JP2002208158A - Liquid crystal element and optical pickup using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high transmittance to light having a specific wavelength and to minimize the fluctuation of light transmittance when a liquid crystal is driven by optimizing the thin film structure of a liquid crystal element. SOLUTION: The relation between the film respective thickness and refractive indices of an ITO film, an insulating film and an alignment film which are constituents of the liquid crystal element are optimized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element for correcting aberration and an optical pickup provided with the same.

[0002]

2. Description of the Related Art In order to facilitate understanding of the prior art, a brief description will be given of the light transmission characteristics of a known optical thin film, the optical structure of a liquid crystal element, and the phase modulation of light using the liquid crystal element. First, consider an optical thin film element in which a transparent thin film is applied in a single layer or in multiple layers on a transparent substrate such as a glass substrate. At this time, since it is made of a transparent material, the light transmittance is constant at about 100% regardless of the wavelength. However, in practice, unless the refractive index of all thin films including the glass substrate is the same, the light transmittance due to the light interference effect depends on the refractive index of the transparent substrate and each thin film, the thickness of each thin film, and the wavelength of light λ. It is determined. That is, the transmittance differs depending on the wavelength. Therefore, under specific conditions, an optical thin film element that transmits only a specific wavelength can be realized, and is well known as an interference filter. In addition, there is scaling in the thickness and wavelength of the thin film with respect to transmittance. That is, when the wavelength becomes 1 / n and the film thickness of the thin film is also made 1 / n, the transmittance is the same.

Next, the optical structure of a general liquid crystal element will be described. The main components are glass substrate, ITO (transparent conductive film), insulating film, alignment film, liquid crystal layer, alignment film, ITO,
The glass substrates are arranged in this order. Glass having a refractive index of about 1.5 is used for the glass substrate. Further, ITO has a refractive index of about 1.7 to 2, a thickness of about 100 to 3000 nm, and an insulating film has a refractive index of about 1.about.2 nm due to restrictions on mass productivity and materials.
The alignment film has a thin film structure of about 7, a film thickness of about 50 to 300 nm, a refractive index of about 1.63, and a film thickness of about 50 to 200 nm. Next, the principle of phase modulation of light using a parallel alignment type or vertical alignment type liquid crystal element will be described. FIGS. 6A and 6B schematically illustrate the operation of the parallel alignment type liquid crystal element. As shown in FIG. 6A, liquid crystal molecules 602 are sandwiched between glass substrates 601 on which a transparent conductive film is applied, and the film thickness is d. Since the orientation axis direction of both glass substrates 601 is the Y-axis direction, the liquid crystal molecules 602 are aligned in parallel with their long-axis directions aligned with the Y-axis direction. When linearly polarized light 603 in the Y-axis direction is incident on this liquid crystal element, it travels while looking at the long axis direction of the liquid crystal molecules 602, so that the optical path length is n1 × d. Here, n1 is a liquid crystal molecule 602
Is the effective refractive index for the linearly polarized light 603.

Next, as shown in FIG. 6B, when a sufficiently high electric field is applied in the Z-axis direction, the liquid crystal molecules 602 are aligned with their long axes in the direction of the electric field. When linearly polarized light 603 in the Y-axis direction enters this liquid crystal element, the light travels while looking at the short-axis direction of the liquid crystal molecules 602, so that the optical path length is n2 × d. Where n2
Is the refractive index of the liquid crystal molecules 602 in the minor axis direction. Therefore, before and after the electric field is applied, the optical path length changes by (n1-n2) × d, and the phase of the incident linearly polarized light 603 becomes 2π (n1-n2) ×
The light is emitted after changing by d ÷ λ (λ is the wavelength of the incident light). An intermediate state can be realized by adjusting the electric field applied to the liquid crystal molecules. Therefore, if liquid crystal molecules are partially driven by using a gradient electric field or the like generated by the divided transparent electrode or high-resistance electrode, it is possible to give a phase distribution to the incident linearly polarized light 603. The basic operation is the same in a vertically aligned liquid crystal element, but when no electric field is applied, FIG.
FIG. 6A shows the state when an electric field is applied in the state shown in FIG.

[0005] In recent years, liquid crystal elements have been attracting attention as aberration correction elements for optical systems in high-density optical disks such as DVDs. This is due to the phase of light caused by coma caused by tilting the optical disc substrate when the liquid crystal element is inserted into the optical path of an optical pickup equipped with a laser light source and an objective lens, and spherical aberration generated when reading a multilayer disc substrate. It is intended to correct the disturbance, that is, the disturbance of the wavefront of light. In accordance with the amount of aberration generated at this time, the liquid crystal element is provided with a refractive index distribution by the divided transparent electrodes to correct the wavefront of the laser light.

[0006]

However, in an optical element for a laser optical system having a limited laser power, such as an optical pickup, the light transmittance is an important factor. In particular, in the case of an optical pickup using a blue laser with a wavelength of about 450 nm, which has recently attracted attention, the luminous efficiency of the blue laser is lower than that of a conventional red laser with a wavelength of about 650 nm. In addition, since laser light generates light having a specific sharp wavelength, light transmittance at that wavelength is important. On the other hand, the liquid crystal element has a thin film structure, and the effective refractive index of the liquid crystal layer changes by further driving. Therefore, when using laser light or monochromatic light, the thin film structure of the liquid crystal element is optimized for the wavelength to be used so that the transmittance is maximized. It is desirable to be.
However, conventional liquid crystal elements have been developed mainly for display applications using an optical shutter effect for white light by using not a phase modulation element but a polarizing plate. Therefore, the thin film structure of the liquid crystal element has not been designed and optimized for the laser optical system. Parallel alignment type liquid crystal, which is a phase modulation type, has been often used for general laser optical systems.However, laser light generally has very large power, so there is no need to consider light transmittance, and thin films of liquid crystal elements are not required. There was no need to optimize the structure.

[0007]

FIG. 7 shows a wavelength 650 when the thickness and the refractive index of ITO are changed in a liquid crystal element.
The simulation result of the light reflection characteristic in monochromatic light of nm is shown. The liquid crystal element includes a glass substrate, ITO, an insulating film, an alignment film, a liquid crystal layer, an alignment film, ITO, and a glass substrate in this order. The refractive index of the glass substrate is 1.49, the refractive index of the insulating film is 1.75, the thickness is 70 nm, the refractive index of the alignment film is 1.63, the thickness is 70 nm, the effective refractive index of the liquid crystal layer is 1.6, and the thickness. Is 5000 nm. In FIG. 7, the Y axis is IT
The film thickness of O, the X axis represents the refractive index of ITO, and the Z axis represents the light reflectance. As shown in FIG. 7, the first reflectance has a minimum value near the thickness of 110 nm of ITO, and is stable even if the refractive index of ITO changes. Also, as the refractive index of ITO is closer to 1.6, the reflectance decreases and the variation with respect to the change of the ITO film thickness also decreases. Therefore, under these conditions, the film thickness of ITO is 1
When the refractive index is 10 nm and the refractive index is 1.6, the light transmittance becomes maximum and stable, but the value that can be selected as the ITO refractive index is actually 1.
7 FIG. 4 also shows that there is a minimum value of the second reflectivity when the thickness of the ITO is around 200 nm.

In actual use of a liquid crystal element, the effective refractive index changes by operating the liquid crystal element. FIG.
FIG. 4 shows a simulation result of light reflection characteristics when the thickness of the insulating film and the effective refractive index of the liquid crystal layer are changed. The ITO film thickness was 110 nm and the refractive index was 1.7.
Is the same as The Y axis represents the thickness of the insulating film, the X axis represents the effective refractive index of the liquid crystal layer, and the Z axis represents the light reflectance. As shown in FIG. 8, a saddle point of the light reflectance exists near the thickness of the insulating film of 70 nm.
Even if the effective refractive index of the liquid crystal layer changes, a high light transmittance is stably realized.

FIG. 9 shows a simulation result of light reflection characteristics when the thickness of the ITO and the thickness of the insulating film are changed in the liquid crystal element. The refractive index of ITO is 1.7, and other conditions are the same as those in FIG. The Y axis is the thickness of the ITO, the X axis is the thickness of the insulating film, and the Z axis is the light reflectance. FIG.
There is a saddle point of light reflectivity, which is represented by the linear expression Y = aX + b, where a is -0.4, b is 60 + 190n or a is -0.6, and b is 170 + 190n (n is an integer). . That is, I approximately satisfies the relationship of this linear expression.
If a set of the TO film thickness and the insulating film thickness is selected, the light transmittance is maximized, and the film is stable even when the film thickness changes. This conditional expression is used when the wavelength of the incident light is 650 nm and cannot be used if the wavelength changes. As described in the description of the light transmission characteristics of the prior art thin film, considering the scaling of the thickness and wavelength of the thin film, the above-mentioned linear expression is Y = aX + b × λ ÷ 65.
It becomes 0 (λ is the wavelength of the incident light described in the unit of nm), and is generalized to the used wavelength. Needless to say, the thickness of the alignment film is also multiplied by λ ÷ 650.

In the above simulation, calculation was performed by a thin-film matrix method which is a known method (for example, Junpei Tsujiuchi, Optical Opinion 2, Asakura Shoten, pp. 49-53). In short, the optical transfer characteristic of a thin film is described by a 2 × 2 matrix described from a refractive index n, a thickness d, and a used wavelength λ of the thin film. Calculate the entire thin film matrix. This is a method of calculating the light reflectance or light transmittance from the result.

[0011]

FIG. 1 shows an example of an embodiment of the present invention. The liquid crystal element of the present invention has the following configuration. That is, the glass substrate 101, which is a pair of transparent substrates in which the ITO 102 and the alignment film 104 are sequentially stacked, faces each other,
A liquid crystal layer 105 is provided between the liquid crystal layer 105 and the alignment film 104, and an insulating film 103 is formed between the alignment film 104 and the ITO 102 on at least one transparent substrate. The liquid crystal element for modulating the phase has a basic configuration. In particular, in the present invention, when the thickness of the liquid crystal layer is 3000 nm or more and 8000 nm or less, the effective refractive index of the liquid crystal molecules with respect to incident light changes in the range of 1.4 or more and 1.8 or less. 1.5 ± 0.2, ITO 1.7 ± 0.1, insulating film 1.75 ± 0.1, alignment film 1.63 ± 0.1, and alignment film thickness 70 ± 30 nm The greatest feature is in providing a liquid crystal element characterized by the following.
More specifically, in this embodiment, optimization is performed under the following conditions. That is, the glass substrate 101
Is a refractive index of 1.49, a thickness of 0.7 mm, ITO is a refractive index of 1.7, a thickness of Y nm, and an insulating film 103 is a refractive index of 1.7.
5, the thickness X nm, the refractive index of the alignment film 104 is 1.63, the thickness is 70 nm, and the effective refractive index of the liquid crystal layer 105 is 1.5 to 1.
Variable up to 7, the thickness is 5000 nm. Here ITO
The film thickness Y of the insulating film 103 and the film thickness X of the insulating film 103 satisfy the following relational expression. Y = aX + b × λ ÷ 650. Here, a is −0.
4, b is 60 + 190n or a is -0.6, b is 1
70 + 190n (n is an integer) and λ is the wavelength of the incident light described in nm. Further, as an application example using the above-described liquid crystal element, a laser light source, an objective lens,
Used in an optical pickup having a liquid crystal element for aberration correction, it is possible to provide a liquid crystal element whose transmittance is optimized by an optimal condition specific to each configuration of the liquid crystal element. It may be possible to satisfy the function as the used optical pickup.

(Embodiment) FIG. 2 shows an embodiment of the present invention. The glass substrate 201 has a refractive index of 1.49, a thickness of 0.7 mm,
TO202 has a refractive index of 1.7, a thickness of 120 nm, and an insulating film 2
03 has a refractive index of 1.75 and a thickness of 70 nm, the alignment film 204 has a refractive index of 1.63 and a thickness of 70 nm, and the liquid crystal layer 205 has a refractive index variable from 1.5 to 1.7. Is optimized for 650 nm.

FIG. 3 shows the liquid crystal device of FIG.
The horizontal axis represents the effective refractive index of the liquid crystal, and the vertical axis represents the light reflectance. FIG. 3 shows that even if the effective refractive index of the liquid crystal changes, the change in the reflectance is suppressed to about 1% at the maximum. Therefore, if a non-reflective coating is applied to the glass substrate of the present liquid crystal element, a transmittance of 99% or more can be theoretically obtained.

FIG. 4 shows another embodiment of the present invention. The glass substrate 401 has a refractive index of 1.49, a thickness of 0.7 mm, and ITO.
402 has a refractive index of 1.7 and a thickness of 74 nm, the insulating film 403 has a refractive index of 1.75 and a thickness of 43 nm, the alignment film 404 has a refractive index of 1.63 and a thickness of 43 nm, and the liquid crystal layer 405 has a refractive index of 1.
Variable from 5 to 1.7, the thickness is 5000 nm, and the transmittance is optimized for the wavelength of the incident light of 400 nm.
That is, the above-mentioned scaling is applied to the liquid crystal element having the structure shown in FIG.
It is 400 times 50/50. However, scaling is meaningless because the glass substrate is not a thin film. Although the liquid crystal layer thickness originally needs to be scaled, it is not subject to scaling because the refractive index fluctuates during driving and the film thickness is determined from the optical conditions used.

FIG. 5 shows another embodiment of the present invention, which is applied to an optical pickup optical system provided with a known liquid crystal aberration correcting mechanism. Laser light 502 emitted from a laser light source 501 having a wavelength of 650 nm is converted into parallel light by a collimating lens 503, passes through a liquid crystal element 504, and is focused on an optical disk 506 by an objective lens 505. The ITO of the liquid crystal element 504 is constituted by divided electrodes and can display an arbitrary pattern. When the optical wavefront aberration occurs due to the inclination of the substrate of the optical disk or the change in thickness, the liquid crystal element 504 is driven by the aberration correction signal 507 to sequentially perform the aberration correction. Since the optical structure of the liquid crystal element 504 is the same as that of the liquid crystal element 50 shown in FIG.
The theoretical light transmittance when the anti-reflection coating is applied to the glass substrate of No. 4 is 99% or more.

FIG. 10 is a diagram showing light reflection characteristics of a liquid crystal optical element not according to the present invention. As the refractive index of ITO:
9 was used. Other conditions are the same as those of the liquid crystal optical element according to the present invention. Compared to FIG. 8 relating to the present invention, the reflectance is 1
The saddle point, which reaches 0% and has little change in reflectance during liquid crystal driving, is not clearly seen.

FIG. 11 is a diagram showing the light reflection characteristics of a liquid crystal optical element not according to the present invention. When the thickness of ITO is 17
It was set to 0 nm. Other conditions are the same as those of the liquid crystal optical element according to the present invention. It can be seen from FIG. 3 relating to the present invention that the reflectance greatly fluctuates to 2% or more. Therefore, it is understood from the above description that the optimization of the individual components constituting the liquid crystal element is important.

[0018]

As is clear from the above description, the use of the liquid crystal device according to the present invention makes it possible to optimize the light transmittance for arbitrary monochromatic light. That is, even when the liquid crystal is driven with a high light transmittance, the fluctuation of the light transmittance can be suppressed low. Also, in the production of the liquid crystal element, the change in transmittance is small even if the film thickness slightly changes. If this liquid crystal element is used as an aberration correction element or an optical wavefront control element in an optical system such as a pickup of an optical disk or a laser printer, the light use efficiency of the optical system is increased, which contributes to a reduction in power consumption of equipment.

In the present invention, the liquid crystal element is optimized for a specific wavelength. However, it can be optimized for a plurality of wavelengths. In that case, the performance is lower than the optimization with a single color.

[Brief description of the drawings]

FIG. 1 is an example showing an embodiment of the present invention.

FIG. 2 is an embodiment of the present invention.

FIG. 3 is a diagram showing light reflection characteristics of a liquid crystal element according to the present invention.

FIG. 4 is another embodiment of the present invention.

FIG. 5 is another embodiment of the present invention.

FIG. 6 is a diagram illustrating the principle of phase modulation of a parallel alignment type liquid crystal element.

FIG. 7 is a diagram illustrating light reflection characteristics of a liquid crystal element.

FIG. 8 is a diagram illustrating light reflection characteristics of a liquid crystal element.

FIG. 9 is a diagram illustrating light reflection characteristics of a liquid crystal element.

FIG. 10 is a diagram illustrating light reflection characteristics of a liquid crystal element.

FIG. 11 is a diagram illustrating light reflection characteristics of a liquid crystal element.

[Explanation of symbols]

 Reference Signs List 101 glass substrate 102 ITO 103 insulating film 104 alignment film 105 liquid crystal layer 501 laser light source 502 laser light 503 collimating lens 504 liquid crystal element 505 objective lens 506 optical disk 507 aberration correction signal 602 liquid crystal molecule 603 linear polarization

Claims (4)

[Claims] A pair of transparent substrates in which ITO and an alignment film are sequentially laminated face each other, a liquid crystal layer is provided between the pair of transparent substrates, and an alignment film on at least one of the transparent substrates and the ITO.
A liquid crystal element for modulating the phase of monochromatic light or laser light on which an insulating film is formed. The liquid crystal layer has a thickness of 3000 nm or more and 8000 nm or less, and an effective refractive index of liquid crystal molecules with respect to incident light is 1 nm. The refractive index with respect to incident light is 1.5 ± 0.2 for the transparent substrate, 1.7 ± 0.1 for ITO, and 1.75 ± 0 for the insulating film. 1. A liquid crystal element characterized by having an alignment film of 1.63 ± 0.1 and an alignment film thickness of 70 ± 30 nm. 2. The relationship between the thickness Ynm of the ITO and the thickness Xnm of the insulating film is approximately Y = aX + b × λ ÷ 6.
50 (λ is the wavelength of incident light described in nm), a is -0.4, b is 60 + 190n or a is-
2. The liquid crystal device according to claim 1, wherein 0.6 and b are 170 + 190n (n is an integer). 3. The film thickness of the ITO is (120 ± 30) ×
λ ÷ 650 nm, insulation film thickness is (70 ± 30) × λ ÷ 65
2. The liquid crystal device according to claim 1, wherein the thickness is 0 nm. 4. An optical pickup having a liquid crystal element for aberration correction, wherein the liquid crystal element is a liquid crystal element.
An optical pickup using a liquid crystal element, which is the liquid crystal element according to any one of the above.