Title of Invention	A SEMICONDUCTOR LASER, A METHOD OF FABRICATION OF SUCH SEMICONDUCTOR LASER AND AN OPTICAL DISK SYSTEM USING THE SAME
Abstract	The semiconductor laser of the present invention comprises an active layer having a quantum well layer and a saturable absorption layer, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV.

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P13858 - 1A- BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a self-oscillation type semiconductor laser used as a light source for an optical disk system, a method for fabricating such a semiconductor laser, and an optical disk device using such a semiconductor laser. 2. Description of the Related Art: With the recent increasing demand for semiconductor lasers in the fields of optical communications, laser printers, optical disk devices, and the like, semiconductor lasers of GaAs and InP, mainly, have been actively studied and developed. In the optical information processing field, a method of recording and reproducing information using light from an AlGaAs semiconductor laser with a wavelength of 780 nm, especially, has been commercialized. Such a method has been widely used for compact disks and the like. In recent years, optical disk devices with a larger memory capacity have been increasingly demanded. With this demand, shorter-wavelength lasers have been requested. An AlGalnP semiconductor laser can oscillate in the red region of wavelengths of 630 to 690 nm, emitting light with the shortest wavelength among those obtained from semiconductor lasers practically available at present. This type of semiconductor laser is therefore highly expected to be a next-generation large-capacity light source for optical information recording, replacing the conventional AlGaAs semiconductor laser. in general, a semiconductor laser generates intensity - 2 - P13858 noise due to the return of light reflected from a disk surface and a temperature change during the reproduction of information from an optical disk, inducing a signal read error. A laser with low intensity noise is therefore indispensable for a light source of an optical disk device. Conventionally, in order to reduce noise, a low-output AlGaAs semiconductor laser for a reproduction-only optical disk device has a structure where saturable absorbers are intentionally formed on both sides of a ridge stripe. With this structure, multiple longitudinal modes can be obtained. In the case where disturbances such as return light and a temperature change arise when a laser is oscillated in a single longitudinal mode, oscillation in an adjacent longitudinal mode is started by a minute change in a gain peak, causing conflict with the oscillation in the original oscillating mode and thus causing noise. When multiple longitudinal modes are used, the change in the intensity of each mode is averaged and is not influenced by the disturbances. Thus, stable low-noise characteristics can be obtained. A method for obtaining further stable self-oscillation characteristics is disclosed in Japanese Laid-Open Patent Publication No. 63-202083. In this publication, a self-oscillation type semiconductor laser has been realized by forming a layer which can absorb output light. Japanese Laid-Open Patent Publication No. 6-260716 reports that the characteristics have been improved by substantially equalizing the energy gaps of an -3- active layer and an absorption layer In particular, the energy gaps of a distortion quantum well active layer and a distortion quantum well saturable absorption layer are substantially equal to each other, so as to obtain good self-oscillation characteristics A similar configuration is described in Japanese Laid-Open Patent Publication No 7-22695 However, the inventors of the present invention have found that good self-oscillation characteristics are not obtained by only substantially equalizing the energy gaps of a saturable absorption layer and an active layer The present invention is aimed at providing a semiconductor laser having stable self-oscillation characteristics effective for noise reduction by examining the energy gap difference between a saturable absorption layer and an active layer, as well as a method for fabricating such a semiconductor laser and an optical disk device using such a semiconductor laser SUMMARY OF THE INVENTION The semiconductor laser of this invention comprises an active layer having a quantum well layer and a saturable absorption layer, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV -4 - P13858 In one embodiment of the invention, a thickness of the saturable absorption layer is in a range of about 10 to about 100 A. In another embodiment of the invention, a plurality of saturable absorption layers are formed. In still another embodiment of the invention, an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 50 to 100 meV. In still another embodiment of the invention, the optical guide layer has a band gap which is larger than a band gap of the saturable absorption layer and smaller than band gaps of the other layers of the cladding structure. In still another embodiment of the invention, a thickness of the optical guide layer is in a range of 300 to 1200Å. In still another embodiment of the invention, the optical guide layer is divided into a plurality of portions in the cladding structure. In still another embodiment of the invention, the optical guide layer is adjacent to the saturable absorption layer in the cladding layer. In still another embodiment of the invention, the saturable absorption layer is doped with impurities of 1 x 1018 cm-3 or more. -5- In still another embodiment of the invention, the active layer has a multiple quantum well structure According to another aspect of the invention, a method for fabricating a semiconductor laser is provided comprising the steps of forming said active layer having a quantum well layer and, forming said cladding structure sandwiching the active layer, the cladding layer comprising said saturable absorption layer and an optical guide layer for increasing a light confinement of the saturable absorption layer, and energy gap of the saturable absorption layer being smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV, characteristics of said laser varying with time but being substantially fixed after a lapse of about one minute , the method comprising the steps of stabilizing said laser where characteristics obtained immediately after laser oscillation are changed to obtain substantially fixed characteristics In one embodiment of the invention, the characteristics are current-light output power characteristics In another embodiment of the invention, the stabilizing step comprises the step of reducing a threshold current by an aging process In another embodiment of the invention, the stabilizing step comprises the step of reducing a threshold current by annealing In still another embodiment of the invention, the threshold current is reduced from a value obtained P13858 - 6 - immediately after the Laser oscillation by 5 mA or more by the stabilizing step. According to still another aspect of the invention, an optical disk device is provided. The device includes a semiconductor laser, a converging optical system for converging a laser beam emitted from the semiconductor laser on a recording medium, and an optical detector for detecting the laser beam reflected from the recording medium, wherein the semiconductor laser includes an active layer having a quantum well layer and a cladding structure sandwiching the active layer, the cladding layer including s saturable absorption layer and an optical guide layer for increasing a light confinement of the saturable absorption layer, and an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV. In one embodiment of the invention, the semiconductor laser oscillates in a single mode when information is recorded on the recording medium, and oscillates in a self-oscillation mode when information recorded on the recording medium is reproduced from the recording medium. In another embodiment of the invention, the optical detector is disposed near the semiconductor laser. In still another embodiment of the invention, the optical detector includes a plurality of photodiodes formed on a silicon substrate, and the semiconductor laser is disposed on the silicon substrate. P13858 - 7 - In still another embodiment of the invention, the silicon substrate includes a concave portion formed on a principal surface thereof and a micromirror formed on a side wall of the concave portion, the semiconductor laser is disposed in the concave portion, and the angle formed between the micromirror and the principal surface is set so that the laser beam emitted from the semiconductor laser proceeds in a direction substantially vertical to the principal surface of the silicon substrate after being reflected from the micromirror. In still another embodiment of the invention, a metal film is formed on a surface of the micromirror. In still another embodiment of the invention, the active layer and the cladding structure are formed of AlxGayIn1-x-yP (0 Alternatively, the semiconductor laser of this invention includes an active layer including a quantum well layer and a saturable absorption layer, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV. In one embodiment of the invention, the thickness of the saturable absorption layer is in the range of about 10 to about 100 A. In another embodiment of the invention, a plurality of saturable absorption layers are formed. P13858 - 8 - In still another embodiment of the invention, an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 50 to 100 meV. In still another embodiment of the invention, the saturable absorption layer is doped with impurities of 1 x 1016 cm-3 or more. In still another embodiment of the invention, distortion is applied to the quantum well layer and the saturable absorption layer. In still another embodiment of the invention, the active layer has a multiple quantum well structure. Alternatively, the semiconductor laser of this invention includes an active layer including a quantum well layer and a cladding structure sandwiching the active layer, wherein the cladding structure includes a saturable absorption layer, and an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV. In one embodiment of the invention, the thickness of the saturable absorption layer is in the range of about 10 to about 100 A. In another embodiment of the invention, a plurality of saturable absorption layers are formed. -9- In still another embodiment of the invention, an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 50 to 100 meV In still another embodiment of the invention, the saturable absorption layer is doped with impurities of 1 x 1018 cm3 or more In still another embodiment of the invention, the active layer has a multiple quantum well structure In still another embodiment of the invention, distortion is applied to the quantum well layer and the saturable absorption layer Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor laser having stable self-oscillation characteristics effective for noise reduction, (2) providing a method for fabricating such a semiconductor laser, and (3) providing an optical disk device using such a semiconductor laser These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Figure 1 is a schematic view illustrating energy gaps - 10 - P13858 Figure 2 is a sectional view of a semiconductor laser of Example 1 according to the present invention. Figure 3 is a band gap energy diagram of the semiconductor laser of Example 1. i Figure 4fl shows the light output power characteristics to indicate Pmax. Figures 4B and 4C show changes in ,the light output power with time when the light output power is more than and less than Pmax, respectively. Figure 5 shows the Tmax and Pmax characteristics of the semiconductor laser of Example 1. Figure 6 shows the relationship between the energy gap difference and the operating current. Figure 7 shows the relationship between the operating current and the life. Figure 8 shows the light output power characteristics of the semiconductor laser of Example 1. Figure 9 shows the change in the light output power with time of the semiconductor laser of Example 1. Figure 10 is a sectional view of a semiconductor laser of Example 2 according to the present invention. Figure 11 is a band gap energy diagram of the semiconductor laser of Example 2. - 11 - P13858 Figures 12A and 12B are light intensity distributions obtained when an optical guide layer is not formed and when it is formed, respectively. Figure 13 shows the light confinement of the semiconductor laser of Example 2. Figure 14 is a band gap energy diagram of a modified semiconductor laser according to the present invention. Figure 15 is a band gap energy diagram showing the positional relationship between a saturable absorption layer and an optical guide layer according to the present invention. Figure 16 is a band gap energy diagram obtained when the saturable absorption layer is formed in the optical guide layer. Figure 17 is a schematic view of an optical disk device according to the present invention. Figure 18 is a perspective view of a laser unit used for the optical disk device according to the present invention. Figure 19 is a schematic view of another optical disk device according to the present invention. Figure 20 illustrates the operation of a hologram element used for the optical disk device according to the present invention. - 12 - P13858 Figure 21 is a plan view of optical detectors used for the optical disk device according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors of the present invention have examined the relationship between the "energy gap difference (AE) between an active layer and a saturable absorption layer" and self-oscillation. As used herein, the energy gap difference (AE) means the "value obtained by subtracting an energy gap (E' gs) between ground levels of the saturable absorption layer from an energy gap (E' ) between ground levels of a quantum well layer of an active layer before laser oscillation (E'ga - E ' gs)" when both the active layer and the saturable absorption layer are of a quantum well structure. The energy gaps E' ga and E'gs and band gaps (Ega, Egs) of these layers are schematically shown in Figure 1. In general, in a semiconductor layer of the quantum well structure, the energy gap between ground levels does not correspond to an energy gap between the bottom of a conduction band and the bottom of a valence band, but an energy difference (E'g) between quantum levels of these bands. The energy gap is therefore larger than a normal band gap (Eg) by about 70 meV. In the case where the active layer is of a quantum well structure while the saturable absorption layer is of a bulk structure, the energy gap difference between the active layer and the saturable absorption layer means the "value obtained by subtracting the band gap (Egs) of the saturable absorption layer from the P13858 - 13 - energy gap (E'ga) between ground levels of the active layer before laser oscillation" According to the present invention, the saturable absorption layer may be of the quantum well structure or of the bulk structure. Accordingly, the "energy gap of the saturable absorption layer" as used herein is defined as follows. That is, it means the " energy gap (E' gs) between ground levels" when the saturable absorption layer is of the quantum well structure, while it means the "band gap (Egs) thereof" when it is of the bulk structure. By using the thus-defined "energy gap of the saturable absorption layer", the "energy gap difference between the active layer and the saturable absorption layer" can be expressed as the "value obtained by subtracting the energy gap of the saturable absorption layer from the energy gap between ground levels of the quantum well layer of the active layer before laser oscillation" The inventors of the present invention have examined and found that stable self-oscillation can be obtained by setting the energy gap difference (AE) between the active layer and the saturable absorption layer at 30 to 200 meV. This is because, m the above range of the energy gap difference, the saturable absorption layer efficiently absorbs laser light while the light absorption is saturated. No self-oscillation is obtained when the energy gap difference (AE) between the active layer and the saturable absorption layer is less than 30 meV. This is probably because, with such small energy gap difference, the saturable absorption layer does not absorb laser light so much. No self-oscillation occurs, either, when the energy gap difference (?E) - 14 - P13858 exceeds 200 meV, because the saturable absorption layer absorbs light too much to exhibit the saturation characteristics. It is found therefore that the appropriate energy gap difference (? E) is in the range of 30 to 200 meV. The present crystal growth technology allows the energy gap of each semiconductor layer and the energy gap difference (?E ) among the layers to be controlled at a precision of several milli-electron volts or less. Therefore, if the energy gap difference (? E) between the active layer and the saturable absorption layer is as large as 10 meV, it is interpreted that the energy gap difference ( ? E ) must have been formed intentionally between the active layer and the saturable absorption layer. Accordingly, when the energy gap difference (?E) between the active layer and the saturable absorption layer is 10 meV or more, the energy gap between the ground levels of the quantum well layer of the active layer and the energy gap of the saturable absorption layer are not "substantially the same" When the energy gap difference (? E) is m the range of 50 to 100 meV, especially, the saturation condition of the saturable absorption layer becomes optimal, allowing for stable self-oscillation even at a high operating temperature. As the energy gap difference (? E) exceeds 100 meV, light absorption by the saturable absorption layer gradually increases, slightly increasing the operating current. The energy gap difference is therefore preferably 100 meV or less. Thus, with an energy gap difference in the range of 50 to 100 meV, the operating current of the semiconductor laser does not - 15 - P13858 increase and good self-oscillation characteristics can be obtained. In particular, this setting of the energy gap difference in the above range is preferable when the semiconductor laser is expected to operate under a comparatively high temperature such as in an application in cars. The carrier density of the saturated absorption layer can be easily increased by reducing the volume of the saturated absorption layer. Laser light output from the active layer is absorbed by the saturable absorption layer, generating pairs of electrons and holes. When the volume of the saturable absorption layer is small, the light absorption thereof per unit volume increases, resulting in an easily increased carrier density thereof. The saturable absorption layer with a high carrier density can be saturated easily, exhibiting excellent saturable absorption effect. Accordingly, more strong and stable self-oscillation characteristics can be obtained with a thinner saturable absorption layer. This11 has been confirmed by the experiments of the inventors of the present invention. The thickness of the saturable absorption layer is preferably in the range of about 10 to 100 Å in order to obtain such strong and stable self-oscillation characteristics. This can also be obtained with a saturable absorption layer of the bulk structure with a thickness of about 100 A or more as far as the energy gap difference is set within the preferable range. A saturable absorption layer with a plurality of separate sub-layers can also be used. In the semiconductor laser of the present invention, an optical guide layer is formed in a cladding - 16 - P13858 structure due to the following reason. When the saturable absorption layer is made as thin as the quantum well layer in order to reduce the volume of the saturable absorption layer, the light confinement within the saturable absorption layer is extremely reduced. As a result, stable self-oscillation is not obtained. The optical guide layer is formed to prevent this problem. If the light confinement of the saturable absorption layer increases to at least about 1.2% while that of the active layer is maintained at 5.0% or more, for example, by using the optical guide layer, stable self-oscillation can be obtained. As described above, the optical guide layer of the present invention is formed to increase the light confinement of the saturable absorption layer. it is disposed at a position apart from the active layer. The optical guide layer of the present invention is therefore largely different from a conventional optical guide layer disposed adjacent to an active layer, to increase the light confinement of the active layer. The positional relationship between the saturable absorption layer and the optical guide layer is optimally determined in consideration of the volume and light confinement of the saturable absorption layer. Now, the present invention will be described by way of examples with reference to the accompanying drawings. - 17 - P13858 (Example 1) Figure 2 is a sectional view of a first example of the semiconductor laser according to the present invention. The semiconductor laser of this example includes an n-type GaAs substrate 10-1 and a semiconductor multilayer structure formed on the GaAs substrate 101. The semiconductor multilayer structure includes an n-type GaAs buffer layer 102, an n-type AlGalnP cladding layer 103, a multiple quantum well active layer 104 made of AlGalnP and GalnP, a first p-type AlGalnP cladding layer 105a, a saturable absorption layer 106 made of p-type GalnP, and a second p-type AlGalnP cladding layer 105b. A stripe ridge (width: about 2.0 to 7.0 pm) extending in a resonator length direction is formed in the upper portion of the second p-type cladding layer 105b. A contact layer 110 is formed on the top surface of the ridge of the second p-type cladding layer 105b. An n-type GaAs current blocking layer 111 is formed on the second p-type cladding layer 105b and the sides of the contact layer 110. A p-type GaAs cap layer 112 is formed over the contact layer 110 and the current blocking layer 111. A p-type electrode 113 is formed on the top surface of the cap layer 112, while an n-type electrode 114 is formed on the bottom surface of the substrate 101. The active layer 104 is of a multiple quantum well structure composed of three pairs of well layers and barrier layers. Herein, the portions of the semiconductor multilayer structure except for the buffer layer, the active layer, the contact layer, the cap layer, and the current blocking layer are called a "cladding structure" as a wnole. In this example, the n-type AlGalnP cladding P13858 - 18 - layer 103, the first p-type AlGalnP cladding layer 105a, the saturable absorption layer 106, and the second p-type AlGalnP cladding layer 105b constitute the cladding structure. The dope level and the film thickness of each semiconductor layer constituting the semiconductor multilayer structure are shown in Table 1 below. Table 1 Name Ref. No. Dope level (cm-3) Thickness Cap layer 112 5 x 1018 3 µm Contact layer 110 1 x 1018 500 Å 2nd p-type cladding layer 105b 1 x 1018 0. 9 µm Saturable absorption layer 106 2 x 1018 50 Å 1st p-type cladding layer 105a 5 x 1017 500 Å Active layer 104 undoped 500 Å Barrier layer 50 Å Well layer 50 Å N-type cladding layer 103 5 x 1017 1.0 µm Buffer layer 102 1 x 1018 0. 3 µm Figure 3 shows the distribution of an Al mole fraction x of (AlxGa1-x)0 5In0 5 of the portions of the semiconductor laser of this example covering the active layer 104 through the saturable absorption layer 106. In - 19 - P13858 thxs example, the Al mole fraction of the n-type cladding layer 103, the first p-type cladding layer 105a, and the second p-type cladding layer 105b is 0.7. Since the quantum well layers of the active layer 104 and the saturable absorption layer 106 are made of Ga0 45In0 S5P and Ga0 40In0 6DP, respectively, they have large lattice constants compared with the surrounding layers, resulting in receiving compression distortion. The energy gap difference between the quantum well layers of the active layer 104 and the saturable absorption layer 106 plays an important role in obtaining stable self-oscillation of the resultant semiconductor laser. In Example 1, the energy gap difference is 57 meV, allowing for stable self-oscillation. The inventors of the present invention have examined the role of the saturable absorption layer and the energy gap difference, and the results will be described with reference to Figures 4A to 4C. Laser oscillation (self-oscillation) is started when the applied current reaches 40 mA as shown in Figure 4A. The self-oscillation is terminated at- a point A of Figure 4A and shifts to normal laser oscillation. The maximum light output power obtained by the self-oscillation is denoted by Pmax. In Figure 4A, Pmax is 4.0 mW. The light output power greatly fluctuates with time as shown in Figure 4C when the applied current is smaller than the current value with which Pmax is obtained, providing self-oscillation with a stable amplitude. However, when the current is larger than that current value, the amplitude of the light output power - 20 - P13858 gradually decreases with time, and eventually shifts to normal laser oscillation, as shown in Figure 4B. The self-oscillation also tends to be terminated when the operating temperature T exceeds a certain level, as in the applied current. The maximum temperature at which the self-oscillation is observed is denoted by Tmax, which is also the temperature at which the self-oscillation is terminated. Figure 5 is a graph illustrating the experimental results showing the relationships among the energy gap difference (meV) as the X axis and the Tmax (temperature at which self-oscillation is terminated) and Pmax (maximum light output power by self-oscillation at room temperature) as the Y axes. As is seen from Figure 5, no self-oscillation was observed when the energy gap difference was 10 meV or 20 meV, but was observed when it was 30 meV. At the energy gap difference of 30 meV, self-oscillation was observed at a temperature as high as 51 °C and a light output power as high as 5 mW was obtained by self-oscillation. The self-oscillation started when the energy gap difference reached 30 meV and was confirmed to have continued until it was 200 meV. In particular, both Tmax and Pmax were high when the energy gap difference was in the range of 50 to 100 meV. This range was therefore confirmed to be the practically preferable range. When the energy gap difference exceeds 100 meV, the absorption of laser light by the saturable absorption layer becomes large due to the large energy gap differ- - 21 - P13858 ence between the active layer and the saturable absorption layer, resulting in slightly increasing the operating current. This will be described with reference to Figure 6. Figure 6 is a graph illustrating the relationship between the energy gap difference (meV) as the X axis and the operating current (mA) as the Y axis. As is observed from this graph, when the energy gap difference exceeds 100 meV, the operating current exceeds 130 mA. Figure 7 is a graph illustrating the relationship between the operating current and the life of the semiconductor laser of this example. This graph is based on measurement results obtained under conditions where the light output power of the semiconductor laser is maintained at 5 mW and the operating temperature is 60°C. It is observed from Figure 7 that the operating current should be set at 130 mA or less to obtain a life of 5000 hours or more. As is observed from Figures 6 and 7, the energy gap difference is preferably 100 meV or less in consideration of the life of the semiconductor laser. The compositions and the thicknesses of the active layer and the saturable absorption layer may be adjusted to set the energy gap difference (AE) between the active layer and the saturable absorption layer within a predetermined range. For example, assuming that the active layer is of the quantum well structure and each well layer thereof is made of Ga0 45In0 55P with a thickness of 50 Å (Eg3: 1.937 eV, ?: 640 nm), an energy - 22 - P13858 gap difference (? E) m the range of 30 to 200 meV can be obtained by adjusting the composition of the saturable absorption layer as shown m Table 2 below. Table 2 (Active layer: GaO45In055P) GaxIn1-xP saturable absorption layer Energy gap difference (? E) Ga0 42In0 58P 30 meV Ga0 4oino eop 50 meV Ga0 34In0 66P 100 meV GaD 24In0 75P 200 meV Table 2 shows the values obtained when the thickness of the saturable absorption layer is 50 A. As the thickness of the saturable absorption layer increases, the energy gap difference (? E) increases. As the Ga mole fraction x of the saturable absorption layer increases, the energy gap difference (? E) decreases. In contrast, as the thickness of the well layers of the active layer increases, the energy gap difference (? E) decreases. As the Ga mole fraction x of the well layers increases, the energy gap difference (? E) increases. Figure 8 is a graph illustrating the current-light output power characteristics of the semiconductor laser of this example. The X axis of the graph represents the current applied to the semiconductor laser - 23 - P13858 (mA), while the Y axis represents the light output power (mW). The threshold current is about 50 mA. A feature of the self-oscillation type semiconductor laser distinguished from normal semiconductor lasers is that the light output power sharply increases at and around the threshold current as is observed from Figure 8. This occurs because, with the existence of the saturable absorption layer, the light output power is not released until a sufficient amount of carriers are accumulated. When the applied current exceeds a certain value, laser oscillation is started, increasing the light output power m proportion to the applied current. Figure 9 shows an output waveform of the semiconductor laser of this example at a point Px of Figure 8. As is observed from Figure 9, the light output power greatly fluctuates during a short period of 2 ns, indicating self-oscillation. In the semiconductor laser according to the present invention, the dope level of the saturable absorption layer is set at 2 x 1018 (cm-3), to reduce the life of carriers. This increases the contribution of spontaneous emission to the temporal change rate of the carrier density, facilitating the self-oscillation. A dope level of 1 x 1018 (cm-3) or more is effective in reducing the life of carriers. The thickness of the saturable absorption layer is not limited to 50 Å as used in this example. The saturable absorption layer can also be of the multiple quantum well structure or of the bulk structure. - 24 - P13858 (Example 2) A second example of the semiconductor laser according to the present invention will be described with reference of Figure 10. The semiconductor laser of this example includes an n-type GaAs substrate 701 and a semiconductor multilayer structure formed on the GaAs substrate 701. The semiconductor multilayer structure includes an n-type GaAs buffer layer 702, an n-type AlGalnP cladding layer 703, a multiple quantum well active layer 704 made of AlGalnP and GalnP, a first p-type AlGalnP cladding layer 705a, an optical guide layer 707, a second p-type AlGalnP cladding layer 705b, a saturable absorption layer 706 made of p-type GalnP, and a third p-type AlGalnP cladding layer 705c. A stripe ridge (width: about 2.0 to 7-0 µm) extending in a resonator length direction is formed in the upper portion of the third p-type cladding layer 705c. A contact layer 710 is formed on the top surface of the ridge of the third p-type cladding layer 705c. An n-type GaAs current blocking layer 711 is formed on the third p-type cladding layer 705c and the sides of the contact layer 710. A p-type GaAs cap,, layer 712 is formed over the contact layer 710 and the current blocking layer 711. A p-type electrode 713 is formed on the top surface of the cap layer 712, while an n-type electrode 714 is formed on the bottom surface of the substrate 701. The active layer 704 is of a multiple quantum well structure composed of three pairs of well layers and barrier layers. In this example, the n-type AlGalnP cladding layer 703, the first p-type AlGalnP cladding layer 705a, - 25 - P13858 the optxcal guide layer 707, the second p-type AlGaInP cladding layer 705b, the saturable absorption layer 706, and the third p-type AlGalnP cladding layer 705c constitute the cladding structure. The semiconductor laser of this example is different from that of Example 1 in that the optical guide layer 707 is formed in the cladding structure as will be described later in detail. Figure 11 shows the distribution of an Al mole fraction x of (AlKGa1-x)0 5In0 5P in the portions of the semiconductor laser of this example covering the n-type cladding layer 703 to the third p-type cladding layer 705c. In this example, the Al mole fraction of the n-type cladding layer 703, the first p-type cladding layer 705a, the second p-type cladding layer 705b, and the third p-type cladding layer 705c is 0.5. The well layers of the active layer 704 and the saturable absorption layer 706 are made of Ga0 45In0 55P and Ga0 40Ino soP' respectively. In this example, as in Example 1, the difference between the ground levels of the saturable absorption layer and the well layers (energy gap difference) is set ,at 57 meV. In order to obtain stable self-oscillation of the semiconductor laser with the configuration shown in Figure 10, the energy gap difference is required to be in the range of 30 to 200 meV, preferably in the range of 50 to 100 meV, as in Example 1. A feature of the semiconductor laser of this example is that the optical guide layer is formed in the cladding structure while the volume of the saturable absorption layer is reduced. As the volume of the - 26 - P13858 saturable absorption layer becomes smaller, the carrier density of the layer can be increased more easily. As the carrier density becomes higher, the light absorption is saturated more easily, exhibiting a more excellent saturable absorption effect. Thus,as the volume of the saturable absorption layer becomes smaller, stronger self-oscillation is obtained. As the volume of the saturable absorption layer is reduced, however, light confinement of the saturable absorption layer decreases. In this example, in order to overcome this problem, the optical guide layer is formed between the active layer and the saturable absorption layer, so that the distribution of laser light can be expanded from the active layer toward the saturable absorption layer, thereby increasing the light confinement of the saturable absorption layer and enhancing the interaction between the saturable absorption layer and light. Thus, the optical guide layer in this example is provided to enhance the light confinement of the saturable absorption layer, which is largely different in the function from conventional optical guide layers provided to enhance the light confinement of the active layer. Next, referring to Figures 12A, 12B, and 13, the function of the optical guide layer m this example will be described. Figures 12A and 12B show the light intensity distributions obtained when no optical guide layer is provided and when an optical guide layer is provided, respectively. As is observed from Figure 12B, two peaks of light intensity appear by forming a semiconductor layer with a small energy gap (high refractive index) compared with other portions of the cladding structure as the optical guide layer at a position apart from the - 27 - P13858 active layer. In other words, the light confinement in both the active layer and the optical guide layer is achieved, allowing light to be effectively distributed to the saturated absorption layer. Figure 13 is a graph showing the dependency of the light confinement upon the thickness of the optical guide layer. The X axis of this graph represents the thickness of the optical guide layer (Å), and the Y axis thereof represents the light confinement (%). The "light confinement" of a layer as used herein means the percentage of the amount of light existing within the layer of the total light amount. It has been found from the experiment results that, in order to obtain stable self-oscillation characteristics, the light confinement of the active layer and the saturable absorption layer is required to be 5% or more and 1.2% or more, respectively. From Figure 13, in order to obtain the above light confinement, the thickness of the optical guide layer should be in the range of 300 to 1200 A. Thus, by forming the optical guide layer in the cladding structure, stable self-oscillation characteristics can be obtained as in Example 1. In this example, the saturable absorption layer is formed at a position apart from the optical guide layer. Alternatively, it may be formed inside the optical guide layer as shown in Figure 14. In this case, also, self-oscillation can be obtained by forming the saturable absorption layer inside the optical guide layer so that the light confinement of the saturable absorption layer is 1.2% or more. - 28 - P13858 In Examples 1 and 2, the active layer xs of the multiple quantum well structure. A semiconductor layer with the stable self-oscillation characteristics can also be realized using an active layer of a single quantum well structure. In this case,the energy gap difference between the ground levels of the quantum well layer and the saturable absorption layer should be in the range of 30 to 200 meV, especially in the range of 50 to 100 meV. The effect of the present invention can also be obtained by a semiconductor laser including a bulk type active layer having no quantum well by setting the energy gap difference at a value within the above range. The positional relationship between the optical guide layer and the saturable absorption layer is not limited to that in Example 2 where the optical guide layer is formed between the saturable absorption layer and the active layer. Referring to Figure 15, the positional relationship between the optical guide layer and the saturable absorption layer will be described. In Figure 15, the saturable absorption layer may be formed at any positions SAl to SA5 shown by dot lines. The position SAl corresponds to the position of the saturable absorption layer adopted in Example 2. While the position SAl is apart from the optical guide layer, the saturable absorption layer may be formed adjacent to the optical guide layer. The positions SA2 to SA4 are the positions of the saturable absorption layer formed inside the optical guide layer. The position SA5 is the position of the saturable absorption layer formed between the optical guide layer and the active layer. Though the position SA5 is apart from the - 29 - P13858 optical guide layer, the saturable absorption layer may be adjacent to the optical guide layer. When the saturable absorption layer is formed inside the optical guide layer, the ground level of the saturable absorption layer varies a little because the height of the barrier of the quantum well as is seen that the saturable absorption layer is different from that obtained when the saturable absorption layer is formed outside the optical guide layer. In the case where the saturable absorption layer is doped with impurities at high density, the impurities inside the saturable absorption layer may adversely affect the active layer if the saturable absorption layer is formed near the active layer. To avoid this problem, the saturable absorption layer is preferably formed apart from the active layer by a distance of 200 A or more when the saturable absorption layer has an impurity density of 1 x 1018 cm-3 or more. A plurality of saturable absorption layers may be formed. For example, saturable absorption layers may be formed at two or more of the positions SAl to SA5 shown in Figure 15. Alternatively, the saturable absorption layer may be of the multiple quantum well structure. As a result of a plurality of saturable absorption layers formed in the cladding structure, however, the total volume of the saturable absorption layers increases, lowering the carrier density of the saturable absorption layers. If a plurality of saturable absorption layers made of a material with a comparatively large refractive index are formed close to one another, light tends to be - 30 - P13858 confined in the portion of the thus closely-formed saturable absorption layers. This reduces the necessity of forming an optical guide layer. Alternatively, a plurality of optical guide layers may be formed separately. For example, a pair of optical guide layers may be formed to sandwich the saturable absorption layer. If the pair of optical guide layers are formed in contact with the saturable absorption layer, the same structure as that obtained when the saturable absorption layer is formed at the position SA3 in Figure 15, i.e., the structure where the saturable absorption layer is formed inside the optical guide layer can be obtained. Incidentally, the thickness of the optical guide layer with the saturable absorption layer formed therein should be the sum of a thickness Tl of a first optical guide portion and a thickness T2 of a second optical guide portion shown in Figure 16. (Example 3) In Example 3, a chip inspection process according to the present invention will be described. In general, a plurality of semiconductor laser elements are formed from one semiconductor wafer. More specifically, after a p-type electrode and an n-type electrode are formed on both surfaces of a semiconductor wafer, the semiconductor wafer is cleaved to obtain a plurality of bars. The cleaved face of each bar is coated with a reflection film. First, in the chip inspection process, a semiconductor laser having characteristics which are not within - 31 - P13858 a predetermined allowance is discarded as defective. For example, when a semiconductor laser as a chip is pulse-driven at room temperature, it is discarded as defective if the threshold current thereof is not within the range of 100 to 200 mA. Then, each of the laser chips which have passed the chip inspection process is sealed in a can and subjected to an assembling process. An aging process is then conducted. The inventors of the present invention have found that, for a semiconductor laser having a saturable absorption layer doped with p-type impurities, the characteristics of the semiconductor laser obtained at the start of the oscillation are different from those obtained one minute or more after the oscillation. It has also been found that the characteristics are stabilized several minutes after the laser oscillation. More specifically, the characteristics are kept ina substantially fixed state about ten minutes after the start of the laser oscillation. For example, assume that the semiconductor laser is driven under the condition of outputting a fixed light power. At this time, while it operates at a driving current of about 100 mA immediately after the start of oscillation, it changes to operate at a driving current of about 70 mA after a lapse of 1 to 10 minutes, in some cases. The above variation in the characteristics occurs within a comparatively short period after the start of laser oscillation, and it hardly occurs after the lapse of this period. This variation in the characteristics is therefore called herein the "initial variation incharac- - 32 - P13858 teristics" In a device or a system including a semiconductor laser as a light source, preferably the operating current of the semiconductor laser should not vary. Accordingly, a process for stabilizing the characteristics (for example, threshold current), preferably an aging process should be conducted. The aging process may be a process where the semiconductor laser is consecutively oscillated at room temperature for 1 to 120 minutes or a process where it is pulse-oscillated at 50 °C for 1 to 120 minutes. This aging process should be conducted after the chip assembling process. It has been found that the characteristics of the semiconductor laser can be stabilized by annealing the semiconductor wafer at 300 to 800 °C for about 10 to 60 minutes before the semiconductor wafer is separated into a plurality of bars, instead of the aging process. This annealing process can be conducted in the wafer state before the assembling process. This eliminates a wasteful process of assembling a defective element by testing and discarding the defective element before the assembling process. Moreover, the process can be conducted for a plurality of semiconductor laser elements simultaneously, not for the individual semiconductor laser elements. Alternatively, this annealing process for stabilizing the characteristics may be conducted after the separation of the wafer into bars. The aging process and the annealing process are particularly effective for a saturable absorption layer doped with p-type impurities (especially, Zn) at high - 33 - P13858 density. In the above examples, AlGalnP semiconductor lasers were specifically described. The present invention is not,limited to this type of semiconductor laser. For example, the present invention is also applicable to AlxGa1-xAs (0 In the case of an AlxGa1-xAs (0 In the case of an AlxGayIn1-x-yN (0 In the case of a MgxZn1-xSySe1-y (0 Mg0 1Zn0 gS0 xSe0 9, for example. (Example 4) In Example 4, an optical disk device according to the present invention will be described with reference to - 34 - P13858 Figure 17. Referring to Figure 17, the optical disk device of this example includes a semiconductor laser 901 of the present invention, a collimator lens 903 for collimating a laser beam 902 (wavelength: 650 nm) emitted from the semiconductor laser 901, a diffraction grating 904 for splitting the parallel beam into three laser beams (only one laser beam is shown in Figure 17), a half prism 905 for allowing a specific component of the laser beam to transmit therethrough or be reflected therefrom, and a condenser lens 906 for converging a laser beam output from the half prism 905 on an optical disk 907. A laser beam spot with a diameter of about 1 µm, for example, is formed on the optical disk 907. As the optical disk 907, not only a read only disk but also a rewritable disk can be used. The laser beam reflected from the optical disk, 907 is first reflected from the half prism 905, is transmitted through a light receiving lens 908 and a cylindrical lens 909, and is incident on a light receiving element 910. The light receiving element 910 which includes a plurality of photodiodes generates an information reproduction signal, a tracking error signal, and a focusing error signal based on the laser beam reflected from the optical disk 907. A driving system 911 drives the optical system based on the tracking error signal and the focusing error signal, so as to adjust the position of the laser beam spot on the optical disk 907. Conventional elements may be used as the above components of the optical disk device of this example - 35 - P13858 except for the semiconductor laser 901. As described in the above examples, the semiconductor laser 901 includes the saturable absorption layer doped with impurities at high density. With such a saturable absorption layer, low-level relative intensity noise can be kept low even when part of the laser beam reflected from the optical disk 907 returns to the semiconductor laser 901 after being transmitted through the half prism 905 and the diffraction grating 904. The semiconductor laser shown in Figure 2 conducts self-oscillation until the light output power is at the level of about 10 mW. As the light output power is increased beyond that level, the oscillation gradually shifts from self-oscillation to a single mode oscillation. For example, the self-oscillation no longer occurs when the light output power is about 15 mW. When information is reproduced from the optical disk, the semiconductor laser should be in the state of self-oscillation where no return light noise arises. When information is recorded on the optical disk, however, self-oscillation is not necessary. Thus, not only low-distortion reproduction of information but also recording are possible by recording information with a light output power of about 15 mW and reproducing information with a light output power of about 5 mW, for example. As described above, in the optical disk device of the present invention, low-distortion reproduction can be attained with a wavelength in the range of 630 to 680 nm without using a circuit component for high-frequency superimposition. - 36 - P13858 In contrast, stable self-oscillation cannot be obtained by the conventional AlGalnP semiconductor lasers with a wavelength of 630 to 680 nm. When such a conventional AlGalnP semiconductor laser is used for an optical disk device, it ,is, required that, a high frequency be superimposed on a driving current to suppress the return noise. This necessitates a large-scale high-frequency superimposing circuit, which is disadvantageous for size reduction of the optical disk device. (Example 5) In Example 5, another optical disk device according to the present invention will be described. The optical disk device of this example uses a laser unit including the semiconductor laser according to the present invention. Specifically, the laser unit includes a silicon substrate having photodiodes and a semiconductor laser mounted on the silicon substrate. A micromirror is formed on the silicon substrate for reflecting a laser beam emitted from the semiconductor laser. Referring to Figure 18, the laser unit will be described in detail. A concave portion 2 is formed in the center of a principal surface la of a silicon substrate 1 (7 mm x 3.5 mm), and a semiconductor laser 3 is disposed on the bottom surface of the concave portion 2. A side wall of the concave portion 2 is tilted serving as a micromirror 4. If the principal surface la of the silicon substrate 1 is in the (100) orientation, a (111) plane may be exposed by anisotropy etching to be used as the micromirror 4. Since the (111) plane is tilted from - 37 - P13858 the (100) plane by 54°, a (111) plane tilted from the principal surface la by 45° can be obtained by using an off substrate tilted from the (100) plane by 9° in a direction. Another (111) plane formed opposite the above (111) plane, used as the micromirror 4 tilts from the principal surface la by 63°. No micromirror is formed in the 63°-tilted (111) plane, but a photodiode for light output power monitoring is formed as will be described later. The (111) plane formed by amsotropy etching is a smooth mirror face, serving as the excellent micromirror 4. In order to enhance the reflection efficiency of the micromirror 4, a metal film which does not absorb laser light easily is preferably formed on at least the tilted plane of the silicon substrate 1. In addition to the photodiode 5 for monitoring light output power from the semiconductor laser 3, 5-part photodiodes 6a and 6b for light signal detection are also formed on the silicon substrate 1. Now, referring to Figure 19, the optical disk device of this example will be described. A laser beam emitted from the semiconductor laser (not shown in Figure 19) of the laser unit 10 with the above-described structure is reflected from the micromirror (not shown in Figure 19) and then split into three beams by a grating formed on the bottom surface of a hologram element 11 (only one beam is shown in Figure 19 for simplification). Then, the laser beam transmits through a quarter wave plate (1/4 ? plate) 12 and an objective lens 13 to be converged on a surface of an optical disk 14. The laser beam reflected from the optical disk 14 is transmitted through the objective lens 13 and the 1/4 ? plate 12, and - 38 - P13858 then is diffracted by a grating formed on the top surface of the hologram element 11. This diffraction forms a minus first-order beam and a plus first-order beam as shown in Figure 20. For example, the minus first-order beam irradiates a light receiving surface-15a of the laser unit located left in Figure 20, while the plus first-order beam irradiates a light receiving surface 15b located right in Figure 20. The pattern of the grating formed on the top surface of the hologram element 11 is adjusted so that the minus first-order beam and the plus first-order beam have different focal distances. As shown in Figure 21, when the laser beam is focused on the optical disk, the shape of a spot of the reflected laser beam formed on the light receiving surface 15a of the laser unit 10 is equal to that formed on the light receiving surface 15b thereof. When the laser beam is not focused on the optical disk, however, the shape of the spot of the reflected laser beam formed on the light receiving surface 15a is different from that formed on the light receiving surface 15b. In this way, the sizes of the light spots formed on the right and left light receiving surfaces are detected as a focusing error signal FES obtained by expression below: FES = (S1 + S3 + S5) - (S2 + S4 + S6) wherein S1 to S3 denote signal intensities output from three center photodiodes of total five photodiodes constituting the light receiving surface 15a, and S4 to S6 denote signal intensities output from three center - 39 - P13858 photodiodes of the total five photodiodes constituting the light receiving surface 15b. When the focusing error signal FES is zero, the laser beam is focused on the optical disk (on focus). The objective lens 13 is driven by an actuator 15r shown in Figure, 19 appropriately so that the focusing error signal FES becomes zero. A tracking error signal TES is determined by expression below: TES = (T1 - T2) + (T3 - T4) wherein Tl and T2 denote the signal intensities output from two outermost photodiodes of the light receiving surface 15a, and T3 and T4 denote the signal intensities output from two outermost photodiodes of the light receiving surface 15b. An information signal RES is determined by expression below: RES = (S1 + S3 + S5) + (S2 + S4 + S6) In this example, the laser unit where the semiconductor laser and the photodiodes are integrally formed is used. However, these elements may be individually formed. The size of the optical disk device can be reduced by using the laser unit where the semiconductor laser and the photodiodes are integrally formed. Moreover, since the photodiodes and the micromirror are preformed in the silicon substrate, only the semiconduc- - 39 - P13858 photodiodes of the total fxve photodiodes constituting the light receiving surface 15b. When the focusing error signal FES is zero, the laser beam is focused on the optical disk (on focus). The objective lens 13 is driven by an actuator 15 shown inFigure 19 appropriately so that the focusing error signal FES becomes zero. A tracking error signal TES is determined by expression below: TES = (Ta - T2) + (T3 - T4) wherein T1 and T2 denote the signal intensities output from two outermost photodiodes of the light receiving surface 15a, and T3 and T4 denote the signal intensities output from two outermost photodiodes of the light receiving surface 15b. An information signal RES is determined by expression below: RES = (S1 + S3 + S5) + (S2 + S4 + S6) In this example, the laser unit where the semiconductor laser and the photodiodes are integrally formed is used. However, these elements may be individually formed. The size of the optical disk device can be reduced by using the laser unit where the semiconductor laser and the photodiodes are integrally formed. Moreover, since the photodiodes and the micromirror are preformed in the silicon substrate, only the semiconduc- - 40 - P13858 tor laser may be optically aligned with respect to the silicon substrate. With this easy optical alignment, the assembling precision increases and the production process can be simplified. Thus, according to the present invention, since the energy gap difference between the active layer and the saturable absorption layer in-the range of 30 to 200 meV can be secured, the saturable absorption layer absorbs -light efficiently and the light absorption is saturated. This makes it possible to provide stable self-oscillation. In particular, the saturation condition of the saturable absorption layer is optimal and the operating current does not increase when the energy gap difference is in the range of 50 to 100 meV, providing good self-oscillation characteristics. Moreover, by forming an optical guide layer, the carrier density of the saturable absorption layer can be easily increased even though the volume thereof is small, providing strong and stable self-oscillation characteristics. Thus, a light source suitable for next-generation large-capacity light information recording can be provided. The optical disk device including such a semiconductor laser is free from intensity noise caused by return of light reflected from the disk surface and temperature change, and thus signal read errors are reduced, providing great significance to the industry. Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the P13858 - 41 - scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed. -42- WE CLAIM 1 A semiconductor laser comprising an active layer having a quantum well layer and a saturable absorption layer, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV 2 A semiconductor laser as claimed in claim 1, wherein the thickness of the saturable absorption layer is in the range of about 10 to about 100 A 3 A semiconductor laser as claimed in claim 1, wherein a plurality of saturable absorption layers are formed 4 A semiconductor laser as claimed in claim 1, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 50 to 100 meV 5 A semiconductor laser as claimed in claim 1, wherein the saturable absorption layer is doped with impurities of 1 x 1018 cm-3 or more 6 A semiconductor laser as claimed in claim 1, wherein distortion is applied to the quantum well layer and the saturable absorption layer 7 A semiconductor laser as claimed in claim 1, wherein the active layer has a multiple quantum well structure 8 A semiconductor laser as claimed in claim 1 comprising a cladding structure sandwiching said active layer, wherein said cladding structure comprises said saturable absorption layer and an optical guide layer for increasing a light confinement of the saturable absorption layer -43- 9 A semiconductor laser as claimed in claim 8, wherein a thickness of the saturable absorption layer is in a range of about 10 to about 100 Å 10 A semiconductor laser as claimed in claim 8, wherein a plurality of saturable absorption layers are formed 11 A semiconductor laser as claimed in claim 8, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 50 to 100 meV 12 A semiconductor laser as claimed in claim 8, wherein the optical guide layer has a band gap which is larger than a band gap of the saturable absorption layer and smaller than band gaps of the other layers of the cladding structure 13 A semiconductor laser as claimed in claim 12, wherein a thickness of the optical guide layer is in the range of 300 to 1200 Å 14 A semiconductor laser as claimed in claim 12, wherein the optical guide layer is divided into a plurality of portions in the cladding structure 15 A semiconductor laser as claimed in claim 12, wherein the optical guide layer is adjacent to the saturable absorption layer in the cladding layer 16 A semiconductor laser as claimed in claim 8, wherein the saturable absorption layer is doped with impurities of 1 x 1018 cm-3 or more 17 A semiconductor laser as claimed in claim 8, wherein the active layer has a multiple quantum well structure -44- 18 A semiconductor laser as claimed in claim 8, wherein the active layer and the cladding structure are formed of AlxGayln1-x-yP (0 1, where x and y are not zero simultaneously) 19 A semiconductor laser as claimed in claim 1 comprising a cladding structure sandwiching said active layer, wherein said cladding structure comprises said saturable absorption layer 20 A'semiconductor laser as claimed in claim 19, wherein the thickness of the saturable absorption layer is in a range of about 10 to about 100 A 21 A semiconductor laser as claimed in claim 19, wherein a plurality of saturable absorption layers are formed 22 A semiconductor laser as claimed in claim 19, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 50 to 100 meV 23 A semiconductor laser as claimed in claim 19, wherein the saturable absorption layer is doped with impurities of 1 x 1018 cm3 or more 24 A semiconductor laser as claimed in claim 19, wherein the active layer has a multiple quantum well structure 25 A semiconductor laser as claimed in claim 19, wherein distortion is applied to the quantum well layer and the saturable absorption layer -45- 26 An optical disk device comprising the semiconductor laser of claim 8, a converging optical system for converging a laser beam emitted from the semiconductor laser on a recording medium, and an optical detector for detecting the laser beam reflected from the recording medium 27 An optical disk device as claimed in claim 26, wherein the semiconductor laser oscillates in a singe mode when information is recorded on the recording medium, and oscillates in a self-oscillation mode when information recorded on the recording medium is reproduced from the recording medium 28 An optical disk device as claimed in claim 26, wherein the optical detector is disposed near the semiconductor laser 29 An optical disk device as claimed in claim 28, wherein the optical detector comprises a plurality of photodiodes formed on a silicon substrate, and the semiconductor laser is disposed on the silicon substrate 30 An optical disk device as claimed in claim 29, wherein the silicon substrate comprises a concave portion formed on a principal surface thereof and a micromirror formed on a sidewall of the concave portion, the semiconductor laser is disposed in the concave portion, and the angle formed between the micromirror and the principal surface is set so that the laser bean emitted from the semiconductor laser proceeds in a direction substantially vertical to the principal surface of the silicon substrate after being reflected from the micromirror 31 An optical disk device as claimed in claim 30, wherein a metal film is formed on a surface of the micro mirror -46- 32 A method for fabricating the semiconductor laser as claimed in claim 8 comprising the steps of forming said active layer having a quantum well layer and, forming said cladding structure sandwiching the active layer, the cladding layer comprising said saturable absorption layer and an optical guide layer for increasing a light confinement of the saturable absorption layer, and energy gap of the saturable absorption layer being smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV, characteristics of said laser varying with time but being substantially fixed after a lapse of about one minute, the method comprising the steps of stabilizing said laser where characteristics obtained immediately after laser oscillation are changed to obtain substantially fixed characteristics 33 A method as claimed in claim 32, wherein the characteristics are current-light output power characteristics 34 A method as claimed in claim 32, wherein the stabilizing step comprises the step of reducing a threshold current by an aging process 35 A method as claimed in claim 32, wherein the stabilizing step comprises the step of reducing a threshold current by annealing 36 A method as claimed in claim 32, wherein the threshold current is reduced from a value obtained immediately after the laser oscillation by 5 mA or more by the stabilizing step 37 A semiconductor laser, substantially as herein described, particularly with reference to, and as illustrated in the accompanying drawings -47- 38 A method for fabricating the semiconductor laser, substantially as herein described, particularly with reference to, and as illustrated in the accompanying drawings 39 An optical disk device incorporating a semiconductor laser, substantially as herein described, particularly with reference to, and as illustrated in the accompanying drawings The semiconductor laser of the present invention comprises an active layer having a quantum well layer and a saturable absorption layer, wherein an energy gap of the saturable absorption layer is smaller than an energy gap between ground levels of the quantum well layer of the active layer by 30 to 200 meV.

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