JP2002040253A - Optical rotation element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical head device which receives no influence of double refraction remaining in an optical disk and has excellent information recording/reproducing characteristics by obtaining an optical rotation element which respectively rotates planes of polarization of linearly polarized light of different two wavelength bands for a CD optical disk and a DVD optical disk by the same angles and loading the optical rotation element therein. SOLUTION: Difference between extraordinary ray refractive index and ordinary ray refractive index of a polymer liquid crystal film 11 held between alignment films 12A, 12B formed on transparent substrates 13A, 13B is made to have a value between 0.05 and 0.25. Further the optical rotation element 101 which rotates two planes of polarization of linearly polarized light by the same angles by twisting an alignment direction of the polymer liquid crystal between both surfaces of the polymer liquid crystal film 11 is obtained and then the optical rotation element 101 is disposed in an optical path between a light source and an objective lens of the optical head device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical rotator and an optical head device.

[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 disk that uses a medium having a high wavelength dependence with respect to reflection and absorption of light as a recording layer of an optical recording medium, and a semiconductor laser used for a CD has a wavelength band of 790 nm in a CD-R or other optical disk. is there. At this time, a semiconductor laser having a wavelength band of 660 nm is used for DVD.

In an optical head device, an optical component whose optical characteristics depend on the polarization state of incident light, such as a polarizing beam splitter, may be used to improve light use efficiency. In some cases, linearly polarized light having a polarization plane that forms a certain angle with respect to the track direction of the optical disc is condensed.

[0005]

Usually, a CD system or D
Recording and recording of information on both CD and DVD optical discs using two linearly polarized light beams whose polarization planes are rotated by a phaser having a desired retardation value with respect to light of any wavelength band for the VD system. When reproduction is performed, good characteristics are shown for one optical disc, but not good for the other.

Specifically, for example, when a phase shifter functioning as a half-wave plate is used for light in the DVD system wavelength band, the linearly polarized light in the DVD system wavelength band has a desired polarization plane. Although it is rotated by the value and transmitted, it does not function as a half-wave plate with respect to the linearly polarized light in the CD wavelength band, and the emitted light is not linearly polarized light but elliptically polarized light. Conversely, a retarder that functions as a half-wave plate for light in the wavelength band for a CD system is D
It does not function as a half-wave plate for light in the VD wavelength band.

[0007] When an unfavorable birefringence distribution exists on an optical disk, the intensity fluctuation of the reflected return light from the optical disk is changed to linearly polarized light when the reflected return light is separated by a polarizing beam splitter and condensed on a photodetector. Elliptically polarized light is larger. Therefore, when the above-described phaser is used, there is a problem that information cannot be recorded / reproduced in the optical head device.

For this reason, for a linearly polarized light in both the wavelength band of the CD system and the DVD system, the rotation angle of the plane of polarization is the same after transmission through the phaser, and the optical element such as an optical rotator can maintain linearity. An optical element has been required.

[0009]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has been made in consideration of the above problem, and includes linearly polarized light P ₁ having a wavelength λ ₁ and linearly polarized light P _{2 having} a wavelength λ ₂ (λ ₁ <λ _2). ) in the polarization rotator that is incident, the difference is 0.05 to linear polarization P ₁ of the polarization rotator extraordinary refractive index on a transparent substrate and ordinary refractive index
A polymer liquid crystal film having a value of 0.25 is formed,
The orientation direction of the polymer liquid crystal is parallel to the film surface of the polymer liquid crystal film and is helically twisted in the film, and the polarization planes of the linearly polarized light P ₁ and the linearly polarized light P ₂ passing through the polymer liquid crystal film are substantially The present invention provides a rotator characterized by being rotated by the same angle.

Further, linearly polarized light having a wavelength lambda ₁ P ₁ and wavelength lambda
Comprising a light source for emitting a _second linearly polarized light _{P 2 (λ 1 <λ 2} ), an objective lens for converging the emitted linearly polarized light P ₁ and the optical recording medium linearly polarized light P _2, information on an optical recording medium Records of
In an optical head device for performing reproduction, an optical head device including the above-described optical rotator in an optical path between a light source and an objective lens is provided.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a rotator 101 of the present invention has a structure in which a polymer liquid crystal film 11 is sandwiched between transparent substrates 13A and 13B. FIG. 1 also shows a state in which linearly polarized light having a wavelength λ transmits through the optical rotator 101 and is emitted with its polarization plane rotated by the optical rotation angle φ.
The optical rotator 101 is manufactured as follows.

A film for an alignment film is applied on the transparent substrates 13A and 13B, and each of them is subjected to a desired alignment treatment.
2A and 12B, the alignment film 12 on the transparent substrate 13A
A is applied with a solution of a liquid crystal monomer which is a birefringent material. Next, the alignment processing direction of the alignment film 12A and the alignment film 12
B so that the direction of the alignment
Layer.

At this time, the molecular direction of the liquid crystal monomer is adjusted at the position where the alignment films 12A and 12B are in contact with the respective alignment processing directions, and the liquid crystal layer is gradually rotated in the thickness direction of the liquid crystal layer for alignment. That is, at the position where the alignment films 12A and 12B are in contact, they are aligned in the cross direction. Finally, a polymer liquid crystal film 11 is obtained by irradiating a light source light for photopolymerization to be polymerized and cured. The orientation direction of the polymer liquid crystal is parallel to the film surface of the polymer liquid crystal film, and at a position in contact with the orientation films 12A and 12B,
It corresponds to each orientation processing direction. Therefore,
The orientation direction of the polymer liquid crystal is spirally twisted in the polymer liquid crystal film 11.

The difference Δn between the extraordinary light refractive index and the ordinary light refractive index of the polymer liquid crystal film 11 takes a value of 0.05 to 0.25. If Δn is smaller than 0.05, the thickness d of the polymer liquid crystal film is increased, and the polymer liquid crystal is not properly aligned, which is not preferable. On the other hand, if Δn is larger than 0.25, the film thickness d must be reduced to 1 to 3 μm in order to obtain an appropriate retardation value, which is not preferable because productivity is reduced. That is, a value of Δn of 0.05 to 0.25 is preferable from the viewpoint of the orientation of the polymer liquid crystal and the productivity of the polymer liquid crystal film.

FIG. 2 shows a rotator polymer liquid crystal film 1 of the present invention.
FIG. 1 shows a state in which linearly polarized light having a wavelength λ is incident and its polarization plane is rotated. In FIG. 2, the polymer liquid crystal film 1
1 of the alignment direction of the polymer liquid crystal on the light incident side alpha _A, when the alignment direction of the light emission side and alpha _B, by aligning the polarization direction of the linearly polarized light incident to the alignment direction alpha _B of the exit side,
A desired optical rotation characteristic is exhibited for linearly polarized light having a wavelength λ, and the optical rotation angle φ is equal to the difference | α _A −α _B | between the orientation direction α _A on the incident side and the orientation direction α _B on the emission side.

Further, the film thickness d of the polymer liquid crystal film 11, which can maintain the linearity of the linearly polarized light transmitted through the optical rotator, is d = λ /
It is determined from the relational expression of (Δn · E). Where the coefficient E
Is given as a function of the optical rotation angle φ, as shown in FIG. 3, and when φ = 90 °, E (90 °) ≒ 1.0, φ = 4
At 5 °, E (45 °) ≒ 2.3.

Further, the optical rotator of the present invention has a wavelength λ
₁ of linear polarization P ₁ and wavelength lambda ₂ of the linearly polarized light P _{2 (λ 1} <
Since λ ₂ ) is incident, the wavelength λ used when designing the optical rotator as described above is defined as λ ₁ <λ <λ ₂ . By limiting the wavelength λ in this manner, the optical rotator can rotate the plane of polarization by substantially the same angle while maintaining the linearity even if the wavelengths of the two different types of linearly polarized light are different. Here, the term “substantially the same angle” means that the two angles are preferably the same, but even if they are different from each other, a deviation that brings about the same effect falls within this range. Specifically, if the deviation is 10 ° or less, the angles are the same.

It is preferable to use an optically flat substrate made of an inorganic material such as a glass substrate as the transparent substrate in order to maintain good transmitted wavefront aberration with respect to a temperature change. Further, a diffraction grating may be formed on the surface of the transparent substrate by using a technique such as photolithography or etching to exhibit a function of diffracting light.

In the optical head device of the present invention shown in FIG. 4, linearly polarized lights having different wavelengths emitted from the semiconductor lasers 1A and 1B are reflected by the beam splitters 2 and 3, respectively, pass through the optical rotator 101, and are parallelized by the collimating lens 4. It becomes light. Before and after the transmission of the optical rotator 101, the linearly polarized lights having different wavelengths are both rotated in the polarization direction by an angle φ, and then condensed on the recording surface of the optical disk 6 by the objective lens 5. The return light reflected on the recording surface of the optical disk 6 becomes parallel light again by the objective lens 5, and is condensed on the photodetector 8 via the collimator lens 4, the optical rotator 101, and the beam splitters 3 and 2. Here, the return light transmitted through the optical rotator 101 has its polarization direction coincident with the polarization direction of the outward path.

In the optical head device of the present invention, since the optical rotator of the present invention is used, the fluctuation of the intensity of the reflected signal light from the optical disk due to the unfavorable birefringence distribution remaining on the optical disk can be reduced, and the optical head device can be stabilized. Record and play back information. In addition, linearly polarized light whose polarization plane is rotated in a direction in which recording / reproducing performance is improved can be incident on a recording / reproducing optical disk having a land / groove structure. Further, since the fluctuation of the optical rotation characteristic depending on the incident angle is smaller than that of the conventional optical rotator using crystal, stable information recording / reproducing characteristics can be obtained. In the above, the optical rotator 101
Is disposed between the beam splitter 3 and the collimating lens 4, but may be disposed between the collimating lens 4 and the objective lens 5.

[0021]

EXAMPLE 1 This example is a specific example of the optical rotator 101 shown in FIG. 1 and has an optical rotation angle φ of 45 °. A polyimide film for an alignment film was applied on transparent substrates 13A and 13B made of glass having a refractive index of 1.5, and subjected to an alignment treatment by rubbing to obtain alignment films 12A and 12B. Next, a solution of a liquid crystal monomer, which is a birefringent material, is applied to the alignment film 12A on the transparent substrate 13A so that the alignment processing direction of the alignment film 12A and the alignment processing direction of the alignment film 12B intersect at 45 °. The transparent substrate 13 </ b> B was overlaid and polymerized and cured by irradiating a light source light for photopolymerization to form the polymer liquid crystal film 11, thereby producing the optical rotator 101. In addition, the polymer liquid crystal film 11
Is that the difference Δn between the extraordinary light refractive index and the ordinary light refractive index is 0.3.
The film thickness d was set to 2.7 μm by using the material of No. 11.

In the optical rotator 101 manufactured as described above, the light incident from the transparent substrate 13A side is linearly polarized light having a wavelength of 685 nm, and its linearity is parallel when its polarization plane is parallel to the alignment direction of the alignment film 12B. While maintaining its polarization plane at 45
It is an optical rotator rotated by ° (see FIG. 2).

The optical rotator of this embodiment is mounted on a CD / DVD compatible optical head device, and has a wavelength of 7 for CD.
Optical rotation characteristics were measured for light of 80 nm and light of a wavelength of 660 nm for DVD. As a result, for the linearly polarized light of 660 nm, the angle of rotation of the plane of polarization was 46 °, and the linear maintenance performance was 0.04 in terms of ellipticity. On the other hand, for linearly polarized light of 790 nm, the angle of rotation of the polarization plane was 40 ° and the ellipticity was 0.12. At both wavelengths, good optical rotation characteristics and straight line maintaining performance were exhibited. Here, the ellipticity is one of the parameters indicating the polarization state of light, and is expressed as a ratio between the short-axis component and the long-axis component of the electric field intensity of the elliptically polarized light. If it is 1, it is circularly polarized light.

Further, the plurality of optical rotators 1 produced in this example
When the transmitted wavefront aberration of No. 01 was measured, each element had a low value of 0.01λ _rms (root mean square deviation) or less at a wavelength of 633 nm. This value is smaller and more stable than the conventional value of 0.015λ _rms of an optical element in which a polycarbonate or the like that induces birefringence is sandwiched by glass or the like.

"Example 2" The optical rotator 101 produced in Example 1
Was installed between the collimating lens 4 and the objective lens 5 in the optical head device as shown in FIG. 66 of DVD system
The two types of linearly polarized light emitted from the semiconductor laser 1A that oscillates linearly polarized light in the 0 nm wavelength band (λ ₁ ) and the semiconductor laser 1B that oscillates linearly polarized light in the CD system 790 nm wavelength band (λ ₂ ) are the optical rotator 101. From the side of the transparent substrate 13A (see FIG. 1).

The semiconductor lasers 1A and 1B were set so that the planes of polarization of the respective linearly polarized lights became parallel. The phase shifter 101 was installed in the optical head device such that the polarization plane and the orientation direction α _B of the polymer liquid crystal on the exit side of the polymer liquid crystal film 11 constituting the optical rotator 101 were parallel (see FIG. 2). ).

In the optical head device configured as described above, the light in the 660 nm wavelength band and the light in the 790 nm wavelength band condensed on the optical disk 6 are both linearly polarized light and have parallel polarization directions. As a result, 1 / rotating the plane of polarization by 45 ° with respect to the conventional single-wavelength linearly polarized light
Compared with the case where a two-wavelength plate is used, the optical head device of the present invention can reduce the fluctuation of the signal light with respect to both CD-based and DVD-based optical discs, and has excellent information recording / reproducing characteristics.

[0028]

As described above, the optical rotator of the present invention maintains a linearly polarized state of laser light in both wavelength bands for a CD optical disk and a DVD optical disk with one optical rotator. While rotating, the plane of polarization of the laser beam can be rotated by the same angle.

By mounting the optical rotator of the present invention on an optical head device, fluctuations of signal light due to an optical disk structure having polarization dependence such as birefringence remaining on the optical disk can be reduced, and the optical disk can reproduce information when reproducing information from the optical disk. An optical head device capable of performing stable signal detection with extremely few reading errors and writing errors during information recording can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an example of the configuration of an optical rotator according to the present invention.

FIG. 2 is a conceptual diagram showing how the optical rotator of FIG. 1 rotates the plane of polarization of linearly polarized light.

FIG. 3 is a graph showing a relationship between a coefficient E and an optical rotation angle φ.

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

[Explanation of symbols]

101: optical rotator 11: polymer liquid crystal film 12A, 12B: alignment film 13A, 13B: transparent substrate 1A, 1B: semiconductor laser 2, 3: beam splitter 4: collimating lens 5: objective lens 6: optical disk 8: photodetector P ₁ , P ₂ : linearly polarized light φ: optical rotation angle d: film thickness of polymer liquid crystal film λ: wavelength of incident light α _A , α _B : orientation direction

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H049 BA08 BA46 BB03 BB62 BC02 BC09 BC21 2H088 EA61 GA06 HA15 KA11 5D119 AA41 AA43 BA01 BB01 BB03 DA01 DA05 EC35 EC45 EC47 FA05 JA12 JA30 JA64

Claims (2)

[Claims] _1. An optical rotator to which linearly polarized light P _{1 having a} wavelength λ ₁ and linearly polarized light P ₂ (λ ₁ <λ ₂ ) having a wavelength λ ₂ are incident. The optical rotator has an extraordinary refractive index and an ordinary light refraction on a transparent substrate. the difference between the rates is formed a polymer liquid crystal film having a value of linear polarization P ₁ to 0.05 to 0.25, the orientation direction of the polymer liquid crystal and parallel to the film surface of the polymer liquid crystal film Linearly polarized light P ₁ that is spirally twisted in the film and transmits through the polymer liquid crystal film
And rotator polarization plane of the linearly polarized light P ₂ is equal to or to substantially the same angle. 2. A light source for emitting the wavelength lambda ₁ of the linearly polarized light P ₁ and wavelength lambda ₂ of the linearly polarized light _{P 2 (λ 1 <λ 2} ), the emitted linearly polarized light P ₁ and linear polarization P ₂ the optical recording An optical head device for recording / reproducing information on / from an optical recording medium, comprising: an objective lens for converging light on a medium; an optical head comprising the optical rotator according to claim 1 in an optical path between the light source and the objective lens. apparatus.