JP3949793B2 - Optical pickup - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup of an optical recording / reproducing apparatus that performs at least one of recording and reproducing information on recording media having different substrate thicknesses.
[0002]
[Prior art]
As an optical pickup for recording and reproducing information on two types of recording media (optical discs) having different substrate thicknesses, for example, an optical pickup disclosed in Japanese Patent Laid-Open No. 6-333255 is known.
[0003]
As shown in FIGS. 3 and 4, the optical pickup disclosed in Japanese Patent Laid-Open No. 6-333255 includes an objective lens 103a designed for an optical disk 131 having a thin substrate thickness and a thick substrate. The objective lens 103 b is designed corresponding to the optical disc 141, and these objective lenses 103 a and 103 b are fixed to the lens holder 104. The lens holder 104 is positioned between the magnets 101a and 101b disposed on the magnetic yoke 102 coupled at one end to the fixing member 108, and four metal suspensions 107a, 107b, 107c, and 107d (only 107a and 107b are shown). ) To be able to translate in the focus direction and the tracking direction.
[0004]
The focusing coil that drives the lens holder 104 in the focus direction and controls the position is wound directly around the lens holder 104, and the tracking coil that drives the lens holder 104 in the tracking direction and controls the position is fixed to the lens holder 104. The magnets 101a and 101b are fixed to the magnetic yoke 102 and constitute a magnetic circuit.
[0005]
On the optical bench 116, a beam separation mirror 112, a beam splitter 113, a semiconductor laser 114, and a photodetector 115 are arranged. The magnetic yoke 102 is fixed to the optical bench 116 by fixing means such as screws. The optical bench 116 is held by a carrier (not shown) and can move in the tracking direction, so that recording can be performed over the entire recording range of the optical disc 131 and the optical disc 141.
[0006]
FIG. 4A shows a condensing state of the objective lenses 103a and 103b and the optical disc 131. In FIG. 3, the light beam 117 emitted from the semiconductor laser 114 is separated into two light beams 117a and 117b by the light beam separation mirror 112, and simultaneously incident on both objective lenses 103a and 103b, and is converged on the recording film 131b. Irradiated.
[0007]
Since the objective lens 103a is designed to correspond to the optical disc 131, the convergent light forms an optimum light spot at the position of the recording film 131b, but the objective lens 103b has optical characteristics, that is, condensing light with the optical disc 131. Since the characteristics are inappropriate, the convergent light is not sufficiently converged. Therefore, it is the reflected light from the objective lens 103a that is received by the photodetector 115, and the focusing error signal and the tracking error signal are detected based on this reflected light, and as a result, recording and reproduction are performed by the objective lens 103a. Become.
[0008]
The reflected light from the objective lens 103b is scattered because it is not converged on the recording film 131b, and is not sufficiently incident on the photodetector 115 (or noise increases).
[0009]
In FIG. 4B, the objective lens 103b is designed in accordance with the optical characteristics of the optical disk 141. Therefore, the convergent light from the objective lens 103b forms an optimum 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 convergent light is not sufficiently converged.
[0010]
Therefore, in the case of the optical disk 141, recording is performed by the objective lens 103b.
4A and 4B, the beam separation mirror 112 is formed with two mirror surfaces 112a and a half mirror surface 112b for separating the light beam 117 into two light beams 117a and 117b. ing. The half mirror surface 112b does not reflect the light beam 117 100%, and is designed with a transmittance that obtains the power of the light beam 117a suitable for projecting onto the optical disc 131 through the objective lens 103a, and is reflected by the half mirror surface 112b. For the light beam 117b, the power suitable for projecting onto the optical disk 141 through the objective lens 103b is obtained.
[0011]
As a result, compatibility with optical disks 131 and 141 having different optical characteristics, particularly, different substrate thicknesses becomes possible.
[0012]
[Problems to be solved by the invention]
However, the optical pickup described above 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, respectively, and always irradiates the optical disk with two light beams 117a and 117b. However, by separating the light beams 117a and 117b by the half mirror surface 112, the light efficiency is reduced to about half. Therefore, in order to perform recording and reproduction, the semiconductor laser 114 has a light having about twice the normal power. The beam must be fired.
[0013]
Further, since the reflected light from the unselected objective lens is scattered, it is detected by the photodetector 115 as stray light. Since the stray light becomes noise, the focusing error signal and the tracking error signal may be affected.
[0014]
The present invention solves the above-described problems, and provides an optical pickup that can perform at least one of recording and reproduction with respect to two types of optical disks having different substrate thicknesses and that has almost no loss of light amount and has high light efficiency. For the purpose.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides light source means for emitting light and polarized light that selectively changes the polarization state of light emitted from the light source means by 90 ° using the TN effect of nematic liquid crystal. A switching means, a polarization beam splitter that switches the light to a first optical path and a second optical path according to the polarization direction of the light that has passed through the polarization switching means, and a first beam path disposed on the first optical path. A first objective lens designed according to a first recording medium having a substrate thickness, and a second objective lens disposed on the second optical path and having a second substrate thickness different from the first substrate thickness In the optical pickup having the second objective lens designed according to the recording medium, the ratio of the P-polarized component reflecting the beam splitter surface of the polarizing beam splitter is Rp, and the ratio of the S-polarized component is Rs. When <Rp <50 <Rs <100 and with constituting said beam splitting surface to be (%), the first optical path or the polarizing beam splitter between said polarization beam splitter first objective lens in which it characterized in that a half-wave plate to said second optical path between the second objective lens with.
Furthermore, in a preferred embodiment of the present invention, the half-wave plate is disposed so as to be joined to the polarizing beam splitter.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The optical pickup according to the first embodiment of the present invention will be described below 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.
[0017]
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 for emitting a light beam. Reference numeral 2 denotes a collimator lens that collimates the light beam emitted from the semiconductor laser 1. 3 is a half mirror for directing a part of parallel light toward the front monitor photodetector 4, and 5 is a nematic liquid crystal 5 c between the two transparent electrode plates 5 a and 5 b, and the molecular orientation of the nematic liquid crystal 5 c up and down the transparent electrode substrate It is a liquid crystal cell interposed so as to be twisted continuously by 90 °. Reference numeral 5d denotes a power supply for switching the molecular orientation of the nematic liquid crystal 5c by applying a voltage to the transparent electrode substrates 5a and 5b.
[0018]
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 two transparent electrode substrates 5a and 5b to a thickness of several μm to several hundreds μm, and the molecular orientation is set to two transparent electrode substrates. It is twisted 90 degrees continuously between 5a and 5b. The liquid crystal cell 5 selectively emits the polarization direction of the incident linearly polarized light by 90 ° by the applied voltage. That is, when the applied voltage is less than a certain threshold value, the nematic liquid crystal 5c maintains the molecular orientation continuously twisted by 90 ° between the two transparent electrode substrates 5a and 5b, and changes the polarization direction of the incident linearly polarized light. Rotate 90 ° for injection. On the other hand, when the applied voltage is equal to or higher than the threshold value, the nematic liquid crystal 5c has its molecular orientation inclined to the electric field direction and loses 90 ° of optical rotation, and is emitted without rotating the polarization direction of the incident linearly polarized light. Let
[0019]
A polarization beam splitter 6 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 = 20%), reflects the S-polarized component by 80% (Rs = 80%), transmits the P-polarized component by 80% (Tp = 80%), and the S-polarized component. 20% transmission (Ts = 20%). The total reflection surface 6b is provided with a coating for totally reflecting light.
[0020]
A half-wave plate 7 is bonded to the beam splitter 6 with an adhesive or the like so as to be positioned 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 to the optical disc through the objective lens 9 parallel to the tangential direction of the information track of the optical disc. Has a function of rotating the plane of polarization of linearly polarized light of the light beam incident thereon.
[0021]
Reference numeral 8 denotes a rising reflection mirror that reflects the light beam emitted from the polarization beam splitter 6 in the direction of 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 disc, the light beam is projected to the objective lens by raising the light beam in a direction perpendicular to the recording surface of the optical disc. Is incident.
[0022]
Reference numerals 9 and 10 are objective lenses. The objective lens 9 is designed for a magneto-optical disk (MO1) having a substrate thickness of 1.2 mm, and the objective lens 10 is for a magneto-optical disk (MO2) having a substrate thickness of 0.6 mm. Designed to. 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 drive coil such as a tracking coil for driving in a direction perpendicular to the tangential direction of the information track (tracking direction) is fixed.
[0023]
Reference numeral 11 denotes a half-wave plate, which is disposed in an optical path between the beam splitter 6 and a photodetector described later. You may join on the surface which emits the reflected light of the beam splitter 6. This half-wave plate has the effect of rotating the polarization plane of the reflected light from the incident optical disk by 45 °.
[0024]
Reference numeral 12 denotes a polarizing beam splitter, which has an action of reflecting S-polarized light and transmitting P-polarized light on the beam splitter surface 12a. Reference numerals 13 and 14 denote photodetectors. The photodetector 13 receives a P-polarized component, and the photodetector 14 receives an S-polarized component.
[0025]
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. Note that 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 that has passed through the half mirror 3 enters the liquid crystal cell 5 and its polarization direction is controlled in accordance with the optical disk on which information is to be reproduced.
[0026]
When the optical disk on which information is to be reproduced is a magneto-optical disk MO1 having a thick substrate, the power source 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 S-polarized light. . This light beam enters the polarization beam splitter 6, is reflected by the beam splitter surface 6 a, and enters the half-wave plate 7. The half-wave plate 7 is set in the direction of its main cross section so that the plane of polarization of the incident S-polarized linearly polarized light is rotated by 90 °, and the incident S-polarized light beam has a plane of polarization of 90. The light beam having the same polarization plane as that of P-polarized light is emitted. This light beam is reflected by the rising reflecting mirror 8 and applied to 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 (direction of the polarization plane) is parallel to the tangential direction of the information track.
[0027]
The light beam reflected from the magneto-optical disk MO1 has its plane of polarization rotated in the range of ± θK (Kerr rotation angle) depending on the direction of magnetization, followed the reverse path and transmitted through the beam splitter surface 6a. And the polarization plane is rotated by 45 °. The light beam is separated into a P-polarized light component and an S-polarized light component by the polarization beam splitter 12 and received by the photodetector 13 and the photodetector 14 respectively. Then, the information signal and each servo signal are detected from the outputs of the photodetectors 13 and 14.
[0028]
Next, when the optical disk on which information is to be reproduced is a magneto-optical disk MO2 having a thin substrate thickness, the power source 5d does not apply a voltage to the transparent electrode substrates 5a and 5b of the liquid crystal cell 5 (or less than the threshold value). Voltage applied). 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 polarization beam splitter 6, passes through the beam splitter surface 6 a, reflects off the total reflection surface 6 b, and exits the beam splitter 6. This light beam is reflected by the rising reflecting mirror 8 and applied to the objective lens 10 for the magneto-optical disk MO2. The light beam is focused 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 P-polarized linearly polarized light, and the direction of the linearly polarized light (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 P-polarized linearly polarized light and parallel to the tangential direction of the information track.
[0029]
The light beam reflected from the magneto-optical disk MO2 is rotated in the range of ± θK (Kerr rotation angle) depending on the direction of magnetization, follows the reverse path, and is reflected by the beam splitter surface 6a. And the polarization plane is rotated by 45 °. The light beam is separated into a P-polarized light component and an S-polarized light component by the polarization beam splitter 12 and received by the photodetector 13 and the photodetector 14 respectively. Then, the information signal and each servo signal are detected from the outputs of the photodetectors 13 and 14.
[0030]
According to the present embodiment, since the polarizing film is formed on the beam splitter surface 6a of the polarizing beam splitter 6 so that Rp <Rs, Tp> Ts, the apparent Kerr rotation angle θk is increased, which is favorable. Information reproduction at the CN ratio is possible. In general, if a phase difference occurs between P-polarized light and S-polarized light in a light beam that is transmitted or reflected by the beam splitter surface 6a of the polarizing beam splitter 6, it becomes elliptically polarized light instead of linearly polarized light, and the CN ratio is lowered. . However, it is difficult to design the beam splitter surface 6a so that the phase difference between transmission and reflection is 0 or an optimum value. On the other hand, in the present embodiment, the phase difference of the light beam transmitted through the beam splitter surface 6a is dealt with by optimally designing the polarizing film of the beam splitter surface 6a, and the level of the light beam reflected by the total reflection surface 6b. The phase difference can be dealt with by optimally designing the reflective film of the total reflection surface 6b, and each phase difference can be corrected to the optimum value.
[0031]
In addition, it is possible to ensure sufficient emission efficiency and irradiate each magneto-optical disk with a light beam, and to sufficiently ensure the CN ratio during information reproduction.
In this embodiment, the characteristics of the beam splitter surface 6a of the polarizing beam splitter 6 are set to Rp = 20% and Rs = 80%. However, the present invention is not limited to this, and Rp and Rs are set to 0 <Rp <respectively. By setting 50 <Rs <100 (%), the same effect can be obtained.
[0032]
Also, various changes can be made to the transmission and reflection in the polarizing beam splitter 6 and the positional relationship between the objective lenses 9 and 10.
[0033]
【The invention's effect】
As described above, according to the present embodiment, an optical pickup that can perform at least one of recording and reproduction with respect to two types of optical disks having different substrate thicknesses and that has almost no loss of light amount and has high light efficiency. Can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an entire optical system of an optical pickup according to the present embodiment.
FIG. 2 is a side view of the periphery of the objective lens in FIG.
FIG. 3 is a perspective view showing a configuration of a conventional optical pickup.
FIG. 4 is a side view of the periphery of an objective lens of a conventional optical pickup.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Collimating lens 3 Half mirror 4 Front monitor photodetector 5 Liquid crystal cell 6 Polarizing beam splitter 7 1/2 wavelength plate 8 Rising reflection mirror 9 Objective lens 10 Objective lens 11 1/2 wavelength plate 12 Polarizing beam splitter 13 Photo detector 14 Photo detector

Claims (2)

Light source means for emitting light, polarization switching means for selectively changing the polarization state of the light emitted from the light source means by 90 ° using the TN effect of nematic liquid crystal, and light passing through the polarization switching means A polarization beam splitter that switches the light to a first optical path and a second optical path according to the polarization direction, and a first recording medium that is disposed on the first optical path and has a first substrate thickness. A first objective lens designed and a second recording medium arranged on the second optical path and designed according to a second recording medium having a second substrate thickness different from the first substrate thickness An optical pickup having an objective lens,
The beam splitter surface is such that 0 <Rp <50 <Rs <100 (%), where Rp is the ratio of the P-polarized component reflecting the beam splitter surface of the polarizing beam splitter and Rs is the ratio of the S-polarized component. As well as
A half-wave plate on the first optical path between the polarizing beam splitter and the first objective lens or on the second optical path between the polarizing beam splitter and the second objective lens An optical pickup characterized by being arranged . The optical pickup according to claim 1, wherein the half-wave plate is disposed so as to be joined to the polarizing beam splitter.

Images