JP4626026B2 - Optical head device - Google Patents

Description

[0001]
The present invention relates to an optical head device.
[0002]
[Prior art]
In an optical head device for recording / reproducing information on / from an optical recording medium such as an optical disk such as a CD or DVD and a magneto-optical disk, light emitted from a semiconductor laser as a light source is condensed on the optical recording medium by a lens. It is reflected by the recording medium and becomes return light. The 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 / reproducing information on CD and DVD optical disks, which are optical recording media having different standards, with the same optical head device. In an optical disc that uses a medium having a high wavelength dependency with respect to reflection / absorption of light as a recording layer of an optical recording medium and is premised on reproduction such as a CD-R, the semiconductor laser used for the CD has a wavelength of 790 nm. is there. At this time, a semiconductor laser having a wavelength band of 660 nm is used for DVD.
[0004]
In addition, 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 in order to improve the light utilization efficiency, and in order to further improve the recording / reproducing performance. In some cases, linearly polarized light with a polarization plane that makes a certain angle with respect to the track direction of the optical disk is condensed.
[0005]
[Problems to be solved by the invention]
Usually, both CD and DVD optical discs using two linearly polarized light whose planes of polarization are rotated by a phaser having a desired retardation value for light in any wavelength band for CD and DVD systems. When information is recorded / reproduced, the one optical disk exhibits good characteristics but the other does not exhibit good characteristics.
[0006]
Specifically, for example, when using a phase shifter that functions as a half-wave plate for light in the wavelength band for DVD, linear polarization of the wavelength band for DVD rotates the plane of polarization by a desired value. However, it does not function as a half-wave plate for 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 the CD system does not function as a half-wave plate for light in the wavelength band for the DVD system.
[0007]
When there is an unfavorable birefringence distribution on the optical disk, the intensity fluctuation of the reflected return light from the optical disk is elliptically polarized compared to the linearly polarized light when the reflected return light is separated by the polarization beam splitter and collected on the photodetector. Is bigger. Therefore, when the above phaser is used, there is a problem that information cannot be recorded / reproduced in the optical head device.
[0008]
For this reason, optical elements such as an optical rotator are used as a phase shifter capable of maintaining the linearity of the rotation angle of the polarization plane after transmission through the phase shifter for linear polarization in both wavelength bands of the CD system and the DVD system. It was sought after.
[0009]
[Means for Solving the Problems]
The present invention has been made in order to solve the above-mentioned problems, and linearly polarized light P ₁ having a wavelength λ ₁ that is a 660 nm wavelength band of a DVD system and linearly polarized light P ₁ having a wavelength λ _{2 that} is a 790 nm wavelength band of a CD system. comprising a light source for emitting _2, an objective lens for focusing the linearly polarized light P ₁ and the linear polarization P ₂ to the optical recording medium, and a photodetector for detecting the reflected return light by the optical recording medium in the optical head device, between the light source and the objective lens comprises a polarization rotator in an optical path between the linearly polarized light P ₁ and the linear polarization P ₂ is common, the polarization rotator, the alignment layer on one surface A pair of transparent substrates, and a polymer liquid crystal film sandwiched between the alignment films of the pair of transparent substrates whose orientation directions cross each other. The polymer liquid crystal film is obtained by polymerizing and curing a liquid crystal monomer. Made of polymer liquid crystal, the linearly polarized light The difference Δn between the extraordinary refractive index and the ordinary refractive index with respect to ₁ has a value of 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 Further, the optical rotator is twisted spirally in the film, and the optical rotator can be rotated so that the polarization planes of the linearly polarized light P ₁ and the linearly polarized light P ₂ transmitted through the polymer liquid crystal film can be rotated by substantially the same angle. Provided is an optical head device that is an optical rotator designed based on a wavelength λ that satisfies the relationship ₁ <λ <λ ₂ .
[0010]
Further, the polymer liquid crystal film, [Delta] n provides the above optical head device having a value of 0.11 with respect to the linearly polarized light P _1.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, an optical rotator 101 used in the optical head device of the present invention has a configuration in which a polymer liquid crystal film 11 is sandwiched between transparent substrates 13A and 13B. FIG. 1 also shows a state in which the linearly polarized light having the wavelength λ is transmitted through the optical rotator 101 and is emitted by rotating its polarization plane by the optical rotation angle φ. The optical rotator 101 is produced as follows.
[0012]
A liquid crystal monomer, which is a birefringent material, is applied to the alignment film 12A on the transparent substrate 13A after coating the alignment film on the transparent substrates 13A and 13B and applying the desired alignment treatment to the alignment films 12A and 12B. Apply the solution. Next, the transparent substrate 13B is overlapped so that the alignment treatment direction of the alignment film 12A and the alignment treatment direction of the alignment film 12B intersect each other.
[0013]
At this time, the molecular direction of the liquid crystal monomer is aligned with the alignment treatment direction at the position where the alignment films 12A and 12B are in contact with each other, and is gradually rotated in the thickness direction of the liquid crystal layer to be aligned. In other words, the alignment films 12A and 12B are aligned with each other in the crossing direction. Finally, the polymer liquid crystal film 11 is formed by being cured by being irradiated with light source light for photopolymerization. The alignment direction of the polymer liquid crystal is parallel to the film surface of the polymer liquid crystal film, and coincides with each alignment treatment direction at a position in contact with the alignment films 12A and 12B. Therefore, the orientation direction of the polymer liquid crystal is twisted in a spiral manner in the polymer liquid crystal film 11.
[0014]
Further, as the polymer liquid crystal film 11, the difference Δn between the extraordinary refractive index and the ordinary refractive index takes a value of 0.05 to 0.25. If Δn is smaller than 0.05, the film thickness d of the polymer liquid crystal film is increased, which leads to poor alignment of the polymer liquid crystal. 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 lowered. That is, Δn is preferably from 0.05 to 0.25 from the viewpoint of the orientation of the polymer liquid crystal and the productivity of the polymer liquid crystal film.
[0015]
FIG. 2 shows a state in which linearly polarized light having a wavelength λ is incident on the rotatory polymer liquid crystal film 11 used in the optical head device of the present invention, and its polarization plane is rotated. In FIG. 2, when the alignment direction of the polymer liquid crystal on the light incident side of the polymer liquid crystal film 11 is α _A and the alignment direction on the light output side is α _B , the polarization direction of the incident linearly polarized light and the alignment direction on the output side By aligning with α _B , a desired optical rotation characteristic is exhibited with respect to linearly polarized light of wavelength λ, and the optical rotation angle φ is the difference between the incident-side orientation direction α _A and the output-side orientation direction α _B | α _A −α _B |
[0016]
The film thickness d of the polymer liquid crystal film 11 that can maintain the linearity of the linearly polarized light transmitted through the optical rotator is determined from the relational expression d = λ / (Δn · E). Here, as shown in FIG. 3, the coefficient E is given as a function of the optical rotation angle φ. When φ = 90 °, E (90 °) ≈1.0, and when φ = 45 °, E (45 °) ≈2.3.
[0017]
Further, as the polarization rotator used for the optical head device of the present invention, since the linear polarization P ₂ of the linearly polarized light P ₁ and wavelength lambda ₂ wavelength _{λ 1 (λ 1 <λ 2} ) are incident, as described above rotator The wavelength λ used when designing is set as λ ₁ <λ <λ ₂ . By limiting the wavelength λ in this way, the optical rotator can rotate the plane of polarization substantially at the same angle while maintaining the linearity even if the wavelengths are different with respect to the two different types of linearly polarized light. Here, “substantially the same angle” means that the two angles should be the same, but a deviation that provides the same effect even if they are different falls within this range. Specifically, if the deviation is 10 ° or less, the angle is the same.
[0018]
In order to maintain good transmitted wavefront aberration with respect to temperature change, it is preferable to use an optically flat substrate made of an inorganic material such as a glass substrate as the transparent substrate. In addition, a diffraction grating may be formed on the surface of the transparent substrate using a technique such as photolithography or etching to develop a function of diffracting light.
[0019]
In the optical head device of the present invention shown in FIG. 4, the linearly polarized light beams having different wavelengths emitted from the semiconductor lasers 1A and 1B are reflected by the beam splitters 2 and 3, respectively, then transmitted through the optical rotator 101, and converted into parallel light by the collimating lens 4. . Before and after transmission through the optical rotator 101, the linearly polarized light having different wavelengths is condensed on the recording surface of the optical disk 6 by the objective lens 5 after the polarization direction is rotated by an angle φ. The return light reflected on the recording surface of the optical disk 6 is converted again into parallel light by the objective lens 5 and is condensed on the photodetector 8 through the collimating 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 matched with the forward polarization direction.
[0020]
In the optical head device of the present invention, since the optical rotator of the present invention is used, fluctuations in the intensity of the reflected signal light from the optical disk due to an undesirable birefringence distribution remaining on the optical disk can be reduced, and stable information Recording and playback are possible. Further, linearly polarized light whose polarization plane is rotated in a direction in which recording / reproducing performance is good can be incident on a recording optical disc having a land / groove structure. In addition, since the fluctuation of the optical rotation characteristic depending on the incident angle is small as compared with a conventional optical rotator using quartz, stable information recording / reproducing characteristics can be obtained.
In the above description, 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】
"Example 1"
This example is a specific example of the optical rotator 101 shown in FIG. 1, and is an optical rotator having an optical rotation angle φ of 45 °.
On the glass transparent substrates 13A and 13B having a refractive index of 1.5, a polyimide film for an alignment film was applied and subjected to an alignment process by rubbing to obtain alignment films 12A and 12B. Next, a solution of a liquid crystal monomer that is a birefringent material is applied to the alignment film 12A on the transparent substrate 13A so that the alignment treatment direction of the alignment film 12A and the alignment treatment direction of the alignment film 12B intersect at 45 °. The transparent substrate 13B was stacked and polymerized and cured by irradiating light source light for photopolymerization to form a polymer liquid crystal film 11, and an optical rotator 101 was produced. Further, as the polymer liquid crystal film 11, a film having a difference Δn between the extraordinary light refractive index and the ordinary light refractive index of 0.11, and the film thickness d was set to 2.7 μm.
[0022]
The optical rotator 101 produced as described above is linearly polarized light having a wavelength of 685 nm and incident from the transparent substrate 13A side, and maintains its linearity when the plane of polarization is parallel to the alignment direction of the alignment film 12B. The optical rotator rotates the plane of polarization by 45 ° (see FIG. 2).
[0023]
The optical rotator of this example was mounted on a CD / DVD compatible optical head device, and the optical rotation characteristics were measured for light with a wavelength of 780 nm for CD and light with a wavelength of 660 nm for DVD. As a result, for the 660 nm linearly polarized light, the optical rotation angle of the plane of polarization was 46 °, and the straight line maintenance performance was 0.04 when expressed by ellipticity. On the other hand, for 790 nm linearly polarized light, the optical rotation angle of the polarization plane was 40 ° and the ellipticity was 0.12. Good optical rotation characteristics and straight line maintenance performance were exhibited at both wavelengths. 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 the ellipticity is 0, linear polarization, If it is 1, it is circularly polarized light.
[0024]
Furthermore, when the transmitted wavefront aberrations of the plurality of optical rotators 101 prepared in this example were measured, all the elements had low values of 0.01λ _rms (root mean square deviation) or less at a wavelength of 633 nm. This value is small and stable compared to the value 0.015λ _rms of the conventional optical element in which polycarbonate or the like in which birefringence is induced is sandwiched between glass or the like.
[0025]
"Example 2"
The optical rotator 101 produced in Example 1 was placed between the collimating lens 4 and the objective lens 5 in the optical head device as shown in FIG. Two types of linearly polarized light emitted from a semiconductor laser 1A that oscillates linearly polarized light in the 660 nm wavelength band (λ ₁ ) of the DVD system and a semiconductor laser 1B that oscillates linearly polarized light in the 790 nm wavelength band (λ ₂ ) of the CD system are: The light enters the optical rotator 101 from the side of the transparent substrate 13A (see FIG. 1).
[0026]
The semiconductor lasers 1A and 1B were installed so that the polarization planes of the respective linearly polarized light were parallel. The phase shifter 101 is installed in the optical head device so that the polarization plane and the alignment direction α _B of the polymer liquid crystal on the exit side of the polymer liquid crystal film 11 constituting the optical rotator 101 are parallel to each other (see FIG. 2). ).
[0027]
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 collected on the optical disk 6 are both linearly polarized light and the polarization directions thereof are parallel. As a result, the optical head device of the present invention can be applied to both CD and DVD optical discs as compared to the case of using a half-wave plate that rotates a 45 ° polarization plane with respect to the conventional linearly polarized light of a single wavelength. Thus, fluctuations in signal light can be reduced, and good information recording / reproducing characteristics have been demonstrated.
[0028]
【The invention's effect】
As described above, the optical rotator used in the optical head device of the present invention maintains the state of linear polarization with respect to laser light in both wavelength bands for CD optical disks and DVD optical disks with a single optical rotator. However, the polarization plane of the laser light can be rotated by the same angle.
[0029]
By mounting the optical rotator of the present invention on an optical head device, fluctuations in signal light due to polarization-dependent optical disk structures such as birefringence remaining on the optical disk can be reduced, and read errors and errors during optical disk information reproduction can be reduced. An optical head device capable of performing stable signal detection with extremely few writing errors during information recording can be realized.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an example of the configuration of an optical rotator used in an optical head device of the present invention.
FIG. 2 is a conceptual diagram showing how the optical rotator of FIG. 1 rotates the polarization plane 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 of 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: Collimator lens 5: Objective lens 6: Optical disk 8: Photo detector 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

Claims (2)

A light source that emits linearly polarized light P ₁ having a wavelength λ _{1 in} the 660 nm wavelength band of the DVD system and linearly polarized light P ₂ having a wavelength λ _{2 in} the 790 nm wavelength band of the CD system, the linearly polarized light P _1, and the linearly polarized light P ₂ In an optical head device comprising: an objective lens that focuses light onto an optical recording medium; and a photodetector that detects return light reflected by the optical recording medium.
Between the light source and the objective lens comprises a polarization rotator in an optical path between the linearly polarized light P ₁ and the linear polarization P ₂ is common,
The optical rotator has a pair of transparent substrates having an alignment film on one surface, and a polymer liquid crystal film sandwiched between the alignment films of the pair of transparent substrates whose alignment directions intersect,
The polymer liquid crystal film is composed of a polymer liquid crystal obtained by polymerizing and curing a liquid crystal monomer, and a difference Δn between an extraordinary refractive index and an ordinary refractive index with respect to the linearly polarized light P ₁ is 0.05 to 0.25. And the orientation direction of the polymer liquid crystal is parallel to the film surface of the polymer liquid crystal film and twisted in a spiral in the film,
Further, the optical rotator satisfies the relationship of λ ₁ <λ <λ ₂ so that the polarization planes of the linearly polarized light P ₁ and the linearly polarized light P ₂ that pass through the polymer liquid crystal film can be rotated by substantially the same angle. An optical head device that is an optical rotator designed based on a wavelength λ . 2. The optical head device according to claim 1, wherein the polymer liquid crystal film has a value of Δn of 0.11 with respect to the linearly polarized light P ₁ .

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