A semiconductor laser device

Abstract: A semiconductor laser device having two or more close-spaced active layers (188a, 188b) each of quantum well structure. These layers are of different thickness and/or different composition. The quantised energy levels of neighbouring wells are displaced relative to each other.

Description: The present invention relates to a semiconductor laser device for emitting laser light at a plurality of different wavelengths, particularly a quantum well type semiconductor laser device that is capable of oscillating at higher quantum levels than the lowest quantum level. In a semiconductor laser it is quite important in many applications for the laser to oscillate at as short a wavelength as possible. Furthermore, lasers which emit laser light at a plurality of different wavelengths are useful in optical communications, and conventionally there are developed a variety of types. Figure 1 is a cross-sectional view showing a prior art quantum well type semiconductor laser such as shown in Appl. Phys. Lett. vol. 39, pp. 134 to 137 (1981). In Figure 1, the reference numeral 1 designates an n⁺ type GaAs substrate, the reference numeral 2 designates an n type Al z Gal 1-z As cladding layer, the reference numeral 3 designates an n type Al z Ga 1-z As parabola type refractive index distribution layer (z gradually changes to y), the reference numeral 4 designates a p type Al x Ga 1-x As quantum well active layer, the reference numeral 5 designates a p type Al y Ga 1-y As parabola type refractive index distribution layer (y gradually changes to z), the reference numeral 6 designates a p type Al z Ga 1-z As cladding layer, and the reference numeral 7 designates a p⁺ type GaAs contact layer. Furthermore, Figure 2 is a cross-sectional view showing a prior art quantum well type semiconductor laser recited in such as Appl. Phys. Lett. vol. 45, pp. 1 to 3 (1984). In Figure 2, the reference numeral 8 designates a p type electrode, the reference numeral 9 designates a p type GaAs substrate, the reference numeral 10 designates a p type Ga 0.65 Al 0.35 As cladding layer, the reference numeral 11 designates a p type Ga 0.7 Al 0.3 As light guide layer, the reference numeral 12 designates a multiple quantum well layer (MQW layer). The reference numeral 13 designates an n type Ga 0.65 Al 0.35 As cladding layer, the reference numeral 14 designates an n type GaAs contact layer, the reference numeral 15 designates a Zn diffusion region, the reference numeral 16 designates a disordered region, the reference numeral 17 designates a silicon dioxide film, and the reference numeral 18 designates an n type electrode. Figure 3 shows a prior art short wavelength semiconductor laser recited in Iwamura et al., Japanese Journal of Applied Physics vol. 24, No. 12, December 1985, pp. L911 to L913. The reference numeral 19 designates an n type electrode, the reference numeral 20 designates an n⁺ type GaAs substrate, the reference numeral 21 designates an n type AlGaAs cladding layer, the reference numeral 22 designates an n type AlGaAs confinement layer, the reference numeral 23 designates an AlGaAs/AlGaAs multi-layer quantum well active layer, the reference numeral 24 designates a p type AlGaAs confinement layer, the reference numeral 25 designates a p type AlGaAs cladding layer, the reference numeral 26 designates a p⁺ type GaAs contact layer, and the reference numeral 27 designates a p type electrode. Herein, the n type AlGaAs cladding layer 22 has an aluminum composition proportion lower than that of the n type AlGaAs cladding layer 21, and the p type AlGaAs cladding layer 24 has an aluminum composition proportion lower than that of the p type AlGaAs cladding layer 25, and thus a SCH structure is produced. A semiconductor laser in which the active layer of the usual double heterostructure laser is replaced by a quantum well structure or a superlattice structure is called as a quantum well type semiconductor laser. A laser which includes a potential well layer in the active layer is called as a single quantum well laser, and a laser which includes a plurality of potential well layers in the active layer is called as a multi-quantum well laser. The operation of the semiconductor laser element having such a quantum well structure active layer will be described below. Figure 4 is a diagram showing the energy level in the quantum well structure. The longitudinal axis designates energy, and the transverse axis designates position perpendicular to the layers. As shown in Figure 4, when a quantum well is constituted by putting a thin semiconductor layer between semiconductor barrier layers having a large energy band gap, then this thin semiconductor layer constitutes a potential well layer, and the intrinsic energy En measured from the bottom (top) of the conduction (valence) band of the electrons (holes) confined in this well is represented by the following Schrodinger equation (1), and forms discrete energy levels. Herein, m e * is an effective mass of electron, is a Planck's constant divided by 2π, and Lz is thickness of the quantum well layer. Herein, the thickness and height of the barrier layer are presumed to be infinity so as to enable one dimensional treating. In this way an electron has quantized energies En, and the electron state density ρ(E) becomes to have a step like distribution (as shown by solid lines) in the quantum well, which electron state density usually has a parabola type distribution (as shown in broken lines) in the bulk crystal. Accordingly, when each of the layers above and below the active layer comprising the quantum well layer are made p type semiconductor cladding layer and n type semiconductor cladding layer having large energy band gaps, respectively, it is possible to confine the carriers (electrons or holes) and the light in the thickness direction. A semiconductor laser constructed in this manner has an n = 1 level (the lowest quantum level) higher than the bottom of the conduction band (read lines in Figure 5) when the active layer is produced by the material which has the same energy band gap as that of the material constituting the active layer of the usual double heterostructure semiconductor laser, and therefore it oscillates at a shorter wavelength because the energy difference is larger than that of a usual double heterostructure semiconductor laser which oscillates by the energy difference between the bottom of the conduction band and the top of the valence band. Furthermore, the quantum well type semiconductor laser has characteristics that the energy levels are discrete and characteristics having a narrow spectrum line width and a good monochromaticity. The prior art device of Figure 1 will be described. At first, an n type Al z Ga 1-z As which will be a cladding layer 2 will be grown on the n⁺ type GaAs substrate 1, and subsequently, an n type Al z Ga 1-z As (z gradually changes to y) parabola type refractive index distribution layer 3, a p type Al x Ga 1-x As quantum well active layer 4, a p type Al y Ga 1-y As (y gradually changes to z) parabola type refractive index distribution layer 5, a p type Al z Ga 1-z As cladding layer 6, and a p⁺ GaAs contact layer 7 will be grown (herein, z > y > x). Figure 6 shows an energy band structure of the cladding layer, the refractive index distribution layer, and the active layer of the semiconductor laser produced in this manner. Herein, at the above and below of the active layer 4 as more apart from the active layer as larger the aluminum composition proportion is, and as lower the refractive index is, whereby the light generated at the active layer 4 is confined within this parabola type refractive index distribution layer. Furthermore, a cavity is produced with the end facet, whereby a laser oscillation is enabled to occur. This type of laser is called as a GRIN-SCH (Graded-index waveguide separate carrier and optical confinements heterostructure) laser because of the structure in which the confinement of the carriers and light are separated. In this prior art device, when carriers (electrons or holes) are injected while a forward bias is applied carriers are confined in the quantum well active layer 4, and the electrons and holes are recombined at the discrete energy levels (n = 1, n = 2, ...) of the conduction band and the valence band, thereby to generate a light emission. In this case, a sharp light generation wavelength peak (λ₁, λ₂, λ₃, ...) in accordance with the energy levels can be obtained. In a general laser the gain at n = 1 becomes larger than the resonator loss, and an oscillation occurs at the quantum level of n = 1. However, when the loss is increased, combinations of carriers not only at between the n = 1 quantum energy levels of the conduction band and the valence band but also those between the quantum energy levels n = 2 thereof become likely to arise as shown in Figure 4, thereby occurring a laser oscillation at the quantum level of n = 2. The oscillation wavelength of n = 2 is fairly short with relative to the oscillation wavelength of n = 1 because the difference between the quantum energy levels n = 1 and n = 2 is fairly large. Next the operation of the prior art example of Figure 2 will be described. Figure 7 shows a diagram for exemplifying the energy band structure of the cladding layer and the multi-quantum well active layer of the prior art device of Figure 2. In this prior art device having a MQW active layer, if the barrier layer is made quite thin, there arises overlapping of energy levels of the potential wells in the active layer, and it is possible to make almost all the electrons positioned at the lowest energy levels of a few quantum wells. From this, in a MQW laser, the half value width of the optical gain spectrum becomes short with relative to the usual DH laser caused by the stepwise state density function ρ(E) as shown in Figure 5, and the threshold value is expected to be lowered because of the higher density of states. When a current is injected into this MQW laser which is biased in a forward direction, the electrons injected from the upper portion n electrode 18 and the positive holes injected from the lower portion p electrode 8 are confined in the GaAs well layer having a small band gap, and electrons and holes are combined to occur a light emission. A light confinement in the thickness direction is conducted by that as larger the aluminum components in the AlGaAs layer becomes as smaller the refractive index becomes, and the confinement in the transverse direction is conducted by that Zn is diffused. Accordingly, if a resonator structure is produced in addition thereto a laser oscillation will arise. Next, the operation of the prior art device of Figure 3 will be described. The gain spectrum of the active layer obtained when a current is applied in the prior art device of Figure 3 becomes as shown by real lines of Figure 8. Herein, n = 1, 2, and 3 designates the peaks corresponding to the lowest, second lowest, and third lowest quantum levels, respectively. When a current is injected the peak corresponding to n = 1 has the largest gain and a laser oscillation occurs at the wavelength corresponding to this quantum level. In a prior art semiconductor laser in such a construction, it is required to make the quantum well layer quite thin or to increase the aluminum composition proportion in order to conduct an oscillation at a short wavelength. However, it is quite difficult to control the thickness of the quantum well layer to be quite thin, and furthermore when the aluminum composition proportion is increased it becomes likely to arise an end surface oxidation thereby to shorten the life time. Furthermore, when the quantum well is made thin and the aluminum composition proportion is increased the threshold current becomes high. These are obstructions against the wavelength shortening by these method. Furthermore in the prior art semiconductor laser with such a construction, the energy level at which the laser oscillates is controlled. Generally, an oscillation occurs at an energy level of n = 1, and an oscillation at a short wavelength of n = 2 is not likely to occur. In a case where an oscillation occurs at the quantum level of n = 2 when the injection current is increased as described in the operation of the prior art device of Figure 4, the oscillation wavelength difference corresponding to the energy difference ΔE₁₂ between the quantum energy levels of n = 1 and n= 2 becomes quite large. For example, in a case of GaAs quantum well type active layer of layer thickness 10nm (100Å), ΔE₁₂ is about 110 meV and the wavelength difference is 57nm (570Å). A prior art semiconductor laser which emits laser light at a plurality of different wavelengths will be described below. Figure 9 shows a laser structure of a transverse junction stripe laser which emits laser light at four different wavelengths which is described in Appl. Phys. Lett. vol. 36, p. 441 (1980). In Figure 9, the reference numeral 28 designates an upper portion electrode, the reference numeral 29 designates a silicon nitride current blocking layer, the reference numeral 30 designates an n type Al y Ga 1-y As cladding layer, the reference numeral 31 designates an n type Al x1 Ga 1-x1 As first active layer, the reference numeral 32 designates an n type Al x2 Ga 1-x2 As second active layer, the reference numeral 33 designates an n type Al x3 Ga 1-x3 As third active layer, the reference numeral 34 designates an n type Al x4 Ga 1-x4 As fourth active layer, the reference numeral 35 designates an n type GaAs substrate, the reference numeral 36 designates a lower portion electrode, the reference numeral 37 designates a Zn diffusion region which is shown by diagonal lines, and the reference numeral 38 designate a Zn driven region which is shown by broken lines. Herein, the aluminum composition proportions of the cladding layer and active layer are in a relation that x₁<x₂<x₃<x₄<y. The device will be operated as follows. In Figure 9 the holes which flow from the p side upper electrode 28 and the electrons which flow from the n side lower electrode 36 proceed along the path designated by the arrows in the figure, and they generate light emissions at the four active layers 31, 32, 33 and 34. Herein, the wavelengths are such that λ₁<λ₂<λ₃<λ₄ because x₁<x₂<x₃<x₄ (λ n designates the wavelength of light which is emitted from the n-th active layer). This prior art multi-wavelength oscillation laser is one which emits a plurality of wavelengths at the same time, and it is impossible to control oscillation by switching them one by one. The present invention is intended to provide a semiconductor laser device which is capable of emitting laser light at a plurality of different wavelengths. The semiconductor laser device of the present invention comprises a plurality of active layers stacked one upon the other. In accordance with the present invention the aforesaid semiconductor laser device is characterised in that the active layers are each of a single quantum well structure having a different respective thickness and/or respective composition such that the quantised levels of adjacent quantum wells are displaced relative to each other. Conveniently the quantised energy levels of respective wells may be displaced so that the n=1 quantum level of one is at a level intermediate the n=1 and n=2 quantum levels of the other. Preferably the n=2 quantum level of the other is at a level which is intermediate the n=1 and n=2 quantum levels of the one just mentioned. It is noted that either or both the thickness and the composition of the active layers can be varied. In use, laser emission at different respective wavelengths can be switched by changing the current injected into the active layers. In the accompanying drawings: Figure 1 is a diagram showing a structure of a prior art quantum well laser; Figure 2 is a diagram showing a structure of another prior art quantum well laser; Figure 3 is a diagram showing a structure of a prior art quantum well laser having a SCH structure; Figure 4 is a diagram showing the energy band gap structure of the quantum well active layer; Figure 5 is a diagram showing the state density and energy levels of the quantum well; Figure 6 is a diagram showing the energy band structure of a semiconductor laser of a GRIN-SCH structure; Figure 7 is a diagram showing the energy band gap structure of a multiple quantum well active layer; Figure 8 is a diagram showing the wavelength vs gain and resonator loss characteristics of a prior art semiconductor laser; Figure 9 is a diagram showing a structure of a prior art multi-wavelength oscillation laser; Figure 10(a) is a diagram showing the structure of the semiconductor laser of a preferred embodiment of the present invention; Figure 10(b) is a diagram showing the energy band structure of the active layer portion of the preferred embodiment of the present invention; Figure 11 is a diagram showing the energy band gap of the quantum well of the preferred embodiment of the present invention; and Figure 12 is a diagram showing the quantum well energy band structure of an alternative of the preferred embodiment of the present invention. So that the present invention may be fully understood, a preferred embodiment thereof will now be described with reference to the drawings. The following description is given by way of example only. Figure 10(a) shows a preferred embodiment of the present invention. In Figure 10(a) which shows a structure of a semiconductor laser of this embodiment, the reference numeral 185 designates an n⁺ type GaAs substrate, the reference numeral 186 designates an n type Al z Ga 1-z As cladding layer, the reference numeral 187 designates an n type Al z Ga 1-z As parabola type refractive index distribution layer (z gradually changes to y), the reference numeral 188a designates a first Al x Ga 1-x As active layer, the reference numeral 189 designates an Al y Ga 1-y As barrier layer, the reference numeral 188b designates a second Al x Ga 1-x As active layer, the reference numeral 190 designates a p type Al y Ga 1-y As parabola type refractive index distribution layer (y gradually changes to z), the reference numeral 191 designates a p type Al z Ga 1-z As cladding layer, and the reference numeral 192 designates a p⁺-GaAs contact layer. Furthermore, Figure 10(b) shows an energy band structure of a semiconductor laser of the embodiment shown in Figure 10(a). This embodiment has a GRIN-SCH structure. Figure 11 shows a band structure in which double quantum well active layers 188a and 188b are arranged each side of the barrier layer 189. These active layers 188a and 188b have different thicknesses L z1 and L z2 , respectively. Herein, the discrete energy levels have larger energies measured from the bottom of the conduction band the smaller the thickness L z becomes as seen from the Schrödinger equation described in the description of the prior art device. Accordingly, when the values of L z1 and L z2 are designed well, an energy level of n = 1 of the active layer 188b can be established between the energy levels of n = 1 and n = 2 of the active layer 188a (L z1 > L z2 ). The device will be operated as follows. When carriers (electrons and holes) are injected into the semiconductor laser having the above-described active layer with said laser being biased in a forward direction the combinations of electrons and holes occur between the n = 1 levels of the conduction band 193 and the valence band 194 of the active layer 188a, thereby resulting in light emission at a first wavelength. When carriers are further injected, combinations occur between the n = 1 levels of the conduction band 193 and the valence band 194 of the active layer 188b, thereby resulting in a light emission of wavelength λ₂. Next, there arise combinations between the n = 2 levels of the conduction band 193 and the valence band 194 of the active layer 188a, thereby resulting in a emission at wavelength λ₃. In this way, when the injection current is increased, it becomes possible to obtain a laser oscillation at wavelengths λ₁, λ₂, λ₃, ..., gradually becoming shorter. The method of producing the semiconductor device shown in Figure 10 will be described. At first, an n type Al z Ga 1-z As cladding layer 186, an n type Al z Ga 1-z As parabola type refractive index distribution layer (z gradually changes to y) 187, an Al x Ga 1-x As quantum well first active layer 188a, an Al y Ga 1-y As barrier layer 189, an Al x Ga 1-x As quantum well second active layer 188b, a p type Al y Ga 1-y As parabola type refractive index distribution layer (y gradually changes to z) 190, a p type Al z Ga 1-z As cladding layer 191, and a p⁺-GaAs contact layer 192 are successively grown on an n⁺ type GaAs substrate 185. Furthermore, an n type electrode is produced on the side of the n⁺-GaAs substrate 185, a p type stripe electrode is produced on the side of the upper p⁺-GaAs contact layer 192, and laser oscillator mirrors are produced vertically with the active layers 188a and 188b by such as cleavages. Thus, a quantum well type semiconductor laser having a plurality of quantum well active layers is produced. In this embodiment, the active layers are constituted by respective quantum wells having different layer thicknesses, and a plurality of laser emissions at different wavelengths with short wavelength differences are obtained which cannot be obtained by a single quantum well active layer. Furthermore, it is possible to switch the plurality of laser emissions by a small injection current difference. Furthermore in the above-illustrated embodiment, the thicknesses of the quantum wells are varied so as to vary the energy levels of the quantum.wells. But as shown in Figure 11, the material composition can be changed without changing the thicknesses of the quantum wells (L z1 = L z2 ) with the same effects as described above. In Figure 12, the reference numeral 195a designates an Al x1 Ga 1-x1 As quantum well layer, and the numeral 195b designates an Al x2 Ga 1-x2 As quantum well layer, whereas x2 > x1. Furthermore, differentiation of quantum well thicknesses and that of quantum well material compositions may be combined with each other. Furthermore in the above-illustrated embodiment there are provided two quantum wells, but there may be provided more than three quantum wells. Furthermore in the above-illustrated embodiment, a GRIN-SCH structure is used for the confinement of laser light, but other confinement methods can be used. Furthermore in the above-illustrated embodiment, GaAs system material is used as material, but such as InP system material can be used with the same effects as described above.

Claims: A semiconductor laser device comprising: a plurality of spaced active layers (188a, 188b; 195a, 195b) stacked one upon the other; which said semiconductor laser device is characterised in that: said active layers (188a, 188b; 195a, 195b) are each of a single quantum well structure having a different respective thickness and/or respective composition such that the quantised energy levels of adjacent quantum wells are displaced relative to each other. A laser device as claimed in claim 1 comprising two spaced active layers (188a, 188b; 195a, 195b) wherein the quantised energy levels of the adjacent quantum wells are displaced relative to each other such that the n=1 energy level of the quantum well of one of the active layers (188b; 195b) lies at a level that is intermediate the n=1 and n=2 energy levels of the quantum well of the other one of the active layers (188a; 195a). A laser device as claimed in claim 2 wherein the n=2 energy level of the quantum well of said other one of the active layers (188a; 195a) lies at a level that is intermediate the n=1 and n=2 energy levels of the quantum well of said one of the active layers (188b; 195b). A laser device as defined in claim 1 or 3, wherein said active layers (188a, 188b) are comprised of the same composition material and have different respective thicknesses (L z1 , L z2 ). A laser device as defined in claims 1 or 3, wherein said active layers (195a, 195b) have the same respective thicknesses (L z1 , L z2 ) and are comprised of different composition material. A laser device as defined in claims 1 or 3, wherein said active layers have different thicknesses and are comprised of different composition material. Use of a laser device as defined in any of claims 1 to 6, wherein oscillations at a plurality of respective wavelengths corresponding to the respective quantum energy levels of the quantum wells are switched by changing the injection current injected into said active layers.

Inventors: FUJIWARA KENZO C O MITSUBISHI, KOJIMA KEISUKE C O MITSUBISHI, MATSUI TERUHITO C O MITSUBISHI, NOMURA YOSHINORI C O MITSUBISH, TOKUDA YASUNORI C O MITSUBISHI, TSUKADA NORIAKI C O MITSUBISHI, FUJIWARA, KENZO, C/O MITSUBISHI DENKI K.K., KOJIMA, KEISUKE, C/O MITSUBISHI DENKI K.K., MATSUI, TERUHITO, C/O MITSUBISHI DENKI K.K., NOMURA, YOSHINORI, C/O MITSUBISHI DENKI K.K., TOKUDA, YASUNORI, C/O MITSUBISHI DENKI K.K., TSUKADA, NORIAKI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: H01S3
IPC codes: H01S5