Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2002-02-25
2004-09-28
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S053260
Reexamination Certificate
active
06798728
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical disk device and a luminescent power control method for semiconductor laser, whereby a laser beam emitted from a laser beam source is focused into a minute light spot to irradiate an optical disk, which is an information recording medium to optically record or reproduce information, and more particularly to a luminescent power control technique for semiconductor laser to permit precise control of the luminescent power of the semiconductor laser according to changes in ambient temperature.
DESCRIPTION OF THE RELATED ART
For optical recording/reproduction device including optical disk devices, the density, recording speed and reliability are progressively increasing. In an optical disk device, a spindle motor or the like is used to turn an optical disk, and a laser beam emitted from a semiconductor laser is focused into a minute light spot of about 1 &mgr;m in diameter to irradiate the optical disk. Recording of information is achieved by modulating the luminescent power of the semiconductor laser into a pulse shape on according to information signals to vary the intensity of the light spot to irradiate the optical disk, and forming a recording mark by using temperature variations occurring on the recording film of the optical disk. Information is reproduced by keeping the luminescent power of the semiconductor laser at a constant low level, detecting the intensity variations and other factors of the reflected light from the optical disk, and converting them into electric signals. Erasion of recording marks is accomplished by irradiating the optical disk, with the luminescent power of the semiconductor laser kept at a constant level between the power level at the time reproduction and the peak level of the recording pulse. In order to achieve information recording in a high density, it is necessary to form minute marks in the same shape all the time, and this requires highly precise and fast control of the luminescent power of the semiconductor laser.
One of the known methods for this control is to divide the pulse of the recording power (in a multi-pulse system) to form minute marks or pits on the recording face of the optical disk, and to vary its power level in multiple values. One example of this method is disclosed in JP-A-2000-244054 as a method for setting a multi-pulse recording waveform and recording power.
FIG. 6A
illustrates the shape of recording marks on an optical disk, wherein reference numeral
101
denotes recording marks, and
102
, a space between recording marks. In
FIG. 6B
, a solid line
103
represents a multi-pulse recording waveform; a horizontal axis
104
, the time; and a longitudinal axis
105
the luminescent power level of the semiconductor laser. When the space
102
is to be recorded, the semiconductor laser is caused to emit light with erasion power of Bias
1
to erase the background mark. To record a mark
101
, the laser power is set to a plurality of levels including Peak
1
, Peak
2
, Bias
2
and Bias
3
, and the plurality of laser power levels are pulse-modulated to uniformize the heat working on the recording mark, resulting in a stable recording mark
101
. Read in the diagram represents the laser power at the time of reproduction. In the recording waveform shown in
FIG. 6
, recording requires setting of a total of five levels of laser power.
The laser power setting method disclosed in JP-A-2000-244054 will be described below. First, the semiconductor laser is caused to emit light, and drive currents to give Peak
1
, Bias
1
and Bias
3
are set. They are counterparts to points P
1
, B
1
and B
3
, respectively, in the graph representing the I-L characteristic in the left part of FIG.
7
. In a linear region
113
a
in which the I-L characteristic can sufficiently approximate a straight line, a linear formula represented by a broken line L
1
is obtained from the two points, P
1
and B
1
, to figure out a semiconductor laser driving amperage which will give Peak
2
. Next, a one-dot chain line L
2
is obtained from the two points, B
1
and B
3
, to figure out a semiconductor laser driving amperage which will give Bias
2
, corresponding to a nonlinear region
113
b
. Thus it is made possible to set five-point power levels from three-point measurement results by approximating a power level corresponding to the linear region and another power level corresponding to the nonlinear region from separate linear formulas and, in each region, obtaining a third point by interpolation into two-point measurement results using a linear expression.
The linear approximation method described above with reference to an example of the prior art whereby a third point is obtained by interpolation into two-point measurement results using a linear expression is suitable for an optical disk device for continuous recording for a long duration, because it does not take a long time for measurement and arithmetic operation and permits real-time recording power correction. However, for an optical disk recorder whose interior is subject to a high temperature rise when used for continuous recording or reproduction for many hours or an optical disk camera often used in a high-temperature ambience, such as outdoors in summer time, the surroundings of the semiconductor laser are highly heated, and even on the high power side of the semiconductor laser the I-L characteristic may deviate from a straight line and become curved. There is a problem that, where the I-L characteristic of the semiconductor laser becomes curved, calculation by linear approximation as in the above-cited example of the prior art is subject to deterioration of the power setting accuracy of the recording pulse waveform, unevenness of recording marks, deterioration in the quality of reproduced signals and resultant difficulty to reproduce information in high density. Nor is there adequate consideration for power correction of the semiconductor laser or speed increase.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the problem noted above, and provide a semiconductor laser luminescent power control unit capable of accurately controlling the luminescent power of the semiconductor laser in an optical recording/reproduction device, such as an optical disk device wherein information is recorded or reproduced optically by focusing a laser beam emitted from a laser beam source into a minute light spot and irradiating therewith an optical disk, which is an information recording medium, even when the surroundings of the semiconductor laser are heated to a high temperature.
In order to solve the problems noted above, according to the invention, there are provided a means to measure the ambient temperature of a semiconductor laser and a means to switch over, when the temperature of the semiconductor laser rises to so high a level that the I-L characteristic on the high power side can no longer be linearly approximated, the method for computing the drive current for a current generating means for driving the semiconductor laser on the basis of the output of a power detecting unit.
More specifically, according to a first aspect of the invention, there is provided an optical disk device provided with a semiconductor laser, a current generator for supplying a D.C. or pulse-shaped drive current to the semiconductor laser, a power detecting unit for detecting the luminescent power of the semiconductor laser, a peak detector for detecting the peak level of the output signal of the power detector, a bottom detector for detecting the bottom level of the output signal of the power detector, an arithmetic and control unit having programs for computing the drive current of the semiconductor laser and controlling the drive current of the current generator according to the result of computation, and a thermal detector for detecting the temperature of the semiconductor laser, wherein the arithmetic and control unit has a plurality of current computing programs for computing the semiconductor laser drive current by differen
Matsuzaki Masanori
Ota Masataka
Sugiyama Hisataka
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