Semiconductor laser

Abstract

This semiconductor laser is a DFB laser comprising: an active layer (103) formed in the shape of a core extending in a waveguide direction over a substrate (101); and a diffraction grating (110) inside a resonator. The semiconductor laser further comprises a non-formation region (111) in which the diffraction grating (110) is not formed, in a width direction perpendicular to the waveguide direction of the active layer (103). The non-formation region (111) is provided in a central portion in the width direction of the active layer (103). Provision of the non-formation region (111) reduces (decreases) the overlap between the diffraction grating (110) and the waveguide mode of light guided using the active layer (103) as a core, and can hence lower the coupling factor.

Description

semiconductor laser

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

[0002] With recent advances in silicon photonics technology and heterogeneous material integration technology, much research has been conducted on semiconductor laser integration technology on silicon photonics using, for example, SOI (Silicon on Insulator) substrates. The most common application of silicon photonics is optical communications, and semiconductor lasers used as light sources for this application are required to oscillate in a single longitudinal mode to suppress the effects of dispersion in the communication path. To achieve a semiconductor laser that oscillates in a single longitudinal mode, a diffraction grating, which is a refractive index modulation structure with a periodicity in the optical axis direction, is generally used (Patent Documents 1 and 2, Non-Patent Documents 1 and 2).

[0003] JP 2018-006440 A JP 2019-204814 A

[0004] T. Aihara et al., "Membrane III-V / Si DFB Laser Using Uniform Grating and Width-Modulated Si "Waveguide", Journal of Lightwave Technology, vol. 38, no. 11, pp. 2961-2967, 2020.

[0005] To fabricate a single-longitudinal-mode semiconductor laser, a periodic change in refractive index is achieved by forming periodic irregularities in the optical confinement (separate confined heterostructure (SCH)) layer adjacent to the active layer. In conventional thin-film III-V lasers integrated on Si, the periodic irregularities are formed in the upper InP cladding layer or the SiN layer formed thereon. The strength of optical resonance (coupling coefficient) due to the diffraction grating is determined by the degree of change in refractive index, which can be changed by adjusting the depth of the recesses.

[0006] However, since the etching process for forming the recesses is performed on the entire surface of the wafer, the depth of all the recesses is substantially the same. Therefore, the depth of the recesses, which determines the coupling coefficient, cannot be arbitrarily set for each element formed at a different location on the wafer in a single etching process. In order to arbitrarily set the depth of the recesses for each element formed at a different location on the wafer, multiple etching processes must be performed, each time changing the etched region. Thus, there is a problem in that it is not easy to form a diffraction grating with a different coupling coefficient for each element formed at a different location on the wafer.

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to make it possible to form a diffraction grating by more easily changing the coupling coefficient for each element formed at a different location on a wafer.

[0008] A semiconductor laser according to the present invention comprises a first cladding layer formed on a substrate, an active layer formed on the first cladding layer in a core shape extending in a waveguiding direction, p-type and n-type semiconductor layers formed in contact with the active layer and sandwiching the active layer, a second cladding layer formed on the active layer, p-electrodes and n-electrodes connected to the p-type and n-type semiconductor layers, and a resonator consisting of a diffraction grating formed on the active layer, and an area where the diffraction grating is not formed is provided in the width direction of the active layer perpendicular to the waveguiding direction.

[0009] As described above, according to the present invention, an area where no diffraction grating is formed is provided in the width direction perpendicular to the waveguiding direction of the active layer, so that diffraction gratings can be formed with easier changes in coupling coefficient for each element formed at a different location within the wafer.

[0010] FIG. 1A is a cross-sectional view showing the configuration of a semiconductor laser according to a first embodiment of the present invention. FIG. 1B is a plan view showing a partial configuration of a semiconductor laser according to the first embodiment of the present invention. FIG. 2 is an explanatory diagram explaining parameters used in calculating the coupling coefficient when an unformed region 111 is provided. FIG. 3A is a characteristics diagram showing the relationship between the unformed region width and the coupling coefficient when the groove depth D of each groove constituting the diffraction grating 110 is set to 20 nm, 30 nm, 40 nm, and 50 nm. FIG. 3B is a characteristics diagram showing the results of normalizing the relationship shown in FIG. 3A with the value when the unformed region width is set to 0. FIG. 3C is a characteristics diagram showing the calculation results of the coupling coefficient and the equivalent refractive index as a waveguide, as well as the positional misalignment E when the unformed region width is 0.2 μm and the groove depth D is 30 nm. FIG. 4 is a plan view showing a partial configuration of another semiconductor laser according to the first embodiment of the present invention. FIG. 5 is a plan view showing a partial configuration of another semiconductor laser according to the first embodiment of the present invention. FIG. 6 is a plan view showing a partial configuration of another semiconductor laser according to the first embodiment of the present invention. Fig. 7A is a cross-sectional view showing the configuration of a semiconductor laser according to embodiment 2 of the present invention. Fig. 7B is a plan view showing a partial configuration of a semiconductor laser according to embodiment 2 of the present invention. Fig. 8 is a plan view showing a partial configuration of another semiconductor laser according to embodiment 2 of the present invention.

[0011] A semiconductor laser according to an embodiment of the present invention will now be described.

[0012] 1A and 1B, a semiconductor laser according to a first embodiment of the present invention will be described. This semiconductor laser is a distributed feedback (DFB) laser that includes an active layer 103 formed in a core shape extending in the waveguiding direction on a substrate 101, and includes a diffraction grating 110 within a resonator.

[0013] This semiconductor laser first has a first cladding layer 102 formed on a substrate 101, and an active layer 103 on the first cladding layer 102. The substrate 101 is made of, for example, Si, and the first cladding layer 102 is made of, for example, silicon oxide. The semiconductor laser also has a p-type semiconductor layer 104 and an n-type semiconductor layer 105 formed in contact with the active layer 103 and sandwiching the active layer 103. The semiconductor laser also has a second cladding layer 106 formed on the active layer 103, and a p-electrode 107 and an n-electrode 108 connected to the p-type semiconductor layer 104 and the n-type semiconductor layer 105.

[0014] In this example, the p-type semiconductor layer 104 and the n-type semiconductor layer 105 are formed by introducing impurities into a semiconductor layer 109 made of, for example, InP. The active layer 103 is formed between the p-type semiconductor layer 104 and the n-type semiconductor layer 105 and is embedded in the semiconductor layer 109. The active layer 103 may have a quantum well structure in which well layers and barrier layers made of, for example, InGaAsP are alternately stacked.

[0015] Here, the diffraction grating 110 is configured by arranging (forming) grooves (slits) extending in a width direction perpendicular to the waveguide direction at multiple locations. The grooves arranged at multiple locations are arranged at predetermined intervals in the waveguide direction to form a lattice pattern. When viewed in a cross section along the groove arrangement direction, the diffraction grating 110 is composed of recesses and protrusions adjacent to the recesses. For example, the diffraction grating 110 can be formed at the interface between the semiconductor layer 109 and the second cladding layer 106. In this case, the diffraction grating 110 can be configured by grooves formed at multiple locations on the top surface of the semiconductor layer 109 above the active layer 103, arranged at predetermined intervals in the waveguide direction. Each of the multiple grooves forming the diffraction grating 110 formed on the top surface of the semiconductor layer 109 is buried in the second cladding layer 106.

[0016] In addition to the above-described configuration, this semiconductor laser includes an unformed region 111, where the diffraction grating 110 is not formed, in the width direction perpendicular to the waveguiding direction of the active layer 103. In the first embodiment, the unformed region 111 is located in the center in the width direction of the active layer 103. By providing the unformed region 111, the overlap between the waveguiding mode of light guided using the active layer 103 as a core and the diffraction grating 110 is reduced (diminished), and therefore the coupling coefficient can be reduced.

[0017] The coupling coefficient can be changed by changing the width of the unformed region 111 perpendicular to the waveguiding direction. Therefore, by changing the width of the unformed region 111 for each element formed at a different location within the wafer, the coupling coefficient can be changed for each element. Thus, according to the first embodiment, it is possible to form a diffraction grating by more easily changing the coupling coefficient for each element formed at a different location within the wafer.

[0018] The effect of reducing the coupling coefficient by providing the unformed region 111 will be described below. The results of calculating the coupling coefficient using the parameters shown in Fig. 2 will be described below. In this calculation, the width of the unformed region 111 (unformed region width), the positional deviation E between the center position of the active layer 103 and the center position of the unformed region 111 in the width direction perpendicular to the waveguiding direction, and the depth D of each groove constituting the diffraction grating 110 were used.

[0019] The coupling coefficient of the diffraction grating was calculated by the film mode matching (FMM) method.

[0020] Figure 3A shows the relationship between the width of the unformed region and the coupling coefficient when the groove depth D is set to 20 nm, 30 nm, 40 nm, and 50 nm. Although there is some calculation error, it was found that the coupling coefficient decreases linearly with increasing width of the unformed region. To visualize the decrease rate, the results are normalized by the value when the width of the unformed region is set to 0 (conventional structure), and are shown in Figure 3B. Within this calculation range, the coupling coefficient changes at a constant rate regardless of the groove depth D, and the coupling coefficient can be reduced to approximately 50% at an unformed region width of approximately 0.44 μm.

[0021] Figure 3C shows the calculation results of the misalignment E, the coupling coefficient, and the equivalent refractive index of the waveguide when the width of the unformed region is 0.2 μm and the groove depth D is 30 nm. Electron beam lithography or the like is used to form the diffraction grating pattern, and it can generally be drawn with a positional accuracy of about 0.05 to 0.1 μm. Even with a positional accuracy of 0.1 μm, it can be seen that the amount of change that the misalignment E causes to the coupling coefficient is negligible. Furthermore, although the equivalent refractive index of the optical waveguide with the active layer 103 as its core changes slightly, the rate of change at a positional accuracy of 0.1 μm is about 0.01%.

[0022] As shown in FIG. 4 , the width of the unformed region 111 can be varied at different positions in the waveguide direction. In the example shown in FIG. 4 , in the region above the active layer 103, the width of the unformed region 111 gradually increases and decreases as the position shifts from one side to the other in the waveguide direction. In this example, the width of the unformed region 111 in the center of the active layer 103 in the waveguide direction is wider than the width of the unformed region 111 at both ends in the waveguide direction. Therefore, the coupling coefficient of the diffraction grating 110 a in the center of the active layer 103 in the waveguide direction is lower than the coupling coefficient at both ends in the waveguide direction. By using the diffraction grating 110 a with a reduced coupling coefficient in part of the waveguide direction, the laser can be operated as a DFB laser with a gradual phase shift.

[0023] 5, the diffraction grating 110b may have a connection region 112 that connects the grooves of the p-type semiconductor layer 104 and the n-type semiconductor layer 105. If the grooves are separated and arranged in a periodically changing lattice pattern in the region of the optical waveguide mode centered on the active layer 103, the diffraction grating will function. The connection region may also extend into the upper part of the active layer 103.

[0024] Furthermore, the planar shape of the portion of each groove constituting the diffraction grating 110b facing the center of the active layer 103 is not limited to a partial rectangle. For example, as shown in FIG. 6A, the planar shape of the portion of each groove constituting the diffraction grating 110b facing the center of the active layer 103 can be semicircular. As shown in FIG. 6B, the planar shape of the portion of each groove constituting the diffraction grating 110b facing the center of the active layer 103 can be triangular. As shown in FIG. 6C, the planar shape of the portion of each groove constituting the diffraction grating 110b facing the center of the active layer 103 can also be corrugated.

[0025] 7A and 7B, a semiconductor laser according to a second embodiment of the present invention will be described. This semiconductor laser is a DFB (Distributed Feedback) laser that includes an active layer 103 formed on a substrate 101 in a core shape extending in the waveguide direction, and includes a diffraction grating 110′ within a resonator.

[0026] Similar to the first embodiment described above, this semiconductor laser includes a first cladding layer 102 formed on a substrate 101, and an active layer 103 on the first cladding layer 102. The substrate 101 is made of, for example, Si, and the first cladding layer 102 is made of, for example, silicon oxide. The semiconductor laser also includes a p-type semiconductor layer 104 and an n-type semiconductor layer 105 formed in contact with the active layer 103 and sandwiching the active layer 103. The semiconductor laser also includes a second cladding layer 106 formed on the active layer 103, and a p-electrode 107 and an n-electrode 108 connected to the p-type semiconductor layer 104 and the n-type semiconductor layer 105.

[0027] In this example, the p-type semiconductor layer 104 and the n-type semiconductor layer 105 are formed by introducing impurities into a semiconductor layer 109 made of, for example, InP. The active layer 103 is formed between the p-type semiconductor layer 104 and the n-type semiconductor layer 105 and is embedded in the semiconductor layer 109.

[0028] In addition to the above-described configuration, this semiconductor laser includes an unformed region 111' where the diffraction grating 110' is not formed, in the width direction perpendicular to the waveguiding direction of the active layer 103. In the second embodiment, the diffraction grating 110 is disposed in the center of the active layer 103 in the width direction, and the unformed regions 111' are disposed on both sides of the diffraction grating 110 so as to sandwich the diffraction grating 110. In the second embodiment as well, the provision of the unformed region 111' reduces (diminishes) the overlap between the waveguide mode of light guided using the active layer 103 as a core and the diffraction grating 110', thereby enabling a reduction in the coupling coefficient.

[0029] The coupling coefficient can be changed by changing the width of the unformed region 111' perpendicular to the waveguiding direction. Therefore, by changing the width of the unformed region 111' for each element formed at a different location within the wafer, the coupling coefficient can be changed for each element. In this way, in the second embodiment as well, it becomes possible to form a diffraction grating by more easily changing the coupling coefficient for each element formed at a different location within the wafer.

[0030] Furthermore, as shown in FIG. 8 , the width of the unformed region 111′a can be changed at different positions in the waveguide direction. In the second embodiment, the width of the diffraction grating 110′a changes at different positions in the waveguide direction. In the example shown in FIG. 8 , in the region above the active layer 103, the width of the unformed region 111′a gradually increases and decreases as the position shifts from one side to the other in the waveguide direction. In this example, the width of the unformed region 111′a in the center of the active layer 103 in the waveguide direction is wider than the width of the unformed region 111′a at both ends in the waveguide direction. Therefore, the width of the diffraction grating 110′a at both ends in the waveguide direction is wider than the width of the diffraction grating 110′a in the center in the waveguide direction.

[0031] As described above, by changing the width of the unformed region 111'a, the coupling coefficient of the diffraction grating 110'a in the central portion in the waveguiding direction of the active layer 103 is lower than the coupling coefficient at both ends in the waveguiding direction. By using the diffraction grating 110'a in which the coupling coefficient is lower in part in the waveguiding direction, it is possible to operate the laser as a DFB laser with a gradual phase shift.

[0032] Although the shape of each groove constituting the diffraction grating in plan view is rectangular, this is not limited to this and the shape of each groove constituting the diffraction grating in plan view can also be a rectangle with rounded vertices.

[0033] As described above, according to the present invention, an area where no diffraction grating is formed is provided in the width direction perpendicular to the waveguiding direction of the active layer, so that it becomes possible to more easily change the coupling coefficient and form diffraction gratings for each element formed at a different location within the wafer.

[0034] 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.

[0035] 101...substrate, 102...first cladding layer, 103...active layer, 104...p-type semiconductor layer, 105...n-type semiconductor layer, 106...second cladding layer, 107...p-electrode, 108...n-electrode, 109...semiconductor layer, 110...diffraction grating, 111...unformed region.

Claims

1. A semiconductor laser comprising: a first cladding layer formed on a substrate; an active layer formed on the first cladding layer in a core shape extending in a waveguiding direction; p-type and n-type semiconductor layers formed in contact with the active layer and sandwiching the active layer; a second cladding layer formed on the active layer; a p-electrode and an n-electrode connected to the p-type and n-type semiconductor layers; and a resonator consisting of a diffraction grating formed on the active layer, wherein an area where the diffraction grating is not formed is provided in the width direction of the active layer perpendicular to the waveguiding direction.

2. A semiconductor laser according to claim 1, wherein the width of said unformed region varies at different positions in said waveguiding direction.

3. A semiconductor laser according to claim 1 or 2, wherein the unformed region is disposed at the center in the width direction of the active layer.

4. A semiconductor laser according to claim 1 or 2, wherein the diffraction grating is disposed in a central portion in the width direction of the active layer, and the unformed regions are disposed on both sides of the diffraction grating so as to sandwich the diffraction grating.

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