JP2001318231A - Polarizing phase correction element and optical head device - Google Patents

Abstract

(57) [Problem] To obtain a polarization phase corrector capable of precisely controlling a phase difference with respect to two linearly polarized lights having different polarization directions and wavelengths, and to realize a stable operation of an optical head device using the same. I do. To A linearly polarized light of wavelength lambda ₁ and wavelength lambda ₂ having a polarization direction perpendicular to the wavelength lambda ₁ of light is light of a transmission wavelength lambda ₂ as to allow correct wavefront aberration is deformed transmitted wavefront The birefringent material layer 12 is processed so that the thickness thereof has rotational symmetry with respect to the optical axis, and the processed concave portion is filled with a filler 13 having a refractive index equal to the ordinary light refractive index of the birefringent material layer 12 to be polarized. This element is used as an optical phase correction element 1 and is mounted on an optical head device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization phase correcting element and an optical head device.

[0002]

2. Description of the Related Art A semiconductor laser having a wavelength of 780 nm and an NA (aperture) for recording / reproducing information on / from a CD-based optical recording medium (hereinafter, an optical recording medium is referred to as an optical disk) including a CD-R for optical writing. An objective lens whose number is 0.45 to 0.5 and an optical disk having a thickness of 1.2 mm are used. On the other hand, a semiconductor laser having a wavelength of 650 nm and an NA of 0.
A 6 objective lens and an optical disk with a thickness of 0.6 mm are used.

In order to record and reproduce information on both CD and DVD optical discs with a compact optical head device, a CD
It is effective to use a total of two semiconductor lasers with different emission wavelengths for DVD and DVD, combine the emitted lights from the semiconductor lasers with a wavelength combining prism, and configure an optical head device using the same objective lens. It is.

However, as described above, since the thickness and the wavelength used for both CD and DVD optical discs are different, when an objective lens optimally designed for one optical disc is used, a large size is required for the other optical disc. There is a problem that spherical aberration occurs and information cannot be recorded / reproduced.

Various systems have been proposed to reduce the spherical aberration that occurs when recording / reproducing information on optical disks having different thicknesses using the same objective lens. Among them, a phase correction region for reducing spherical aberration is formed at the center of the phase correction element, and NA = 0.6 for the wavelength for DVD and for the wavelength for CD at the periphery. N
The method using a phase correction element having a diffraction grating for limiting the aperture to A = 0.45 is useful because an objective lens designed for DVD can be used for CD.

FIG. 2 is a side view showing the structure of the optical head device according to the present invention. The basic structure is the same as that of the conventional optical head device. The mounted configuration will be described.

[0007] In FIG. 2, the optical axes of the outgoing lights from the semiconductor laser 3 A having a wavelength of 780 nm and the semiconductor laser 3 B having a wavelength of 650 nm are aligned by a wavelength synthesizing prism 9. The light is collimated and enters the phase correction element 10 and the objective lens 6. Further, the outgoing light is condensed on the information recording surface of the optical disk 7 by the objective lens 6, and the reflected light (emitted light) reflected on the information recording surface is again converted into parallel light by the objective lens 6 and passes through the phase correction element 10. After that, the light is converged by the collimating lens 5. The reflected light reflected by the beam splitter 4 and having its optical axis deflected by 90 ° is converted to a photodetector 8.
And is converted to an electric signal. The phase correction element 10 is fixed to the objective lens 6 by a holder, and is usually driven by an actuator (not shown).

An objective lens optimally designed for a DVD optical disk with NA = 0.6 is replaced with a CD with NA = 0.45.
A conventional phase correction element (FIG. 8A, plan view, FIG. 8B, cross-sectional view) for reducing spherical aberration generated when used in a system optical disk will be described.

The phase correction element 10 is formed by directly processing the surface of a light-transmitting substrate 41 having an isotropic refractive index such as glass, or by processing a film formed on the surface, so that the cross section has an uneven shape and a step is formed. The phase correction region 42 of d _p is formed in an annular shape. Here, processing the concavo-convex portion so that the optical path difference based on step d _p of the uneven shape with respect to the wavelength lambda ₁ of the incident light for DVD is an integral multiple of lambda _1. By this processing, the wavelength λ ₁
Because there is no substantial phase difference for the incident light of
The phase correction element does not affect the wavefront of the transmitted light, and a favorable state in which the wavefront aberration is small with respect to the DVD-based optical disc is maintained.

On the other hand, in order to reduce the wavefront aberration generated for a CD optical disk, the above-described uneven step is formed in an annular shape in the area of NA = 0.45. Here, the area of NA = 0.45 means an inner area including a boundary represented by a numerical aperture of 0.45. With this formation, Marechal's RMS (Root Mean) is applied to the incident light in the area of NA = 0.45 at the wavelength λ ₂ for CD.
(Square) The phase correction function of not more than the wavefront aberration reference value of 0.07λ _rms is exhibited by using the objective lens integrally with the phase correction element.

As a result, the use of such a phase correction element 10 makes it possible to record and reproduce information on a DVD-based optical disc and a CD-based optical disc.

[0012]

However, the above-mentioned position complementation
When the positive element is used, the step d of the uneven shape of the phase correction area
_pOptical path difference based on₁Add irregularities so as to be an integral multiple of
RM for CD optical discs
0.04λ S wavefront aberration value_rmsIt is difficult to reduce
It was difficult.

As a result, it has been difficult to stably maintain a low wavefront aberration value in an optical head device composed of a plurality of optical components. In particular, in a writing optical disk such as a CD-R used with NA = 0.5,
Since high light condensing property on the optical disc, that is, smaller residual wavefront aberration is required, the wavefront aberration value is not sufficiently small to realize stable information recording.

In view of the above situation, an object of the present invention is to provide an optical head device equipped with a light source that emits light of two wavelengths and one shared objective lens for recording and reading information on optical disks having different thicknesses. An object of the present invention is to provide a polarization phase correction element and an optical head device capable of stably reproducing.

[0015]

According to the present invention, there is provided a polarization phase correcting element which uses light having wavelengths of λ ₁ and λ ₂ (λ ₁ ≠ λ ₂ ). Is λ
Comprising a birefringent material layer having a refractive index different for the _first linear polarized light and the linearly polarized light in the polarization direction perpendicular to the polarization has a direction lambda ₂ of the linearly polarized light, the main optical axis when the birefringent material layer optical crystal However, in the case of a polymer material, the molecular orientation axes are aligned in one direction, so that the birefringence can be corrected from the optical axis of the transmitted light of the birefringent material layer toward the periphery so that the wavefront aberration of the transmitted light can be corrected. A polarizing phase compensator, wherein the thickness of the material layer changes and the thickness has rotational symmetry with respect to the optical axis.

Further, wavelength λ ₁ and wavelength λ ₂ (λ ₁ ≠ λ ₂ )
Of a polarizing phase correction element to be used by the incidence of light, with respect to said polarizing phase correcting element lambda ₁ of the linearly polarized light and the linearly polarized light in the polarization direction orthogonal with the polarization direction lambda ₂ of the linearly polarized light A polymer material layer, which is a birefringent material layer having a uniform refractive index and a uniform thickness, is provided from the optical axis of the transmitted light of the polymer material layer toward the periphery so as to correct the wavefront aberration of the transmitted light. Therefore, the present invention provides a polarizing phase correction element, wherein the direction of the molecular orientation axis in the polymer material layer changes and the direction of the molecular orientation axis has rotational symmetry with respect to the optical axis.

Furthermore, two light sources for emitting light of wavelengths λ ₁ and λ ₂ , respectively, and a light source for condensing the emitted lights of λ ₁ and λ ₂ on two kinds of optical recording media having different thicknesses, respectively. In an optical head device for recording / reproducing information on / from an optical recording medium provided with a common objective lens, the above-mentioned polarizing phase correction element is provided in an optical path between the two light sources and the optical recording medium. Optical head device is provided.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION A D = NA = 0.6 designed to provide good aberration characteristics for an optical disk which is an optical recording medium having a wavelength of λ ₁ = 650 nm and a thickness of 0.6 mm.
FIG. 3C shows the wavefront aberration generated when the VD objective lens is used on a CD optical disk having a wavelength of λ ₂ = 780 nm and a thickness of 1.2 mm and a NA of 0.5.
In FIG. 3, NA is plotted on the horizontal axis, and wavefront aberration is plotted on the vertical axis. FIG. 3 shows a two-dimensional cross-sectional shape of the phase difference distribution, but in fact, it has a three-dimensional distribution rotationally symmetric with respect to the vertical axis (NA = 0). FIG. 3 (1)
FIGS. 3A and 3B show the case of the polarization phase correcting element of the present invention and the case of the conventional phase correcting element, respectively, as described later.

The configuration of the polarization phase correcting element of the present invention, which reduces the wavefront aberration generated in the CD optical disk as described above and does not cause the wavefront aberration of the DVD optical disk, and its operation and effects will be specifically described below. Will be described.

In the present invention, the birefringent material layer is calcite,
It may be formed of an optical crystal such as LiNbO ₃ or LiTaO ₃ . Further, it may be formed of a polymer liquid crystal obtained by polymerizing and curing a liquid crystal, or a polymer material such as polycarbonate which exhibits birefringence by uniaxial stretching.

In the first embodiment of the polarization phase correcting element of the present invention, the main optical axis is aligned in one direction in the layer in the case of an optical crystal, and the molecular orientation axis is aligned in the case of a polymer material containing a liquid crystal. One that is aligned in one direction within the layer and that has a birefringent material layer thickness that varies from location to location as described below. Hereinafter, a case where the birefringent material layer is a polymer liquid crystal obtained by polymerizing and curing a liquid crystal will be described.

In FIG. 1 showing a polarizing phase correction device of the first aspect of the present invention, extraordinary refractive isotropic light transmitting substrate 11 on the ordinary refractive index have a refractive index of glass or the like with n _o rate is assumed to form a polymer liquid crystal layer 12 of n _e. The polymer liquid crystal layer is obtained by polymerizing and curing a liquid crystal monomer.

First, a solution of a liquid crystal monomer before polymerization and curing is applied on an alignment film on a light-transmitting substrate 11 which has been subjected to an alignment treatment, and the alignment vector (molecular alignment axis) of the liquid crystal is determined in a specific direction in the coating surface. After alignment, the polymer is cured by irradiation with light such as ultraviolet rays to form a polymer liquid crystal layer. Next, when light is incident on the polymer liquid crystal layer from a light source such as a semiconductor laser, the thickness of the polymer liquid crystal layer changes as the transmitted light moves away from the optical axis toward the periphery, and the thickness is Etching is performed so as to have rotational symmetry with respect to the axis. The shape of the polymer liquid crystal layer having the rotational symmetry is a shape that corrects the wavefront aberration of the transmitted light.

Alternatively, an alignment film is formed on the surface of a mold processed into a depth distribution shape that is rotationally symmetric with respect to the optical axis, and a liquid crystal monomer solution is applied to the light-transmitting substrate 11 on which the alignment film is formed and the mold. The liquid crystal substrate 1
After being oriented so as to be aligned in a specific direction in the plane of 1, the substrate is polymerized and cured by irradiation with light such as ultraviolet rays. Thereafter, when the mold is released, a polymer liquid crystal layer whose thickness is rotationally symmetric with respect to the optical axis is formed.

The concave portion of the polymer liquid crystal layer 12 formed as described above is filled with a transparent filler 13 having an isotropic refractive index n _s , and the transparent substrate glass having an isotropic refractive index is used. By sandwiching the polymer liquid crystal layer 12 between the two glass substrates 11 and 14, the polarizing phase correction element 1 is manufactured. Refractive index n _s of the filler is selected material so as to be substantially equal to the ordinary refractive index n _o of the liquid crystal polymer layer.

[0026] The polarizing phase correction device 1 when the linearly polarized light of wavelength lambda ₁ that corresponds to the ordinary refractive index n _o of the liquid crystal polymer layer is incident, the refractive index difference between the polymer liquid crystal layer 12 and the filler 13 Therefore, the incident light is transmitted while maintaining the wavefront state without generating a spatial phase difference distribution. Therefore, this polarization phase correcting element 1 is integrated with the objective lens, and FIG.
If the disposed in the optical head device, the change in wavefront aberration for the wavelength lambda ₁ of the light does not occur.

On the other hand, when linearly polarized light of wavelength λ ₂ having a polarization direction orthogonal to the polarization direction of linearly polarized light of wavelength λ ₁ is incident, the refractive index of the polymer liquid crystal layer acting on the light of wavelength λ ₂ becomes extraordinary light. since the refractive index n _e, the refractive index difference between the filler 13 and the polymeric liquid crystal layer 12 and the (n _e -n _s), polymer liquid crystal layer 12
A phase difference distribution proportional to the spatial distribution of the thickness is generated. Therefore, the change in wavefront aberration for the wavelength lambda ₂ in the optical head device occurs.

At this time, the spatial phase difference distribution generated in the polarization phase correcting element 1 when linearly polarized light having a wavelength of λ ₂ is incident is generated as shown in FIG. By forming the thickness distribution of the polymer liquid crystal layer 12 so as to cancel the phase difference distribution, the RMS wavefront aberration value can be reduced to a value of 0.04λ _rms or less.

In the second embodiment of the polarizing phase correction element of the present invention, in the case of a polymer material containing liquid crystal, the direction in which the alignment vector (molecular alignment axis) is aligned in the layer depends on the location as described below. different. However, the thickness of the polymer material (birefringent material) layer is constant regardless of the location. Hereinafter, a case where the birefringent material layer is a polymer liquid crystal obtained by polymerizing and curing a liquid crystal will be described.

As shown in FIG. 4, the polarizing phase corrector 2 according to the second embodiment of the present invention differs from the first embodiment in that the thickness of the polymer liquid crystal layer, which is the birefringent material layer 22, is uniform. Yes No filler is used. In addition, the orientation vector of the polymer liquid crystal layer is aligned in a certain direction in the plane at each interface between the two glass substrates 21 and 24, which are translucent substrates, and the polymer liquid crystal layer 22. However, the orientation vector direction in the thickness direction of the polymer liquid crystal layer 22 changes spatially and differs depending on the location, and the orientation vector direction has rotational symmetry with respect to the optical axis, so that the interface with the glass substrate is The second embodiment is different from the first embodiment in that the transmitted light has a phase difference distribution when the light having the same polarization direction as the orientation vector direction is transmitted.

There are various methods for spatially distributing the orientation vector direction in the thickness direction. For example, as shown in FIG. 4, a liquid crystal monomer solution is sandwiched between a glass substrate 21 and a glass substrate 24 so as to have a constant thickness, so that the orientation vector direction of the liquid crystal is oriented in a specific direction in the plane. Align. That is, on the liquid crystal side of the glass substrate 24, an annular electrode 25 divided in an annular shape and an alignment film (not shown) subjected to alignment processing are formed. On the liquid crystal side of the glass substrate 21, a solid electrode 26 connected in a plane and an alignment film (not shown) subjected to an alignment process are formed. Then, a liquid crystal monomer solution is sandwiched between the glass substrates 21 and 24 so as to be in contact with both alignment films, so that the alignment vector directions are aligned in a specific direction.

When a different AC voltage is applied to each liquid crystal monomer solution layer between each ring of the ring electrodes 25 and the solid electrode 26, as shown in FIG. The direction of each orientation vector for each layer is inclined, and the phase difference of the transmitted light in each orbicular zone forms a spatial phase distribution that cancels out the wavefront aberration of the CD optical disc shown in (3) of FIG. In this way, the orientation vector direction is rotationally symmetric with respect to the optical axis, and the radius of each annular zone and the applied voltage are determined. Thereafter, a polymer liquid crystal layer 2 is polymerized and cured by applying a voltage and irradiating light such as ultraviolet rays.
Let it be 2.

This method is preferable because the transparent electrode pattern and the applied voltage can be freely set, so that the spherical aberration of the CD-type optical disk can be reduced with high accuracy and the element manufacturing process can be simplified as compared with the first embodiment.

In FIG. 4, in order to incline the alignment vector of the liquid crystal with respect to the normal direction of the glass substrate sandwiching the liquid crystal layer,
Ring-shaped electrodes are formed on the liquid crystal side surface of the translucent substrate, and different voltages are applied to each ring-shaped zone. However, the connected solid electrodes for voltage application are deformed into a predetermined shape and arranged outside the element, and the electrode spacing at each corresponding position between the deformed electrode and the opposing electrode is changed to change the liquid crystal. The effective voltage applied to the layers may have a voltage distribution similar to that of the device of FIG.

FIG. 5 shows a method of manufacturing an element having the above-described specific structure. Electrode substrate 2 so that the distance between the electrodes varies depending on the location.
After processing the surface of No. 8, electrode substrates 28 and 30 on which solid electrodes 27 and 29 are formed are prepared. Electrode substrate 28,
Elements (cells) in which a solution of a liquid crystal monomer is sandwiched between translucent substrates 21 and 24 that have been subjected to an alignment treatment are arranged between 30 so that the alignment vectors of the liquid crystal are aligned in a specific direction. Thereafter, by applying a predetermined AC voltage between the solid electrodes 27 and 29, a spatially inclined liquid crystal orientation vector distribution similar to that of FIG. 4 can be realized. Further, the liquid crystal monomer is polymerized and cured to form the polymer liquid crystal layer 22.

According to this method, since it is not necessary to form a patterned transparent electrode inside the polarization phase corrector, the polarization phase corrector can be realized with an inexpensive material. Further, since the electrodes are connected and not divided electrodes, a smooth phase difference distribution for correcting the wavefront aberration corresponding to the phase difference distribution shown in (3) of FIG. 3 can be realized, so that more precise correction can be performed. .

Thus, as in the first embodiment, the second
Mode also wavelength λ_TwoPhase corrector for linearly polarized light
The spatial phase difference distribution that occurs in the
Wavefront yield shown in Fig. 3 (3) when used in a disc
In order to cancel the difference, the orientation vector distribution of the polymer liquid crystal layer
To form an RMS wavefront aberration of 0.04λ.
_rmsIt can be reduced to the following values. Also, the wavelength λ_TwoLinearly polarized light
Wavelength λ with orthogonal polarization directions₁The light of the phase difference distribution
Does not occur.

In each of the first and second embodiments, since the polarization directions of the incident lights of the wavelengths λ ₁ and λ ₂ are orthogonal to each other and both use a birefringent material, the light of one wavelength is used for the other. , A spatial phase difference distribution can be formed independently for the light.
As a result, accurate wavefront aberration correction can be performed, and the target 0.
It can be reduced to an RMS wavefront aberration value of 04λ _rms or less.

In the first and second aspects, the polarization phase correction for reducing spherical aberration generated when an objective lens having a reduced wavefront aberration with respect to a DVD optical disc is used for a CD optical disc. The case of the element has been described. Conversely, an objective lens having a reduced wavefront aberration with respect to a CD optical disk may be used as a polarization phase correction element for reducing spherical aberration generated when the objective lens is used for a DVD optical disk. Alternatively, when an objective lens that retains spherical aberration for both DVD and CD optical disks is used, a polarization phase correction element that reduces spherical aberration for both optical disks can be used.

The polarization phase correction area for generating a spatial phase difference distribution is formed in the area of NA = 0.6 for a DVD optical disk in the polarization phase correction element. In the area of NA = 0.45 for the CD optical disc for reproduction,
For a CD optical disc for recording, it is preferable to form a polarization phase correction area in the area of NA = 0.5 because spherical aberration can be reduced with a relatively simple configuration.
It may be formed in the area of 0.6.

In the case where the polarization phase correction region is formed in the region where NA = K (K <0.6), a polarization diffraction grating for limiting the aperture is formed in a region other than the region where NA = K. C
This is more preferable for avoiding light rays that increase the wavefront aberration in a D-type optical disc.

In FIG. 6, which shows an example of the polarization phase correction element according to the third embodiment of the present invention, the periphery of the polarization phase correction area 20, that is, NA = 0.5 (or NA = 0.45)
Is formed in a region other than the region (a), the polarizing diffraction grating 31 transmits an incident light of one of the two wavelengths in a straight line, and diffracts an incident light of the other wavelength. Function as

The polarizing diffraction grating 31 also has two forms A and B as in the case of the polarizing phase correction region 20. That is, Form A, the polymer liquid crystal layer is oriented vector aligned in a specific direction, after the cross section is processed to a diffraction grating periodic uneven shape, refractive index approximately equal to n and the ordinary refractive index n _o of the liquid crystal polymer _{This is a} polarizing diffraction grating formed by filling a concave portion with a filler (s) (an irregular shape of a polymer liquid crystal layer). Form B
Are those in which the polymer liquid crystal layer has no irregularities but the orientation vector direction of the liquid crystal changes spatially (for each region), that is, a region where the orientation vector direction is parallel to the translucent substrate surface and aligned in the thickness direction. This is a polarizing diffraction grating in which regions inclined with respect to the thickness direction of the molecular liquid crystal layer alternately form a periodic structure (uniform thickness of the polymer liquid crystal layer).

In any of the embodiments, no diffraction occurs when linearly polarized light (first linearly polarized light) having a wavelength λ _{1 in} a polarization direction orthogonal to the direction of the orientation vector of the polymer liquid crystal is incident.
As diffraction when the incident linearly polarized light having a wavelength lambda ₂ in the polarization direction perpendicular to the polarization direction of the wavelength lambda ₁ (second linear polarization) occurs, the wavelength lambda ₁ and the polarization direction and orientation of the polymer liquid crystal of the wavelength lambda ₂ What is necessary is just to set a vector direction.

In the case of such a polarizing diffraction grating, if the incident light is the first linearly polarized light, a high straight transmissivity can be obtained almost independently of the wavelength. Further, by the phase difference between the high refractive index portions and the low refractive index portion of the diffraction grating acts on second linearly polarized light of the incident wavelength lambda ₂ is substantially lambda _2/2, the wavelength lambda Most of the incident light of No. ₂ can be diffracted to reduce straight transmitted light.

That is, specifically, the aforementioned polymer liquid crystal
In the case of layer unevenness, extraordinary light refractive index n _eAnd the refractive index n_sLattice
Assuming that a step formed by the portion is d, the optical path difference (n_e-N_s)
× d is λ_Two/ 2. In addition, the aforementioned polymer
In the case of the liquid crystal layer having a uniform thickness, the polymer liquid crystal layer has a uniform thickness d.
In the case of, the orientation vector of the liquid crystal is in the thickness direction of the polymer liquid crystal layer.
The average refraction in the thickness direction of the grating portion inclined with respect to_m
Then, the optical path difference (n_m-N_o) × d is λ_Two/ 2
Average refractive index n_mAnd d may be adjusted.

Therefore, the first linearly polarized incident light (wave
Long λ₁) Transmits straight through the polarizing diffraction grating region, but the second
Linearly polarized incident light (wavelength λ_Two) Indicates the polarization grating area
Is not diffracted by the lens and does not travel straight through.
Polarizing grating area is wavelength λ ₁And wavelength λ_TwoWaves against the light
An aperture limiting filter having high selectivity is obtained. Where
In the return path, the diffraction
Grating pitch so that the reflected light does not enter the photodetector.
By designing the grating pattern, the diffracted light can become stray light.
And can be avoided.

As a result, in the polarization phase correcting element,
By using the polarizing phase correction region and the polarizing diffraction grating region together, an objective lens optimized for DVD can be used as a CD.
= 0.45 or NA = 0.
Since the light that increases the wavefront aberration in the region other than the region 5 is excluded, the CD described in the first and second embodiments is eliminated.
To the level of the RMS wavefront aberration value in a system optical disc.

The aperture limiting filter using the polarizing diffraction grating shown in the third embodiment can be manufactured by the same process as the polarizing phase correction element shown in the first and second embodiments, so that the number of steps can be reduced. Therefore, the number of parts does not increase.

Instead of the polarizing diffraction grating, an aperture limiting filter made of a conventional dielectric multilayer interference film is provided with NA = 0.0 on the light-transmitting substrate surface on the light incident side of the polarizing phase correction element.
It may be formed in a region excluding the region of 45 or NA = 0.5.

In the optical head device shown in FIG.
If the output polarization direction of the output polarization direction and wavelength lambda ₂ of the semiconductor laser 3B of the _first semiconductor laser 3A are parallel, the incident polarization direction of the polarizing phase correction element 1 or 2 is the wavelength lambda ₁ and wavelength lambda ₂ In the optical path between the semiconductor laser 3A and the wavelength synthesizing prism 9 or in the optical path between the semiconductor laser 3B and the wavelength synthesizing prism 9 so as to be orthogonal to each other, a phase in which the polarization direction of the polarized light emitted from the semiconductor laser is rotated by 90 °. It is preferable to arrange a half-wave plate (not shown) as the plate. Such a phase plate is obtained, for example, by stretching polycarbonate or the like.

Further, the phase difference distribution generated by the polarization phase correcting element of the present invention may deviate from the rotational symmetry with respect to the optical axis to the extent described below. That is, when aberration components other than spherical aberration remain due to the optical components used in the optical head device and the recording / reproducing performance of information on the optical disc is insufficient, such aberration components are reduced. What is necessary is just to deform | transform the spatial phase difference distribution of a polarization phase correction element, deviating from rotational symmetry.

For example, the wavelength λ for DVD₁Incident light
Incident on the axis of the object lens, but the wavelength λ for CD_TwoThe light of
The light may enter from outside the optical axis of the objective lens. like this
, The rotational symmetry axis of the phase difference distribution of the polarizing phase compensator
Is shifted from the optical axis of the objective lens, and the
Spherical aberration of CD-based optical discs is
It is preferable to change the phase difference distribution so as to be small.
The configuration for deforming such a phase difference distribution has a wavelength λ₁of
Light emission point and wavelength λ_TwoLight emission point is 100-300μ
Uses two-wavelength semiconductor lasers, etc., spaced about m apart
The wavelength λ ₁Light on the optical axis of the objective lens
The wavelength λ_TwoLight becomes off-axis light and the wavefront aberration is minimized.
It is effective to become.

Alternatively, in order to correct the phase difference distribution of the deviation from the rotational symmetry with respect to the optical axis of the light emitted from the semiconductor laser having the wavelength λ ₂ , the phase difference distribution of the polarization phase correcting element is changed from a circular shape. It may be elliptical.

[0055]

[Embodiment 1] This embodiment will be described with reference to FIG. On the glass substrate 11 ordinary refractive index n _o = 1.50, using the filler 13 of polymeric liquid crystal and the refractive index n _s = 1.50 for extraordinary refractive index n _e = 1.60, the polymer liquid crystal layer 12 The polymer liquid crystal layer 12 was formed by an etching process so that the thickness change had rotational symmetry with respect to the optical axis (thick vertical line).

Let the objective lens for DVD with NA = 0.6 be N
The wavefront aberration generated when used for CD-R at A = 0.5 is as shown in (3) of FIG. 3, and the maximum value W _{max of the} phase difference is shown.
Was about 0.75λ ₂ . The wavelength λ ₂ of the semiconductor laser for CD was 785 nm.

In order to reduce the phase difference of the transmitted light of wavelength λ ₂ for CD, the polymer liquid crystal layer 12 has a cross-sectional shape as shown in FIG.
Substantially equal each step d _N to the shape of the transmitted wavefront of (a) consists of step equal N stages (2N + 1) numbers of the step is formed of the annular stepped, optical path difference for each step (n _e -n _s) × d _N to W _max
/ (N + 1).

In this example, N = 15 and (n_e-N_s) × d
_N= W_max/(N+1)=36.8 nm
Step d of stairs_N= 368 nm was processed into 15 steps. this
The phase difference of the incident light due to the step is C shown in FIG.
To cancel the spherical aberration of D-type optical disc,
Radius r of (2N + 1) orb-shaped steps that generate a phase difference _n
(N = 1 to 31) was determined.

When the thus-obtained polarizing phase correction element 1 is mounted on an optical head device, the spherical aberration of a CD optical disk is reduced as shown in FIG. At this time, the calculated value of the RMS wavefront aberration is 0.014λ _rms , and C
Even when used for writing optical disks such as DR, the value was sufficiently small. Further, the light wavelength lambda ₁ having a polarization direction orthogonal to the linearly polarized light of wavelength lambda _2, did not produce a phase difference distribution.

For comparison, the wavefront aberration when a conventional phase correction element is used is shown in FIG. The calculated value of the RMS wavefront aberration at this time is 0.056λ _rms , and it can be seen that the aberration reduction effect of (1) in FIG. 3 is superior to (2) in FIG.

Example 2 This example will be described with reference to the configuration of FIG. In this example, the transparent solid electrode 29 has a thickness of 0.2 mm.
It differs from the configuration of FIG. 5 in that it is formed on the inner surface of a 5 mm glass substrate 21. Using a glass substrate 24 of the glass substrate 21 and the electrode is not formed thick 0.1 mm, the ordinary refractive index n _o = 1.50, the solution of liquid crystal monomer of the extraordinary refractive index n _e = 1.60 A sandwiched cell was produced. These refractive index values are values for each of two different wavelengths of light having orthogonal polarization directions. Note that, on the inner surfaces of the glass substrate 21 and the glass substrate 24, an alignment film that has been subjected to an alignment process so that the alignment vector direction of the liquid crystal is aligned with a certain direction in the plane is formed.

The shape of the projection of the electrode substrate 28 outside the cell is made to cope with the spherical aberration distribution shown in FIG. 3C, and the solid electrode 27 and the glass substrate 21 formed on the surface of the projection of the electrode substrate 28 are formed. The effective voltage of about 300 V is applied to the transparent electrode 29 on the inner surface of
Was applied to form a directional distribution of the orientation vectors of the liquid crystal in the thickness direction of the cell as shown in FIG. That is, the orientation vector direction is set to have rotational symmetry with respect to the optical axis (dashed line).

Thereafter, the liquid crystal monomer was polymerized and cured by irradiating ultraviolet rays from the glass substrate 21 side, thereby fixing the directional distribution of the orientation vector of the liquid crystal. As a result of mounting the polarizing phase correction element thus obtained on an optical head device,
The spherical aberration of the CD optical disk is reduced to the same extent as in Example 1,
The value was sufficiently small even when used for a writing optical disc such as a CD-R.

[Example 3] A description will be given with reference to FIG. 6 in which a polarizing diffraction grating is formed in a region other than the region of NA = 0.5 in the polarizing phase correction element of Example 1. In this example, the material and manufacturing process of the polarizing phase correction region 20 are the same as those in Example 1, and the ordinary refractive index n _o in which the orientation vectors are aligned in a specific direction.
= 1.50, the polymer liquid crystal layer of the extraordinary refractive index n _e = 1.60, the grating pitch in the peripheral region of the polarizing phase correction region 20 is processed into a polarizing diffraction grating 31 of 30 [mu] m.

In the region of the polarizing diffraction grating 31, after the polymer liquid crystal layer is processed into a diffraction grating having a concave and convex shape having a periodic cross section, the concave portion is substantially equal to the ordinary light refractive index n _o = 1.50 of the polymer liquid crystal. A polarizing diffraction grating formed by filling with a filler having a refractive index of n _s was obtained. Here, the wavelength λ ₂ for CD
= Relative extraordinarily polarized light 785 nm, the step d of the unevenness of the diffraction grating region _{d = 0.5 × λ 2 / (} n e -n s) = 392
Assuming 5 nm, the diffraction efficiency of ± 1st order diffracted light is 70% in total
As a result, the zero-order transmitted light was 5% or less.

On the other hand, for ordinary light polarized light of wavelength λ ₁ for DVD (having a polarization direction orthogonal to the extraordinary light polarized light of wavelength λ ₂ ), it does not act as a diffraction grating, and therefore has a high zero-order transmission of about 97%. It became light. Therefore, such a polarizing diffraction grating restricts the aperture to NA = 0.5 for extraordinary light polarized light having a wavelength of λ ₂ = 785 nm for CD, but has a polarizing property for light having a wavelength of λ ₁ for DVD. The diffraction grating does not function as a diffraction grating and functions as a wavelength-selective aperture limiting filter that does not change the phase.

When the extraordinary light having the wavelength λ ₂ for CD is diffracted by the polarizing diffraction grating 31 on the outward path and the returning path, the diffracted light generated on the outward path and the diffracted light generated on the returning path are combined. There may be stray light returning to the light receiving area of the same photodetector as the zero-order light that is not diffracted. As shown in FIG. 6, in order to avoid such stray light from being mixed into the photodetector, as shown in FIG.
Is preferably such that there is no twice rotational symmetry with respect to the optical axis of the transmitted light.

As described above, by using the polarization phase correction element in which the polarization diffraction grating 31 is formed integrally with the polarization phase correction area 20 by integrally driving with the objective lens 6 of the optical head device shown in FIG. Since the light that increases the wavefront aberration in the area other than the area of NA = 0.5 of the objective lens 6 in the CD optical disk is eliminated, the RMS wavefront aberration value of the CD optical disk is substantially reduced to 0.014λ _rms. did it.

Example 4 FIG. 7 shows an example in which the polarization phase correcting element of Example 3 is mounted on an optical head device. Configuration of the optical head device of FIG. 7 differs from the arrangement of FIG. 2, the wavelength lambda ₁ = 65
A semiconductor laser having a wavelength of 0 nm and a semiconductor laser having a wavelength of λ ₂ = 780 nm are arranged as one two-wavelength semiconductor laser 3 arranged in a single unit with their light emitting points separated by about 100 μm. , Miniaturization of equipment
The number of parts was reduced.

When the polarization directions of the outgoing lights of the wavelengths λ ₁ and λ ₂ are not orthogonal but parallel, two wavelengths are required in order to make the polarization directions of the incident lights of the respective wavelengths to the polarization phase correcting element 1 orthogonal. Λ between the semiconductor laser 3 for use and the polarization phase correcting element 1
It may be disposed polycarbonate phase plate (not shown) for giving a phase difference of (5/2) lambda ₁ with respect to _1. Since the linearly polarized light of wavelength lambda ₁ transmitted through the phase plate is the polarization direction is rotated 90 degrees, the linearly polarized light of wavelength lambda ₂ does not rotate, the wavelength lambda ₁
And the polarized light of wavelength λ ₂ are orthogonal to each other.

With the optical head device using the two-wavelength semiconductor laser as described above, the number of components is reduced and the size and weight can be reduced as compared with the configuration shown in FIG. By using the optical head device of the present invention, a stable operation can be realized in the reproduction of a DVD optical disc and the recording and reproduction of information on a CD optical disc.

In this embodiment, the diffraction grating for generating three beams for tracking error correction is not used. However, if this diffraction grating is used, the polarization direction of the linearly polarized light having the wavelength λ ₁ and the wavelength λ ₂ is orthogonal. It is effective to reduce the number of parts if integrated with a wave plate to be formed.

In FIGS. 2 and 7, the case where a prism is used as the beam splitter 4 has been described. However, an arrangement in which a hologram beam splitter is used instead of the prism and the diffracted light on the return path is guided to the photodetector 8 is shown. It may be. In such a case, the photodetector 8 may be arranged near the semiconductor laser 3A or 3B (FIG. 2) or 3 (FIG. 7) so that the semiconductor laser and the photodetector are integrated in the same package. This is preferable in reducing the size of the device. 7 is a collimating lens, 6 is an objective lens, and 7 is an optical disk.

FIGS. 2 and 7 show the case where the photodetector 8 is used as an optical signal detector for both DVD and CD with the same configuration. However, the photodetector for DVD and the photodetector for CD are respectively used. They may be arranged separately.

[0075]

As described above, since the polarizing phase compensator of the present invention has a birefringent material layer, it utilizes the birefringence to make use of two incident lights having different polarization directions and wavelengths. Light of one wavelength can be transmitted as it is, and light of the other wavelength can precisely control the phase difference.

Therefore, when the polarization phase correcting element of the present invention is used in an optical head device, light of two different wavelengths can be stably used in recording and reproducing information on and from two types of optical recording media such as optical disks having different thicknesses. A compact and lightweight optical head device with a reduced number of components and a reduced operation can be realized.

[Brief description of the drawings]

FIG. 1 is a conceptual side view showing a cross section of a polarizing phase corrector according to a first embodiment of the present invention and a transmitted wavefront of light transmitted through the polarizing phase corrector; FIG. (B) Conceptual side view for incident light of the other wavelength.

FIG. 2 is a side view showing an example of the optical head device of the present invention.

FIG. 3 is a graph showing an example of a wavefront aberration generated when a CD optical disk is used in an optical head device optimally designed for a DVD optical disk. (1) A graph showing an example in which the polarizing phase correction element of the present invention is mounted. (2) A graph showing an example when a conventional phase correction element is mounted. (3) A graph showing an example in which a polarizing phase correction element is not mounted.

FIG. 4 is a conceptual side view showing a cross section of a polarization phase correction element according to a second embodiment of the present invention and a transmitted wavefront of light transmitted through the polarization phase correction element, and (a) a concept for incident light of one wavelength. (B) Conceptual side view for incident light of the other wavelength.

FIG. 5 is a schematic side view showing a method for manufacturing a polarization phase corrector according to another example of the second embodiment of the present invention.

FIG. 6 is a schematic plan view showing a polarizing phase correction element according to a third embodiment of the present invention.

FIG. 7 is a side view showing another example of the optical head device of the present invention.

FIG. 8 is a schematic diagram showing an example of a conventional phase correction element;
(A) The top view of a phase correction element, (b) The sectional view of a phase correction element.

[Explanation of symbols]

 1, 2: polarizing phase corrector 3: semiconductor laser for 2 wavelengths 3A, 3B: semiconductor laser 4: beam splitter 5: collimating lens 6: objective lens 7: optical disk (optical recording medium) 8: photodetector 9: wavelength Synthetic prism 10: Phase correction element 11, 14, 21, 24: Transparent substrate (glass substrate) 12, 22: Birefringent material layer (polymer liquid crystal layer) 13: Filler 20: Polarizing phase correction region 25: Ring electrodes 26, 27, 29: Solid electrodes 28, 30: Electrode substrate 31: Polarizing diffraction grating 42: Phase correction area

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H049 BA06 BA42 BB05 BC01 BC06 BC08 BC21 2H099 AA05 BA17 CA05 5D119 AA41 BA01 BB13 CA16 EC01 EC47 EC48 EC49 FA08 JA09 JA22

Claims (4)

[Claims] 1. A polarizing phase compensating element for using light having a wavelength of λ ₁ and a wavelength of λ ₂ (λ ₁ ≠ λ ₂ ) incident thereon, wherein said polarizing phase compensating element comprises linearly polarized light of λ ₁ and said λ _1. A birefringent material layer having a different refractive index for linearly polarized light having a polarization direction orthogonal to the polarization direction of linearly polarized light λ ₂ is provided, and when the birefringent material layer is an optical crystal, a main optical axis is a polymer material. Sometimes, the molecular orientation axes are aligned in one direction, and the thickness of the birefringent material layer increases from the optical axis of the transmitted light of the birefringent material layer toward the periphery so as to correct the wavefront aberration of the transmitted light. A polarizing phase correction element, wherein the polarization phase correction element changes and the thickness has rotational symmetry with respect to the optical axis. 2. A polarization phase corrector for use by making light of wavelengths λ ₁ and λ ₂ (λ ₁ ≠ λ ₂ ) incident thereon, wherein the polarization phase corrector is a linearly polarized light of λ ₁ and Equipped with a polymer material layer that is a birefringent material layer with a uniform thickness and a different refractive index for linearly polarized light of λ ₂ having a polarization direction orthogonal to the polarization direction of linearly polarized light, and can correct wavefront aberration of transmitted light As described above, the direction of the molecular orientation axis in the polymer material layer changes from the optical axis of the transmitted light of the polymer material layer toward the periphery, and the molecular orientation axis direction has rotational symmetry with respect to the optical axis. What is claimed is: 1. A polarizing phase correction element, comprising: 3. A polarizing diffraction grating having different diffraction efficiencies with respect to linearly polarized light having a wavelength of λ ₁ and linearly polarized light having a wavelength of λ ₂ is further formed around the birefringent material layer. 3. The polarizing phase correction element according to claim 1. 4. A light source for emitting light of wavelengths λ ₁ and λ ₂ , respectively, and a light source for converging emitted lights of λ ₁ and λ ₂ on two types of optical recording media having different thicknesses. 4. An optical head device for recording / reproducing information on / from an optical recording medium provided with a common objective lens, wherein the polarization phase correcting element according to claim 1, 2 or 3 comprises: Optical head device installed in the optical path between the two.