Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-04-20
2004-01-06
Korzuch, William (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112180
Reexamination Certificate
active
06674709
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head apparatus that performs recording or reproduction of optical information recording media.
2. Description of the Prior Art
Generally, a rewritable type optical disc must monitor the quantity of incident light to the recording surface of the disc to secure the signal recording quality with high accuracy. For this reason, the accuracy of a system that monitors the light quantity using light output from the posterior end face of a laser chip used in a reproduction-only optical head is not high, and therefore it is necessary to monitor the light quantity using light radiated from the anterior end face of the laser chip (hereinafter referred to as “anterior light”).
On the other hand, while optical discs are attracting attention as large-capacity information memories, optical head apparatuses need to attend a demand for high-speed recording or reproduction of optical discs. To meet this demand, it is necessary to increase the speed of modulation of a semiconductor laser light source and at the same time improve responsivity of the above described monitoring of the anterior light.
A conventional optical pick up will be explained with reference to the attached drawings below.
FIG. 14
shows an example of an outlined configuration of a conventional optical head, apparatus. A diverging beam
802
radiated from a semiconductor laser light source
801
passes through a parallel flat plate
803
placed diagonally to the optical axis and is converted to a parallel beam
805
by a collimate lens
804
.
This collimated beam
805
is partially reflected by a polarized beam splitter
806
and enters into a photodetector
809
. A beam
810
, the major portion of the collimated beam
805
, passes through the polarized beam splitter
806
and is converted to a circularly polarized beam by a ¼ wavelength plate
811
, and then condensed into an optical disc
814
through an objective lens
813
mounted on an actuator
812
.
The beam reflected by the optical disc
814
passes through the objective lens
813
and is converted by the ¼ wavelength plate
811
to a linearly polarized beam, which is orthogonal to the polarization plane of the outgoing radiation beam of the semiconductor laser light source
801
and entered into the polarized beam splitter
806
.
Since the polarization plane of the incident beam entered into the polarized beam splitter
806
is orthogonal to the first half of the optical path, the incident beam is reflected by the polarized beam splitter
806
, diffracted by a hologram element
815
, branched into a positive 1st-order diffracted light
817
and negative 1st-order diffracted light
818
with the optical axis of the incident light as an axis of symmetry, then condensed by a detection lens
817
, entered into signal detectors
820
and
821
, respectively, to detect control signals such as focusing and tacking, and RF signals.
On the other hand, photodetector
809
that detects light reflected by the polarized beam splitter
806
acts as an output light quantity monitor of the semiconductor laser light source
801
.
Here, the reason why the parallel plate
803
is placed diagonally to the optical axis of the incident beam between the semiconductor laser light source
801
and collimate lens
804
will be explained. Generally, as for a semiconductor laser used for a light source of the optical head apparatus, from the standpoint of an optical characteristic, mode west of an oscillated beam of a semiconductor laser element
901
differs between the semiconductor composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane) as shown in FIG.
15
.
That is, while the mode west is a point that matches a specular surface
902
within the perpendicular (Y-Z axial plane), it is a point inside an activated layer
903
of the semiconductor laser element
901
, that is, a point at a certain depth from the specular surface
902
into the resonator within the composition plane (X-Z axial plane).
Therefore, the converging point of the oscillated beam differs between the composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane), and thus an “astigmatic difference”
904
in optical terms is produced.
When an astigmatic difference occurs, the beam spot is distorted into a flat, vertically or horizontally oblong spot. Therefore, the beam spot spans mutually neighboring recording tracks of an optical disc, causing a problem of deteriorating a signal characteristic.
It is for this reason that in
FIG. 14
, the parallel plate
803
is placed inclined at a predetermined angle in the reverse direction in order to correct the astigmatism of the light beam radiated from the semiconductor laser
801
.
Moreover, another method proposed to correct such astigmatism of a light beam is canceling out the astigmatism of the light spot by inserting a cylindrical lens in the same optical path of the laser beam.
BRIEF SUMMARY OF THE INVENTION
Object of the Invention
The above described conventional optical head apparatus has the following problems:
Generally, when recording a signal on a rewritable type optical disc, it is necessary to secure sufficient optical power on the disc, and therefore the light utilization efficiency of the optical head must be secured.
However, the configuration of the above described conventional example performs no beam shaping, and therefore abandons a portion of light in the outer regions for reasons related to the design of the objective lens, which means a loss of light quantity.
Furthermore, a part of the beam within the effective aperture is reflected and used by the photodetector
809
to monitor the light quantity, which increases the loss all the more. To avoid this, lowering the light quantity to be conducted to the light quantity monitor and increasing the light quantity within the effective aperture will deteriorate the S/N ratio of the monitor signal.
Moreover, increasing the speed of laser modulation requires the responsivity of the anterior light monitor itself to be improved. For this reason, it is preferable to reduce the photoreception area of the photodetector and input a condensed beam in order to improve the response frequency characteristic of optical detection.
However, exposing the photodetector to an excessively condensed beam will increase the light intensity per unit area of the detector surface, increasing the carrier density on the photoreception surface of the detector, which then becomes saturated causing the traveling speed of carriers to slow down. That is, condensing the beam on the detector excessively may cause a problem of deteriorating the response frequency characteristic of optical detection.
Furthermore, all the above described methods to correct the astigmatism of a light beam produced by an astigmatic difference among the semiconductor laser elements above must provide special parts such as a transparent parallel plate and cylindrical lens separately, causing an additional problem of unavoidably increasing the number of parts, hence cost increase.
In addition, since the photodetector for an RF signal, focusing or tracking control signals is provided apart from the photodetector for laser light quantity monitoring, which increases the number of parts and complicates the optical system, making it difficult to reduce the size of the optical head.
The present invention has been implemented taking into account these problems of the conventional optical head apparatus and it is an object of the present invention to provide an optical head apparatus with high light utilization efficiency.
It is another object of the present invention to provide a compact optical head apparatus.
It is still another object of the present invention to provide an optical head apparatus with an excellent response frequency characteristic of optical detection.
SUMMARY OF THE INVENTION
Therefore one aspect of the present invention is an optical head apparatus, comprising:
a semiconductor laser light source;
a
Asada Jun-ichi
Matsumiya Hiroaki
Momoo Kazuo
Nagashima Kenji
Nishiwaki Seiji
Korzuch William
RatnerPrestia
Vuong Bach
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