JP2913947B2 - Quantum well semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a quantum well semiconductor laser. Specifically, the present invention relates to a quantum well semiconductor laser having a Fabry-Perot resonator structure.

[0002]

2. Description of the Related Art In a conventional general semiconductor laser, the emission wavelength shifts to a longer wavelength side (positive characteristic) because the band gap of the active layer decreases with an increase in ambient temperature.
Is known, and the wavelength variation with temperature (for example, about 0.
27 nm / ° C).

If the emission wavelength of laser light emitted from a semiconductor laser has temperature dependence, for example, when focusing or collimating the laser light using a lens, a problem arises that the focal position is shifted due to a change in temperature.
For this reason, the semiconductor laser is required to have a small wavelength variation (temperature stability) with respect to a change in the ambient temperature.

On the other hand, OQE (Optical Quantum Electron)
ics); 1991, Vol. 14 (published by the Institute of Electronics, Information and Communication Engineers)
According to a paper published in, a semiconductor laser with a single quantum well structure having two or more quantum levels in a quantum well is used.
It has been reported that a change in wavelength exhibits a negative characteristic in a specific temperature range by optimizing the cavity length.

FIG. 8 is a diagram for explaining this negative characteristic in principle. Considering a two-level single quantum well structure as shown in FIG. 8A, the temperature characteristic of the emission wavelength is as shown in FIG.
It is shown in (b). If the transition between the quantum levels is not taken into account, the emission wavelength λ of the laser light stimulated by the electrons at the lowest level (ground level) E ₀ shows a positive characteristic.
As shown by the broken line A in FIG. 3B, when the temperature T increases, the emission wavelength λ increases. Similarly, the upper level (first excitation level)
The emission wavelength λ of the laser light stimulated by the electrons at E ₁ also shows a positive characteristic, but the energy is larger than the lowest level E ₀ , so that a characteristic like a broken line B shifted to a shorter wavelength side than the broken line A. become. However, when the resonator length is shortened, the transition to the upper quantum level E ₁ is more likely to occur, and the electrons at the lowest level E ₀ at a low temperature gradually increase in the upper level E _{1 as the} temperature T increases. Transition to. Since the semiconductor laser also emits light in this transition process, the curve showing the temperature characteristic of the emission wavelength λ shifts from the broken line A to the broken line B and is represented by a characteristic curve such as a real curve C. Therefore, a negative characteristic is exhibited during the transition from the top of the broken line A to the broken line B, and the emission wavelength λ becomes shorter as the temperature T increases.

[0006]

In the quantum well laser exhibiting the negative characteristic as described above, the emission wavelength is determined at the boundary region R1 at which the positive characteristic shifts to the negative characteristic or at the boundary region R2 at which the negative characteristic shifts to the positive characteristic. The temperature fluctuation of λ is reduced. However, such a quantum well laser has a problem that the wavelength fluctuation is small and the region is narrow.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a quantum well type anti-conductor laser having high temperature stability over a wide temperature range. is there.

[0008]

According to a quantum well semiconductor laser of the present invention, at least one quantum well having two or more quantum levels is provided in a Fabry-Perot quantum well semiconductor laser. The reflectance at the intermediate emission wavelength is higher than the reflectance at the emission wavelength corresponding to the quantum level and the reflectance at the emission wavelength corresponding to the quantum level higher by one than the quantum level. The raised end face coat is applied to at least one of the emission end faces.

[0009]

According to the present invention, in a quantum well structure having two or more quantum levels, the reflectance is lower than the reflectance at each emission wavelength corresponding to the lower quantum level and the higher quantum level. Since the end face coat with higher reflectance at the emission wavelength is applied, laser operation in the middle emission wavelength region becomes dominant, and shifts from a lower quantum level to a higher quantum level. In the process, a region having a small wavelength variation occurs in a wide range.

[0010]

FIG. 1 is a sectional view showing the structure of a multiple quantum well semiconductor laser device 1 having a Fabry-Perot resonator structure according to one embodiment of the present invention. This is n-GaAs
For example, molecular beam epitaxial growth (MB
The N-AlGaAs lower cladding layer 3, the active layer 4 having a thickness of 3 μm, the p-AlGaAs upper cladding layer 5, and the p-GaAs cap layer 6 are sequentially grown by the method E). And an n-type electrode 8 provided on the lower surface of the substrate 2.

The active layer 4 has a multiple quantum well structure (a quad quantum well structure in FIGS. 2 and 3), and has an SCH structure in which well layers and barrier layers are alternately stacked. Has become. This quantum well structure is realized by changing the Al composition x of Al _x Ga _{1 -x} As.
FIG. 2 shows the Al composition ratio x in the vicinity of the active layer 4. In the upper and lower clad layers 5, 3, the Al composition is x = 0.6, whereas in the barrier layers 10a, 10b, the Al composition x =.
0.3, and in the well layers 9a to 9d, the Al composition x = 0 (GaAs
s). The thickness (well width) of the GaAs well layers 9a to 9d is S1 = 120 °, and the thickness (barrier width) of the AlGaAs barrier layer 10b between the well layers 9a to 9d is S0 = 80 °.

FIG. 3 shows a multiple quantum well structure of the active layer 4 formed by changing the Al composition as shown in FIG. 2. In each of the well layers 9a to 9d, a discrete quantum level E ₀ and E ₁ are generated. This lowest quantum level E ₀
Corresponds to an emission wavelength of 850 nm, and the higher quantum level E ₁ corresponds to an emission wavelength of 790 nm.

The cavity length of the semiconductor laser device 1 is 300 μm, and both emission end faces (cleavage faces) 11 have an emission wavelength λ corresponding to the lowest quantum level E ₀ as shown in FIG. = 850 nm, the emission wavelength corresponding to the higher quantum level E ₁ corresponding to the higher quantum level E ₁ , and the reflectance at the emission wavelength corresponding to the lowest quantum level E ₀ and the higher quantum level E ₁ at the emission wavelength corresponding to the lowest quantum level E _0. An end face coat 12 made of a dielectric film such as SiO ₂ or Al ₂ O ₃ is applied so that the reflectivity is higher than the reflectivity at the corresponding emission wavelength. For example, FIG. 4, the central wavelength lambda ₀ = 820 nm facet coating 12 of SiO ₂ and Al ₂ O ₃ and five layers one by lamination with a film thickness of lambda _0/4, respectively, in
(Calculated value) in the range of 790 nm to 850 nm.

FIG. 5 is an explanatory diagram showing the temperature characteristics of the emission wavelength λ according to the above embodiment. As described above, two quantum levels E ₀ and E ₁ exist in the active layer 4, and the reflection at each emission wavelength corresponding to the lowest quantum level E ₀ and the higher-order quantum level E _1. Since the reflectance at the intermediate emission wavelength is higher than the reflectance, the laser operation in the intermediate emission wavelength region becomes dominant. As a result, when the temperature rises, the transition to the higher-order quantum level E ₁ becomes easier, but a large temperature change is required before the transition from the lowest quantum level E ₀ to the higher-order quantum level E ₁ . As the region of the negative characteristic becomes longer, the gradient of the negative characteristic becomes gentler, and the wavelength variation of the emission wavelength λ decreases over a wide temperature range as shown by a curve G in FIG.

FIG. 6 is a diagram showing the relationship between the reflectance and the wavelength in another end face coat 12 used in the semiconductor laser device 1 according to the first embodiment. This is because an intermediate wavelength 820 nm between the emission wavelengths (850 nm and 790 nm) corresponding to the two quantum levels E ₀ and E ₁ is set as a center wavelength λ _0, and AlG having different compositions is used.
shows the reflectance of the GaAs lambda _0/4 of the thickness facet coating 12 formed 15 layers alternately (the calculation result). In this reflectivity curve, the reflectivity at the intermediate emission wavelength is significantly higher than the reflectivity at the emission wavelength corresponding to the lowest quantum level E ₀ and the higher-order quantum level E ₁ .

FIG. 7 is a sectional view of a multiple quantum well semiconductor laser device 21 having a Fabry-Perot resonator structure according to another embodiment of the present invention. As in this embodiment, the end face coat 12 may be any one of the emission end faces 11.

Although the above embodiment has been described using an AlGaAs-based material, the present invention is not limited only to this material system.
The present invention is also applicable to sP-based materials and AlGaInP-based materials for visible light lasers.

[0018]

According to the present invention, since the laser operation in the intermediate emission wavelength region becomes dominant, the wavelength variation in a wide range during the transition from the lower quantum level to the higher quantum level. A small area of As a result, a semiconductor laser having a small wavelength fluctuation over a wide temperature range can be manufactured.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a structure of a Fabry-Perot multiple quantum well semiconductor laser device according to one embodiment of the present invention.

FIG. 2 shows Al in an active layer of the semiconductor laser device according to the first embodiment;
It is a figure which shows the change of a composition.

FIG. 3 is a diagram showing a potential and a quantum level in the multiple quantum well structure according to the first embodiment;

FIG. 4 is a diagram showing a relationship between a reflectance of an end face coat and a wavelength in the semiconductor laser device according to the first embodiment.

FIG. 5 is a diagram showing a temperature characteristic of an emission wavelength in the embodiment.

FIG. 6 is a diagram showing the relationship between the reflectance and wavelength of another end face coat used in the semiconductor laser device of the above.

FIG. 7 is a sectional view showing the structure of a Fabry-Perot multiple quantum well semiconductor laser device according to another embodiment of the present invention.

FIG. 8A is a diagram showing a two-level single quantum well;
(B) is a diagram showing the temperature characteristics of the emission wavelength.

[Explanation of symbols]

Reference Signs List 4 active layer 9a to 9d well layer 10a, 10b barrier layer 11 emission end face 12 end face coat E ₀ , E ₁ quantum level

Claims (1)

(57) [Claims] In a Fabry-Perot quantum well semiconductor laser, at least one quantum well having two or more quantum levels is provided. At least one of the emission end faces has a reflectivity at an intermediate emission wavelength that is higher than the reflectance at an emission wavelength corresponding to a quantum level higher by one than the quantum level. A quantum well type semiconductor laser characterized by the above-mentioned.