JP3154726B2 - Semiconductor laser and disk device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a low-noise self-excited oscillation type semiconductor laser used for a light source of an optical disk system and the like, and an optical disk device using the semiconductor laser.

2. Description of the Related Art In recent years, demand for semiconductor lasers (laser diodes) has increased in the fields of optical communication, laser printers, optical disks, and the like, and research and development of various semiconductor laser devices, mainly GaAs-based and InP-based semiconductor laser devices, have been actively conducted. It has been advanced. In the field of optical information processing, a system for recording / reproducing information using an AlGaAs-based semiconductor laser (780 nm-Band AlGaAs Laser Diodes) having a lasing wavelength of 780 nm as a light source has been put into practical use. It has become widespread as a disk recording / playback system.

However, recently, there has been a strong demand for an increase in the storage capacity of these optical disks, and accordingly, a semiconductor laser device capable of emitting laser light of a shorter wavelength has been required.

According to the AlGaInP-based semiconductor laser device, 630 to 690 nm
(Red region) laser oscillation is possible. In the present specification, “AlGaInP” means (Al _x Ga _1-x ) _0.6 In
_0.5 P (0 ≦ x <1) is simply represented.
Since this semiconductor laser device can emit laser light of the shortest wavelength among various semiconductor laser devices that are currently in practical use, they have been widely used in the past.
It is promising as a next-generation light source for large-capacity optical information recording, replacing AlGaAs-based semiconductor lasers.

For evaluation of a semiconductor laser device, not only the wavelength of the laser beam but also intensity noise and temperature characteristics are important factors. In particular, when the semiconductor laser device is used as a light source for reproducing an optical disk, it is extremely important that the intensity noise is small. This is because the intensity noise induces an error in reading a signal recorded on the optical disk. The intensity noise of the semiconductor laser element is caused not only by the temperature change of the element, but also by a part of the light reflected from the surface of the optical disk returning to the semiconductor laser element. Therefore, even if such reflected light returns, a semiconductor laser element with low intensity noise is indispensable as a light source for reproducing an optical disk.

Conventionally, AlGaAs was used as a low-power light source exclusively for reproduction of optical discs.
When a system-based semiconductor laser device is used, a saturable absorber is intentionally formed on both sides of a ridge stripe in the device in order to reduce noise. By employing such a structure, the longitudinal mode (Longitudinal mode) of laser oscillation can be multiplied. When the laser oscillates in the single longitudinal mode, if a disturbance such as feedback of laser light to the element or a temperature change enters, a slight change in the gain peak causes the laser to approach the longitudinal mode that was oscillating up to that point. In the vertical mode, laser oscillation starts and there is competition with the original oscillation mode (Mode competitio
Cause n). This is the cause of noise, and when multiplying the vertical mode, the intensity change of each mode is averaged,
Moreover, since it does not change due to disturbance, stable low noise characteristics can be obtained.

Further, Japanese Patent Application Laid-Open No. 63-202083 discloses a prior art capable of obtaining a stable self-excited oscillation characteristic. In this prior art, a self-sustained pulsation type laser diode (Self-Sustained Pulsation type laser diode) is provided by providing a layer capable of absorbing light generated in an active layer.
Has been realized.

Japanese Patent Application Laid-Open No. Hei 6-260716 describes that the characteristics of the red semiconductor laser device are improved by making the band gap of the active layer substantially equal to the band gap of the absorption layer. FIG. 4 is a schematic sectional view showing a conventional self-pulsation type semiconductor laser device disclosed in Japanese Patent Application Laid-Open No. 6-260716. Hereinafter, this semiconductor laser device will be described with reference to FIG.

In FIG. 4, a buffer layer 202 made of n-type GaInP, a cladding layer 203 made of n-type AlGaInP, and a strained quantum well active layer (st) made of GaInP are formed on a substrate 1601 made of n-type GaAs.
A rained quantum well active layer) 204 is sequentially formed. Here, the clad layer 203 includes a strained quantum well saturable absorptive layer.
A ridge-shaped cladding layer 206 and a contact layer 20 made of p-type GaInP are formed thereon.
7 are formed. Both sides of the ridge-shaped cladding structure 206 and the contact layer 207 are buried with a current blocking layer 208 made of an n-type GaAs layer. Further, a cap layer 209 made of p-type GaAs is formed on the contact layer 207 and the block layer 208. A p-type electrode 210 is formed on the cap layer 209, and an n-electrode 211 is formed on the substrate 201 side. I have.

FIG. 5 is an energy band diagram of the strained quantum well saturable absorber layer 205, in which a barrier layer 301 made of (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P and a Ga _x In _1-x P (film thickness of 100 °, strain) +0.5 to 1.0
%) Are alternately stacked, and in the conventional example, three well layers 302 are formed. Here, the band gap of the strained quantum well active layer 204 is substantially equal to that of the strained quantum well saturable absorption layer 205. With this configuration, an attempt is made to obtain good self-excited oscillation characteristics.

The gain characteristics of the AlGaInP-based semiconductor differ greatly from the gain characteristics of the AlGaAs-based semiconductor, and as a result, it has become clear that self-excited oscillation is difficult. FIG. 6 shows the carrier density dependence of the gain for GaInP and GaAs. GaAs and GaInP are AlGaAs semiconductor laser and AlG, respectively.
aA material mainly used for the active layer of a semiconductor laser.

In order to achieve self-sustained pulsation, it is required that the ratio of the increase in gain to the increase in carrier density (the slope of the gain curve) is large. However, the slope of the gain curve of GaInP is G
Since the slope of the gain curve of aAs is smaller, it was found that self-sustained pulsation was relatively difficult to achieve.

Further, according to the experimental results of the present inventors, in the case of a red semiconductor laser (AlGaInP-based semiconductor laser), the band gap between the active layer and the saturable absorption layer is made equal to each other due to the difference in the gain characteristics described above. It has been found that it is difficult to obtain a stable self-sustained pulsation simply by doing.

According to the experimental results of the present inventors, in the case of a red semiconductor laser (AlGaInP-based semiconductor laser), the difference in gain characteristics makes it possible to achieve stable operation by simply equalizing the band gap between the active layer and the saturable absorption layer as in the conventional example. It was found that it was difficult to obtain the self-excited oscillation.

The carrier lifetime can be shortened by increasing the impurity doping amount. This is because the recombination speed is proportional to the product of the probability of occupation of electrons and holes. However, in order to shorten the carrier lifetime, an impurity doping amount of 1 × 10 ¹⁸ cm ⁻³ or more is required. The inventors' experiments have shown that adding a layer with a high doping level in the immediate vicinity of the active layer reduces reliability. This is p
This is due to the diffusion of Zn as a type dopant. Therefore, high doping effective for self-sustained pulsation becomes a problem from the viewpoint of reliability.

The present invention has been made in view of such a point, and particularly, by setting the degree and thickness of doping of the saturable absorption layer and the spacer layer constituting the semiconductor laser optimally, a stable self-pulsation characteristic can be obtained. An object of the present invention is to provide a semiconductor laser having high reliability.

Another object is to provide an optical disk device using such a semiconductor laser.

DISCLOSURE OF THE INVENTION The self-pulsation type semiconductor laser of the present invention is a self-pulsation type semiconductor laser including an active layer and a cladding structure sandwiching the active layer, wherein the cladding structure is doped with impurities. A saturable absorbing layer, wherein the saturable absorbing layer
The heterobarrier leak electron density from the active layer to the saturable absorbing layer is set at a distance of 1 × 10
¹⁸ cm ^-3 or more.

In a preferred embodiment, the impurity doped in the saturable absorption layer is p-type.

Preferably, a spacer layer is provided between the active layer and the saturable absorption layer, and the thickness of the spacer layer is preferably 500 ° or less.

In a preferred embodiment, the height and thickness of the barrier in the conduction band of the spacer layer are set so that the heterobarrier leak electron density is 1 × 10 ¹⁸ cm ⁻³ or more.

In one embodiment, the height of the barrier in the conduction band of the spacer layer is higher than the height of the barrier in the n-type portion of the cladding structure.

In one embodiment, the height of the barrier in the conduction band of the spacer layer is lower than the height of the barrier in the n-type portion of the cladding structure.

In one embodiment, the saturable absorber layer is formed of a compressively-strained quantum well.

In one embodiment, the active layer and the saturable absorption layer are both formed of strained quantum wells having a compressive strain.

In one embodiment, the active layer and the saturable absorption layer are both formed of a strained quantum well having a tensile strain.

The spacer layer preferably has an impurity concentration of 1.0 × 10 ¹⁷ cm ⁻³ or more and 1.0 × 10 ¹⁸ cm ⁻³ or less.

The saturable absorption layer preferably has an impurity concentration of 1.0 × 10 ¹⁸ cm ⁻³ or less.

The heterobarrier leak electron density is preferably 4 × 10 ¹⁸ cm ⁻³ or less.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a sectional structural view of a first embodiment of a semiconductor laser device according to the present invention, and FIG. 1B is a diagram showing a configuration of an Al composition of a main part thereof.

FIG. 2A is a diagram showing a change in carrier lifetime with respect to the photoexcited carrier density of a saturable absorption layer doped with a p-type impurity at a concentration of 5 × 10 ¹⁷ cm ⁻³ , and FIG. 2B is a diagram schematically showing a heterobarrier leak. FIG.

FIG. 3 is a diagram showing the dependence of the hetero barrier leak electrons on the spacer layer thickness.

FIG. 4 is a schematic sectional view of a conventional self-pulsation type semiconductor laser.

FIG. 5 is an energy band diagram of the strained quantum well saturable absorption layer.

 FIG. 6 is a diagram showing gain characteristics of GaAs and GaInP.

FIGS. 7A, 7B and 7C are compositional diagrams near the active layer in the second embodiment of the semiconductor laser device according to the present invention.

FIG. 8 is a sectional structural view of a semiconductor laser device according to a third embodiment of the present invention.

FIG. 9 is a diagram showing a composition structure near an active layer in a third embodiment of the semiconductor laser device according to the present invention.

FIG. 10 shows the composition x of the (Al _x Ga _1-x ) _0.5 In _0.5 P spacer layer.
FIG. 7 is a diagram showing how the heterobarrier leak electron density changes with respect to the thickness of the spacer layer.

FIG. 11 is a diagram schematically showing the configuration of an embodiment of the optical disk device according to the present invention.

FIG. 12 is a perspective view of a laser unit used in the optical disk device according to the present invention.

FIG. 13 is a diagram schematically showing the configuration of another embodiment of the optical disc device according to the present invention.

FIG. 14 is a diagram showing the function of the hologram element used in the embodiment of the optical disk device according to the present invention.

FIG. 15 is a plan view showing a photodetector used in an embodiment of the optical disk device according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION A semiconductor laser according to the present invention has a saturable absorbing layer doped with an impurity, and has a spacer layer made of a material having a band gap larger than the band gaps of the active layer and the saturable absorbing layer. (With a thickness of 50 nm or less) between the active layer and the saturable absorption layer. From the viewpoint of reliability, the impurity concentration of the spacer layer is suppressed to 1 × 10 ¹⁸ cm ⁻³ or less.

In the semiconductor laser of the present invention, by adjusting the thickness and band gap of the spacer layer, an appropriate amount of electrons is injected from the active layer to the saturable absorption layer, thereby achieving self-pulsation. More specifically, by arranging the saturable absorption layer at a position close to the active layer while setting the impurity concentration of the saturable absorption layer relatively low, electrons injected from the active layer into the saturable absorption layer are reduced. Increasing the amount, thereby shortening the lifetime of the electrons in the saturable absorber layer and exhibiting self-sustained pulsation. The density of electrons injected into the saturable absorption layer is 1 × 10
It becomes higher than ¹⁸ cm ^-3, even if the impurity concentration of the saturable absorbing layer is smaller than 1 × 10 ¹⁸ cm ^-3, we that self-oscillation can be achieved has been found.

Conventionally, it has been considered that the saturable absorbing layer should be provided at a position separated from the active layer by, for example, 1000 ° or more. The reason is that if the saturable absorption layer is too close to the active layer, it has been considered that the saturable absorption layer has a gain and self-excited oscillation does not occur. However, according to the study of the present inventors, even if the saturable absorbing layer is brought close to the active layer, if the density of electrons injected into the saturable absorbing layer is within a predetermined range, no gain occurs, and instead, It has been found that self-sustained pulsation is likely to occur because the life of electrons is shortened.

The density of electrons injected into the saturable absorbing layer is 5 × 10
^{If the height} is higher than ¹⁸ cm ^-3, the density of electrons injected into the saturable absorption layer is 5 × 10 ¹⁸ cm because the saturable absorption layer has a gain.
^-3 or less, particularly preferably 4 × 10 ¹⁸ cm ^-3 or less.

Hereinafter, embodiments of the semiconductor laser device according to the present invention will be described with reference to the drawings.

FIG. 1A shows a sectional structure of a first embodiment of a semiconductor laser according to the present invention. This semiconductor laser is an n-type Ga
An As substrate 101 and a semiconductor multilayer structure formed on the GaAs substrate 101 are provided. This semiconductor multilayer structure includes an n-type GaInP buffer layer 102, an n-type AlGaInP cladding layer 103, and a GaInP active layer.
104, p-type AlGaInP spacer layer 105, p-type GaInP saturable absorption layer 106, first p-type AlGaInP cladding layer 107, p-type G
An aInP etching stop layer 108 and a second p-type AlGaInP cladding layer 109 are included.

The second p-type AlGaInP cladding structure 109 has a stripe shape (width: about 2.0 to 7.0 μm) extending in the resonator length direction.

The contact layer 1 is formed on the upper surface of the second p-type cladding layer 109.
10 are formed. On both sides of the second p-type cladding layer 109 and the contact layer 110, an n-type GaAs current blocking layer 1 is provided.
11 are formed. On the contact layer 110 and the current block layer 111, a p-type GaAs cap layer 112 is formed. A p-electrode 113 is formed on the upper surface of the cap layer 112, and an n-electrode 114 is formed on the back surface of the substrate 101.

In the present specification, a buffer layer,
The remaining portion excluding the active layer, the contact layer, the cap layer, and the current blocking layer will be referred to as a “cladding structure” as a whole. In the case of this embodiment, n-type AlGaIn
The P cladding layer 103, the saturable absorption layer 106, the p-type GaInP etching stop layer 108, the first p-type AlGaInP cladding layer 107, and the second p-type AlGaInP cladding layer 109 constitute a cladding structure.

When a voltage is applied between the p-electrode 113 and the n-electrode 114 to cause a current (drive current) to flow from the p-electrode 113 to the n-electrode 114 for laser oscillation, the current flows through the contact layer 110 and the second p-electrode. It is confined by the current blocking layer 111 so as to flow through the mold cladding layer 109. Therefore, the current flows in a region (current injection region) directly below the second p-type cladding layer 109 in the active layer 104, and does not flow in a region directly below the current blocking layer 111. Light is generated in the current injection region of the active layer 104, and the light spreads to some extent outside the current injection region. A part of this light interacts with the saturable absorption layer 106 to cause self-pulsation.

The doping levels and thicknesses of the respective semiconductor layers constituting the laminated structure of this embodiment are as follows.

Since the spacer layer 105, the saturable absorption layer 106, and the first p-type cladding layer 107 located near the active layer 104 have a low doping concentration of 5 × 10 ¹⁷ cm ⁻³ , the active layer 104 The solid phase diffusion of impurities into the element is small, and the reliability of the device is not impaired. The composition of these layers is shown in FIG.
Is shown in The p-type cladding layer 107 and the n-type cladding layer 103 and the spacer layer 105 using a _{(Al 0.7 Ga 0.8) 0.5 In} 0.5 P, an active layer 104, the saturable absorbing layer 106 and the etch stop layer 108 Ga _0.5 In _0.5 P is used.

FIG. 2A shows a change in carrier lifetime with respect to the photoexcited carrier density of the saturable absorption layer 106 doped with a p-type impurity at a concentration of 5 × 10 ¹⁷ cm ⁻³ . The photoexcited carrier density is the density of electron-hole pairs excited by light. In the present specification, as shown in FIG.
Excess electrons injected from 3 into the saturable absorption layer 106 beyond the active layer 104 are called "heterobarrier leak electrons". Figure 2A
In FIG. 7, a plurality of curves are shown using the density of “heterobarrier leak electrons” as a parameter.

FIG. 2A shows that when the heterobarrier leak electron density increases, the recombination velocity of electron-hole pairs in the saturable absorption layer 106 increases, and the carrier lifetime tends to decrease. As the carrier life is shorter, the contribution of spontaneous emission to the time change rate of carriers increases, so that self-sustained pulsation can be easily generated and relative noise can be reduced. According to experiments by the inventors, it was found that the life time is desirably about 7 nanoseconds or less. FIG. 2A shows that when the heterobarrier leak electron density is 1.0 × 10 ¹⁸ cm ⁻³ or more, the carrier lifetime with respect to the photoexcited carrier density is 7 nm or less, and thus stable self-pulsation can be performed.

In this figure, a p-type impurity is added to the saturable absorption layer in 5 × 10 ¹⁷
but it is obtained by doping cm ^-3 concentration, by increasing the concentration of the saturable absorbing layer, even when the 1 × 10 ¹⁸ cm ^-3, any hetero barrier leakage electron density 1.0 × 10 ¹⁸ cm ^-3 or more For example, the carrier lifetime in the saturable absorption layer becomes shorter, the contribution of spontaneous emission to the time change rate of carriers increases, self-sustained pulsation can be easily generated, and relative noise can be reduced. However, from the viewpoint of reliability, it is not preferable to increase the doping level of the spacer layer. This is because the spacer layer is adjacent to the active layer. According to experiments by the inventors, when the impurity concentration of the spacer layer exceeds 1.0 × 10 ¹⁸ cm ⁻³ , the impurity may diffuse from the spacer layer to the active layer, and the reliability of the semiconductor laser device may decrease. all right. Therefore, the impurity concentration of the spacer layer is 1.0 × 10 ¹⁸ c
It is preferable to set m- ³ or less. On the other hand, if the impurity concentration is too low, the electric resistance of the saturable absorbing layer will increase. The impurity concentration of the spacer layer is 1.0 × 10 ¹⁷ cm ^-3 or more,
It is preferably 1.0 × 10 ¹⁸ cm ⁻³ or less.

As shown in FIG. 2B, the heterobarrier leak electrons are electrons in which the electrons injected from the n-type cladding layer 103 into the active layer 104 diffuse into the p-type cladding layer 107 without being sufficiently confined in the active layer 104. Means The hetero barrier leak electron density largely depends on the thickness of the spacer layer 105.

In the AlGaInP system, since the heterobarrier in the conduction band is small, heterobarrier leak electrons are particularly remarkably present. FIG. 3 shows the theoretically determined thickness of the spacer layer 105 and the saturable absorption layer 106.
Shows the relationship with the density of the hetero-barrier leak electrons injected into the substrate. If the band gap of the spacer layer is small, the amount of electrons leaking to the saturable absorption layer increases. Even under the condition that the band gap between the n-type cladding layer 103 and the spacer layer 105 is the same, the heterobarrier leak electrons leaking from the active layer 104 to the saturable absorbing layer 106 tend to increase as the thickness of the spacer layer 105 decreases. Can be seen. And a thickness of approximately 500Å of the spacer layer 105 (Å), 1.0 × 10 ¹⁸ to the aforementioned 7 nanoseconds or less of the carrier lifetime is obtained
Is obtained. As a result, self-sustained pulsation can be achieved without doping the saturable absorption layer 106 with a concentration as high as 1.0 × 10 ¹⁸ cm ⁻³ or more, and both low noise and high reliability can be achieved.

Embodiment 2 As described above, in the semiconductor laser device of Embodiment 1, the density of electrons (heterobarrier leak electron density) injected into the saturable absorption layer 106 is controlled by changing the thickness of the spacer layer 105. Was done. In this embodiment, the hetero barrier leakage electron density is controlled by changing the height of the barrier in the conduction band of the spacer layer 105, not the thickness of the spacer layer 105.

7A to 7C show the band structure of the conduction band of the n-type cladding layer 103, the active layer 104, the spacer layer 105, the saturable absorption layer 106, and the p-type cladding layer 107.

FIG. 7A shows a structure similar to that of the first embodiment, in which the thickness d of the spacer layer 105 is 500 ° and ΔE is 0.16 eV. FIG. 7B shows a structure in which the band gap of the spacer layer is increased and the thickness is reduced. FIG. 7C shows a structure in which the band gap of the spacer layer is reduced and the thickness is increased.

In the example of FIG. 7B, (Al _x Ga _{1 -x} ) _0.5 In _0.5 P spacer layer 10
The composition x of 5 is 1.0 and the thickness is 200 °. If the thickness is smaller than this, a large amount of electrons leak to the saturable absorbing layer due to a tunnel effect, and the saturable absorbing layer has a gain. Further, when the thickness of the spacer layer exceeds 600 °, a sufficiently large heterobarrier leak electron density cannot be obtained. For this reason, the thickness of the spacer layer is preferably 200 to 600 °.

In the example of FIG. 7C, (Al _x Ga _{1 -x} ) _0.5 In _0.5 P spacer layer 10
The composition x of 5 is 0.35 and the thickness is 1000 °.

FIG. 10 shows the composition x of the (Al _x Ga _1-x ) _0.5 In _0.5 P spacer layer.
FIG. 9 is a diagram showing how the hetero barrier leak electron density changes with respect to the thickness (t) of the spacer layer. As described above, the heterobarrier leak electron density is 1.
It is preferably 0 × 10 ¹⁸ cm ⁻³ or more. In order to prevent gain from being generated in the saturable absorption layer, 4.0
It is preferably at most × 10 ¹⁸ cm ⁻³ . Therefore, in the graph of FIG. 10, the spacer is formed so as to have an Al composition (x) and a thickness (t) within the range of 1.0 × 10 ¹⁸ cm ⁻³ or more and 4. × 10 ¹⁸ cm ⁻³ or less. Is preferably performed.

Third Embodiment Hereinafter, a third embodiment of the semiconductor laser device according to the present invention will be described with reference to FIGS. Since the semiconductor laser of this embodiment uses a multiple quantum well structure for the active layer, the efficiency is high and a higher optical output can be obtained.

As shown in FIG. 8, this semiconductor laser has an n-type
It has a GaAs substrate 801 and a semiconductor laminated structure formed on the GaAs substrate 801. This semiconductor laminated structure is composed of n-type GaInP
Buffer layer 802, n-type AlGaInP cladding structure 803, multiple quantum well active layer 804, p-type AlGaInP spacer layer 805, p-type GaInP saturable absorption layer 806, light guide layer 814, first p-type A
lGaInP cladding layer 807, p-type GaInP etching stop layer 80
8, including a second p-type AlGaInP cladding layer 809.

The second p-type AlGaInP cladding layer 809 has a stripe shape (width: about 2.0 to 7.0 μm) extending in the resonator length direction.

The contact layer 8 is formed on the upper surface of the second p-type cladding layer 809.
10 are formed. On both sides of the second p-type cladding layer 809 and the contact layer 810, an n-type GaAs current blocking layer 8 is provided.
11 are formed. A p-type GaAs cap layer 812 is formed on the contact layer 810 and the current block layer 811. A p-electrode 813 is formed on the upper surface of the cap layer 812, and an n-electrode 814 is formed on the back surface of the substrate 801. Active layer 80
Reference numeral 4 has a multiple quantum well structure including three well layers and barrier layers.

The type of each semiconductor layer constituting this semiconductor laser device,
The thickness and the impurity concentration are the same as those of the first embodiment. The features of the semiconductor laser device of this embodiment are as follows.

1) As the saturable absorption layer, a quantum well saturable absorption layer (thickness: 30 to 150 degrees) 806 is used.

2) The multiple quantum well active layer 804 is used as the active layer.

3) At a position adjacent to the saturable absorption layer 806 (Al _0.05 G
a _0.5 ) Light guide layer composed of _0.5 In _0.5 P (thickness: 300 mm to 15 mm)
00Å) 814 are provided.

Hereinafter, the semiconductor laser device of this embodiment will be described in more detail with reference to FIG.

As is clear from FIG. 9, in the present embodiment, the light guide layer 814 having a refractive index smaller than the refractive index of the saturable absorption layer 806 and larger than the refractive indexes of the spacer layer 805 and the first p-type cladding layer 807. It is provided near the saturable absorption layer 806.

When the saturable absorption layer 806 is thinned so as to have a quantum well structure, self-excited oscillation cannot be generated as it is because the light confinement coefficient is extremely reduced.

In the present embodiment, saturable absorption is achieved by disposing an optical guide layer 814 made of (Al _0.05 Ga _0.5 ) _0.5 In _0.5 P having a higher refractive index than other portions of the cladding structure near the saturable absorption layer 806. The confinement factor of layer 806 is increased. When the confinement coefficient in the saturable absorption layer 806 is made at least about 1.5% or more by inserting the light guide layer 814, stable self-pulsation can be generated.

When the saturable absorption layer 806 is a quantum well, the confinement coefficient cannot be set to a size required for self-pulsation without the light guide layer 814 because the thickness is thin. When the number of saturable absorption layers 806 is increased to increase the confinement coefficient, the volume of the saturable absorption layers 806 is increased, the carrier density is reduced, and self-pulsation does not occur. Therefore, by providing the light guide layer 814 on the saturable absorption layer 806, self-pulsation can be newly realized.

The band gap of the light guide layer 814 is equal to the saturable absorption layer 806.
Is preferably larger than the band gap of the spacer layer 805. However, if the band gap of the light guide layer 814 is too close to the band gap of the saturable absorption layer 806, light is confined too much in the saturable absorption layer 806, and the light absorption saturation characteristic is not exhibited.

The multiple quantum well active layer 804 includes three quantum well layers,
The thickness of each quantum well layer is 50 mm. The light guide layer 814 for the quantum well saturable absorption layer 806 has a composition x = 0.5 and a thickness of 150.
It is formed from a 0 ° layer. This thickness has been found to be effective above 200 mm.

Maximum light output (P _max ) of the semiconductor laser device of this embodiment
By introducing the quantum well structure into the multiple quantum well active layer 804, the maximum optical output of the semiconductor laser device using the bulk active layer could be increased by about 20%. Further, the threshold current is reduced, and operation is possible even at high temperatures. Further, the quantum well saturable absorption layer 806 may be provided near the multiple quantum well active layer 804 as long as the dopant is not diffused.

With the semiconductor laser of this embodiment, a self-excited oscillation phenomenon is obtained, and a relative noise intensity (RIN) of −130 dB / Hz or less is also obtained. As described above, the characteristics of the semiconductor laser of the present embodiment adopt a novel structure of a quantum well active layer, a spacer layer, a low-concentration saturable absorbing layer, and a light guide layer.

Fourth Embodiment A fourth embodiment of the semiconductor laser device according to the present invention will be described.

The semiconductor laser device of this embodiment has the same configuration as the semiconductor laser device of FIG. The characteristic point of this embodiment is that various strains are given to the active layer and / or the saturable absorbing layer as shown in Table 2 below.

As shown in Table 2, the semiconductor laser of this embodiment shows the correspondence between the oscillation mode and the absorption mode when compressive strain and tensile strain are applied to the active layer and the saturable absorbing layer.

When no strain is applied to the active layer, the laser beam oscillates in the TE mode. Therefore, compressive strain is also applied to the saturable absorber layer to
Try to absorb the mode. The saturable absorption layer absorbs both TE and TM modes when no strain is applied, but if the laser oscillation mode is made easy to absorb, the saturable absorption layer increases because the absorption efficiency increases. Even when the thickness (volume) of the absorption layer is reduced, the effect of self-pulsation can be easily obtained.

Similarly, as shown in Table 2, when compressive strain is applied to the active layer, the active layer oscillates in the TE mode. Therefore, compressive strain is applied to the saturable absorber layer to increase the TE mode absorption efficiency. vice versa,
When a tensile strain is applied to the active layer, oscillation occurs in the TM mode. Therefore, a tensile strain is also applied to the saturable absorbing layer to increase the TM mode absorption efficiency.

By combining the mode of laser oscillation with the mode of the saturable absorption layer due to the strain of the active layer, the absorption efficiency of laser light in the saturable absorption layer increases, and the thickness of the saturable absorption layer is reduced. Even if the size is reduced, the self-excited oscillation characteristics can be obtained.

In each of the embodiments of the present invention, the case where an AlGaInP-based material is used is described, but the present invention can be applied to a semiconductor laser made of any material.

Embodiment 5 Next, an optical disc device according to the present invention will be described with reference to FIG.

This optical disc apparatus includes a semiconductor laser element 901 according to the present invention, a collimator lens 903 for converting laser light (wavelength 650 nm) 902 emitted from the semiconductor laser element 901 into parallel light, and three laser lights for the parallel light. (Only one laser beam is shown in the figure)
A half prism 905 that transmits / reflects a specific component of the laser light, and a condenser lens 906 that collects the laser light emitted from the half prism 905 on an optical disk 907. On the optical disc 907, for example, a laser beam spot having a diameter of about 1 μm is formed. The optical disc 907 is not limited to a read-only disc, but may be a rewritable disc.

The reflected laser light from the optical disc 907 is reflected by the half prism 905, passes through the light receiving lens 908 and the cylindrical lens 909, and enters the light receiving element 910. The light receiving element 910 has a plurality of divided photodiodes, and generates an information reproduction signal, a tracking signal, and a focus error signal based on the laser light reflected from the optical disc 907. The drive system 911 drives the optical system based on the tracking signal and the focus error signal to adjust the position of the laser light spot on the optical disc 907.

In this optical disc device, the semiconductor laser element 901
Known components may be used for the other components. As described above, the semiconductor laser device 901 of this embodiment has the saturable absorption layer doped at a high concentration. For this reason, a part of the laser light reflected from the optical disc 907 passes through the half prism 905 and the diffraction grating 904, and the semiconductor laser element 901
, The low noise relative intensity noise is maintained at a low level.

According to the semiconductor laser device shown in FIG.
Self-excited oscillation occurs up to the level of mW, but when the optical output is increased beyond that level, the oscillation state gradually changes from self-excited oscillation to single mode oscillation. For example, when the optical output is about 15 mW, no self-oscillation occurs. When reproducing information recorded on an optical disk, the semiconductor laser element should be in a state where no return light noise occurs due to self-excited oscillation. However, when information is recorded on the optical disk, it is not necessary to perform self-excited oscillation. For example, by recording information with an optical output of about 15 mW and reproducing information with an output of about 5 mW, not only low distortion reproduction of information but also recording is possible.

As described above, according to the optical disk device of the present invention, the wavelength is 630 to 680 without using circuit components for high frequency superposition.
Reproduction with low distortion is achieved in the nm band.

On the other hand, the conventional AlGaInP-based semiconductor laser device having a wavelength of 630 to 680 nm could not generate stable self-sustained pulsation. By doing so, it was necessary to suppress return light noise. For that purpose, a large-sized high frequency superimposing circuit is required, which is not suitable for downsizing the optical disk device.

Embodiment 6 Next, another embodiment of the optical disk device according to the present invention will be described.

This optical disk apparatus is an apparatus using a laser unit including the above-described semiconductor laser element according to the present invention. This laser unit includes a silicon substrate on which a photodiode is formed, and a semiconductor laser device mounted thereon. Further, a micromirror that reflects laser light emitted from the semiconductor laser element is formed on the silicon substrate.

First, this laser unit will be described with reference to FIG. As shown in FIG. 12, a concave portion 2 is formed in the center of a main surface 1a of a silicon substrate (7 mm × 3.5 mm) 1, and a semiconductor laser device 3 is arranged on the bottom surface of the concave portion 2. One side surface of the concave portion 2 is inclined and functions as a micro mirror 4. When the main surface 1a of the silicon substrate 2 has the plane orientation (100), the (111) plane is exposed by anisotropic etching, and can be used as the micromirror 4. Since the (111) plane is inclined by 54 ° from the (100) plane, if an off-substrate whose main surface 1a is inclined by 9 ° in the <110> direction from the (100) plane is used, 45 ° is inclined with respect to the main surface 1a.゜ An inclined (111) plane is obtained. The (111) plane provided at a position facing the (111) plane is inclined by 63 ° with respect to the substrate main surface 1a. The micromirror 4 is not formed on this surface, and a light output monitoring photodiode 5 described later is formed. Since the (111) surface formed by anisotropic etching is a smooth mirror surface, it functions as an excellent micromirror 4. However, in order to increase the reflection efficiency of the micromirror 4, a metal film that does not easily absorb laser light is used. It is preferable to vapor-deposit at least on the inclined surface of the silicon substrate 1.

On the silicon substrate 1, in addition to the photodiode 5 for monitoring the optical output of the semiconductor laser element 3, five-division photodiodes 6a and 6b for detecting an optical signal are formed.

With reference to FIG. 13, an optical disk device according to the present embodiment will be described. The laser light emitted from the semiconductor laser element (not shown in FIG. 13) of the laser unit 10 having the above-described structure is a micro mirror (not shown in FIG. 13).
After being reflected by the hologram element 11, the beam is separated into three beams by a grating formed on the lower surface of the hologram element 11 (only one beam is shown in the figure for simplicity). After that, the laser beam is applied to a quarter-wave plate (1 / 4λ plate)
The light passes through 2 and the objective lens 13 and is focused on the optical disk 14. The laser light reflected from the optical disk 14 is transmitted through the objective lens 13 and the / 4λ plate 12, and then diffracted by the grating formed on the upper surface of the hologram element 11.
By this diffraction, as shown in FIG. 14, -1 order light and + order light are formed. For example, the -1 order light is applied to the light receiving surface 15a located on the left in the figure, and the +1 order light is applied to the light receiving surface 15b located on the right in the figure. The pattern of the grating formed on the upper surface of the hologram element 11 is adjusted so that the focal lengths of the −1st order light and the + 1st order light are different.

As shown in FIG. 15, when the laser beam is focused on the optical disc, the light receiving surface 15
The shape of the spot of the reflected laser light formed on a becomes equal to the shape of the spot of the reflected laser light formed on the light receiving surface 15b. When the laser light is not focused on the optical disk, the shape of the spot of the reflected laser light formed on the light receiving surface of the laser unit differs between the two light receiving surfaces 15a and 15b.

The size of the light spot formed on the left and right light receiving surfaces is detected as the focus error signal FES as follows.

FES = (S1 + S3 + S5)-(S2 + 4S + S6) Here, as shown in FIG. 15, S1 to S3 are signals output from the central three photodiodes among the five photodiodes constituting the light receiving surface 15a. Meaning strength,
S4 to S6 mean the signal intensities output from the central three photodiodes among the five photodiodes forming the light receiving surface 15b. Focus error signal FE
When S is zero, the laser light is on focus on the optical disc. The objective lens 13 is driven by the actuator 15 in FIG. 13 so that the focus error signal FES becomes zero.

The tracking error signal TES is obtained as follows.

TES = (T1−T2) + (T3−T4) Here, T1 and T2 mean the signal intensities output from the two photodiodes at both ends of the five photodiodes constituting the light receiving surface 15a. T3 and T4 mean the signal intensities output from the two photodiodes at both ends of the five photodiodes forming the light receiving surface 15b.

 The information signal RES is obtained as follows.

RES = (S1 + S3 + S5) + (S2 + S4 + S6) In this embodiment, the laser unit in which the semiconductor laser element and the photodiode are integrated is used, but these may be separated.

As described above, by using the laser unit in which the semiconductor laser element and the photodiode are integrally formed, the size of the optical disk device can be reduced. Further, since the photodiode and the micromirror are formed in advance on the silicon substrate, the optical alignment only needs to be performed for the alignment of the semiconductor laser element with respect to the silicon substrate. Since the optical alignment is thus easy, the assembling accuracy is high and the manufacturing process is simplified.

INDUSTRIAL APPLICABILITY As described above, the semiconductor laser of the present invention is capable of controlling the density of electrons injected into the saturable absorption layer beyond the barrier of the spacer layer by adjusting the thickness of the spacer layer. Can be
As a result, the carrier lifetime can be reduced, realizing stable self-sustained pulsation characteristics, and eliminating the need to dope the saturable absorber layer at a high concentration to obtain Can also be prevented,
Highly reliable laser characteristics can be obtained.

Continuation of the front page (56) References JP-A-8-204282 (JP, A) JP-A-6-260716 (JP, A) JP-A-7-22695 (JP, A) JP-A-7-86676 (JP) JP-A-4-328337 (JP, A) JP-A-6-350187 (JP, A) JP-A-5-243682 (JP, A) IEEE Photonics Technology Letters Vol. 7, No. 12, (1995) ) P. 1406-1408 IEICE Technical Report LQE95-2 (1995) pp. 7-12 (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (18)

(57) [Claims] 1. A self-pulsation type semiconductor laser comprising an active layer and a clad structure sandwiching the active layer, wherein the clad structure includes a saturable absorbing layer doped with impurities. The distance between the saturable absorbing layer and the active layer is such that the heterobarrier leak electron density from the active layer to the saturable absorbing layer is 1 × 10 ¹⁸ cm ⁻³ or more. Self-excited oscillation type semiconductor laser. 2. The self-pulsation type semiconductor laser according to claim 1, wherein the impurity doped in the saturable absorption layer is p-type. 3. The self-pulsation type semiconductor laser according to claim 1, wherein a spacer layer is provided between said active layer and said saturable absorption layer, and said spacer layer has a thickness of 500 ° or less. 4. The height and thickness of the barrier in the conduction band of the spacer layer are such that the heterobarrier leak electron density is 1 × 10
4. The self-pulsation type semiconductor laser according to claim 3, which is set to be ¹⁸ cm ^-3 or more. 5. The self-pulsation type semiconductor laser according to claim 4, wherein a height of a barrier in a conduction band of the spacer layer is higher than a height of a barrier in an n-type portion of the cladding structure. 6. The self-pulsation type semiconductor laser according to claim 4, wherein the height of the barrier in the conduction band of the spacer layer is lower than the height of the barrier in the n-type portion of the cladding structure. 7. The self-pulsation type semiconductor laser according to claim 1, wherein said saturable absorption layer is formed of a quantum well having a compressive strain. 8. The self-pulsation type semiconductor laser according to claim 1, wherein said active layer and said saturable absorption layer are both formed of strained quantum wells having compressive strain. 9. The self-pulsation type semiconductor laser according to claim 1, wherein said active layer and said saturable absorption layer are both formed of strained quantum wells having tensile strain. 10. The spacer layer has an impurity concentration of 1.0 × 10
^2. The self-excited oscillation type semiconductor laser according to claim 1, which has a size of ¹⁷ cm ^-3 or more and 1 × 10 ¹⁸ cm ^-3 or less. 11. The saturable absorption layer has an impurity concentration of 1 × 10
^2. The self-excited oscillation type semiconductor laser according to claim 1, which has a ^{diameter of 18} cm ^-3 or less. 12. The method according to claim 1, wherein said hetero barrier leak electron density is 4 ×.
4. The self-sustained pulsation type semiconductor laser according to claim 3, wherein the diameter is 10 ¹⁸ cm ^-3 or less. 13. A semiconductor laser device, a condensing optical system for condensing laser light emitted from the semiconductor laser device on a recording medium, and a photodetector for detecting laser light reflected by the recording medium. An optical disc device comprising: a self-pulsation type semiconductor laser comprising: an active layer; and a cladding structure sandwiching the active layer, wherein the cladding structure is doped with impurities. Wherein the distance between the saturable absorbing layer and the active layer is such that the heterobarrier leak electron density from the active layer to the saturable absorbing layer is 1 × 10 ¹⁸ cm ^−3. An optical disk device characterized in that the distance is as described above. 14. The semiconductor laser device oscillates in a single mode when recording information on the recording medium, and operates in a self-excited oscillation mode when reproducing information recorded on the recording medium. 14. The optical disc device according to 13. 15. The optical disk device according to claim 13, wherein said photodetector is arranged near said semiconductor laser device. 16. The photodetector has a plurality of photodiodes formed on a silicon substrate, and the semiconductor laser device is disposed on the silicon substrate.
16. The optical disc device according to 15. 17. The silicon substrate has a concave portion formed on a main surface thereof, and a micromirror formed on one side surface of the concave portion of the silicon substrate. After the laser light emitted from the semiconductor laser device is reflected by the micromirror, the micromirror and the main surface are moved in a direction substantially perpendicular to the main surface of the silicon substrate. 17. The optical disk device according to claim 16, wherein an angle with the optical disk is set. 18. The optical disk device according to claim 17, wherein a metal film is formed on a surface of said micromirror.