JP4622160B2 - Diffraction grating integrated optical rotator and optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diffraction grating integrated optical rotator and an optical head device, and more particularly to a diffraction grating integrated optical rotator using a polymer liquid crystal and an optical head device for recording and reproducing information on an optical recording medium.
[0002]
[Prior art]
In an optical head device that records and reproduces information on an optical recording medium (hereinafter referred to as an “optical disk”) such as a CD, a DVD, and a magneto-optical disk, light emitted from the semiconductor laser is condensed on the optical recording medium by a lens. The condensed emitted light is reflected by the optical recording medium and becomes return light. The outgoing light that has become the return light is guided to the light receiving element by the beam splitter, and the information on the optical recording medium is converted into an electrical signal.
[0003]
Further, in the optical head device, the polarization state of the polarization plane of the light from the semiconductor laser is controlled using elements such as a half-wave plate, a quarter-wave plate, and a polarization beam splitter, and the light use efficiency And the performance of recording and reproduction can be improved.
[0004]
[Problems to be solved by the invention]
In an optical head device, a diffraction grating for tracking servo of an optical disk and a half-wave plate for controlling the polarization direction may be mounted. In order to reduce the size of the optical head device and improve productivity, it is desired to integrate a diffraction grating and a half-wave plate. However, in assembling the optical head device, there is a step of rotating the diffraction grating around the optical axis to match the diffracted light with the track on the optical disk. That is, when the diffraction grating and the half-wave plate are integrated, the half-wave plate is also rotated together with the diffraction grating in the above process, so that light having a desired polarization plane direction can be obtained as the element outgoing light. I could not.
[0005]
Therefore, as an optical element to be used in place of the half-wave plate, there has been a demand for an optical element that rotates the polarization plane by a desired value without largely changing the polarization plane direction with respect to the rotation around the optical axis. .
[0006]
[Means for Solving the Problems]
The present invention has been made in order to solve the above-described problems, and includes a transparent substrate, a diffraction grating having an uneven cross-sectional shape formed on the transparent substrate, and a diffraction grating on the opposite side of the diffraction grating on the transparent substrate. And an optical rotator composed of a polymer liquid crystal film formed on a surface, wherein the orientation direction of liquid crystal molecules in the optical rotator is twisted about an axis in the thickness direction of the liquid crystal film.
[0007]
A light source that emits linearly polarized light; an objective lens that condenses the light emitted from the light source on the optical recording medium; and a detector that detects the outgoing light collected by the objective lens and reflected by the optical recording medium. An optical head device comprising the above-described diffraction grating integrated optical rotator is provided in an optical path between a light source and an objective lens.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
As an example of the diffraction grating integrated optical rotator of the present invention, as shown in FIG. 1, the diffraction grating having a concavo-convex cross section formed on the transparent substrate 11 and the diffraction grating of the transparent substrate 11 opposite to the diffraction grating are provided. A polymer liquid crystal film 13 is integrally formed on the surface with the alignment film 12A interposed therebetween. The alignment direction of the liquid crystal molecules of the polymer liquid crystal film 13 is twisted around the axis of the thickness direction of the liquid crystal film. Further, the transparent substrate 14 is superimposed on the polymer liquid crystal film 13 with the alignment film 12B interposed therebetween, and the diffraction grating integrated optical rotator 101 is configured.
In FIG. 1, linearly polarized light having a wavelength λ is incident on and transmitted through the diffraction grating integrated optical rotator 101 from above in the paper, thereby rotating the polarization plane by the optical rotation angle φ and generating and emitting diffracted light. Show.
[0009]
The diffraction grating integrated optical rotator 101 is manufactured as follows. As the transparent substrate 11, an optically isotropic medium such as glass or quartz glass can be used. The diffraction grating formed on the transparent substrate 11 is produced by processing quartz glass formed on the glass surface using photolithography and etching techniques when glass is used as the material of the transparent substrate 11. On the other hand, when quartz glass is used as the material of the transparent substrate 11, it may be produced directly on the quartz glass substrate using photolithography and etching techniques. The grating plane pattern of the diffraction grating may be a straight simple grating or a curved hologram grating, and can be selected depending on the application.
[0010]
After the alignment film is applied on the transparent substrate 11 and the transparent substrate 14 on which the diffraction gratings are formed as described above and subjected to a desired alignment treatment to form the alignment films 12A and 12B, the alignment film on the transparent substrate 14 is obtained. A solution of a liquid crystal monomer that is a birefringent material is applied to 12B. Next, the transparent substrate 11 on which the diffraction grating is formed is overlaid so that the alignment treatment direction of the alignment film 12A and the alignment treatment direction of the alignment film 12B intersect each other.
[0011]
At this time, the monomer molecular direction of the liquid crystal monomer is aligned with each alignment treatment direction at a position in contact with the alignment films 12A and 12B, and is gradually rotated (twisted) around the axis in the thickness direction of the liquid crystal layer. And align. That is, the direction of the monomer molecules matches the alignment processing direction at the position in contact with the alignment films 12A and 12B. Finally, the polymer liquid crystal film 13 is obtained by being polymerized and cured by irradiation with light source light for photopolymerization. The alignment direction of the liquid crystal molecules of the polymer liquid crystal film is parallel to the film surface of the 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 13.
[0012]
Moreover, it is preferable to use the polymer liquid crystal film 13 having a difference Δn between the extraordinary refractive index and the ordinary refractive index of 0.05 to 0.25 for the following reason. When Δn is smaller than 0.05, the film thickness d of the polymer liquid crystal film is increased, and alignment defects of the polymer liquid crystal are liable to occur. On the other hand, if Δn is greater than 0.25, the film thickness d must be reduced to 1 to 3 μm in order to obtain an appropriate retardation value, which may reduce productivity.
[0013]
FIG. 2 shows a state in which linearly polarized light having a wavelength λ is incident from the back right side of the figure and is transmitted to the left front side when the polarization plane is rotated to the polymer liquid crystal film 13 of the optical rotator according to the present invention. ing. In FIG. 2, the alignment direction of the liquid crystal molecules on the light incident side of the polymer liquid crystal film 13 is α _A , and the alignment direction on the light emission side is α _B. At this time, by aligning the polarization direction of the incident linearly polarized light with the orientation direction α _A on the incident side (horizontal direction in FIG. 2), a desired optical rotation characteristic is exhibited with respect to the linearly polarized light of wavelength λ, and its 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 exit side.
[0014]
The film thickness d of the polymer liquid crystal film 13 that can maintain the linearity of the linearly polarized light that passes through the optical rotator can be obtained 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≈1.13, when φ = 70 °, E≈1.06, φ When = 45 °, E≈1.01. Here, ≒ means almost equal.
[0015]
The diffraction grating integrated optical rotator of the present invention has almost no polarization plane of incident linearly polarized light even when the element is rotated around the optical axis when adjusting the direction of diffracted light by the diffraction grating formed on the element. The desired value can be maintained without rotating. Therefore, according to the present invention, an element in which the diffraction function and the optical rotation function are integrated can be realized. Further, since it is necessary 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.
[0016]
FIG. 4 is a side view showing an example of an optical head device of the present invention on which the above-described diffraction grating integrated optical rotator 101 is mounted. In the optical head device of the present invention, the linearly polarized light emitted from the semiconductor laser 1 as the light source passes through the diffraction grating integrated optical rotator 101 and the polarization beam splitter 2 in this order, and then becomes parallel light by the collimator lens 3. The light is condensed on the information recording surface of the optical disk 5. Here, since the light passes through the diffraction grating integrated optical rotator 101, the polarization direction is rotated by the angle φ by the optical rotator part (polymer liquid crystal film) of the diffraction grating integrated optical rotator 101, and the diffraction grating part. Is divided into a plurality of lights (not shown). The light reflected on the information recording surface of the optical disk 5 becomes parallel light again by the objective lens 4, and is condensed on the photodetector 6 through the collimator lens 3 and the polarization beam splitter 2.
[0017]
In the optical head device of the present invention, since the diffraction grating integrated optical rotator 101 in which the diffraction gratings are integrated is used, the number of components can be reduced as compared with the conventional optical head device. Also, when adjusting the condensing position of the diffracted light used for tracking of the optical disk generated by the diffraction grating part on the optical disk, the polarization direction of the polarization plane of the emitted light is adjusted even if the diffraction grating integrated optical rotator is rotated. It is constant. As a result, it is possible to obtain a stable amount of outgoing light from the polarization beam splitter 2 and to record and reproduce information stably. In addition, the optical head device of the present invention has high productivity because it has a small number of components and can be easily assembled and adjusted.
[0018]
【Example】
"Example 1"
This example is a specific example of the diffraction grating integrated optical rotator 101 shown in FIG. 1, and is an optical rotator having an optical rotation angle φ of 70 °.
[0019]
The surface of the glass transparent substrate 11 having a refractive index of 1.5 was processed, and a simple rectangular diffraction grating having an uneven cross-sectional shape and a period of 20 μm was formed using photolithography and etching. Next, a polyimide film for an alignment film was applied to the transparent substrate 11 and the transparent substrate 14 (refractive index 1.5), and an alignment process was performed 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 12B on the transparent substrate 14, and the alignment treatment direction of the alignment film 12A and the alignment treatment direction of the alignment film 12B cross each other at 70 °. The transparent substrate 11 was stacked and polymerized and cured by irradiating light source light for photopolymerization to form a polymer liquid crystal film 13 to produce a diffraction grating integrated optical rotator 101. In the polymer liquid crystal film 13, liquid crystal molecules are twisted around an axis in the thickness direction. Moreover, as the polymer liquid crystal film 13, a film having a difference Δn between the extraordinary refractive index and the ordinary refractive index of 0.067 was used, and the film thickness d was set to 5.7 μm.
[0020]
The diffraction grating integrated optical rotator 101 manufactured as described above has linearity when light incident from the transparent substrate 14 side is linearly polarized light having a wavelength of 405 nm and its polarization plane is parallel to the alignment direction of the alignment film 12B. This functioned as an optical rotator that rotated the plane of polarization by 70 °. In addition, the diffraction grating integrated optical rotator of this example is insensitive to the rotational shift around the optical axis, and the rotation angle of the polarization plane of linearly polarized light is 70 ± 0.1 with respect to the rotation of the element of ± 5 °. °.
[0021]
By adjusting the direction of the diffracted light by the diffraction grating formed on the element by preparing the diffraction grating integrated optical rotator as described above, even if this element is rotated around the optical axis, the incident linearly polarized light The angle at which the plane of polarization of the light rotates does not change and can maintain a desired value. Therefore, according to the present invention, an element in which the diffraction function and the optical rotation function are integrated can be realized.
[0022]
Furthermore, when the transmitted wavefront aberration of the plurality of diffraction grating integrated optical rotators 101 prepared in this example was 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.
[0023]
"Example 2"
As shown in FIG. 4, the diffraction grating integrated optical rotator 101 produced in Example 1 was installed between the semiconductor laser 1 and the polarization beam splitter 2 of the optical head device. Light emitted from the semiconductor laser 1 that oscillates linearly polarized light having a wavelength of 405 nm is incident on the diffraction grating integrated optical rotator 101 from the transparent substrate 14 side with the polarization plane direction coincident with the alignment direction of the alignment film 12B (FIG. 1, FIG. (See FIG. 2).
[0024]
When the optical head device is configured in this way, the diffraction grating integrated optical rotator 101 is rotated by ± 3 °, and the diffracted light from the diffraction grating formed on the diffraction grating integrated optical rotator 101 is reflected on the optical disk 5. The condensing position at was adjusted. At this time, the polarization plane direction of the outgoing light from the diffraction grating integrated optical rotator 101 was approximately 70 ° with respect to the incident polarized light. As a result, the variation in the amount of light of the linearly polarized light emitted from the diffraction grating integrated optical rotator 101 after passing through the polarizing beam splitter 2 can be suppressed to within ± 1.5%, and excellent information recording and reproducing characteristics are exhibited. It was. Further, in the optical head device of this example, the number of components is small and assembly adjustment can be easily performed.
[0025]
【The invention's effect】
As described above, the diffraction grating integrated optical rotator of the present invention is capable of adjusting rotation of the diffracted light by the diffraction grating formed on the transparent substrate constituting the element, while adjusting the rotational position around the optical axis. Does not change the polarization plane direction.
[0026]
By mounting the diffraction grating integrated optical rotator of the present invention on an optical head device, the rotation margin around the optical axis of the element is increased when the element is mounted, and the adjustment of the characteristics can be facilitated to improve the productivity. In addition, since the diffraction grating integrated optical rotator of the present invention integrates the diffraction function and the optical rotation function, the number of parts of the optical head device can be reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an example of the configuration of a diffraction grating integrated optical rotator of the present invention.
FIG. 2 is a conceptual diagram showing how the optical rotator in the present invention 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 according to the embodiment.
FIG. 4 is a side view showing an example of the configuration of the optical head device of the present invention.
[Explanation of symbols]
101: diffraction grating integrated optical rotator 11: transparent substrate 12A, 12B: alignment film 13: polymer liquid crystal film 1: semiconductor laser 2: polarizing beam splitter 3: collimating lens 4: objective lens 5: optical disk 6: photodetector

Claims (5)

  An optical rotation comprising a transparent substrate, a diffraction grating having an uneven cross-sectional shape formed on the transparent substrate, and an optical rotator made of a polymer liquid crystal film formed on a surface opposite to the diffraction grating on the transparent substrate. A diffraction grating integrated optical rotator in which the orientation direction of liquid crystal molecules in the optical element is twisted around the axis of the thickness direction of the liquid crystal film. The film thickness d of the polymer liquid crystal film is expressed by the following equation, where Δn is the difference between the extraordinary refractive index and the ordinary light refractive index of the polymer liquid crystal film, and λ is the wavelength of incident light. 1. A diffraction grating integrated optical rotator according to 1.
          d = λ / (Δn · E) where 1 <E ≦ 1.13 The polymer liquid crystal film includes a first transparent substrate on which a first alignment film is formed, and a second transparent substrate on which a second alignment film is formed on a surface opposite to the surface on which the diffraction grating is formed. 3. The diffraction grating integrated optical rotator according to claim 1, wherein the orientation direction of the polymer liquid crystal is different between the first alignment film surface and the second alignment film surface. 4. The diffraction grating integrated optical rotator according to claim 1, wherein the polymer liquid crystal film is obtained by polymerizing and curing liquid crystal monomer molecules by light irradiation. Light comprising: a light source that emits linearly polarized light; an objective lens that condenses the light emitted from the light source on the optical recording medium; and a detector that detects the emitted light collected by the objective lens and reflected by the optical recording medium. 5. An optical head device, wherein the diffraction grating integrated optical rotator according to claim 1 is installed in an optical path between a light source and an objective lens.

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