JPH08321659A - Manufacture of semiconductor laser array - Google Patents

Abstract

PURPOSE: To provide a method of manufacturing a semiconductor laser array, which lessens an irregularity in the characteristics of each resonator and integrates a diffraction grating, an optical waveguide layer, active layers and a clad layer on a substrate in a wide wavelength range. CONSTITUTION: A laser array is constituted of an LD 1 and an LD 2. The array is mainly constituted of a diffraction grating, 100, an InGaAsP optical waveguide layer 12, active layers 31 and 32 and a P-type InP clad layer on an N-type InP substrate 11. The active layers are active layers 31 and 32, which are respectively formed into a multiple quantum well structure consisting of InGaAsP well and barrier layers having layer thicknesses different from each other. Here, the MQW active layer 31 consists of well layers 33 of a layer thickness of 50Å and barrier layers 34 of a layer thickness of 50Å as the enlarged figure of the layer 31 is shown in a diagram D and the MQW active layer 32 consists of well layers 35 of a layer thickness of 100Å and barrier layers 36 of a layer thickness of 100Å as the enlarged figure of the layer 32 is shown in a diagram E. In this case, the pitch of the diffraction grating is the same to both layers 31 and 32. In such a way, the structure of each resonator constituting the array can be formed by the one-time formation of the diffraction grating and a one-time epitaxial growth.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an integrated multi-wavelength distributed feedback semiconductor laser array which is a light source required for wavelength division multiplexing optical communication.

[0002]

2. Description of the Related Art Recently, optical multiplex communication has been actively researched and developed as a large capacity optical communication. A plurality of semiconductor lasers with different oscillation wavelengths are required for such a light source for wavelength-division communication, but lasers with different oscillation wavelengths are integrated on the same substrate from the standpoint of downsizing the light source, adjusting the optical axis, etc. The research and development of the multi-wavelength integrated laser array is also active. In order to increase the multiplex density in such a multi-wavelength light source, it is preferable that the multi-wavelength light source is composed of a distributed feedback semiconductor laser (hereinafter referred to as DFB-LD) having stable single axis mode oscillation even at the time of high speed modulation.

In the DFB-LD array, as a relatively easy method of changing the oscillation wavelength of each LD, there is a method of changing the pitch Λ of the diffraction grating forming each LD in each LD [Reference H. Okuda. et.al. JAPAN GAY READY
Physics (Jpn.J.Appl.Phys.) 23 (1984) L904].

FIG. 6A is a plan view of an LD array in which a plurality of LDs are integrated on the same substrate. Two typical here
Only two LDs (1,2) are shown. 6B and 6C are sectional views aa ′ and b of the LD1 and LD2 in the capacity direction.
It is an epitaxial structure figure of -b '. Where 11 is n
Type InP substrate, 12 n-InGaAsP optical waveguide layer (bandgap wavelength λ _g = 1.1 μm), 13 InGa
AsP active layer (λ _g = 1.3 μm), 14 is p-type InP
It is a clad layer. Diffraction gratings 20 and 21 having pitches Λ ₁ = 3940Å and Λ ₂ = 3850Å are formed on the n-type InP substrate 11, respectively.

The oscillation wavelength λ of the LD is n _eff, which is the effective refractive index,
When N is the diffraction order, it is determined by λ = 2n _eff · Λ / N (1). When N = 2 and n _eff are set to 3.30, the oscillation wavelengths of the LD are λ ₁ = 1.30 μm and λ ₂ respectively.
= 1.27 μm, and an oscillation wavelength difference of 30 nm can be obtained with two LDs. Thus, the oscillation wavelength of the DFB-LD can be changed by changing the pitch of the diffraction grating in each LD.

However, the bandgap wavelength λ _g of the active layer 13 of each LD 1, 2 of this DFB-LD array,
That is, since the gain peak is the same as 1.3 μm,
The difference between the oscillation wavelength and the gain peak becomes a problem. That is, FIGS. 7A and 7B show the oscillation spectra of LD1 and LD2. In LD1, the oscillation wavelength corresponds to the gain peak, whereas in LD2, the oscillation wavelength determined by the diffraction grating pitch is large with respect to the gain peak. It can be seen that the wavelength is shifted to the short wavelength side. Such a shift causes an increase in oscillation threshold current, a decrease in light emission efficiency, and deterioration of temperature characteristics. Further, when the difference between the two becomes large, the DFB mode no longer oscillates.

Further, as a manufacturing problem, in order to manufacture diffraction gratings having different pitches in different regions on the same substrate by a commonly used two-beam interference exposure method, a masking process and a two-light interference exposure process other than the selected region are performed. May have to be repeated in multiple stages. Such a complicated process causes not only a decrease in yield but also deterioration and variation in device characteristics.

[0008]

As described above, in the DFB-LD array according to the method of obtaining the oscillation wavelength difference by changing the diffraction grating pitch in the conventional example, the oscillation wavelength of the LD and the active layer bandgap determined by the diffraction grating pitch. Due to the shift of the gain peak determined by the above, there is a problem of deterioration or variation in the LD characteristics of the array.

Further, a very complicated process is required to form diffraction gratings having different pitches in the selected region, resulting in a decrease in yield and deterioration in characteristics.

An object of the present invention is to provide a method of manufacturing a multi-wavelength semiconductor laser array in which lasers having a stable single wavelength with a small variation in characteristics of each resonator are integrated in a wider wavelength range.

[0011]

SUMMARY OF THE INVENTION The present invention comprises a semiconductor laser array having at least two first and second distributed feedback laser resonators, each of which emits laser light of a single oscillation wavelength. A step of forming a diffraction grating having the same pitch on a semiconductor substrate, a step of forming a first resonator region and a second resonator region having different stripe widths on the substrate, The second resonator region includes an optical waveguide layer, a well layer, and a quantum well active layer that is a periodic structure of a barrier layer, and the optical waveguide layer, the well layer, and the barrier layer of the first resonator region are formed. A method of manufacturing a semiconductor laser array includes a step of simultaneously forming a film having a thickness smaller than those of the optical waveguide layer, the well layer, and the barrier layer in the second resonator region.

By the above-mentioned means, the oscillation wavelength difference is obtained by the effective refractive index difference in each distributed feedback laser capacity forming the array, and the gain peak also shows the same shift as the oscillation wavelength shift due to the quantum size effect. It is possible to provide a distributed feedback laser array in which deterioration of laser characteristics due to a shift between a wavelength and a gain peak is suppressed by a very easy means.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment in which a distributed feedback (DFB) LD array according to the present invention uses an InGaAsP / InP material will be described below. FIG. 1 shows this array structure, and A to C are plan views A of two LDs (LD1, LD2) as in the case of FIG. 6 showing a conventional example.
2A and 2B are cross-sectional basic structure diagrams B and C in the direction of the capacity.
Like the conventional example, the diffraction grating 10 is formed on the n-InP substrate 11.
0, InGaAsP optical waveguide layer 12, active layers 31, 32,
It is mainly composed of a p-InP clad layer.

The difference from the conventional example is that in the conventional example.
Is the pitch of the diffraction gratings 20 and 21 is Λ ₁, Λ₂Active unlike
Although the composition and layer thickness of the layer 13 were the same,
In the embodiments, the active layers have different layer thicknesses, InG
aAsP well layer (λ_g= 1.3 μm) and InGaAsP
Barrier layer (λ_g= 1.05 μm)
(MQW) structure active layers 31 and 32. MQW here
The active layer 31 has a thickness of 50 Å as shown in the enlarged view of FIG. 1D.
It consists of 5 pairs of well layer 33 and barrier layer 34 of 50 Å, MQ
The W active layer 32 is 100% as shown in the enlarged view of FIG. 1E.
It consists of 5 pairs of Å well layer 35 and 100 Å barrier layer 36.
It In this case, the diffraction grating pitch is Λ ₀= 400
It is the same with 0Å.

The oscillation wavelength of the DFB-LD depends on the effective refractive index of the guided mode of the LD according to the equation (1) in the conventional example (FIG. 2A). In FIG. 1, n _eff of LD1 is 3.18, while that of LD2 is 3.25. Due to this difference in n _eff , the respective oscillation wavelengths are 1.27 μm in LD1 and LD
2, the difference between 1.30 μm and 30 nm is obtained.

On the other hand, in the structure of the present invention, LD1, L
Since the active layers 31 and 32 of D2 are MQW layers with well layer thicknesses of 50 Å and 100 Å, respectively, the band gap energy, that is, the gain peak, of both is different due to the quantum size effect as shown in FIG. 2B. That is, in LD1, it is 1.27 μm, whereas in LD2 it is 1.30 μm. FIG. 3 shows oscillation spectra of two LDs of the present invention. A corresponds to LD1 and B corresponds to LD2. The gain peak almost coincides with the oscillation wavelength.
There is almost no difference between the two as in the conventional example shown in FIG. This is because the oscillation wavelength shift due to the change in n _eff and the gain peak shift due to the quantum size effect occur in the same direction with respect to the change in the well layer thickness.

As described above, in the DFB-LD array of the present invention, the deviation between the oscillation wavelength and the gain peak is small, the variation in the characteristics of each LD in the array is small, and all good current-
The optical output characteristics and temperature characteristics are shown.

Next, a manufacturing process of the DFB-LD array having the structure of the present invention will be described. First, as shown in FIG. 4A, a diffraction grating with a pitch Λ ₀ = 4000Å is formed on an n-type InP substrate by a two-beam interference exposure method. Next, as shown in FIG. 4B, a width S is formed on the substrate in the direction perpendicular to the diffraction grating.
_A plurality of mesa stripes 35 and 36 having different ₁ and S ₂ are formed by ordinary photolithography. On this mesa substrate, as shown in FIG. 3C by liquid phase epitaxial growth, I
The nGaAsP optical waveguide layer 11, the InGaAsPMQW active layer (31, 32), and the p-InP layer 14 are sequentially formed.
According to the liquid phase epitaxy method, the thickness of the epitaxial layer on the mesa stripe is thinner than that of the flat portion and largely depends on the width of the mesa stripe.

FIG. 5 shows the dependency of the well layer L _Z of the MQW active layer and the effective refractive index on the mesa stripe width S. Both the well layer thickness and the effective refractive index decrease as the mesa stripe width decreases. When S ₁ is 10 μm and S ₂ is 20 μm, the well layer thicknesses 33 and 35 in the MQW layers 31 and 32 shown in FIGS. 1D and 1E are 50 Å and 100 Å, respectively, and the effective refractive index n _eff of the active layer is 3.18, respectively. 3.25
And different. As shown in FIGS. 2 and 3, the oscillation wavelength shift due to the difference in effective refractive index and the gain peak shift due to the difference in well layer thickness exhibit the same behavior. Therefore, a multi-wavelength DFB having good characteristics with a small deviation between the oscillation wavelength and the gain peak. An LD array can be obtained.

As described above, in this manufacturing method, an integrated wavelength DFB-LD array having a small variation in characteristics can be obtained by a very simple process of one diffraction grating formation process and basically one epitaxial growth process. it can.

By the way, in this embodiment, for simplicity,
Although the two-wavelength integrated element has been described as an example, the same applies to a multi-wavelength LD array having three or more wavelengths. Also DFB
The structure of -LD is a structure in which a diffraction grating exists under the active layer, but the same applies to the DFB-LD structure having a diffraction grating on the active layer. Although the liquid phase method has been described as the epitaxial growth method, similar effects can be obtained by selecting the conditions in other methods such as the MOVPE method and the MBE method. InGaAs as a material
The P / InP system was explained, but AlGaAs / GaA
The same applies to other III-V group semiconductors such as s series.

[0022]

As described above, according to the present invention, a multi-wavelength integrated laser array can be formed by a very simple process in which each resonator structure constituting the array can be formed by one-time production of a diffraction grating and one-time epitaxial growth. In addition, the difference between the gain peak wavelength determined by the well layer thickness and the oscillation wavelength determined by the total film thickness of the optical waveguide layer, the well layer, and the barrier layer is constant in each resonator. This is a remarkable effect that it is possible to provide a multi-wavelength semiconductor laser array in which lasers having a stable single wavelength with a small variation are integrated in a wider wavelength range.

[Brief description of drawings]

FIG. 1 shows a structure of a DFB-LD array according to an embodiment of the present invention, FIG. 1A is a plan view thereof, and FIGS. 1B and 1C are optical axis directions aa ′ and bb ′. Sectional views, and Figures D and E are enlarged sectional views of the MQW layer in Figures B and C, respectively.

FIG. 2A is a diagram showing an effective refractive index dependency of an oscillation wavelength, and B is a well layer thickness dependency of a gain peak.

FIG. 3A and B are two representative LDs of the array of the present invention.
Figure showing the oscillation spectrum of

4A, 4B and 4C are perspective views and sectional views showing a manufacturing process of a DFB-LD array according to the present invention.

FIG. 5 is a diagram showing the dependence of the effective refractive index and the well layer thickness on the mesa stripe width.

6 shows a structure of a conventional laser, FIG. 6A is a plan view thereof, and FIGS. 6B and 6C are aa ′ and bb ′ of FIG.
Line cross section

7A and 7B are diagrams showing the oscillation spectrum of FIG.

[Explanation of symbols]

 1 LD1 2 LD2 11 n-type InP substrate 12 InGaAsP optical waveguide layer 14 p-type InP clad layer 31 MQW active layer 32 MQW active layer 33 well layer 35 well layer 100 diffraction grating

Claims (1)

[Claims] 1. A semiconductor laser array comprising at least two first and second distributed feedback laser resonators, wherein each resonator emits laser light of a single oscillation wavelength different from each other. Forming a diffraction grating with the same pitch, forming a first resonator region and a second resonator region having different stripe widths on the substrate, in the first and second resonator regions, An optical waveguide layer, a well layer, and a quantum well active layer that is a periodic structure of a barrier layer, and the thickness of the optical waveguide layer, the well layer, and the barrier layer in the first cavity region is the same as that of the second cavity region. A method of manufacturing a semiconductor laser array, comprising the step of simultaneously forming a film that is thinner than the optical waveguide layer, the well layer, and the barrier layer in the resonator region.