JPH05110187A - Array type semiconductor laser and manufacture of the same - Google Patents

Abstract

PURPOSE:To integrate distributed feedback type semiconductor lasers having the same characteristics except the oscillation wavelength are integrated on the same substrate. CONSTITUTION:MESAs in different heights and widths are sequentially arranged in the form of a stripe on a InP substrate 1. A light waveguide layer 2 is epitaxially grown by the liquid phase growth method so that the surface becomes flat. A diffraction grating 11 is then formed on the surface of waveguide layer 2. A waveguide layer 3, an active layer 4, an untimelt-back layer 5 and a p-type clad layer 6 are epitaxially grown on the waveguide layer 2. With such constitution, since the height of diffraction grating 11 is uniform for each laser 18, 19, 20 and the effective refractive index is different, distributed feedback type semiconductor lasers having the equalized laser characteristics other than the oscillation wavelength can be integrated on the same substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an array type semiconductor laser device having a plurality of distributed feedback type semiconductor laser parts having different oscillation wavelengths.

[0002]

2. Description of the Related Art Oscillation wavelength λ of distributed feedback semiconductor laser
Is given by the Bragg condition shown in equation (1).

Λ = 2 · N _eff · Λ / M (1) where N _eff is the effective refractive index inside the optical waveguide, Λ is the diffraction grating period, and M is the order (M = 1, 2, 3, ...).
Therefore, in order to fabricate distributed feedback semiconductor lasers of different wavelengths on the same substrate, the effective refractive index of the optical waveguide of a unit distributed feedback semiconductor laser is set to the effective refractive index of the optical waveguide of another unit distributed feedback semiconductor laser. Ways have been done to make it different from rates. This is realized by making the distance between the diffraction grating and the active layer in the unit distributed feedback type semiconductor laser different from the distance between the diffraction grating and the active layer in other unit distributed feedback type semiconductor lasers (see, for example, Japanese Patent Laid-Open No. Hei 2-
71573). This will be described below with reference to the drawings.

FIGS. 6 and 7 are views for explaining a conventional array type semiconductor laser. 6 is a perspective view, 28 is a substrate, 29 is a buffer layer, 30 is a lower cladding layer, 31 is an active layer, 32 is a waveguide layer, 33 is a diffraction grating, 34 is an upper cladding layer, 35 is a contact layer, 36 is an insulating layer, 37
Is an n-type electrode, 38 is a p-type electrode, 39 and 40 are cuts in a sectional configuration view, and 41 and 42 are unit distributed feedback semiconductor lasers.

FIG. 7 (a) is a sectional view of the distributed feedback semiconductor laser at 39, and FIG. 7 (b) is a sectional view of the distributed feedback semiconductor laser at 40. The difference between FIGS. 7A and 7B is that the waveguide layer 32
Is the thickness of.

The operation of the conventional array type semiconductor laser having the above structure will be described below.

Since the distributed feedback semiconductor lasers 41 and 42 have different thicknesses of the waveguide layer 32, the effective refractive indexes of the respective optical waveguides have different values. Distributed feedback semiconductor laser 4
The effective refractive indices of the optical waveguides 1 and ₄₂ are N _eff1 and N _eff , respectively.
_{As eff2} , when the period of the diffraction grating is Λ, the distributed feedback semiconductor lasers 41 and 42 oscillate at different wavelengths λ ₁ and λ ₂ , respectively, as can be seen from equation (1).

[0008]

However, in the conventional structure, the diffraction grating 33 is formed on the surface of the waveguide layer 32 where the step between the distributed feedback lasers 41 and 42 is formed. The diffraction grating is formed by photolithography, but when a resist is applied to the surface of the waveguide layer 32 having a step by a spinner, the step is affected by the step.
As shown in (a), the thickness of the resist applied on the upper part and the lower part of the step is different. When interference exposure is performed in this state, the exposure state of the resist differs between the upper part and the lower part of the step, and the shape of the diffraction grating obtained after development and etching becomes different between the upper part and the lower part of the step. Since the resist thickness becomes thicker in the lower part of the step than in the upper part, the height of the obtained diffraction grating becomes lower than that in the upper part of the step as shown in FIG. 8B. Since the coupling coefficient representing the degree of coupling between the light propagating in the waveguide and the diffraction grating is proportional to the height of the diffraction grating, the coupling coefficients of the waveguides formed above and below the step have different values. Differences in coupling coefficient appear as differences in laser characteristics such as threshold current value and modulation distortion. That is, when distributed feedback semiconductor lasers are arrayed by the method of the conventional example, the characteristics of the respective lasers differ in addition to the wavelength.

In view of the above points, an object of the present invention is to provide an array type semiconductor laser having a plurality of distributed feedback type semiconductor lasers having the same characteristics other than the oscillation wavelength on the same substrate and a manufacturing method thereof.

[0010]

The present invention firstly provides a monolithic laser array type semiconductor laser device in which a unit semiconductor laser section is formed of a distributed feedback type semiconductor laser,
A compound semiconductor substrate and a resonator of the unit semiconductor laser section are provided on the substrate, the resonator including at least an active layer, a first waveguide layer and a second waveguide layer, and the first conductive layer. The waveguide layer and the second waveguide layer are adjacent to each other, and
The index of refraction of the waveguide layer has a value between the index of refraction of the substrate and the index of refraction of the first waveguide layer, and the diffraction grating has a refractive index of the first waveguide layer and the second waveguide layer. An array-type semiconductor provided between the first and second semiconductor laser sections, wherein the second waveguide layer of the unit semiconductor laser section has a layer thickness different from a second waveguide layer of another unit semiconductor laser. It is a laser.

Second, a step of etching the compound semiconductor substrate to form mesa stripes having different heights and widths, and a compound semiconductor film to be a second waveguide layer on the substrate so that the surface becomes flat. And a step of epitaxially growing by a liquid phase growth method. A method of manufacturing an array-type semiconductor laser, comprising: a step of forming a diffraction grating on the surface of the second waveguide layer; and a step of epitaxially growing a first waveguide layer and an active layer on the diffraction grating. Is.

[0012]

According to the present invention, the effective refractive index of the optical waveguide of each distributed feedback semiconductor laser is made different by making the layer thickness of the second waveguide layer different in each distributed feedback semiconductor laser by the above-mentioned structure. A plurality of distributed feedback semiconductor lasers having different oscillation wavelengths can be obtained. Further, the layer thickness of the first waveguide layer adjacent to the active layer is the same in each distributed feedback semiconductor laser, and a diffraction grating is formed between the first waveguide layer and the second waveguide layer. Therefore, the distance between the diffraction grating and the active layer can be made the same in each distributed feedback semiconductor laser. This makes it possible to form a diffraction grating on the second waveguide layer, which is a flat surface, and to eliminate the above-mentioned variation in characteristics among the distributed feedback semiconductor lasers. As a method of making the layer thickness of the second waveguide layer partially different and performing epitaxial growth so that the surface thereof becomes flat, mesas having different heights and widths are sequentially arranged in a stripe shape and formed on a semiconductor substrate. On top of that, a second waveguide layer LPE
Method is used for epitaxial growth. According to the LPE method, the layer thickness of the epitaxial layer on the mesa stripe is thinner than the layer thickness of the portion other than the mesa stripe, and largely depends on the width of the mesa stripe. By utilizing this, by appropriately selecting the height and width of the mesa stripe with respect to the set different layer thicknesses, it is possible to grow an epitaxial layer having a partially different layer thickness so as to have a flat surface. As described above, it is possible to form a plurality of distributed feedback semiconductor lasers having the same characteristics other than the oscillation wavelength on the same substrate. Also,
The first waveguide layer and the active layer are formed by metalorganic vapor phase epitaxy (MOVP).
When the epitaxial growth is performed by the method E), the layer thickness of the active layer becomes uniform on each mesa stripe even if the surface is not completely flat when the second waveguide layer is epitaxially grown by a predetermined layer thickness. As a result, the active layer thickness becomes the same in each distributed feedback semiconductor laser, so the threshold current values become the same.

[0013]

1 is a perspective sectional view for explaining the structure of an array type semiconductor laser according to a first embodiment of the present invention. 1
Is an n-type InP substrate, 2 is an n-type InGaAsP waveguide layer with a wavelength composition of 1.1 μm, 3 is n with a wavelength composition of 1.3 μm
Type InGaAsP waveguide layer, 4 has a wavelength composition of 1.55μ
m non-doped InGaAsP active layer, 5 p-type InGaAsP anti-melt back layer with wavelength composition of 1.3 μm, 6 p-type InP clad layer, 7 p-type InP current blocking layer, 8 n-type InP Current blocking layer 9, p-type InP buried layer, 10 p-type InGaAsP
Contact layer, 11 is a diffraction grating with a pitch of 2360Å, 1
2 is an n-side electrode, 13 is a p-side electrode, 14 is a separation groove, 15,
Reference numerals 16 and 17 are cuts in the sectional configuration diagram, 18, 19 and 20 are distributed feedback semiconductor lasers, 21, 22 and 23 are mesas, and 27.
Is an insulating film. 2 (a), (b), and (c) show distributed feedback semiconductor lasers 18, 19, and 20, respectively.
It is the figure shown in the state cut by.

The structure of the array type semiconductor laser of this embodiment is such that the waveguide layer 2, the waveguide layer 3, the active layer 4, and the
Three laser resonator structures in which an anti-meltback layer 5 and a p-type cladding layer 6 are laminated are formed, and a diffraction grating 11 is formed between each of the waveguide layers 2 and 3. The only difference between the laser resonator structures is that the layer thickness of the waveguide layer 2 is different from d ₁ , d ₂ , and d ₃ , respectively.

The operation of the array type semiconductor laser of this embodiment having the above-mentioned structure will be described below.

Distributed feedback type semiconductor lasers 18, 19, 20
Since the thickness of the waveguide layer 2 is different from d ₁ , d ₂ , and d ₃ , the effective refractive index of the optical waveguide of each laser is N _eff1 , N _eff2 , and N _eff .
It will be different from _eff3 . Then, the oscillation wavelengths λ ₁ , λ ₂ , and λ ₃ are determined from the Bragg condition (equation (1)) described above. If d ₁ > d ₂ > d _{3 at} this time, N _eff1 > N _eff2 > N _{eff 3}
Therefore, the relation of the oscillation wavelength is λ ₁ > λ ₂ > λ ₃ . FIG. 3 shows the relationship between the layer thickness of the waveguide layer 2 and the oscillation wavelength.

In the array type semiconductor laser of the embodiment shown in FIG. 3, for example, d ₁ , d ₂ and d ₃ of the waveguide layer 2 are 0.25 μm, 0.15 μm and 0.09 μm, respectively, the waveguide layer 3 and the active layer 4 are formed. , And the anti-meltback layer 5 has thicknesses of 0.1 μm, 0.15 μm, and 0.1 μm, respectively, the effective refractive indices N _eff1 , N _eff2 , and N _eff of the optical waveguides of the distributed feedback semiconductor lasers 18, 19, and 20. _eff3 is 3.300,
It becomes 3.297 and 3.292, and the oscillation wavelength is 1.558μ.
m, 1.556 μm, and 1.554 μm.

4 and 5 are perspective views showing a method of manufacturing an array type semiconductor laser according to an embodiment of the present invention.

First, a mesa 24 having a width of 30 μm and a height of 0.05 μm, and a width of 10 are formed on an n-type (100) plane InP substrate.
The mesa 25 having a height of 0.15 μm and a width of 6 μm and the mesa 26 having a height of 0.21 μm are wet-etched (H ₂
SO ₄ : H ₂ O ₂ : H ₂ O = 5: 1: 1) (Fig. 4 (a)). On top of this, a wavelength composition of 0.3 μm, 1.1 μm InGa
The AsP waveguide layer 2 is epitaxially grown by the LPE method (FIG. 4 (b)). As described above, since the growth rate is different between the upper part of each mesa and the other part, the surface of the waveguide layer 2 becomes flat, and the waveguide layer 2 on the mesas 24, 25, and 26 is flattened.
Layer thicknesses of 0.25 μm, 0.15 μm and 0.0, respectively.
It becomes 9 μm. Next, a pitch 2360 is formed on the surface of the waveguide layer 2.
The diffraction grating 11 of Å is formed (FIG. 4 (c)). On top of that, an n-type InGaAsP waveguide layer 3 having a wavelength composition of 1.3 μm,
The InGaAsP active layer 4 having a wavelength composition of 1.55 μm, the p-type InGaAsP anti-meltback layer waveguide layer 5 having a wavelength composition of 1.3 μm, and the p-type InP clad layer 6 are 0.1 μm, 0.15 μm, and 0.1 μm, respectively. , 0.5 μm
Is epitaxially grown to the thickness shown in Fig. 5 (a). Mesa etching is performed on the thus obtained multilayer semiconductor crystal. At this time, the center of each mesa 21, 22 and 23 is made of InP.
It is aligned with the center of the mesas 24, 25, 26 formed on the substrate. The width of the mesas 21, 22 and 23 is 1.3 μm (FIG. 5 (b)). Then, a p-type InP current blocking layer 7, an n-type InP current blocking layer 8 and a p-type InP are formed on the entire surface.
Buried layer 9, p-type InGaAsP contact layer 10
Are grown epitaxially. Next, the isolation groove 14 is formed by wet etching, and the insulating film 27 is deposited on the entire surface. Further, a contact window is opened in the insulating film 27, and a p-type I
A p-type electrode 13 made of Au / Zu is formed on the nGaAsP contact layer 10, and Au / S is formed under the n-type InP substrate 1.
An n-type electrode 12 made of n is formed (FIG. 1).

By using such a method, only the waveguide layers 2 of the distributed feedback semiconductor lasers 18, 19 and 20 can have different layer thicknesses, and distributed feedback semiconductors that can obtain different oscillation wavelengths can be obtained. Lasers 18, 19,
An array type semiconductor laser having 20 formed on the same substrate can be manufactured. Further, since the diffraction grating is formed on the waveguide layer 2 having a flat surface, it becomes possible to form a diffraction grating having a uniform height over the entire surface, and the distributed feedback semiconductor lasers 18, 19 and 20 have oscillation wavelengths. The other characteristics are the same except for the difference.

Although the active layer 4 has a bulk structure in this embodiment, it may have a quantum well structure. The anti-melt back layer 5 is not necessary depending on the growing method (for example, MOVPE method). The wavelength may be other than the 1.55 μm band. InP and InGaAs are used to form the semiconductor layer.
InGaAs, GaAs, AlGaAs as well as P
Alternatively, other semiconductors may be used. Although the buried structure is shown in this embodiment, a ridge type structure or another structure may be used.

[0022]

As described above, according to the present invention,
An array type semiconductor laser having a plurality of distributed feedback type lasers having different oscillation wavelengths on the same semiconductor substrate can be obtained. Further, since the diffraction grating is formed on a flat surface, the characteristics of each laser are the same except the oscillation wavelength. In this way, an array type semiconductor laser having the same laser characteristics other than the oscillation wavelength can be obtained, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a perspective sectional view of an array type semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a sectional structural view of an array type semiconductor laser in a cavity direction of an embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the layer thickness of the waveguide layer 2 and the oscillation wavelength.

FIG. 4 is a perspective view illustrating a method for manufacturing an array type semiconductor laser according to an embodiment of the present invention.

FIG. 5 is a perspective view illustrating a method for manufacturing an array type semiconductor laser according to an embodiment of the present invention.

FIG. 6 is a perspective view of a conventional array type semiconductor laser.

FIG. 7 is a cross-sectional structural view of a conventional array type semiconductor laser in a cavity direction.

FIG. 8 is a diagram illustrating a problem of a conventional array type semiconductor laser.

[Explanation of symbols]

1 n-type InP substrate 2 wavelength composition 1.1 μm n-type InGaAsP waveguide layer 3 n-type InGaAsP waveguide layer 1.3 μm wavelength composition 4 non-doped InGaAs with wavelength composition 1.55 μm
P active layer 5 p-type InGaAsP anti-meltback layer with wavelength composition of 1.3 μm 6 p-type InP clad layer 7 p-type InP current block layer 8 n-type InP current block layer 9 p-type InP buried layer 10 p Type InGaAsP contact layer 11 Pitch 2360 Å diffraction grating 12 n-side electrode 13 p-side electrode 14 separation groove 15 cut of cross-sectional configuration diagram 16 cut of cross-sectional configuration diagram 17 cut of cross-sectional configuration diagram 18 distributed feedback semiconductor laser 19 distributed feedback type Semiconductor laser 20 Distributed feedback semiconductor laser 21 Mesa 22 Mesa 23 Mesa 24 Mesa 25 Mesa 26 Mesa 27 Insulating film 28 Substrate 29 Buffer layer 30 Lower clad layer 31 Active layer 32 Waveguide layer 33 Diffraction grating 34 Upper clad layer 35 Contact layer 36 Insulating layer 37 n-type electrode 38 p-type electrode 39 cross section Cut 40 of composition diagram Cut of cross-sectional configuration diagram 41 Distributed feedback semiconductor laser 42 Distributed feedback semiconductor laser 43 Resist 44 Thin resist portion 45 Thick resist portion 46 High diffraction grating portion 47 High diffraction grating height Lower part

Claims (2)

[Claims] 1. A monolithic laser array type semiconductor laser device in which a unit semiconductor laser section is formed of a distributed feedback type semiconductor laser, comprising a compound semiconductor substrate and a resonator of the unit semiconductor laser section on the substrate. The resonator includes at least an active layer, a first waveguide layer and a second waveguide layer, the first waveguide layer and the second waveguide layer are adjacent to each other, and the second waveguide layer Has a value between the refractive index of the substrate and the refractive index of the first waveguide layer, and a diffraction grating is provided between the first waveguide layer and the second waveguide layer. An array type semiconductor laser, wherein the layer thickness of the second waveguide layer of the unit semiconductor laser section is different from the layer thickness of the second waveguide layer of another unit semiconductor laser. 2. A step of etching a compound semiconductor substrate to form a plurality of mesa stripes having different heights and widths,
A step of epitaxially growing a compound semiconductor film to be a second waveguide layer on the substrate by a liquid phase epitaxy method so that the surface becomes flat. A method of manufacturing an array-type semiconductor laser, comprising: a step of forming a diffraction grating on the surface of the second waveguide layer; and a step of epitaxially growing a first waveguide layer and an active layer on the diffraction grating. ..