JPS6114787A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To improve the laser characteristic and the characteristic yield of an element by forming phase shifting regions of different thickness partly along a laser resonance axis direction and forming a diffraction grating over the entire laser resonance direction in a waveguide range, thereby reducing an oscillation threshold current. CONSTITUTION:A groove 2 of 0.2mum is formed on a substrate 1. A buffer 3, a non-doped active layer 4, and a light guide layer 5 corresponding to 1.3mum of light emitting wavelength is sequentially laminated 0.1mum at approx. 0.2mum, 0.2mum and 0.1mum at the groove 2 at the flat part. A diffraction grating 6 is formed on the surface of the layer 5, and a P type InP clad layer 7 is grown thereon. The thus formed double hetero structure (DH) semiconductor wafer is mesa etched, buried in normal step to form a DFB-BHLD. This phase shifting region 8 is formed to alter the wavelength of the light wave in the region 8 to vary the equivalent refractive index.

Description

[Detailed description of the invention] (Technical field of invention) The present invention relates to a distributed feedback semiconductor laser.

(Prior art and its problems) The development of a distributed feedback semiconductor laser (DPE3-LD) is underway as a semiconductor light source that exhibits stable single-axis mode oscillation even during high-speed modulation and can widen the transmission band of optical fiber communications. ing. DFB-LD has a wavelength selection mechanism using a diffraction grating with an appropriate pitch, and Gl)
The results show that even when modulated at a high speed of /S level, oscillation occurs at a single wavelength.

By the way, in a normal DF'13-LD, if there is no n: plane reflection, the threshold gain for the two axial modes sandwiching the Bragg wavelength is equal, so basically the
It is known that axial mode oscillation occurs. If at least one of the output end faces is at an angle η with the reflecting 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 H-side end face reflectance is 0, the diffraction grating phase at the reflecting end face is /. / Case 2
The threshold gains for the two axial modes are equal. Even in a state where the phases are close to each other, the threshold gain difference between the two becomes small, so biaxial mode oscillation occurs, and as the injected current is increased and the optical output is increased, the difference between the two axial modes increases. Mode skipping may occur. Furthermore, since the period of the diffraction grating is about 240 OA, it is impossible to control the phase of the diffraction grating as described above at the reflective end face formed by the cleavage.

(Object of the invention) The object of the invention is the above-mentioned viewpoint! 59. It is an object of the present invention to provide a distributed feedback semiconductor laser which can stably obtain single-axis mode oscillation and has improved characteristics.

(Structure of the Invention) The structure of the distributed feedback semiconductor laser according to the present invention includes 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 on a semiconductor substrate. In a distributed feedback semiconductor laser having a laminated structure having at least the active layer;
The waveguide region made of the optical power layer has a phase shift region having a partially different thickness along the laser resonance axis direction,
It is characterized in that the diffraction grating is formed within the waveguide region throughout the laser resonance axis direction.

(Operation and principle of the present invention) Conventional distributed feedback semiconductor laser (DPB-LD)
In contrast, in the present invention, by forming a region inside the element that shifts the phase of the diffraction grating, single-axis mode oscillation is achieved at the black wavelength for an appropriate amount of phase shift. In this case, the threshold gain can also be significantly lowered compared to the normal case.

In the present invention, the diffraction grating is formed in the usual manner, and the isolateral refractive index of the waveguide region is partially changed to shift the phase of the light wave. That is, regions having different layer thicknesses are formed within the waveguide region to provide a phase cyst region.

In a normal DPB-LD, at the Bragg wavelength, the phases of the light waves shift by π in one round trip, so they cancel each other out. In other words, it cannot oscillate at the Bragg wavelength, which is the so-called stop band. Therefore, if the phase of the light wave is shifted by π by forming portions with different equilateral refractive indexes along the laser resonance axis direction as described above, the Bragg wavelength will be shifted by 2π in one round trip. Can oscillate at Bragg wavelength.

At this time, K simultaneously reduces the threshold gain.

(Example) The present invention will be described in more detail below using drawings showing examples. FIG. 1 shows an embodiment of the present invention, in which a groove 2 of 1 clHO .mu.m and a depth of 0.2 .mu.m is formed on an n-InP substrate 1. As shown in FIG. In addition, an n-InP buffer layer 3,
Non-Dogue I no, which corresponds to an emission wavelength of 1.55 μm,
ie ()a O,4+ ABokai o P O,10 active layer 4, corresponding to emission wavelength 1.3 pm r) In
ayt 4 Ga 118 ASo, atpo, ss
The thickness of the light guide layer 5 is 0.1 μm in each flat part.

Approximately 0.2 ttm and 0.2 μm in the two groove parts, respectively.
n, 0. The layers are sequentially stacked by about IAtn. Width 10μ
When the diameter is around m, the growth rate inside the groove 2 becomes a little high, so that a substantially flat surface can be obtained as a whole as a whole. Although some depressions may be formed in this part, there will be no problem in the subsequent manufacturing process. The surface of the light guide layer 5 has a period of 240O.
A. A diffraction grating 6 with a depth of about 700 A is formed by laser interference exposure, and a p-InP cladding layer 7 is formed on top of the diffraction grating 6.
grew. The p-InP cladding layer was grown at a low i degree of about 550° C. to prevent thermal deterioration of the diffraction grating 6. The double heterostructure (DH) semiconductor wafer thus produced was subjected to mesa etching, and a DFB-BHLD was produced by performing buried growth using a normal process. By forming such a phase shift region 8, the 1-equivalent refractive index changes, so the wavelength of the light wave within the waveguide region 8 changes. In the case of this embodiment, the phase shift region 8
Since the thickness of the active layer is increased in , the wavelength is shortened there, and an appropriate phase shift occurs compared to the case where the phase shift region 8 is not provided. By selecting the above parameters, the phase of the light wave changes by approximately π with respect to the Bragg wavelength, and stable single-axis mode oscillation was observed near the Bragg wavelength. In this way, in the FB-BHLD, an element was cut out by cutting both sides of the HC to a length of 25 Q pm, and the loss threshold current at room temperature CW was 20 tyt A, and the 9-minute quantum efficiency from one side was 2.
5%, a maximum single-axis mode output of 3 QmW, and a maximum single-axis mode CW oscillation temperature of 100'Ca degrees were obtained with good reproducibility.

Additionally, DI'i''B with such a phase shift mechanism
-LD, the influence of the phase of the diffraction grating 6 on the reflective end face is also small. In ordinary DFB-IJ, there are many elements in which mode skipping occurs due to phase conditions at the reflective end face. -As an example, when we arbitrarily cut out a single wafer and investigated the characteristic yield, we found that a normal DF
In the B-LD, a 35-chi element exhibits mode skipping due to the phase condition of the reflective end face at an output level within lomW, but in the DFB-LD, the proportion decreases to 5 chips, and a single mode skip occurs due to the phase shift shown in the example. The homogeneity yield of uniaxial mode oscillation has been significantly improved.

In the embodiments of the present invention, InP is used as the substrate, and InGa is used as the substrate.
Although AsP is used as the active layer and the optical guide layer, the semiconductor material used is of course not limited to this, and GaAA'
As/GaAs system, In0aAs/InAlA
There is no problem in using other semiconductor materials such as , S-based, etc.

In addition, in the embodiment, the entire phase shift region 7 was formed to cause an equivalent refractive index difference mainly due to the difference in the film thickness of the active JVj4, but the active layer was flattened and a method was used to create a difference in the film thickness of the guide layer 4. Good too.

Furthermore, it is also effective to use a mesa instead of a groove and reduce the film thickness in that area.

In the embodiment, the film thickness is increased in the phase shift region 8, but the same effect can be obtained even if the film thickness is formed to be decreased.

(Effects of the Invention) A feature of the present invention is that in a distributed feedback semiconductor laser, a phase shift region is formed and a diffraction grating is uniformly formed throughout the waveguide region. This makes it possible to stably excavate a single mode near the Bragg wavelength, achieving a reduction in the oscillation threshold current, and greatly improving the laser characteristics and device characteristic yield of the DFB-LD.
was able to obtain.

[Brief explanation of the drawing]

FIG. 1 shows a sectional view of a DFB-LD which is an embodiment of the present invention. In the figure, 1 is an n-InP substrate, 2 is a groove, and 3 is an n-InP substrate.
P buffer layer, 4 active layers, 5 optical guide layer,
6-digit diffraction grating, 7 is p-InP cladding layer,
Each of the 8 phase shift areas is completed. 71-1 Figure

Claims (1)

[Claims] A 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, A waveguide region consisting of at least the active layer and the optical guide layer has a phase shift region partially different in thickness along the laser resonance axis direction, and within the waveguide region, the entire laser resonance axis direction is A distributed feedback semiconductor laser characterized in that the above-mentioned diffraction grating is formed.