Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2002-01-02
2003-11-04
Ip, Paul (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S029011
Reexamination Certificate
active
06643303
ABSTRACT:
This application is a U.S. National Phase Application of PCT International Application PCT/JP01/00848.
1. Technical Field
The present invention relates to an optical information processing apparatus for recording, reproducing or erasing information on an optical medium or magneto-optical medium such as an optical disk or an optical card, and more particularly an optical head apparatus which emits laser beams having different wavelengths and a laser driving apparatus used in the optical head apparatus.
2. Background Art
An optical memory technology which uses an optical disk having a pit-shaped pattern as a memory medium having a high density and a large capacity is finding extending application in a digital audio-disk, a video disk, a document file disk and a data file and the like.
By the optical memory technology, information is recorded and reproduced on an optical disk with high accuracy and high reliability by way of a finely condensed light beam. Recording and reproducing operations depend only on an optical system used for the optical memory technology.
Basic functions of an optical head apparatus which is a principal part of the optical system are classified roughly into converging for forming a diffraction limited fine spot, focal point control and tracking control for the above described optical system, and pit signal detection. These functions are realized by combination of various kinds of optical systems and photoelectric conversion detecting systems depending on purposes and applications.
On the other hand, an optical disk having a high density and a large capacity which is referred to as DVD has recently been put to practical use and spotlighted as an information medium which permits processing a large amount of information such as animations. The DVD optical disk adopts a smaller pit size on an information recording surface so as to have a recording density higher than that on a compact disk (hereinafter abbreviated as a CD) which is a conventional optical disk. Accordingly, an optical head apparatus which records and reproduces (information on) the DVD optical disk uses a light source and a converging lens having a wavelength and a numerical aperture (hereinafter abbreviated as NA) for determining a spot diameter which are different from those for the CD. For reference, a wavelength of a light source is approximately 0.78 &mgr;m and an NA of a converging lens is 0.45 for the CD, whereas a wavelength of a light source is approximately 0.63 to 0.68 &mgr;m and an NA is approximately 0.6 for the DVD optical disk.
For recording and reproducing (information on) two kinds of optical disks of the CD and the DVD optical disk by one optical information processing apparatus, an optical head apparatus having two optical systems is required. Since a CD-R which has recently been used frequently as a derivative form of the CD permitting additional writing uses a reflecting film on a disk which is optimalized to a wavelength of 0.78 &mgr;m and does not allow information reproduction at the wavelength for the DVD in particular, an optical information processing apparatus which records and reproduces information not only on DVD and CD but also on the CD-R must have two light sources having wavelengths of 0.78 &mgr;m and 0.63 to 0.68 &mgr;m.
From demands for compact and thin configuration and a lower cost of the optical head apparatus, on the other hand, an optical system tends to be used commonly for the CD and DVD, and there are adopted, for example, a system which switches only converging lenses between two kinds for the CD and DVD optical disk and a system which uses also a converging lens commonly while mechanically or optically enlarging only an NA for the DVD optical disk and reducing the NA for the CD.
Furthermore, there has recently been developed a technology which integrates also a light source by forming an infrared laser emitting layer and a red laser emitting layer in a chip of semiconductor laser (for example, LD: laser diode). Furthermore, there has been developed a two-wavelength light source which has two kinds of semiconductor laser chips mounted in a single package.
The above described two light sources will hereinafter be referred to collectively as two-wavelength LDs for convenience. Description will now be made of a driving method for the two-wavelength LD.
FIG. 10
is a block diagram of a conventional LD driving circuit, in which reference numeral
1
denotes a semiconductor laser, reference numeral
2
denotes a photodiode which is a photodetector for monitoring an output from the semi conductor laser. Reference numeral
3
denotes a common terminal, reference numeral
4
denotes a variable resistor, reference numeral
5
denotes an operational amplifier and reference numeral
6
denotes an automatic power control circuit (hereinafter abbreviated as APC circuit).
Description will now be made of operations of the LD driving circuit which is configured as described above. In
FIG. 10
, the common terminal
3
is maintained at a positive potential with a power source apparatus (not shown). When the APC circuit
6
starts operating, a current is supplied to the semiconductor laser
1
to emit rays from the semiconductor laser
1
. When the photodiode
2
receives a portion of the rays emitted from the semiconductor laser
1
, a current corresponding to an intensity of the emitted rays is supplied from the common terminal
3
to ground by way of the photodiode
2
and the variable resistor
4
, thereby generating a voltage across both ends of the variable resistor
4
. The voltage of the variable resistor
4
is detected by the operational amplifier
5
and fed back to the APC circuit
6
. The APC circuit
6
controls the current supplied to the semiconductor laser
1
so that the voltage of the variable resistor
4
is a predetermined voltage, thereby maintaining an optical output from the semiconductor laser
1
constant. A resistance value of the variable resistor
4
is adjusted so that the optical output from the semiconductor laser
1
has a desired value.
Though the above described conventional example uses an output monitoring photodetector for a semiconductor laser, a two-wavelength LD which has recently been under development uses only an output monitoring photodetector for two light sources in most cases as shown in FIG.
11
.
FIG. 11
is a circuit block diagram of the two-wavelength LD, in which reference numeral
1
a
denotes a first semiconductor laser and reference numeral
1
b
denotes a second semiconductor laser. Reference numeral
2
denotes a photodiode which is a photodetector for monitoring outputs from the semiconductor lasers. Reference numeral
3
denotes a common terminal. Due to demands for a low cost and a compact configuration of the light source, the circuit uses only one photodetector
2
and only one output terminal of the photodetector
2
. Though
1
a
and
1
b
are two semiconductor lasers in
FIG. 11
, these lasers may be separate light emitting layers of a single laser chip.
When the above described LD driving circuit is used as a circuit for driving such a two-wavelength LD as that shown in
FIG. 11
, the circuit has such a configuration as that shown in FIG.
12
.
FIG. 12
is a block diagram of a circuit for driving the two-wavelength LD, in which members identical to those in
FIG. 11
are denoted by identical reference numerals with no description in particular. In
FIG. 12
, reference numerals
4
a
and
4
b
denote variable resistors, reference numerals
5
a
and
5
b
denote operational amplifiers, reference numerals
6
a
and
6
b
denote APC circuits, and reference numerals
7
a
and
7
b
denote electronic switches.
Semiconductor laser driving operations in
FIG. 12
will be described briefly. For driving first semiconductor laser
1
a
, the electronic switch
7
a
is set in a connected condition, whereas the electronic switch
7
b
is set in a disconnected condition.
When the first semiconductor laser
1
a
is allowed by the APC circuit
6
a
to start emitting rays, a current is supplied to a photodetector
2
Ito Tatsuo
Kadowaki Shin-ichi
Kamei Tomotada
Komma Yoshiaki
Yamamoto Hiroaki
Ip Paul
Nguyen Phillip
RatnerPrestia
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