Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-11-20
2003-02-04
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06516016
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device having a current confinement structure and an index-guided structure. The present invention also relates to a process for producing a semiconductor laser device having a current confinement structure and an index-guided structure. Further, the present invention relates to a solid-state laser apparatus which includes as an excitation light source a semiconductor laser device having a current confinement structure and an index-guided structure. The solid-state laser apparatus may include provision for generating a second harmonic.
2. Description of the Related Art
(1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 &mgr;m band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that each semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.102 discloses a semiconductor laser device which emits light in the 0.98 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al
0.48
Ga
0.52
As lower cladding layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, an Al
0.2
Ga
0.8
As/In
0.2
Ga
0.8
As double quantum well active layer, an undoped Al
0.2
Ga
0.8
AS optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al
0.67
Ga
0.33
As etching stop layer, a p-type Al
0.48
Ga
0.52
As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al
0.67
Ga
0.33
As etching stop layer by the conventional photolithography and selective etching, and an n-type Al
0.7
Ga
0.3
As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl
2
gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.
However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.
(2) In addition, IEEE Journal of Selected Topics in Quantum Electronics, vol. 29, No. 6, 1993, pp.1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al
0.4
Ga
0.6
As lower cladding layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al
0.2
Ga
0.8
As optical waveguide layer, a p-type Al
0.4
Ga
0.6
As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al
0.4
Ga
0.6
As upper cladding layer by the conventional photolithography and selective etching, and an n-type In
0.5
Ga
0.5
P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.
However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having a different V-group component, on such an upper cladding layer.
(3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.189 discloses an all-layer-Al-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 &mgr;m band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by the conventional photolithography and selective etching, and an n-type In
0.5
Ga
0.5
P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.
The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.
As in the above example, the conventional current semiconductor laser devices which contain a current confinement structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 &mgr;m band, are difficult to produce, or have poor characteristics, and are unreliable in high output power operation.
(4) High-power semiconductor laser devices having a broad light-emitting area are employed as excitation light sources in conventional solid-state laser apparatuses, in which output laser light is emitted from a solid-state laser crystal. In particular, some solid-state laser apparatuses further include a nonlinear crystal which converts a fundamental wave emitted from the solid-state laser crystal into a second harmonic, and such solid-state laser apparatuses are widely used.
In the above solid-state laser apparatuses, the semiconductor laser devices as excitation light sources are required to emit laser light with very high output power. In order to achieve the high output power, semiconductor laser devices in which an active layer has a width of 10 &mgr;m or greater are used, while the widths of the active layers in single-mode laser devices are usually about 3 &mgr;m. Therefore, a number of high-order transverse modes are mixed in oscillated light, and when the oscillation power is increased, the modes of oscillated light are liable to change to different modes due to spatial hole burning of carriers, which is caused by high density distribution of photons in the resonant cavity. At the same time, near-field pattern, far-field pattern, and oscillation spectrum vary. In addition, since efficiencies of current-to-light conversion are different between different transverse modes, the optical output power further varies. This phenomenon is called a kink in the current-optical output power characteristic of a semiconductor laser device.
Thus, the following problems arise.
When the above high-power semiconductor laser device is used as an excitation light source in a solid-state laser apparatus, at least one component coupled with an oscillation mode of the solid-state laser resonator is utilized as an excitation light from among oscillated light generated by the semiconductor laser device and condensed by a lens system into a solid-state laser crystal. Therefore, the output intensity varies greatly with changes of the transverse modes. In addition, since the absorption spectrum of the solid-state laser crystal has a fine absorption spectrum structure in a narrow wavelength band,
Fukunaga Toshiaki
Wada Mitsugu
Fuji Photo Film Co. , Ltd.
Ip Paul
Zahn Jeffrey
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