Semiconductor laser device

Abstract: Disclosed herein is a semiconductor laser device of the field modulation type having a structure in which the threshold carrier density for laser oscillation is reduced so as to enable an effective action of a modulated electric field applied externally on an active region for radiating light, thereby enabling an extremely high speed modulation. A quantum structure which does not fulfill the charge neutrality condition for free-carriers or a strained super lattice structure is adopted as the structure in which the threshold carrier density is reduced.

Description: 1. Field of the Invention This invention relates to a semiconductor laser device capable of being used for optical fiber communication which needs ultra high speed optical modulation, or for optical logic circuits, optical connection, etc., utilized in optical computers. 2. Description of the Related Art Hitherto, a method of modulating the intensity of output power from a quantum well by applying an electric field to a quantum well structure in the lamination direction of the structure to vary the quantum states has been practiced, and discussed in Oyo Butsuri [Journal of Applied Physics (Japan)], Vol. 55, p. 210, 1986, and similarly in EP-A-0 276 140 and US-A-4 700 353, which disclose the features included in the first part of claim 1. The principle of modulation used in the method will be explained below while referring to FIGS. 2A and 2B. Referring to FIG. 2A, there are shown the quantum level (broken line) and the wave functions of electrons and holes, under zero applied electric field. Both the electrons and holes are localized in the quantum well layer, and the wave functions thereof are each symmetrical with respect to the center of the well. When an electric field is applied in the lamination direction of the quantum well structure, on the other hand, the energy bands are inclined as shown in FIG. 2B, resulting in that the symmetricity of the wave functions is lost. That is, the wave function of electrons is localized to the left side where the energy is lower, whereas the wave function of holes is localized to the right side, as shown in the figure. In other words, the position of electrons and the position of holes differ from each other on a spatial basis, as shown in FIG. 2B. In this specification, the difference between the peak position of the wave function of electrons and the peak position of the wave function of holes is defined as the "distance between the position of electrons and the position of holes", as shown in FIG. 2B. Thus, when an electric field is applied externally in the lamination direction of the quantum well structure as mentioned above, the wave function of electrons and the wave function of holes are spatially separated from each other in the quantum well. That is, the "distance between the position of electrons and the position of holes" is increased. As a result, the spatial overlap of the wave function of electrons and the wave function of holes is decreased, with the result of a decrease in the probability of optical transition, namely, in the oscillation strength. In short, the intensity of output power is varied depending on whether an electric field is present or not. The speed of modulation, or the rate of variation in the intensity of output power depending on the presence or absence of an electric field, is extremely high-- of the order of picoseconds. Thus, it is possible to increase the modulation speed by one or two orders of magnitude, as compared with the speed attained in a direct modulation system of semiconductor laser according to the prior art. SUMMARY OF THE INVENTION When the above-mentioned modulation system is applied to a semiconductor laser having an ordinary quantum well structure, the presence of electrons and holes localized in the quantum well cancels the electric field applied to the well (screening effect), thereby weakening the net strength of the applied electric field. That is, the carrier density required for laser oscillation is about 2 × 10¹⁸ cm⁻³, and, under such a high carrier density as this, the modulation of intensity of output power of a quantum well by application of an electric field becomes impossible due to the screening. It was found, as a result of the present inventors' studies, that the above-mentioned modulation method according to the prior art is usable only in a spontaneous light radiation condition, namely, before laser oscillation, and it is difficult to apply the method to modulation of a semiconductor laser device. The present invention contemplates a solution to the above-mentioned technical problems involved in the prior art. It is accordingly an object of the present invention to provide a novel structure of semiconductor laser device which is capable of modulating the intensity of output power from a quantum well by application of an electric field, even in a laser oscillation condition, as shown in FIGS. 2A and 2B. This object is met by the semiconductor laser device defined in claim 1. The potential well comprising the region not fulfilling the charge neutrality condition results in a reduction of the threshold carrier density for laser oscillation, as described in detail below, which ensures an effective action of a modulated electric field, thereby enabling the ultra high speed optical modulation in a laser oscillation mode. In an embodiment of the invention, the active region having a structure in which the distance between the position of electrons and the position of holes is varied in the potential well by the electric field applied from the electric field applying means. The position of the electrons and the position of the holes may be controlled by an electric field to vary the spatial overlap of the wave functions which respectively describe the electrons and the holes, as shown in FIGS. 2A and 2B, so as to control the radiation of light based on recombination of the carriers. Electric field modulation is selected for realizing an ultra high speed optical modulation in a laser oscillation mode. By adoption of the active region in which the threshold carrier density for laser oscillation is such a carrier density as not to cause effective screening of a modulated electric field, a typical value of the carrier density being less than 1 × 10¹⁸ cm⁻³, it is possible to realize the ultra high speed optical modulation by an electric field. According to a specific embodiment of the present invention, the above-mentioned region not fulfilling the charge neutrality condition for free-carriers is formed by selective introduction of a conductivity type impurity into the potential well. In this case, the density of the conductivity type impurity is desirably not less than 1 × 10¹⁸ cm⁻³. The introduction of an n-type impurity, which has a greater effect on reduction of the threshold carrier density for laser oscillation, is particularly suitable for the electric field modulation used in the present invention. The impurity density is desirably not less than 1 × 10¹⁸ cm⁻³ in the case of an n-type impurity, and not less than 2 × 10¹⁸ cm⁻³ in the case of a p-type impurity. Especially, a remarkable effect is obtained with an n-type impurity used in a density of not less than 2 × 10¹⁸ cm⁻³ or with a p-type impurity used in a density of not less than 5 × 10¹⁸ cm⁻³. One advantage of the present invention is that it is possible to realize a semiconductor laser device which is capable of varying a quantum state by application of an electric field to a potential well, thereby modulating the intensity of output power at high speed in a laser oscillation condition. Another advantage of the present invention is that it is possible to realize a semiconductor laser device in which the spectral chirping at the time of modulation is extremely small. Though the limit of transmission distance in the conventional optical communication is determined by the wavelength dispersion of an optical fiber, the use of the present invention reduces the influence of the wavelength dispersion, thereby enabling dispersion-free transmission. Still another advantage of the present invention is that it is possible to realize a semiconductor laser device suitable for ultra high speed optical fiber communication or for optical connection in computers. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing one embodiment of the semiconductor laser device according to the present invention; FIGS. 2A and 2B are diagrams for illustrating the fundamental principle of operation of the invention, particularly illustrating the behavior of electrons and holes under zero applied electric field and under a non-zero applied electric field, respectively; FIG. 3 is a diagram showing the relationship between doping density and threshold carrier density; FIGS. 4, 5A and 5B are sectional views showing other embodiments of the invention; FIGS. 6A to 6H are diagrams showing various forms of a potential well structure provided in an active region employed in the semiconductor laser device according to the invention; FIG. 7 is a system chart showing an embodiment of the invention in which the semiconductor laser device according to the invention is used for optical fiber communication; FIG. 8 is a diagram showing an arrangement of peripheral circuit for the semiconductor laser device according to the invention; FIG. 9 is a cross-sectional view of the device shown in FIG. 4, taken along the direction of a cavity; and FIG. 10 is a diagram showing the relationship between the distance between the position of electrons and the position of holes and optical output power. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A system which does not fulfill the charge neutrality condition for free-carriers in a potential well means a system which satisfies the following inequality: (n₀ + n₁) ≠ (p₀ + p₁) where n₁ and p₁ are the densities of injected electrons and holes, and n₀ and p₀ are the densities of electrons and holes under an uninjected condition. However, the system not fulfilling the charge neutrality condition, referred to in the present invention, means a system in which the deviation of the above inequality from the corresponding equality is typically not less than 1 × 10¹⁸ cm⁻³, that is, | (n₀ + n₁) - (p₀ + p₁) | > 1 × 10¹⁸ cm⁻³. The present inventors have presented the idea that an introduction of an impurity into a quantum well structure enables a considerable lowering in the carrier density necessary for laser oscillation, namely, the threshold carrier density [Shin-Gaku Giho (Technical Research Reports of the Institute of Electronics and Communication Engineers of Japan), OQE86-63]. The present invention realizes the idea in an electric field modulation type semiconductor laser device. The lowering of the threshold carrier density is based on the effect of impurity doping, that is, the effect of reducing the stimulated absorption term and increasing the optical gain at a given injected carrier density. This effect is shown in FIG. 3. Because a laser oscillation condition is obtained at a low carrier density, an electric field externally applied to the well for the purpose of modulation is not canceled by the movement of carriers localized in the well. As a result, it is possible to control the spatial overlap of the wave function of electrons and the wave function of holes. FIGS. 2A and 2B show the distributions of the electrons and holes localized in a potential well, in the absence of any applied electric field and in the presence of an applied electric field, respectively. Thus, it is possible to perform a high-speed modulation by an electric field. The shapes of the impurity-doped portion and the potential well may take a variety of forms, as exemplified in FIGS. 6A to 6H. Particularly, each of the figures shows a system in which the distribution of a conductivity type impurity introduced into the potential well or into the vicinity of the well is partialized to develop such a potential field that the carriers localized in the well are separated according to the polarity thereof. By forming this structure, it is possible to separate easily the electrons and holes from each other. Besides, as shown in FIGS. 6F, 6G and 6H, it is also an effective means for accomplishing the same purpose to maintain the distance between electrons and holes by providing a step in the potential well, thereby partializing the distribution of, for example, the electrons. The modulation in the present invention is carried out by externally applying an electric field for modulation to the potential well by a means for applying electric field. It is possible to render the laser oscillation substantially zero, by application of a modulated electric field. The expression "substantially zero" used herein means that it is possible to eliminate stimulated emission (all the emissions other than spontaneous emission). The means for applying electric field may comprise, for instance, at least one pair of elecrodes disposed respectively on the upper and lower sides of the potential well. FIG. 10 shows the relationship of the distance between the position of electrons and the position of holes, with optical output power. The data given here was obtained where the thickness of the quantum well layer was 10 nm. The two curves show the values obtained with potential well structures corresponding to FIG. 6A and FIG. 6F, respectively. In the graph corresponding to FIG. 6F, the thickness of the well layer is the total thickness of the stepped well (two-stage well). As shown in the figure, the optical output power decreases with an increase in the distance. The difference between the structure of FIG. 6A and the structure of FIG. 6B, as seen in FIG. 10, is based on the stepped well (two-stage well) structure of FIG. 6F, in which electrons are liable to drop into the lower well portion (the deepest portion) of the stepped well, resulting in a partialized distribution of electrons. Some preferred embodiments of the present invention will now be explained while referring to the drawings. In FIG. 1, an undoped GaAℓAs clad layer 102, a GaAs quantum well layer 103 with 5 to 20 nm thickness and an undoped GaAℓAs clad layer 104 are grown on a semi-insulating InP substrate 101. In this case, the GaAs quantum well layer 103 is formed with two regions, namely, an undoped GaAs layer 103b and an n-type GaAs layer 103a doped with an n-type impurity, which is Se, Te or Si, in an impurity density of 1 × 10¹⁸ to 1 × 10¹⁹ cm⁻³. This structure corresponds to FIG. 6A. That is to say, in this embodiment a charge neutrality condition is established in which the electron density in the well is higher than the hole density in the well. Thereafter, a projected stripe having a width of 0.5 to 5 µ m and penetrating at least the GaAs quantum well layer 103 is formed. Then, a p-GaAℓAs buried layer 105 and an n-GaAℓAs buried layer 106 are formed, followed by formation of a p-current injection electrode 107 and an n-current injection electrode 108. Further, the semi-insulating substrate 101 in the region corresponding to the projected stripe is removed by selective etching, and electrodes for applying electric field 109 and 110 are formed. Finally, the system thus obtained is cleaved to obtain a cavity length of 100 to 500 µm. The device produced as above on an experimental basis performs laser oscillation with 2 to 15 mA. FIG. 8 shows a driving system, or peripheral circuit, for this embodiment. Electrodes 107, 108, 109 and 110 correspond to those in FIG. 1, the electrode 108 being grounded. A current injection source 841 having a function of controlling the current injection quantity by a variable resistor 840 is connected to the electrode 107. The electrodes 109 and 110 for applying an electric field to the quantum well are connected with an electric field application source 842. When the electrode for applying electric field 109 was grounded and a voltage of 2V was applied to the other electrode for applying electric field 110 during laser oscillation at an output of 1 to 5 mW, the laser oscillation ceased and the optical output power was reduced to zero. It was thus possible to modulate the intensity of laser output power according to the presence or absence of an electric field applied to the electrode 110. The switching speed in this case was about 5 ps, the value being restricted by the CR time constant of the device. Another embodiment of the semiconductor laser device according to the present invention is illustrated in FIG. 4, and FIG. 9 shows a cross-sectional view of the device, taken along the direction of a cavity. This embodiment exemplifies an application of the semiconductor laser device of the invention to an InGaAsP DFB laser. An undoped InP layer 402 with 0.05 to 0.3 µ m thickness, an InGaAs or InGaAsP quantum well layer 403 with 5 to 15 nm thickness and an undoped InGaAsP optical guide layer 405 with 0.05 to 0.3 µ m thickness are grown on an n-InP substrate 401. Then, a grating 440 with a period of about 240 nm is formed on the optical guide layer 405. Thereafter, a p-InP optical confinement layer 406 is grown. Here, the quantum well layer 403 is formed with two regions, namely, a p-type region 403a and an undoped region 403b. The doping density of the p-type region 403a is about 2 × 10¹⁸ to 1 × 10¹⁹ cm⁻³. This structure corresponds to FIG. 6B. Next, a projected stripe having a width of 0.5 to 3 µm and penetrating the grown layers is formed, and then a p-InP buried layer 417 and an n-InP buried layer 418 are grown. Then, a p-current injection electrode 407 and an n-current injection electrode 408 are formed, and electrodes for applying electric field 409 and 410 are formed. Finally, the system thus obtained is cut to a device length of 100 to 1000 µm. The formation direction (the direction of grooves) of the grating is orthogonal to the stripe. In this device, the quantum well layer 403 is sandwiched in a p/n junction, and an electric field is therefore applied to the quantum well layer 403 due to a built-up potential, without any voltage applied between the electrodes 409 and 410. Accordingly, when a voltage of + 1 V is applied to the electrode 410, the energy band in the quantum well layer becomes flat. The device produced as above on an experimental basis performs laser oscillation with 5 to 10 mA. In this case, the spectrum obtained indicates a single mode, which reflects the DFB structure. When a voltage of - 2 to - 3 V is applied to the electrode for applying electric field 410 during laser oscillation at an optical output power of 5 mW effected by grounding the electrode for applying electric field 409 and applying + 1 V to the electrode 410, the laser oscillation ceases and the optical output power is reduced to zero. This switching takes place very rapidly, in 2 to 3 ps. In addition, the spectral chirping in this case is extremely small and is exactly the same as the chirping due to a limit of modulation. A further embodiment of the present invention is illustrated in FIGS. 5A and 5B. This embodiment has a structure resembling a two-dimensional electron gas hetero-bipolar transistor having an emitter, a base and a collector. After a p-GaAℓAs layer 502, a modulation doped multiple quantum well active layer 503, an n-GaAℓAs 503 and a p-GaAs 505 are grown on a p-GaAs substrate 501, an emitter electrode 507 is formed. The modulation doped multiple quantum well active layer 503, as shown in detail in the circle of FIG. 5B, has a periodic structure comprising quantum well layers, each of which is composed of an undoped Ga 0.9 Aℓ 0.1 As layer 503b and an n doped GaAs layer 503c, and n doped Ga 0.7 Aℓ 0.3 As barrier layers 503a. That is, each quantum well layer is formed of two materials differing in bandgap energy, and the GaAs having the smaller bandgap energy is doped with an n-type impurity. This structure corresponds to FIG. 6H. Next, the emitter electrode 507 and the p-GaAs 505 in the other regions than the above are removed, a base electrode 508 is formed, and a collector electrode 509 is provided on the lower side. In this device, the application of electric field to the multiple quantum well is carried out according to the presence or absence of a voltage applied to the collector electrode 509. Carrier injection into the multiple quantum well is effected through the emitter and base electrodes, and a modulation of the intensity of laser output power similar to the modulation in the above-mentioned embodiment is achieved. Other than the forms of quantum well structure shown in the three embodiments above, a variety of quantum well structures are applicable. The applicable forms of quantum well structure are shown respectively in FIGS. 6A to 6H. Each of the figures shows the edge of the conduction band when an electric field is applied to the quantum well structure. To avoid complication, one of the quantum wells is shown in each figure. All these structures are extremely effective for the respective embodiments shown in FIGS. 1, 4, 5A and 5B. FIG. 7 illustrates an embodiment in which each of the above-mentioned semiconductor laser devices of the present invention is applied to optical communication. To the semiconductor laser device 707 are connected a carrier injection electrode 708 and a signal generator for applied electric field 709. Laser light 730 modulated by the optical device 707 is transmitted through an optical fiber 731, and an optical output 732 from the fiber is converted by an optical detector 706 into an electric signal, which is discriminated by a receiver 705. In this embodiment, it is possible to realize a transmission speed of 100 Gbit/s and a fiber length of 40 km. Although the invention has been described with reference to an ordinary double hetero semiconductor laser device, it is to be appreciated that other semiconductor laser devices are applicable, including a buried hetero semiconductor laser device, a distributed Bragg reflection semiconductor laser device, a distributed feedback semiconductor laser device, and the like.

Claims: A semiconductor laser device comprising an active region (103) having a potential well, means (107, 108) for injecting carriers into the active region (103) to cause radiation of light, a resonator for obtaining laser light from the radiated light, and means (109, 110) for applying an electric field to the active region (103) in order to modulate the optical output power of the laser, characterised in that the potential well comprises a region which does not fulfill the charge neutrality condition for free carriers such that: |(n₀ + n₁) - (p₀ + p₁)| > 1 * 10¹⁸ cm⁻³ where n₁ and p₁ are the densities of injected electrons and holes, and n₀ and p₀ are the densities of electrons and holes under an uninjected condition. The device of claim 1, wherein the region which does not fulfill the charge neutrality condition for free carriers is formed by selective introduction of a conductivity type impurity into the potential well, or into the vicinity of the potential well in the active region (103). The device of claim 2, wherein the conductivity type impurity is an n-type impurity. The device of any one of claims 1 to 3, wherein said electric field is applied in such a direction that the distance between the position of electrons localized in the potential well and that of holes localized in the potential well is increased. The device of any one of claims 1 to 4, wherein the non-fulfillment of the charge neutrality condition for free carriers is established by an asymmetry in the shape of the potential well. The device of any one of claims 1 to 5, wherein the optical output power is modulated by the applied electric field while a substantially constant driving current in the vicinity of the threshold current for radiation of laser light is injected. The device of any one of claims 1 to 6, wherein the active region (103) has a structure in which the distance between the position of electrons and that of holes in the potential well is varied by the applied electric field. The device of claim 7, wherein the electric field applying means (109, 110) varies the spatial overlap of the wave functions which respectively describe the electrons and the holes, so as to control the radiation of light based on re-combination of the carriers. The device of claim 7 or 8, wherein the threshold carrier density for laser oscillation of the active region (103) has such a value as to cause no effective screening of the applied electric field. The device of any one of claims 1 to 9, wherein said resonator comprises a cleaved facet or a grating provided on an optical guide layer adjacent to at least one side of the active region layer.

Inventors: CHINONE NAOKI, OHTOSHI TSUKURU, SASAKI SHINJI, TSUCHIYA TOMONOBU, UOMI KAZUHISA, CHINONE, NAOKI, OHTOSHI, TSUKURU, SASAKI, SHINJI, TSUCHIYA, TOMONOBU, UOMI, KAZUHISA
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: H01S3
IPC codes: H01S5