JPH046217Y2 - - Google Patents

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a distributed feedback semiconductor laser device that emits light by distributed feedback coupling based on at least periodic changes in refractive index, and in particular, to a distributed feedback semiconductor laser device that emits light by distributed feedback coupling due to periodic changes in refractive index, and in particular, a distributed feedback semiconductor laser device that emits light using distributed feedback coupling caused by periodic changes in refractive index. The present invention relates to a distributed feedback surface emitting semiconductor laser device configured as follows.

Prior Art In general, research and development is progressing on distributed feedback (DFB) type semiconductor laser devices as a laser device suitable for obtaining single mode oscillation during high-speed modulation as a light source for long-distance optical communication. A species distribution feedback semiconductor laser device is constructed by covering the surface of a semiconductor layer with a diffraction grating formed on its surface with a further semiconductor layer, and uses longitudinal mode light emission that resonates with periodic changes in the refractive index caused by the diffraction grating. However, problems remain in achieving a single mode with good reproducibility, and there are various problems such as the need to make the structure asymmetric and shorten the resonator.

In other words, in a conventional distributed feedback semiconductor laser device using a diffraction grating, another semiconductor crystal layer is grown on the surface of the semiconductor layer on which the diffraction grating has been formed. The shape of the diffraction grating on the base surface was distorted, making it difficult to preserve the diffraction grating. Further, since a diffraction grating is formed near the active layer of the light emitting region due to carrier concentration, there is a risk that irregularities may occur in the crystal of the semiconductor layer formed on the underlying surface on which the diffraction grating is formed. Furthermore, since the change in the equivalent refractive index due to the diffraction grating cannot be made large, the coupling coefficient for distributed feedback cannot be made large. This increases the resonator length, making it difficult to achieve a single mode. Moreover, the diffraction grating is
In addition to the narrow grating spacing, it is difficult to cut the grating end faces, and it is difficult to form reflective end faces with good phase matching.Furthermore, the longitudinal mode that can resonate is at a wavelength determined by the diffraction grating. Conventional distributed feedback semiconductor laser devices using a diffraction grating have many drawbacks, such as the possibility that two types of longitudinal mode oscillation may occur simultaneously because there are two at the front and the rear.

On the other hand, in order to obtain a single mode by shortening a semiconductor laser device that oscillates by distributed feedback and obtaining a single mode, it is known that it is suitable to configure the semiconductor laser device to emit surface light. Attempts have been made to develop a distributed Bragg reflection (DBR) type semiconductor laser device in which Bragg reflectors with a multilayer structure are provided on both end faces of a semiconductor layer to perform Bragg reflection. In addition to the aforementioned problem common to distributed semiconductor laser devices, such as achieving a single mode with good reproducibility, the Bragg reflector, which has a multilayer structure, is much thicker than a normal reflector made of thin metal layers. In addition to making it difficult to shorten the cavity length of a semiconductor laser device, there is also the inherent problem that it is difficult to match the resonant light-emitting layer of the multilayer structure with the Bragg reflection layer. It has not yet been put to practical use, and a semiconductor laser device that performs single longitudinal mode oscillation that has good performance and is capable of high-speed modulation has not yet been realized.

Note that some distributed feedback surface-emitting semiconductor laser devices utilize interface reflection without providing any reflecting means on both end faces of a multilayer structure, but interface reflection is uncertain and stable laser oscillation is not possible. is difficult to obtain, and in order to cause laser oscillation with sufficient distributed feedback, it is necessary to have a multilayer structure with an extremely large number of layers, resulting in a disadvantage that the device becomes long.

Summary of the invention The purpose of the invention is to eliminate the above-mentioned conventional drawbacks, easily inject current into a multilayered semiconductor layer to create a short cavity, and increase the threshold of laser oscillation by distributed feedback resonance in a single longitudinal mode. It is an object of the present invention to provide a distributed feedback surface emitting semiconductor laser device which can perform strong surface emitting light with good reproducibility.

That is, the distributed feedback type surface emitting semiconductor laser device of the present invention has active semiconductor layers formed of group elements forming crystals and different combinations of group elements, and p-type and n-type active semiconductor layers forming p-n junctions therebetween.
A plurality of semiconductor layers are sequentially and alternately laminated on a semiconductor substrate, and are arranged coaxially on at least one of both end faces parallel to each of the layers to confine a current path in a direction perpendicular to each of the layers. By providing an electrode layer that also serves as a reflective mirror surface and injecting current perpendicularly into each layer through the electrode layer, periodic changes in the optical refractive index and optical amplification gain in the plurality of layers and both upper and lower ends can be observed. It is configured to emit light by resonating with reflection between surfaces, the wavelength of the resonant light emission is λ, the refractive index of the active semiconductor layer is _n1 , and the refractive index of the P-type and n-type semiconductor layers is _n2. The active semiconductor layer has a layer thickness of λ/4n ₁ , and the p-type and n-type semiconductor layers have a layer thickness of λ/4n ₂ .

Embodiments The present invention will be described in detail below with reference to the drawings.

First, an example of the basic configuration of the distributed feedback surface emitting semiconductor laser device of the present invention is shown in FIG. The semiconductor laser device with the configuration shown in the figure is composed of a combination of group elements that can be grown as crystals, and has an active semiconductor layer, such as a GaInAsP layer, that forms an active region and emits light by concentration of carriers, and a layer that can grow crystals as well. On both end faces of a semiconductor device having a multilayer structure in which pn junction semiconductor layers made of other possible combinations of group elements and group elements and contributing to current injection, such as pn junction InP layers, are stacked alternately in multiple stages, A p-type and an n-type semiconductor layer, for example, an InP layer, are deposited on a semiconductor substrate, for example, an n-type
A thin metal electrode layer is provided on both end faces of a transparent semiconductor device having such a structure, which is provided on an InP substrate, and also serves as a reflecting mirror, and a voltage is applied to the transparent semiconductor device. Single longitudinal mode light emission that resonates with periodic changes in index and optical amplification gain is extracted from one end face.

In other words, the distributed feedback with the configuration shown in Figure 1 is as follows:
This occurs due to the multi-layer stacking of the GaInAsP layer 40 serving as the active region and the pn junction semiconductor layer consisting of the p-type InP layer 50 and the n-type InP layer 60, and it resonates and emits light through a path 10 indicated by an arrow in the center of the figure. do. Also,
As shown in the figure, a crystal-grown n-type InP layer 35 is deposited on an n-type InP substrate 30, and the above-mentioned GaInAsP layer 40, p-type InP layer 50 and n-type
InP layers 60 are sequentially and alternately crystal-grown and deposited in a laminated manner, and a p-type InP layer 70 is crystal-grown and deposited on the final GaInAsP layer 40 of this laminated structure. Let λ be the wavelength of resonant light emission from such a laminated structure,
The refractive index of the GaInAsP layer is n ₁ , and the refractive index of the InP layer is
n ₂ , the thickness of the GaInAsP layer 40 is λ/4n ₁ , and consists of p-type and n-type InP layers 50 and 60.
The layer thickness of the pn junction InP layer is set to λ/4n ₂ so that the reflected lights from the boundary surfaces of each layer are in phase with each other and cause multiple interference. In addition, in order to flow current through such a laminated structure, n-type and p-type are coated on both end faces.
As shown in the figure, the InP layers 35 and 70 are made thicker than each layer in the stacked structure. A semiconductor device having such a configuration is provided on an n-type InP substrate having an appropriate thickness, for example, about 10 microns, and an insulating layer, for example, having an opening provided preferably in the center to limit the region of the resonance path 10, is provided. Through the SiO ₂ layer 80,
For example, a transparent p-side (+) electrode thin layer 100 made of an alloy of Au/Zn or Au/Cr is deposited on the upper end surface, and is similarly provided in the center part to form a resonance path 1.
An insulating layer with an opening defining a region of 0, e.g.
For example, Au _/ Sn or
N-side (-) electrode thin layer 11 made of Au/Ge alloy
0 is adhered to the lower end surface.

Next, to explain the operation of the distributed feedback surface emitting semiconductor laser device of the present invention having the basic configuration shown in the first diagram, first, a voltage is applied between the electrode thin layers 100 and 110 at both ends to cause a current to flow through the multilayer semiconductor device. When flowing, the carrier is initially located at the top stage.
Confined within the GaInAsP layer 40, as a result,
A population inversion of the carrier is formed, forming a GaInAsP active region which becomes a laser medium. Then,
Due to the leakage current of the pn junction semiconductor layers 50 and 60 or the tunnel current due to reverse bias, carriers are sequentially confined in each GaInAsP layer 40 located below the second stage, and the active region is sequentially formed. The light generated by the activation of the GaInAsP layer 40 at each stage is transmitted to the GaInAsP layer 40 and the InP layers 50 and 60.
It resonates with the periodic changes in the refractive indexes n ₁ and n ₂ and the periodic changes in the optical amplification gain, and causes laser oscillation at a wavelength selectively determined by the periodic changes. According to such a laminated structure of semiconductor layers, a large coupling coefficient can be obtained between the layers, so the resonator length required for laser oscillation can be shortened, and therefore, the oscillation mode due to different wavelengths can be reduced. Since the spacing can be widened, single mode laser oscillation can be easily obtained. In addition, by appropriately adjusting the layer thickness of the p-type InP layer 70 provided between the reflective electrode thin layer 100 on the upper end surface, reflected light with good phase matching can be obtained on the reflective mirror surfaces at both ends. By reducing the threshold value of the injected current that causes laser oscillation, laser oscillation can be easily caused.

Next, the manufacturing process of the semiconductor laser device of the present invention having the basic configuration shown in FIG. 1 will be explained with reference to FIGS. On top of this, an n-type InP layer 35 with an appropriate thickness to allow current to flow is deposited by crystal growth, and on top of that, a GaInAsP layer 40 and a p-type InP layer 35 are deposited.
After repeatedly and sequentially stacking the layer 50 and the n-type InP layer 60, the uppermost GaInAsP layer 40 is coated with an appropriate layer thickness to adjust the phase matching with the reflective electrode layer and to apply a current. p type for flowing
An InP layer 70 is deposited by crystal growth. Then,
As shown in FIG. 2b, an insulating layer is formed on the top surface of the multilayer semiconductor device constructed as described above.
After depositing the SiO ₂ layer 80, circular openings corresponding to the resonant paths 10 are formed by etching at suitable intervals for later cutting into a plurality of laser emitting elements. Next, as shown in FIG. 2c, the lower surface of the n-type InP layer 30 is polished by a conventional method to make the thickness an appropriate value, for example, about 100 microns as described above, and then as shown in FIG. 2d. As shown, a SiO ₂ layer 90 serving as an insulating layer is deposited on the polished lower end surface of the n-type InP substrate 30 in the same manner as on the upper end surface, and each opening formed in the SiO 2 layer 80 on the upper end surface is filled with the SiO ₂ layer 90. Similar openings are formed by etching to face each other. Thereafter, as shown in FIG. 2e, a thin alloy layer such as Au/Zn, Au/Cr, etc. is deposited over the entire surface of the SiO ₂ layer 80 on the upper end surface, and the SiO ₂ layer 80 on the lower end surface is coated on the entire surface.
A thin alloy layer such as Au/Sn or Au/Ge is deposited over the entire surface of layer 90 to form a p-side (+) electrode layer 100 and an n-side (-) electrode layer 110, respectively.
Next, the SiO ₂ layers 80 and 90 on the upper and lower end surfaces are cut into a plurality of blocks so that the respective openings are located in the center, and the blocks are cut into a plurality of blocks having the desired shape and dimensions as shown in FIG. 2f. A plurality of semiconductor laser light emitting devices are completed at the same time.

Next, FIG. 3 shows another example of the structure of the semiconductor laser device of the present invention, which increases the effect of current injection through a reverse biased pn junction in the basic structure shown in FIG. The configuration shown in the third figure is between the p-n junction semiconductor layer of each stage, the p-type InP layer 50 and the n-type InP layer 60 in the basic configuration shown in the first figure.
By interposing the p ⁺ type InP layer 55 and the n ⁺ type InP layer 65, a reverse-biased super-step p-n junction is formed to promote current injection due to the tunnel effect, reducing the injection current required to cause laser emission. The threshold value is reduced and powerful laser emission can be reliably and easily obtained.

In addition, in order to promote current injection, such super-step
The p-n junction has a narrow energy band gap,
It is better to form a dissimilar interface between the active layer and the active layer, where current can easily flow, but doping at a high concentration near the active layer, which requires a certain thickness for laser oscillation, may cause problems with crystallinity and optical loss. But it's not a good idea.

In the configuration examples of the semiconductor laser device of the present invention shown in FIGS. 1 and 3, the semiconductor substrate 30 is formed using an n-type InP semiconductor, but the semiconductor substrate 30 is formed using a p-type InP semiconductor. You can also. In this case, the combinations of p-type and n-type of each semiconductor layer and each electrode layer in the configurations shown in the first and third figures are all reversed.

Furthermore, in all of the configuration examples described above, the active semiconductor layer 40 is formed of a GaAs semiconductor, and the pn junction semiconductor layers 50, 60,
Both end semiconductor layers 35, 70 and semiconductor substrate 30
are formed of InP semiconductors, but these semiconductor materials are not limited to the compositions of the examples mentioned above.
Different combinations of group elements capable of crystal growth and group elements can be arbitrarily used. For example, the semiconductor substrate 30 is formed of a GaAs semiconductor, the active semiconductor layer 40 is formed of a GaAs semiconductor, and a pn junction semiconductor layer is formed. Even if semiconductors of any desired composition are appropriately combined as long as crystal growth is possible, such as forming 50 and 60 with GaAlAs semiconductors, the same effects of the present invention as described above can be obtained.

Next, we will explain the results of examining the laser oscillation conditions in the distributed feedback surface emitting semiconductor laser device of the present invention configured as described above. Assuming that the changes occur in the form of a sine wave, and the magnitude of each change is n _d and α _d , the coupling coefficient K between layers, where the real part is K _r and the imaginary part is K _i , is as follows ( 1) It is expressed by the formula.

K=K _r +jK _i =πn _d /λ+jα _d /2 (1) where λ is the wavelength of resonant light emission.

On the other hand, the following relationship (2) holds among the element length L, optical amplification gain α, detuning ratio δ, optical propagation constant γ, and coupling coefficient K of the multilayer semiconductor element.

(α−jδ)L=±KLcoshγL (2) Using these equations (1) and (2), the threshold value of the optical amplification gain α necessary to obtain laser oscillation is calculated, and the results are shown in Figure 4. The result of calculating the relationship between the threshold gain α and the element length L is shown in FIG.

Therefore, the magnitude of the change in refractive index n _d actually obtained with the configuration shown in Figure 1 is determined by the combination of the GaInAsP semiconductor layer and the InP semiconductor layer.
When resonance emission of 1.6 μm is obtained, n _d =0.185
Therefore, Ga _1-x Al _x in the other composition examples mentioned above
In the combination of As semiconductor and GaAs semiconductor
When = 0.7, n _d = 0.226. Therefore, the real part K _r of the coupling coefficient K is 3624 cm −1 and 7978 cm ⁻¹ for the above combination example, respectively.
cm ^-1 is expected. In addition, in the example of the former combination, when the element length L is set to L = 6 μm, the parameters in the calculation results shown in Figure 5
K _r L = 2.17, and the gain α = 1180 cm ^-1 . Note that it is desirable for the resonant light emitting element to have a gain α<400 cm ⁻¹ , but when the gain α is set in this range, the required element length L is about 10 μm. Therefore, the layer thickness of each semiconductor layer in the multilayer structure according to the structure shown in the first drawing is about 0.3 μm, and since 10 to 30 layers are usually stacked, the element length L is about 3 to 9 μm.

Effects As is clear from the above explanation, according to the present invention, the distributed semiconductor laser device is configured in a multilayer structure to emit surface light, so that resonant light emission in a single longitudinal mode is ensured by shortening the cavity. In addition, since the light output is extracted perpendicularly to the end face of the multilayer semiconductor light emitting device, it is possible to arrange a large number of such semiconductor light emitting devices on a single wafer to construct a two-dimensional laser array. This is extremely suitable, and furthermore, since layer thickness control in a multilayer semiconductor device is easy, phase matching between reflecting surfaces can be reliably and easily obtained by such film thickness control.

Therefore, the distributed feedback surface emitting semiconductor laser device of the present invention has the special effect of being widely applicable in constructing optical integrated circuits that are expected to be put to practical use, and has extremely important significance.

[Brief explanation of the drawing]

FIG. 1 is a side sectional view showing an example of the basic configuration of the distributed feedback type surface emitting semiconductor laser device of the present invention, and FIGS. 2 a to 2 f sequentially show the manufacturing process of the semiconductor laser device of the present invention having the configuration shown in FIG. 1. 3 is a side sectional view showing another configuration example of the semiconductor laser device of the present invention, and FIGS. 4 and 5 are characteristic curves showing examples of calculation results of various characteristics of the semiconductor laser device of the present invention, respectively. It is a diagram. 1... p-type region, 10... optical resonance path, 20...
...Light output, 30...n-type InP substrate, 35,60...
...n-type InP layer, 40...GaInAsP layer, 50, 70
... p-type InP layer, 55 ... p ⁺ type InP layer, 65 ...
n ⁺ type InP layer, 80,90...SiO ₂ layer, 100,1
10... Electrode thin layer, 120... Metal thin layer.

Claims (1)

[Claims for Utility Model Registration] 1. A plurality of layers of active semiconductor layers each consisting of a group element forming a crystal and a mutually different combination of group elements, and p-type and n-type semiconductor layers forming a p-n junction between them. an electrode that is configured by sequentially and alternately laminating layers on a semiconductor substrate, and is coaxially disposed on at least one of both end faces parallel to each of the layers, and serves as a reflective mirror surface configured to confine a current path in a direction perpendicular to each of the layers; By providing layers and injecting current perpendicularly into each layer through the electrode layer, light is emitted by resonating with the periodic changes in the optical refractive index and optical amplification gain in the multiple layers and the reflection between the upper and lower end surfaces. The wavelength of the resonant light emission is λ, and the refractive index of the active semiconductor layer is n ₁
, the refractive index of the P-type and n-type semiconductor layers is _n2 , and the layer thickness of the active semiconductor layer is λ/
4n ₁ , and the layer thickness of the P-type and n-type semiconductor layers is λ/4n ₂ . 2. The p-type or n-type semiconductor layer having a layer thickness different from λ/4n ₂ is inserted into the plurality of layers stacked alternately in order to match the phase of the reflected light between the mirror surfaces at both ends. A distributed feedback surface emitting semiconductor laser device according to claim 1, characterized in that it is a utility model registered.