode 109 are protected by a protective layer 110. The protective layer 110 on the second semiconductor layer 104b functions as an upper cladding layer.

[0058] The above-described configuration is the same as that of the above-described embodiment 1. In embodiment 2, a core 112 is provided that is embedded in the lower cladding layer 102 and formed along the active layer 103, and an optical waveguide is provided by the core 112. The core 112 is formed from the first region 121 to the second region 122, and is arranged in a region that couples with the mode field of the active layer 103.

[0059] In the second embodiment, a diffraction grating 105a is formed in the core 112. Therefore, the diffraction grating 105a is formed in a region that couples with the mode field of the active layer 103. The diffraction grating 105a is formed at a uniform pitch from the first region 121 to the second region 122. The diffraction grating 105a is provided with a refractive index change at a uniform pitch in the waveguiding direction and without a phase shift from first region 121 to first region 121. In this example, the diffraction grating 105a is formed on the upper surface of the core 112 on the active layer 103 side.

[0060] The core 112 may be a channel type having a rectangular cross section perpendicular to the optical axis direction, or a rib type having a slab layer, or may be made of silicon, hydrogenated amorphous silicon, or the like.

[0061] According to the above-described configuration, laser light oscillated in the active layer 103 can be coupled to the optical waveguide formed by the core 112. With this configuration, the semiconductor laser according to the embodiment can be used as a light source for silicon photonics using a Si optical waveguide (not shown) optically connected to the optical waveguide formed by the core 112. Furthermore, by forming an optical waveguide having a supermode using the active layer 103 and the core 112, it is possible to enhance light confinement in the core 112 and reduce internal loss.

[0062] As described above, in the second embodiment as well, the cross-sectional equivalent refractive index is different between the first region 121 and the second region 122. Therefore, simply by providing the diffraction grating 105a having a uniform period without periodic variation, the resonator can be provided with regions having different Bragg wavelengths.

[0063] As described above, the difference in the Bragg wavelengths between the first region 121 and the second region 122, which is determined from the refractive index difference between the active layer 103 and the claddings above and below the active layer 103 and the period of the diffraction grating 105, is set to about half the stop band width of the first region 121 (second region 122). With this configuration, when the second region 122 is considered to be a distributed Bragg reflector (DBR) portion, the reflectance is not uniform for the two oscillation modes of the first region 121, and one of the modes becomes the oscillation mode in the first region 121. In this way, the semiconductor laser according to the second embodiment also achieves single-mode operation based on a principle similar to that of the DR laser.

[0064] In the second embodiment described above, a diffraction grating with a uniform pitch also enables single-mode laser oscillation while suppressing spatial hole burning, and stable single-mode characteristics can be obtained in a structure in which the product of the length L in the optical axis direction and the coupling coefficient κ of the diffraction grating is large.

[0065] When the active layer 103 has a multiple quantum well structure, the quantum wells in this region of the active layer 103 can be disordered and deactivated by, for example, implanting ions into the second region 122. Disordering the quantum wells changes the band gap while maintaining the average refractive index in the second region 122, suppressing the gain in the second region 122 and reducing optical loss. Disordering the quantum wells suppresses the generation of photons caused by carriers injected into the active layer 103 in the second region 122, and prevents destabilization of oscillation characteristics due to mode competition caused by different Bragg wavelengths between the first region 121 and the second region 122.

[0066] Furthermore, by increasing the degree of disordering of the quantum wells described above, it is possible to change the structure to one that is transparent (has little light absorption) to the oscillation wavelength determined by the first region 121 without injecting carriers into the second region 122. Therefore, by using the above-described configuration, a structure is created that does not allow ineffective current that does not contribute to oscillation to flow, and it becomes possible to drive the laser with less power.

[0067] The above-described disordering of the quantum wells can be achieved by forming an insulating layer made of an insulating material such as SiO2 on the surface of the second semiconductor layer 104b by p-CVD or the like, and then performing a heat treatment to diffuse point defects generated at the interface between the second semiconductor layer 104b and the insulating layer to the active layer 103. Also, as shown in Figure 4, the quantum wells can be disordered by diffusing point defects generated in the second semiconductor layer 104b by ion implantation into the active layer 103 by performing a heat treatment. The above-described configuration for disordering the quantum wells can be combined with the first and second embodiments described above.

[0068] Third Embodiment Next, a semiconductor laser according to a third embodiment of the present invention will be described with reference to FIG.

[0069] In the above-described first and second embodiments, the active layer 103 in the second region 122 is made thicker than the active layer 103 in the first region 121, so that the equivalent refractive index of the second region 122 is different from the equivalent refractive index of the first region 121.

[0070] In contrast to this, in the third embodiment, the active layer 103 formed in the second region 122 has a width different from that of the active layer 103 formed in the first region 121, thereby making the equivalent refractive indexes of the two different. In Fig. 5, the width of the active layer 103 changes discontinuously from the first region 121 to the second region 122, but by gradually changing the width between these regions into a tapered shape, adiabatic mode conversion can be achieved.

[0071] The third embodiment described above can be combined with the first and second embodiments described above.

[0072] [Fourth Embodiment] Next, a semiconductor laser according to a fourth embodiment of the present invention will be described with reference to Fig. 6. In the fourth embodiment, a third region 123 is provided on the side of the first region 121 opposite to the second region 122 in the optical axis direction. The third region 123 is continuous with the first region 121. The second region 122 and the third region 123 sandwich the first region 121.

[0073] The active layer 103 is also formed from the first region 121 to the third region 123. The diffraction grating 105 is formed at a uniform pitch from the first region 121 to the third region 123 in addition to the second region 122. The third region 123 has a different equivalent refractive index from the first region 121.

[0074] For example, the active layer 103 formed in the third region 123 has a different thickness or width from the active layer 103 formed in the first region 121. Furthermore, for example, the active layer 103 formed in the third region 123 has a different composition from the active layer 103 formed in the first region 121.

[0075] In the first, second, and third embodiments described above, the second region 122, which has a different equivalent refractive index, is provided on one end of the first region 121, and the second region 121 functions as a DBR section (filter). In the fourth embodiment, the third region 123, which has a different equivalent refractive index, is provided on the other end of the first region 121, and the pitch of the diffraction grating 105 in this region is also uniform. For example, by locating one of the two modes at the stop band edge in the first region 121 at the center of the Bragg wavelengths of the second region 122 and the third region 123, it is possible to more strongly confine light within the resonator and aim to lower the threshold value of laser oscillation.

[0076] This configuration can also be combined with the configuration of embodiment 2. In addition to the second region 122, the quantum wells of the active layer 103 can also be disordered and inactivated in the third region 123.

[0077] Next, specific design guidelines for realizing the semiconductor laser according to the embodiment will be described using calculation results.

[0078] FIG. 7 shows the optical axis direction along the entire active layer with a length of 80 μm and a wavelength of κ=500 cm -1 7 shows the relationship between the total length of the first and second regions, the length of the second region being x μm, and the threshold gain of the entire semiconductor laser including the first and second regions, when the total length of the first and second regions is 80 μm.

[0079] The emission spectra from the front and rear of the resonator for each value of x are shown in Fig. 8. x = 0 is a simple uniform diffraction grating, and in this configuration the threshold gains on both ends of the stop band of the DFB laser are equal.

[0080] As shown in Figure 7, when the length x of the second region is increased to approximately 10 to 15 μm, the threshold gain of the first mode decreases slightly, while the threshold gain of the second mode increases. This can be understood as being due to the second region acting as a mirror for one of the oscillation modes in the first region. Furthermore, when the value of x is increased to approximately 30 μm, the threshold gain of the first mode increases. This can be understood as being due to the insufficient confinement of light in the optical axis direction within the resonator as the first region becomes shorter. Furthermore, it is expected that competition between the two modes will occur as the ratio of the first region to the second region approaches 1:1. From the above calculations, when the combined length of the first and second regions is approximately 80 μm, the appropriate length of the second region is considered to be approximately 10 to 15 μm.

[0081] In this calculation, we have described a design for obtaining stable single-mode oscillation by controlling the oscillation peak wavelength determined by the first region to approximately the center of the reflection spectrum of the second region. However, by controlling the oscillation peak wavelength determined by the first region to a region where the reflection spectrum of the second region changes sharply with wavelength, it is possible to vary the threshold carrier density due to slight wavelength fluctuations during direct modulation, thereby suppressing fluctuations in carrier concentration in the active layer and thereby achieving the detuned loading effect, which achieves high-speed operation.

[0082] Next, structural changes required to achieve the desired refractive index change necessary to obtain the effects of the semiconductor laser according to the embodiment will be described using calculation results. Figure 9 shows the results of calculations using the film mode matching method to determine the equivalent refractive index of the cross-sectional mode when the thickness of the active layer is changed. Assuming thickness changes due to selective growth, the calculation was performed assuming a basic structure with an active layer thickness of 100 nm, and first and second semiconductor layers made of InP above and below the active layer with thicknesses of 125 nm, and these thicknesses always changing proportionally.

[0083] In the case of selective growth, the refractive index of the active layer changes slightly along with the thickness in the second region, but for simplicity, it is assumed to be constant here. The width of the active layer was set to 0.8 μm. The refractive index of the active layer was set to 3.4, assuming InGaAsP. The refractive index of the first and second semiconductor layers made of InP was set to 3.169. The protective layers, which serve as the lower and upper cladding layers, were made of SiO2 and had a refractive index of 1.45.

[0084] For example, if the center of the Bragg wavelength of the first region is designed to be 1300 nm, the center of the Bragg wavelength of the second region must be set to approximately 1305 nm or 1295 nm to achieve a detuning of 5 nm. Because the oscillation wavelength is proportional to the equivalent refractive index, the refractive index change required for the above detuning is approximately 0.11, or approximately 0.4%. Figure 9 shows that the thickness change required to achieve this level of refractive index change is approximately 3 nm.

[0085] It is not easy to control the thickness difference by processing. On the other hand, crystal growth allows for the formation of quantum wells with thickness control on the order of 0.1 nm, and thickness changes by selective growth are also highly accurate and reproducible. Therefore, thickness control of 3 nm is quite feasible.

[0086] Figure 10 shows the results of calculating the equivalent refractive index when the width of the active layer is changed in the same manner as above. The thickness of the active layer was set to 100 nm. The thicknesses of the first and second semiconductor layers, made of InP above and below the active layer, were set to 125 nm. It can be seen that a change in the refractive index of approximately 0.11 between the first and second regions requires changing the width of the active layer by 200 to 300 nm. The width of the active layer varies depending on factors such as the accuracy of lithography and the undercutting effect during etching, but it is relatively easy to achieve an accuracy of several tens of nanometers in either case. The width controllability of 200 to 300 nm required for a typical structure of a semiconductor laser according to the embodiment is fully achievable.

[0087] As described above, according to the embodiment of the present invention, an active layer is formed from the first region to the second region, a diffraction grating is formed at a uniform pitch from the first region to the second region, and the second region has an equivalent refractive index different from that of the first region. Therefore, stable single-mode characteristics can be obtained in a structure in which the product of the length in the optical axis direction and the coupling coefficient of the diffraction grating is large.

[0088] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0089] 101...substrate, 102...lower cladding layer, 103...active layer, 104a...first semiconductor layer, 104b...second semiconductor layer, 105...diffraction grating, 106...n-type semiconductor layer, 107...p-type semiconductor layer, 108...n-type electrode, 109...p-type electrode, 110...protective layer.

Claims

1. A semiconductor laser comprising: an active layer formed from a first region on a substrate to a second region continuous with the first region; a diffraction grating formed at a uniform pitch from the first region to the second region on the substrate; a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer; an n-type electrode connected to the n-type semiconductor layer; and a p-type electrode connected to the p-type semiconductor layer, wherein the second region has a different equivalent refractive index from the first region.

2. A semiconductor laser according to claim 1, wherein the active layer formed in the second region has a different thickness or width from the active layer formed in the first region.

3. A semiconductor laser according to claim 1, wherein the active layer formed in the second region has a different composition from the active layer formed in the first region.

4. A semiconductor laser according to any one of claims 1 to 3, wherein the active layer is formed from the first region opposite the second region to a third region continuous with the first region, the diffraction grating is formed at a uniform pitch from the first region to the third region in addition to the second region, and the third region has an equivalent refractive index different from that of the first region.

5. A semiconductor laser according to claim 4, wherein the active layer formed in the third region has a thickness or width different from that of the active layer formed in the first region.

6. A semiconductor laser according to claim 4, wherein the active layer formed in the third region has a different composition from the active layer formed in the first region.

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