Semiconductor laser

Abstract

This semiconductor laser comprises, above a substrate (101), an active layer (103) that extends in the waveguide direction and that is formed in a core shape. The semiconductor laser also comprises a diffraction grating (110) in a resonator. A first clad layer (102) is formed on the substrate (101). The semiconductor laser has the active layer (103) above the first clad layer (102). The semiconductor laser also comprises a second clad layer (106) formed above the active layer (103). The diffraction grating (110) is formed on the side of the first clad layer (102) of the resonator or on the side of the second clad layer (106) and is formed separately from the boundary area between the core of the resonator and the first clad layer (102) or the second clad layer (106).

Description

Technical Field

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

Background Art

[0002] With the increase in communication traffic on the Internet and the like, higher speed and larger capacity of optical fiber transmission are required. In response to this demand, the development of digital coherent communication technology using coherent optical communication technology and digital signal processing technology has progressed, and 100G systems have been put into practical use. In such a communication system, a single-mode semiconductor laser is required as a light source for transmitting and receiving stations.

[0003] As a typical structure of an optical resonator for single-mode operation, a diffraction grating having a λ / 4 phase shift has been used. In this structure, phase inversion is caused by a phase shifter formed in a part of the diffraction grating provided in the resonator, enabling single-mode oscillation at the Bragg wavelength. This laser is called a λ / 4 shift DFB (Distributed Feedback) laser and has already been put into practical use. Also, for extending the transmission distance and expanding the transmission capacity, higher optical output and narrower linewidth of the DFB laser are required.

[0004] For higher optical output and narrower linewidth of the DFB laser, lengthening the DFB laser is effective. However, lengthening the DFB laser is limited by the following two problems.

[0005] First, when the DFB laser is lengthened, there is a problem that the oscillation mode becomes unstable due to the influence of spatial hole burning. When the DFB laser is lengthened, light is strongly localized in the phase shift region. In this strong light localization region, a large amount of carriers are consumed, resulting in a decrease in carrier density. Such a phenomenon in which a carrier distribution occurs in the resonator due to the light intensity distribution in the laser is called spatial hole burning. The change in carrier density causes a change in refractive index. As a result, a distribution occurs in the refractive index inside the resonator. The refractive index distribution leads to a decrease in the reflectivity of the optical resonator and a decrease in mode selectivity, making the oscillation mode of the laser unstable.

[0006] Second, when the DFB laser is lengthened, there is a problem that the reflectivity as a resonator increases, making it more susceptible to the influence of losses inside the resonator and resulting in a decrease in external quantum efficiency. Due to this problem, as a result, the increase in optical output is limited.

[0007] As a solution to these problems, it is effective to reduce the coupling coefficient of the diffraction grating as the DFB laser is lengthened. That is, by reducing the coupling coefficient of the diffraction grating, while lengthening the DFB laser, the reflectivity as a resonator can be kept constant, and the above problems can be avoided.

[0008] Generally, the diffraction grating is formed by forming periodic unevenness in a part of the active layer made of a III-V group compound semiconductor constituting the semiconductor laser (see Non-Patent Document 1). If the change in this unevenness is small, the coupling coefficient of the diffraction grating decreases.

[0009] However, in the formation of the diffraction grating, extremely fine processing is required, and there is a limit to reducing the change in unevenness. That is, there is a limit to reducing the coupling coefficient, and accordingly, there is a limit to lengthening the DFB laser. For example, the change in unevenness in the diffraction grating for making the coupling coefficient 20 cm -1 is extremely small, less than 5 nm, and it is obvious that it is difficult to manufacture.

Prior Art Documents

Non-Patent Documents

[0010] [Non-Patent Document 1] 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. [Summary of the Invention] [Problems to be Solved by the Invention]

[0011] As described above, in order to increase the high optical output and narrow the linewidth of a DFB laser, it is effective to increase the length of the DFB laser. However, in the conventional technology, there is a problem that it is not easy to increase the length of the DFB laser.

[0012] The present invention has been made to solve the above problems, and an object thereof is to enable easy elongation of a DFB laser. [Means for Solving the Problems]

[0013] The semiconductor laser according to the present invention includes a first cladding layer formed on a substrate, an active layer formed in a core shape extending in a waveguide direction on the first cladding layer, a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer with the active layer interposed therebetween, a second cladding layer formed on the active layer, and a p electrode and an n electrode connected to the p-type semiconductor layer and the n-type semiconductor layer, and is a semiconductor laser having a diffraction grating in a resonator, wherein the diffraction grating is formed on the side of the first cladding layer or the second cladding layer of the resonator and is formed away from the boundary region between the core of the resonator and the first cladding layer or the second cladding layer.

[0014] Also , halfThe method for manufacturing a semiconductor laser is the method for manufacturing a semiconductor laser described above, and the distance between the diffraction grating and the resonator is controlled by controlling the thickness of the layer between the diffraction grating and the resonator.

Advantages of the Invention

[0015] As described above, according to the present invention, since the diffraction grating is formed away from the boundary region between the core of the resonator and the first cladding layer or the second cladding layer, it becomes possible to easily lengthen the DFB laser.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2A

Figure 2B

Figure 3A

Figure 3B

Figure 3C

Figure 4A

Figure 4B

Figure 4C

Figure 4D

Figure 4E

Figure 5A

Figure 5B

Figure 5C

Figure 6A

Figure 6B

Figure 6C

Figure 6D

Figure 6E

Figure 6F

DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, a semiconductor laser according to an embodiment of the present invention will be described with reference to FIG. 1. This semiconductor laser is a DFB (Distributed Feedback) laser that includes an active layer 103 formed in a core shape extending in the waveguide direction on a substrate 101 and includes a diffraction grating 110 in a resonator.

[0018] First, a first cladding layer 102 is formed on the substrate 101, and the active layer 103 is provided on the first cladding layer 102. The substrate 101 is composed of, for example, Si, and the first cladding layer 102 is composed of, for example, silicon oxide. Further, a p-type semiconductor layer 104 and an n-type semiconductor layer 105 formed in contact with the active layer 103 with the active layer 103 interposed therebetween are provided. Also, 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 are provided.

[0019] 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 121 made of, for example, InP. Also, between the p-type semiconductor layer 104 and the n-type semiconductor layer 105, the active layer 103 is formed by being embedded in the semiconductor layer 121.

[0020] In addition to the above-described configuration, in the semiconductor laser according to the embodiment, the diffraction grating 110 is formed on the side of the first cladding layer 102 or the second cladding layer 106 of the resonator, and is formed away from the boundary region between the core of the resonator and the first cladding layer 102 or the second cladding layer 106. In the semiconductor laser according to the embodiment, the diffraction grating 110 is disposed in the first cladding layer 102 or the second cladding layer 106 in the thickness direction (lamination direction). In this example, the diffraction grating 110 is formed on the side of the second cladding layer 106 of the resonator and is formed away from the boundary region between the active layer 103 serving as the core of the resonator and the first cladding layer 102. The boundary region is, for example, the region between the second cladding layer 106 and the active layer 103.

[0021] Here, a conventional laterally injected DFB laser will be described with reference to FIG. 2A. The conventional DFB laser is a laterally injected DFB laser having substantially the same configuration as the semiconductor laser described with reference to FIG. 1 (see Non-Patent Document 1). In the conventional DFB laser, the diffraction grating 310 in the resonator is formed in the boundary region between the active layer 103 and the second cladding layer 106. In the example shown in FIG. 2A, the diffraction grating 310 is formed at the interface between the semiconductor layer 121 and the second cladding layer 106. The diffraction grating 310 is composed of periodic grooves arranged in the waveguide direction, formed in the semiconductor layer 121 above the active layer 103. Also, as shown in FIG. 2B, there is a configuration in which the diffraction grating 310a is formed by periodic grooves formed in a SiN layer formed in contact with the semiconductor layer 121.

[0022] Next, the calculation results of the coupling coefficients of the respective diffraction gratings in each of the semiconductor lasers (DFB lasers) described above are shown in FIGS. 3A, 3B, and 3C. FIG. 3A shows the calculation results in the conventional DFB laser described with reference to FIG. 2A. FIG. 3B shows the calculation results in the conventional DFB laser described with reference to FIG. 2B. FIG. 3C shows the calculation results in the semiconductor laser according to the embodiment described with reference to FIG. 1.

[0023] For example, as shown in FIG. 3A, when a groove is formed in the semiconductor layer 121 to form the diffraction grating 310, even if the depth of the groove of the diffraction grating 310 is only 10 nm, the coupling coefficient (kappa) is 360 cm -1 and becomes a large value. In this case, in order to realize a stable single mode, the DFB length is about 50 μm, and high optical output and narrow linewidth cannot be realized.

[0024] Also, as shown in FIG. 3B, in the case of FIG. 2B in which the diffraction grating 310a is formed in a SiN layer formed in contact with the semiconductor layer 121, a lower coupling coefficient can be realized as compared with the configuration of FIG. 2A. In this configuration, the thickness of the silicon nitride layer becomes the groove depth of the diffraction grating 310a. For example, when the depth (thickness) of the groove of the diffraction grating 310a is 10 nm, the coupling coefficient is 75 cm -1However, even in this case, in order to realize a stable single mode, the DFB length is limited to about 240 μm, and similarly, high optical output and narrow linewidth cannot be expected.

[0025] In contrast, according to the embodiment, as shown in FIG. 3C, a smaller coupling coefficient can be realized. In FIG. 3C, the horizontal axis represents the spatial gap between the active layer 103 and the diffraction grating 110. As shown in FIG. 3C, it can be seen that by increasing the distance between the active layer 103 and the diffraction grating 110, the coupling coefficient decreases while approaching 0 cm -1 to zero.

[0026] Therefore, by determining an arbitrary DFB length and setting an optimal interval according to the determined DFB length, a high optical output and narrow linewidth laser can be realized without increasing the reflectivity of the diffraction grating 110 too much. For example, when the interval is 300 nm, the coupling coefficient is 1 cm -1 so that the DFB length can be set to 10 mm. At this length, much higher optical output and narrower linewidth than before can be expected.

[0027] Next, another semiconductor laser according to the embodiment of the present invention will be described with reference to FIGS. 4A, 4B, 4C, 4D, and 4E.

[0028] For example, as shown in FIG. 4A, a light coupling layer 111 formed in a core shape extending along the active layer 103 and embedded in the first cladding layer 102 in a state capable of optical coupling with the active layer 103 can be further provided. In this example, the diffraction grating 110 is formed by being embedded in the second cladding layer 106 on the side where the light coupling layer 111 is not formed.

[0029] Also, for example, as shown in FIG. 4B, the diffraction grating 110a can be formed by being embedded in the first cladding layer 102 on the side where the optical coupling layer 111 is formed. In this example, the diffraction grating 110a is formed between the active layer 103 and the optical coupling layer 111. In this case, the diffraction grating 110a is disposed at a location away from the boundary region between both the optical coupling layer 111 and the active layer (resonator) and the cladding layer. The boundary region between the optical coupling layer 111 and the cladding layer is the interface between the optical coupling layer 111 and the cladding layer.

[0030] Also, for example, as shown in FIG. 4C, the diffraction grating 110b can be formed by being embedded in the second cladding layer 106 on the side where the optical coupling layer 111 is formed. Also in this example, the diffraction grating 110b is formed between the active layer 103 and the optical coupling layer 111.

[0031] Also, for example, as shown in FIG. 4D, the diffraction grating 110c can be formed by being embedded in the first cladding layer 102 on the side where the optical coupling layer 111 is formed, and the diffraction grating 110b can be formed on the side where the active layer 103 is not formed as viewed from the optical coupling layer 111. Further, as shown in FIG. 4E, the diffraction grating 110d can be formed by being embedded in the second cladding layer 106 on the side where the optical coupling layer 111a is formed, and the diffraction grating 110d can be formed on the side where the active layer 103 is not formed as viewed from the optical coupling layer 111a.

[0032] Also, the diffraction gratings 110, 110a, 110b, 110c, and 110d can be formed to have the same width as the optical coupling layer 111 in the waveguide direction.

[0033] As described above, by providing the optical coupling layer 111, the active layer 103 and the optical coupling layer 111 are combined to form a supermode as a waveguide mode (reference). Therefore, by adjusting the width and thickness of the optical coupling layer 111, the optical confinement in the active layer 103, the p-type semiconductor layer 104, and the n-type semiconductor layer 105 can be freely adjusted. For example, when the width of the optical coupling layer 111 is increased and the effective refractive index of the optical waveguide formed by the optical coupling layer 111 is relatively increased with respect to the optical waveguide (gain waveguide) by the active layer 103, the optical confinement in the optical waveguide formed by the optical coupling layer 111 increases, while the optical confinement in the active layer 103, the p-type semiconductor layer 104, and the n-type semiconductor layer 105 decreases. It is known that this can reduce the optical absorption loss and is effective in increasing the external quantum efficiency as compared with a semiconductor laser not provided with the optical coupling layer 111 (reference).

[0034] Hereinafter, in the structure having the optical coupling layer 111a described with reference to FIG. 4E, the effect of the diffraction grating 110d will be described with reference to FIGS. 5A, 5B, and 5C. Hereinafter, the case where the optical coupling layer 111a is made of SiN will be described as an example. Note that FIG. 5A shows the result when a groove is directly formed on the upper surface of the optical coupling layer to form a diffraction grating, rather than the configuration of the semiconductor laser according to the embodiment. Further, FIG. 5C shows the result when a groove is directly formed on the side surface of the optical coupling layer to form a diffraction grating, rather than the configuration of the semiconductor laser according to the embodiment. FIG. 5C shows the calculation result of the structure having the optical coupling layer 111a described with reference to FIG. 4E.

[0035] As shown in FIG. 5C, according to the semiconductor laser described with reference to FIG. 4E, since the diffraction grating 110d is disposed at a position away from the interface (boundary region) between the optical coupling layer 111a and the second cladding layer 106, it can be seen that a relatively low coupling coefficient can be realized by increasing the distance between the boundary region and the diffraction grating 110d. That is, the same effect as described above can be obtained even in a DFB laser forming a supermode waveguide.

[0036] Next, a method for manufacturing a semiconductor laser according to an embodiment of the present invention will be described with reference to FIGS. 6A to 6F. Hereinafter, the manufacturing method will be described by taking the semiconductor laser described with reference to FIG. 4E as an example.

[0037] First, as shown in FIG. 6A, a first cladding layer 102 made of, for example, silicon oxide (SiO2) is formed on a substrate 101 made of Si. For example, the first cladding layer 102 can be formed by thermally oxidizing the surface of the substrate 101 made of Si. Next, as shown in FIG. 6B, an active layer 103, a p-type semiconductor layer 104, and an n-type semiconductor layer 105 embedded in a semiconductor layer 121 are formed on the first cladding layer 102 by, for example, a III-V semiconductor such as InP (reference document). The active layer 103 can have, for example, a multiple quantum well structure.

[0038] Next, as shown in FIG. 6C, a first dielectric layer 131, an optical coupling layer forming layer 132, a second dielectric layer 133, and a diffraction grating forming layer 134 are sequentially formed on the semiconductor layer 121. For example, the first dielectric layer 131 and the second dielectric layer 133 can be made of SiO2, and the optical coupling layer forming layer 132 and the diffraction grating forming layer 134 can be made of SiN.

[0039] In the formation of these layers, film formation and the like are carried out under low-temperature conditions of 500°C or lower so as not to damage the already formed III-V compound semiconductor layers such as the semiconductor layer 121 and the active layer 103. For example, as the film formation method described above, an electron cyclotron resonance (ECR) plasma CVD (Chemical Vapor Deposition) method can be used. Also, deuterium silane can be used as the source gas for film formation. Thereby, a SiN layer with suppressed light absorption in the communication wavelength band can be formed.

[0040] Note that any one of the first dielectric layer 131, the optical coupling layer forming layer 132, the second dielectric layer 133, and the diffraction grating forming layer 134 can be formed by a sputtering method as long as the thickness is 100 nm or less.

[0041] Next, by using a known lithography technique, a mask pattern for forming a diffraction grating is formed on the diffraction grating forming layer 134, and the diffraction grating forming layer 134 is etched by dry etching using this mask pattern, so that, as shown in FIG. 6D, a diffraction grating layer 135 is formed on the second dielectric layer 133.

[0042] In the above-described etching process, in the thickness direction of the second dielectric layer 133, a part may be over-etched from the upper surface. However, as will be described later, since the diffraction grating layer 135 is embedded with a layer of the same material as the second dielectric layer 133, the above-described problem of over-etching is avoided. In the conventional configuration, if over-etching occurs, the groove depth of the diffraction grating becomes larger than the design value, and the formed diffraction grating has a coupling coefficient larger than the design, making it difficult to form a diffraction grating with a low coupling coefficient. Further, in the conventional configuration, if an attempt is made to form a shallow groove depth, the etching time is shortened, but such short-time control is difficult. On the other hand, according to the above-described embodiment, the depth of the groove of the diffraction grating is determined by the thickness of the diffraction grating forming layer, and it is a feature that the problem of the groove depth changing due to over-etching can be avoided.

[0043] Next, by patterning the diffraction grating layer 135, the second dielectric layer 133, the optical coupling layer forming layer 132, and a part of the first dielectric layer 131 in the thickness direction, as shown in FIG. 6E, a diffraction grating 110d and an optical coupling layer 111a are formed, and a mesa in which the second dielectric layer 133a is sandwiched therebetween is formed. For example, a mesa-shaped mask pattern is formed by lithography on the diffraction grating layer 135, and by using this mask pattern to collectively dry-etch and process the above-described layers, the state shown in FIG. 6E can be achieved.

[0044] Next, the second cladding layer 106 is formed as shown in FIG. 6F by depositing the same dielectric material as the first dielectric layer 131 and the second dielectric layer 133 so as to embed the mesa portion formed by the diffraction grating 110d and the optical coupling layer 111a. The deposited dielectric material layer and the second dielectric layer 133a and the first dielectric layer 131a are integrated to form the second cladding layer 106. Thereafter, by forming each electrode, the semiconductor laser shown in FIG. 4E is obtained.

[0045] The thickness direction interval between the optical coupling layer 111a and the diffraction grating 110d and the groove depth of the diffraction grating 110d can be determined (controlled) by the thickness of the second dielectric layer 133 and the diffraction grating formation layer 134. In this example, the second dielectric layer 133 becomes the layer between the diffraction grating 110d and the resonator, and by controlling this thickness, the interval between the diffraction grating 110d and the resonator can be controlled. Therefore, compared with the conventional structure in which the groove depth is determined by the etching amount and the etching time, according to the embodiment, a semiconductor laser with a diffraction grating having high precision and a low coupling coefficient can be fabricated.

[0046] Also, in addition to the thickness of the second dielectric layer 133 and the diffraction grating formation layer 134, the coupling coefficient of the diffraction grating can be controlled by controlling the refractive indices of the first dielectric layer 131, the optical coupling layer formation layer 132, the second dielectric layer 133, and the diffraction grating formation layer 134. For example, in the formation (deposition) of each layer, the refractive index can be controlled by the flow rate of the source gas containing nitrogen and oxygen, and the degree of freedom in controlling the coupling coefficient can be increased.

[0047] As described above, according to the present invention, the diffraction grating is formed away from the boundary region between the core of the resonator and the first cladding layer or the second cladding layer and is arranged in the first cladding layer or the second cladding layer. Therefore, the coupling coefficient of the diffraction grating can be more easily made low, and the elongation of the DFB laser can be easily implemented.

[0048] Note that the present invention is not limited to the embodiments described above, and it is obvious that many modifications and combinations can be implemented by those with ordinary knowledge in the art within the technical idea of the present invention.

[0049] [References] T. Aihara et al., "Membrane buried-heterostructure DFB laser with an optically coupled III-V / Si waveguide", Optics Express, vol. 27, no. 25, pp. 36438-36448, 2019.

Explanation of Reference Numerals

[0050] 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, 110... Diffraction grating, 121... Semiconductor layer.

Claims

1. A first cladding layer formed on a substrate, an active layer formed in a core shape extending in a waveguide direction on the first cladding layer, a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer with the active layer sandwiched therebetween, a second cladding layer made of a dielectric formed on the active layer, and a p electrode and an n electrode connected to the p-type semiconductor layer and the n-type semiconductor layer, comprising, a semiconductor laser having a diffraction grating in a resonator, wherein the diffraction grating is formed on the side of the second cladding layer of the resonator and is formed away from a boundary region between the core of the resonator and the second cladding layer, further comprising an optical coupling layer formed in a core shape extending along the active layer and embedded in the first cladding layer or the second cladding layer in a state capable of optical coupling with the active layer, wherein the diffraction grating is formed by being embedded in the second cladding layer. A semiconductor laser characterized by this.

2. In the semiconductor laser according to Claim 1, the diffraction grating is formed between the active layer and the optical coupling layer. A semiconductor laser characterized by this.

3. In the semiconductor laser according to Claim 1, the diffraction grating is formed on the side where the active layer of the optical coupling layer is not formed. A semiconductor laser characterized by this.

4. In the semiconductor laser according to any one of Claims 1 to 3, the diffraction grating is formed to have the same width as the optical coupling layer in the waveguide direction. A semiconductor laser characterized by this.

Images