Optical pickup device

Details Optical pickup device Optical pickup device

2000-06-13

2003-04-22

Kim, Robert H. (Department: 2882)

Radiant energy

Photocells; circuits and apparatus

Photocell controls its own optical systems

C250S225000, C369S110020, C369S112160

Reexamination Certificate

active

06552317

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device for effecting the reproduction of an optical recording medium. More particularly, the present invention concerns an optical system for separating emergent light from a light source and return light from the optical recording medium.
As an optical pickup device for effecting the reproduction of an optical recording medium such as a compact disk (CD), an optical pickup device of a polarizing system is known which is arranged to be capable of separating emergent light and return light by causing the emergent light from the laser light source and the return light from the optical recording medium to pass through a quarter-wave phase difference plate (quarter-wave plate). For example, as shown in
FIGS. 10 and 11
, a device in which a polarizing beam splitter
11
(polarizing and separating element), a phase difference plate
12
, and an objective lens
16
are disposed at positions midway in the optical path from a laser light source to a photodetector is arranged such that the light emitted from a laser light source
13
constituted by a laser diode passes through the polarizing beam splitter
11
and the phase difference plate
12
, and is then applied to the recording surface of an optical recording medium
5
as a spot of light, while the return light from the optical recording medium
5
passes through the phase difference plate
12
and the polarizing beam splitter
11
again. The return light from the optical recording medium
5
, when passing through the phase difference plate
12
, is converted to laser light whose polarization direction differs 90° from the polarization direction of the emergent light from the laser light source
13
, and is guided to a photodetector
14
disposed in a direction different from that of the laser light source
13
by the polarizing beam splitter
11
.
In addition, there are cases in which, as shown in
FIGS. 1 and 2
, a diffractive element
21
(hologram element) serving as a polarizing and separating element is disposed between the laser light source
13
and the phase difference plate
12
, and the return light from the optical recording medium
5
is guided to the photodetector
14
by this diffractive element
21
.
In either one of these optical pickup devices
1
A and
1
B of the polarizing system, as a development of the optical system and the state of polarization of the light are schematically shown in
FIGS. 12A and 12B
, the linearly polarized light emitted from the laser light source
13
is converted to circularly polarized light by the phase difference plate
12
, and the return light (circularly polarized light) from the optical recording medium
5
is converted to linearly polarized light whose polarization direction differs 90° from the polarization direction of the linearly polarized light emitted from the laser light source
13
by the phase difference plate
12
, and is guided to the photodetector
14
disposed in a direction different from that of the laser light source
13
.
Accordingly, with the optical pickup devices
1
A and
1
B of this type, there is an advantage in that the emergent light from the laser light source
13
can be applied effectively to the optical recording medium
5
, and the return light from the optical recording medium
5
can be guided to the photodetector
14
with high efficiency.
However, in the above-described optical pickup device, the effective quantity of light (represented as EQL in the figure) becomes 100% when the amount of birefringence &dgr; of the optical recording medium
5
is 0. In a case where the optical recording medium
5
itself has birefringence, a change occurs in the polarized state of the light due to this birefringence as well, so that there is the problem that the effective quantity of light drops below 100%.
For example, as shown in
FIG. 12C
, if the optical recording medium
5
itself has an amount of birefringence &dgr; corresponding to the quarter wavelength in the back-and-forth movement of the light, the light already becomes linearly polarized light when it is reflected by the optical recording medium
5
. Consequently, when the return light reflected by the optical recording medium
5
passes through the phase difference plate
12
again, the linearly polarized light becomes circularly polarized light, and the effective quantity of light drops to 50%. Further, as shown in
FIG. 12D
, if the optical recording medium
5
has an amount of birefringence &dgr; corresponding to the &lgr;/2 (hereinafter, wavelength is represented by &lgr;) in the back-and-forth movement of the light, the return light, when passing through the phase difference plate
12
, is converted at this point of time to the linearly polarized light whose polarization direction is the same as that of the linearly polarized light emitted from the laser light source
13
. Consequently, the return light and the emergent light from the laser light source
13
cannot be separated from each other, so that the effective quantity of light reaching the photodetector
14
becomes 0%.
The relationship between the amount of birefringence &dgr; of such an optical recording medium
5
and the detected quantity of light can be expressed as the relationship such as the one indicated by the dotted line L
0
in FIG.
5
. Namely, if the quantity of light detected by the photodetector
14
when the amount of birefringence &dgr; of the optical recording medium
5
is 0 is set as 1, the intensity of the signal detected by the photodetector
14
continues to drop when the amount of birefringence &dgr; of the optical recording medium
5
shifts from 0 to &lgr;/2. When the amount of birefringence &dgr; of the optical recording medium
5
reaches &lgr;/2, the signal intensity becomes 0.
Generally, the substrate of the optical recording medium
5
is fabricated by injection molding, and since the resin flows radially outward from the central side of the optical recording medium
5
, the optical recording medium
5
is likely to have birefringence whereby the refractive index differs between the radial direction and the circumferential direction. When the signal intensity was measured from the central side to the radially outward side of the optical recording medium
5
to confirm its actual state, there was an optical recording medium exhibiting the characteristic shown in
FIG. 13
as an example which exhibited extreme birefringence. In the characteristic shown in
FIG. 13
, the signal intensity was initially at an extremely low level on the central side of the disk, and exhibited a minimum value at a position offset slightly toward the radially outward side therefrom, and the signal intensity became gradually higher in a region extending from that position toward the radially outermost side. If consideration is given on the basis of this result, it can be seen that, in the disk having the characteristic shown in
FIG. 13
, there is a region P where the amount of birefringence &dgr; is &lgr;/2 on the slightly radially outward side from the center, and that the amount of birefringence d&dgr; becomes gradually smaller on the further radially outward side. Although this example is an extreme one, it can be estimated that disks which are manufactured by the same manufacturing method show a similar tendency, and it is conceivable that such disks generally have certain quantities of birefringence in the entire radial direction.
In contrast, it is conceivable to use an optical pickup device which is arranged such that the optical recording medium
5
itself and the phase difference plate
12
function as a single phase difference plate by orienting the direction of the anisotropic axis (hereinafter simply referred to as the azimuth) of the phase difference plate
12
in the direction of the birefringence of the optical recording medium
5
, and in which, instead of the quarter-wave phase difference plate, a phase difference plate having a phase difference with the amount of phase difference offset from the quarter wavelength by a portion cor

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