Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-08-03
2002-07-02
Edun, Muhammad (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112010, C369S044120, C369S112070
Reexamination Certificate
active
06414930
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head of an optical recording/reproducing apparatus, particularly to a thin optical head having preferable optical characteristics.
2. Description of the Related Art
An optical head is used as an important component for reading a signal from an optical recording medium such as an optical disk or optical card memory, that is, a compact disk (CD) or DVD. It is preferable that an optical head is provided with not only a signal detecting function but also a control mechanism such as a focus servo or tracking servo in order to fetch a signal from an optical recording medium.
FIG. 19
shows a conventional typical optical head. As shown in
FIG. 19
, a laser beam
2
emitted from a semiconductor laser
1
serving as a light source is changed to parallel rays by a collimator lens
3
, passes through a focus/track error signal detecting optical element
8
constituted of a holographic optical element, then the optical axis of the light is bent up to 90° and the light enters an objective lens
4
. The laser beam
2
condensed on an optical disk
11
by the objective lens
4
is reflected and goes back through the original optical path, changed to parallel rays by the objective lens
4
, and reflected by a mirror
20
to enter the focus/track error signal detecting optical element
8
. The laser beam
2
entering the focus/track error signal detecting optical element
8
is divided in the element
8
and condensed on a photodetector. Thereby, a reproduced signal and focus and track error signals serving as servo signals are read.
As shown in
FIG. 19
, the height of the optical head is shown by the sum of a WD (working distance), thickness of the objective lens
4
, space from the bottom face of the objective lens
4
up to the top of the mirror
20
, and height of the mirror
20
.
To decrease the height of an optical head, the minimum value of the sum of the WD, lens thickness, and space is almost determined by the type of the optical disk
11
. In the case of a DVD, even if respectively setting the WD, lens thickness, and space to a minimum value of 1.1 mm, it is necessary to set the height of the mirror
20
to a value larger than a beam diameter, that is, it is necessary to set the height to at least 3 mm. Therefore, in this case, the height of the optical head is estimated as at least 6.3 mm and thus, it is difficult to further decrease the height of an optical head.
SUMMARY OF THE INVENTION
The present invention is made to solve the above problems of the prior art and its object is to provide an optical head which can be decreased in thickness and which has preferable optical characteristics.
One aspect of the present invention is an optical head comprising:
a diffractive optical element set in the optical path between a light source and an information recording medium; and
refractive optical means which is set in the optical path and into which the optical axis of the light emitted from the light source diagonally comes; wherein
diffraction angle change of the diffracted light emitted from the diffractive optical element due to the wavelength fluctuation of the emitted light and refraction angle change of the refracted light emitted from the refractive optical means occur in the direction in which the diffraction angle change and refraction angle change are canceled.
For example, when using a semiconductor laser beammaking it possible to constitute a thin optical head and serving as the light emitted from a light source, it is possible to obtain a preferable condensed spot on an optical disk surface even if central wavelengths of the emitted light are changed due to spread of a wavelength band of approx. 2 nm due to a high-frequency superimposition module or change of environmental temperatures.
Another aspect of the present invention is an optical head, wherein the diffractive optical element is integrated with the refractive optical means.
Thereby, for example, the structure is stabilized and alignment is easily made.
Still another aspect of the present invention is an optical head, wherein collimator means is included which changes the light emitted from the light source substantially to parallel rays and makes the parallel rays enter the diffractive optical element.
Thereby, for example, the diffraction efficiency and diffraction-angle change value of the light coming into a diffractive optical element become equal over the entire surface.
Yet another aspect of the present invention is an optical head, wherein a focus/track error signal detecting optical element is set between the light source and the refractive optical means and the diffractive optical element is integrated with the focus/track error signal detecting optical element.
Thereby, for example, the structure is stabilized and alignment is easily made.
Still yet another aspect of the present invention is an optical head, wherein the diffractive optical element is a uniform-period grating.
Thereby, for example, a diffractive optical element is easily aligned or fabricated.
A further aspect of the present invention is an optical head, wherein
the diffractive optical element is set in a convergence-light optical path or divergence-light optical path, and
the period of the diffractive optical element differs depending on places in accordance with the convergent degree or divergent degree of the light coming into the diffractive optical element.
“A still further aspect” of the present invention is an optical head, wherein the period is adjusted so as to be further lengthened substantially from the central portion toward the circumferential portion of the diffractive optical element.
Thereby, for example, it is possible to accurately uniform the diffraction angle change value of the diffracted light emitted from a diffractive optical element over the entire surface.
A yet further aspect of the present invention is_an optical head, wherein the diffractive optical element is set in a convergent-light optical path or divergent-light optical path having a numerical aperture of 0.39 or less and the period of the diffractive optical element is uniform.
Thereby, for example, a diffractive optical element is easily aligned or fabricate.
A still further aspect of the present invention is an optical head, wherein the diffractive optical element is set in an optical path nearby the light source.
Thereby, for example, it is possible to decrease the area and the cost of a diffractive optical element.
One aspect of the present invention is an optical head, wherein the refractive optical means is an optical element having three planes for receiving or reflecting light and the diffractive optical element is formed on at least one of the three planes of the refractive optical means.
Thereby, for example, the structure is stabilized and alignment is easily made.
Another aspect of the present invention is an optical head, wherein the diffractive optical element is the reflective type and is formed on the reflective plane of the refractive optical means.
Thereby, for example, the diffraction efficiency of a diffractive optical element is improved.
Still another aspect of the present invention is an optical head, wherein the refractive optical means is a prism made of low-dispersion transparent material having three planes for receiving or reflecting light.
Yet another aspect of the present invention is an optical head, wherein the transparent material has an Abbe number of 50 or more.
Thereby, for example, because the period of a diffractive optical element is lengthened, the element is easily fabricated, a high diffraction efficiency is obtained, and it is possible to cancel influences of wavelength fluctuation in a wide wavelength region.
Still yet another aspect of the present invention is an optical head, wherein
the refractive optical means is a prism made of transparent material having a refractive index n, and
one of the base angles of the prism is substantially a right angle and the other &thgr; of the base angles has an angle &thgr; sub
Matsuzaki Keiichi
Saimi Tetsuo
Shiono Teruhiro
Edun Muhammad
Ratner & Prestia
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