Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-06-24
2003-06-24
Young, W. R. (Department: 2655)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S112030, C369S112280
Reexamination Certificate
active
06584060
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical pick-up device having a diffraction component for recording/reading information on an optical recording medium such as an optical disk, an optical card, or an optical tape.
2. Description of Related Art
In the field of an optical pick-up device for recording/reading information on an optical recording medium such as a Compact Disk (CD), demands for simplification of structure, assembly, or adjustment, and demands for reduction of costs have been increasing in recent years.
FIG. 21
illustrates a background optical pick-up device which employs a holographic diffraction component. The optical pick-up device includes a semiconductor laser light source
511
for emitting laser light having a wavelength of 780 nm, a photodetector unit
531
, the holographic diffraction component
553
, a mirror
565
, a reflective surface
566
, a diffraction grating
541
for generating 3-division light beams, and an objective lens
521
. The laser light source
511
, the photodetector unit
531
, and the holographic diffraction component
553
are integrated in a body tube, and are optically adjusted to form a block.
A light beam emitted from the laser light source
511
is transmitted through the diffraction grating
541
for generating 3-division light beams and the holographic diffraction component
553
, is reflected by the mirror
565
for bending the optical path, is transmitted by the objective lens
521
, converges on a recording pit-surface of an optical disk
101
, and is reflected by the recording pit-surface.
The returning light beam thus reflected by the recording pit-surface is transmitted through the objective lens
521
, is reflected by the mirror
565
, and is incident onto the holographic diffraction component
553
, where two kinds of +1st order diffracted returning light are generated by a holographic surface
554
having two different holographic patterns with different pitches. These two kinds of beams are incident on the reflective surface
566
at an angle equal to or more than a critical angle, are thereby reflected by the total internal reflection, are transmitted through the transmitting surface after the total internal reflection, and arrive in the photodetector unit
531
. Thereby, information signals, focusing-error signals, and tracking-error signals are detected.
According to the background optical pick-up device as shown in
FIG. 21
, the laser light source or the photodetector is capable of being substituted to another one having a different specification. Therefore, modification by substitution of the component to another type of high-speed photodetector or by adoption of a new type of semiconductor laser light source can be easily achieved. Further, initial investments are not expensive for manufacturing products according to this embodiment.
Another background optical pick-up device is disclosed in the Japanese Laid-Open Patent Publication No. 3-225636, which employs a birefringent diffraction component having a birefringent crystal for achieving high efficiency of light-utilization.
FIG. 22
illustrates the background optical pick-up device which includes a semiconductor laser light source
511
as a light source, a birefringent diffraction grating
541
for separating a light beam
502
emitted from the semiconductor laser light source
511
into three beams, a collimator lens
523
and an objective lens
521
, a birefringent holographic diffraction component
555
, a quarter-wave plate
625
, a 6-division photodetector unit
532
for detecting the diffracted returning light beam out of the optical axis, and a photodetector
533
.
The collimator lens
523
and an objective lens
521
are used for an imaging optical system. The light beam from the diffraction grating
541
is collimated into parallel light beam and converges on an optical disk
101
through the imaging optical system. The birefringent holographic diffraction component
555
diffracts and separates the returning light reflected by the optical disk
101
out of the optical axis of the imaging system. The quarter-wave plate
625
is disposed between the optical disk
101
and the diffraction grating
541
or between the optical disk
101
and the birefringent holographic diffraction component
555
.
In this optical pick-up device, the birefringent holographic diffraction component is used for simplifying the structure. Further, a uniaxial structure of this optical pick-up device achieves miniaturization thereof and reduction in weight. In addition, reproduction of signals of the optical disk at high efficiency is achieved, when the birefringent holographic diffraction component and the birefringent diffraction grating are employed.
As described above, by combining the semiconductor laser light source, the photodetector, and the holographic diffraction component, an optical pick-up device is provided having features accompanied by a miniaturized structure, a reduced weight, and a simplified method for adjustment. However, in the background optical pick-up device for recording/reading, there still remain problems as follows.
(1) High efficiency in utilizing light is desired for recording information on an optical recording medium. In an optical pick-up device for reading information on an optical recording medium such as a Compact Disc (CD), which is normally used for reproduction only, there are few problems regarding efficiency in utilizing light. In this case, the focal length of the collimator lens may be long.
In contrast, in the optical pick-up device for recording a re-writable or write-once optical recording medium such as a Compact Disc ReWritable (CD-RW), a Compact Disc Recordable (CD-R), a Digital Video Disc Recordable (DVD-R), or a Digital Video Disc ReWritable (DVD-RW), a collimator lens having a a large numerical aperture and a short focal length is frequently used for collecting light from the semiconductor laser light source with little loss, in order to secure a high power of light on the surface of the optical disk.
However, when such a collimator lens having a short focal length is used, a space for disposing a mirror for reflecting the diffracted returning light, etc., becomes narrow. Therefore, the mirror is required to be miniaturized. In this case, mass-productivity is deteriorated due to difficulties in assembling such a miniaturized component accurately.
(2) A large separation angle is required. As described above, in achieving an optical pick-up device capable of recording, the collimator lens having a large numerical aperture and a short focal length, for example, a numerical aperture of 0.3 and a focal length of 10 mm, is frequently used. In this case, for securing a package interval between a laser diode package and a photodetector package within such a short distance, a large diffraction angle of the diffraction grating should preferably be employed, from this point of view.
For achieving the large diffraction angle of the diffraction grating, a diffraction grating having a short pitch should be employed. In this case, however, due to restrictions in fabricating a grating having such a short pitch, it is difficult to form an ideal grating structure having the short pitch.
As a result, a separation property for the polarized light or diffraction efficiency is generally deteriorated, and a S/N ratio of signals is reduced. With this context, there have been limitations in employing a diffraction grating having a reduced pitch, or large diffraction angle, in the background optical pick-up device.
(3) Divergent transmitted light is imposed on aberration due to anisotropy of substrate crystal. When the birefringent crystal substrate such as a thin lithium niobate substrate is disposed in a divergent optical path, because the refractive index depends on a propagation direction of the light, the transmitted light is imposed on aberration.
Therefore, when the birefringent crystal is disposed between the laser diode light source and the collimator lens, it is preferred that the abe
Akiyama Hiroshi
Nakayama Masahiko
Oohchida Shigeru
Dickstein , Shapiro, Morin & Oshinsky, LLP
Ricoh & Company, Ltd.
Young W. R.
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