Optical pickup

Details Optical pickup Optical pickup

2001-03-26

2002-12-17

Edun, Muhammad (Department: 2653)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S112240, C369S112140

Reexamination Certificate

active

06496453

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup for optically recording and reproducing information onto and from optical disks.
2. Description of the Prior Art
The structure of a conventional optical pickup will be described with reference to FIG.
20
. The optical pickup shown in
FIG. 20
has two laser light sources of different wavelengths, that is, a semiconductor laser
101
of a wavelength of 660 nm that performs recording and reproduction onto and from high-density media such as digital versatile disks (DVDs), and a semiconductor laser
115
of a wavelength of 780 nm that performs reproduction from media such as compact disks (CDs).
To an actuator movable member
111
of the optical pickup, two kinds of objective lenses
107
and
118
are attached, and switching between the two kinds of objective lenses is made by the actuator movable member
111
rotating about a rotation shaft
120
. The two kinds of objective lenses
107
and
118
are switched between when recording or reproduction onto or from a disk
108
with a base material thickness of 0.6 mm such as a DVD is performed and when reproduction from a disk
109
with a base material thickness of 1.2 mm such as a CD is performed by the different light sources.
First, an optical system that performs recording and reproduction onto and from media such as DVDs will be described. A light beam emitted from the semiconductor laser
101
of high power is converted into a parallel beam by a collimator lens
102
, and is incident on a mirror
103
. On the mirror
103
, a wavelength selective film is formed where light of a wavelength of 660 nm is mostly transmitted and partly reflected and light of a wavelength of 780 nm is substantially totally reflected. Therefore, an extremely small part of the light beam incident on the mirror
103
is reflected and most of the light beam is transmitted. The reflected part of the light beam is directed to a photodetector
110
, where the light quantity of the light beam is detected. By doing this, the emission power of the semiconductor laser
101
is monitored to satisfy the function of keeping constant the power on the disk surface in recording and reproduction.
The light transmitted by the mirror
103
is reflected at a reflecting mirror
106
, and is transmitted by a polarizing hologram
104
provided on the actuator movable member
111
. The polarizing hologram is formed by forming a grating with a depth d on a substrate of an anisotropic material such as lithiumniobate and filling an isotropic material (refractive index n
1
) in the grooves of the grating. Generally, when the phase difference between light passing through the grooves and light passing between the grooves is &phgr;, the transmittance is represented by cos
2
(&PHgr;/2). When the refractive indices of the substrate for polarized light parallel to the grating grooves and polarized light vertical to the grating grooves are n
1
and n
2
, respectively, for the polarized light parallel to the grating grooves, since &phgr;=0, the transmittance is 1. For the polarized light vertical to the grating grooves, since &phgr;=2&pgr;(n
1
−n
2
) d/&lgr;, by setting the depth d so that &phgr;=&pgr;, the transmittance is 0 and the polarized light is completely diffracted.
Therefore, by considering the polarization direction of the light beam emitted from the semiconductor laser
101
and the bearing of the grooves of the polarizing hologram
104
, the light beam from the light source can be transmitted without diffracted when passing through the polarizing hologram
104
. The transmitted light is converted from linearly polarized light to circularly polarized light by a quarter wave plate
105
, is aperture-limited by an objective lens attachment hole of the actuator movable member
111
, is incident on the objective lens
107
, and is condensed on the signal surface of the disk
108
with a material thickness of 0.6 mm.
The light beam reflected at the disk
108
passes through the objective lens
107
, and is transmitted by the wave plate
105
. Since the light beam is converted into linearly polarized light orthogonal to the direction in which the light beam is polarized on the way to the disk
108
at this time, most of the light beam is diffractivey branched by the polarizing hologram
104
. These diffracted light beams are reflected at the reflecting mirror
106
, are transmitted by the mirror
103
, pass through the collimator lens
102
, and are condensedly incident on a photodetector
117
integrated with the laser
101
. By use of variations in the quantity of this light, a servo signal and an RF signal such as a focus signal or a tracking signal can be obtained.
Next, an optical system that performs reproduction from media such as CDs by use of the other semiconductor laser
115
will be described. A light beam emitted from the semiconductor laser
115
is diffractively branched to ±first order light and to zero order light by passing through a glass hologram
114
not depending on polarization. These light beams are condensed by a collimator lens
113
, are reflected at a mirror
112
, the mirror
103
and the reflecting mirror
106
, are incident on the objective lens
118
provided on the actuator movable member
111
, and are condensed as three spots on the signal surface of the disk
109
with a base material thickness of 1.2 mm. The main beam spot is used for an RF signal, and the two sub beam spots, for three beam tracking.
The light beam reflected at the disk
109
passes through the objective lens
108
, the reflecting mirror
106
, the mirror
103
, the mirror
112
and the collimator lens
113
, is further diffractively branched by the glass hologram
114
, and is condensedly incident on a photodetector
119
integrated with the semiconductor laser
115
. By use of variations in the quantity of this light, a servo signal and an RF signal can be obtained.
In optical pickups that perform recording and reproduction of highly dense signal pits such as those on DVDs, it is required to form very small and high-quality light condensation spots on the optical disk surface. Generally, the size of a light condensation spot depends on the numerical aperture (NA) of the objective lens, the light wavelength &lgr;, and the light intensity at the aperture pupil end of the objective lens, that is, the rim intensity. The light source wavelength &lgr; and the NA of the objective lens are defined by a specification or the like, for example, for DVDs; &lgr; is approximately 650 nm, and NA is 0.6. However, to ensure the spot quality commensurate with the numerical aperture, it is necessary to secure a sufficient rim intensity.
Generally, a light beam emitted from a semiconductor laser has an elliptical far field pattern. For this reason, in the optical pickup structure shown as a conventional example, when the rim intensity in the direction of minor axis of the far field pattern is ensured, the rim intensity is considerably high in the direction of the major axis, that is, the amount of eclipse, due to the aperture limitation, of the light beam incident on the objective lens is considerably large in the direction of the major axis.
In an optical pickup for recording, high optical power is required on the disk surface; a power of approximately 12 to 17 mW is necessary as the objective lens exit light quantity. Therefore, it is necessary to cause the optical pickup to operate within the emission power rating of the laser light source by maximizing the transmission efficiency of the optical system. However, in the conventional structure, since the amount of eclipse in the direction of major axis of the emission far field pattern is large, the light quantity loss is large, so that to ensure the power on the disk surface, the margin for the maximum rating is small even though a high-power laser is used.
FIG. 21
is a view showing the structure of another conventional optical pickup for preventing such a light quantity loss. Since the structure of

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