Semiconductor laser control circuit and laser light source

Details Semiconductor laser control circuit and laser light source Semiconductor laser control circuit and laser light source

2000-10-27

2002-12-03

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular component circuitry

For driving or controlling laser

C372S038100, C372S038070, C372S029015, C372S026000, C250S205000, C369S059160

Reexamination Certificate

active

06490302

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser control circuit for adjusting a modulated laser power of each level to a target value when the output modulation of emitted laser radiation is performed among multi-value levels.
Examples of known recording mediums storing optically rewritable information thereon include phase-change storage media and magneto-optical recording media. In writing information onto a phase-change storage medium, an information layer of the medium is irradiated with a focused laser beam, thereby partially heating and fusing the information layer. The highest temperature the information layer can reach due to the heat applied thereto or the cooling process of the layer differs depending on the intensity of the laser radiation incident thereto. Thus, the optical characteristics of the information layer, such as the refractive index thereof, are locally modifiable by modulating the intensity of the laser radiation emitted. More specifically, if the intensity of the laser radiation is higher than a predetermined reference level, part of the information layer of the recording medium that has been irradiated with the radiation is rapidly cooled from an elevated temperature so as to be amorphized. If the intensity of the laser radiation is relatively low on the other hand, the irradiated part of the information layer of the recording medium is gradually cooled from an intermediate to high temperature and therefore crystallized. The amorphized part of the information layer of the recording medium is called a “mark”, while the crystallized part is called a “space”. That is to say, the mark and space have mutually different optical characteristics in terms of their refractive indices, for example. Accordingly, binary data is storable in the information layer of the recording medium by arranging the marks and spaces in a specific pattern. As used herein, the laser radiation for use in information recording will be called “write radiation”.
In reading out information stored on a phase-change storage medium, the information layer thereof is irradiated with a laser radiation beam with an intensity low enough to not cause any phase change in the information layer and a radiation beam, which is reflected from the information layer, is detected. As used herein, the laser radiation for use in information readout will be called “readout radiation”. The mark, or the amorphized part of the information layer of the recording medium, has a relatively low reflectance, while the space, or the crystallized part of the information layer of the recording medium, has a relatively high reflectance. Accordingly, by recognizing the difference in the amount of the radiation reflected from the mark and space, a read signal can be obtained.
Information can be recorded on such a recording medium by a pulse position modulation (PPM) or pulse width modulation (PWM) technique. A recording technique which uses PWM is also called a “mark edge recording” technique.
According to the PPM recording technique, marks are recorded with the space between the marks varied, and information to be written is assigned to positions of the marks. Each of these marks is represented as a pulse with a relatively short, constant pulse width. In contrast, according to the PWM technique, marks of various lengths are recorded with the space between the marks also varied, and information to be written is represented by edge positions of the marks and spaces with a variety of lengths. Generally speaking, the density of the information recorded can be higher with the PWM technique than with the PPM technique.
In performing a PWM recording, longer marks are recorded compared to the PPM recording. If long marks are recorded on a phase-change storage medium, however, the widths of those marks might be non-uniform, because the information layers of media of this type may accumulate and dissipate heat in various manners and their recording sensitivities may be greatly different from each other. It is also known that if the information layer is continuously irradiated with radiation for a long time to record a long mark therein, then the second half of the long mark is likely to increase its width because too much heat is accumulated in that part. To avoid such an unfavorable increase in mark width, a write strategy, by which the radiation is irradiated for recording purposes as a greater number of pulses each with an even short width, was adopted. Methods and apparatuses of this type, that is to say, multi-pulse mark recording methods and apparatuses, are disclosed in U.S. Pat. Nos. 5,490,126 and 5,636,194, for example.
In order to write data onto a recording medium without any error, it is necessary to maintain an intensity of laser radiation with which the recording medium is irradiated at an appropriate level. Such an appropriate level varies depending on the kind of a recording medium. After the recording medium is inserted into a recording/reproducing apparatus, the optimization of the intensity or power level of laser radiation is automatically performed. In order to maintain the power levels of the modulated laser radiation at the target levels, however, it is necessary to always or periodically monitor the power levels of the modulated laser radiation, and to control a laser light source (a semiconductor laser) so that the optical power cannot shift from a target value.
Hereinafter, with reference to FIG.
9
and FIGS.
10
(
a
) to (
h
), a prior-art semiconductor laser control circuit will be described.
FIG. 9
shows a configuration of a prior-art semiconductor laser control circuit. The control circuit controls a current for driving a semiconductor laser
1
, so that laser radiation having a pulse waveform shown in FIG.
10
(
d
) can be emitted from the semiconductor laser
1
.
An intensity (power level) of the laser radiation emitted from the semiconductor laser
1
is detected and converted into a current signal by a monitoring light detector
2
. The current signal is then converted into a voltage signal by a current-to-voltage converter
3
. The voltage signal is input into a peak-power detector
4
, a bottom-power detector
5
, and a sample-and-hold circuit
6
shown in FIG.
9
.
The peak-power detector
4
detects an envelope of a peak-power level in a wave-form of the input voltage signal. The bottom-power detector
5
detects an envelope of a bottom-power level. The sample-and-hold circuit
6
detects a bias-power level of laser radiation.
Outputs of the peak-power detector
4
, the bottom-power detector
5
, and the sample-and-hold circuit
6
are input into peak-power current controller
7
, bottom-power current controller
12
, and bias-power current controller
17
, respectively.
The peak-power current controller
7
compares the output of the peak-power detector
4
with a predetermined reference peak-power voltage
8
, and controls a value of a current flowing out of a peak-power current source
9
. The current is supplied to the semiconductor laser
1
via a switch
11
which is opened and closed in accordance with a peak-power modulation signal
10
shown in FIG.
10
(
a
).
The bottom-power current controller
12
compares the output of the bottom-power detector
5
with a predetermined reference peak-power voltage
13
, and controls a value of a current flowing out of a bottom-power current source
14
. The current is supplied to the semiconductor laser
1
via a switch
16
which is opened and closed in accordance with a bottom-power modulation signal
15
shown in FIG.
10
(
b
).
The bias-power current controller
17
compares the output of the bias-power sample-and-hold circuit
6
with a predetermined reference bias-power voltage
18
, and controls a value of a current flowing out of a bias-power current source
19
. The current is supplied to the semiconductor laser
1
via a switch
21
which is opened and closed in accordance with a bias-power modulation signal
20
shown in FIG.
10
(
c
).
The semiconductor laser
1
is driven by a combination of curr

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