Coherent light generators – Particular operating compensation means
Reexamination Certificate
2001-11-19
2004-09-14
Wong, Don (Department: 2828)
Coherent light generators
Particular operating compensation means
C372S038100
Reexamination Certificate
active
06792013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic equipment such as an optical disk device or a rewritable CD-ROM device, which performs writing and reading operation by using a laser diode as a light source. More particularly, it relates to an auto power control circuit for a laser diode or an auto laser power control (ALPC) circuit which is used to control a power to be supplied to the laser diode to keep the optical output from the laser diode constant.
2. Description of the Related Art
An optical disk device or a rewritable CD-ROM device uses a laser diode as a light source, and performs data writing and reading operation by irradiation of a laser light onto a disk. However, the optical output from the laser diode varies widely according to ambient the temperature, the operating period of time and so on. As is understood from
FIG. 6
, for example, even if the laser diode is driven by a certain current IF, the optical output power Po significantly varies according to the temperatures Tc of 50° C., 25° C., 0° C. and −25° C. It may thus occur that the oscillating operation of the laser diode stops or the optical output from the laser diode becomes too large with result of destruction. It is therefore required to control the driving current to be supplied to the laser diode in order to obtaing a substantially constant optical output power.
To this end, the ALPC circuit is provided to detect the optical output from the laser diode and to then control the driving current flowing through the laser diode such that the optical output from the laser diode is kept constant.
The description will be made on the fundamental ALPC circuit with reference to FIG.
7
. This ALPC circuit
100
has a laser diode (LD)
1
, a photodiode (PD)
2
, a current-to-voltage converter or an I/V converter
3
, an operational amplifier
4
and a current booster
5
. The photodiode
2
is used to detect the optical output
101
from the LD
1
and thus generates a detected current Is that is representative of the power of the optical output
101
. This current Is flows through a resistor RM in the I/V converter
3
so that an optical detection voltage Vd corresponding to the optical output detection current Is is generated across the resistor RM. This voltage Vd is supplied to an inverting input terminal (−) of the operational amplifier
4
having a non-inverting input terminal (+) supplied with a reference voltage Vref. The current booster
5
is constituted by a PNP transistor Q and an resistor RL connected between the emitter thereof and a power voltage line Vcc. The base of the transistor Q is connected to the output of the operational amplifier
4
, and the collector thereof is connected to LD
1
to supply a driving current IF thereto.
In this manner, the driving current IF of the LD
1
is controlled such that the optical detection voltage Vd becomes equal to the reference voltage Vref. The optical output from the LD
1
is thus controlled to be constant. For example, when the optical output
101
from the LD
1
increases due to the temperature variations, the optical detection current Is from the photodiode PD
2
increases accordingly. The increase in the optical detection current Is makes larger the voltage drop across the resistor RM to lower the optical detection voltage Vd. In response thereto, the operational amplifier
4
increases the base potential of the transistor Q, so that the driving current IF is made small. The optical output power
101
of the LD
1
is thus decreased.
Based on the above ALPC circuit
1001
an optical disk device according to the prior art is equipped with an ALPC circuit
100
as shown in FIG.
8
. It is to be noted that in the optical disk device, the required optical output from a laser diode differs according to the operation modes such as a write operation mode, an erase operation mode and a read operation mode. Therefore, the auto laser power control circuit
1000
is provided with a WRITE block
10
, an ERASE block
20
and a READ block
30
, one of which is brought into an active state in accordance with the operation mode to be currently initiated to control the driving current of the laser diode (LD)
1
during each operation mode. These blocks are substantially identical in configuration with one another. Accordingly, a description will given only to the WRITE block
107
. It is noted that the same constituents as those shown in
FIG. 7
are indicated by the same reference numerals to omit further description thereof.
The current flowing through PD
2
in response to the optical output from the LD
1
driven by the current booster
5
is supplied to the WRITE, ERASE and READ blocks
10
,
20
and
30
, each of which thus includes the I/V converter
3
. The conversion voltage V
1
is supplied through a resistor R
1
to the operational amplifier
4
, differently from FIG.
7
. The resistor R
1
determines the gain of the operational amplifier
4
together with a resistor R
2
connected between the output and the non-inverting terminal of the operational amplifier
4
. Such gain is set to be a considerable value, 100 for example, because a high sensitivity is required for this kind of device. However, such high gain then may sometime cause undesirable overshoot and/or undershoot in the output of the operational amplifier
4
. A capacitor C is therefore connected in parallel to the resistor R
2
, thereby solving such a problem.
As is further distinct from FIG. 7, the reference voltage V2 to be supplied to the amplifier
4
is derived from digital data. Specifically, the reference voltage digital data WRCUR is supplied from a system controller (not shown) in the write operation mode and is converted into a reference voltage V2 by a D/A converter
6
, which the voltage is then supplied to the amplifier
4
as a reference voltage Vref shown in
FIG. 7 through a
switch SW
0
provided between the D/A converter
6
and the operational amplifier
4
. The switch SW
0
is controlled by a write operation mode signal C
0
that assumes an active level in the write operation mode and an inactive level in the other modes. The active level of the signal C
0
causes the switch SW
0
to select the voltage converted by the D/A converter
6
and supplies it to the operational amplifier
4
as the reference voltage V2. When the signal C
0
indicates a mode other than the write operation mode, on the other hand, the switch SW
0
selects and supplies the ground potential to the operation amplifier
4
, so that the LD
1
is maintained uncontrolled from the WRITE block
10
even if the malfunction of the current booster
5
occurs. As is readily understood from the foregoing, each of the other blocks
20
and
30
is activated by the corresponding signal to the control signal C
0
in the same manner with the unique digital data for reference voltage to corresponding mode. Each of the outputs WLD, ELD and RLD of the blocks
10
,
20
and
30
is then supplied to the current booster
5
. Although not shown, the current booster
5
is constructed to one of the signals WLD, ELD and RLD in response to the operation mode to be currently executed.
Thus, the LD
101
is controlled to output a laser with a substantially constant power in the respective operation modes.
It has been, however, recognized by the inventor that the ALPC circuit
100
has the problem that the shift in operation from one mode to another mode to be a relatively long period of time to deteriorate a high speed operation. This problem becomes remarkable upon the operation being moved from the erase or read mode to the write mode. This will be described below in details with reference to
FIG. 9
which shows the signal voltage waveforms of respective parts in the WRITE block
10
in case where the operation mode is shifted from write to read, and then back to write.
In the write operation mode shown on the left-hand side of
FIG. 9
, the signal C
0
assumes a high level as an active level, so that the reference voltage V2 based on the data WRCUR is supplied to
Ishiwata Hiromasa
Saito Akihiro
McGinn & Gibb PLLC
Vy Hung Tran
Wong Don
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