JPH11120606A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pickup which is capable of executing recording and reproducing to and from two kinds of optical disks varying in substrate thicknesses and has good light efficiency. SOLUTION: The beam splitter surface 6a of the optical pickup consisting of a semiconductor laser 1, a liquid crystal cell 5, a polarization beam splitter 6, an objective lens 9 for the optical disk of the large substrate thickness and an objective lens 10 for the optical disk of the small substrate thickness is constituted so as to attain 0<Rp<50<Rs<100(%) when the ratio of the P polarized light component reflecting the beam splitter surface 6a of the polarization beam splitter 6 is defined as Rp and the ratio of the S polarized light component as Rs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical pickup of an optical recording / reproducing apparatus which performs at least one of recording and reproduction of information on recording media having different substrate thicknesses.

[0002]

2. Description of the Related Art As an optical pickup for recording and reproducing information on and from two types of recording media (optical disks) having different substrate thicknesses, for example, an optical pickup disclosed in Japanese Patent Application Laid-Open No. Hei 6-333255 is known.

As shown in FIGS. 3 and 4, an optical pickup disclosed in Japanese Patent Application Laid-Open No. 6-333255 includes an objective lens 103a designed for an optical disk 131 having a small substrate thickness, An objective lens 103b designed for a thick optical disk 141; and these objective lenses 103a and 103b are fixed to a lens holder 104. Lens holder 104
Is a magnetic yoke 10 connected at one end to a fixing member 108.
2 and magnets 101a and 101b arranged on
Between the four metal suspensions 107a, 10
7b, 107c and 107d (only 107a and 107b are shown) are supported so as to be able to translate in the focus direction and the tracking direction.

A focusing coil for driving and controlling the position of the lens holder 104 in the focus direction is directly wound around the lens holder 104, and a tracking coil for driving and controlling the position of the lens holder 104 in the tracking direction is fixed to the lens holder 104. . Magnet 101
a and 101b are limited to the magnetic yoke 102, and constitute a magnetic circuit.

The optical table 116 has a beam separating mirror 11
2. A beam splitter 113, a semiconductor laser 114, and a photodetector 115 are provided. Magnetic yoke 1
Numeral 02 is fixed to the optical table 116 with fixing means such as screws. The optical table 116 is held by a carrier (not shown),
The optical disk 131 and the optical disk 141 can be moved in the tracking direction and can be recorded in the entire recording range.

In FIG. 4A, the objective lenses 103a, 103
3B shows the light condensing state of 3b and the optical disk 131. In FIG. 3, a light beam 117 emitted from a semiconductor laser 114 is split into two light beams 1 by a light beam separating mirror 112.
17a and 117b, separated into two objective lenses 103a,
Simultaneously incident on the recording film 131b, the recording film 131b is irradiated as convergent light.

The objective lens 103a is designed to correspond to the optical disk 131, so that the convergent light forms an optimum light spot at the position of the recording film 131b. That is, the converged light is not sufficiently converged because the light-collecting characteristics are inappropriate. Therefore, the light received by the photodetector 115 is the reflected light from the objective lens 103a, and the focusing error signal and the tracking error signal are detected based on the reflected light, and as a result, the information is recorded and reproduced by the objective lens 103a. Become.

[0008] The reflected light from the objective lens 103b is not converged on the recording film 131b and is scattered, so that the light is not sufficiently incident on the photodetector 115 (or noise is increased).

In FIG. 4B, the objective lens 1
03b is designed according to the optical characteristics of the optical disc 141, so that the convergent light from the objective lens 103b forms an optimal light spot at the position of the recording film 141b.
Since the objective lens 103a is inappropriate for the optical characteristics of the optical disk 141, the converged light is not sufficiently converged.

Accordingly, in the case of the optical disk 141, recording is performed by the objective lens 103b. Further, in FIGS. 4A and 4B, the light beam 117 is applied to the beam separation mirror 112 by two light beams 117.
a and two mirror surfaces 112a for separation into 117b
And a half mirror surface 112b. The half mirror surface 112b does not reflect 100% of the light beam 117,
The light beam 117b is designed with a transmittance to obtain the power of the light beam 117a suitable for projecting onto the optical disc 131 via the objective lens 103a, and the light beam 117b reflected on the half mirror surface 112b is transmitted to the optical disc 141 via the objective lens 103b. The power suitable for projection is obtained.

As a result, compatibility can be achieved with optical disks 131 and 141 having different optical characteristics, particularly, different substrate thicknesses.

[0012]

However, the above-described optical pickup has the following problems. The half mirror surface 112b is used to separate the light beam emitted from the semiconductor laser 114 into the objective lenses 103a and 103b, and the two light beams 117 are always used.
a and 117b are irradiated on the optical disk. However, two light beams 117 are generated by the half mirror surface 112.
a and 117b, the light efficiency is reduced by about half.
14 must emit a light beam having approximately twice the power of a normal light beam.

The reflected light from the unselected objective lens is scattered, so that the photodetector 115 detects it as stray light. Since the stray light becomes noise, the focusing error signal and the tracking error signal may be affected.

The present invention solves the above-mentioned problems, and can perform at least one of recording and reproduction with respect to two types of optical discs having different substrate thicknesses, and has almost no loss of light quantity and has good optical efficiency. The purpose is to provide.

[0015]

In order to achieve the above object, the present invention provides a light source means for emitting light, and a polarization state of the light emitted from the light source means, using a TN effect of a nematic liquid crystal. Polarization switching means for selectively changing the light by 90 °, a polarization beam splitter for switching the light to a first optical path and a second optical path according to the polarization direction of the light passing through the polarization switching means, A first objective lens arranged on an optical path and designed according to a first recording medium having a first substrate thickness, and arranged on the second optical path and different from the first substrate thickness A second objective lens designed in accordance with a second recording medium having a second substrate thickness, wherein a ratio of a P-polarized light component reflecting the beam splitter surface of the polarizing beam splitter is Rp, S polarized light When the minute percentage of the Rs, 0
The beam splitter surface was configured such that <Rp <50 <Rs <100 (%).

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical pickup according to a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a schematic view showing the entire optical system, and FIG. 2 is a side view around the objective lens.

First, the configuration of the optical pickup will be described with reference to FIGS. Reference numeral 1 denotes a semiconductor laser as a light source that emits a light beam. Reference numeral 2 denotes a collimator lens that converts a light beam emitted from the semiconductor laser 1 into parallel light. 3
Is a half mirror for directing a part of parallel light toward the front monitor photodetector 4, and 5 is two transparent electrode plates 5a and 5b.
This is a liquid crystal cell in which the nematic liquid crystal 5c is interposed so that the molecular orientation is continuously twisted up and down by 90 ° above and below the transparent electrode substrate. 5d is the transparent electrode substrate 5a, 5
This is a power supply for applying a voltage to the switch b to switch the molecular orientation of the nematic liquid crystal 5c.

The liquid crystal cell 5 will be described in more detail. In the liquid crystal cell 5, a nematic liquid crystal 5c having a positive dielectric anisotropy is sandwiched between the two transparent electrode substrates 5a and 5b to a thickness of several μm to several hundred μm, and the molecular orientation of the liquid crystal cell is adjusted to the two transparent electrode substrates. 90 ° is continuously twisted between 5a and 5b. The liquid crystal cell 5 selectively rotates the polarization direction of the incident linearly polarized light by 90 ° by the applied voltage and emits the light.
That is, when the applied voltage is less than a certain threshold value, the nematic liquid crystal 5c maintains the molecular orientation twisted continuously by 90 ° between the two transparent electrode substrates 5a and 5b, and changes the polarization direction of the incident linearly polarized light. Rotate 90 ° to eject. On the other hand, when the applied voltage is equal to or higher than the threshold value, the nematic liquid crystal 5c loses its 90 ° optical rotation due to its molecular orientation tilting in the direction of the electric field, and emits without rotating the polarization direction of the incident linearly polarized light. Let it.

Reference numeral 6 denotes a polarizing beam splitter, which has a beam splitter surface 6a and a total reflection surface 6b. The beam splitter surface 6a reflects the P-polarized component by 20% (Rp = 2
0%), 80% reflection of the S-polarized component (Rs = 80%), P
It has the property of transmitting 80% of the polarized light component (Tp = 80%) and transmitting 20% of the S polarized light component (Ts = 20%).
The total reflection surface 6b is provided with a coating for totally reflecting light.

Reference numeral 7 denotes a half-wave plate, which is bonded to the beam splitter 6 with an adhesive or the like so as to be located in an optical path between the beam splitter surface 6a and an objective lens 9 described later. The half-wave plate 7 is used to make the direction of linear polarization of the light beam irradiated on the optical disk via the objective lens 9 parallel to the tangential direction of the information track of the optical disk. Has the function of rotating the plane of polarization of linearly polarized light of the light beam incident on the.

Reference numeral 8 denotes a rising reflection mirror that reflects the light beam emitted from the polarization beam splitter 6 toward the objective lens. Since the light beam emitted from the polarization beam splitter 6 travels in a plane parallel to the recording surface of the optical disk, the light beam is set up in a direction perpendicular to the recording surface of the optical disk to cause the light beam to reach the objective lens. Is incident.

Reference numerals 9 and 10 denote objective lenses. The objective lens 9 is designed for a magneto-optical disk (MO1) having a substrate thickness of 1.2 mm. The objective lens 10 has a substrate thickness of 0.6 mm.
For the magneto-optical disk (MO2). Although not shown, the objective lenses 9 and 10 are held by a holder, and a focusing coil and an optical disc for driving the objective lenses 9 and 10 in a direction (focusing direction) perpendicular to the recording surface of the optical disc. A driving coil such as a tracking coil for driving in a direction (tracking direction) perpendicular to the tangential direction of the information track is fixed.

Reference numeral 11 denotes a half-wave plate, which is arranged in an optical path between the beam splitter 6 and a photodetector described later. The beam splitter 6 may be joined on a surface from which reflected light is emitted. The half-wave plate has a function of rotating the polarization plane of the reflected light from the optical disk by 45 °.

Reference numeral 12 denotes a polarizing beam splitter, which has a function of reflecting an S-polarized component and transmitting a P-polarized component on the beam splitter surface 12a of the reflected light from the optical disk.
Reference numerals 13 and 14 denote photodetectors. The photodetector 13 receives an S-polarized component, and the photodetector 14 receives a P-polarized component.

Next, the operation will be described. The divergent light beam emitted from the semiconductor laser 1 is converted into a parallel light beam by the collimator lens 2. In addition, the light beam emitted from the semiconductor laser 1 will be described as S-polarized linearly polarized light. The parallel light beam enters the half mirror 3, where a part of the light beam is reflected and received by the front monitor photodetector 4. The emission power of the semiconductor laser 1 is controlled according to the amount of light received by the front monitor photodetector 4. The light beam transmitted through the half mirror 3 enters the liquid crystal cell 5, and its polarization direction is controlled according to the optical disk from which information is to be reproduced.

If the optical disk from which information is to be reproduced is a magneto-optical disk MO1 having a thick substrate, the power supply 5d
Applies a voltage equal to or higher than the threshold value to the transparent electrode substrates 5a and 5b of the liquid crystal cell 5, and emits the light as S-polarized light. This light beam enters the polarization beam splitter 6, is reflected by the beam splitter surface 6a, and enters the half-wave plate 7. 1 /
The two-wavelength plate 7 sets the plane of polarization of the incident linearly polarized S-polarized light to 9
The direction of the main cross section is set so as to be rotated by 0 °, and the incident S-polarized light beam is rotated by 90 ° in the polarization plane and emitted as a light beam having the same polarization plane as P-polarized light. Is done. This light beam is reflected by the rising reflection mirror 8 and is irradiated on the objective lens 9. The light beam is focused and irradiated on the magneto-optical disk MO1 by the objective lens 9, and a light spot is formed on the recording surface of the magneto-optical disk MO1. This light spot is P-polarized linearly polarized light, and the direction of the linearly polarized light (the direction of the polarization plane) is parallel to the tangential direction of the information track.

The light beam reflected from the magneto-optical disk MO1 has its polarization plane rotated within the range of ± θK (Kerr rotation angle) depending on the direction of magnetization, follows the reverse path, and transmits through the beam splitter plane 6a, and The light enters the wave plate 11 and its polarization plane is rotated by 45 °. The light is separated into a P-polarized component and an S-polarized component by the polarization beam splitter 12 and received by the photodetector 13 and the photodetector 14, respectively. Then, information signals and respective servo signals are detected from the outputs of the photodetectors 13 and 14.

Next, when the optical disk from which information is to be reproduced is a magneto-optical disk MO2 having a small substrate thickness, the power supply 5d does not apply a voltage to the transparent electrode substrates 5a and 5b of the liquid crystal cell 5 (or a threshold voltage). Voltage below the value).
Therefore, the S-polarized linearly-polarized light beam incident on the liquid crystal cell 5 is converted into a P-polarized linearly-polarized light beam and emitted. This light beam enters the polarizing beam splitter 6, passes through the beam splitter surface 6a, and is totally reflected by the total reflection surface 6b.
Is reflected and exits the beam splitter 6. This light beam is reflected by the rising reflection mirror 8 and irradiates the objective lens 10 for the magneto-optical disk MO2. The light beam is condensed and irradiated on the magneto-optical disk MO2 by the objective lens 10, and a light spot is formed on the recording surface of the magneto-optical disk MO2. This light spot is linearly polarized light of P polarization, and the direction of the linearly polarized light (the direction of the polarization plane) is parallel to the tangential direction of the information track. That is,
The light beams incident on the magneto-optical disk MO1 and the magneto-optical disk MO2 are linearly polarized light of P polarization, and are parallel to the tangential direction of the information track.

The light beam reflected from the magneto-optical disk MO2 has its polarization plane rotated within a range of ± θK (Kerr rotation angle) depending on the direction of magnetization, follows an opposite path, and is reflected by the beam splitter plane 6a. The light enters the wave plate 11 and its polarization plane is rotated by 45 °. The light is separated into a P-polarized component and an S-polarized component by the polarization beam splitter 12 and received by the photodetector 13 and the photodetector 14, respectively. Then, information signals and respective servo signals are detected from the outputs of the photodetectors 13 and 14.

According to the present embodiment, Rp <Rs, Tp> on the beam splitter surface 6a of the polarization beam splitter 6.
Since the polarizing film is formed so as to be Ts, the apparent Kerr rotation angle θk increases, and information can be reproduced with a good CN ratio. Generally, the polarization beam splitter 6
When a phase difference occurs between the P-polarized light and the S-polarized light in the light beam transmitted or reflected by the beam splitter surface 6a, the light beam becomes not the linearly polarized light but the elliptically polarized light, and the CN ratio decreases.
However, it is difficult to design the beam splitter surface 6a such that the phase difference in transmission and reflection is zero or an optimum value. On the other hand, in the present embodiment, the beam splitter surface 6
The phase difference of the light beam that passes through a is dealt with by optimally designing the polarizing film on the beam splitter surface 6a, and the phase difference of the light beam reflected on the total reflection surface 6b is adjusted by the reflection film on the total reflection surface 6b. It is possible to cope with the problem by designing it optimally, and to correct each phase difference to an optimal value.

Furthermore, it is possible to irradiate each magneto-optical disk with a light beam while securing sufficient emission efficiency, and it is also possible to ensure a sufficient CN ratio during information reproduction. In the present embodiment, the polarization beam splitter 6
Rp = 20% and Rs
= 80%, but without being limited to this, R
p and Rs are respectively 0 <Rp <50 <Rs <100
The same effect can be obtained by setting (%).

Various changes can also be made to the transmission and reflection of the polarization beam splitter 6 and the positional relationship between the objective lenses 9 and 10.

[0033]

As described above, according to the present embodiment, at least one of recording and reproduction can be performed on two types of optical discs having different substrate thicknesses, and loss of light amount is almost eliminated, and light efficiency is improved. A good optical pickup can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating an entire optical system of an optical pickup according to an embodiment.

FIG. 2 is a side view around an objective lens in FIG. 1;

FIG. 3 is a perspective view showing a configuration of a conventional optical pickup.

FIG. 4 is a side view showing the periphery of an objective lens of a conventional optical pickup.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Collimator lens 3 Half mirror 4 Front monitor photodetector 5 Liquid crystal cell 6 Polarization beam splitter 7 1/2 wavelength plate 8 Start-up reflection mirror 9 Objective lens 10 Objective lens 11 1/2 wavelength plate 12 Polarization beam splitter 13 Photodetector 14 Photodetector

Claims (1)

[Claims] 1. A light source means for emitting light, a polarization switching means for selectively changing a polarization state of light emitted from the light source means by 90 ° using a TN effect of a nematic liquid crystal, and the polarization switching means. A polarizing beam splitter that switches the light between a first optical path and a second optical path according to the polarization direction of the light that has passed through the means; and a first beam splitter disposed on the first optical path and having a first substrate thickness. A first objective lens designed according to a recording medium; and a second objective medium arranged on the second optical path and having a second substrate thickness different from the first substrate thickness. And a ratio of a P-polarized light component reflecting the beam splitter surface of the polarizing beam splitter to Rp, and a ratio of an S-polarized light component to Rs.
Wherein the beam splitter surface is configured to satisfy 0 <Rp <50 <Rs <100 (%).