JP2001331962A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical head device wherein two kinds of light different in wavelength is used, a high light-condensing performance of two kinds of light is obtained in one optical system and electric noises are a little. SOLUTION: An aperture restricting element 101 is composed of a central part through which both two linear polarized light beams whose polarization planes are orthogonal each other pass as they are and a peripheral part wherein one light beam is diffracted, the other light beam transmits and a diffraction grating whose central axis is not a twice rotational symmetric axis is formed. The aperture restricting element is interposed between a collimator lens 4 and an objective lens 5 in the device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device for recording and reproducing information on an optical recording medium.

[0002]

2. Description of the Related Art In an optical head device for recording / reproducing information on an optical recording medium such as an optical disk such as a CD or a DVD or a magneto-optical disk, light emitted from a semiconductor laser as a light source is applied to the optical recording medium by a lens. The light is condensed and reflected by the optical recording medium to become return light. This return light is guided to a light receiving element, which is a photodetector, using a beam splitter, and information on the optical recording medium is converted into an electric signal.

[0003] CD / DVD compatible optical head devices have been commercialized for recording and reproducing information on CD and DVD optical disks, which are optical recording media of different standards, using the same optical head device. In an optical head device that uses a medium having a high wavelength dependency with respect to the reflection and absorption of light as a recording layer of an optical recording medium, and a semiconductor laser used for a CD has a 790 nm wavelength band in a CD-R or the like. Things. At this time, a DVD uses a semiconductor laser having a wavelength band of 660 nm.

[0004]

The CD and DVD optical disks have different standards, that is, the wavelength band used for recording and reproduction, the thickness of the substrate (optical disk), and the recording density. It is necessary to change the numerical aperture of the optical system from time to time. Therefore, there has been proposed a method of switching an objective lens for condensing outgoing light from a semiconductor laser on the information recording surface of a CD or DVD optical disk. However, since the number of components increases, the size and weight of the optical head device can be reduced. Have difficulty.

There has also been proposed a method in which a birefringent crystal is used as an aperture limiting element to form a polarization hologram lens having a curved diffraction grating pattern in an aperture limiting area to limit the aperture. However, it is difficult for the birefringent crystal to narrow the diffraction grating pitch, and the light diffracted by the polarization hologram lens diverges without being condensed on the photodetector. Had become.

[0006]

Means for Solving the Problems The present invention has been made to solve the above problems, the wavelength lambda ₁ of the linearly polarized light P ₁ and wavelength lambda ₂ of the linearly polarized light P ₂ (provided that λ ₁ ≠ λ ₂ ), An objective lens for condensing the linearly polarized light P ₁ and the linearly polarized light P ₂ on the optical recording medium, and an optical path between the light source and the objective lens, and the linearly polarized light P ₁ is transmitted. Comprises an aperture limiting element that limits transmission of linearly polarized light P ₂ ,
An optical head device for recording / reproducing information on / from an optical recording medium, wherein an aperture limiting element is integrated with a transparent substrate, and a peripheral portion surrounding a central portion of the aperture limiting element is formed of a laminated polymer liquid crystal. The central axis perpendicular to the surface of the aperture limiting element is not a rotationally symmetric axis twice with respect to the plane pattern of the diffraction grating. Linearly polarized light P whose wavefronts are orthogonal to each other
₁ and linearly polarized light P ₂ are both transmitted in the central portion,
Wherein in the peripheral portion to provide an optical head device, wherein a linearly polarized light P ₂ linearly polarized light P ₁ is diffracted is transmitted without being diffracted.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIG.
The optical head device according to the first embodiment of the present invention is an optical head device for recording / reproducing information on an optical disk 6 of DVD and CD, and uses a linearly polarized light P ₁ having a wavelength λ ₁ for DVD as a light source. Two types of light sources are provided: a semiconductor laser 1A that emits light and a semiconductor laser 1B that emits linearly polarized light P ₂ (where λ ₁ ≠ λ ₂ ) of a wavelength λ ₂ for CD.

The linearly polarized light beams P ₁ and P ₂ emitted from the semiconductor lasers 1A and 1B are reflected by the beam splitters 2 and 3, respectively, pass through the collimator lens 4, pass through the aperture limiting element 101, and pass through the objective lens 5 Is focused on the optical disk 6. The light reflected by the optical disk 6 is finally collected on the photodetector 8.

As shown in FIG. 2A, the aperture limiting element 101 forms a diffraction grating pattern having an uneven cross section on a birefringent material 12 laminated on a transparent substrate 11, and then removes the unevenness of the diffraction grating. With the adhesive 13 filled so as to fill,
It is configured to be fixed to the transparent substrate 14.

As the transparent substrates 11 and 14, an optically isotropic medium such as glass or quartz glass can be used. As the birefringent material 12, a polymer liquid crystal is used. The alignment film such as polyimide applied to the surface of the transparent substrate 11 is subjected to an alignment treatment such as rubbing to form an alignment film. A solution of a liquid crystal monomer is applied on this alignment film, and the optical axes of the liquid crystal molecules are aligned in the direction of the alignment treatment. In this state, a birefringent material 12 is laminated on the transparent substrate 11 by irradiating a photopolymerization curing agent previously contained in the liquid crystal monomer solution with a light source for photopolymerization to polymerize the liquid crystal monomer. (Form).

[0011] The thickness d of the birefringent material 12, the difference between the ordinary refractive index n _o and extraordinary refractive index n _e of the birefringent material 12 n _e -
It is determined from n _o (= Δn) and the wavelength λ ₂ subject to aperture restriction. By setting d = λ ₂ / 2Δn, zero-order diffracted light that passes straight through is not generated.

As shown in FIG. 2B, the birefringent material 12 laminated on the transparent substrate 11 is subjected to photolithography and etching techniques to form a central region R of the aperture limiting element as shown in FIG. to form a ₁ and a region R ₂ of the peripheral portion of the aperture limiting element. Region R ₁ of the central portion of the aperture limiting element, two different together transmits linearly polarized light of wavelength region R ₂ of the peripheral portion of the aperture limiting element is a diffraction grating that limits the transmission of linearly polarized light according to the wavelength is there.

A central axis perpendicular to the surface of the aperture limiting element 101 and passing through the center of the element is defined as a rotation axis. Then, with respect to the plane pattern of the diffraction grating, when the central axis perpendicular to the surface of the aperture limiting element is not a rotationally symmetric axis twice, that is, when rotated 180 ° around the central axis, the plane pattern becomes the original Do not overlap with the plane pattern. At this time, the linearly polarized light P ₂ having the wavelength λ ₂ diffracted on the forward path travels along an optical path different from the zero-order light even after being diffracted on the return path, and does not reach the photodetector.

The plane pattern of the diffraction grating is not limited to two divisions as shown in FIG.
Division or other division may be used, as long as the central axis of the aperture limiting element 101 is not twice a rotationally symmetric axis with respect to the plane pattern of the diffraction grating.

If the diffracted light generated by the aperture limiting element enters the photodetector as stray light, accurate signal detection cannot be performed. Therefore, the diffraction grating pattern and the diffraction grating pitch are determined so that the diffracted light does not enter. Although it depends on the arrangement of the optical system and the photodetector, it is preferable to set the diffraction grating pitch to 40 μm or less because a sufficient diffraction angle at which the diffracted light does not enter the photodetector as stray light can be obtained.

The adhesive 13 is preferably an acrylic, epoxy, or polyester UV-curable or thermosetting adhesive with good workability, but is not limited thereto. The refractive index n _s of the adhesive 13 is substantially equal to the ordinary refractive index n _o of the birefringent material 12.

The aperture limiting element 10 constructed as described above
1, the wavelength λ at which the birefringent material 12 becomes ordinary light
_{For one} linearly polarized light P ₁ , the birefringent material 12 and the adhesive 1
Diffraction grating consisting of 3 showed no diffraction effect for n _o = n _s, not subject to aperture limitation. On the other hand, the linearly polarized light P _{2 having the} wavelength λ ₂ that becomes an extraordinary light with respect to the birefringent material 12 exhibits a diffraction effect, does not generate the 0th-order diffracted light that transmits straight, and is restricted by the aperture.

FIG. 3A shows a case where the objective lens 5 has a large numerical aperture and the linearly polarized light P ₁ having a wavelength λ ₁ for DVD transmits through the aperture limiting element 101, and FIG. 3 shows a state in which linearly polarized light P ₂ having a wavelength λ ₂ for CD is transmitted through the aperture limiting element 101 when the numerical aperture of the objective lens 5 is limited to a small value. The same reference numerals in FIG. 3 as those in FIG. 3 denote the same elements.

When the aperture limiting element 101 is mounted on an optical head device, as shown in FIGS. 3 (a) and 3 (b), in order to limit the aperture according to the wavelength, a wavelength λ incident on the aperture limiting element 101 is used. ₁ of linear polarization P ₁ and wavelength lambda ₂ of the linearly polarized light P ₂
It is preferable to dispose the semiconductor lasers 1A and 1B such that the polarization planes are orthogonal to each other (see FIG. 1).

That is, in the optical head device according to the first embodiment of the present invention, the above-described aperture limiting element 101 is disposed between the collimator lens 4 and the objective lens 5 so that the structure shown in FIG.
As shown in FIG. 3A and FIG. 3B, since the numerical aperture of the objective lens is effectively limited according to the wavelength of the linearly polarized light and the linearly polarized light is condensed on the optical disk 6, even optical disks having different thicknesses can be used. One optical system can achieve high light-collecting ability.

[Second Embodiment] An optical head device according to a second embodiment of the present invention shown in FIG. 4 employs a light source for recording and reproducing information on a DVD and a CD optical disk 6 in the same manner as the first embodiment. a head device, as a light source, a semiconductor laser 1A and CD that emits linearly polarized light P ₁ of the wavelength lambda ₁ for DVD
The semiconductor laser 1B for emitting a linearly polarized light P ₂ having a wavelength lambda ₂ of use
Are provided.

However, in the present embodiment, the aperture limiting element 102 is provided between the collimating lens 4 and the objective lens 5, and the semiconductor lasers 1A and 1B have outgoing linearly polarized planes of polarization. Immediately before the light enters the aperture limiting element 102, they are arranged so as to be parallel to each other. The same reference numerals in FIG. 4 as those in FIG. 4 denote the same elements.

As shown in FIG. 5A, the aperture limiting element 102 forms a diffraction grating pattern having an uneven cross section on the birefringent material 12 laminated on the transparent substrate 11 and fills the unevenness of the diffraction grating. Substrate 14 in which a birefringent organic thin film 15 is adhered with an adhesive 16 with the adhesive 13 filled as described above
It is fixed and configured.

The birefringent organic thin film 15 is a birefringent film whose optical axes are aligned in the stretching direction by stretching an organic material such as polycarbonate. When m ₁ and m ₂ are natural numbers, a phase difference of (m ₁ −１ ／) · λ ₁ occurs when light of wavelength λ ₁ passes through the birefringent organic thin film 15, while wavelength λ 1 _When light ₂ passes through the birefringent organic thin film 15, m ₂
A birefringent organic thin film 15 such that a phase difference of λ ₂ is generated;
Has been adjusted.

The adhesive 16 is preferably an acrylic, epoxy, or polyester UV-curable or thermosetting adhesive, like the adhesive 13, and the adhesive 16 has a refractive index of It is preferable to select a material that is substantially equal to the refractive organic thin film 15 because the occurrence of transmitted wavefront aberration due to uneven thickness of the birefringent organic thin film can be prevented. That is, the transparent substrate 14 integrated with the birefringent organic thin film 15
It includes a linear polarization P ₁ and linear polarization P ₂ of the wavelength lambda ₂ wavelength lambda ₁ is, when incident on the respective polarization parallel, have a phase difference generating function of emitting by orthogonal polarization.

The birefringent material 12 is laminated and processed on the transparent substrate 11 in the same manner as in the first embodiment.
As shown in the region R ₂ of the peripheral portion of the region R ₁ and the opening limiting element in the central portion of the aperture limiting element is formed. The region R _{1 at} the center of the aperture limiting element transmits both linearly polarized lights of two different wavelengths, and the region R _{1 at} the periphery of the aperture limiting element.
Reference numeral ₂ denotes a diffraction grating that limits transmission of linearly polarized light according to the wavelength. With respect to the plane pattern of the diffraction grating, the plane pattern of the diffraction grating is formed such that the central axis of the aperture limiting element 102 does not coincide with two rotationally symmetric axes for the same reason as in the first embodiment.

The aperture limiting element 10 constructed as described above
An optical head apparatus of the second embodiment of the present invention which were installed 2, polarization is parallel to the wavelength lambda ₁ together lambda ₂ (provided that lambda ₁ ≠
When the respective linearly polarized lights P ₁ and P _{2 of} λ ₂ ) enter the aperture limiting element 102, the numerical aperture of the objective lens is effectively limited according to the wavelength of the light and converge on the optical disc 6. Even with optical disks having different thicknesses, a single optical system can realize a high light-collecting ability.

[0028]

EXAMPLE 1 This example is a specific example of an aperture limiting element (FIG. 2) mounted on the optical head device (FIG. 1) of the first embodiment. Polyimide is applied in the form of a film on a transparent substrate 11 having a refractive index of 1.5, and a rubbing alignment treatment is performed to form an alignment film (not shown). Then, the liquid crystal monomer applied on the alignment film is photopolymerized and cured. A polymer liquid crystal was formed, and this was used as a birefringent material 12. Refractive index of the polymer liquid crystal, an extraordinary refractive index after photopolymerization curing n _e of about 1.6, the ordinary refractive index n _o of about 1.5
It is.

The polymer liquid crystal formed on the transparent substrate 11 is subjected to photolithography and etching techniques as shown in FIG.
As shown in (b), the region R _{1 at} the center of the aperture limiting element having a numerical aperture of 0.5 and the depth of the unevenness of the diffraction grating are 3.95.
[mu] m, the diffraction grating pitch to form a region R ₂ of the peripheral portion of the aperture limiting element consisting of 15μm linear grating.

In the plane pattern of the diffraction grating, as shown in FIG. 2B, the grating directions of the two types of linear gratings form an angle of 90 °. Therefore, with respect to the plane pattern of the diffraction grating, the central axis that is perpendicular to the plane of the aperture limiting element and passes through the center of the element is not an axis of rotational symmetry twice.

[0031] Next, the refractive index n _s is by using an adhesive 13 of substantially equal acrylic UV-curable and ordinary refractive index n _o of the liquid crystal polymer, as shown in FIG. 2 (a), liquid crystal polymer An aperture limiting element 101 was fabricated by fixing the substrate to a transparent substrate 14 having a refractive index of 1.5 so as to fill the processed unevenness.

The aperture limiting element 101 manufactured as described above
Shows a transmittance of 96% or more in all regions (R ₁ and R ₂ ) when linearly polarized ordinary light having a wavelength of 660 nm is incident on
When linearly polarized extraordinary light having a wavelength of 790 nm is incident, the central region (R ₁ ) having a numerical aperture of 0.5 exhibits a transmittance of 96% or more in the central region (R ₁ ) and the diffraction grating in the peripheral region (R ₂ ). The generated straight transmitted light (zero-order light) was 3% or less. That is, a wavelength-selective aperture limiting element that limits aperture only for linearly polarized light having a wavelength of 790 nm was obtained.

Example 2 This example is a specific example of the aperture limiting element (FIG. 5) mounted on the optical head device (FIG. 4) of the second embodiment. Similarly to Example 1, as the birefringent material 12, after forming on the transparent substrate 11 of the liquid crystal polymer, 5 (b), the diffraction grating and the region R ₁ of the center portion of the aperture limiting element thereby forming a region R ₂ of the peripheral portion of the aperture limiting element having a pattern.

Next, as shown in FIG. 5A, a transparent substrate 14 having a refractive index of 1.5 is placed on a transparent substrate 14 for a DVD optical disk.
A polycarbonate birefringent organic thin film 15 having a retardation value of 1650 nm with respect to linearly polarized light in a wavelength band of 60 nm was fixed with an acrylic adhesive 16 to have a phase difference generating function. Then, the birefringent organic thin film 15
If, during the birefringent material 12, the refractive index n _s is by using an adhesive 13 of substantially equal acrylic UV-curable and ordinary refractive index n _o of the liquid crystal polymer, so as to fill the unevenness of the diffraction grating pattern The aperture limiting element 102 was manufactured by fixing.

The aperture limiting element 102 manufactured as described above
Is a linearly polarized light having a wavelength of 660 nm and a wavelength of 790
Even when the polarization planes of the linearly polarized light of nm are parallel, the polarization planes of the two linearly polarized lights can be made orthogonal by transmitting the birefringent organic thin film 15, and exhibit the same wavelength-selective aperture limiting characteristic as in Example 1. Was.

Example 3 This example is a specific example of the optical head device (FIG. 4) of the second embodiment on which the aperture limiting element 102 manufactured in Example 2 is mounted. An optical head apparatus of this embodiment, an optical head apparatus for recording and reproducing information of an optical disc 6 of DVD and CD, as a light source, a semiconductor laser 1A that emits linearly polarized light P ₁ of the wavelength lambda ₁ for DVD The semiconductor laser 1B has two types of light sources that emit linearly polarized light P ₂ having a wavelength λ ₂ for CD, and the planes of polarization of the linearly polarized lights P ₁ and P ₂ that emit the semiconductor lasers 1A and 1B are:
Two semiconductor lasers were arranged so as to be parallel to each other immediately before entering the aperture limiting element 102. The aperture limiting element 102 manufactured in Example 2 was provided between the collimating lens 4 and the objective lens 5.

The linearly polarized lights P ₁ and P ₂ emitted from the semiconductor lasers 1A and 1B are reflected by the beam splitters 2 and 3, respectively, and transmitted through the collimator lens 4. Thereafter, the linearly polarized lights P ₁ and P ₂ are subjected to aperture limitation according to the wavelength by the aperture limiting element 102, and are condensed by the objective lens 5 on the information recording surfaces of DVD optical discs or CD optical discs having different thicknesses. Was. That is, the optical head device of the present example is applicable to optical disks having different thicknesses.
One optical system showed high light-gathering ability.

Further, since the central axis is not a rotationally symmetric axis twice with respect to the plane pattern of the diffraction grating, the linearly polarized light P ₂ having the wavelength λ ₂ diffracted on the outward path is also diffracted on the return path. The light travels on an optical path different from the zero-order light and does not reach the photodetector. For this reason, the diffracted light does not overlap with the electric signal as noise, and the noise is reduced. Further, since the diffraction grating pitch of the aperture limiting element 102 was as narrow as 15 μm, highly accurate signal detection could be performed without diffracted light entering the photodetector.

[0039]

According to the optical head device of the present invention which uses linearly polarized lights having two different wavelengths, an aperture limiting element utilizing polarization characteristics is arranged between a collimator lens and an objective lens, so that the light can be transmitted. It is possible to effectively limit the numerical aperture of the objective lens according to the wavelength and realize a high light-collecting ability with a single optical system for optical disks having different thicknesses.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a configuration of an optical head device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an aperture limiting element mounted on the optical head device according to the first embodiment of the present invention, (a) a sectional view, and (b) a plan view.

3A and 3B are diagrams illustrating a state in which light transmitted through an aperture limiting element and an objective lens is focused on an optical disk, FIG. 3A is a side view illustrating a case where the numerical aperture of the objective lens is large for DVD, and FIG.
FIG. 3 is a side view showing a case where the numerical aperture of the objective lens is small for a CD.

FIG. 4 is a conceptual diagram showing a configuration of an optical head device according to a second embodiment of the present invention.

FIG. 5 is a diagram showing an aperture limiting element mounted on an optical head device according to a second embodiment of the present invention, (a) a sectional view, and (b) a plan view.

[Explanation of symbols]

101, 102: Aperture limiting element 1A, 1B: Semiconductor laser 2, 3: Beam splitter 4: Collimating lens 5: Objective lens 6: Optical disk 8: Photodetector 11, 14: Transparent substrate 12: Birefringent material 13: Adhesion Agent 15: Birefringent organic thin film R ₁ : Central area of aperture limiting element R ₂ : Area of peripheral area of aperture limiting element

Continued on front page F-term (reference) 2H049 AA03 AA33 AA37 AA43 AA45 AA48 AA50 AA57 AA65 5D119 AA41 BA01 BB01 BB04 EC45 EC47 FA08 JA31 JA58

Claims (2)

[Claims] And 1. A light source for emitting a wavelength lambda ₁ of the linearly polarized light P ₁ and wavelength lambda ₂ of the linearly polarized light P ₂ (provided that λ ₁ ≠ λ _2), the linearly polarized light P ₁ and the linear polarization P ₂ to the optical recording medium An optical recording medium comprising: an objective lens for condensing light; and an aperture limiting element installed in an optical path between the light source and the objective lens and transmitting linearly polarized light P ₁ but restricting transmission of linearly polarized light P _2. An optical head device for recording / reproducing information, wherein an aperture limiting element is integrated with a transparent substrate, and a diffraction grating made of laminated polymer liquid crystal is provided around a central portion of the aperture limiting element. With respect to the plane pattern of the diffraction grating, the central axis perpendicular to the surface of the aperture limiting element is not a rotationally symmetric axis twice, and the polarization planes incident on the aperture limiting element are orthogonal to each other. Linearly polarized light P ₁ and linearly polarized light P ₂ Both the transmitted optical head device, wherein the linearly polarized light P ₂ linearly polarized light P ₁ is diffracted is transmitted without being diffracted in the peripheral portion. Wherein said transparent substrate has a phase difference generating function, by transmitting the linearly polarized light P ₁ and the linear polarization P ₂ having parallel polarization each other polarization of the linearly polarized light P ₁ and the linear polarization P ₂ The optical head device according to claim 1, wherein the wavefronts are orthogonal.