Coherent light generators – Particular active media – Semiconductor
Patent
1997-12-19
2000-09-12
Davie, James W.
Coherent light generators
Particular active media
Semiconductor
372 45, H01S 319
Patent
active
06118800&
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a low-noise semiconductor laser used as a light source of an optical disk system, or the like.
BACKGROUND ART
In recent years, the demand for semiconductor lasers has been increasing in the field of optical communication, laser printers, optical disks, and the like, and research and development therefor have mainly been actively conducted for those of the GaAs and InP types. In the field of optical information processing, a system for recording/reproducing information using, particularly, AlGaAs type semiconductor laser light having a wavelength of about 780 nm has been put to practical use, and widely used for compact disks, and the like.
When reproducing an optical disk by using a semiconductor laser as a light source, an intensity noise is generated due to feedback of reflected light from a disk surface or due to temperature variation. Such an intensity noise induces a signal read error. Therefore, a semiconductor laser with a low intensity noise is essential as a light source for optical disks.
Conventionally, in an AlGaAs type semiconductor laser with a low output which is used as a light source for a read-only optical disk, noise reduction has been attempted by employing a structure where a saturable absorber is intentionally formed on each side of a ridge stripe to reduce noise. By employing such a structure, multiple longitudinal modes are realized. If a disturbance due an optical feedback, temperature variation, or the like, is introduced while a semiconductor laser is oscillating in a single longitudinal mode, adjacent longitudinal modes start oscillating due to a slight change in the gain peak. Those modes which start oscillating as described above will conflict with the original oscillation mode, thus presenting a cause of a noise. On the other hand, when multiple longitudinal modes are realized in such a way as described above, intensity variation between different modes is averaged, while the intensity variation due to a disturbance will not be generated, whereby it is possible to obtain a stable low-noise characteristic.
Moreover, as an alternative method, an attempt to obtain a more stable self-sustained pulsation characteristic is shown in Japanese Laid-open Publication No. 63-202083. Specifically, a self-sustained pulsation type semiconductor laser is realized by providing a layer which can absorb output light.
Furthermore, Japanese Laid-open Publication No. 6-260716 reports that an operating characteristic of a semiconductor laser was improved by making the bandgap of an active layer and the bandgap of an absorbing layer substantially equal to each other. The publication refers particularly to a red semiconductor laser as an example.
FIG. 16 is a cross-sectional view schematically showing the structure of the self-sustained pulsation type semiconductor laser disclosed in the above Japanese Laid-open Publication No. 6-260716.
Specifically, an n-type GaInP buffer layer 1302 and an n-type AlGaInP cladding layer 1303 are formed on an n-type GaAs substrate 1301, and a GaInP strained quantum well (MQW) active layer 1304 is formed in the cladding layer 1303. A strained quantum well saturable absorbing layer 1305 is further formed in the cladding layer 1303. A cladding layer 1306 and a p-type GaInP contact layer 1307 are formed into a ridge-like shape above the cladding layer 1303. Both sides of the ridge formed of the cladding layer 1306 and the contact layer 1307 are buried with an n-type GaAs current blocking layer 1308. Moreover, a p-type GaAs cap layer 1309 is formed on the contact layer 1307 and the current blocking layer 1308, a p-electrode 1310 is formed on the cap layer 1309 and, on the other hand, an n-electrode 1311 is formed on the reverse surface of the substrate 1301.
The above Japanese Laid-open Publication No. 6-260716 attempts to obtain a good self-sustained pulsation characteristic by the structure as described above.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, a semiconductor laser includ
REFERENCES:
patent: 5608752 (1997-03-01), Goto et al.
patent: 6014394 (2000-01-01), Tomita
International Search Report dated Jul. 4, 1997.
H. Adachi et al., "Self-Sustained Pulsation in 650-nm-Band AlGaInP Visible-Laser Diodes with Highly Doped Saturable Absorbing Layer", IEEE Photonics Technology Letters, vol. 7, No. 12, 3 pages and cover, Dec. 1995.
R. C. P. Hoskens et al., "Self-pulsating lasers with quantum well saturable absorber", American Institute of Physics, Appl. Phys. Lett. 67 (10), pp. 1343-1345, Sep. 4, 1995.
Adachi Hideto
Kidoguchi Isao
Kumabuchi Yasuhito
Davie James W.
Matsushita Electric - Industrial Co., Ltd.
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