Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-03-20
2004-08-17
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S050121
Reexamination Certificate
active
06778575
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly, it relates to an AlGaInP-based high-output red semiconductor laser device.
2. Description of the Prior Art
An AlGaInP-based high-output red semiconductor laser device is generally known as a semiconductor laser device applicable to a recordable DVD system or the like. In order to improve the recording speed in the recordable DVD system, the intensity of a laser beam applied to the disk must be improved. In order to improve the intensity of the laser beam applied to the disk, the coupling efficiency between an objective lens for focusing the beam on the disk and the laser beam must be improved while the output of the semiconductor laser device as the light source must be increased.
Improvement of the coupling efficiency between the objective lens and the laser beam is now studied. In general, the objective lens is provided in response to an angle of horizontal divergence of the laser beam, and hence a vertical laser beam component having a larger angle than the horizontal divergence angle is applied outward beyond the objective lens. In this case, the coupling efficiency between the objective lens and the laser beam is reduced. In order to improve the coupling efficiency between the objective lens and the laser beam, therefore, the vertical beam divergence angle must be reduced. In other words, the ratio of the vertical beam divergence angle to the horizontal beam divergence angle (aspect ratio: vertical beam divergence angle/horizontal beam divergence angle) must be approached to 1.0 with respect to an active layer of the semiconductor laser device.
In order to increase the output of the semiconductor laser device, the level of COD (catastrophic optical damage: deterioration of an end face emitting the laser beam) must indispensably be improved. It is known that COD is caused in the following cycle: When a current is injected in a laser end face having surface states in high density, non-radiative recombination results through the surface states. Therefore, the laser end face generates heat. An energy gap is reduced in the active layer on the laser end face due to this heat, to increase light absorption. Thus, heat is further generated. The temperature of the laser end face is increased due to this cycle, whereby crystals are fused to break the laser end face as a result.
A method employing a window structure by Zn diffusion is generally known as a method of suppressing such COD. This method is disclosed in IEEE Journal of Quantum Electronics, Vol. 29, No. 6, pp. 1874 to 1877 (1993), for example. In the conventional method employing a window structure, an impurity is introduced into a region of an active layer of a laser device close to a cavity end face thereby disordering a quantum well structure of the active layer. Therefore, a band gap of the region of the active layer close to the cavity end face is spread as compared with the remaining regions, thereby reducing light absorption on the cavity end face. Thus, temperature increase can be suppressed on the laser end face, thereby suppressing COD.
A method of reducing optical density on a portion of an active layer close to a cavity end face by enlarging the area of an emission spot is also known as another method of suppressing COD. In this case, the area of the emission part is so enlarged as to reduce the vertical beam divergence angle.
When the vertical beam divergence angle is reduced, a kink (bend of a current-light output characteristic) originating from higher transverse mode hardly results. When the vertical beam divergence angle is reduced, therefore, not only suppression of the aforementioned COD but also improvement of the light output can be attained.
In general, the output of the semiconductor laser must be increased along with improvement of the coupling efficiency between the objective lens and the laser beam in order to improve the intensity of the laser beam applied to the disk, as hereinabove described. In order to improve the coupling efficiency between the objective lens and the laser beam, the aspect ratio, i.e., the ratio of the vertical beam divergence angle to the horizontal beam divergence angle, must be reduced. In order to increase the output of the semiconductor laser device, the COD level or the kink level must be improved. It is generally known that the aspect ratio can be reduced, and the COD and kink levels can be improved by reducing the vertical beam divergence angle. In order to reduce the vertical beam divergence angle, the emission spot may be enlarged.
When the emission spot is enlarged in a conventional semiconductor laser device having a loss guided structure confining transverse light by absorbing light in a current blocking layer in order to reduce the vertical beam divergence angle, however, light absorption in the current blocking layer is increased to reduce inclination (slope efficiency) of the current-light output characteristic. Therefore, an operating current for obtaining a constant light output is disadvantageously increased. When the operating current is increased, light output is readily saturated (light output thermal saturation) by heat generation, and hence it is difficult to improve the light output. Thus, it is difficult to increase the output by reducing the vertical beam divergence angle in the conventional semiconductor laser device having a loss guided structure.
To this end, generally known is a method of reducing light absorption in a current blocking layer through a real refractive index guided structure transparentizing the current blocking layer with respect to a laser beam. According to this real refractive index guided structure, a light confinement layer consisting of a material having a smaller refractive index than a cladding layer is provided to cover the side surfaces of the cladding layer of a ridge portion thereby confining transverse light due to difference between refractive indices. Thus, the current blocking layer absorbs no light, to hardly cause light output thermal saturation resulting from light absorption in the current blocking layer.
When the area of an emission spot is enlarged for reducing the vertical beam divergence angle in a conventional semiconductor laser device having such a real refractive index guided structure, however, the ratio of the light component (optical confinement factor) present in the portion of the active layer to the overall area of the light is reduced. Therefore, the light hardly attains a gain, leading to difficulty in lasing. It is known that a threshold current is thus increased to increase an operating current, readily leading to light output thermal saturation.
Thus, when the emission spot is enlarged for reducing the vertical beam convergence angle in the conventional laser device having a real refractive index guided structure, light output thermal saturation readily results from reduction of the optical confinement factor although light output thermal saturation hardly results from light absorption in the current blocking layer, and hence it has been regarded as difficult to improve a kink light output and obtain a high maximum light output. Consequently, there has been developed no red laser device of a real refractive index guided structure having a low aspect ratio with a vertical beam divergence angle of not more than 20.0°.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser device having a real refractive index guided structure capable of obtaining a high kink light output and a high maximum light output also when reducing a vertical beam divergence angle.
Another object of the present invention is to implement a low aspect ratio in the aforementioned semiconductor laser device.
Noting the aforementioned point, the inventors have made various experiments and deep study, to find out that a high kink light output and a high maximum light output can be obtained in a semiconductor laser device having a real refractive in
Hiroyama Ryoji
Inoue Daijiro
Nomura Yasuhiko
Takeuchi Kunio
Sanyo Electric Co,. Ltd.
Vy Hung Tran
Wong Don
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