Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2001-12-27
2003-12-09
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S216000, C369S112010
Reexamination Certificate
active
06660986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information reading and recording apparatus for recording media, such as an optical disk or the like used in an optical disk drive.
2. Description of the Related Art
Currently, in information input/output apparatuses using light, readout of information is performed by producing a recording pit by converging light emitted from a laser beam source on a track groove provided on the disk-shaped recording medium such as a CD (compact disk), as a micro spot, recording the presence or the absence of the pit as information, and then irradiating the track with the micro spot to detect the presence or absence of the pit on the track by reflected light.
Recently, DVDs (digital video disks) having a recording capacity of about 7 times that of CDs have become widely used to meet the demand for increased recording capacity. Increase in recording capacity means improvement of the recording density, which depends upon the number of recording pits that can be formed on the recording medium (hereinafter referred to as a disk). In DVDs, decreasing the size of the recording pit, that is, decreasing the diameter of the spot of light irradiated on the disk is one of the factors in increasing the density. The size of the spot to be irradiated on the disk is proportional to the wavelength of the laser and is inversely proportional to the numerical aperture of the objective lens. Accordingly, for decreasing the size of the recording pit, it is required to shorten the wavelength of the laser and to increase the numerical aperture of the objective lens.
However, DVDs are strongly required to be compatible with CDs from the viewpoint of backward comparability of software. Originally, an optical head device was provided with one laser beam source with a wavelength of 635-650 nm and one objective lens having a numerical aperture of about 0.6 for DVDs and another laser beam source with a wavelength of 780 nm and another objective lens having a numerical aperture of about 0.45 for CDs so as to maintain the compatibility between the both disks.
However, when the numerical aperture of the objective lens is increased, the convergence state of the light beam deteriorates due to coma aberration with respect to the inclination of the optical disk. Since coma aberration is proportional to the third power of the numerical aperture of the objective lens and to the thickness of the disk protection substrate, DVDs are designed to have a disk protection substrate with a thickness of 0.6 mm, which is half that of CDs.
When the thickness of the substrate deviates from the designed value, the position where light passing through the inward portion of the objective lens converges deviates from the position where light passing through the outward portion thereof converges depending on spherical aberration. Therefore, when a CD is read by the use of an objective lens having a numerical aperture of 0.6, which is optimally designed for the thickness of the substrate of a DVD, it is necessary to correct spherical aberration by limiting the luminous flux in the outward portion incident on the lens or by slightly diverging the incident angle at the lens.
Accordingly, while one objective lens can be used commonly for the DVD and the CD with the necessary correction of spherical aberration, two laser beam sources each having a different wavelength from other have to be provided for compatibility with a write-once CD. This is because the reflective recording layer of the write-once CD is formed of an organic dye material and thus has a reflection coefficient as low as 6% for light beam having a wavelength of 635-650 nm, that is a wavelength appropriate to the DVD.
Thus, since the current DVD optical head apparatus is equipped with two laser beam sources respectively with a wavelength of 635-650 nm for the DVD and a wavelength of 780 nm for the CD, and since light beams from the two light sources are to be guided to the two objective lenses thereby requiring additional parts such as a prism, aperture control means, or the like for respective light beams, downsizing and cost reduction of the apparatus are very difficult to realize.
In order to solve the problems described above, various optical pickup apparatuses shown in
FIG. 9
to
FIG. 12
have been proposed. Following is an outline of the conventional optical pickup apparatuses.
FIG. 9
is a block diagram of a first conventional example, which includes laser beam sources
91
and
12
to emit laser beam with a wavelength of 650 nm for the DVD and a wavelength 780 nm for the CD, respectively, and a wavelength selection prism
92
for making the respective laser beams travel along the same optical path. There is provided a half mirror
11
for reflecting and guiding the laser beam to a collimating lens
13
and also for passing therethrough and guiding to a photo-detector
90
the laser beam reflected at a disk
18
and returning therefrom. There is also provided a reflection mirror
15
for directing to an objective lens
16
or
17
the laser beam having passed through the collimating lens
13
so that the laser beam is guided from the objective lens
16
or
17
to the disk
18
. The disk
18
, that is, the DVD or the CD, is placed on a drive mechanism (not shown) according to the application, and is rotated by the drive mechanism.
The objective lens
16
has a high numerical aperture (high NA) for DVDs, and the objective lens
17
has a low numerical aperture (low NA) for CDs. The drive mechanism (not shown) is adapted to switch between the objective lens for the DVD and that for the CD.
The laser beam reflected at and returning from the disc
18
passes through the half mirror
11
and is received by the photo-detector
90
that converts it into an electrical signal.
FIGS. 8A
to
8
C are explanatory drawings of the wavelength selection prism
92
above described. The wavelength selection prism
92
is provided with an optical path control film
80
having characteristics shown in FIG.
8
C. The optical path control film
80
characteristically blocks light having a wavelength of 700 nm or below and allows light having a wavelength of 750 nm or above to pass through. Therefore, while light
81
with a wavelength of 780 nm incident on the optical path control film
80
is not blocked by the optical path control film
80
and thus travels straight through as shown in
FIG. 8A
, light
82
with a wavelength of 650 nm incident on the optical path control film
80
from the orthogonal direction to the light
81
is blocked by the optical path control film
80
and reflected by 90 degrees to be directed along the same optical path as the light
81
with a wavelength of 780 nm, as shown in FIG.
8
B.
The operation of the optical pickup apparatus shown in
FIG. 9
is described below. A laser diode (wavelength; 650 nm)
91
for DVDs and another laser diode (wavelength: 780 nm)
12
for CDs, as light sources, are disposed orthogonal to each other so that respective light beams are guided into the same optical path by the wavelength selection prism
92
. Then, the optical axis of the light beam is reflected by 90 degrees at the half mirror
11
, and the light beam is converted into a parallel pencil by the collimating lens
13
. The light beam formed in a parallel pencil is directed by the reflection mirror
15
toward the surface of the disk
18
where a recording layer exists, thereby made incident on the objective lens
16
or
17
.
A suitable objective lens between the objective lens
16
with a high NA for DVDs and the objective lens
17
with a low NA for CDs is selected, switched and set in place by the drive mechanism (not shown). When reading an DVD, the laser diode
91
for DVDs oscillates, and the objective lens
16
with a high NA for DVDs is placed in the optical path to converge the light beam onto a disk (DVD)
18
a.
When reading a CD, the laser diode
12
for CDs oscillates and the objective lens
17
with a low NA for CDs is placed in the optical path to converge the light beard on
Egawa Motoji
Kitamura Atsushi
Matsumoto Kozo
Lee Patrick J.
Minebea Co. Ltd.
Oliff & Berridg,e PLC
Porta David
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