JP2000163780A - Semiconductor laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the return light noise of an optical disk by allowing plural semiconductor laser elements for emitting the laser beams of different wavelengths to have the same mitigation vibration frequency. SOLUTION: The two semiconductor laser elements 2 and 3 are closely arranged at a prescribed interval on a sub mounting substrate 4 and mounted so as to make the emission plane of the laser beams be the same plane. In this case, in order to make the return optical noise lowest, the parameter and shape of respective layers for constituting the semiconductor laser elements 2 and 3 are optimized. When defining the delay time of return light as tp0, a self exciting vibration pulse number as fp0 and the mitigation vibration frequency as fR, the return light noise becomes lowest when a standardized distance tp0fp0 is 0.2-0.4. Normally, since it is fp0≈fR, it becomes lowest when fp0fR is in the range of 0.2-0.4. When the mitigation vibration frequency fR of the semiconductor laser elements is made the same, tp0fR becomes the range of 0.2-0.4 and the return light noise becomes lowest.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a CD (Compa)
ct Disc) and DVD (Digital Video)
The present invention relates to a semiconductor laser device that emits a multi-wavelength laser beam used as a light source of an optical disk device that reproduces a plurality of optical disks typified by o Disc), and particularly relates to return light noise of the semiconductor laser device.

[0002]

2. Description of the Related Art Since an optical disk is reproduced by using a laser beam of a specific wavelength, in order to reproduce an optical disk written in a different format, a laser beam of a different specific wavelength corresponding to that is required. . Usually, a semiconductor laser element is used as a light source of the laser light, and the wavelength of the laser light is determined by the material of the semiconductor laser element. Therefore, one of the different formats, or
Reproduction of a plurality of optical discs requires a plurality of semiconductor laser elements having different laser wavelengths corresponding to them. At this time, in the case of one optical disc, the format is different on both sides, and in the case of a plurality of optical discs,
In this case, the format is set independently.

When one or a plurality of optical discs of different formats are reproduced, a single optical disc apparatus has been actively developed to reproduce the optical disc in order to reduce the cost and size. . In particular, recording information on CD and DVD optical disks is reproduced by one optical disk device. As a light source for reproducing a CD optical disc, an AlGaAs semiconductor laser element is used.
As a light source for reproducing an optical disk for VD, InGa
An AlP-based semiconductor laser device is used. Therefore, an AlGaAs semiconductor laser device and InGaAl
A semiconductor laser device having a P-based semiconductor laser device mounted thereon or a semiconductor laser device having an AlGaAs-based semiconductor laser device and an InGaAlP-based semiconductor laser device simultaneously formed on the same semiconductor substrate is required. Hereinafter, an optical disk device using a conventional semiconductor laser device will be described with reference to FIG.

FIG. 4 is a diagram schematically showing an optical disk device using a conventional semiconductor laser device. First, the configuration of the optical disk device will be described. The optical disk device includes a semiconductor laser device 27 and the semiconductor laser device 2.
A lens system 25 and an optical disk 26 are sequentially arranged above the optical disk 7. The semiconductor laser device 27 includes an AlGaAs-based semiconductor laser element 28 and an InG
The aAlP-based semiconductor laser elements 29 are placed close to each other with a predetermined space therebetween, and the laser light emitting surfaces of the respective semiconductor laser elements are mounted on the same plane.

Next, the operation of the optical disk device when the optical disk 26 is for a CD will be described. In this case,
An AlGaAs-based semiconductor laser device 28 is used.
The laser light emitted from the semiconductor laser element 28 is applied to an optical disk 26 via a lens system 25, and reflected light including recording information recorded on the optical disk 26 is collected via a lens system 25 to a light receiving element (not shown). The recorded information is signal-processed and reproduced. Optical disc 26 is DV
In the case of D, InGaAlP-based semiconductor laser 29
Is used, and reproduction can be performed in the same manner as in the case of a CD.

[0006]

SUMMARY OF THE INVENTION However, AlG
When the laser light emitted from the aAs-based semiconductor laser element 28 and the InGaAlP-based semiconductor laser element 29 is applied to the optical disk 26 and reflected and returned, a gap between the laser light applied to the optical disk 26 and the return light is generated. Return optical noise caused by the interference of As measures for reducing the return light noise, an AlGaAs semiconductor laser device 28 and an InGaAlP semiconductor laser device 29 are used.
By using a self-oscillation type or by using an external oscillation circuit.
GaAs-based semiconductor laser device 28 and InGaAlP
It is conceivable that a high frequency is superimposed on the drive current of the semiconductor laser element 29 to reduce the coherence.

However, when the self-pulsation type semiconductor laser device 28 and the semiconductor laser device 29 are used, the following problems occur. Generally, as shown in FIG.
Semiconductor lasers - for optical feedback noise when laser device is of the self-pulsation type, the delay time of the return light and t _p0, the number of self-oscillation pulse as f _p0, the return light delay time t _p0 of the returning light when the normalized distance normalized by the self-excited vibration pulse period 1 / f _p0 of the absence of the t _p0 f _p0, when the normalized distance t _p0 f _p0 is between 0.2 and 0.4, It is known that it becomes the lowest and increases outside the range (Preliminary Proceedings of the Spring Association of Applied Physics 1997, 31a-NG-9). In FIG. 5, ◇ indicates that the emission output of the semiconductor laser device is 2 mw,
Shows the case where the emission output of the semiconductor laser device is 5 mw. The noise intensity _with respect to the normalized distance t _p0 f _p0 shows the same tendency when the emission output of the semiconductor laser device is 2 mw or 5 mw.

In FIG. 4, the semiconductor laser device 2
8 and the optical distance between the semiconductor laser element 29 and the optical disk 26 is H, and the speed of light is c, the laser light travels back and forth between the semiconductor laser element 28 and the semiconductor laser element 29 and the optical disk 26. Light delay time t
_p0 is (2H / c). The number of self-excited vibration pulses f
T _p0 = 1 / f between _p0 and the self-excited oscillation pulse period T _p0
There is a relationship of _p0 .

Here, the number of self-excited oscillation pulses f _p0 is usually
It is known that the relationship is substantially equal to the relaxation oscillation frequency f _R of the semiconductor laser device. That is, f _p0 ≒ f _R.
Generally, the relaxation oscillation frequency f _R is expressed as follows. f _R = [(aξη _sep p _out / eVη ₁ ) / 2π] ^1/2 (1) where a is the differential gain, ξ is the optical confinement ratio, η _sep
Is the front slope efficiency, p _out is the front output, e is the elementary charge, V is the active region volume, and η ₁ is the internal differential quantum efficiency. The active region volume V is the product of the cross-sectional area of the active layer of the semiconductor laser device for laser emission and the cavity length. As a result, the normalized distance t _p0 f _p0 becomes t _p0 f _R , and the return optical noise becomes the lowest when t _p0 f _R is in the range of 0.2 to 0.4.

The differential gain a, the optical confinement ratio ξ, the front slope efficiency η _sep , the front output p _out , the active region volume V, and the internal differential quantum efficiency η ₁ are determined by the type and structure of the material of the semiconductor laser device. -Based semiconductor laser device 28 and InGaAlP-based semiconductor laser device 29
Since the materials have different materials and structures, they have different relaxation oscillation frequencies f _R , that is, the number of self-excited oscillation pulses f _p0 . Therefore, when a semiconductor laser element 28 and the semiconductor laser device 29 of InGaAlP system AlGaAs system in the optical disc apparatus, can suppress the normalized distance t _p0 f _p0 in the range of 0.2 to 0.4 No return light noise could be reduced.

In the case where a high frequency is superimposed on the driving current of the semiconductor laser device by using the external oscillation circuit, the following problem arises. As shown in the equation (1), the relaxation oscillation frequency f _R has different modulation bands because it depends on the type and structure of the material of the semiconductor laser device. It is known that the return light noise of a semiconductor laser element decreases as the degree of modulation of the laser light increases, because the coherence decreases. Using a high frequency superimposing circuit,
AlGaAs-based semiconductor laser device 28 and InGaAl
When an attempt is made to superimpose a high frequency on the P-based semiconductor laser element 29, for example, the AlGaAs-based semiconductor laser element 2
8, the modulation degree of the InGaAlP-based semiconductor laser element 29 is low. For this reason, there has been a problem that the return light noise is low when reproducing the optical disk for DVD, but large when reproducing the optical disk for CD.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has been made in consideration of the return light noise of one or a plurality of optical discs reproduced by a semiconductor laser device emitting different laser wavelengths. It is an object of the present invention to provide a semiconductor laser device capable of reducing the noise.

[0013]

SUMMARY OF THE INVENTION The invention of a semiconductor laser device according to the present invention uses a plurality of self-excited oscillation type semiconductor laser elements mounted on a substrate and emitting laser beams of different wavelengths. A semiconductor laser device for performing reproduction, wherein the plurality of semiconductor laser elements have the same relaxation oscillation frequency.

According to a second aspect of the present invention, there is provided a semiconductor laser device for reproducing an optical disk by using a plurality of self-excited oscillation type semiconductor laser elements which emit laser beams of different wavelengths formed on the same semiconductor substrate. The plurality of semiconductor laser devices have the same relaxation oscillation frequency.

According to a third aspect of the present invention, in the semiconductor laser device according to the first or second aspect, the distance between the semiconductor laser element and the optical disk is H, the speed of light is c, the relaxation oscillation frequency of the semiconductor laser element. when the f _R a, wherein the plurality of semiconductor laser _{elements, f R × (2H / c} ) ≒ 0.2
０．４0.4.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a semiconductor laser device according to the present invention will be described with reference to FIG. FIG. 1 is a perspective view showing a semiconductor laser device of the present invention. 2A and 2B are diagrams showing a configuration of a semiconductor laser device mounted on the semiconductor laser device of the present invention, wherein FIG. 2A is a perspective view of an AlGaAs semiconductor laser device, and FIG.
It is a perspective view of a system semiconductor laser element. FIG. 3 is a schematic diagram showing an optical disk device using the semiconductor laser device of the present invention. The same components as those of the conventional example are denoted by the same reference numerals, and detailed description thereof will be omitted. According to the semiconductor laser device of the present invention, instead of the conventional semiconductor laser elements 28 and 29, as described above, f _R × (2H /
c) The semiconductor laser device 2 and the semiconductor laser device 3 having a relaxation oscillation frequency f _R such that t _p0 f _R is between 0.2 and 0.4.

A semiconductor laser device of the present invention and an optical disk device using the semiconductor laser device will be described with reference to FIGS. First, the semiconductor laser device of the present invention will be described. As shown in FIG. 1, a semiconductor laser device 1 of the present invention
aAs-based semiconductor laser device 2 and InGaAlP-based semiconductor laser device 3 are closely arranged at a predetermined interval;
In addition, they are mounted so that the emission surfaces of the laser beams are flush with each other. Semiconductor laser device 2 and semiconductor laser device 3
Is a self-pulsation type, the resonator length of the semiconductor laser element 2 is L ₀ , and the resonator length of the semiconductor laser element 3 is L ₁
It is. In the semiconductor laser device 1, the parameters and shapes of the layers constituting the semiconductor laser device 2 and the semiconductor laser device 3 are optimized to minimize return light noise when used in an optical disk device.

Next, the structures of the AlGaAs semiconductor laser device 2 and the InGaAlP semiconductor laser device 3 will be described with reference to FIG. First, AlGaAs
The configuration of the s-based semiconductor laser device 2 will be described. As shown in FIG. 2A, an n-type G
aAs buffer layer 6, n-type Al _0.6 Ga _0.4 As cladding layer 7, an undoped Al _0.13 Ga _0.87 As active layer 8, p-type Al _0.6 Ga _0.4 As cladding layer 9, p-type Al _0.2 Ga _0.8
As etching stop layers 10 are sequentially stacked.
Further, a pair of trapezoidal ridge portions 13 each comprising a p-type Al _0.6 Ga _0.4 As clad layer 11 and a p-type GaAs cap layer 12 sequentially laminated on the p-type Al _0.2 Ga _0.8 As etching stop layer 10 are sandwiched. The n-type GaAs current confinement layers 14 are formed. Here, the length of the upper end 13a of the trapezoidal ridge 13 is 3 μm,
The length of 3b is 5 μm. A p-type GaAs contact layer 15 and a p-type ohmic electrode 16 are sequentially formed on the ridge 13 and the pair of n-type GaAs current confinement layers 14 and 14. Note that the n-type G
An n-type ohmic electrode 17 is formed on the aAs substrate 2.

On the other hand, the configuration of the InGaAlP-based semiconductor laser device 3 is as follows. The same components as those of the AlGaAs semiconductor laser device 2 are denoted by the same reference numerals. (1
An n-type GaAs buffer layer 6 and an n-type (Al _0.7 Ga _0.3 ) _0.5 I on an n-type GaAs substrate 5 which is 12 ° off from the (00) plane.
n _0.5 P cladding layer 18, an undoped Ga _{0. 5} In _0.5 P
An active layer 19, a p-type (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P cladding layer 20, and an (Al _0.2 Ga _0.8 ) _0.5 In _0.5 P etching stop layer 21 are sequentially stacked. Further, a trapezoidal ridge in which a p-type (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P clad layer 22 and a p-type GaAs cap layer 12 are sequentially stacked on the (Al _0.2 Ga _0.8 ) _0.5 In _0.5 P etching stop layer 21. A portion 23 and a pair of n-type GaAs current confinement layers 14 that sandwich the ridge portion 23 are formed. Here, the length of the upper end 23a of the trapezoidal ridge 23 is 3 μm, and the length of the lower end 23b is 6 μm. The p-type GaAs contact layer 1 is formed on the ridge portion 23 and the pair of n-type GaAs current confinement layers 14, 14.
5, p-type ohmic electrodes 16 are sequentially formed. Note that an n-type ohmic electrode 17 is formed on the n-type GaAs substrate 2 on the side opposite to the stacking direction.

The semiconductor laser device 2 and the semiconductor laser device 3 are operated by injecting a current from the p-type ohmic electrode 16 to the n-type ohmic electrode 17 so that undoped Al _0.13 Ga corresponding to each ridge portion 13 is formed.
_0.87 Undoped Ga _0.5 In _0.5 P active layer 19 corresponding to light emitting region A (shaded area) of As active layer 8 and ridge 23
The laser light is emitted from the light emitting region B (shaded area). Note that the Al _0.13 Ga _0.87 As active layer 8 corresponding to the pair of n-type GaAs current confinement layers 14 and 14 and the undoped Ga
_Since the region of the _0.5 In _0.5 P active layer 19 becomes a saturable absorption band, it does not emit laser light. Here, the active region volume V contributing to the relaxation oscillation frequency f _R is the semiconductor laser device 2
In this case, the product is the product of the sectional area S ₀ of the light emitting region A and the cavity length L ₀ , and in the semiconductor laser device 3, the product of the sectional area S _{1 of} the light emitting region B (hatched portion) and the cavity length L ₁ .

Next, the configuration and operation when the semiconductor laser device of the present invention is used for an optical disk device will be described with reference to FIG. The optical disk device is a semiconductor laser device 2
And a semiconductor laser device 1 on which a semiconductor laser device 3 is mounted, a mirror 24 for reflecting laser light emitted from these semiconductor laser devices, and an optical disk 26 disposed above the mirror 24 via a lens system 25. Consists of

The operation of the optical disk device is performed as follows. When the optical disk 26 is for a CD,
An AlGaAs-based semiconductor laser device 2 is used. The laser light emitted from the semiconductor laser device 2 is
4, the light is reflected upward by a lens system 25 and irradiates an optical disk 26 via a lens system 25. Reflected light including recording information recorded on the optical disk 26 is condensed via a lens system 25 to a light receiving element (not shown). The signal processing of the information is performed and the reproduction is performed. If the optical disk 26 is for DVD, I
An nGaAlP-based semiconductor laser device 3 is used, and a CD
The reproduction can be performed in the same manner as in the case of the application.

Here, return light noise when the semiconductor laser device of the present invention is used for an optical disk device will be described with reference to FIGS. As described above, the delay time of the return light is t _p0 , the number of self-excited oscillation pulses is f _p0 , and the relaxation oscillation frequency f
_Assuming that _R is the return light noise, the normalized distance t _p0 f _p0 is _.0 .
It is lowest when it is between 2 and 0.4. Also, usually
Since f _p0 ≒ f _R, it becomes the lowest when t _p0 f _R is in the range of 0.2 to 0.4. On the other hand, the relaxation oscillation frequency f
_R is a value determined by the material except for the active region volume V, as shown in the equation (1), and can be adjusted by the active layer region volume V.

Undoped Al of the semiconductor laser device 2_0.13
Ga_0.87The thickness of the As active layer 8 is 65 nm, and the cavity length L₀
Is 146 μm, and the undoped Ga of the semiconductor laser device 3 is
_0.5In _0.5The thickness of the P active layer 19 is 0.1 μm, and the resonator length
L₁Is 210 μm, the semiconductor laser element 2 and the half
Relaxation oscillation frequency f of the semiconductor laser element 3_RAre both 1.5
GHz. Further, the semiconductor laser device 2 and the semiconductor laser
The distance l between the laser element 3 and the optical disk 26 is 30 mm
, The delay time t of the return light_p0Is 2 × 10^-Tenin s
is there. From the above, t_p0f_RBecomes 0.3,
Since it is within the range of 0.2 to 0.4, return optical noise is low.
Can be suppressed.

As described above, the delay time t _p0 of the return light is uniquely determined when the distance between the semiconductor laser element 2 and the semiconductor laser element 3 and the optical disk 29 is determined. If the frequency f _R is the same, t _p0 f _R is in the range of 0.2 to 0.4.
A semiconductor laser device having the lowest return light noise can be obtained. Here, the case where two semiconductor laser elements are formed on the submount substrate 4 has been described.
A semiconductor laser device that emits two or more different laser wavelengths may be mounted. Furthermore, the same effect can be obtained by forming a plurality of semiconductor laser devices emitting different laser wavelengths on the same semiconductor substrate in the same manner as in the embodiment of the present invention.

[0026]

According to the semiconductor laser device of the present invention, a plurality of self-oscillation type semiconductor laser elements mounted on a sub-mount substrate and emitting laser beams of different wavelengths are used. A semiconductor laser device for performing reproduction, wherein the plurality of semiconductor laser elements have the same relaxation oscillation frequency, so that t _p0 f _R can be in the range of 0.2 to 0.4. Therefore, return light noise can be reduced. A similar effect can be obtained in the case of a semiconductor laser device including a plurality of self-oscillation type semiconductor laser elements that emit laser beams of different wavelengths formed on the same semiconductor substrate.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a semiconductor laser device of the present invention.

FIG. 2 is a perspective view showing a semiconductor laser element used in the semiconductor laser device of the present invention.

FIG. 3 is a diagram schematically showing an optical disk device using the semiconductor laser device of the present invention.

FIG. 4 is a diagram schematically showing an optical disk device using a conventional semiconductor laser device.

FIG. 5 is a diagram showing the relationship between the noise intensity of the semiconductor laser device and the normalized distance.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser device, 2, 3 ... Semiconductor laser element, 4
... submount substrate (substrate), 5 ... n-type GaAs substrate,
6 ... n-type GaAs buffer layer, 7 ... n-type Al_{0. 6}Ga_0.4
As clad layer, 8 ... undoped Al_0.13Ga_0.87As
Active layer, 9 ... p-type Al_0.6Ga_0.4As cladding layer, 10
... p-type Al_0.2Ga_0.8As etching stop layer, 11
... p-type Al_0.6Ga_0.4As clad layer, 12 ... p-type Ga
As cap layer, 13, 23 ... ridge, 14 ... n-type G
aAs current confinement layer, 15 ... p-type GaAs contact layer,
16 ... p-type ohmic electrode, 17 ... n-type ohmic electrode
Pole, 18 ... n-type (Al_0.7Ga_0.3)_0.5In_0.5P crack
Doped layer, 19 ... undoped Ga _0.5In_0.5P active layer, 20
... p-type (Al_0.7Ga_0.3)_0.5In_0.5P cladding layer, 2
1 ... (Al_0.2Ga_0.8)_0.5In_0.5P etching stock
Layer, 22... P-type (Al_0.7Ga_0.3)_0.5In_0.5P class
Head layer, 24 mirror, 25 lens system, 26 optical disk
School

Claims (3)

[Claims] 1. A semiconductor laser device for reproducing an optical disk by using a plurality of self-oscillation type semiconductor laser elements mounted on a substrate and emitting laser beams of different wavelengths, wherein the plurality of semiconductors are A semiconductor laser device, wherein the laser elements have the same relaxation oscillation frequency. 2. A semiconductor laser device for reproducing an optical disk by using a plurality of self-oscillation type semiconductor laser elements which emit laser beams of different wavelengths formed on the same semiconductor substrate. Wherein the relaxation oscillation frequency is the same. When wherein said semiconductor laser device and a distance between the optical disk H, light velocity c, and relaxation oscillation frequency of the semiconductor laser element and f _R, the plurality of semiconductor laser elements, f _R × 3. The method according to claim 1, wherein a relationship of (2H / c) ≒ 0.2 to 0.4 is satisfied.
13. The semiconductor laser device according to claim 1.