Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-08-02
2002-04-09
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06370176
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gallium nitride (GaN) group semiconductor laser device incorporated as a light source in an optical system of an optical pickup apparatus for an optical disk and the like.
2. Description of the Related Art
GaN group (GaInAlN) semiconductors are used as materials for semiconductor laser devices having an emission wavelength in a range of ultraviolet to green light wavelengths. A semiconductor laser device using GaN group semiconductors is described in MRS Internet J. Nitride Semiconductor Res. Vol. 2 (1997) Art. 5, for example.
FIG. 4
shows a cross-sectional view of this conventional semiconductor laser device. Referring to
FIG. 4
, the semiconductor laser device includes a sapphire substrate
201
, a GaN buffer layer
202
, an n-type GaN contact layer
203
, an n-type In
0.05
Ga
0.95
N layer
204
, an n-type Al
0.08
Ga
0.92
N cladding layer
205
, an n-type GaN guide layer
206
, an active layer
207
of a multi-quantum well (MQW) structure composed of In
0.15
Ga
0.85
N quantum well layers and In
0.02
Ga
0.98
N barrier layers, a p-type Al
0.2
Ga
0.8
N layer
208
, a p-type GaN guide layer
209
, a p-type Al
0.08
Ga
0.92
N cladding layer
210
, a p-type GaN contact layer
211
, a p-side electrode
212
, and an n-side electrode
213
. Specifically, the MQW structure active layer
207
is composed of a total of seven layers, i.e., four In
0.15
Ga
0.85
N quantum well layers each having a thickness of 3.5 nm and three In
0.02
Ga
0.98
N barrier layers each having a thickness of 7 nm, which are alternately stacked. In this conventional semiconductor laser device, the p-type Al
0.08
Ga
0.92
N cladding layer
210
and the p-type GaN contact layer
211
are etched to form a stripe-shaped ridge for narrowing current to be injected.
When a GaN group semiconductor laser device is used as a light source of an optical disk system, such a laser device is of a self-oscillation type which outputs modulated light power for injection of a constant current, so as to prevent an occurrence of data read error due to noise generated during data read. A semiconductor laser device of this type is described in Japanese Laid-Open Publication No. 9-191160.
FIG. 5
shows a cross-sectional view of this conventional semiconductor laser device. Referring to
FIG. 5
, the semiconductor laser device includes an n-type SiC substrate
221
, an n-type AlN buffer layer
222
, an n-type AlGaN cladding layer
223
, an n-type GaN optical guide layer
224
, an In
0.05
Ga
0.95
N quantum well active layer
225
having a thickness of 10 nm, a p-type GaN optical guide layer
226
, a p-type AlGaN cladding layer
227
, a p-type In
0.1
Ga
0.9
N saturable absorption layer
228
having a thickness of 5 nm, a p-type GaN contact layer
209
, a p-side electrode
230
, and an n-side electrode
231
. In this conventional semiconductor laser device, part of light generated in the active layer
225
is absorbed by the saturable absorption layer
228
. This changes the absorption coefficient of the saturable absorption layer
228
, and with this change of the absorption coefficient, the intensity of laser-oscillated light emitted from the active layer
225
cyclically changes.
As a result, the interference of output light from the semiconductor laser device decreases. If a semiconductor laser device having a low interference is used as a light source of an optical disk system, output light from the semiconductor laser device does not interfere with return light which has directly returned to the active region of the semiconductor laser device after being reflected from a disk. This suppresses generation of noise and thus prevents occurrence of data read error.
When the semiconductor laser device with the above construction is incorporated as a light source in an optical system of an optical pickup apparatus for an optical disk and the like, a tracking servo mechanism is required to accurately focus a spot of a laser beam emitted from the semiconductor layer device on a pit array formed on a surface of the disk. This tracking servo mechanism normally employs a technique called a three-beam method for detecting a displacement of a spot from a pit.
FIG. 6
schematically shows an optical pickup apparatus employing the above technique. Referring to
FIG. 6
, laser light
242
emitted from a semiconductor layer device
241
is split into three beams by a diffraction grating
243
. The split beams pass through a non-polarizing beam splitter
244
and a collimator lens
245
to be collimated. The collimated beams are then focused by an object lens
246
on an information recording surface of a disk
247
on which a pit array is formed. The three beams focused and reflected from the information recording surface of the disk
247
are guided back to the non-polarizing beam splitter
244
via the object lens
246
and the collimator lens
245
, to be received respectively by photodiodes
248
,
249
, and
250
. The photodiode
248
functions to read a signal representing a pit array recorded on the information recording surface of the disk
247
, while the photodiodes
249
and
250
function to detect a displacement of a spot of a laser beam from a pit. The positions of the object lens
246
and the like are adjusted in accordance with the outputs from the photodiodes
249
and
250
, so that a spot of a laser beam can be accurately focused on a pit array formed on the surface of the disk.
In the above three-beam method, three beams reflected from the information recording surface of the disk
247
are not only reflected from the non-polarizing beam splitter
244
to be received by the photodiodes
248
,
249
, and
250
, but partly pass through the non-polarizing beam splitter
244
to be incident on the diffraction grating
243
. The incident converged beam is divided into three beams by the diffraction grating
243
to illuminate the surface of the semiconductor laser device
241
as return light. In
FIG. 6
, the illumination positions of the three return beams are denoted by A, B, and C.
FIG. 7
is a front view of the semiconductor laser device
241
for illustrating the illumination positions of the three return beams. At the illumination position A, the return beam directly returns to the active region of the semiconductor laser device
241
. The illumination positions B and C of the return beams are away from the illumination position A downward and upward, respectively, by a distance of about 20 &mgr;m to 50 &mgr;m. In the conventional semiconductor laser device shown in
FIG. 5
, the saturable absorption layer
228
is provided for suppressing generation of noise due to interference between output light of the laser device and return light at the illumination position A.
The conventional GaN group semiconductor laser device has the following problems.
Three return beams are produced in the case of employing the three-beam method shown in
FIGS. 6 and 7
. Among the three beams, the return beam at the illumination position B is incident on the substrate of the semiconductor laser chip. If the substrate is made of a material having a small absorption coefficient with respect to laser light, such as sapphire and silicon carbide, the return beam at the illumination position B is subjected to multiple reflection inside the substrate forming an interference pattern. The conventional GaN group semiconductor laser device uses sapphire or silicon carbide as a material of the substrate. Further, no layer for absorbing laser light is formed between the active layer and the substrate. It has been found, therefore, that an interference pattern formed by the return beam at the illumination position B and the laser light in the active region interact with each other, resulting in influencing the intensity of the output light of the semiconductor laser device.
A disk is rotated in an optical disk system so that data is read from the disk. During the rotation, the disk tends to tilt slightly, and the angle of this tilt varies with the rotation of the disk. This
Davie James W.
Renner Otto Boisselle & Sklar
Sharp Kabushiki Kaisha
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