JPH04372185A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser exhibiting low noise for return light of transverse fundamental-mode oscillation impinging on the surface of a wafer. CONSTITUTION:A quantum well active layer 23 formed onto a compound semiconductor substrate 21 through a first clad layer 22 and a mesa-Shaped second clad layer 24 formed onto the active layer are provided. In a semiconductor laser, the second clad layer has self-alignment type double-hetero structure, in which a current block layer 30 is buried, a first grating 27 is shaped in the direction of a resonator in the base section of the mesa-shaped second clad layer, a second grating 28 is formed at a top section near an end face on the output side, and a high reflecting film 33 is formed on the other end face.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a distributed reflection type (Distr
ibuted Bragg Reflector :D
The present invention relates to a surface-emitting type semiconductor laser (BR), and particularly relates to a semiconductor laser that can oscillate with high output, low noise, and in a single fundamental mode.

0002

2. Description of the Related Art FIG. 2 shows, for example, K. Applied Physics Letters by K. Kojima et al.
et al. Appl. Phys. Lett. ，
50(24), pp1705-1707, 1987
) is a perspective view showing the structure of a conventional surface-emitting DBR laser published in .

1 in the figure is an n-type GaAs substrate. On this substrate 1, an n-type AIGaAs cladding layer (lower cladding layer) 2, an active layer 3, and a p-type AIGaAs rib-type cladding layer (upper cladding layer) 4 are formed. An n-type electrode 5 is formed on the back surface of the substrate 1. A grating 6 is selectively provided on the upper cladding layer 4.
is formed. A dielectric film 7 is formed on the upper grading layer 4 where the grating 6 is not present. A p-type electrode 9 is formed on the dielectric film 7. Note that 10 in the figure is an active region for obtaining single fundamental mode oscillation.

Next, the operation will be explained. The light propagating in the resonator mainly exists in the active layer 3, but some of it leaks into the upper and lower cladding layers 2 and 4. When a diffraction grating 6 is provided in the upper cladding layer, the phase of the reflected (scattered) light from each diffraction grating is determined by
The direction of travel of the light is determined. The spacing between each diffraction grating is λ/2
When it is an integral multiple of (λ: wavelength), the light returns to the original direction, but the larger the multiple, the more the light travels in various directions, and when it is an even multiple of λ/2, the light also travels in a direction perpendicular to the substrate 1. It turns out. The laser of this conventional example performs surface emitting operation based on the above-mentioned principle, and at this time, in order to convert the lateral mode of the oscillated laser light into the fundamental mode, the stripe width is usually narrowed to 3 to 4 μm. .

[0005]

[Problems to be Solved by the Invention] However, in conventional DBR surface-emitting semiconductor lasers, the width of the stripes in the active region 10 is narrow, so when trying to increase the injection current to obtain high output, the end face is destroyed due to heat. , or optical damage (
There was a problem that COD) occurred and the output was rate-limited.

[0006] Furthermore, with respect to the return light, in a DBR surface emitting type semiconductor laser, the fact that the oscillation wavelength is stable greatly contributes to the coherence, which increases noise. Therefore, in the use of conventional semiconductor lasers, an isolator is introduced,
The wavefront of the returned light is rotated to prevent interference. However, the introduction of an isolator increases the cost of the system and requires adjustment of the optical axis system, making it unsuitable for optical information processing systems. The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to obtain a semiconductor laser that has low noise with respect to return light that oscillates in a transverse fundamental mode and is emitted to the surface of a wafer.

[0007]

[Means for Solving the Problems] A semiconductor laser according to the present invention is a self-aligned type semiconductor laser comprising a mesa-shaped cladding layer formed on a quantum well active layer, and the cladding layer is embedded with a current blocking layer. It has a double heterostructure,
A grating groove is formed on the bottom of the mesa-shaped cladding layer in the cavity direction, a grating is also formed on the mesa top of the mesa stripe near one end face on the optical output side, and a high reflection coating is formed on the other end face. It is characterized by being

[0008] Furthermore, in the above structure, if an active mass having a narrower band gap than that of the compound semiconductor substrate is formed, only the mesa stripe portion of the compound semiconductor substrate is removed and a grating is formed on the back surface to emit light from the back surface. This is a semiconductor laser characterized in that it can be extracted.

[0009]

[Operation] In the surface-emitting semiconductor laser with a diffraction grating region configured as above, what are the regions with grating regions on the mesa stripes (diffraction regions) and the regions with grating regions on both sides of the mesa stripes with bottoms (active regions)? , are joined smoothly in a tapered shape. In the active region, by changing the remaining amount on both sides of the mesa stripe bottom of the P-type first cladding layer 4 from 0.2 μm to 0.4 μm, when the active layer has a double quantum well structure of 10 nm GaAs, horizontal and horizontal The refractive index difference in the direction is 3× from 1×10-2
10-3, a big change. Therefore, by forming a grating with a period of a standing wave for the oscillation wavelength λ0, the change in refractive index is reduced from 1×10−2 to 3×10−3.
However, since there is wavelength dependence in the grating period, the peaks of the grating as a supersaturated absorption layer have wavelength selectivity. Furthermore, although the spectral width is widened by self-sustained pulsation, only the spectrum that matches the grating period is coupled and guided in the vertical direction due to the wavelength filter effect of the grating formed on the mesa stripe of the diffraction region. Therefore, a high-output, wavelength-stable surface-emitting laser is expected to exhibit self-sustained oscillation, and low noise can be expected.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface emitting semiconductor laser according to an embodiment of the present invention will be described with reference to FIG.

21 in the figure is a Si-doped (100) n-type GaAs substrate. Here, the impurity concentration of this substrate 21 is 2×10 18 cm −3 . On the substrate 21, an n-type AI0.
45Ga0.55As first cladding layer 22 (Si doped, impurity concentration 1 x 1017 cm-3), quantum well type active layer 23 (double quantum well; GaAs quantum well width 10 nm,
AI0.3 Ga0.7 As barrier width 5 nm: non-doped), thickness 0.7 μm p-type AI0.45 Ga0.5
5As mesa-shaped second cladding layer 24 (Mg dove, impurity concentration 7 x 1017 cm-3), p-type A with a thickness of 0.5 μm
I0.3 Ga0.7 As third cladding layer 25 (Mg
A p+ type GaAs cap layer 26 (Mg dove, impurity concentration 7×10 18 cm −3 ) having a thickness of 0.5 μm is formed.

A first grating 2 is disposed in the resonator direction (arrow X direction) on the bottom surface of the mesa-shaped second cladding layer 24.
7 is formed. The first grating 27 is formed only in the active region A, and has a grating period of 251n.
m, depth 0.2 μm. Said second cladding layer 24
A second grating 28 is formed on the top of the grating 28 . The second grating 28 is formed only in the diffraction region B, has a grating period of 251 nm, and has a depth of 0.2 μm.
It is. SiO2 is formed on the second grating 28.
A protective film 29 made of is formed. 30 in the diagram is
An n-type GaAs current blocking layer 30 (Si-doped, impurity concentration 2×10 18 cm
-3) is embedded. Furthermore, a p-type electrode 31 is formed on the front side of the active region A, and an n-type electrode 32 is formed on the back side.
is formed. Further, a high reflection film 33 is formed on the side surfaces of the substrate 21, the first to third cladding layers, the active layer 23, the current blocking layer 30, the p-type electrode 31, and the n-type electrode 32. Next, a method for manufacturing the semiconductor laser having the above structure will be explained.

First, an n-type first cladding layer 22, a quantum well type active layer 23, a p-type second cladding layer 24, and a p-type third cladding layer are formed on a (100) n-type GaAs substrate 21 by the MOVPE method. A layer 25 and finally a p+ type cap layer 26 are successively grown. Next, after applying SiO2 to a thickness of 0.3 μm over the entire surface of the wafer, stripes with a width of 5 μm are formed. Using this stripe as a mask, the first cladding layer 22 is etched away by phosphoric acid etching, leaving 0.4 μm of the first cladding layer 22. Next, a first grating 27 having a grating period of 251 nm and a depth of 0.2 μm is formed only in the active region A by interference exposure method. After that, MPVP
An n-type current blocking layer 30 is formed by a selective growth method using E.
Embed the mesa stripe with.

After removing the SiO2 film, only the mesa stripe region of the diffraction region is exposed by etching to expose the surface of the third cladding layer 25, and again by interference exposure method, a grating with a period of 251 nm and a depth of 0.2 μm is formed on the mesa stripe.
A second grating 28 is formed. Thereafter, a protective film 29 made of SiO2 is formed on the second grating 28, a p-type electrode 31 is formed only in the active region, and an n-type electrode 32 is formed on the back surface. After the cleavage, a high reflection film 33 is formed on the end face of the active region, and the cleavage face on the grating side end face is etched to lower the reflectance, resulting in a surface emitting laser according to an embodiment of the present invention.

In the surface emitting semiconductor laser device having such a structure, carriers injected from the p-type electrode 31 are injected only into the central mesa stripe portion by the current blocking layer 30. Since this structure uses a quantum well structure, the wavelength steepness of its differential gain reduces waveguide loss to 1/2 to 1/3 compared to a normal bulk crystal. Therefore, the light generated by carrier recombination within the active layer efficiently repeats feedback between the high-reflection end face and the distributed reflector for the above-mentioned reason, and oscillates as a laser.

According to the semiconductor laser having the above structure, the thickness of the bottomed portion of the mesa strip of the p-type AIGaAs mesa-shaped second cladding layer 24 changes periodically in the cavity direction. Therefore, by matching this period with the wavelength of the standing wave in the resonator, the loss changes periodically in the direction of the resonator, and the refractive index difference in the horizontal and lateral directions changes from 10-2 to 3 x 10-3. has changed to. When the refractive index difference is changed in the horizontal and lateral directions, self-sustained oscillation occurs because a temporally saturable absorption region is created for the lateral diffusion of injected carriers. However, in the resonator direction, self-sustained pulsation is promoted for a wavelength determined by the period of the diffraction grating, so the broadening of the wavelength spectrum due to self-sustained pulsation is suppressed.

Further, the light reflected by the highly reflective end face is coupled to the diffraction region B in a tapered manner, and its wavelength is limited by the secondary grating formed on the p-type AIGaAs third cladding layer 25. In other words, it has a wavelength filter function. Here, when the spacing between the diffraction gratings is an integer multiple of λ/2 (λ is the oscillation wavelength), it returns to the original direction, but the larger the multiple, the more it moves in various directions, and when it is an even multiple of λ/2, it returns to the original direction. , the light also travels in a direction perpendicular to the substrate 21. Since the active region A is made as long as 300 μm, the spectral line width expands due to self-sustained oscillation, but in the diffraction region B, it is coupled at an even multiple of λ/2, but it seems that the self-oscillation pulse width can be sufficiently coupled. Since it is a high-order grating, it has a wide spectral linewidth rather than a narrow spectrum like a distributed reflector laser, but it can emit high-efficiency, high-output laser light in the in-plane direction that is resistant to return light noise.

[0018] In the above embodiment, a case was described in which a GaAs substrate was used as the compound semiconductor substrate, but the present invention is not limited to this. For example, an InP-based substrate may be used. At this time, by using, for example, InGaAsP, which has a narrower band gap than InP, as the material of the active layer, it is possible to emit light from the back surface side of the substrate (lower side in FIG. 1). In this way, by emitting light from the back side of the substrate, better heat dissipation performance can be obtained compared to the above embodiment.

[0019]

[Effects of the Invention] As detailed above, the semiconductor laser of the present invention has a structure that allows use of MOVPE technology with excellent controllability and mass production, and also has an active layer having a quantum well structure and a steep differential quantum gain. It is a device with high efficiency and good temperature characteristics.

Furthermore, the quantum well structure reduces the optical confinement coefficient within the active layer, that is, strengthens the effect of light seeping into the cladding layer, and increases the efficiency of coupling with the grating region.

Furthermore, in the horizontal direction, the refractive index difference in the remaining amount on both sides of the mesa of the upper mesa-like cladding layer can be changed largely depending on the depth of the grating, and the lateral diffusion effect of carriers can be easily suppressed. A saturable absorption region can be formed, and self-sustained oscillation can be expected. Furthermore, the spectral line width generated by self-sustained pulsation can be efficiently emitted in the diffraction region toward the wafer surface. As described above, according to the present invention, it is possible to realize a laser with high efficiency, high output, good temperature characteristics, stable wavelength, and low noise with respect to return optical noise.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a partially developed surface-emitting semiconductor laser according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional surface-emitting semiconductor laser.

[Explanation of symbols]

21...n-type GaAs substrate, 22...first cladding layer, 23
...Quantum well type active layer, 24...Mesa-shaped second cladding layer, 2
5... Third cladding layer, 26... Cap layer, 27... First grating, 28... Second grating, 29... Protective film, 30... Current blocking layer, 31... P-type electrode, 32... n
Type electrode, 33...high reflection film.

Claims (2)

[Claims] 1. A quantum well active layer formed on a compound semiconductor substrate via a first cladding layer, and a mesa-shaped second cladding layer formed on the active layer, wherein the second cladding layer is a self-aligned double heterostructure in which the second mesa-shaped
A first grating is formed on the bottom of the cladding layer in the direction of the cavity, a second grating is formed on the top near the output side end face, and a high reflection film is formed on the other end face. Features of semiconductor laser. 2. The semiconductor laser according to claim 1, wherein the bandgap of the active layer is narrower than the bandgap of the compound semiconductor substrate.