JPS6177381A - Integrated distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To stably obtain single axial mode oscillation, and to contrive to improve the characteristics by a method wherein the title device has a phase control region having a phase control layer of larger energy gap than that of the active layer, and this phase control region is provided with independent electrodes. CONSTITUTION:A diffraction circuit 2 is formed on an N-InP substrate 1, and an N-In0.71Ga0.26As0.61P0.39 photo guide layer 3, a non-doped In0.59Ga0.41As0.90P0.10 active layer 4, and a P-InP clad layer 10 are successively laminated thereon. The photo guide layer 3 and the active layer 4 are grown at a relatively low temperature in order to prevent the disappearance of the diffraction grating 2, and then the this diffraction grating 2 is preserved also after crystal growth. The phase control layer 5 is laminated by selectively etching the active layer 4 and the photo guide layer 3 only in the part serving as the phase control region 6. At this time crystal growth is carried out at a normal temperature, and the diffraction grating 2 almost disappears. The phase control layer 5 is formed to a level enough to smoothly couple light waves. Thereafter, crystal growth is carried out into a buried structure by mesa etching by a normal process, and DFB electrodes 8 and 9 and a phase control electrode 7 are independently formed; accordingly, a desired DFB-LD is obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an integrated distributed feedback semiconductor laser. Distributed feedback semiconductor laser (DFB-LD) is a semiconductor light source that can be made large.
) is being developed. DFB-LD has a wavelength selection mechanism using a diffraction grating with an appropriate pitch, and has a wavelength selection mechanism of Gb/
Results have been obtained in which stable oscillation occurs at a single wavelength even when modulated at m-level high speeds.

[Problem that the invention seeks to solve]

By the way, it is known that in a normal DFB-LD, when there is no end face reflection, the threshold gain for the two axial modes sandwiching the Bragg wavelength is equal, so basically biaxial mode oscillation occurs. ing. When at least one output end face is a reflective end face, the relationship between the oscillation wavelength sandwiching the Bragg wavelength and the threshold gain becomes asymmetrical, resulting in oscillation in one axial mode. In that case, when the end face reflectance on one side is 00, if the diffraction grating phase on the reflecting surface is close to π/2, 3π/2, the difference in threshold gain for the two axial modes will be small, so the two axial modes will be uniform. oscillation is likely to occur. Moreover, the diffraction grating period is 240
At about 0 A, it is impossible to control the above-mentioned diffraction grating phase on the reflecting surface formed by the "interfolds".

SUMMARY OF THE INVENTION In view of the above, an object of the present invention is to provide an integrated distributed feedback semiconductor laser that can stably obtain single-axis mode oscillation and has improved characteristics.

[Means for solving problems]

The present invention provides at least an active layer 4 on a semiconductor substrate l;
In an integrated distributed feedback semiconductor laser having a laminated structure with a light guide layer 3 having a larger energy gap than the active layer 4 and having a diffraction grating 2 formed on one surface, the integrated distributed feedback semiconductor laser has a larger energy gap than the active layer 4 . It is characterized in that it includes a phase control region 6 having a phase control layer 5 with a large gap, and that an independent electrode 7 is provided in the phase control region 6.

[Effect]

In contrast to the conventional DFB-LD, in the present invention, by forming a region inside the element to control the phase of the light wave,
By appropriately setting the amount of phase shift, single-axis mode oscillation is achieved at the flag wavelength. In this case, the threshold gain can also be significantly lowered compared to the normal case.

The effect of phase shift has been reported by Mr. Umiya et al. (Electron, Lett, 20. p3
26 (1984 units.

In the present invention, in order to perform such an appropriate phase shift, the diffraction grating is formed in the usual manner, and a phase control region partially transparent to light waves is formed. Normal DFB-
In an LD, the phases of light waves shift by π radians in one round trip at the Bragg wavelength, so they cancel each other out. That is, it does not oscillate at the Bragg wavelength, and this becomes a so-called stop band. Therefore, if a phase control region is formed along the laser resonance axis direction as described above and the phase of the light wave is shifted by π radians, the Bragg wavelength will be shifted by 2π in one round trip, and therefore oscillation will occur at the Bragg wavelength. At this time, the threshold gain is also reduced and the oscillation threshold current is reduced.

Improvements in temperature characteristics, quantum efficiency, etc. are expected. [Example] The present invention will be explained in more detail below using drawings showing examples.

FIG. 1 shows an embodiment according to the present invention.
A diffraction grating 2 is formed on a P substrate l, and n-Ino4Gao, tsA!, corresponding to an emission wavelength of 1.3 μm is formed thereon. 1
oatPo, se optical guide layer 3, emission wavelength 1.55 μm
Non-doped Ino, 5eGao-stA8+ corresponding to
aoPo,t. An active layer 4, a p-InP cladding layer 1O, etc. are sequentially formed. The diffraction grating 2 was formed by a two-beam interference exposure method using a He--Cd gas laser and a chemical etching method. The cycle was 2,400 people and the depth was about 1,000. The growth of the optical guide layer 3, active layer 4, etc. was carried out at a relatively low temperature of 600° C. or lower in order to prevent the disappearance of the diffraction grating 2. As a result, even after crystal acid prolongation, 500 to 600
Diffraction grating 2 was stored at depth A. Phase control area 6
The active layer 4 and the optical guide layer 3 were selectively etched only in the portions where the wavelength was 1.3 μm, and the phase control layer 5 having a composition having an emission wavelength of 1.3 μm was laminated. At this time, crystal growth was carried out at a normal temperature of about 640"c, and the diffraction grating 2 almost disappeared. The phase control layer 5 was formed at a height that allows the light waves to combine smoothly. Thereafter, the mesa The desired DFB-W was obtained by etching and crystal growth in the buried structure to independently form a DFB electrode 8°9 and a phase control electrode 7. The width of the active layer in the buried structure was set to 1.5 degrees. In addition,
In order to provide good insulation between the electrodes when forming the electrodes, a semi-insulating groove layer may be formed by forming etching grooves between the electrodes or by irradiating protons.

The DFB-LD fabricated as described above has an element length of 250 mm.
.mu.m, and the characteristics were evaluated by cutting out so that a phase control region 6 with a length of 20 .mu.m was placed approximately at the center thereof. As a result, by setting the control current flowing through the phase control region 6 to several mA, the oscillation threshold current is 20 mA at room temperature CW.
A device with characteristics of a single-sided differential quantum efficiency of 30 cm and a maximum single-axis mode output of 30 mW was obtained with good reproducibility.

By appropriately setting the control current, the intensity ratio between the main mode and the sub-mode could always be kept at about 30 to 40 dB.

In addition, by changing the control current, a wavelength change on the order of IO was observed, and the change in optical output at that time was very small.

Furthermore, a DFB-LD having such a phase control region 6
In this case, the influence of the phase of the diffraction grating 2 on the reflective end face is also small. In ordinary DFB-LDs, there are many elements in which mode skipping occurs due to phase conditions at the reflective end face. - As an example, when we cut arbitrary pieces from one wafer and investigated the characteristic yield, we found that in a normal DFB-LD, a 35-chi element exhibits mode skipping due to the reflection end face phase condition at an output level of 10 mW or less. However, in the phase control DFB-W shown in the example, the ratio was reduced to 5, and the homogeneity yield of single-axis mode oscillation was significantly improved.

Note that in the above embodiments, InP is used as the substrate, and InGa is used as the substrate.
Although a semiconductor material with a wavelength of 1 μm in which AsP is used as an active layer is shown, the semiconductor material used in the present invention is of course not limited to this, and may be GaAA'As/GaAa type.

There is no problem in using other semiconductor materials such as InGaAs/InAJAs. In addition, in the embodiment, the substrate l
Although the diffraction grating 2 is directly formed on the substrate and the light guide layer 3 and the active layer 4 are grown thereon, the active layer and the light guide layer may be laminated in advance and the diffraction grating may be formed on the light guide layer. .

Although the phase control layer 5 was grown after the optical guide layer 3 and the active layer 4 were further laminated, this order may be reversed.

In addition, in the example, the characteristics were evaluated using the DFB electrodes 8 and 9 in common, but the three electrodes were driven separately, and one DFB
A region may also be used as a modulator.

〔Effect of the invention〕

A feature of the present invention is that a phase control region transparent to the oscillation wavelength is formed in an integrated distributed feedback semiconductor laser. This has enabled stable single-axis mode oscillation near the Bragg wavelength, reduced the oscillation threshold current, improved temperature characteristics, and improved quantum dynamics. DFB with significantly improved characteristic yield
This has the effect that LD can be obtained.

[Brief explanation of drawings]

FIG. 1 shows a DFB-LDO cooid layer which is an embodiment of the present invention, 4 is an active layer, 5 is a phase control layer, 6 is a phase control region,
7 is a control electrode, 8 and 9 are DFB electrodes, i. and p-InP cladding layer respectively. Patent Applicant: NEC Corporation 1: ITIP substrate 2: Diffraction grating 3: Optical layer 4:) layer 5: phase control layer 6: phase control 9a region 7: phase control ii gutter

Claims (1)

[Claims] (1) An integrated distributed feedback semiconductor laser having a laminated structure on a semiconductor substrate, including at least an active layer and an optical guide layer having a larger energy gap than the active layer and having a diffraction grating formed on one surface. An integrated distributed feedback semiconductor laser comprising a phase control region having a phase control layer having at least a larger energy gap than the active layer, and an independent electrode provided in the phase control region.