JP2924433B2 - Semiconductor laser and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser used for optical communication and a method of manufacturing the same.

[0002]

2. Description of the Related Art The coherent optical transmission system has attracted attention as a long-distance transmission system because it has higher receiving sensitivity than the direct detection system. Among them, F which is highly feasible
In the SK method, a semiconductor laser is directly modulated to change an oscillation frequency. There is a demand for a single-wavelength light source that has a small frequency fluctuation, that is, a narrow oscillation spectrum line width, a high frequency modulation (FM) modulation efficiency, and a flat FM response characteristic. Distributed feedback semiconductor laser (DFB) as a light source for coherent transmission
−LD), and some transmission experiment results have been reported so far. Among them, a DFB-LD having an active layer having a quantum well structure (hereinafter abbreviated as MQW-DFB-LD) is particularly promising as a light source for coherent transmission because it operates in a narrow spectral linewidth reflecting the effect of reducing the linewidth enhancement coefficient. Have been watched.

[0003]

As a long-distance, large-capacity light source for coherent transmission, the inventors of the present invention actually manufactured and evaluated a λ / 4 shift MQW-DFB-LD, and as a result, found that a large number of devices were obtained. Showed good results in both line width and FM response characteristics, but it was recognized that some devices had low FM modulation efficiency. In a device having a good FM response, the low frequency dip frequency is 1 to 50 kHz, and 200 to 300 MHz / in a frequency range of 1 MHz to 2 GHz.
Although a high FM modulation efficiency of about mA was shown, some elements have a low FM modulation efficiency of about 100-150 MHz / mA or less, and some have a sharp decrease in FM modulation efficiency from a high frequency region near 1 GHz. For example, 2.5 Gb /
s, it was not easy to obtain a desired frequency displacement.

In view of the above, it is an object of the present invention to provide a DFB-LD having a high FM modulation efficiency over a wide frequency range.
To provide a semiconductor laser structure for stably realizing the above and a method for manufacturing the same.

[0005]

Means for Solving the Problems A semiconductor laser provided by the present invention to solve the above-mentioned problems and a method of manufacturing the same are as follows. (1) The phase shift region is located adjacent to the active layer that undergoes radiative recombination.
Feedback type semiconductor laser with a diffraction grating
The active layer is a well layer, a barrier layer and
A quantum well structure comprising a guide layer, wherein the phase shift
In the vicinity of the region, the guide layer and the barrier layer
A semiconductor laser having a high conduction band lower end. (2) The half where the diffraction grating having the phase shift region is formed
Forming a striped insulating film on a conductive substrate;
In the region sandwiched between the insulating films by vapor phase epitaxy
Selectively growing a semiconductor layer such as an active layer
In the method for manufacturing a semiconductor laser, the insulating film may
In the vicinity of the phase shift area, the width is narrower than in other areas.
A method for manufacturing a semiconductor laser, comprising:

[0006]

The present inventors obtained the following findings as a result of evaluating and analyzing the FM response characteristics of MQW-DFB-LD. (1) As a result of evaluating the frequency characteristics of the FM response, κL dependence as shown in FIG. 2 was recognized. This is thought to be due to the following two effects. One is that the larger the element of κL, the more the electric field of light concentrates on the phase shift position at the center of the element, and the higher the photon density inside the resonator, the higher the FM modulation efficiency in the low frequency region due to the effect of the nonlinear gain. Second, the equivalent phase shift amount fluctuates due to the effect of axial hole burning, and the threshold gain also changes accordingly, so that the wavelength fluctuation amount, that is, the FM modulation efficiency is further increased. In such a phenomenon, when the fluctuation of the injection current occurs, it takes time for the electric field distribution inside the resonator and the carrier density distribution to be balanced as a whole, before the steady state is settled. Since the response time is longer than the lifetime, the FM modulation efficiency is reduced in a high frequency range. (2) As shown in FIG. 3, an experimental result was obtained in which the FM modulation efficiency increased with an increase in the thickness of the active waveguide including the MQW active layer and the guide layers on both sides. It is considered that this is because the amount of carriers overflowing to the guide layer increases, so that the amount of change in the refractive index there increases, and at the same time, the amount of change in the gain as viewed from the entire device becomes relatively small.

[0007] From the above, the thickness of the guide layer and the κL
Is considered to be small, it is possible to realize FM modulation characteristics with high FM modulation efficiency and at the same time flat frequency response. However, if the value of κL is too small, the threshold gain increases, so that the threshold current,
Therefore, the operating current increases.

Therefore, the gist of the present invention is to reduce the effect of axial hole burning without increasing the threshold gain too much.
An object is to realize a flat FM response characteristic. For that purpose, the interaction between the electric field and the carrier may be reduced near the phase shift region where the electric field of light is concentrated. From the knowledge of the above (2), it can be seen that carrier leakage to the guide layer has a great effect on the FM response characteristics, and it is important to generate a certain amount of carrier leakage in order to increase the FM modulation efficiency. is there. Therefore, the energy gap of the guide layer should be increased in the area where the electric field is concentrated.
Therefore, if the amount of carrier leakage is reduced, the effect of axial hole burning is suppressed even if the supply of light to the diffraction grating is large to a certain extent. And a FM modulation characteristic having a flat frequency response can be realized.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to the drawings showing embodiments. FIG. 1 shows a DFB according to an embodiment of the present invention.
It is a top view for showing a manufacturing process of LD, and an element sectional view. To manufacture such a DFB-LD, first,
A diffraction grating (period 2400 °) was formed on the surface (10
0) A SiO ₂ film having a thickness of about 2000 ° is formed on a p-InP substrate 1 having a plane orientation by a CVD method, and FIG.
As shown in the figure, the width of the wide part is 8 μm and the width of the narrow part is 6 μm
Is patterned in parallel stripes at intervals of 2 μm,
A SiO ₂ mask 2 is formed. Here, the longitudinal direction of the mask 2 was the same direction as the repetition direction of the diffraction grating, and the narrow portion was 100 μm in length and 900 μm repeatedly. The phase shift region was located at the center of the narrow portion of the mask. The p-InGaAsP guide layer 3 having an emission wavelength of 1.3 μm is formed at a thickness of 1500 ° and the non-doped InP spacer layer 4 is formed at a portion sandwiched between the wide areas of the mask 2 by MOVPE on the substrate 1. 5
At 00 °, the MQW active waveguide layer 5 including the guide layers on both sides and the n-InP clad layer 6 are grown to a thickness of 3000 ° (FIG. 1B). The diffraction grating depth after growth is about 250mm,
The coupling coefficient was about 30 cm ^-1 . The MQW active waveguide layer 5 has a thickness of 70 .ANG.
バ リ ア barrier layer (1.3 μm-composition InGaAsP), and a guide layer (1.
3 μm composition InGaAsP). Further, a part of the mask 2 is etched in a stripe shape,
In the region of 6 μm in width covering the W active waveguide layer 5, n-
The InP cladding layer 7 is 2 μm thick, n-InGaAsP
A contact layer 8 (1.3 μm composition InGaAsP) was grown to a thickness of 0.5 μm (FIG. 1C). At this time, in the phase shift region at the center of the device, the guide layer thickness on both sides of the MQW well layer is about 1300 °, and the liquid crystal composition is 1.25 μm.
When laser oscillation was performed, the amount of carrier leakage in that portion was relatively small, and a structure in which axial hole burning was suppressed was obtained. After forming the SiO ₂ film 9 on the entire surface of the laser wafer grown as described above and opening only the portion of the contact layer 8 on the upper surface of the MQW active layer, electrodes are formed on both the growth layer side and the substrate side. MQW-D
Obtain FB-LD.

[0010] The MQW-DFB-LD thus produced
Is set to a length of 90 so that the phase shift region is located at the center of the device.
Cut to 0 μm, and form AR coating films on both end faces,
When the characteristics were evaluated, at an oscillation wavelength of 1.55 μm band,
A device with good performance with an oscillation threshold current of 25 mA, a differential efficiency of 0.2 W / A, a maximum output of 50 mW or more, and a spectral line width of about 0.5-1 MHz at a light output of 30 mW was obtained with good reproducibility. In addition, when the FM modulation characteristics were evaluated, a relatively high FM modulation efficiency of about 300 MHz / mA was obtained because the guide layer sandwiching the MQW well layer was sufficiently thick, and leakage to the guide layer occurred in the phase shift portion where the electric field was concentrated. However, since the amount of carriers is small, the effect of axial hole burning is also reduced, and the 3 dB band 5 GHz
An FM modulation characteristic having a very flat frequency response with z was obtained.

In the embodiment of the present invention, an element having a wavelength band of 1.2 to 1.6 μm using InP as a substrate is shown.
Of course, the material system used is not limited to this.
Other materials such as AlAs may be used. In the embodiment, the semiconductor layer having a different composition and thickness is grown automatically by changing the width of the mask for selective growth by using the selective MOVPE technique. It is possible to use a method of partially etching the semiconductor layer and growing a semiconductor layer having a different composition and thickness there.

[0012]

As described above, the DFB-LD of the present invention
The energy of the guide layer sandwiching the MQW well layer
An FM having a flat frequency response characteristic in which axial hole burning is suppressed by increasing the gap to obtain high FM modulation efficiency and reducing the amount of leaked carriers in a portion where an electric field is concentrated. Modulation characteristics were realized. Such a single wavelength LD has a Gb / s FSK.
This is useful when applied to a coherent transmission system.

[Brief description of the drawings]

FIG. 1 shows an MQW-DFB-LD according to an embodiment of the present invention.
(A) and cross-sectional views (b) and (c) for showing a manufacturing process of (a).

FIG. 2 is a diagram showing FM response characteristics of elements having different κL, and shows that as κL is larger, FM modulation efficiency in a low frequency range is higher and a decrease amount in a higher frequency range is larger.

FIG. 3 is a diagram showing the dependence of the FM modulation efficiency on the active waveguide layer at a modulation frequency of 2 GHz. The number of MQW well layers is 3, and the guide layers on both sides occupy most of the active waveguide layer thickness. ing. This shows that the FM modulation efficiency increases as the thickness of the active waveguide layer increases.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 mask 3 guide layer 4 spacer layer 5 MQW active waveguide layer 6 n-InP layer 7 clad layer 8 contact layer 9 insulating film 10, 11 electrode

Claims (2)

(57) [Claims] 1. A adjacent to the active layer emitting recombination, a phase
Distributed feedback type with diffraction grating with shift region
In the semiconductor laser, the active layer is a well layer, a barrier,
Having a quantum well structure comprising a layer and a guide layer,
The guide layer and the barrier layer are closer to the phase shift region than to the other regions.
A semiconductor laser characterized by having a high conduction band lower end of a rear layer.
The. 2. A diffraction grating having a phase shift region is formed.
Forming a striped insulating film on the semiconductor substrate,
Vapor phase formation occurs in the region sandwiched between the stripe-shaped insulating films.
A method for selectively growing semiconductor layers such as active layers by the long method
The method of manufacturing a semiconductor laser, comprising:
Is wider than the other regions in the vicinity of the phase shift region.
Method for manufacturing semiconductor laser characterized by narrowing the width
Law.