A SEMICONDUCTOR LASER DEVICE

Abstract: A semiconductor laser having an active layer (104) of quantum well structure, where the resonator loss is enhanced thereby to conduct an oscillation at a high quantum level.

Description: The present invention relates to a quantum well type semiconductor laser device, and more particularly to one which is capable of oscillating at a higher quantum level than the lowest quantum level. In a semiconductor laser it is quite important in many practical applications to make the laser oscillate at as short a wavelength as possible. Furthermore, a laser device which can emit light at each of a plurality of different wavelengths is useful in optical communication. 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 a quantum well type semiconductor laser. A laser which includes a potential well layer in the active layer is called a single quantum well laser, and a laser which includes a plurality of potential well layers in the active layer is called a multi-quantum well laser. The operation of the semiconductor laser element having a quantum well structure active layer will be described below. Figure 1 is a diagram showing the energy levels of a quantum well structure. The longitudinal axis designates energy, and the transverse axis designates position perpendicular to the layers. As shown in Figure 1 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 a step like distribution as shown in figure 2. The electron state density usually has a parabola type distribution, also as shown. Accordingly, when both of the layers respectively above and below the active quantum well layer are made a p type semiconductor cladding layer and an n type semiconductor cladding layer each having large energy band gaps, it is possible both to confine the carriers (electrons or holes) and to confine the light in the thickness direction, so that as a result a quantum well type semiconductor laser is produced. A semiconductor laser constructed in this manner has an n = 1 level (the lowest quantum level) higher than the bottom of the conduction band. When the active layer is produced of material which has the same energy band gap as that of material constituting the active layer of the usual double heterostructure semiconductor laser, 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. An example of a quantum well type semiconductor laser is described in European Patent Application EP-A-0194335. The laser device, which is described with reference to figure 3 thereof, comprises a substrate layer of n-type GaAs, a GaAs buffer layer of thickness 0.1 to 0.5 µm, a lower cladding layer of Ga 1-x Al x As : x = 0.4 and thickness 1 µm, a quantum well active layer structure comprising three sub layers, one of Ga 1-z Al z As : 0.1<z<0 and thickness less than 20 nm sandwiched between two sub-layers of Ga 1-y Al y As : y = 0.2 each of thickness 10 nm, an upper cladding layer of Ga 1-x Al x As : x - 0.4 and thickness 1 nm, and a contact layer of dopant enriched p-type GaAs layer of thickness 0.1 to 0.5 µm. A common electrode is provided on the lower surface of the substrate and a contact structure is provided on the upper undivided surface of the contact layer. This contact structure comprises two spaced apart strip like electrodes which are arranged co-linearly and are of different respective lengths. The longer one of these stripes corresponds to the laser active region, while the shorter one corresponds to a loss control region. Laser loss is controlled by varying the current injected into the laser by means of this shorter stripe. A multi-quantum well active layer structure can be substituted for the active layer described. The present invention also provides a quantum well type semiconductor laser and is an alternative to that just described. In accordance with a first aspect of the present invention there is provided a method of operating a current injection semiconductor laser, which laser includes an active layer of a quantum well structure, at least one current electrode by which means current can be injected into said active layer, and a voltage electrode by which means a reverse bias voltage may be applied and an electric field developed across a portion (B) of said laser, which method comprises: injecting current into said active layer to cause said laser to emit radiation at a first resonant wavelength corresponding to a quantized level transition in said active layer; and applying a reverse bias voltage to said voltage electrode to cause a shift in the exciton absorption spectrum of said portion (B) of said laser so that whilst current is injected into said active layer said laser is caused to emit radiation at a second resonant wavelength corresponding to a quantized level transition different from said first resonant wavelength. In accordance with a second aspect of the present invention there is provided a current injection semiconductor laser system for performing the method of claim 1, which system comprises: a current injection semiconductor laser having an active layer of a quantum well structure, at least one current electrode by which means current can be injected into said active layer, and a voltage electrode by which means a reverse bias voltage may be applied and an electric field developed across a portion (B) of said laser; a current source connected to said at least one current electrode for injecting current into said active layer of the laser to stimulate emission individually at each one of a first resonant wavelength corresponding to a quantized level transition and a second resonant wavelength corresponding to another quantized level transition respectively; and a voltage source connected to said voltage electrode for supplying a reverse bias voltage which shall cause a shift in the exciton absorption spectrum of said portion (B) of said laser sufficient that resonance at said first resonant wavelength shall be suppressed in favour of resonance at said second resonant wavelength. In accordance with a third aspect of the present invention there is provided a current injection semiconductor laser said laser comprising: a quantum well active layer; adjacent cladding layers of opposite conductivity type, respectively below and above said active layer; an electrode structure overlying said active layer, which electrode structure is comprised of at least two spaced electrodes, namely a current electrode and a voltage electrode; and an electrical contact layer located between said electrode structure and one of said cladding layers, this electrical contact layer being subdivided and separated by means of insulation to isolate said current electrode and said voltage electrode, wherein that portion (B) of said laser underlying said voltage electrode exhibits an exciton absorption spectrum which is capable of a shift, in the presence of a reverse bias voltage applied to said voltage electrode, that is sufficient to cause said laser to change resonant wavelength. In the accompanying drawings: Figure 1 is a diagram showing a typical energy band gap structure of a quantum well active layer; Figure 2 is a diagram showing the state density and energy levels of this quantum well; Figure 3 is a diagram showing the structure of a semiconductor laser, an embodiment of the present invention; Figure 4 is a diagram for exemplifying the operation of this embodiment; and Figure 5 is a diagram showing an alternative embodiment; So that this invention shall be better followed, preferred embodiments thereof will be described now in particular detail and reference will be made to the accompanying drawing just listed. The description that follows is intended by way of example only. Figure 3 shows a first embodiment of the present invention. In Figure 3, the reference numeral 137 designates a lower electrode, the reference numeral 138 designates an n type GaAs substrate, the reference numeral 139 designates a lower n type AlGaAs cladding layer, the reference numeral 140 designates a GaAs quantum well active layer, the reference numeral 141 designates an upper p type AlGaAs cladding layer, the reference numeral 142 designates a p type GaAs contact layer, the reference numeral 143 designates an electrode for current injection, the reference numeral 144 designates an electrode for loss modulation, and the reference numeral 145 designates an insulating region for electrically separating the current injection portions A and B. The device will be operated as follows. Figure 4 shows a gain spectrum of this first embodiment. n = 1, 2, 3, ... designate the gain peaks corresponding to the transitions at the first, second, and third quantum levels of the electrons and holes, respectively. Figure 4 shows a case where an oscillation of transition corresponding to n = 1 is occured due to that the gain of n = 1 is balanced with the resonator loss. Figure 4 shows an oscillation spectrum in this case. Furthermore, it is known that the exciton peak of the absorption spectrum (n = 1, 2, 3, ...) is shifted by 20 to 30 meV to the high energy side with relative to the respective laser oscillation wavelength. Next, a reverse bias is applied to the loss modulation portion so as to shift the exciton peak to the oscillation wavelength of n = 1 as shown in Figure 4 (the real lines show the absorption spectrum when the applied voltage is 0, and the broken lines show an absorption spectrum when the reverse bias voltage application is conducted). By this, a resonator internal loss is induced against the laser oscillation wavelength corresponding to n = 1, and this laser oscillation is supressed. As a result, the consumption of population caused by the induced emission from the quantum level n = 1 is gone, and the carriers become to occupy the quantum level of n = 2. As shown in Figure 4 the shift of the exciton peak corresponding to n = 2 is small, and the resonator internal loss due to the reverse bias voltage application has almost no effect on the corresponding laser oscillation. Accordingly, a laser oscillation corresponding to this transition arises. Figure 4 shows an oscillation spectrum of this case. In this way, when the reverse bias is not applied a laser oscillation corresponding to the quantum level n = 1 is conducted, and when the reverse bias is applied a laser oscillation corresponding to the quantum level n = 2 is conducted. As described above, in this embodiment an electrode for applying a reverse bias to the same layer structure as that to which a gain current is injected is provided in a semiconductor laser having a quantum well structure, whereby the internal loss is selectively enhanced in accordance with the oscillation wavelength by the control of the voltage applied from this electrode, whereby an oscillation at a high quantum level is enabled, and switching of the oscillation wavelengths is possible by this control. In the above-described first embodiment the loss modulation portion B is provided at one side of laser end surfaces, but this may be provided at the central portion of the laser as in the alternative embodiment shown in Figure 5. Furthermore, the active layer may be of a multiple quantum well structure, having a plurality of quantum well layers, instead of a single well structure. Furthermore, the present invention may be applied to a semiconductor laser of a ridge waveguide type, a BH (Buried Heterostructure) type, a CSP (Channeled Substrate Planer) type, or a superlattice disordered type. Furthermore in the above-illustrated embodiments, the laser oscillations of n = 1 and n = 2 are described, but the present invention may be applied to one conducting a laser oscillation corresponding to a further higher quantum level. The layer thickness of the quantum well active layer and the stripe width of the light waveguide of the semiconductor laser have not been described yet in detail. However, the layer thickness typically may be less than 30 nm Also the stripe width may be less than 10 nm to increase the loss of the light transmission and to enhance the occupation proportion of the higher quantum levels by making it difficult for the injected carriers to be subjected to energy relaxation. Furthermore, as a method of producing such a narrow stripe a method recited in the Journal of Applied Physics vol.58, p.4515 (1985) can be used in order to accomplish a lowering of the threshold value. However, the method of producing the narrow stripe is not limited to that method, and any method capable of obtaining a narrow stripe width active layer can be used. That is, as the transverse direction confinement, a ridge waveguide, a buried heterostructure, a channeled substrate, or a general superlattice disordering by impurity diffusion can be used. In the illustrated embodiment a laser using GaAs/AlGaAs is described, but InP/InGaAsP can be also used with the same effects as described above. As is evident from the foregoing description, the resonator internal loss is enhanced to some degree thereby to control the balance between the gain and the loss, whereby laser oscillation at a higher quantum level is favoured. This enables conducting an oscillation at a short wavelength, and switching the oscillation wavelength by a switching of bias voltage. An integrated semiconductor laser for wavelength multiple light communication oscillating at different wavelengths is easily obtained at a low cost, and the utilization efficiency thereof is quite significant.

Claims: A method of operating a current injection semiconductor laser, which laser includes an active layer of a quantum well structure (140), at least one current electrode (143) by which means current can be injected into said active layer (140), and a voltage electrode (144) by which means a reverse bias voltage may be applied and an electric field developed across a portion (B) of said laser, which method comprises: injecting current into said active layer (140) to cause said laser to emit radiation at a first resonant wavelength corresponding to a quantized level transition in said active layer; and applying a reverse bias voltage to said voltage electrode to cause a shift in the exciton absorption spectrum of said portion (B) of said laser so that whilst current is injected into said active layer (140) said laser is caused to emit radiation at a second resonant wavelength corresponding to a quantized level transition different from said first resonant wavelength. A current injection semiconductor laser system for performing the method of claim 1, which system comprises: a current injection semiconductor laser having an active layer (140) of a quantum well structure, at least one current electrode (143) by which means current can be injected into said active layer (140), and a voltage electrode (144) by which means a reverse bias voltage may be applied and an electric field developed across a portion (B) of said laser; a current source connected to said at least one current electrode (143) for injecting current into said active layer (140) of the laser to stimulate emission individually at each one of a first resonant wavelength corresponding to a quantized level transition and a second resonant wavelength corresponding to another quantized level transition respectively; and a voltage source connected to said voltage electrode for supplying a reverse bias voltage which shall cause a shift in the exciton absorption spectrum of said portion (B) of said laser sufficient that resonance at said first resonant wavelength shall be suppressed in favour of resonance at said second resonant wavelength. A current injection semiconductor laser for use in the system of claim 2, said laser comprising: a quantum well active layer (140); adjacent cladding layers (139,141) of opposite conductivity type, respectively below and above said active layer (140); an electrode structure (143,144) overlying said active layer (140), which electrode structure is comprised of at least two spaced electrodes, namely a current electrode (143) and a voltage electrode (144); and an electrical contact layer (142) located between said electrode structure (143,144) and one (141) of said cladding layers (139,141), this electrical contact layer (142) being subdivided and separated by means of insulation (145) to isolate said current electrode (143) and said voltage electrode (144), wherein that portion (B) of said laser underlying said voltage electrode (144) exhibits an exciton absorption spectrum which is capable of a shift, in the presence of a reverse bias voltage applied to said voltage electrode (144), that is sufficient to cause said laser to change resonant wavelength. A laser as claimed in claim 3 wherein that portion (B) of said laser underlying said voltage electron (144) exhibits an exciton absorption spectrum having a peak absorption offset from a resonant wavelength of said laser, which peak can be shifted to said resonant wavelength by application of said reverse bias voltage. A laser as claimed in either one of the preceding claims 3 or 4 wherein said change in resonant wavelength is a change between the n=1 and n=2 quantum state resonant wavelengths. A laser as claimed in any of claims 3-5 wherein said electrode structure (143, 144) is comprised of a single current injection electrode (143) and a single voltage bias electrode (144). A laser as claimed in any of claims 3-5 wherein said electrode structure (143, 144) is comprised of a pair of current injection electrodes (143, 143) and a single voltage bias electrode (144) located between said pair of electrodes (143, 143). A laser as claimed in any of claims 3 to 7 wherein said active layer (140) is of a single quantum well structure. A laser as claimed in any of claims 3 to 7 wherein said active layer (140) is of a multiple quantum well structure.

Inventors: 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 DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: H01S3
IPC codes: H01S5