JP2004234757A - Optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a phase plate whose wave front aberration is small even under conditions of high temperature and high humidity and to make an optical head device to be an optical head device whose optical characteristics are excellent even under conditions of high temperature and high humidity by mounting the phase plate in the optical head device. <P>SOLUTION: The phase plate is formed by stacking respectively resin birefringent layers 201 and 202 having respectively a high-molecular liquid crystal layer between contiguous glass substrates of glass substrates 101, 102 and 103 which are optically isotropic substrates and this phase plate is installed in an optical path existing among the light sources and the objective lens of the optical head device. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device, and more particularly to an optical head device equipped with a phase plate for controlling the polarization state of laser light from a light source.
[0002]
[Prior art]
An optical head device is used for writing optical information on an optical recording medium such as an optical disk and a magneto-optical disk and reading optical information from the optical recording medium. For example, as shown in the conceptual diagram of the optical head device of the present invention in FIG. 6, the optical head device is a semiconductor laser that is a light source 5 or 6 on a recording surface of a disk-shaped optical recording medium (hereinafter referred to as optical disk 3). In the case of the light source 6, the emitted light is changed in the traveling direction by the beam splitter prism 4, and then condensed by using the objective lens 2 to write / read information. Here, the light receiving system is omitted for simplicity.
[0003]
As the semiconductor laser that is the light source, CD and DVD that are two types of optical disks require semiconductor lasers that emit two different wavelengths of 790 nm and 660 nm, respectively. It is necessary to arrange a quarter-wave plate at the position of the laminated phase plate 1 in the present invention in a common optical path through which light of two different wavelengths emitted from the light sources 5 and 6 passes. Conventionally, as a wave plate used in such an arrangement, two phase plates having birefringence are laminated with their optical axis directions different (intersecting) to form a quarter wave plate at two different wavelengths. Broadband wave plates that function as:
[0004]
[Patent Document 1]
JP-A-11-149015 [0005]
[Problems to be solved by the invention]
In this conventional example, as shown in FIG. 5, the wave plate has two birefringent films 209 and 210 interposed between two glass substrates 109 and 110 with three adhesive layers 307, 308, and 309 interposed therebetween. And had a laminated structure. However, when the birefringent film is made of resin, there are two resin films and three adhesive layers between the two glass substrates, and the thickness of the resin layer is increased in total.
[0006]
As described above, when the resin layer is sandwiched thickly between the glass substrates, there are three problems. The first problem is reliability. When the wave plate is stored under high temperature and high humidity, a large amount of moisture is contained due to the thick resin layer. Thereby, the transmitted wavefront aberration of the element (wavelength plate) is deteriorated. The second problem is that distortion occurs when these resin layers are laminated with an adhesive, thereby deteriorating wavefront aberration and causing a reduction in production yield. The third problem is that when the element (wavelength plate) is cut or laminated together with other optical components, the soft resin layer is thick, so that the element is easily distorted and the wavefront aberration is deteriorated.
[0007]
These second and third problems are considered to be because the relatively soft resin layer is thick and easily distorted. The present invention has been made to solve these problems of the prior art.
[0008]
[Means for Solving the Problems]
The present invention includes a light source, an objective lens that condenses laser light emitted from the light source onto an optical recording medium, a photodetector that detects laser light reflected by the optical recording medium, and a light source and an objective lens. Alternatively, in an optical head device including a phase plate that controls the polarization state of laser light in the optical path between the photodetector and the objective lens, the phase plate is composed of two or more resin birefringent layers whose optical axis directions intersect each other. And an optically isotropic substrate that is transparent and has lower moisture permeability than the resin birefringent layer is disposed between the laminated resin birefringent layers. A head device is provided.
[0009]
The resin birefringent layer and the optically isotropic substrate are alternately laminated to form a laminate, and both outer sides of the laminate are constituted by the optically isotropic substrate. An optical head device is provided.
[0010]
The above-described optical head device is provided in which at least one of the resin birefringent layers is a polymer liquid crystal film.
[0011]
Furthermore, there is provided the above optical head device in which the two resin birefringent layers are alternately laminated with three glass isotropic optical substrates and no adhesive material.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The phase plate in the optical head device of the present invention is an example in which a plurality of resin birefringent layers having different optical axis directions, that is, crossing the optical axis directions are laminated. A broadband wave plate that functions as a four-wave plate will be described.
[0013]
In the optical head device of the present invention shown in FIG. 6, the laser beams emitted from the light sources 5 and 6 are, for example, two laser beams having different wavelengths, and two laser beams, for example, are disposed between the light source 5 and the objective lens 2. A broadband laminated phase plate 1 for controlling the phase state is installed. This wide-band laminated phase plate has two birefringent resin birefringent layers stacked so as to intersect their optical axes, and the resin birefringent layer on which laser light first enters is laminated. The retardation value Rd ₁ is larger than the retardation value Rd _{2 of} the second incident birefringent resin layer, and the ratio of the retardation values, that is, Rd ₁ / Rd ₂ is 1.8 to 2.2. ing.
[0014]
Here, if the ratio is taken as the phase difference instead of the retardation value, the same value is obtained. That is, the phase difference can be expressed as Rd × 2π / λ using the retardation value Rd and the wavelength λ.
[0015]
With this configuration, the broadband phase plate functions as a substantially quarter-wave plate for linearly polarized laser light of any wavelength that is transmitted, and the linearly polarized light can be made substantially circularly polarized.
[0016]
In the present invention, for example, an optically isotropic substrate that is transparent and has lower moisture permeability than the resin birefringent layer is disposed between two resin birefringent layers. Here, the moisture permeability of the material can be known, for example, by measuring the water absorption rate of the material. That is, the mass of the material sample is measured, and then the material sample is immersed in distilled water for a certain period of time at a constant temperature to be hydrated. Then, the mass is measured again, and the amount of water absorption is known from the mass change before and after the immersion. The water absorption rate is obtained from the ratio between the mass change (water absorption amount) and the mass of the material sample before immersion.
[0017]
The resin birefringent material used for the laminated phase plate 1 is preferably a polymer liquid crystal polymerized in a state where liquid crystal is aligned, or a polycarbonate film having birefringence by stretching, for example. Furthermore, an organic polymer film having birefringence by stretching can also be used.
[0018]
Furthermore, as an optically isotropic substrate that is transparent and has low moisture permeability compared to the resin birefringent layer, a resin substrate such as an epoxy resin, a polyester resin, a fluorine resin, or a thermoplastic urethane resin is preferably used. . More preferably, a glass substrate is used, and the moisture permeability is much lower than that of the resin birefringent layer, and smoothness is easily secured. Hereinafter, a case where a glass substrate is used as the optically isotropic substrate will be described.
[0019]
An example of the laminated cross-sectional structure of the phase plate used in the optical head of the present invention is shown in FIG. In this example, a case where a resin film such as polycarbonate is used as the resin birefringent material will be described. As shown in FIG. 4, the adhesive layer 303, the phase plate 207, the adhesive layer 304 and the adhesive layer 305, the phase plate 208, and the adhesive layer 306 were laminated between the adjacent substrates of the three glass substrates 106, 107, and 108, respectively. . At this time, the thickness of the resin layer including the two adhesive layers is about 80 μm per layer.
[0020]
The optical axes of the two resin birefringent layers can be adjusted by the bonding angle, the phase difference is adjusted by using the resin birefringent layer of the film having the desired phase difference, and the characteristics of the phase plate after lamination Can be adjusted.
[0021]
The conventional configuration example using the resin birefringent layer of the film as described above is an adhesive layer 307, a film 209, an adhesive layer 308, a film 210, an adhesive between two glass substrates 109 and 110 as shown in FIG. The layer 309 was stacked. In this structure, the thickness of the resin birefringent layer including the adhesive layer was as extremely thick as about 160 μm. Hereinafter, the resin birefringent layer may be abbreviated as a birefringent layer.
[0022]
If it is thick, it absorbs moisture after storage under high temperature and high humidity, which degrades the wavefront aberration of the transmitted light, or the wavefront aberration yield due to distortion of the resin birefringent layer at the time of creation is very low and expensive. It was. Further, there is a problem that the phase plate is easily distorted by processing such as cutting the thick birefringent layer or attaching it to other optical components.
[0023]
According to the experiments by the present inventors, the deterioration of wavefront aberration after storage under high temperature and high humidity is improved to about 1/2 by inserting a glass substrate between two resin birefringent layers. I found out. Although the thickness of the entire birefringent layer is almost the same in the configuration of FIG. 4 of the present invention and the conventional configuration of FIG. 5, glass is obtained by dividing the birefringent layer into two and arranging a glass substrate in the middle. This is presumably because the substrate served as a moisture permeable barrier layer and the amount of moisture entering the birefringent layer was reduced.
[0024]
Moreover, as shown in FIG. 3, the structure which remove | eliminated the glass substrates 106 and 108 of the both sides of the structure in FIG. 4 may be sufficient. This is preferable because the entire thickness of the phase plate can be reduced and the weight can be reduced. In addition, it is preferable to perform antireflection coating (AR coating) on the surfaces of the resin birefringent layers 205 and 206 because surface reflection can be reduced. Reference numeral 105 denotes a glass substrate, and 301 and 302 denote adhesive layers.
[0025]
Next, an example in which a polymer liquid crystal is used as a material for the resin birefringent layer will be described. As shown in FIG. 1, polymer liquid crystal layers 201 and 202 are sandwiched between adjacent substrates of three glass substrates 101, 102, and 103, respectively. The polymer liquid crystal layer can be polymerized by injecting a low-molecular liquid crystal between two glass substrates whose surfaces are aligned and photopolymerizing with UV light or the like.
[0026]
In the example shown in FIG. 1, alignment processing is performed on both surfaces of the central glass substrate 102, and alignment processing is performed on the surfaces of the two glass substrates 101 and 103 on both sides facing the liquid crystal layer. Liquid crystal molecules are aligned in the direction of this alignment treatment. By setting this alignment direction to a desired angle, the angle formed by the optical axes of the two liquid crystal layers can be changed. Here, for example, the alignment treatment may be performed by applying polyimide to the surface of the glass substrate to form a thin film and rubbing, or rubbing the glass substrate directly without forming a polyimide thin film.
[0027]
The three glass substrates 101, 102, and 103 that have been subjected to the alignment treatment as described above are bonded together through a spacer having a size of several μm, liquid crystal is injected into the gap between the two, and polymerized by UV light to be polymerized. The two resin birefringent layers 201 and 202 may be laminated through a glass substrate without using an adhesive layer.
[0028]
The phase difference 2π (Δn × d) / λ of each birefringent layer can be obtained by adjusting the thickness (d) of the liquid crystal layer or changing the birefringence amount (Δn) of the resin birefringent layer. A phase difference can be obtained.
[0029]
As shown in FIG. 1, the thickness of the birefringent layer can be minimized by laminating two birefringent layers 201 and 202 of polymer liquid crystal without using an adhesive layer. This phase plate has a thin birefringent layer, which has been a problem in the past due to wavefront aberration degradation after storage under high temperature and high humidity, yield reduction due to wavefront aberration, phase plate cutting and other optical components. This is most preferable because distortion due to sticking hardly occurs.
[0030]
Further, by removing the two glass substrates 101 and 103 on both sides of the phase plate shown in FIG. 1, polymer birefringent resin layers 203 and 204 are laminated on both sides of the glass substrate 104 as shown in FIG. It is good also as a structure. This is preferable because the entire thickness of the phase plate can be reduced and the weight can be reduced. Further, it is preferable to apply a non-reflective coating (AR coating) to the surfaces of the resin birefringent layers 203 and 204 because surface reflection can be reduced.
Further, as a combination of two resin birefringent layers, one may be a polymer liquid crystal and the other a film birefringent layer.
[0031]
The relationship between the phase difference of these phase plates and the optical axis direction is as follows. In order to convert linearly polarized light into circularly polarized light as a quarter wavelength plate at two different wavelengths as described above, The phase difference of the refracting layer is approximately ½ of the average of the two wavelengths used, and is approximately twice the phase difference of the resin birefringent layer on the exit side. For example, it can be realized by setting the angle to about 15 degrees and for example about 75 degrees.
[0032]
Further, for example, in a two-wavelength phase plate that transmits and uses two types of linearly polarized light of 660 nm band for DVD and 790 nm wavelength band for CD, the polarization direction is rotated by 45 ° with respect to the linearly polarized light of 660 nm wavelength band, A two-wavelength phase plate that circularly polarizes linearly polarized light with respect to linearly polarized light in the 790 nm wavelength band is as follows. That is, the optical axis directions of two resin birefringent layers intersect at an angle in the range of 19 to 29 °, and one resin birefringent layer has a phase difference in the range of 533 to 652 nm, and the other The resin birefringent layer can be realized by giving a phase difference in the range of 356 to 434 nm.
[0033]
Thus, polarization control at each wavelength can be performed by combining two types of resin birefringent layers having different optical axis directions.
[0034]
【Example】
An optical head device using a phase plate in which a glass substrate 102 is sandwiched between two layers of resin birefringent layers 201 and 202 of a polymer liquid crystal as shown in FIG. 1 will be described.
Polyimide was applied to one side of the two glass substrates 101 and 103 and both sides of the glass substrate 102, and alignment treatment was performed by rubbing. The rubbing direction was the same as the incident polarization direction, and the opposing surfaces were the same, and 75 degrees and 15 degrees, respectively. The glass substrates 101 and 102 were bonded to each other with a gap of about 3 μm and the glass substrates 102 and 103 with a gap of about 6 μm. This gap was maintained by using a spherical spacer.
[0035]
Liquid crystal before polymerization is injected into this gap and polymerized by UV polymerization to form a polymer and birefringence, which is made of three layers of resin birefringent layers 201 and 202 of a polymer liquid crystal that is twice as thick. A phase plate was prepared by laminating without using an adhesive or an adhesive between adjacent substrates.
[0036]
The wavefront aberration of this phase plate is approximately 0.01 λ _rms or less, and a phase plate with good wavefront aberration can be obtained with a good yield.
Linearly polarized light having two wavelengths of 660 nm and 790 nm was incident on the phase plate from the glass substrate 103 side. As a result, it was confirmed that the light of both wavelengths can obtain a good circularly polarized light having an ellipticity of 0.95, and this phase plate sufficiently functions as a quarter-wave plate at the above two wavelengths. .
[0037]
This quarter-wave plate was mounted at the position of the laminated phase plate 1 of the optical head device shown in the schematic diagram of FIG. 6, and as a result of recording / reproducing experiments, both DVD and CD showed good recording / reproducing characteristics. Indicated.
[0038]
In addition, as a result of testing the quarter-wave plate under high temperature and high humidity, it was confirmed that the change in wavefront aberration was 0.003λ _rms or less, which was a practically acceptable level.
[0039]
【The invention's effect】
As described above, since the phase plate in the present invention is laminated with a plurality of resin birefringent layers sandwiched between glass substrates, it absorbs moisture even when used under high temperature and high humidity conditions. Can be suppressed. Therefore, it is possible to reduce the wavefront aberration of the transmitted light of the resin birefringent layer that occurs with the absorption of moisture. Furthermore, since moisture absorption is suppressed, the number of defective products is reduced and production yield is improved.
[0040]
Therefore, when the optical head device is configured by combining this phase plate with other optical components, an optical head device having good optical characteristics with less wavefront aberration can be obtained. Furthermore, the production yield of the optical head device is also improved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an example of a phase plate using three glass substrates in the present invention.
FIG. 2 is a cross-sectional view showing an example of a phase plate using one glass substrate in the present invention.
FIG. 3 is a cross-sectional view showing another example of a phase plate using one glass substrate in the present invention.
FIG. 4 is a cross-sectional view showing another example of a phase plate using three glass substrates in the present invention.
FIG. 5 is a cross-sectional view showing an example of a conventional phase plate.
FIG. 6 is a conceptual diagram showing a configuration example of an optical head device of the present invention.
[Explanation of symbols]
101, 102, 103, 104, 105, 106, 107, 108, 109, 110: Glass substrates 201, 202, 203, 204: Polymer liquid crystal layers 205, 206, 207, 208, 209, 210: Birefringence made of resin Layers 301, 302, 303, 304, 305, 306, 307, 308, 309: adhesive layer 1: laminated phase plate 2: objective lens 3: optical disk 4: beam splitter prism 5, 6: light source

Claims (4)

A light source, an objective lens for condensing the laser light emitted from the light source onto the optical recording medium, a photodetector for detecting the laser light reflected by the optical recording medium, and between or between the light source and the objective lens In the optical head device having a phase plate for controlling the polarization state of the laser beam in the optical path between the objective lens and the objective lens, the phase plate is formed by laminating two or more resin birefringent layers intersecting the optical axis direction. An optical head device, wherein an optically isotropic substrate that is transparent and has a lower moisture permeability than the resin birefringent layer is disposed between the laminated resin birefringent layers. 2. The resin birefringent layer and the optically isotropic substrate are alternately laminated to constitute a laminate, and both outer sides of the laminate are constituted by the optically isotropic substrate. Optical head device. 3. The optical head device according to claim 1, wherein at least one of the resin birefringent layers is a polymer liquid crystal film. 4. The optical head device according to claim 1, wherein the two resin birefringent layers are alternately laminated with three glass substrates which are the optically isotropic substrates and without using an adhesive material. .

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