JPH0656904B2 - Integrated distributed feedback semiconductor laser - Google Patents

Description

The present invention relates to an integrated distributed feedback semiconductor laser.

[Prior art]

A distributed feedback semiconductor laser (DFB-LD) is being developed as a semiconductor light source that exhibits stable single-axis mode oscillation even during high-speed modulation and can have a large transmission band in optical fiber communication. The DFB-LD has a wavelength selection mechanism using an appropriate pitch diffraction grating, and the result is that it oscillates stably at a single wavelength even when modulated at a high speed of Gb / s level.

[Problems to be solved by the invention]

By the way, it is known that the normal DFB-LD basically oscillates in the biaxial mode because the threshold gains for the two axial modes sandwiching the Bragg wavelength are equal when there is no end face reflection. . When at least one of the output end faces is a reflection end face, the oscillation wavelength and the threshold gain sandwiching the Bragg wavelength become asymmetrical, and oscillation occurs in one axis mode. Even in that case, when the reflectivity on one side is 0 and the diffraction grating phase on the reflecting surface is close to π / 2 and 3π / 2, the difference in threshold gain between the two axial modes becomes small, so the biaxial mode It becomes easy to oscillate. Moreover, the diffraction grating period is about 2400Å, and it is impossible to control the above-mentioned diffraction grating phase on the reflecting surface formed by "cleavage".

An object of the present invention is to provide an integrated distributed feedback type semiconductor laser which can obtain stable single axis mode oscillation and have improved characteristics.

[Means for solving problems]

The present invention relates to an integrated distributed feedback semiconductor laser having a laminated structure of an active layer and an optical guide layer having a larger energy gap than the active layer and a diffraction grating formed on one surface on a semiconductor substrate. In the integrated circuit, a phase control region is provided, the phase control layer in the phase control region is a semiconductor layer having a larger energy gap than the active layer, and an independent electrode is provided in the phase control region. Distributed feedback semiconductor laser.

[Action]

In contrast to the conventional DFB-LD, in the present invention, a region for controlling the phase of the light wave is formed inside the element, so that the phase shift amount is appropriately set and single axis mode oscillation is performed at the Bragg wavelength. In this case, the threshold gain can be greatly reduced as compared with the normal case.

The effect of phase shift was reported by Umiya et al. (Electron. Lett. 20 , p326 (1984)).

In the present invention, in order to carry out such an appropriate phase shift, the diffraction grating is usually formed so as to form a phase control region which is partially transparent to a light wave. In an ordinary DFB-LD, the phases of the light waves at the Bragg wavelength are offset by π radian in one round trip, 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, as described above, if a phase control region is formed along the laser resonance axis direction and the phase of the light wave is shifted by π radian, two round trips at the Bragg wavelength occur.
It will be deviated by π, and will therefore oscillate at the Bragg wavelength. At this time, at the same time, the threshold gain is also reduced, and it is expected that the oscillation threshold current will be reduced, the temperature characteristics will be improved, and the quantum efficiency will be improved.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to the drawings illustrating embodiments.

FIG. 1 shows an embodiment according to the present invention. First, the diffraction grating 2 is formed on the n-InP substrate 1, and then the emission wavelength 1.3
n-In _{0 · 22} Ga _{0 · 28} As _{0 · 61} P _{0 · 39} light guide layer 3 corresponding to μm, undoped In _{0 · 59} Ga corresponding to emission wavelength of 1.55 μm
_{0 · 41} As _{0 · 90} P _{0 · 10 The} active layer 4, the p-InP cladding layer 10, 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 2400Å and the depth was 1000Å. The growth of the light guide layer 3, the active layer 4, etc. was carried out at a relatively low temperature of 600 ° C. or lower in order to prevent the diffraction grating 2 from disappearing. As a result, the diffraction grating 2 was stored at a depth of 500 to 600 Å even after crystal growth. Only the active layer 4 and the light guide layer 3 were selectively etched only in the portion to be the phase control region 6, and the phase control layer 5 having an emission wavelength of 1.3 μm was laminated. At this time, crystal growth was performed at a normal temperature of about 640 ° C., and the diffraction grating 2 almost disappeared. The phase control layer 5 is formed at a height such that light waves are smoothly coupled. After that, mesa etching is performed by a normal process, crystal growth is performed in the embedded structure, and the DFB electrode 8,
9. The phase control electrode 7 was independently formed to obtain the desired DFB-LD. In the embedded structure, the width of the active layer was 1.5 μm.
In order to ensure good insulation between the electrodes when forming the electrodes, an etching groove may be formed between the electrodes, or a semi-insulating semiconductor layer may be formed by performing proton irradiation.

The DFB-LD manufactured as described above was cut out so that the device length was 250 μm and the phase control region 6 having a length of 20 μm was arranged at the central portion thereof, and the characteristics were evaluated. As a result, by setting the control current flowing in the phase control region 6 to several mA,
At room temperature CW, an element having characteristics of oscillation threshold current of 20 mA, differential quantum efficiency of 30% from one side and maximum single axis mode output of 30 mW was obtained with good reproducibility. By setting the control current appropriately, the intensity ratio between the main mode and the sub mode could be kept at about 30 to 40 dB. In addition, a wavelength change of about 10 Å was observed by changing the control current, and the change of the optical output at that time was also very small.

Further, in the DFB-LD having such a phase control region 6, the influence of the phase of the diffraction grating 2 on the reflection end face is small. In the usual DFB-LD, the number of elements that cause mode skipping was reduced due to the phase condition at the reflection end face. As an example, when the characteristic yield was investigated by arbitrarily cutting out from one wafer, 35% of the elements were 10% in the normal DFB-LD.
The mode jump due to the reflection end face phase condition was shown at the output level within mW, but in the phase control DFB-LD shown in the example, the ratio was reduced to 5%, and the uniform yield of single axis mode oscillation was significantly increased. Improved.

In the above embodiments, the semiconductor material of the wavelength 1 μm band, which uses InP as the substrate and InGaAsP as the active layer, is shown. However, the semiconductor material used in the present invention is not limited to this, and GaAlAs / GaAs series Other semiconductor materials such as InGaAs / InAlAs can be used without any problem. In the embodiment, the diffraction grating 2 is formed directly on the substrate 1, and the light guide layer 3 and the active layer 4 are grown on the diffraction grating 2. However, the active layer and the light guide layer are laminated in advance, and the light guide layer is formed on the light guide layer. A diffraction grating may be formed. Further, the phase control layer 5 was grown after the optical guide layer 3 and the active layer 4 were laminated, but the order may be reversed. Further, in the embodiment, the characteristics were evaluated with the DFB electrodes 8 and 9 in common, but three electrodes were driven separately,
The DFB region may be used as a modulator.

〔The invention's effect〕

A feature of the present invention is that in the integrated distributed feedback semiconductor laser, a phase control region transparent to the oscillation wavelength is formed. This makes it possible to stably oscillate the single axis mode near the Bragg wavelength, reduce the oscillation threshold current, improve temperature characteristics, improve quantum efficiency, etc.
DFB-LD with greatly improved laser characteristics and device characteristic yield
It has the effect of being able to obtain

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view in the resonator direction of a DFB-LD which is an embodiment of the present invention. In the figure, 1 is an InP substrate, 2 is a diffraction grating, 3 is an optical guide layer,
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, and 10 is a p-InP cladding layer.

Claims (1)

[Claims] 1. An integrated distributed feedback type having a laminated structure of an active layer and an optical guide layer having a larger energy gap than the active layer and a diffraction grating formed on one surface on a semiconductor substrate. In the semiconductor laser, a phase control region is provided, and the phase control layer in the phase control region comprises a semiconductor layer having a larger energy gap than the active layer,
An integrated distributed feedback semiconductor laser, wherein independent electrodes are provided in the phase control region.