Optical pickup

Details Optical pickup Optical pickup

1997-10-20

2001-01-30

Edun, Muhammad (Department: 2753)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S110040, C369S044370, C369S044230

Reexamination Certificate

active

06181666

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup for recording and reproducing information on and from a recording medium such as an optical card and an optical disc.
The present invention also relates to an optical pickup for reproducing information recorded on the recording medium, such as the optical card and the optical disc or the like, especially to a multi-track read type optical pick up for reproducing plural tracks, simultaneously.
2. Related Art Statement
As a conventional optical pickup, there is an optical pickup in which in order to perform tracking control by, for example, 3 beam method, or to reproduce plural tracks simultaneously, a light beam from a semiconductor laser is collimated by a collimator lens to form a parallel luminous flux, this flux is divided into a plurality of light beams by means of a diffraction grating, these light beams are irradiated with a predetermined positional relation through an inward and return way separating optical element and an objective lens, and these return light are lead to a detecting optical system by separating the return light from the inward way through the objective lens.
In such an optical pickup, in order to increase utility efficiency of the light beam from the semiconductor laser, it is advantageous to utilize a polarizing beam splitter as an inward and return way separating optical element. To this end, for example, in the inward way, it is necessary to dispose the semiconductor laser in such a manner that a polarizing plane of emitting light thereof is made a predetermined direction for the polarizing beam splitter.
While, even though the semiconductor laser is disposed in such a way, for example, in the case of shaping a light beam of an ellipsoidal section emitted from the semiconductor laser into a circular section by a beam shaping prism, the semiconductor laser can not be disposed in such a way. As a countermeasure in this case, it is conceived that the semiconductor laser is disposed so as to shape the emitted light in the beam shape having circular section by the beam shaping prism and the polarizing direction thereof can be rotated by 90° by disposing a ½ wave plate between the semiconductor laser and the polarizing beam splitter.
In this case, however, as shown in
FIG. 24
, provided that a ½ wave plate
152
is disposed behind a diffraction grating
151
with an azimuth of 45°, an incident light beam Bin of linear polarization (inner direction in paper) from the semiconductor laser is incident on the ½ wave plate
152
by separating the light beam into, for example 0 ordered light B
0
and ±1 ordered diffraction lights B+1, B−1 by the diffraction grating
151
. Even though the 0 ordered light B
0
is converted into a linear polarization having 90° rotated polarization plane, the ±1 ordered diffraction lights B+1, B−1 are not linearly polarized under the influence of incident angle dependency of the ½ wave plate
152
and are converted into right-handed and left-handed ellipsoidal polarization. Therefore, in the latter polarization beam splitter (not shown), there is a problem that an optical property for the ii ordered diffraction lights B+1, B−1 is decreased and thus primary problem can not be attained. Such a problem becomes remarkable in case of obtaining more high ordered diffraction light, in the diffraction grating
151
.
In the case that the recording semiconductor laser and the reproducing semiconductor laser are used, light beams from these semiconductor lasers are irradiated on the polarization beam splitter, light beam from one semiconductor laser is transmitted or reflected through or on the polarization beam splitter, and the light beam from the other semiconductor laser is reflected or transmitted to c ose them in the same optical path, and these light beams are irradiated on the recording medium through common objective lens, the above problem arises in the same manner even in case of separating light beam from at least one of semiconductor lasers into plural sub-beams and of irradiating them on the polarization beam splitter.
When the recording semiconductor laser and the reproducing semiconductor laser are used, the light beams from these semiconductor lasers are composed substantially in the same optical path by the polarization beam splitter, and are irradiated on the recording medium through the common objective lens, the disposition of both semiconductor lasers can be considered so as to make polarizing planes of respective emitted lights orthogonal.
In case of disposing both semiconductor lasers in such a way, when light beams of ellipsoidal sectional shape emitted from respective semiconductor lasers are not subjected to a beam shaping to obtain light beam of circular sectional shape, the direction of respective ellipsoidal spots formed on the recording medium have major axis direction orthogonal to the tracks in recording time, and have major axis direction parallel to the tracks in reproducing time, and thus become orthogonal to each other, so that the strength distribution of respective spots on the recording medium are not all the same. Therefore, resolution and resolving power are not all the same at recording time and reproducing time and thus recording and reproducing with high precision can not be performed.
As a conventional optical pickup, there is provided an optical pickup in which in order to follow, for example, a recording beam or a reproducing beam to a predetermined track of a recording medium precisely, to record information while verifying, and to reproduce the information recorded on plural tracks of the recording medium, a plurality of light beams are irradiated on the recording medium in the form of spot shape and in a predetermined positional relation, the light reflected on the recording medium is received on corresponding light receiving element, thereby detecting required signals.
However, in the above conventional optical pickup, in some recording mediums, there is a problem that the return lights of the light beam irradiated at an adjacent position of the recording medium surface are also incident on other light receiving elements, so that these return lights are subjected to interference phenomena with each other, thereby causing large amounts of cross-talk, and thus the required signals can not be detected with the requisite precision.
For example, as in an optical card
161
shown in
FIG. 25
, the recording medium formed by laminating an adhesive layer
163
, a recording layer
164
and a transparent layer
165
on a substrate
162
of resin, successively, has 400 &mgr;m thickness of the adhesive layer
163
, a few tenths to a few hundredths nm thickness of the recording layer
164
and 400 &mgr;m thickness of the transparent layer. When plural light beams are condensed on the recording layer
164
of such an optical card
161
through the transparent layer
165
in a predetermined positional relation so as not to with respective reflected lights, if the condensed points of adjacent light beams on the recording layer
164
are closed, a cross-talk for which the reflected light of one light beam reflected on a boundary plane between the adhesive layer
163
and the substrate
162
through the recording layer
164
is entered into the return light of adjacent other light beams, thereby affecting unfavorable influence on the detection of the required signal. Moreover, in the case shown in
FIG. 25
, adjacent light beams irradiated on the optical card
161
are reflected somewhat on the surface of the transparent layer
165
, so that the return lights thereof are interfered with each other on the corresponding light receiving element. However, the surface of the transparent layer
165
is sufficiently separated from the recording layer
164
, so that the return lights on the surface are sufficiently diffused, and thus there is substantially no problem.
As the conventional optical pickup, there are two types of pickups; on

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