Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-11-02
2001-06-12
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S059110
Reexamination Certificate
active
06246659
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser light power control method for use in recording on optical disks, such as a CD-R, CD-RW, DVD-R, DVD-RAM and M0, to control the recording laser light power to follow a predetermined reference power value, and a laser diode driving circuit using such a laser light power control method. More particularly, the present invention relates to an improved technique which can control the recording laser light power from a laser diode with high precision by accurately detecting the laser light power in cases where each recording pulse is provided in a train of divided pulses.
In recording or reproducing data on an optical disk by use of laser light power, it is necessary to control, with high precision, recording or reproducing laser light power that is predetermined depending on the optical disk used. To this end, the so-called ALPC (Automatic Laser Power Control) technique has been used which constantly detects the laser light power during the recording or reproduction operation and performs control to provide the predetermined recording or reproducing laser light power on the basis of the detected power value.
In
FIG. 2
, there is shown one example of a conventional laser diode driving circuit for optical recording based on such an ALPC technique. Laser diode
10
emits laser light
12
for recording or reproducing data to or from an optical disk. The emitted laser light
12
from the diode
10
is received by a monitor diode
14
provided within an optical pickup, and an output electric current from the monitor diode
14
is converted into a voltage signal via a current-to-voltage converter
16
. Peak value detector circuit
18
detects a peak value of the output voltage from the current-to-voltage converter
16
; the detected peak value represents laser light power that is actually irradiated onto the optical disk. Offset detector circuit
20
detects a difference or offset between the detected peak value from the peak value detector circuit
18
and a predetermined reference laser light power value and thereby outputs an offset voltage value representative of the offset. Value of an electric current to drive the laser diode
10
is then controlled in accordance with the offset voltage from the offset detector circuit
20
so that the laser light
12
is constantly controlled to provide predetermined recording laser light power.
In
FIG. 3
, there is shown another example of the conventional laser diode driving circuit, which includes a laser diode
10
, monitor diode
14
and current-to-voltage converter
16
similar to those of FIG.
2
. In the example of
FIG. 3
, an output voltage from the current-to-voltage converter
16
is sent to an analog gate circuit
22
, where it is sampled in response to a sampling pulse that is generated at predetermined timing corresponding to a recording pulse. The sampled voltage value is held by a hold circuit
24
; the thus-held voltage value represents laser light power that is actually irradiated onto the optical disk. Offset detector circuit
20
detects a difference or offset between the voltage value held in the hold circuit
24
and a target laser light power value and thereby outputs an offset voltage value representative of the offset. Value of an electric current to drive the laser diode
10
is then controlled in accordance with the offset voltage from the offset detector circuit
20
so that the laser light
12
is constantly controlled to provide predetermined recording or reproducing power.
Another-type laser diode driving circuit has been known, which is designed to constantly detect a value of a driving current flowing through the laser diode and control the laser-driving current value to follow a predetermined reference value for the recording or reproduction purpose.
Among various known techniques for recording data on an optical disk is the so-called “divided pulse recording”, which is characterized by dividing each recording pulse, for forming a single pit on the optical disk, into a train of smaller-width pulses (hereinafter called “divided pulses”). This divided pulse recording technique has the advantage that it can effectively minimize errors in the pit width and length due to excessive heat accumulation.
However, in cases where the laser diode driving circuit of
FIG. 2
or
3
is employed in the divided pulse recording, each of the divided pulses tends to have too small a width with the result that the current-to-voltage converter
16
is unable to appropriately follow the pulse frequency, which would result in the output waveform of the converter
16
loosing necessary sharpness, i.e., becoming dull. Such a dull output waveform of the converter
16
would prevent accurate detection of the laser light power, and thus the laser light power could not be controlled with high precision. Further, the laser diode driving circuit of
FIG. 3
could not achieve high-speed, high-density recording using the divided pulse recording technique, because of a limited switching speed of the analog gate circuit
22
.
Furthermore, with the above-mentioned conventional technique which detects a value of a driving current flowing through the laser diode and controls the driving current value to follow a predetermined reference value for recording or reproduction, it was not possible to control the laser light power with high precision due to the fact that a “driving-current vs. output-laser-lightpower” characteristic of the laser diode would greatly vary due to thermal drift and various other physical changes occurring with the passage of time.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a laser light power control method which, in divided pulse recording, can accurately detect laser light power and thereby control the laser light power with high precision, as well as a laser diode driving circuit using such a method.
According to an aspect of the present invention, there is provided a method of controlling laser light power to be used for recording information on an optical disk in accordance with a mark-length recording scheme using recording laser light power emitted from a laser diode driven by a recording signal including recording pulses, which comprises the steps of: providing each of the recording pulse in divided pulses, one of the divided pulses having a greater pulse width than the other divided pulses; detecting the recording laser light power at predetermined timing corresponding to the one divided pulse having the greater pulse width; and controlling an electric current for driving the laser diode in such a manner that the recording laser light power detected by the step of detecting follows a predetermined reference power value.
Because of the arrangement that one of the divided pulses in the recording pulse has a greater pulse width than the other divided pulse and the recording laser light power is controlled at predetermined timing corresponding to such a greater-width divided pulse, a circuit for detecting laser light power need not have a high-speed response characteristic. Thus, the laser light power control method of the present invention can accurately detect the laser light power and thereby control the laser light power to appropriately follow a predetermined reference value with high precision.
As shown in
FIG. 4
, the greater-width divided pulse can be placed selectively at a central position (FIG.
4
A), leading or fore position (
FIG. 4B
) or trailing or rear position (
FIG. 4C
) of the divided pulse train. However, placing the greater-width divided pulse at the central position or leading position of the divided pulse train is more preferable in that it can minimize the possibility of a rear end portion of a pit being excessively expanded rearward under the influence of residual heat.
Further, such a greater-width divided pulse may be placed only in a particular divided pulse train for forming a selected pit length, rather than being placed in every divided pulse train irrespective of the pit length.
Honda Kazuhiko
Suzuki Yoshiaki
Chu Kim-Kwok
Pillsbury & Winthrop LLP
Tran Thang V.
Yamaha Corporation
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