JP7709640B2 - Semiconductor laser - Google Patents

Description

The present invention relates to a semiconductor laser.

The increase in communication traffic on the Internet and elsewhere has created a demand for faster and larger capacity optical fiber transmission. In response to this demand, progress has been made in the development of digital coherent communication technology that utilizes coherent optical communication technology and digital signal processing technology, and 100G systems have been put into practical use. In such communication systems, single-mode semiconductor lasers are required as local light sources for transmission and reception.

A diffraction grating with a λ/4 phase shift has been used as a typical structure for an optical resonator to achieve a single mode. In this structure, a phase shifter formed in part of the diffraction grating provided in the resonator inverts the phase, enabling single-mode oscillation at the Bragg wavelength. This type of laser is called a λ/4 shifted DFB (Distributed Feedback) laser, and is already in practical use. In addition, in order to extend the transmission distance and expand the transmission capacity, DFB lasers are required to have higher optical output and narrower linewidth.

In order to increase the optical output and narrow the linewidth of a DFB laser, it is effective to lengthen the DFB laser. However, lengthening a DFB laser is limited by the following two problems:

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

Secondly, when the DFB laser is lengthened, the reflectivity of the resonator increases, making it more susceptible to losses inside the resonator and reducing the external quantum efficiency. This problem ultimately limits the ability to increase optical output.

An effective solution to these problems is to reduce the coupling coefficient of the diffraction grating as the DFB laser is lengthened. In other words, by reducing the coupling coefficient of the diffraction grating, the reflectivity of the resonator can be kept constant while the DFB laser is lengthened, and the above problems can be avoided.

Generally, a diffraction grating is made by forming periodic irregularities in part of an active layer made of III-V compound semiconductors that make up a semiconductor laser (see Non-Patent Document 1). If the change in this irregularity is small, the coupling coefficient of the diffraction grating decreases.

However, the formation of a diffraction grating requires extremely fine processing, and there is a limit to how small the change in the unevenness can be made. In other words, there is a limit to how small the coupling coefficient can be, and therefore there is a limit to how long the DFB laser can be made. For example, the change in the unevenness in a diffraction grating to make the coupling coefficient 20 cm ^-1 is extremely small, at 5 nm or less, and it is clearly difficult to manufacture.

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.

As mentioned above, increasing the length of a DFB laser is an effective way to increase the optical output and narrow the linewidth of the DFB laser, but with conventional technology, there was a problem in that it was not easy to increase the length of a DFB laser.

The present invention has been made to solve the above problems, and aims to make it easier to lengthen DFB lasers.

The 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 the waveguiding direction, a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer and sandwiching the active layer, 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 with a diffraction grating in a resonator, the diffraction grating being formed on the first cladding layer side or the second cladding layer side of the resonator and being formed away from the boundary region between the core of the resonator and the first cladding layer or the second cladding layer.

Furthermore , a method for manufacturing a semiconductor laser is the above-mentioned method for manufacturing a semiconductor laser, in which the thickness of the layer between the diffraction grating and the resonator is controlled to control the distance between the diffraction grating and the resonator.

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, making it easy to lengthen the DFB laser.

FIG. 1 is a cross-sectional view showing the structure of a semiconductor laser according to an embodiment of the present invention. FIG. 2A is a cross-sectional view showing the configuration of a conventional DFB laser. FIG. 2B is a cross-sectional view showing the configuration of a conventional DFB laser. FIG. 3A is a characteristic diagram showing the calculation results of the coupling coefficient of the diffraction grating in the conventional DFB laser described with reference to FIG. 2A. FIG. 3B is a characteristic diagram showing the calculation results of the coupling coefficient of the diffraction grating in the conventional DFB laser described with reference to FIG. 2B. FIG. 3C is a characteristic diagram showing a calculation result of a coupling coefficient of a diffraction grating in a semiconductor laser according to an embodiment of the present invention. FIG. 4A is a cross-sectional view showing the configuration of another semiconductor laser according to an embodiment of the present invention. FIG. 4B is a cross-sectional view showing the configuration of another semiconductor laser according to an embodiment of the present invention. FIG. 4C is a cross-sectional view showing the configuration of another semiconductor laser according to an embodiment of the present invention. FIG. 4D is a cross-sectional view showing a configuration of another semiconductor laser according to an embodiment of the present invention. FIG. 4E is a cross-sectional view showing a configuration of another semiconductor laser according to an embodiment of the present invention. FIG. 5A is a characteristic diagram showing a calculation result of a coupling coefficient of a diffraction grating when a diffraction grating is formed by directly forming grooves on the upper surface of an optical coupling layer in a structure having an optical coupling layer. FIG. 5B is a characteristic diagram showing a calculation result of the coupling coefficient of a diffraction grating when a diffraction grating is formed by directly forming grooves on the side surface of the optical coupling layer in a structure having an optical coupling layer. FIG. 5C is a characteristic diagram showing the calculation results of the coupling coefficient of the diffraction grating 110d in the structure having the optical coupling layer 111. FIG. 6A is a cross-sectional view showing a state of a semiconductor laser in the middle of a process for explaining a method for manufacturing a semiconductor laser according to an embodiment of the present invention. FIG. 6B is a cross-sectional view showing a state of the semiconductor laser in the middle of a process for explaining a method for manufacturing a semiconductor laser according to an embodiment of the present invention. FIG. 6C is a cross-sectional view showing a state of the semiconductor laser in the middle of a process for explaining a method for manufacturing the semiconductor laser according to the embodiment of the present invention. FIG. 6D is a cross-sectional view showing a state of the semiconductor laser in the middle of a process for explaining a method for manufacturing the semiconductor laser according to the embodiment of the present invention. FIG. 6E is a cross-sectional view showing a state of the semiconductor laser in the middle of a process for explaining a method for manufacturing a semiconductor laser according to an embodiment of the present invention. FIG. 6F is a cross-sectional view showing a state of the semiconductor laser in the middle of a process for explaining a method for manufacturing a semiconductor laser according to an embodiment of the present invention.

A semiconductor laser according to an embodiment of the present invention will be described below with reference to Figure 1. This semiconductor laser is a distributed feedback (DFB) laser that has an active layer 103 formed in a core shape extending in the waveguiding direction on a substrate 101 and has a diffraction grating 110 in the resonator.

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, 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.

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 InP, for example. 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 121.

In addition to the above-mentioned configuration, in the semiconductor laser according to the embodiment, the diffraction grating 110 is formed on the first cladding layer 102 side or the second cladding layer 106 side 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 (stacking direction). In this example, the diffraction grating 110 is formed on the second cladding layer 106 side of the resonator, and is formed away from the boundary region between the active layer 103, which is 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.

Here, a conventional lateral current injection type DFB laser will be described with reference to FIG. 2A. The conventional DFB laser is a lateral current injection type DFB laser having a configuration similar to that of the semiconductor laser described with reference to FIG. 1 (see Non-Patent Document 1). In the conventional DFB laser, a 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 formed in the semiconductor layer 121 above the active layer 103 and is composed of periodic grooves arranged in the waveguiding direction. In addition, as shown in FIG. 2B, there is also a configuration in which the diffraction grating 310a is formed from periodic grooves formed in a SiN layer formed in contact with the semiconductor layer 121.

Next, the calculation results for the coupling coefficients of each diffraction grating in each of the semiconductor lasers (DFB lasers) described above are shown in Figures 3A, 3B, and 3C. Figure 3A shows the calculation results for the conventional DFB laser described using Figure 2A. Figure 3B shows the calculation results for the conventional DFB laser described using Figure 2B. Figure 3C shows the calculation results for the semiconductor laser according to the embodiment described using Figure 1.

3A, when a diffraction grating 310 is formed by forming grooves in the semiconductor layer 121, the coupling coefficient (kappa) is as large as 360 cm ^-1 even if the groove depth of the diffraction grating 310 is only 10 nm. 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.

2B, in which the diffraction grating 310a is formed on a SiN layer formed on and in contact with the semiconductor layer 121, a lower coupling coefficient can be achieved compared to the configuration of FIG. 2A. In this configuration, the thickness of the silicon nitride layer is the groove depth of the diffraction grating 310a, and when the groove depth of the diffraction grating 310a is 10 nm, for example, the coupling coefficient is 75 cm ^-1 . However, even in this case, the DFB length is limited to about 240 μm in order to achieve a stable single mode, and high optical output and narrow linewidth cannot be expected.

In contrast, according to the embodiment, a smaller coupling coefficient can be realized as shown in Fig. 3C, in which the horizontal axis represents the spatial gap between the active layer 103 and the diffraction grating 110. As shown in Fig. 3C, by increasing the gap between the active layer 103 and the diffraction grating 110, the coupling coefficient decreases as it approaches 0 cm ^-1 .

Therefore, by determining an arbitrary DFB length and setting an optimal spacing according to the determined DFB length, a high optical output and narrow linewidth laser can be realized without excessively increasing the reflectance of the diffraction grating 110. For example, if the spacing is set to 300 nm, the coupling coefficient becomes 1 cm ^-1 , so the DFB length can be set to 10 mm. With this length, a much higher optical output and narrower linewidth than conventional ones can be expected.

Next, other semiconductor lasers relating to embodiments of the present invention will be described with reference to Figures 4A, 4B, 4C, 4D, and 4E.

For example, as shown in FIG. 4A, an optical coupling layer 111 may be further provided that is embedded in the first cladding layer 102 in a state in which it can be optically coupled to the active layer 103 and is formed in a core shape extending along the active layer 103. In this example, the diffraction grating 110 is embedded in the second cladding layer 106 on the side where the optical coupling layer 111 is not formed.

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 103 (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.

4C, the diffraction grating 110b can be embedded in the second cladding layer 106 on the side where the optical coupling layer 111 is formed. In this example, the diffraction grating 110b is also formed between the active layer 103 and the optical coupling layer 111.

4D, the diffraction grating 110c can be 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. As shown in FIG. 4E, the diffraction grating 110d can be 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.

In addition, 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 waveguiding direction.

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, if the width of the optical coupling layer 111 is widened and the effective refractive index of the optical waveguide made of the optical coupling layer 111 is increased relative to the optical waveguide (gain waveguide) made of the active layer 103, the optical confinement in the optical waveguide made of 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. This is known to be effective in reducing the optical absorption loss compared to a semiconductor laser without the optical coupling layer 111 and increasing the external quantum efficiency (reference).

In the following, the effect of the diffraction grating 110d in the structure having the optical coupling layer 111a described with reference to FIG. 4E will be described with reference to FIG. 5A, FIG. 5B, and FIG. 5C. In the following, the optical coupling layer 111a is made of SiN. Note that FIG. 5A shows the result when a diffraction grating is formed by forming a groove directly on the upper surface of the optical coupling layer, not the configuration of the semiconductor laser according to the embodiment. Also, FIG. 5C shows the result when a diffraction grating is formed by forming a groove directly on the side surface of the optical coupling layer, not 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.

As shown in Figure 5C, in the semiconductor laser described with reference to Figure 4E, the diffraction grating 110d is disposed at a location away from the interface (boundary region) between the optical coupling layer 111a and the second cladding layer 106, so it can be seen that a relatively low coupling coefficient can be achieved by increasing the distance between the boundary region and the diffraction grating 110d. In other words, the same effect as described above can be obtained in a DFB laser that forms a supermode waveguide.

Next, a method for manufacturing a semiconductor laser according to an embodiment of the present invention will be described with reference to Figures 6A to 6F. Below, the manufacturing method will be described using the semiconductor laser described with reference to Figure 4E as an example.

First, as shown in Fig. 6A, a first cladding layer 102 made of, for example, silicon oxide (SiO ₂ ) 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 are formed on the first cladding layer 102, the active layer 103 being embedded in a semiconductor layer 121, using, for example, a III-V group semiconductor such as InP (reference literature). The active layer 103 can have, for example, a multiple quantum well structure.

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.

In forming these layers, deposition is performed at a low temperature of 500°C or less so as not to destroy the already formed III-V compound semiconductor layers such as the semiconductor layer 121 and the active layer 103. For example, the above-mentioned deposition method may be an electron cyclotron resonance (ECR) plasma CVD (chemical vapor deposition) method. Deuterium silane may be used as the source gas for deposition. This allows the formation of a SiN layer that suppresses light absorption in the communication wavelength band.

In addition, any 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 so long as the thickness is 100 nm or less.

Next, a mask pattern for forming a diffraction grating is formed on the diffraction grating forming layer 134 using known lithography techniques, and the diffraction grating forming layer 134 is etched by dry etching using this mask pattern to form a diffraction grating layer 135 on the second dielectric layer 133, as shown in FIG. 6D.

In the above-mentioned etching process, the second dielectric layer 133 may be partially over-etched from the upper surface in the thickness direction. However, as described later, the diffraction grating layer 135 is embedded with a layer of the same material as the second dielectric layer 133, so the above-mentioned 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 larger coupling coefficient than the design, making it difficult to form a diffraction grating with a low coupling coefficient. In addition, in the conventional configuration, if a shallow groove depth is to be formed, the etching time is shortened, but such a short time control is difficult. In contrast, according to the above-mentioned embodiment, the groove depth of the diffraction grating is determined by the thickness of the diffraction grating formation layer, and the problem of the groove depth changing due to over-etching can be avoided.

Next, 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 are patterned to form the diffraction grating 110d and the optical coupling layer 111a, and a mesa is formed with the second dielectric layer 133a sandwiched between them, as shown in Figure 6E. For example, a mesa-shaped mask pattern is formed on the diffraction grating layer 135 by lithography, and the above-mentioned layers are dry-etched together using this mask pattern to process them, resulting in the state shown in Figure 6E.

Next, the same dielectric material as the first dielectric layer 131 and the second dielectric layer 133 is deposited so as to bury the mesa portion formed by the diffraction grating 110d and the optical coupling layer 111a, thereby forming the second cladding layer 106, as shown in FIG. 6F. The deposited layer of dielectric material is integrated with the second dielectric layer 133a and the first dielectric layer 131a to form the second cladding layer 106. After this, the electrodes are formed to obtain the semiconductor laser shown in FIG. 4E.

The distance between the optical coupling layer 111a and the diffraction grating 110d in the thickness direction 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 forming layer 134. In this example, the second dielectric layer 133 is a layer between the diffraction grating 110d and the resonator, and by controlling the thickness of this layer, the distance between the diffraction grating 110d and the resonator can be controlled. Therefore, compared to the conventional structure in which the depth of the groove is determined by the etching amount and etching time, according to the embodiment, a semiconductor laser with a diffraction grating having high accuracy and a low coupling coefficient can be manufactured.

In addition to controlling the thicknesses of the second dielectric layer 133 and the diffraction grating forming layer 134, the coupling coefficient of the diffraction grating can also be controlled by controlling the refractive indexes 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. For example, in the formation (deposition) of each layer, the refractive index can be controlled by the flow rate of the raw material gas containing nitrogen and oxygen, increasing the degree of freedom in controlling the coupling coefficient.

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 disposed within the first cladding layer or the second cladding layer, so that the coupling coefficient of the diffraction grating can be more easily reduced, and the length of the DFB laser can be easily increased.

It is to be understood that the present invention is not limited to the embodiments described above, and that many modifications and combinations can be implemented by a person having ordinary knowledge in the art within the technical concept of the present invention.

[Reference] 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.

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 (4)

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;
a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer and sandwiching the active layer;
a second cladding layer made of a dielectric material formed on the active layer;
a p-electrode and an n-electrode connected to the p-type semiconductor layer and the n-type semiconductor layer,
A semiconductor laser having a diffraction grating in a resonator,
the diffraction grating is formed on the second clad layer side of the resonator and is formed away from a boundary region between a core of the resonator and the second clad layer ,
an optical coupling layer that is embedded in the first cladding layer or the second cladding layer in a state capable of optically coupling with the active layer and is formed in a core shape extending along the active layer;
The semiconductor laser , wherein the diffraction grating is embedded in the second cladding layer . 2. The semiconductor laser according to claim 1 ,
4. A semiconductor laser, comprising: a first layer and a second layer, the first layer including a first insulating layer and a second insulating layer; 2. The semiconductor laser according to claim 1 ,
4. A semiconductor laser, comprising: a first insulating layer formed on said first insulating film; a second insulating layer formed on said first insulating film; The semiconductor laser according to any one of claims 1 to 3 ,
4. A semiconductor laser, comprising: a diffraction grating having a width equal to that of the optical coupling layer in a waveguiding direction;